Cyclic nucleotides and cardiac function

Moderate but not severe hypothermia increases intracellular cyclic AMP through preserved production and reduced elimination

Rewarming from accidental hypothermia could be complicated by acute cardiac dysfunction but providing supportive pharmacotherapy at low core temperatures is challenging. Several pharmacological strategies aim to improve cardiovascular function by increasing cAMP in cardiomyocytes as well as cAMP and cGMP levels in vascular smooth muscle, but it is not clear what effects temperature has on cellular elimination of cAMP and cGMP. We therefore studied the effects of differential temperatures from normothermia to deep hypothermia (37 °C-20 °C) on cAMP levels in embryonic H9c2 cardiac cells and elimination of cAMP and cGMP by PDE-enzymes and ABC-transporter proteins. Our experiments showed significant elevation of intracellular cAMP in H9c2-cells at 30 °C but not 20 °C. Elimination of both cAMP and cGMP through ABC transport-proteins and PDE-enzymes showed a temperature dependent reduction. Accordingly, the increased cardiomyocyte cAMP-levels during moderate hypothermia appears an effect of preserved production and reduced elimination at 30 °C. This correlates with earlier in vivo findings of a positive inotropic effect of moderate hypothermia.

Structure-Function Relationships and Modifications of Cardiac Sarcoplasmic Reticulum Ca2+-Transport

Sarcoplasmic reticulum (SR) is a specialized tubular network, which not only maintains the intracellular concentration of Ca2+ at a low level but is also known to release and accumulate Ca2+ for the occurrence of cardiac contraction and relaxation, respectively. This subcellular organelle is composed of several phospholipids and different Ca2+-cycling, Ca2+-binding and regulatory proteins, which work in a coordinated manner to determine its function in cardiomyocytes. Some of the major proteins in the cardiac SR membrane include Ca2+-pump ATPase (SERCA2), Ca2+-release protein (ryanodine receptor), calsequestrin (Ca2+-binding protein) and phospholamban (regulatory protein). The phosphorylation of SR Ca2+-cycling proteins by protein kinase A or Ca2+-calmodulin kinase (directly or indirectly) has been demonstrated to augment SR Ca2+-release and Ca2+-uptake activities and promote cardiac contraction and relaxation functions. The activation of phospholipases and proteases as well as changes in different gene expressions under different pathological conditions have been shown to alter the SR composition and produce Ca2+-handling abnormalities in cardiomyocytes for the development of cardiac dysfunction. The post-translational modifications of SR Ca2+ cycling proteins by processes such as oxidation, nitrosylation, glycosylation, lipidation, acetylation, sumoylation, and O GlcNacylation have also been reported to affect the SR Ca2+ release and uptake activities as well as cardiac contractile activity. The SR function in the heart is also influenced in association with changes in cardiac performance by several hormones including thyroid hormones and adiponectin as well as by exercise-training. On the basis of such observations, it is suggested that both Ca2+-cycling and regulatory proteins in the SR membranes are intimately involved in determining the status of cardiac function and are thus excellent targets for drug development for the treatment of heart disease.

Therapeutic efficacy of phentolamine in the management of severe hand, foot and mouth disease combined with pulmonary edema

The aim of this study was to examine the effects of phentolamine on severe hand, foot and mouth disease (HFMD) combined with pulmonary edema (PE). From May 2008 to December 2012, 53 children with severe HFMD plus PE were enrolled in the treatment group, receiving phentolamine intravenously at a loading dose of 5 µg/kg/min. The control group comprised 52 children with the same disease who did not receive phentolamine infusion. Data concerning creatine kinase (CK), CK-MB, cardiac troponin I (cTnI), heart rate, systolic blood pressure (SBP) and the duration of ventilation dependence and hospitalization were collected. Adverse events were also recorded. It was found that the phentolamine-treated patients exhibited significantly lower CK, CK-MB and cTnI levels, heart rate and SBP than the controls (P<0.01 for all parameters). The average duration of ventilator dependence and hospitalization was significantly shorter (P<0.01) in the phentolamine group than in the control group. It was also found that the overall mortality rate was lower in the phentolamine group (5.8%) than in the control group (11.5%). No adverse events were observed in either group. Thus, these results offer preliminary evidence that phentolamine reduces mortality and relieves the symptoms of EV71-induced PE. Phentolamine is a potential therapeutic agent for this highly lethal disorder.

Development of a high-throughput screening paradigm for the discovery of small-molecule modulators of adenylyl cyclase: identification of an adenylyl cyclase 2 inhibitor

Adenylyl cyclase (AC) isoforms are implicated in several physiologic processes and disease states, but advancements in the therapeutic targeting of AC isoforms have been limited by the lack of potent and isoform-selective small-molecule modulators. The discovery of AC isoform-selective small molecules is expected to facilitate the validation of AC isoforms as therapeutic targets and augment the study of AC isoform function in vivo. Identification of chemical probes for AC2 is particularly important because there are no published genetic deletion studies and few small-molecule modulators. The present report describes the development and implementation of an intact-cell, small-molecule screening approach and subsequent validation paradigm for the discovery of AC2 inhibitors. The NIH clinical collections I and II were screened for inhibitors of AC2 activity using PMA-stimulated cAMP accumulation as a functional readout. Active compounds were subsequently confirmed and validated as direct AC2 inhibitors using orthogonal and counterscreening assays. The screening effort identified SKF-83566 [8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrobromide] as a selective AC2 inhibitor with superior pharmacological properties for selective modulation of AC2 compared with currently available AC inhibitors. The utility of SKF-83566 as a small-molecule probe to study the function of endogenous ACs was demonstrated in C2C12 mouse skeletal muscle cells and human bronchial smooth muscle cells.

