Blockade of the ATP-sensitive potassium channel modulates reactive hyperemia in the canine coronary circulation

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Effect of ascorbic acid on forearm reactive hyperaemia in patients with hypercholesterolaemia

BACKGROUND

This study was designed to research the effect of hypercholesterolaemia and ascorbic acid on forearm blood flow (FBF) reactive hyperaemia (RH). Reactive hyperaemia seems to be at least partly endothelium-dependent. Endothelial dysfunction has been described in patients with hypercholesterolaemia, and has been reversed with ascorbic acid administration.

METHOD

Forearm blood flow was studied with venous occlusion plethsmography in 26 healthy volunteers and 46 hypercholesterolaemic patients. Hypercholesterolaemic patients were divided into two groups. Group A comprised 25 patients, who received ascorbic acid and group B comprised 21 patients, who received placebo. All subjects underwent measurement of FBF at baseline and during RH (phase A). Forearm blood flow during RH was measured every 15 seconds for three minutes. Subsequently patients in group A received 2 g of ascorbic acid orally in the form of effervescent tablets, and patients in group B received placebo orally in the same form. Forearm blood flow measurements at baseline and during RH were repeated two hours later (phase B).

RESULTS

Maximal percent increase of FBF was significantly higher in healthy subjects than in hypercholesterolaemic patients (139.1+/-12.1% versus 73.1+/-11.0% respectively, P<0.05). Duration of RH was smaller in hypercholesterolaemic patients compared to normal subjects (60.9+/-17.1 seconds versus 105.6+/-10.2 seconds, P<0.05). Administration of ascorbic acid but not of placebo increased the duration of RH (69.1+/-11.1 seconds versus 104.1+/-12.2 seconds, P<0.05) but not of peak RH FBF.

CONCLUSION

Hypercholesterolaemia seems to impair both the early and late phase of RH. Ascorbic acid improves only the duration of RH, possibly due to its antioxidant effect on endothelium.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Contribution of nitrergic nerve in canine gingival reactive hyperemia

Reactive hyperemia reflects a compensatory vasodilation response of the local vasculature in ischemic tissue. The purpose of this study is to clarify the mechanism of regulation of this response in gingival circulation by using pharmacological analysis of reactive hyperemia and histochemical analysis of gingival tissue. Application of pressure to the gingiva was used to create temporary ischemia, and gingival blood flow was measured after pressure release. Reactive hyperemia increased in proportion to the duration of pressure. Systemic hemodynamics remained unaffected by the stimulus; therefore, the gingival reactive hyperemia reflected a local adjustment in circulation. Gingival reactive hyperemia was significantly suppressed by nitric oxide (NO) synthase inhibitors, especially the neural NO synthase-selective antagonist 7-nitroindazole, but not by anticholinergic drugs, β-blockers, or antihistaminergic drugs. Moreover, immunohistochemical staining for neural NO synthase and histochemical staining for NADPH diaphorase activity were both positive in the gingival perivascular region. These histochemical and pharmacological analyses show that reactive hyperemia following pressure release is mediated by NO-induced vasodilation. Furthermore, histochemical analysis strongly suggests that NO originates from nitrergic nerves. Therefore, NO may play an important role in the neural regulation of local circulation in gingival tissue ischemia.

Metabolic hyperemia requires ATP-sensitive K+ channels and H2O2 but not adenosine in isolated mouse hearts

We have previously demonstrated that adenosine-mediated H2O2 production and opening of ATP-sensitive K(+) (KATP) channels contributes to coronary reactive hyperemia. The present study aimed to investigate the roles of adenosine, H2O2, and KATP channels in coronary metabolic hyperemia (MH). Experiments were conducted on isolated Langendorff-perfused mouse hearts using combined pharmacological approaches with adenosine receptor (AR) knockout mice. MH was induced by electrical pacing at graded frequencies. Coronary flow increased linearly from 14.4 ± 1.2 to 20.6 ± 1.2 ml·min(-1)·g(-1) with an increase in heart rate from 400 to 650 beats/min in wild-type mice. Neither non-selective blockade of ARs by 8-(p-sulfophenyl)theophylline (8-SPT; 50 μM) nor selective A2AAR blockade by SCH-58261 (1 μM) or deletion affected MH, although resting flow and left ventricular developed pressure were reduced. Combined A2AAR and A2BAR blockade or deletion showed similar effects as 8-SPT. Inhibition of nitric oxide synthesis by N-nitro-l-arginine methyl ester (100 μM) or combined 8-SPT administration failed to reduce MH, although resting flows were reduced (by ∼20%). However, glibenclamide (KATP channel blocker, 5 μM) decreased not only resting flow (by ∼45%) and left ventricular developed pressure (by ∼36%) but also markedly reduced MH by ∼94%, resulting in cardiac contractile dysfunction. Scavenging of H2O2 by catalase (2,500 U/min) also decreased resting flow (by ∼16%) and MH (by ∼24%) but to a lesser extent than glibenclamide. Our results suggest that while adenosine modulates coronary flow under both resting and ischemic conditions, it is not required for MH. However, H2O2 and KATP channels are important local control mechanisms responsible for both coronary ischemic and metabolic vasodilation.

Interactions between A(2A) adenosine receptors, hydrogen peroxide, and KATP channels in coronary reactive hyperemia

Myocardial metabolites such as adenosine mediate reactive hyperemia, in part, by activating ATP-dependent K(+) (K(ATP)) channels in coronary smooth muscle. In this study, we investigated the role of adenosine A(2A) and A(2B) receptors and their signaling mechanisms in reactive hyperemia. We hypothesized that coronary reactive hyperemia involves A(2A) receptors, hydrogen peroxide (H(2)O(2)), and KATP channels. We used A(2A) and A(2B) knockout (KO) and A(2A/2B) double KO (DKO) mouse hearts for Langendorff experiments. Flow debt for a 15-s occlusion was repaid 128 ± 8% in hearts from wild-type (WT) mice; this was reduced in hearts from A(2A) KO and A(2A)/(2B) DKO mice (98 ± 9 and 105 ± 6%; P < 0.05), but not A(2B) KO mice (123 ± 13%). Patch-clamp experiments demonstrated that adenosine activated glibenclamide-sensitive KATP current in smooth muscle cells from WT and A(2B) KO mice (90 ± 23% of WT) but not A(2A) KO or A(2A)/A(2B) DKO mice (30 ± 4 and 35 ± 8% of WT; P < 0.05). Additionally, H(2)O(2) activated KATP current in smooth muscle cells (358 ± 99%; P < 0.05). Catalase, an enzyme that breaks down H(2)O(2), attenuated adenosine-induced coronary vasodilation, reducing the percent increase in flow from 284 ± 53 to 89 ± 13% (P < 0.05). Catalase reduced the repayment of flow debt in hearts from WT mice (84 ± 9%; P < 0.05) but had no effect on the already diminished repayment in hearts from A(2A) KO mice (98 ± 7%). Our findings suggest that adenosine A(2A) receptors are coupled to smooth muscle KATP channels in reactive hyperemia via the production of H(2)O(2) as a signaling intermediate.

Effects of autologous, cross-linked erythrocytes on isolated hypoperfused rabbit heart dynamics

The present study was aimed at assessing the effects of either red blood cells (RBC) or RBC cross-linked with the bifunctional dimethyl suberimidate reagent (C-RBC) on contractile force (CFo), heart rate (HR) and coronary flow (CF) of the isolated rabbit heart hypoperfused with RBC suspensions under 30 mm Hg constant pressure. RBC or C-RBC caused a rapid and marked reduction of CF, CFo and HR. In RBC-treated hearts, however, reperfusion with Tyrode solution partially restored the initial myocardial parameters, while in C-RBC-treated hearts a rapid impairment of diastolic relaxation with a subsequent, steady and increasing heart contracture was observed. Histological analysis showed that in C-RBC-perfused hearts either capillaries or precapillary arterioles were occluded by C-RBC in spite of extensive washings with Tyrode solution. These findings indicate that C-RBC impair coronary circulation markedly and irreversibly.

Coronary artery reperfusion: The ADP receptor P2Y(1) mediates early reactive hyperemia in vivo in pigs

The physiological mechanisms that regulate reactive hyperemia are not fully understood. We postulated that the endothelial P2Y(1) receptor that release vasodilatory factors in response to ADP might play a vital role in the regulation of coronary flow. Intracoronary flow was measured with a Doppler flow-wire in a porcine model. 2-MeSADP (10(-5) M), ATP (10(-4) M) or UTP (10(-4) M) alone or as co-infusion with a selective P2Y(1) receptor blocker, MRS 2179 (10(-3) M) was locally delivered through the tip of a coronary angioplasty balloon. In separate pigs the coronary artery was occluded with the balloon for 10 min. During the first and tenth minutes of coronary ischemia, 2.5 ml of MRS 2179 (10(-3) M) was delivered distal to the occlusion in 8 pigs, 10 pigs were used as controls. MRS 2179 fully inhibited the 2-MeSADP-mediated coronary flow increase (P < 0.05) with no effect on UTP, indicating selective P2Y(1) inhibition. ATP-mediated flow increase was significantly inhibited by MRS 2179. During reactive hyperemia following coronary occlusion, flow increased by nearly sevenfold. MRS 2179, however, reduced the post-ischemic hyperemia by a mean of 46% during the period 1-2.5 min following balloon deflation (P < 0.05), which corresponds to peak velocity flow during reperfusion. In conclusion, MRS 2179, a selective P2Y(1) receptor blocker, significantly reduces the increased coronary flow caused both by 2-MeSADP and reactive hyperemia in coronary arteries. Thus, ADP acting on the endothelial P2Y(1) receptor may play a major role in coronary flow during post-ischemic hyperemia.

