Roles of nitric oxide and adenosine in the regulation of coronary conductance in the basal state and during reactive hyperemia

Protective effects of (6R)-5,6,7,8-tetrahydro-l-biopterin on local ischemia/reperfusion-induced suppression of reactive hyperemia in rat gingiva

We herein investigated the regulatory mechanism in the circulation responsible for rat gingival reactive hyperemia (RH) associated with ischemia/reperfusion (I/R). RH was analyzed using a laser Doppler flowmeter. RH and I/R were elicited by gingival compression and release with a laser Doppler probe. RH increased in a time-dependent manner when the duration of compression was between 30 s and 20 min. This increase was significantly suppressed by N^ω-nitro-l-arginine-methyl-ester (l-NAME), 7-nitroindazole (7-NI), and 2,4-diamino-6-hydroxypyrimidine (DAHP). However, RH was markedly inhibited following 60 min of compression. This inhibition was significantly decreased by treatments with superoxide dismutase (SOD), (6R)-5,6,7,8-tetrahydro-l-biopterin (BH₄), and sepiapterin. The luminescent intensity of superoxide anion (O₂^•−)-induced 2-methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo-[1,2-a] pyrazine-3-one (MCLA) was markedly decreased by SOD and BH₄, but only slightly by sepiapterin. BH₄ significantly decreased O₂^•− scavenging activity in a time-dependent manner. These results suggested that nitric oxide (NO) secreted by the nitrergic nerve played a role in regulating local circulation in rat gingiva. This NO-related regulation of local circulation was temporarily inhibited in the gingiva by the I/R treatment. The decrease observed in the production of NO, which was caused by suppression of NO synthase (NOS) activity subsequent to depletion of the NOS co-factor BH₄ by O₂^•−, played a partial role in this inhibition.

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.

Pattern differences between distributions of microregional myocardial flows in crystalloid- and blood-perfused rat hearts

Regional myocardial flow distributions in Langendorff rat hearts under Tyrode and blood perfusion were assessed by tracer digital radiography (100-μm resolution). Flow distributions during baseline and maximal hyperemia following a 60-s flow cessation were evaluated by the coefficient of variation of regional flows (CV; related to global flow heterogeneity) and the correlation between adjacent regional flows (CA; inversely related to local flow randomness). These values were obtained for the original images (642 pixels) and for coarse-grained images (322, 162, and 82 blocks of nearby pixels). At a given point in time during baseline, both CV and CA were higher in blood ( n = 7) than in Tyrode perfusion ( n = 7) over all pixel aggregates ( P < 0.05, two-way ANOVA). During the maximal hyperemia, CV and CA were still significantly higher in blood ( n = 7) than in Tyrode perfusion ( n = 7); however, these values decreased substantially in blood perfusion and the CV and CA differences became smaller than those at baseline accordingly. During basal blood perfusion, the 60-s average flow distribution ( n = 7) showed a smaller CV and CA than those at a given point in time ( P < 0.05, two-way ANOVA). Coronary flow reserve was significantly higher in blood than in Tyrode perfusion. In conclusion, the flow heterogeneity and the local flow similarity are both higher in blood than in Tyrode perfusion, probably due to the different degree of coronary tone preservation and the presence or absence of blood corpuscles. Under blood perfusion, temporal flow fluctuations over 60-s order are largely involved in shaping microregional flow distributions.

Role of nitric oxide in post-ischemic gingival hyperemia in anesthetized dogs

The possible involvement of nitric oxide (*NO) in the preservation of blood flow to the canine gingiva after compression of gingival tissue was studied. Gingival blood flow, gingival tissue oxygen partial pressure (PO2), external carotid arterial blood pressure and external carotid arterial blood flow were monitored before, during, and after compression of gingival tissue in the presence and absence of the nitric oxide synthase inhibitor, Nomega-nitro-L-arginine-methyl-ester (L-NAME). Compression of gingival tissue resulted in an immediate decrease in gingival blood flow and tissue PO2. After the compression of gingival tissue, hyperemia was observed in the gingiva, which depended on the duration of ischemia. Gingival tissue PO2 slowly recovered during hyperemia. Pretreatment with L-NAME (60 mg/kg, i.a.) significantly suppressed reactive hyperemia in gingival tissue. The L-NAME-suppressed reactive hyperemia was partially reversed by treatment with L-arginine (60 mg/kg, i.a.). In addition, *NO was detected using an *NO selective electrode during interruption of blood flow and during reactive hyperemia in the gingiva. These results suggest that *NO contributes to the vasodilation during reactive hyperemia in gingival tissue, and aids in the maintenance of homeostasis in gingival circulation.

Endogenous and exogenous coronary vasodilatation are attenuated in cardiac hypertrophy: a morphological defect?