Therapeutic efficacy of milrinone in the management of enterovirus 71-induced pulmonary edema

Hand, foot, and mouth disease and herpangina are the major clinical manifestations of enterovirus 71 (EV71) infections. Brain-stem encephalitis and pulmonary edema are severe complications that can lead to death. This study was designed to evaluate the potential therapeutic effect of milrinone, a phosphodiesterase (PDE) inhibitor, in the treatment of patients with EV71-induced pulmonary edema. We conducted a historically controlled trial of 24 children with severe EV71-induced pulmonary edema from April 1998-June 2003 in southern Taiwan. Patients were divided into groups treated before and after the introduction of milrinone therapy. Etiological diagnosis was established by viral cultures and confirmed by specific immunofluorescence and neutralization tests. All 24 patients were below 5 years of age. The mortality was lower in the milrinone-treated vs. nontreated group (36.4% vs. 92.3%, P=0.005). Sympathetic tachycardia was decreased in patients treated with milrinone compared to controls (144 +/- 17/min vs. 206 +/- 26/min, P=0.004). A marked decrease in IL-13 (77 +/- 9 pg/ml vs. 162 +/- 88 pg/ml, P=0.001) was observed in milrinone-treated patients compared to controls. There was a significant reduction in white blood cell (10,838 +/- 4,537/mm3 vs. 19,475 +/- 7,798/mm3, P=0.009) and platelet (257 +/- 45 x 10(3)/mm3 vs. 400 +/- 87 x 10(3)/mm3, P=0.001) counts in milrinone-treated patients compared to controls. These results were associated with improvement in sympathetic regulation and decrease in IL-13 production. Milrinone therapy may provide a useful therapeutic approach for this highly lethal disorder.

Regulation of L-type Ca2+ channels in the heart: overview of recent advances

Regulation of L-type Ca2+ channels is complex, because many factors, such as phosphorylation, divalent cations, and proteins, specified or unspecified, have been shown to affect the channel activities. An additional complication is that these factors interact with one another to achieve final outcomes. Recent molecular technologies have helped to shed light on the mechanisms governing the activity of L-type Ca2+ channels. In this review article, three major topics concerning regulation of L-type Ca2+ channels in the heart are discussed, i.e. c-AMP dependent channel phosphorylation, role of magnesium (Mg2+), and the phenomenon of channel run-down.

Synthesis of benzoxazinone derivatives: a new route to 2 (N phthaloylmethyl)-4H-3,1-benzoxazin-4-one

A new method has been designed to prepare the known benzoxazinone derivative 2-(N-phthaloylmethyl)-4H-3,1-benzoxazin-4-one (4). The acyl chloride derivative N-phthaloylglycine reacts with anthranilic acid in chloroform, in the presence of triethylamine, to give an intermediate that is then reacted with cyanuric chloride, used as a cyclization agent, to produce the benzoxazinone derivative.

Effect of olprinone, a phosphodiesterase III inhibitor, on arterial wall distensibility: differentiation between aorta and common carotid artery

Although the effects of phosphodiesterase III (PDE III) inhibitors as vasorelaxants have been well documented, there are only few data on the wall response of different arteries. We evaluated the artery-specific effect of olprinone (OP), one of the PDE III inhibitors, on the major branches of human arteries and peripheral circulation. In 14 healthy subjects (average age: 57.5 +/- 21.2 years), systolic and diastolic diameters (Ds and Dd, respectively) and the time velocity integral (VI) of flow velocity patterns were measured by M-mode and Doppler echocardiography in the carotid artery (CA), the ascending aorta (asAo), the abdominal aorta (abAo), and the left ventricular outflow tract. Blood pressure (BP) was simultaneously measured using a cuff sphygmomanometer. Measurements were taken before and 20min after a bolus injection of OP (0.2 microg/kg). Distensibility (Ds - Dd), stiffness parameter beta (In(systolic BP/diastolic BP)/(Ds/Dd - 1)), cardiac output (CO: (Flow Area) x VI x HR at left ventricular outflow), selective flow volume (FV: (Flow Area) x VI x HR at CA or abAo), and vascular resistance (VR: mean BP/(CO or FV)) were then calculated. The distensibility increased significantly after OP administration (P = 0.0015), but that of the asAo or abAo did not change. Although there was a significant increase in CO (P = 0.001) and a significant decrease in systemic VR (P = 0.001) following OP administration, the FV and VR of both CA and abAo did not change significantly. The selectiveness of the effect of OP was demonstrated in terms of the CA wall distensibility. This was thought to be attributable to the differences in the structural components or the reactivity of smooth muscle cells to OP.

Cell coupling and impulse propagation in the failing heart

Cell coupling and impulse propagation were investigated in the ventricle of cardiomyopathic hamsters at an advanced stage of heart failure. An appreciable decline in junctional conductance was found, a phenomenon in part related to activation of the plasma and cardiac renin-angiotensin systems. Decreased expression of connexin43 or an alteration of junctional proteins also might be implicated in the decreased cell coupling. Morphologic abnormalities such as fibrosis, necrosis, and rupture of cell contacts contribute to the decline of conduction velocity or to the blockade of impulse propagation in some areas of the ventricle, creating the conditions for anisotropic conduction and cardiac arrhythmias. The decrease in membrane potential found in myopathic cells is related in part to depression of Na-KATPase activity, and the lack of action of beta-adrenergic agonists on junctional conductance is explained by down-regulation of beta receptors and an abnormality of adenyl cyclase.

Effect of the Yang tonifying herbs on myocardial beta-adrenoceptors of hypothyroid rabbits

New Zealand White female rabbits were randomly divided into three groups, each contained six rabbits, i.e. thyroidectomized and untreated rabbits (group 1), thyroidectomized rabbits treated by the Yang tonifying herbs (group 2) and sham thyroidectomized rabbits as controls (group 3). The myocardial beta-adrenoceptor density (Bmax) and affinity (Kd) of each group of rabbits were determined by radioligand binding assay technique on the thirtieth postoperative day and the data were handled by using a computer program of the Woolf plot with weight regression. Moreover, the serum levels of thyroxine (T4) and triiodothyronine (T3) of each group of rabbits were measured by radioimmunoassay technique and their heart rates (HR) were also recorded on the preoperative and thirtieth postoperative day. The results showed that the Bmax, T4, T3 and HR in group 1 were lower significantly than that in group 3 (P < 0.01-0.001), but the change of Kd in group 1 was not significant; the deviation of the indices from the normal value in group 2 was less remarkably than in group 1 other than T4.