Contribution of adenosine A(2A) and A(2B) receptors to ischemic coronary dilation: role of K(V) and K(ATP) channels

This study was designed to elucidate the contribution of adenosine A(2A) and A(2B) receptors to coronary reactive hyperemia and downstream K(+) channels involved. Coronary blood flow was measured in open-chest anesthetized dogs. Adenosine dose-dependently increased coronary flow from 0.72 ± 0.1 to 2.6 ± 0.5 mL/minute/g under control conditions. Inhibition of A(2A) receptors with SCH58261 (1 μm) attenuated adenosine-induced dilation by ∼50%, while combined administration with the A(2B) receptor antagonist alloxazine (3 μm) produced no additional effect. SCH58261 significantly reduced reactive hyperemia in response to a transient 15 second occlusion; debt/repayment ratio decreased from 343 ± 63 to 232 ± 44%. Alloxazine alone attenuated adenosine-induced increases in coronary blood flow by ∼30% but failed to alter reactive hyperemia. A(2A) receptor agonist CGS21680 (10 μg bolus) increased coronary blood flow by 3.08 ± 0.31 mL/minute/g. This dilator response was attenuated to 0.76 ± 0.14 mL/minute/g by inhibition of K(V) channels with 4-aminopyridine (0.3mm) and to 0.11 ± 0.31 mL/minute/g by inhibition of K(ATP) channels with glibenclamide (3 mg/kg). Combined administration abolished vasodilation to CGS21680. These data indicate that A(2A) receptors contribute to coronary vasodilation in response to cardiac ischemia via activation of K(V) and K(ATP) channels.

Role of potassium channels in coronary vasodilation

K+ channels in coronary arterial smooth muscle cells (CASMC) determine the resting membrane potential ( Em) and serve as targets of endogenous and therapeutic vasodilators. Em in CASMC is in the voltage range for activation of L-type Ca2+ channels; therefore, when K+ channel activity changes, Ca2+ influx and arterial tone change. This is why both Ca2+ channel blockers and K+ channel openers have such profound effects on coronary blood flow; the former directly inhibits Ca2+ influx through L-type Ca2+ channels, while the latter indirectly inhibits Ca2+ influx by hyperpolarizing Em and reducing Ca2+ channel activity. K+ channels in CASMC play important roles in vasodilation to endothelial, ischemic and metabolic stimuli. The purpose of this article is to review the types of K+ channels expressed in CASMC, discuss the regulation of their activity by physiological mechanisms and examine impairments related to cardiovascular disease.

Regulation of Coronary Blood Flow During Exercise

Exercise is the most important physiological stimulus for increased myocardial oxygen demand. The requirement of exercising muscle for increased blood flow necessitates an increase in cardiac output that results in increases in the three main determinants of myocardial oxygen demand: heart rate, myocardial contractility, and ventricular work. The approximately sixfold increase in oxygen demands of the left ventricle during heavy exercise is met principally by augmenting coronary blood flow (∼5-fold), as hemoglobin concentration and oxygen extraction (which is already 70–80% at rest) increase only modestly in most species. In contrast, in the right ventricle, oxygen extraction is lower at rest and increases substantially during exercise, similar to skeletal muscle, suggesting fundamental differences in blood flow regulation between these two cardiac chambers. The increase in heart rate also increases the relative time spent in systole, thereby increasing the net extravascular compressive forces acting on the microvasculature within the wall of the left ventricle, in particular in its subendocardial layers. Hence, appropriate adjustment of coronary vascular resistance is critical for the cardiac response to exercise. Coronary resistance vessel tone results from the culmination of myriad vasodilator and vasoconstrictors influences, including neurohormones and endothelial and myocardial factors. Unraveling of the integrative mechanisms controlling coronary vasodilation in response to exercise has been difficult, in part due to the redundancies in coronary vasomotor control and differences between animal species. Exercise training is associated with adaptations in the coronary microvasculature including increased arteriolar densities and/or diameters, which provide a morphometric basis for the observed increase in peak coronary blood flow rates in exercise-trained animals. In larger animals trained by treadmill exercise, the formation of new capillaries maintains capillary density at a level commensurate with the degree of exercise-induced physiological myocardial hypertrophy. Nevertheless, training alters the distribution of coronary vascular resistance so that more capillaries are recruited, resulting in an increase in the permeability-surface area product without a change in capillary numerical density. Maintenance of α- and ß-adrenergic tone in the presence of lower circulating catecholamine levels appears to be due to increased receptor responsiveness to adrenergic stimulation. Exercise training also alters local control of coronary resistance vessels. Thus arterioles exhibit increased myogenic tone, likely due to a calcium-dependent protein kinase C signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, training augments endothelium-dependent vasodilation throughout the coronary microcirculation. This enhanced responsiveness appears to result principally from an increased expression of nitric oxide (NO) synthase. Finally, physical conditioning decreases extravascular compressive forces at rest and at comparable levels of exercise, mainly because of a decrease in heart rate. Impedance to coronary inflow due to an epicardial coronary artery stenosis results in marked redistribution of myocardial blood flow during exercise away from the subendocardium towards the subepicardium. However, in contrast to the traditional view that myocardial ischemia causes maximal microvascular dilation, more recent studies have shown that the coronary microvessels retain some degree of vasodilator reserve during exercise-induced ischemia and remain responsive to vasoconstrictor stimuli. These observations have required reassessment of the principal sites of resistance to blood flow in the microcirculation. A significant fraction of resistance is located in small arteries that are outside the metabolic control of the myocardium but are sensitive to shear and nitrovasodilators. The coronary collateral system embodies a dynamic network of interarterial vessels that can undergo both long- and short-term adjustments that can modulate blood flow to the dependent myocardium. Long-term adjustments including recruitment and growth of collateral vessels in response to arterial occlusion are time dependent and determine the maximum blood flow rates available to the collateral-dependent vascular bed during exercise. Rapid short-term adjustments result from active vasomotor activity of the collateral vessels. Mature coronary collateral vessels are responsive to vasodilators such as nitroglycerin and atrial natriuretic peptide, and to vasoconstrictors such as vasopressin, angiotensin II, and the platelet products serotonin and thromboxane A2. During exercise, ß-adrenergic activity and endothelium-derived NO and prostanoids exert vasodilator influences on coronary collateral vessels. Importantly, alterations in collateral vasomotor tone, e.g., by exogenous vasopressin, inhibition of endogenous NO or prostanoid production, or increasing local adenosine production can modify collateral conductance, thereby influencing the blood supply to the dependent myocardium. In addition, vasomotor activity in the resistance vessels of the collateral perfused vascular bed can influence the volume and distribution of blood flow within the collateral zone. Finally, there is evidence that vasomotor control of resistance vessels in the normally perfused regions of collateralized hearts is altered, indicating that the vascular adaptations in hearts with a flow-limiting coronary obstruction occur at a global as well as a regional level. Exercise training does not stimulate growth of coronary collateral vessels in the normal heart. However, if exercise produces ischemia, which would be absent or minimal under resting conditions, there is evidence that collateral growth can be enhanced. In addition to ischemia, the pressure gradient between vascular beds, which is a determinant of the flow rate and therefore the shear stress on the collateral vessel endothelium, may also be important in stimulating growth of collateral vessels.

Voltage-dependent K+ channels regulate the duration of reactive hyperemia in the canine coronary circulation

We previously demonstrated a role for voltage-dependent K(+) (K(V)) channels in coronary vasodilation elicited by myocardial metabolism and exogenous H(2)O(2), as responses were attenuated by the K(V) channel blocker 4-aminopyridine (4-AP). Here we tested the hypothesis that K(V) channels participate in coronary reactive hyperemia and examined the role of K(V) channels in responses to nitric oxide (NO) and adenosine, two putative mediators. Reactive hyperemia (30-s occlusion) was measured in open-chest dogs before and during 4-AP treatment [intracoronary (ic), plasma concentration 0.3 mM]. 4-AP reduced baseline flow 34 +/- 5% and inhibited hyperemic volume 32 +/- 5%. Administration of 8-phenyltheophylline (8-PT; 0.3 mM ic or 5 mg/kg iv) or N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mg/min ic) inhibited early and late portions of hyperemic flow, supporting roles for adenosine and NO. 4-AP further inhibited hyperemia in the presence of 8-PT or L-NAME. Adenosine-induced blood flow responses were attenuated by 4-AP (52 +/- 6% block at 9 microg/min). Dilation of arterioles to adenosine was attenuated by 0.3 mM 4-AP and 1 microM correolide, a selective K(V)1 antagonist (76 +/- 7% and 47 +/- 2% block, respectively, at 1 microM). Dilation in response to sodium nitroprusside, an NO donor, was attenuated by 4-AP in vivo (41 +/- 6% block at 10 microg/min) and by correolide in vitro (29 +/- 4% block at 1 microM). K(V) current in smooth muscle cells was inhibited by 4-AP (IC(50) 1.1 +/- 0.1 mM) and virtually eliminated by correolide. Expression of mRNA for K(V)1 family members was detected in coronary arteries. Our data indicate that K(V) channels play an important role in regulating resting coronary blood flow, determining duration of reactive hyperemia, and mediating adenosine- and NO-induced vasodilation.