Reactive hyperaemia (RH) following brief ischaemia is reduced in hypertrophied hearts, and this may contribute to reduced coronary flow reserve. We studied vasodilatation during RH and in response to exogenous stimuli in control and hypertrophied hearts and explored the mechanisms underlying RH. Vascular reactivity was assessed in isolated hypertrophied hearts (55+/-3 days after aortic banding or sham operation) by constructing dose-response curves to acetylcholine (ACh), sodium nitroprusside (SNP) and adenosine. Reactive hyperaemic vasodilatation was assessed after global ischaemia (5-120 s) in the presence/absence of L -NAME, 8-phenyltheophylline (8-PT) and glibenclamide. Purine release and NO overflow in the coronary perfusate were analysed. Aortic constriction increased heart/body weight ratio (47%), myocyte size (19%) and arteriolar wall thickness (51%), all P<0.01. Coronary reserve was reduced in hypertrophy (105+/-8%v 182+/-12%, P<0.01). Dose response curves for ACh, SNP and adenosine were reduced in hypertrophy (69%, 86% and 68%, all P<0.01) v shams; however ED(50)values were unchanged. The peak flow and duration of RH were also attenuated (50%, P<0.001) in hypertrophy. While purine washout during RH was related to the duration of preceding ischaemia, nitrate washout was not. RH experiments in the presence of L -NAME, 8-PT and glibenclamide indicated that RH is mediated by combined actions of K(ATP)channels>adenosine>NO in both groups. RH is mediated by similar mechanisms in control and hypertrophied hearts. All vasodilatation was similarly attenuated in hypertrophy, independent of endothelial activation. We hypothesize that increased arteriolar wall thickness may limit vasodilator responses to all stimuli in hypertrophy.

Myocardial blood flow and coronary reserve in chronically anemic fetal lambs

Chronic fetal anemia produces large compensatory increases in coronary blood flow in the near-term fetal lamb. To determine if increased coronary flow in anemic fetuses is associated with decreased coronary flow reserve or, alternatively, an increase in coronary conductance, we measured maximal coronary artery conductance during adenosine infusion before and during anemia. Isovolemic hemorrhage over 7 days reduced hematocrit from 30.6 +/- 2. 7 to 15.8 +/- 2.4% (P < 0.02) and the oxygen content from 7.3 +/- 1. 4 to 2.6 +/- 0.4 ml/dl (P < 0.001). Coronary blood flow increased from control (202 +/- 60) to 664 +/- 208 ml. min(-1). 100 g(-1) with adenosine to 726 +/- 169 ml. min(-1). 100 g(-1) during anemia and to 1,162 +/- 250 ml. min(-1). 100 g(-1) (left ventricle) during anemia with adenosine infusion (all P < 0.001). Coronary conductance, determined during maximal vasodilation, was 18.2 +/- 7.7 before and 32.8 +/- 11.9 ml. min(-1). 100 g(-1). mmHg(-1) during anemia (P < 0. 001). Coronary reserve, the difference between resting and maximal myocardial blood flow interpolated at 40 mmHg, was unchanged in control and anemic fetuses (368 +/- 142 and 372 +/- 201 ml/min). Because hematocrit affects viscosity, anemic fetuses were transfused with blood to acutely increase the hematocrit back to control, and conductance was remeasured. Coronary blood flow decreased 57.3 +/- 18.9% but was still 42.6 +/- 18.9% greater than control. We conclude that in chronically anemic fetal sheep coronary conductance is increased and coronary reserve is maintained, and this is attributed in part to angiogenesis as well as changes in viscosity.

Influence of chronic nitric oxide inhibition of coronary blood flow regulation: a study of the role of endogenous adenosine in anesthetized, open-chested dogs

The effect of chronic inhibition of endothelium-derived nitric oxide (NO) synthesis on the regulation of coronary blood flow (CBF) is yet to be elucidated. A chronic canine model of inhibited NO synthesis was created and the role of adenosine in the regulation of coronary blood flow in this model was examined. Dogs were fed a diet supplemented with 40 mg/kg per day N(G)-nitro-L-arginine methyl ester (L-NAME group, n=8) or a regular diet without L-NAME supplementation (control group, n=8) for 4 weeks. The experiments were performed in an anesthetized, open-chest state and the results were compared in the L-NAME and control groups. Chronic L-NAME treatment significantly increased arterial pressure. Neither basal CBF in the left anterior descending artery nor heart rate differed between the L-NAME and control groups. In the L-NAME group, the response of CBF to intracoronary acetylcholine and adenosine was blunted, but that to glyceryl trinitrate was not. In addition, myocardial reactive hyperemia following 20 sec coronary occlusion was blunted in the L-NAME group. During atrial pacing at a rate 60 beats/min faster than the sinus rate, CBF increased to a similar degree in the L-NAME and control groups, and systolic wall thickening (SWT) changed similarly in both groups. Intracoronary 8-phenyltheophylline (8-PT), an adenosine receptor blocker, decreased basal CBF in the L-NAME group but not in the control group. In the L-NAME group, pacing-induced increase in CBF was abolished and SWT deteriorated after 8-PT administration. Basal myocardial adenosine release was significantly increased in the L-NAME group compared with the control group. It is concluded that in anesthetized, open-chest dogs with chronic inhibition of NO synthesis, adenosine may play a compensatory role in the regulation of coronary blood flow, as concomitant blockade of adenosine causes deterioration of coronary circulation and cardiac function.