Comparison of inotropic and chronotropic effects of vasoactive intestinal peptide in isolated dog atria

The positive chronotropic and inotropic effects of vasoactive intestinal peptide, VIP, were studied in an isolated canine right atrial preparation. Atria were removed, maintained in a bath, and perfused with Tyrode's solution. Contractile force and atrial depolarization were measured. VIP (18.8-600 pmol) was injected into a cannulated sinoatrial nodal artery and dose response curves were obtained. The mean EC50 was similar for the inotropic and the chronotropic responses (136 and 144 pmol, respectively). Time courses of the onset and of recovery from the responses were measured. Times for onset of VIP effects were similar but, once the effect was initiated, rate of development of the response and recovery time from the responses were dose dependent. The increases in atrial rate lasted two to four times longer than did the increases in contractile force. Recovery from the chronotropic and inotropic responses to VIP differ, suggesting that the intracellular responses are coupled differently to the receptors. The responses to VIP were compared to those of 100 pmol isoproterenol, another positive chronotropic and inotropic agent. Isoproterenol was a slightly more potent chronotropic and inotropic agent than VIP. Desensitization of the responses was determined. Repeated exposures to VIP decreased the chronotropic response but not the inotropic response to VIP. There was no significant decrease in responsiveness to isoproterenol.

Differences in response to activation of adenylyl cyclase by various stimulants in human myocardium

The response of adenylyl cyclase complex in human atrial tissue removed at corrective surgery of normoxemic and hypoxemic congenital heart defects in children to various stimulants was evaluated and related to the oxygenation state of the myocardium. When comparing response to stimulation in normoxemic and hypoxemic atria a higher basal as well as stimulated adenylyl cyclase activity was found in hypoxemic atria; an insignificant stimulatory effect of isoprenaline in normoxemic hearts became significant in the atria of hypoxemic patients. Hypoxemic samples also showed two times higher activity when the total catalytic activity was evaluated by the stimulation with forskolin. Higher stimulatory effect of Gpp/NH/p was also observed in hypoxemic than in normoxemic state. Increased adenylyl cyclase activity might represent one of adaptive mechanisms to hypoxemia in patients with congenital heart defects.

Modulating L-type calcium current affects discontinuous cardiac action potential conduction

We have used pairs of cardiac cells (i.e., one real guinea pig ventricular cell and a real-time simulation of a numerical model of a guinea pig ventricular cell) to evaluate the effects on action potential conduction of a variable coupling conductance in combination with agents that either increase or decrease the magnitude of the L-type calcium current. For the cell pairs studied, we applied a direct repetitive stimulation to the real cell, making it the "leader" cell of the cell pair. We have demonstrated that significant delays in action potential conduction for a cell pair can occur either with a decreased value of coupling conductance or with an asymmetry in size such that the follower cell is larger than the leader cell. In both conditions we have shown that isoproterenol, applied to the real cell at very low concentrations, can reversibly decrease the critical coupling conductance (below which action potential conduction fails) for a cell pair with fixed cell sizes, or, for a fixed value of coupling conductance, increase the maximum allowable asymmetry in cell size for successful conduction. For either of these effects, we were able to show that treatment of the real cell with BayK 8644, which more specifically increases the magnitude of the L-type calcium current, was able to mimic the actions of isoproterenol. Treatment of the leader cell of the cell pair (the real cell) with nifedipine, which selectively lowers the magnitude of the L-type calcium current, had effects opposite those of isoproterenol or BayK 8644. The actions of nifedipine, isoproterenol, and BayK 8644 are all limited to conditions in which the conduction delay is on the order of 5 ms or more, whether this delay is caused by limited coupling conductance or by asymmetry in size of the cells. This limitation is consistent with the time course of the L-type calcium current and suggests that the effects of calcium channel blockers or beta-adrenergic blocking drugs, in addition to being selective for regions of the heart that depend on the L-type calcium current for the upstroke of the action potential, would also be somewhat selective for regions of the heart that have discontinuous conduction, either normally or because of some pathological condition.

Thiopentone inhibits beta-adrenergic responses in myocardial tissue

The purpose of this study was to determine the importance of inhibition of beta-adrenergic function in thiopentone-induced myocardial depression. Using an isolated, electrically stimulated rat left atria model, contractile dose-response curves to thiopentone (200 microM, 400 microM, 600 microM, 800 microM) were shifted to the right in preparations treated with 10(-3)M dibutyryl cyclic adenosine monophosphate (cAMP) compared with atria stimulated with 10(-6) M dibutyryl cyclic isoprenaline, demonstrating that inhibition of beta-adrenergic mechanisms by thiopentone is physiologically important. Depression by thiopentone was similar in atria treated with 10(-5) M forskolin compared with preparations stimulated with 10(-6) M isoprenaline, indicating that thiopentone does not block beta-adrenergic receptors. It is concluded that thiopentone depresses myocardial function by several mechanisms, one of which involves inhibition of the adenyl cyclase cascade. The adenyl cyclase enzyme is a likely site where thiopentone inhibits the system; however, other components of the cascade may also be involved.