Cellular and molecular mechanisms regulating vascular tone. Part 2: regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells

Understanding the physiological mechanisms regulating vascular tone would lead to better circulatory management during general anesthesia. This two-part review provides an overview of current knowledge about the cellular and molecular mechanisms regulating the contractile state of vascular smooth muscle cells (i.e., vascular tone). The first part reviews basic mechanisms controlling the cytosolic Ca2+ concentration in vascular smooth muscle cells, and the Ca2+-dependent regulation of vascular tone. This second part reviews the regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells-including Rho/Rho kinase, protein kinase C, arachidonic acid, Ca2+/calmodulin-dependent protein kinase II, caldesmon, calponin, mitogen-activated protein kinases, tyrosine kinases, cyclic nucleotides, Cl- channels, and K+ channels.

The ADP receptor P2Y(1) mediates t-PA release in pigs during cardiac ischemia

BACKGROUND

The endothelial ADP receptor P2Y(1) is responsible for a large part of the reactive hyperemia following cardiac ischemia. Tissue plasminogen activator (t-PA) increases during reactive hyperemia. We postulated that the release of t-PA during reactive hyperemia could be mitigated through blocking the coronary endothelial P2Y(1) receptor.

METHODS

t-PA was measured in peripheral arterial blood and locally in the venous blood from the coronary sinus in a porcine model. The stable ADP analogue 2-MeSADP (10(-5) M), alone or as co-infusion with a selective P2Y(1) receptor blocker, MRS2179 (10(-3) M) was locally delivered in the left anterior descending artery through the tip of a coronary angioplasty balloon. In separate pigs the coronary artery was occluded with the balloon for 10 min. During the first and tenth minute of coronary ischemia, 2.5 ml of MRS2179 (10(-3) M) was delivered distal to the occlusion in 8 pigs, 10 pigs were used as controls.

RESULTS

2-MeSADP increased levels of t-PA in the coronary sinus, which could be significantly inhibited by co-infusion with MRS2179. During cardiac ischemia and reperfusion, t-PA increased significantly, an effect that could be significantly inhibited by MRS2179.

CONCLUSIONS

Intra coronary administered MRS2179, a selective P2Y(1) receptor blocker, significantly reduces the increased levels of t-PA caused by both 2-MeSADP and cardiac ischemia in coronary arteries. Thus, ADP acting on the endothelial P2Y(1) receptor may mediate release of t-PA during ischemia and post-ischemic hyperemia, an effect that may counteract some of the platelet activating effects of ADP. Abbreviated Abstract We postulated that the release of t-PA during post ischemic reactive hyperemia could be mitigated through blocking the coronary endothelial P2Y(1) receptor in a porcine model. The ADP analogue 2-MeSADP (10(-5) M), alone or as co-infusion with a the P2Y(1) receptor blocker, MRS2179 (10(-3) M) was locally delivered in the left anterior descending artery. In separate pigs the coronary artery was occluded for ten min. During the first and tenth min of coronary ischemia, 2.5 ml of MRS2179 (10(-3) M) was delivered distal to the occlusion. t-PA was measured in peripheral arterial blood and also from the Coronary Sinus. We found that levels of t-PA in the blood from the coronary Sinus increased during infusion with 2-MeSADP as well as during ischemia and reperfusion. In pigs treated with MRS2179 levels of t-PA in the Coronary Sinus were significantly reduced during both coinfusion with 2-MeSADP and during ischemia/reperfusion. Thus, ADP acting on the endothelial P2Y(1) M receptor may mediate release of t-PA during ischemia and post-ischemic hyperemia, an effect that may counteract some of the platelet activating effects of ADP.

Effects of the blockade of cardiac sarcolemmal ATP-sensitive potassium channels on arrhythmias and coronary flow in ischemia–reperfusion model in isolated rat hearts

Activation of ATP-sensitive K+ channels (K ATP) during ischemia leads to arrhythmias and blockade of these channels exert antiarrhythmic action. In this study, we investigated the effects of HMR1098, a sarcolemmal K ATP channel blocker and 5-hydroxydeconoate (5-HD), a mitochondrial K ATP channel blocker on cardiac function and arrhythmias in isolated rat hearts. The hearts were subjected to 30 min coronary occlusion, followed by 30 min reperfusion. In the preischemic period, both HMR 1098 and 5-HD slightly increased coronary perfusion pressure. Coronary occlusion increased the perfusion pressure and decreased the left ventricular developed pressure (LVDP) in both control and drug-treated hearts. However, inhibition of LVDP was greater and recovery of the perfusion pressure was lower in 30 micromol/l HMR1098 and 100 micromol/l 5-HD-treated hearts compared to control (P < 0.05). HMR1098, at 3 micromol/l, but not at 30 micromol/l, significantly reduced the ratio of bigeminis, couplets and salvos (P < 0.05). Ventricular tachycardia and ventricular fibrillation were not prevented by HMR1098, at both concentrations, and with 5-HD (100 micromol/l). These results suggest that blockade of sarcK ATP and mitoK ATP channels exert weak antiarrhythmic action, but reduce the recovery of coronary perfusion and contractile force, implying that both types of K(ATP) channels have beneficial role in the recovery of ischemic rat myocardium.

Reduced coronary reserve in response to short-term ischaemia and vasoactive drugs in ex vivo hearts from diabetic mice

AIM

The aim of the present study was to compare the coronary flow (CF) reserve of ex vivo perfused hearts from type 2 diabetic (db/db) and non-diabetic (db/+) mice.

METHODS

The hearts were perfused in the Langendorff mode with Krebs-Henseleit bicarbonate buffer (37 degrees C, pH 7.4) containing 11 mmol L(-1) glucose as energy substrate. The coronary reserve was measured in response to three different interventions: (1) administration of nitroprusside (a nitric oxide donor), (2) administration of adenosine and (3) production of reactive hyperaemia by short-term ischaemia.

RESULTS

Basal CF was approximately 15% lower in diabetic when compared with non-diabetic hearts (2.1 +/- 0.1 vs. 2.6 +/- 0.2 mL min(-1)). The maximum increase in CF rate in response to sodium nitroprusside and adenosine was significantly lower in diabetic (0.6 +/- 0.1 and 0.9 +/- 0.1 mL min(-1) respectively) than in non-diabetic hearts (1.2 +/- 0.1 and 1.4 +/- 0.1 mL min(-1) respectively). Also, there was a clear difference in the rate of return to basal CF following short-term ischaemia between diabetic and non-diabetic hearts. Thus, basal tone was restored 1-2 min after the peak hyperaemic response in non-diabetic hearts, whereas it took approximately 5 min in diabetic hearts.

CONCLUSION

These results show that basal CF, as well as the CF reserve, is impaired in hearts from type 2 diabetic mice. As diabetic and non-diabetic hearts were exposed to the same (maximum) concentrations of NO or adenosine, it is suggested that the lower coronary reserve in type 2 diabetic hearts is, in part, because of a defect in the intracellular pathways mediating smooth muscle relaxation.

Mediators of coronary reactive hyperaemia in isolated mouse heart

1. Mechanisms regulating coronary tone under basal conditions and during reactive hyperaemia following transient ischaemia were assessed in isolated mouse hearts. 2. Blockade of NO-synthase (50 muM L-NAME), K(ATP) channels (5 muM glibenclamide), A(2A) adenosine receptors (A(2A)ARs; 100 nM SCH58261), prostanoid synthesis (100 muM indomethacin), and EDHF (100 nM apamin+100 nM charybdotoxin) all reduced basal flow approximately 40%. Effects of L-NAME, glibenclamide, and apamin+charybdotoxin were additive, whereas coadministration of SCH58261 and indomethacin with these inhibitors failed to further limit flow. 3. Substantial hyperaemia was observed after 5-40 s occlusions, with flow increasing to a peak of 48+/-1 ml min(-1) g(-1). Glibenclamide most effectively inhibited peak flows (up to 50%) while L-NAME was ineffective. 4. With longer occlusions (20-40 s), glibenclamide alone was increasingly ineffective, reducing peak flows by approximately 15% after 20 s occlusion, and not altering peak flow after 40 s occlusion. However, cotreatment with L-NAME+glibenclamide inhibited peak hyperaemia by 70 and 25% following 20 and 40 s occlusions, respectively. 5. In contrast to peak flow changes, sustained dilation and flow repayment over 60 s was almost entirely K(ATP) channel and NO dependent (each contributing equally) with all occlusion durations. 6. Antagonism of A(2A)ARs with SCH58261 reduced hyperaemia 20-30% whereas inhibition of prostanoid synthesis was ineffective. Effects of A(2A)AR antagonism were absent in hearts treated with L-NAME and glibenclamide, supporting NO and K(ATP)-channel-dependent effects of A(2A)ARs. 7. EDHF inhibition alone exerted minor effects on hyperaemia and only with longer occlusions. However, residual hyperaemia after 40 s occlusion in hearts treated with L-NAME+glibenclamide+SCH58261+indomethacin was abrogated by cotreatment with apamin+charybdotoxin. 8. Data support a primary role for K(ATP) channels and NO in mediating sustained dilation after coronary occlusion. While K(ATP) channels (and not NO) are also important in mediating initial peak flow adjustments after brief 5-10 s occlusions, their contribution declines with longer 20-40 s occlusions. Intrinsic activation of A(2A)ARs is important in triggering K(ATP) channel/NO-dependent hyperaemia. Synergistic effects of combined inhibitors implicate interplay between mediators, with compensatory changes occurring in K(ATP) channel, NO, and/or EDHF responses when one is individually blocked.

Endothelin-1 acts via protein kinase C to block KATP channels in rabbit coronary and pulmonary arterial smooth muscle cells

We investigated the effects of the vasoconstrictor endothelin-1 (ET-1) on the whole-cell ATP-sensitive K+ (KATP) currents of smooth muscle cells that were isolated enzymatically from rabbit coronary artery (CASMCs) and pulmonary artery (PASMCs). The size of the KATP current did not differ significantly between CASMCs and PASMCs. ET-1 reduced the KATP current in a concentration-dependent manner, and this inhibition was greater in PASMCs than in CASMCs (half-inhibition values of 12.20 nM and 1.98 nM in CASMCs and PASMCs, respectively). However, the level of inhibition induced by other vasoconstrictors (angiotensin II, norepinephrine, and serotonin) were not significantly different between CASMCs and PASMCs. Pretreatment with the protein kinase C (PKC) inhibitors staurosporine (100 nM) and GF 109203X (1 microM) prevented ET-1-induced inhibition of the KATP current in both arterial smooth muscle cell preparations. The PKC activators phorbol-12,13-dibutyrate (PDBu) and 1-olelyl-2-acetyl-sn-glycerol (OAG) reduced the KATP current in dose-dependent manner. Although the numbers of ET receptors were not significantly different between the 2 arterial smooth muscle cell preparations, the effects of PDBu and OAG were greater on PASMCs. ET-1-induced inhibition of the KATP current was unaffected by the PKA inhibitor Rp-cAMPs (100 microM) and PKA inhibitory peptide (5 microM).

Mechanism of vasodilation to adenosine in coronary arterioles from patients with heart disease

Adenosine is a key myocardial metabolite that elicits coronary vasodilation in a variety of pathophysiological conditions. We examined the mechanism of adenosine-induced vasodilation in coronary arterioles from patients with heart disease. Human coronary arterioles (HCAs) were dissected from pieces of the atrial appendage obtained at the time of cardiac surgery and cannulated for the measurement of internal diameter with videomicroscopy. Adenosine-induced vasodilation was not inhibited by endothelial denudation, but A(2) receptor antagonism with 3,7-dimethyl-1-propargylxanthine and adenylate cyclase (AC) inhibition with SQ22536 significantly attenuated the dilation. In contrast, A(1) receptor antagonism with 8-cyclopentyl-1,3-dipropylxanthine significantly augmented the sensitivity to adenosine. Moreover, dilation to A(2a) receptor activation with 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adenosine hydrochloride was reduced by the A(1) receptor agonist (2S)-N(6)-(2-endo-norbornyl)adenosine. The nonspecific calcium-activated potassium (K(Ca)) channel blocker tetrabutylammonium attenuated adenosine-induced dilation, as did the intermediate-conductance K(Ca) blocker clotrimazole. Neither the large-conductance K(Ca) blocker iberiotoxin nor small-conductance K(Ca) blocker apamin altered the dilation. In conclusion, adenosine endothelium independently dilates HCAs from patients with heart disease through a receptor-mediated mechanism that involves the activation of intermediate-conductance K(Ca) channels via an AC signaling pathway. The roles of A(1) and A(2) receptor subtypes are opposing, with the former being inhibitory to AC-mediated dilator actions of the latter. These observations identify unique fundamental physiological characteristics of the human coronary circulation and may help to target the use of novel adenosine analogs for vasodilation in perfusion imaging or suggest new strategies for myocardial preconditioning.

Matching coronary blood flow to myocardial oxygen consumption

At rest the myocardium extracts approximately 75% of the oxygen delivered by coronary blood flow. Thus there is little extraction reserve when myocardial oxygen consumption is augmented severalfold during exercise. There are local metabolic feedback and sympathetic feedforward control mechanisms that match coronary blood flow to myocardial oxygen consumption. Despite intensive research the local feedback control mechanism remains unknown. Physiological local metabolic control is not due to adenosine, ATP-dependent K(+) channels, nitric oxide, prostaglandins, or inhibition of endothelin. Adenosine and ATP-dependent K(+) channels are involved in pathophysiological ischemic or hypoxic coronary dilation and myocardial protection during ischemia. Sympathetic beta-adrenoceptor-mediated feedforward arteriolar vasodilation contributes approximately 25% of the increase in coronary blood flow during exercise. Sympathetic alpha-adrenoceptor-mediated vasoconstriction in medium and large coronary arteries during exercise helps maintain blood flow to the vulnerable subendocardium when cardiac contractility, heart rate, and myocardial oxygen consumption are high. In conclusion, several potential mediators of local metabolic control of the coronary circulation have been evaluated without success. More research is needed.

Nitric oxide and H2O2 contribute to reactive dilation of isolated coronary arterioles

The role of metabolic factors derived from cardiac muscle in the development of reactive hyperemia after brief occlusions of the coronary circulation seems to be well established. However, the contribution of occlusion-induced changes in hemodynamic forces to eliciting reactive hyperemia is less known. We hypothesized that in isolated coronary arterioles changes in intraluminal pressure and flow, during and after release of occlusion (O/R), themselves via activating intrinsic mechanosensitive mechanisms, elicit release of vasoactive factors resulting in reactive dilations. Thus in isolated coronary arterioles (diameter: 88 ± 8 μm) changes in diameter to changes in pressure or pressure plus flow (P+F) during and after a brief period (30, 60, and 120 s) of O/R of cannulating tube were measured by videomicroscopy. In response to both types of O/R, diameter first decreased, then, subsequently increased during occlusions. When only pressure was changed (from 80-10-80 mmHg), after release of occlusion, peak dilations increased as a function of the duration of occlusions. After flow was established (30 μl/min), O/R elicited changes in both pressure and flow (from 80-10-80 mmHg and from 0 to 30 μl/min). In these conditions, after the release of occlusions, not only the peak but also the duration of reactive dilation increased significantly as a function of the length of occlusions. The dilations during, and peak dilations after occlusions both in pressure and P+F protocols were significantly reduced by the inhibition of NO synthase with Nω-nitro-l-arginine-methyl-ester (l-NAME) or by endothelium removal, whereas duration of postocclusion dilations were reduced by l-NAME or by endothelium removal only in P+F protocols. Furthermore, in both protocols, catalase significantly reduced the peak but not the duration of reactive dilations. Thus, mechanosensitive mechanisms that are sensitive to deformation, pressure, stretch, and wall shear stress elicit release of NO and H2O2, resulting in reactive dilation of isolated coronary arterioles.

Reactive hyperemia following coronary balloon angioplasty, but not dipyridamole-induced hyperemia, predicts resolution of exercise-induced ST-segment depression

OBJECTIVES

To characterize delayed restoration of coronary blood flow following successful percutaneous transluminal coronary angioplasty (PTCA).

BACKGROUND

Delayed restoration of coronary blood flow following successful PTCA is common and likely the result of multiple factors. Temporary myocardial ischemia and dipyridamole administration both result in increased coronary blood flow, but by different mechanisms. The relationship between these phenomena and exercise-induced ST-segment depression after PTCA was investigated to determine if any correlation existed.

METHODS

Forty consecutive patients with single-vessel coronary artery disease underwent treadmill exercise testing before and after PTCA. The percentage change in coronary blood flow before and after 90 s balloon inflation was assessed. After a new steady state had been reached, dipyridamole was infused and changes in coronary blood flow were again determined. The relationship between changes in coronary blood flow and the presence of ST-segment depression during exercise testing after PTCA was determined.

RESULTS

Peak coronary blood flow induced by reactive hyperemia was significantly greater than that in the steady state after balloon inflation (48.5+/-38.8 compared with 15.1+/-13.2 ml/min, P<0.0001). Dipyridamole administration also resulted in significant increases in coronary blood flow (15.1+/-13.2 ml/min compared with 31.0+/-24.9 ml/min, P<0.0001). ST-segment depression after PTCA was significantly less than before (0.10+/-0.07 mV compared with 0.19+/-0.08 mV, P<0.001). Further, reactive hyperemia, but not dipyridamole-induced hyperemia, correlated with attenuation of exercise-induced ST-segment depression after PTCA (r=0.62, P<0.0001).

CONCLUSIONS

Reactive hyperemia following temporary coronary occlusion recreates local conditions associated with delayed resolution of myocardial ischemia following successful PTCA. Further, this phenomenon appears to be distinct from changes in coronary blood flow induced by dipyridamole.

Comparison of the micro- and macro-vascular effects of glimepiride and gliclazide in metformin-treated patients with Type 2 diabetes: a double-blind, crossover study

AIMS

To compare the metabolic and vascular effects of two sulphonylureas (SU), gliclazide (specific for the pancreatic [SUR1] receptor) and glimepiride (a nonspecific agent that also binds to vascular and cardiac [SUR2] receptors), during chronic administration in metformin-treated patients with Type 2 diabetes (T2DM).

METHODS

A randomized, double-blind, crossover study of gliclazide 80 mg BID and glimepiride 2 mg OD, each for 4 weeks as add-on therapy to metformin, with a 4-week washout period. Patients attended four study mornings after first dose and 4 weeks' SU treatment for measurements of arterial distensibility (Ax), pressor responsiveness to i.v. angiotensin II (ANGII), and cutaneous microvascular vasodilator responses to iontophoresis of acetylcholine (ACh) and sodium nitroprusside (SNP).