Effects of intracoronary infusion of an inotropic agent, E-1020 (loprinone hydrochloride), on cardiac function: evaluation of left ventricular contractile performance using the end-systolic pressure-volume relationship

The phosphodiesterase (PDE) III inhibitor, E-1020 (loprinone hydrochloride), has positive inotropic and vasodilating effects. This study evaluated the positive inotropic effect of intracoronary E-1020 in eight patients with coronary artery disease and hypertensive heart disease. A direct intracoronary infusion of the PDE III inhibitor minimizes its vasodilating effect. After baseline hemodynamic measurements and coronary arteriography, a micromanometer-tipped 8F conductance catheter was introduced into the left ventricle to determine the hemodynamic effects of E-1020. Saline and vehicle were infused into the left main coronary artery at a rate of 2 ml/min. The dose of intracoronary E-1020 increased from 2.5 to 5.0 and 7.5 micrograms/min. The inotropic effect of E-1020 was defined as the change in the slope of the end-systolic pressure-volume relationship (Emax), which was independent of afterload and preload. Emax significantly increased at infusion rates of 7.5 micrograms/min from control. Peak +dP/dt increased at an infusion rate of 5.0 micrograms/min or higher, while left-ventricular end-diastolic pressure (LVEDP) decreased significantly at a rate of 5.0 and 7.5 micrograms/min. Intracoronary infusion of E-1020 at a rate of 2.5 micrograms/min produced a plasma concentration of 20 ng/ml, which was identical to the minimum effective plasma concentration seen in previous study by intra venous infusion. However, at a plasma concentration of 20 ng/ml, E-1020 has more vasodilating effects than inotropic effects. Clinically, E-1020 appears to have a positive inotropic effect that depends on the extent of myocardial perfusion.

Cyclic GMP accumulation in normal and diabetic primary culture adult rat ventricular cardiomyocytes: a minor role for nitric oxide in phosphorylase activation

The potential role of nitric oxide in the diabetes-induced hypersensitive activation of glycogen phosphorylase by epinephrine was investigated in adult rat ventricular cardiomyocytes. Pretreatment of normal and diabetic-derived cells with 1 mM sodium nitroprusside significantly diminished the phosphorylase activation response by nearly 20% in both normal and diabetic myocytes but failed to alter the hypersensitivity of the diabetic cells. Nitroprusside increased cGMP levels in both normal and diabetic myocytes although the effect was more pronounced in the diabetic cells. Epinephrine did not alter cellular cGMP content and cGMP levels were consistently lower in diabetic myocytes when compared with normal myocytes. Preincubation of ventricular myocytes with the nitric oxide synthase inhibitor L-iminoethyl ornithine did not affect phosphorylase activation. These data indicate that nitric oxide plays a minor role in phosphorylase activation by epinephrine in rat cardiomyocytes and suggest that signal transduction via nitric oxide is not affected by the onset of diabetes.

Relation of myocardial oxygen consumption and function to high energy phosphate utilization during graded hypoxia and reoxygenation in sheep in vivo

This study investigates the relation between myocardial oxygen consumption (MVO2), function, and high energy phosphates during severe hypoxia and reoxygenation in sheep in vivo. Graded hypoxia was performed in open-chested sheep to adjust PO2 to values where rapid depletion of energy stores occurred. Highly time-resolved 31P nuclear magnetic resonance spectroscopy enabled monitoring of myocardial phosphates throughout hypoxia and recovery with simultaneous MVO2 measurement. Sheep undergoing graded hypoxia (n = 5) with an arterial PO2 nadir of 13.4 +/- 0.5 mmHg, demonstrated maintained rates of oxygen consumption with large changes in coronary flow as phosphocreatine (PCr) decreased within 4 min to 40 +/- 7% of baseline. ATP utilization rate increased simultaneously 59 +/- 20%. Recovery was accompanied by marked increases in MVO2 from 2.0 +/- 0.5 to 7.2 +/- 1.9 mumol/g per min, while PCr recovery rate was 4.3 +/- 0.6 mumol/g per min. ATP decreased to 75 +/- 6% of baseline during severe hypoxia and did not recover. Sheep (n = 5) which underwent moderate hypoxia (PO2 maintained 25-35 mmHg for 10 min) did not demonstrate change in PCr or ATP. Functional and work assessment (n = 4) revealed that cardiac power increased during the graded hypoxia and was maintained through early reoxygenation. These studies show that (a) MVO2 does not decrease during oxygen deprivation in vivo despite marked and rapid decreases in high energy phosphates; (b) contractile function during hypoxia in vivo does not decrease during periods of PCr depletion and intracellular phosphate accumulation, and this may be related to marked increases in circulating catecholamines during global hypoxia. The measured creatine rephosphorylation rate is 34 +/- 11% of predicted (P < 0.01) calculated from reoxygenation parameters, which indicates that some mitochondrial respiratory uncoupling also occurs during the rephosphorylation period.

Influence of ischaemia and reperfusion on cardiac signal transduction. G protein content, adenylyl cyclase activity, cyclic AMP content, and forskolin and dibutyryl cyclic AMP-induced inotropy in the rat Langendorff heart

We investigated whether post-receptor alterations contribute to the diminished beta-adrenergic inotropic effects in the rat Langendorff heart following ischaemia (I) and reperfusion (R). We quantitated immunodetectable Gs and Gi protein alpha-subunit content, basal and stimulated adenylyl cyclase activity and cyclic AMP (cAMP) content in normoxic, ischaemic (30 min) and ischaemic reperfused (30 min) hearts. In addition, we measured the inotropic response of normoxic and reperfused Langendorff hearts to forskolin and dibutyryl cAMP (db-cAMP). Immunodetectable Gs and Gi alpha-subunits were unaltered by I or R. Basal adenylyl cyclase activity was decreased during I, but recovered during R. In membranes from normoxic hearts, isoprenaline, GTP, Gpp(NH)p, NaF, forskolin or Mn2+ enhanced adenylyl cyclase activity. This increase in activity was diminished in ischaemic hearts, but could be restored by R. cAMP content decreased time-dependently during I and did not recover by R, indicating ATP depletion. Forskolin and db-cAMP induced an inotropic response in normoxic hearts, which was virtually abolished after I and R. We conclude that adenylyl cyclase responsiveness is impaired during I. Since adenylyl cyclase responsiveness recovers during R, whereas inotropic responses to forskolin and db-cAMP are virtually absent in reperfused hearts, an additional mechanism downstream of cAMP formation appears to be defective during R, which prevents recovery of inotropic responses to hormonal stimulation.