RESULTS

Glycaemic responses were similar (e.g. serum fructosamine was 315 vs 329 micro mol l-1 after 4 weeks), and there was no change in augmentation index during treatment with either SU (9.1 vs 9.8 mmHg after 4 weeks [95% confidence interval -8.1, 10.5]). Similarly, there were no differences between treatments in pressor responsiveness (e.g. PD10[dose of agonist required to increase mean BP by 10 mmHg] for ANGII was 1.37 vs 1.68 ng kg-1 min-1[-4.3, 6.9]) or cutaneous microvascular vasodilator responses (peak ACh response 68 +/- 36 vs 63 +/- 34 perfusion units [-82.7, 79.1]).

CONCLUSIONS

There is no evidence that SUR1-specific and nonspecific SUs have differential effects on arterial distensibility, endothelial function or vasodilator mechanisms in metformin-treated patients with T2DM.

Inhibition of vascular ATP-sensitive K+ channels does not affect reactive hyperemia in human forearm

The extent to which ATP-sensitive K(+) channels contribute to reactive hyperemia in humans is unresolved. We examined the role of ATP-sensitive K(+) channels in regulating reactive hyperemia induced by 5 min of forearm ischemia. Thirty-one healthy subjects had forearm blood flow measured with venous occlusion plethysmography. Reactive hyperemia could be reproducibly induced (n = 9). The contribution of vascular ATP-sensitive K(+) channels to reactive hyperemia was determined by measuring forearm blood flow before and during brachial artery infusion of glibenclamide, an ATP-sensitive K(+) channel inhibitor (n = 12). To document ATP-sensitive K(+) channel inhibition with glibenclamide, coinfusion with diazoxide, an ATP-sensitive K(+) channel opener, was undertaken (n = 10). Glibenclamide did not significantly alter resting forearm blood flow or the initial and sustained phases of reactive hyperemia. However, glibenclamide attenuated the hyperemic response induced by diazoxide. These data suggest that ATP-sensitive K(+) channels do not play an important role in controlling forearm reactive hyperemia and that other mechanisms are active in this adaptive response.

On the role of mechanosensitive mechanisms eliciting reactive hyperemia

We hypothesized that changes in hemodynamic forces such as pressure (P) and flow (F) contribute importantly to the development of reactive hyperemia. To exclude the effects of vivo factors, isolated rat skeletal muscle arterioles ( approximately 130 microm) were utilized. We found that changes in P or P + F following occlusions elicited reactive dilations (RD). The peak of RD (up to approximately 45 microm), but not the duration of RD, increased to changes in P (80 to 10, then back to 80 mmHg) as a function of the length of occlusions (30, 60, and 120 s). However, changes in P + F (80-10 -80 mmHg + 25-0-25 microl/min) increased both the peak and duration of RD (from approximately 25 to 90 s) with longer occlusions. When only P changed, inhibition of nitric oxide synthesis or endothelium removal (E-) reduced only the peak of RD, whereas when P + F were changed, both the peak and duration of RD became reduced. Inhibition of stretch-activated cation channels by gadolinium reduced the peak but enhanced the duration of RD (both to P or P + F) that was unaffected by N(G)-nitro-l-arginine methyl ester (l-NAME) or by E-. When only P changed, inhibition of tyrosine kinases by genistein reduced peak RD but did not affect the RD duration. However, when P + F changed, genistein reduced both the peak and the duration of RD, additional l-NAME reduced the peak RD, but did not affect the duration of RD. Thus in isolated arterioles an RD resembling the characteristics of reactive hyperemia can be generated that is elicited by deformation, stretch, pressure, and flow/shear stress-sensitive mechanisms and is, in part, mediated by nitric oxide.

Coronary metabolic adaptation restricted by endothelin in the dog heart

Endothelin elicits long-lasting vasoconstriction in the coronary bed. This remarkable spastic response raises the question whether or not the metabolic adaptive mechanisms of the coronaries are activated under endothelin effect. The role of the compensatory mediators adenosine and inosine was investigated before and after intracoronary (i.c.) administration of endothelin-1 (ET-1, 1.0 nmol) using 1-min reactive hyperemia (RH) tests on in situ dog hearts (n=15) with or without blocking the ATP-sensitive potassium (K+(ATP)) channels by glibenclamide (GLIB, 1.0 micromol min(-1), i.c.). The release of adenosine and inosine via the coronary sinus was measured by HPLC during the first minute of RH. Endothelin-1 reduced baseline coronary blood flow (CBF) and RH response (hyperemic excess flow (EF) control vs. ET-1: 81.7+/-13.6 vs. 43.4+/-10.9 ml, P<0.01), while it increased the net nucleoside release (adenosine, control vs. ET-1: 58.9+/-20.4 vs. 113.7+/-39.4 nmol, P<0.05; inosine: 242.1+/-81.8 vs. 786.9+/-190.8 nmol, P<0.05). GLIB treatment alone did not change baseline CBF but also reduced RH significantly and increased nucleoside release (EF control vs. GLIB: 72.1+/-11.7 vs. 31.9+/-5.5 ml, P<0.01; adenosine: 18.8+/-4.6 vs. 63.0+/-24.8 nmol, P<0.05; inosine: 113.0+/-37.2 vs. 328.2+/-127.5 nmol, P<0.05). Endothelin-1 on GLIB-treated coronaries further diminished RH and increased nucleoside release (EF: 21.5+/-8.0 ml, P<0.05 vs. GLIB; adenosine: 75.3+/-28.1 nmol, NS; inosine: 801.9+/-196.6 nmol, P<0.05 vs. GLIB). The data show that ET-1 reduces metabolic adaptive capacity of the coronaries, and this phenomenon is due to decreased vascular responsiveness and not to the blockade of ischemic mediator release from the myocardium. The coronary effect of ET-1 may partially be dependent on K+(ATP) channels.

Vasodilator responses to adenosine and hyperemia are mediated by A(1) and A(2) receptors in the cat vascular bed

Hemodynamic responses to adenosine, the A(1) receptor agonists N(6)-cyclopentyladenosine (CPA) and adenosine amine congener (ADAC), and the A(2) receptor agonist 5'-(N-cyclopropyl)-carboxamido-adenosine (CPCA) were investigated in the hindquarter vascular bed of the cat under constant-flow conditions. Injections of adenosine, CPA, ADAC, CPCA, ATP, and adenosine 5'-O-(3-thiotriphosphate) (ATPgamma S) into the perfusion circuit induced dose-related decreases in perfusion pressure. Vasodilator responses to the A(1) agonists were reduced by the A(1) receptor antagonists KW-3902 and CGS-15943, whereas responses to CPCA were reduced by the A(2) antagonist KF-17837. Vasodilator responses to adenosine were reduced by KW-3902, CGS-15943, and by KF-17837, suggesting a role for both A(1) and A(2) receptors. Vasodilator responses to ATP and the nonhydrolyzable ATP analog ATP gamma S were not attenuated by CGS-15943 or KF-17837. After treatment with the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester, the cyclooxygenase inhibitor sodium meclofenamate, or the ATP-dependent K(+) (K) channel antagonists U-37883A or glibenclamide, responses to adenosine and ATP were not altered. Responses to adenosine, CPA, and CPCA were increased in duration by rolipram, a type 4 cAMP phosphodiesterase inhibitor, but were not altered by zaprinast, a type 5 cGMP phosphodiesterase inhibitor. When blood flow was interrupted for a 30-s period, the magnitude and duration of the reactive vasodilator response were reduced by A(1) and A(2) receptor antagonists. These data suggest that vasodilator responses to adenosine and the A(1) and A(2) agonists studied are not dependent on the release of cyclooxygenase products, nitric oxide, or the opening of K channels in the regional vascular bed of the cat. The present data suggest a role for cAMP in mediating responses to adenosine and suggest that vasodilator responses to adenosine and to reactive hyperemia are mediated in part by A(1) and A(2) receptors in the hindquarter vascular bed of the cat.

Control of coronary blood flow during exercise

Under normal physiological conditions, coronary blood flow is closely matched with the rate of myocardial oxygen consumption. This matching of flow and metabolism is physiologically important due to the limited oxygen extraction reserve of the heart. Thus, when myocardial oxygen consumption is increased, as during exercise, coronary vasodilation and increased oxygen delivery are critical to preventing myocardial underperfusion and ischemia. Exercise coronary vasodilation is thought to be mediated primarily by the production of local metabolic vasodilators released from cardiomyocytes secondary to an increase in myocardial oxygen consumption. However, despite various investigations into this mechanism, the mediator(s) of metabolic coronary vasodilation remain unknown. As will be seen in this review, the adenosine, K(+)(ATP) channel and nitric oxide hypotheses have been found to be inadequate, either alone or in combination as multiple redundant compensatory mechanisms. Prostaglandins and potassium are also not important in steady-state coronary flow regulation. Other factors such as ATP and endothelium-derived hyperpolarizing factors have been proposed as potential local metabolic factors, but have not been examined during exercise coronary vasodilation. In contrast, norepinephrine released from sympathetic nerve endings mediates a feed-forward betaadrenoceptor coronary vasodilation that accounts for approximately 25% of coronary vasodilation observed during exercise. There is also a feed-forward alpha-adrenoceptor-mediated vasoconstriction that helps maintain blood flow to the vulnerable subendocardium when heart rate, myocardial contractility, and oxygen consumption are elevated during exercise. Control of coronary blood flow during pathophysiological conditions such as hypertension, diabetes mellitus, and heart failure is also addressed.

ATP-dependent K(+) channels contribute to local metabolic coronary vasodilation in experimental diabetes

This study tested whether ATP-dependent K(+) channels (K(ATP) channels) are an important mechanism of functional coronary hyperemia in conscious, instrument-implanted diabetic dogs. Data were collected at rest and during exercise before and after induction of diabetes with alloxan monohydrate (40-60 mg/kg intravenously). K(ATP) channels were inhibited with glibenclamide (1 mg/kg intravenously). In nondiabetic dogs, arterial plasma glucose concentration increased from 4.8 +/- 0.3 to 21.5 +/- 2.2 mmol/l 1 week after alloxan injection. In nondiabetic dogs, exercise increased myocardial oxygen consumption (MVO(2)) 3.4-fold, myocardial O(2) delivery 3.0-fold, and heart rate 2.4-fold. Coronary venous PO(2) decreased from 19.9 +/- 0.8 mmHg at rest to 14.8 +/- 0.8 mmHg during exercise. Diabetes significantly reduced myocardial O(2) delivery and lowered coronary venous PO(2) from 16.3 +/- 0.6 mmHg at rest to 13.1 +/- 0.9 mmHg during exercise. Glibenclamide did not alter the slope of the coronary venous PO(2) versus MVO(2) relationship in nondiabetic dogs. In diabetic dogs, however, glibenclamide further reduced myocardial O(2) delivery; coronary venous PO(2) fell to 9.0 +/- 1.0 mmHg during exercise, and the slope of the coronary venous PO(2) versus MVO(2) relationship steepened. These findings indicate that K(ATP) channels contribute to local metabolic coronary vasodilation in alloxan-induced diabetic dogs.

Vascular effects of glibenclamide vs. glimepiride and metformin in Type 2 diabetic patients

AIMS

Glibenclamide attenuates the protective responses to opening of vascular ATP-sensitive potassium (K(ATP)) channels during ischaemia. Therefore, glibenclamide treatment of Type 2 diabetes mellitus may have hazardous cardiovascular effects when used under conditions of ischaemia. Glimepiride and metformin seem to lack such characteristics. Based on these data, we hypothesized that, in contrast to glibenclamide, chronic treatment of Type 2 diabetic patients with glimepiride or metformin will not impair the vasodilator function of K(ATP) opening in vivo.

METHODS

Two groups of 12 Type 2 diabetes mellitus patients participated in a double-blind randomized cross-over study consisting of two 8-week periods, in which treatment with orally administered glibenclamide (15 mg/day) was compared with either glimepiride or metformin (6 mg and 1500 mg/day, respectively). At the end of each treatment period, the increase in forearm blood flow (FBF, venous occlusion plethysmography) in response to intra-arterial administered diazoxide (K(ATP) opener), acetylcholine (endothelium-dependent vasodilator) and dipyridamole (adenosine-uptake blocker) and to forearm ischaemia was measured.

RESULTS

There were no significant differences in vasodilator responses to diazoxide, acetylcholine, dipyridamole and forearm ischaemia after glibenclamide compared with glimepiride and metformin.

CONCLUSIONS

Chronic treatment of Type 2 diabetes mellitus with glimepiride or metformin has similar effects on vascular K(ATP) channels compared with chronic glibenclamide treatment.

Time course of forearm arterial compliance changes during reactive hyperemia

Ultrasonic studies have shown that arterial compliance increases after prolonged ischemia. The objective of the present study was to develop an alternative plethysmographic method to investigate compliance, exploring validity and clinical applicability. Forearm pulse volume (FPV) and blood pressure (BP) were used to establish the FPV-BP relationship. Forearm arterial compliance (FAC) was measured, and the area under the FAC-BP curve (FAC(AUC)) was determined. The time course curve of compliance changes during reactive hyperemia was obtained by continuous measurements of FAC(AUC) for 20 s before and for 300 s after arterial occlusion. This technique allows us to effectively assess compliance changes during reactive hyperemia. Furthermore, the selected measurement protocol indicated the necessity for continuous measurements to detect "true" maximal FAC(AUC) changes. On multivariate analysis, preischemic FAC(AUC) was mainly affected by sex, peak FAC(AUC) was affected by sex and systolic BP, percent changes were affected by plasma high-density and low-density lipoprotein cholesterol, peak time was affected by age and body mass index, and descent time was affected by plasma triglyceride levels. The proposed technique is highly sensitive and well comparable with the generally accepted echotracking system. It may thus be considered as an alternative tool to detect and monitor compliance changes induced by arterial occlusion.

The return of increased blood pressure after discontinuation of antihypertensive treatment is associated with an impaired post-ischemic skin blood flow response

OBJECTIVE

To assess the post-ischemic skin blood flow response after withdrawal of antihypertensive therapy in hypertensive patients with normal blood pressure during treatment.

DESIGN AND METHODS

Twenty hypertensive patients (group A) with a normal clinic blood pressure (<140/ 90 mmHg) receiving antihypertensive treatment (any monotherapy; one pill per day for at least 6 months) had their treatment discontinued. Before medication withdrawal and 2, 4, 12 and 24 weeks thereafter, the following measurements were made: clinic blood pressure, home blood pressure (three times per week, morning and evening) and skin blood flow response to a 5 min forearm arterial occlusion (using laser Doppler flowmetry). The patients were asked to perform an ambulatory blood pressure recording at any time if home blood pressure was > or =160/95 mmHg on two consecutive days, and treatment was initiated again, after determination of the skin hyperemic response, if daytime ambulatory blood pressure was > or =140/90 mmHg. The same studies were performed in 20 additional hypertensive individuals in whom antihypertensive treatment was not withdrawn (group B). The allocation of patients to groups A and B was random.

RESULTS

The data fom 18 patients in group A who adhered strictly to the procedure were available for analysis. Seven of them had to start treatment again within the first 4 weeks of follow-up; four additional patients started treatment again during the next 8 weeks (group A1). The seven other patients remained untreated (group A2). The skin hyperemic response decreased significantly in patients in group A1 and returned to baseline values at the end of the study, when there were again receiving antihypertensive treatment. In patients in group A2 a significant attenuation of the hyperemic response was also observed. This impaired response was present even at the end of the 6 month follow-up, at which time the patients were still untreated but exhibited a significantly greater blood pressure than before drug discontinuation. The hyperemic response of patients who did not stop treatment (group B) did not change during the course of the study.

CONCLUSIONS

Our findings show a decrease in the postischemic skin blood flow response after withdrawal of antihypertensive treatment in hypertensive patients. This impaired response may be due to the development of endothelial dysfunction, vascular remodeling, or both, and might contribute to the return of blood pressure to hypertensive values after withdrawal of antihypertensive therapy.

Effects of sulfonylurea derivatives on ischemia-induced loss of function in the isolated rat heart

This study determined whether sulfonylurea derivatives affect cardiac function prior to and after a mild ischemic incident (stunning). This was investigated using an isolated, erythrocyte-perfused, working rat heart model. In total, 11 groups were studied: five increasing (clinically relevant) concentrations of the classical glibenclamide (range 0.005-4 micromol l(-1)), five increasing concentrations of the newly developed glimepiride (range 0.005-0.8 micromol l(-1)), and one control group. Pre-ischemically, glibenclamide and glimepiride reduced coronary blood flow concentration dependently to 55.2+/-4.5% and 58.5+/-5.5%, respectively (P<0.001). Twenty minutes after a 12-min ischemic incident, these reductions of flow were even more pronounced (to 38.3+/-6.7% and 45.8+/-5.8%, P<0.001). This shows that both sulfonylureas reduce coronary blood flow at concentrations slightly higher than therapeutic ones. In the control group, the ischemic incident significantly lowered cardiac function by 22.2+/-2.9%. In the therapeutic range, glimepiride, but not glibenclamide, significantly reduced this ischemia-induced cardiac functional loss to 4.9+/-1.2% (P<0.01). Therefore, we suggest that both sulfonylureas and in particular glimepiride can be used safely in patients with type 2 diabetes mellitus, as long as the coronary vascular system is not compromised. Because of the obvious vasocontrictor response to sulfonylurea derivatives, these drugs must be used with caution in patients with a reduced coronary reserve.

Sulfonylurea treatment of type 2 diabetic patients does not reduce the vasodilator response to ischemia

OBJECTIVE

Sulfonylureas block the activation of vascular potassium-dependent ATP channels and impair the vasodilating response to ischcmia in nondiabetic individuals, but it is not know whether this occurs in type 2 diabetic patients under chronic treatment with these drugs. Glimepiride, a new sulfonylurea, apparently has no cardiovascular interactions. The aim of our study was to compare the effect of the widely used compound glibenclamide, the pancreas-specific glimepiride, and diet treatment alone on brachial artery response to acute forearm ischemia.

RESEARCH DESIGN AND METHODS

Brachial artery examination was performed by a high-resolution ultrasound technique on 20 type 2 diabetic patients aged mean +/- SD) 67 +/- 2 years and on 18 nondiabetic patients matched for age, hypertension, and dislipidemia. Diabetic subjects underwent three separate evaluations at the end of each 8-week treatment period, during which they received glibenclamide, glimepiride, or diet alone according to crossover design. Scans were obtained before and after 4.5 min of forearm ischemia. Postischemic vasodilation and hyperemia were expressed as percent variations in vessel diameter and blood flow.