Involvement of A pertussis Toxin Sensitive G-Protein in the Inhibition of Inwardly Rectifying K Currents by Platelet-Activating Factor in Guinea-Pig Atrial Cardiomyocytes

Platelet-activating factor (PAF) inhibits single inwardly rectifying K⁺ channels in guinea-pig ventricular cells. There is currently little information as to the mechanism by which these channels are modulated. The effect of PAF on quasi steady-state inwardly rectifying K⁺ currents (presumably of the I_K1 type) of auricular, atrial and ventricular cardiomyocytes from guinea-pig were studied. Applying the patch-clamp technique in the whole-cell configuration, PAF (10 nM) reduced the K⁺ currents in all three cell types. The inhibitory effect of PAF occurred within seconds and was reversible upon wash-out. It was almost completely abolished by the PAF receptor antagonist BN 50730. Intracellular infusion of atrial cells with guanine 5′-(β-thio)diphosphate (GDPS) or pretreatment of cells with pertussis toxin abolished the PAF dependent reduction of the currents. Neither extracellularly applied isoproterenol nor intracellularly applied adenosine 3′,5′-cyclic monophosphate (cyclic AMP) attenuated the PAF effect. In multicellular preparations of auricles, PAF (10 nM) induced arrhythmias. The arrhythmogenic activity was also reduced by BN 50730. The data indicate that activated PAF receptors inhibit inwardly rectifying K⁺ currents via a pertussis toxin sensitive G-protein without involvement of a cyclic AMP-dependent step. Since I_K1 is a major component in stabilizing the resting membrane potential, the observed inhibition of this type of channel could play an important role in PAF dependent arrhythmogenesis in guinea-pig heart.

Modulation of cytosolic free calcium concentration by ?1-adrenoceptors in rat atrial cells

The effects of alpha 1-adrenoceptor stimulation by phenylephrine (PE) and beta-adrenoceptor stimulation by isoprenaline (ISO) on Ca2+ current (ICa) and free intracellular Ca2+ concentration ([Ca2+]i) were studied in isolated atrial myocytes from rat hearts. PE did not significantly affect the magnitude of ICa, whereas large increases of peak ICa were observed in response to ISO. In electrically driven cells, PE evoked a concentration-dependent, gradual increase in diastolic [Ca2+]i and, initially, an increase in the height of peak [Ca2+]i transients. When the diastolic [Ca2+]i was increased to a greater extent, the amplitude of [Ca2+]i transients was decreased. Simultaneous measurements of [Ca2+]i and membrane potential showed that the increase in diastolic [Ca2+]i was associated with a depolarization of the membrane, and the greater amplitude of [Ca2+]i transients with a prolongation of the action potential (AP). The PE-induced increase in diastolic [Ca2+]i was eliminated when the cells were voltage-clamped at the original resting membrane potential (RP); under these conditions, an increase in [Ca2+]i transients was observed in response to PE. ISO usually caused larger increases in the amplitude of [Ca2+]i transients with only minor changes in diastolic [Ca2+]i. These results suggest that PE and ISO increase the amplitude of [Ca2+]i transients in rat atrium in different ways. The increase in [Ca2+]i transients in response to beta-adrenoceptor stimulation is commonly thought to be mediated by a greater conductance of voltage-dependent Ca2+ channels causing a greater Ca2+ influx and a release of more Ca2+ from the sarcoplasmic reticulum during the AP. The increase in diastolic [Ca2+]i in response to PE is probably a consequence of the depolarization of the membrane, possibly involving the voltage-dependent Na(+)-Ca2+ exchange mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulatory guanine nucleotide-binding protein and adenylate cyclase activities in Bio 14.6 cardiomyopathic hamsters at the hypertrophic stage

The Bio 14.6 cardiomyopathic Syrian hamster is an animal model of human idiopathic cardiomyopathy. The pathogenesis of the disease in this animal has not yet been clearly elucidated. It is well known that alpha- and beta-adrenergic receptors are increased in the myocardium of this animal, but that isoprenaline does not produce an augmented response. We examined the activity of cardiac stimulatory GTP-binding protein (Gs), which couple with beta-adrenergic receptors to stimulate adenylate cyclase, in Bio 14.6 cardiomyopathic hamsters at 90 and 160 days of age. The cardiac norepinephrine concentration was significantly increased in Bio 14.6 hamsters compared with control hamsters (F1B) at 90 days of age (1,739 +/- 120 vs 1,470 +/- 161 ng/g wet tissue weight, p less than 0.05). Cardiac forskolin-stimulated adenylate cyclase activities at 90 and 160 days of age were lower in the cardiomyopathic hamsters than in the F1B controls (90 days old: 98 +/- 24 vs 122 +/- 29 pmol/min/mg protein, p less than 0.05; 160 days old: 74 +/- 13 vs 124 +/- 28 pmol/min/mg protein, p less than 0.01). Cardiac Gs activities at 90 and 160 days of age were significantly lower in Bio 14.6 hamsters than those in F1B hamsters (90 days old: 204 +/- 42 vs 259 +/- 49 pmol/min/mg protein, p less than 0.05; 160 days old: 156 +/- 39 vs 211 +/- 60 pmol/min/mg protein, p less than 0.05). We thus demonstrated functional defects in cardiac Gs protein and adenylate cyclase activity in the Bio 14.6 cardiomyopathic hamsters at 90 to 160 days of age (the hypertrophic stage of cardiomyopathy).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of flosequinan upon isoenzymes of phosphodiesterase from guinea-pig cardiac and vascular smooth muscle