RESULTS

Postischemic vasodilation and hyperemia were, respectively, 5.42 +/- 0.90 and 331 +/- 38% during glibenclamide, 5.46 +/- 0.69 and 326 +/- 28% during glimepiride, and 5.17 +/- 0.64 and 357 +/- 35% during diet treatment (NS). These results were similar to those found in the nondiabetic patients (6.44 +/- 0.68 and 406 +/- 42%, NS).

CONCLUSIONS

In type 2 diabetic patients, the vasodilating response to forearm ischemia was the same whether patients were treated with diet treatment alone or with glibenclamide or glimepiride at blood glucose-lowering equipotent closes.

Investigation of mechanisms that mediate reactive hyperaemia in guinea-pig hearts: role of K(ATP) channels, adenosine, nitric oxide and prostaglandins

1. Reactive hyperaemia is a transient vasodilatation following a brief ischaemic period. ATP-dependent K(+) (K(ATP)) channels may be important in mediating this response, however it is unclear whether mitochondrial K(ATP) channels contribute to this in the heart. 2. We examined the involvement of K(ATP) channels and the relative role of mitochondrial channels as mediators of coronary reactive hyperaemia and compared them to mechanisms involving NO, prostaglandins and adenosine in the guinea-pig isolated heart. 3. Reactive hyperaemic vasodilatation (peak vasodilator response and flow debt repayment) were assessed after global zero-flow ischaemia (5 -- 120 s) in the presence of nitro-L-arginine methyl ester (L-NAME, 10(-5) M, n=9), 8-phenyltheophylline (8-PT, 10(-6) M, n=12) and indomethacin (10(-5) M, n=12). 4. Glibenclamide (10(-6) M, n=12) a non-selective K(ATP) channel inhibitor and 5-hydroxy-decanoic acid (5-HD, 10(-4) M, n=10) a selective mitochondrial K(ATP) channel inhibitor were also used. The specificity of the effects of glibenclamide and 5-HD (n=6 each) were confirmed using pinacidil (38 nmol -- 10 micromol) and diazoxide (42 nmol -- 2 micromol). Glibenclamide was most effective in blocking the hyperaemic response (by 87%, P<0.001) although 5-HD and 8-PT also had a marked effect (40% inhibition, P<0.001 and 32%, P<0.001, respectively). L-NAME and indomethacin had little effect. 5. Perfusion with L-NAME and glibenclamide significantly reduced baseline coronary flow (22%, P<0.01 and 33%, P<0.01) while 8-PT, indomethacin and 5-HD had no effect. 6. K(ATP) channels are the major mediators of the coronary reactive hyperaemic response in the guinea-pig. Although mitochondrial K(ATP) channels contribute, they appear less important than sarcolemmal channels.

Adenosine therapy: a new approach to chronic heart failure

Both the prevention and attenuation of chronic heart failure (CHF) are important issues for cardiologists. There are three different strategies to prevent patients from deleterious sequels. The first strategy is to remove the causes of CHF if possible; the second is to attenuate the events that may lead to CHF, such as myocardial ischaemia and reperfusion injury, cardiomyopathy and myocarditis, cardiac hypertrophy and ventricular remodelling; the third is to prevent or attenuate the progression of CHF. Adenosine has a number of actions which merit it as a possible cardioprotective and therapeutic agent for CHF. Firstly, adenosine induces collateral circulation via inducing growth factors and triggering ischaemic preconditioning, both of which induce ischaemic tolerance in advance. Adenosine is also known to reduce the release of noradrenaline, production of endothelin and attenuate the activation of renin-angiotensin system all of which are believed to cause cardiac hypertrophy and remodelling. Secondly, exogenous adenosine is known to reduce the severity of ischaemia and reperfusion injury. Thirdly, adenosine is reported to counteract neurohumoral factors, i.e., cytokine systems, known to be related to the pathophysiology of CHF. Recently, we revealed that adenosine metabolism is changed in patients with CHF and increases in adenosine levels may aid to reduce the severity of CHF. Thus, there are many potential mechanisms for cardioprotection attributable to adenosine and we postulate the use of adenosine therapy will be beneficial in patients with CHF.

Evaluation of endothelium-dependent vasodilation in the human peripheral circulation

Flow-mediated vasodilation (FMD) in the brachial artery measured by ultrasound, and the increase in forearm blood flow (FBF) induced by local infusion of a muscarinic-receptor agonist have both frequently been used to evaluate endothelium-dependent vasodilation (EDV) in the human forearm. The present study intended to evaluate the relationship between these techniques and to investigate if vasodilation induced by the muscarinic receptor-agonist methacholine (MCh) was owing to production of nitric oxide (NO). FMD during hyperaemia was assessed by ultrasound and FBF was measured by venous occlusion plethysmography during local infusion of MCh or L-arginine in the human forearm. Both these methods were applied in 26 individuals. In another 12 individuals forearm arterial and venous plasma concentrations of nitrate/nitrite (NOx) were measured together with FBF before and during local MCh infusion. While the change in brachial artery diameter induced by sublingually given nitroglycerine and the vasodilatory response to sodium nitroprusside (SNP) given locally in the forearm were significantly correlated (r = 0.70, P < 0.01), FMD showed no relationship with the vasodilation evoked by MCh (r = -0.03) or L-arginine (r = 0.04). The five-fold increase in FBF during MCh infusion was associated with a significant increase in venous plasma NOx concentrations (P < 0.05) and a more than 11-fold increase in forearm NOx-release (P < 0.01). Thus, a significant relationship between the two methods regarding the evaluation of endothelium-independent vasodilation evoked by NO-donors was found, but no relationship was found between the two methods regarding the evaluation of endothelium-dependent vasodilation. Furthermore, vasodilation induced by MCh in the forearm seems to be induced by NO-release.

Role of pertussis toxin-sensitive G protein in metabolic vasodilation of coronary microcirculation

We have previously demonstrated that pertussis toxin (PTX)-sensitive G protein (G(PTX)) plays a major role in coronary microvascular vasomotion during hypoperfusion. We aimed to elucidate the role of G(PTX) during increasing metabolic demand. In 18 mongrel dogs, coronary arteriolar diameters were measured by fluorescence microangiography using a floating objective. Myocardial oxygen consumption (MVO(2)) was increased by rapid left atrial pacing. In six dogs, PTX (300 ng/ml) was superfused onto the heart surface for 2 h to locally block G(PTX). In eight dogs, the vehicle (Krebs solution) was superfused in the same way. Before and after each treatment, the diameters were measured during control (130 beats/min) and rapid pacing (260 beats/min) in each group. Metabolic stimulation before and after the vehicle treatment caused 8.6 +/- 1. 8 and 16.1 +/- 3.6% dilation of coronary arterioles <100 microm in diameter (57 +/- 8 microm at control, n = 10), respectively. PTX treatment clearly abolished the dilation of arterioles (12.8 +/- 2. 5% before and 0.9 +/- 1.6% after the treatment, P < 0.001 vs. vehicle; 66 +/- 8 microm at control, n = 11) in response to metabolic stimulation. The increases in MVO(2) and coronary flow velocity were comparable between the vehicle and PTX groups. In four dogs, 8-phenyltheophylline (10 microM, superfusion for 30 min) did not affect the metabolic dilation of arterioles (15.3 +/- 2.0% before and 16.4 +/- 3.8% after treatment; 84.3 +/- 11.0 microm at control, n = 8). Thus we conclude that G(PTX) plays a major role in regulating the coronary microvascular tone during active hyperemia, and adenosine does not contribute to metabolic vasodilation via G(PTX) activation.

Coronary vascular K+ATP channels contribute to the maintenance of myocardial perfusion in dogs with pacing-induced heart failure

The functional role of coronary vascular ATP-sensitive potassium (K+ATP) channels in the regulation of coronary blood flow (CBF) has not been determined in chronic heart failure (CHF). To test the hypothesis that K+ATP channels contribute to myocardial perfusion in HF, we examined the effects of intracoronary infusion of glibenclamide, an inhibitor of K+ATP channels, on basal CBF in control and CHF dogs. CHF was produced in mongrel dogs by pacing the right ventricle for 4 weeks. Under anesthesia, CBF in the left anterior descending coronary artery, other hemodynamic and metabolic parameters, or regional myocardial blood flow were measured. Basal CBF was less in CHF dogs than in controls. Glibenclamide at the graded doses (5, 15 and 50 microg x kg(-1) x min(-1) decreased CBF in both control and CHF dogs. The percentage decrease in CBF with glibenclamide at 50 microg x kg(-1) x min(-1) was greater (p<0.01) in CHF dogs than in controls. The greater decrease in CBF with glibenclamide at 50microg x kg(-1) x min(-1) was associated with myocardial ischemia. Glibenclamide decreased myocardial blood flow in each sublayer of the myocardium in the 2 groups. These results suggest that the basal activity of coronary vascular K+ATP channels is increased in CHF dogs but not in controls. This may contribute to the maintenance of myocardial perfusion in CHF.