The effect of flosequinan and its sulphone metabolite BTS 53,554, on phosphodiesterase isoenzymes isolated from guinea-pig cardiac and vascular smooth muscle using DEAE-cellulose chromatography was investigated. Zaprinast and milrinone showed peak I and peak III selectivity, and IBMX non-selective activity respectively, against both cardiac and vascular smooth muscle isoenzymes, as expected for these reference inhibitors. Flosequinan and BTS 53,554 demonstrated non-selective inhibition with similar potency against both cardiac and vascular smooth muscle isoenzymes and, overall, were the least potent compounds tested. The high inhibitory concentrations observed (IC50 peak III 660 microM for cardiac tissue and 230 microM for vascular smooth muscle with flosequinan) relative to its clinically effective plasma concentration (10 microM) questions the relevance of phosphodiesterase inhibition to the efficacy of flosequinan in heart failure.

Immunodetectable levels of the inhibitory guanine nucleotide-binding regulatory proteins in failing human heart: discordance with measurements of adenylate cyclase activity and levels of pertussis toxin substrate

Human hearts with idiopathic dilated cardiomyopathy have diminished adenylate cyclase activity and increased amounts of the alpha-subunit of the inhibitory guanine nucleotide-binding regulatory protein (alpha Gi) as measured by pertussis toxin catalyzed ADP-ribosylation. We utilized specific antisera against synthetic peptides corresponding to amino sequences deduced from cDNA's encoding the three alpha Gi subspecies to compare the immunologic and bioactivity levels of Gi in failing and non-failing human hearts. The various antisera detected three peptides with Mr 42,000, 38,000, and 37,000. Only the Mr 42,000 peptide co-migrated with the pertussis toxin substrate. Although functional activity of alpha Gi was increased in the particulate fractions of the failing heart as measured by inhibition of guanine nucleotide-stimulated adenylate cyclase activity and the quantity of pertussis toxin substrate was also increased, there were not associated changes in the levels of immunodetectable Gi. Therefore, the increased functional activity of Gi in the failing human heart as assessed by adenylate cyclase measurements cannot be explained by a relative increase in the among of Gi protein.

Depolarization-induced influx of sodium in response to phenylephrine in rat atrial heart muscle

1. The effects of alpha 1-adrenoceptor stimulation on transmembrane potential, currents and ion fluxes were investigated in multicellular preparations and/or single cells obtained from the left atrium of rat hearts. 2. In multicellular preparations, phenylephrine caused a concentration-dependent positive inotropic effect, an increase in action potential duration, and a decrease in resting potential; the effects were antagonized by phentolamine. 3. In the presence of phenylephrine (100 mumol/1), two levels of resting potential were observed when the preparations were, alternately, electrically stimulated or kept at rest (-74 +/- 1 mV during activity and -62 +/- 4 mV at rest; mean +/- S.E.M.; n = 9). 4. In resting preparations, the depolarization in response to phenylephrine was eliminated in low-Na+ solution (12 mmol/l) and antagonized by tetrodotoxin (10 mumol/l). 5. The phenylephrine-induced depolarization was also seen in nominally Ca(2+)-free solution and in the presence of (-)-devapamil (1 mumol/l). 6. The alkylating agent N-ethyl-maleimide (30 mumol/l) abolished the depolarizing effect of phenylephrine. 7. Phorbol 12,13-dibutyrate (10 mumol/l) also abolished the depolarizing effect of phenylephrine. 8. Phenylephrine caused a significant increase of 22Na+ uptake in resting preparations and of 45Ca2+ uptake in beating preparations. 9. The depolarizing effect of phenylephrine was also observed in single atrial myocytes. Steady-state membrane currents in response to 500 ms depolarizing and hyperpolarizing voltage clamp steps were decreased. The cross-over of I-V curves under control and test conditions was at about -70 mV. The effects of phenylephrine were antagonized in the presence of phentolamine. 10. After suppression of potassium currents by substitution of CsCl for internal and external KCl ([KCl]o), phenylephrine had no effect on membrane currents. 11. In conclusion, we presume the following sequence of events in response to phenylephrine in rat atrial heart muscle. First, the stimulation of alpha 1-adrenoceptors decreases the K+ conductance thereby producing a depolarization in the presence of an inward current. Second, the change of the membrane potential in the depolarizing direction induces a TTX-sensitive Na+ window current which further propels the depolarization. Third, the increase in Na+ influx may increase Ca2+ influx by activating the Na(+)-Ca2+ exchange in mechanism. The greater influx of Ca2+ may contribute to the positive inotropic effect in response to phenylephrine.

Mechanism of receptor-mediated modulation of the delayed outward potassium current in guinea-pig ventricular myocytes

1. Receptor-mediated modulation of the delayed outward potassium current (IK) was investigated in guinea-pig single ventricular cells by using whole-cell voltage clamp and intracellular dialysis. 2. Isoprenaline increased IK in a dose-dependent manner with a half-maximum dose of 1.8 X 10(-8) M. Isoprenaline (10(-6) M) maximally increased IK by a factor of 2.85. This effect did not depend on the concentration of intracellular Ca2+ [( Ca2+]i). 3. External application of 10(-5) M-forskolin and internal application of 5 X 10(-5) M-cyclic AMP or 5 X 10(-6) M of the catalytic subunit of cyclic AMP-dependent protein kinase (PKA) also increased IK about 3-fold. The effect of isoprenaline on IK was masked by previous application of cyclic AMP. 4. All the above phosphorylating agents increased the amplitude of IK without a significant change in the current kinetics. 5. In the presence of 10(-5) M-forskolin, an additional application of 10(-8) M-12-O-tetradecanoylphorbol-13-acetate, an activator of protein kinase C (PKC), produced a further increase in IK, suggesting that the active sites of PKA and PKC on the IK channel are different. 6. Acetylcholine (10(-6) M) suppressed IK when the current was previously enhanced by 2 X 10(-8) M-isoprenaline, but had little effect in the absence of isoprenaline. 7. We conclude that beta-adrenergic modulation of IK is mediated by cyclic AMP-dependent phosphorylation but not by an increase in [Ca2+]i, that PKA and PKC enhance IK independently, and that acetylcholine antagonizes beta-adrenergic stimulation of IK most probably by inhibiting adenylate cyclase.

Decreased bioactivity of the guanine nucleotide-binding protein that stimulates adenylate cyclase in hearts from cardiomyopathic Syrian hamsters

We investigated regulation of cardiac adenylate cyclase in 29-d-old BIO 14.6 Syrian hamsters, which inherit cardiomyopathy as an autosomal recessive trait. Pharmacologic stimulation of adenylate cyclase in cardiac membranes with isoproterenol, fluoride ion, guanine nucleotide, forskolin, and manganous ion indicated that there was defective coupling of the guanine nucleotide-binding protein that stimulates adenylate cyclase (Gs) to adenylate cyclase. Cyc complementation assays revealed congruent to 50% less Gs activity in cardiac and skeletal muscle from cardiomyopathic hamsters. Despite this decrease in functional Gs, there were no changes in immunologic levels of the alpha-subunit of Gs (alpha Gs) or in levels of mRNA encoding alpha Gs. The defect in Gs bioactivity was limited to cardiac and skeletal muscle, occurred only in animals homozygous for the dystrophic trait, and was demonstrable before any cardiac abnormalities were evident on light microscopy. By contrast, cardiac levels of beta-adrenergic receptors were not different in cardiac membranes from BIO 14.6 hamsters. We conclude that a functional defect in alpha Gs may contribute to a contractile abnormalities in the cardiomyopathic BIO 14.6 hamster. However, the etiology of the alpha Gs defect remains obscure.

[Enzyme activity of cardiac glycogen metabolism: study of an in situ hypoxia protocol in the rat]

Myocardial hypoxia, induced by arrest of the artificial ventilation of anaesthetized open-chest rats, was utilized in order to study some aspects of the regulation of myocardial glycogen metabolism. Atenolol, a cardioselective beta-adrenergic receptor antagonist, and verapamil, an inhibitor of sarcolemmal calcium transfer, were used to determine the respective role of adenosine 3', 5'-cyclic monophosphate (cAMP) and calcium in the activation of the enzymes of glycogen phosphorolysis and synthesis. Glycogen degradation is reduced by atenolol treatment, as a consequence of a reduced activation of glycogen phosphorylase. Verapamil treatment has no significant effect, neither on the enzyme activation nor on the glycogen utilization. The activation of glycogen synthase, expressed by the conversion of the enzyme from the D to the I form, which results from the decrease in glycogen stores during hypoxia, is lowered under the effect of both drugs. However, in the beta-blocker treatment case, this effect results from a lower glycogen depletion while this effect is more specific in hearts from rats treated with verapamil. Under the effect of verapamil, the reduction of synthase activation, for a similar depletion of glycogen stores, was confirmed by experiments using isolated rat hearts submitted to ischaemia. These results show that: 1. the glycogenolysis in the hypoxic myocardium in situ is mainly controlled by a cAMP-dependent enzyme conversion or by metabolic allosteric effectors; 2. the activation of myocardial glycogen synthase, which is essentially correlated to the reduction of glycogen stores, is also calcium-dependent and most probably totally cAMP-independent.

New mechanisms for positive inotropic agents: focus on the discovery and development of imazodan

Intense efforts during the last decade to identify a useful positive inotropic agent to replace digitalis for the treatment of congestive heart failure have led to the discovery of several dozen potential substitutes, of which a number are currently undergoing clinical trials. In addition to producing a variety of new therapeutic entities, research in this area has also yielded valuable new information regarding the fundamental events that regulate calcium homeostasis and contractile function in the cardiac cell. For example, several of these new inotropic agents, including the calcium-channel stimulator BAY-k 8644, the sodium-channel stimulator DPI-201-186, and the sodium-calcium exchange inhibitor dichlorobenzamil, have provided considerable insight into the role of sodium and calcium in regulating contractility and the molecular events that mediate potential-dependent ion channels. Likewise, the discovery and development of agents like imazodan, amrinone, enoximone, and other selective type III phosphodiesterase inhibitors have provided new information regarding multiple molecular forms of cyclic nucleotide phosphodiesterase, compartmentation of cyclic AMP, and the importance of soluble vs. membrane-bound phosphodiesterases.

Biochemical aspects of inhibition of cardiovascular low (Km) cyclic adenosine monophosphate phosphodiesterase

Multiple isozymes of cyclic nucleotide phosphodiesterase (PDE) exist in mammalian cells. At least 5 major types of PDE isozymes have been identified; they differ by substrate affinity, maximal activity, intracellular regulation or mechanism of pharmacologic inhibition. A low Michaelis constant (Km) cyclic adenosine monophosphate (cAMP) PDE, whose activity is inhibited by submicromolar concentrations of cyclic guanosine monophosphate and stimulated by cAMP-mediated phosphorylation, is present in both cardiac muscle and vascular smooth muscle. This PDE isozyme (referred to as peak IIIc PDE) is sensitive to selective inhibition by amrinone, milrinone, imazodan, CI-930, piroximone, and numerous other PDE inhibitors. The subcellular distribution of cardiac PDE IIIc varies according to species; it is found in the soluble fraction of guinea pig myocardium, in the particulate fraction of canine myocardium, and in both fractions of primate (simian and human) myocardium. Another PDE isozyme, which is sensitive to inhibition by rolipram and is less sensitive to inhibition by PDE IIIc inhibitors, is found in cardiac muscle of some species (i.e., soluble fractions of rat and canine myocardium) and is apparently not related to direct regulation of positive inotropy. Both positive inotropy and vasorelaxation by milrinone and other PDE IIIc inhibitors can be linked to inhibition of PDE IIIc and activation of the cAMP system. These significant relations are similar to those obtained for other cAMP-related positive inotrope/vasodilators (such as beta-adrenoreceptor agonists). Moreover, an increased rate of ventricular relaxation (lusitropy), which is apparent with PDE IIIc inhibitors, may also be attributable to activation of the cAMP system.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of arachidonic acid metabolites and adrenergic nervous system