Role of K(ATP)(+) channels and adenosine in the control of coronary blood flow during exercise

The present study was designed to examine the role of ATP-sensitive potassium (K(ATP)(+)) channels during exercise and to test the hypothesis that adenosine increases to compensate for the loss of K(ATP)(+) channel function and adenosine inhibition produced by glibenclamide. Graded treadmill exercise was used to increase myocardial O(2) consumption in dogs before and during K(ATP)(+) channel blockade with glibenclamide (1 mg/kg iv), which also blocks adenosine mediated coronary vasodilation. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous values by using a previously tested mathematical model (Kroll K and Stepp DW. Am J Physiol Heart Circ Physiol 270: H1469-H1483, 1996). Coronary venous O(2) tension was used as an index of the balance between O(2) delivery and myocardial O(2) consumption. During control exercise, myocardial O(2) consumption increased approximately 4-fold, and coronary venous O(2) tension fell from 19 to 14 Torr. After K(ATP)(+) channel blockade, coronary venous O(2) tension was decreased below control vehicle values at rest and during exercise. However, during exercise with glibenclamide, the slope of the line of coronary venous O(2) tension vs. myocardial O(2) consumption was the same as during control exercise. Estimated interstitial adenosine concentration with glibenclamide was not different from control vehicle and was well below the level necessary to overcome the 10-fold shift in the adenosine dose-response curve due to glibenclamide. In conclusion, K(ATP)(+) channel blockade decreases the balance between resting coronary O(2) delivery and myocardial O(2) consumption, but K(ATP)(+) channels are not required for the increase in coronary blood flow during exercise. Furthermore, interstitial adenosine concentration does not increase to compensate for the loss of K(ATP)(+) channel function.

Coronary microcirculation: physiology and pharmacology

Coronary microvessels play a pivotal role in determining the supply of oxygen and nutrients to the myocardium by regulating the coronary flow conductance and substance transport. Direct approaches analyzing the coronary microvessels have provided a large body of knowledge concerning the physiological and pharmacological characteristics of the coronary circulation, as has the rapid accumulation of biochemical findings about the substances that mediate vascular functions. Myogenic and flow-induced intrinsic vascular controls that determine basal tone have been observed in coronary microvessels in vitro. Coronary microvascular responses during metabolic stimulation, autoregulation, and reactive hyperemia have been analyzed in vivo, and are known to be largely mediated by metabolic factors, although the involvement of other factors should also be taken into account. The importance of ATP-sensitive K(+) channels in the metabolic control has been increasingly recognized. Furthermore, many neurohumoral mediators significantly affect coronary microvascular control in endothelium-dependent and -independent manners. The striking size-dependent heterogeneity of microvascular responses to all of these intrinsic, metabolic, and neurohumoral factors is orchestrated for optimal perfusion of the myocardium by synergistic and competitive interactions. The regulation of coronary microvascular permeability is another important factor for the nutrient supply and for edema formation. Analyses of collateral microvessels and subendocardial microvessels are important for understanding the pathophysiology of ischemic hearts and hypertrophied hearts. Studies of the microvascular responses to drugs and of the impairment of coronary microvessels in diseased conditions provide useful information for treating microvascular dysfunctions. In this article, the endogenous regulatory system and pharmacological responses of the coronary circulation are reviewed from the microvascular point of view.

Near-infrared monitoring of myocardial oxygenation during ischemic preconditioning

BACKGROUND

Ischemic preconditioning has been advocated as a method of cardioprotection for minimally invasive direct coronary artery bypass. This study was performed to estimate the cardioprotective effect of ischemic preconditioning before ischemia by examining the changes in myocardial tissue oxygenation and also to examine whether adenosine triphosphate-sensitive potassium channel opener enhances the cardioprotective effect of ischemic preconditioning.

METHODS

Myocardial ischemia was induced in three groups of 6 dogs by temporary occlusion of the left anterior descending coronary artery. Group 1 dogs received a 30-minute coronary occlusion and subsequent 3-hour reperfusion. Groups 2 and 3 dogs underwent three periods of 5-minute coronary occlusion and 5-minute reperfusion and then received 30-minute sustained ischemia and 3-hour reperfusion. In group 3, nicorandil was administered during the procedure. Myocardial oxygenation was measured using three-wavelength near-infrared spectroscopy. Myocardial blood flow was measured by the colored microsphere method.

RESULTS

During ischemic preconditioning the myocardial tissue oxygen saturation decreased rapidly at coronary occlusion and increased at reperfusion. It was increased stepwise at the second and third coronary occlusion. Myocardial oxygen saturation during 30-minute sustained ischemia was significantly higher in groups 2 and 3 than in group 1 (p < 0.05). The myocardial tissue hemoglobin concentration showed similar changes to myocardial oxygen saturation. During 30-minute sustained ischemia, it was significantly higher in group 2 than in group 1 (p < 0.001), and it was significantly higher in group 3 than in groups 1 and 2 (p < 0.05). Regional myocardial blood flow showed no difference after 30 minutes of sustained ischemia among the three groups. Troponin-T levels were significantly lower in groups 2 and 3 than in group 1 (p < 0.01).

CONCLUSIONS

Ischemic preconditioning had beneficial effects on myocardial oxygenation during sustained ischemia, and the protected state of the myocardium could be monitored with the use of near-infrared spectroscopy. Ischemic preconditioning coupled with nicorandil administration might provide protection for minimally invasive direct coronary bypass.

Hemodynamic changes caused by glibenclamide in isolated, working, erythrocyte perfused rat heart

Glibenclamide-induced cardiac hemodynamic changes before and after ischemia have been frequently studied. In general a Langendorff buffer perfused heart model was used to examine these effects. However these models used protein-free buffer perfusates. To improve clinical relevance and thereby enhance extrapolation to the in vivo condition we studied the effects of glibenclamide on cardiac hemodynamics using a working, erythrocyte perfused, rat heart model, where the perfusate was enriched with albumin. The results show a dose-dependent decline in CBF in normoxia and at the end of reperfusion (after an ischemic period) with glibenclamide treatment compared to control. Cardiac functional recovery improved with 1 and 4 mumol.L-1 glibenclamide concentrations. From this study it seems that there is a marked decrease in CBF but this did not result in impaired myocardial function after a period of ischemia, so it appears that there are no startling side effects of glibenclamide in the ischemic rat heart.

Reactive hyperemia in skin after cardiopulmonary bypass

OBJECTIVE

To study reactive hyperemia (RH) using a transcutaneous PO2/PCO2 combination electrode heated to 37 degrees C and tissue reflectance spectrophotometry in patients before and after cardiopulmonary bypass (CPB) to determine whether microcirculatory function of skin is altered.

DESIGN

Prospective study.

SETTING

Anesthesiology and critical care unit of a university hospital.

PARTICIPANTS

Eight patients undergoing elective CPB under mild hypothermia.

INTERVENTIONS

To produce RH, blood flow to the forearm was prevented by inflation of a cuff to 300 mmHg for an interval of 5 minutes.

MEASUREMENTS AND MAIN RESULTS

Measurements were obtained on the day prior to surgery (DPS), on the day of surgery (DOS) rewarmed to 37 degrees C in the intensive care unit (ICU), and on the first (POD 1) and the third postoperative days (POD 3). The following parameters were recorded: preocclusive baseline cutaneous PO2, and PCO2 (B-PtcO2, B-PtcCO2), and microvascular hemoglobin saturation (B-HbO2); postischemic peak of PtcO2, PtcCO2, and HbO2; and 10 minutes after release of the cuff occlusion posthyperemic PtcO2, PtcCO2, and HbO2. B-PtcO2 was 3.5 +/- 1.2 mmHg on DPS, 2.6 +/- 0.7 mmHg on DOS, 1.5 +/- 0.3 mmHg on POD 1, and 3.5 +/- 3.5 mmHg on POD 3. B-PtcCO2 increased significantly from 40.1 +/- 2.5 mmHg to 52.2 +/- 2.0 mmHg on DOS (p = 0.01) and to 48.9 +/- 3.6 mmHg on POD 1 (p = 0.02). On POD 3, B-PtcCO2 was 40.6 +/- 2.6 mmHg. B-HbO2 declined from a preoperative value of 42.4% +/- 8.6% to 37.1% +/- 14.7% on DOS and further to 21.7% +/- 4.8% on POD 1, which was significantly different (p = 0.03). On POD 3, B-HbO2 still remained lower (30.7% +/- 6.2%) compared with the preoperative value. RH (deltaPtcO2, deltaHBO2) was quantified as the differences between peak PtcO2, HBO2 and B-PtcO2, B-HBO2. DeltaPtcO2 was 13.0 +/- 2.3 on DPS, 11.3 +/- 2.9 on DOS, 12.6 +/- 2.6 on POD 1, and 11.5 +/- 3.5 on POD 3. DeltaHBO2 was 42.0 +/- 5.6 on DPS, 40.0 +/- 7.1 on DOS, 49.9 +/- 2.5 on POD 1, and 52.9 +/- 6.4 on POD 3. The elimination rate of carbon dioxide from skin (ECO2) was calculated as difference between peak PtcCO2 and PtcCO2 after 3 minutes of reperfusion divided by the difference between peak PtcCO2 and B-PtcCO2. ECO2 was 1.0 +/- 0.2 kPa/min on DPS, 0.7 +/- 0.1 kPa/min on DOS, and 0.8 +/- 0.1 kPa/min on POD 1 and POD 3.

CONCLUSION

Cutaneous microcirculation assessed by RH is well preserved during the immediate postoperative period in patients undergoing uncomplicated coronary artery surgery with CPB.