Arachidonic acid, which is esterified mainly at the Sn-2 position of phospholipids, is deacylated in response to a wide variety of neurohormonal stimuli, including sympathetic nerve stimulation and catecholamines. In most cardiovascular tissues, arachidonic acid is metabolized via the cyclooxygenase pathway into prostaglandins. The kind and quantity of prostaglandins formed in response to adrenergic stimuli vary in different tissues. For example, PGE2 is the major product of arachidonic acid formed in the spleen and kidney and PGI2 in the heart in response to sympathetic nerve stimulation and norepinephrine. The sequence of events, including the type of adrenergic receptor involved in the action of catecholamines on PG synthesis, varies from tissue to tissue. For example, PG synthesis elicited by adrenergic stimulation in the heart is mediated via activation of beta-1 adrenergic receptors, whereas in the kidney it is linked to stimulation of alpha-1 adrenergic receptors. Moreover, prostaglandin synthesis elicited by alpha-1 adrenergic receptor activation in the kidney requires only partially extra- but mainly intracellular Ca++ and calmodulin. However, beta-1 adrenergic receptor stimulated PG synthesis in the heart is absolutely dependent upon extracellular Ca++ but not on calmodulin. Furthermore, alpha-1 but not beta-1 adrenergic receptor stimulated PG synthesis is inhibited by mepacrine, a phospholipase A2 and C inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal relationship of contractility and myocardial potassium balance following beta-adrenergic stimulation of the in situ pig heart

Lower intracellular Na+ during beta-adrenergic stimulation provides an increased driving force for Na-Ca exchange, which might attenuate the inotropic response. Since (1) Na+ reduction is coupled to K+ uptake, and (2) K+ uptake lags behind the positive inotropic response to isoproterenol, we could examine the effect of Na-Ca exchange by comparing cardiac contractility and K+ balance following intracoronary isoproterenol infusion (0.6-0.8 microgram min-1). In 8 open-chest pigs, potassium concentrations were continuously measured by PVC-valinomycin mini-electrodes in arterial blood (a), and in myocardial venous blood in a shunt from the coronary sinus (cs) to the right atrium. Shunt flow, aortic flow, a left ventricular segment length and left ventricular pressure (LVP) were also continuously recorded. 64 (41-85)% (median and 95% confidence interval) of the LV dP/dt increase occurred within 1 min; thereafter contractility rose slowly. During the first minute of isoproterenol infusion, there was a small net myocardial K+ release, which then reversed to K+ accumulation. A maximum a-cs K+ concentration difference of 0.20 (0.09-0.39) mM occurred at 3.0 (2.0-4.25) min, falling to 0.05 (0.01-0.10) mM after 6.5 (3.75-8.75) min, at which point accumulated myocardial K+ uptake was 135 (27-219) mumol 100 g-1. Heart rate remained unchanged and intramural ECG indicated no sign of ischemia during the first 1.5 min of isoproterenol infusion. At 6.25 (5.0-8.0) min after stop of isoproterenol, LV dP/dt was 12 (9-24)% lower than before infusion (P less than 0.02) whereas myocardial K+ content remained higher than control. Thus, the monovalent cation shift succeeding the positive inotropic response was not associated with reduced contractility, but could explain the undershoot of LV dP/dt after stopping isoproterenol.

Changes in cyclic nucleotides during the calcium paradox in the isolated rat heart

Reperfusion of hearts with a Ca2+-containing medium after a perfusion period in Ca2+-free medium results in irreversible cell damage (calcium paradox). In this investigation we have studied coronary flow and cyclic AMP and cyclic GMP levels after several periods of Ca2+-free perfusion in isolated rat hearts. We also investigated the effects of papaverine (Pap), noradrenaline (NA), acetylcholine (ACh) and absence of inorganic phosphate during Ca2+-free perfusion on coronary flow (CF) and cyclic nucleotide levels. Inability of the heart to recover contractile activity with development of contracture during the reperfusion period was accepted as indicative of the calcium paradox. Ca2+-free perfusion alone and NA and absence of inorganic phosphate during the Ca2+-free perfusion period increased CF, whereas Pap and ACh decreased it. However, only Ca2+-free perfusion and NA elevated cyclic AMP. On the other hand, Pap and ACh increased cyclic GMP (with a transient rise of cyclic AMP in Pap infusion), and absence of inorganic phosphate decreased both cyclic AMP and cyclic GMP. Pap, ACh and absence of phosphate prevented the calcium paradox. Our study suggests that increased cyclic AMP during the Ca2+-free perfusion may contribute, with the other factors, to the occurrence of the calcium paradox.

Arrhythmogenic effects of theophylline in human atrial tissue

We studied the effects of theophylline on the transmembrane action potential and the contractile force of human atrial fibers obtained from the hearts of 15 patients, undergoing corrective open-heart surgery. Atrial fibers were perfused with Tyrode solution and driven electrically at a constant rate of 60 beats per min. Theophylline (0.1-1 mM) steepened the diastolic depolarization, increased the amplitude of oscillatory potential during diastole and facilitated the development of spontaneous slow response action potentials. These arrhythmogenic effects of theophylline were suppressed after diltiazem (0.1-0.3 microM) pretreatment. The present findings provide the electrophysiologic evidence that abnormal atrial automaticity as a result of triggered activity may be the underlying cause for atrial ectopic activity and multifocal atrial tachycardia in patients taking theophylline.