Effects of calcium channel agonism by Bay-K-8644 on ventricular fibrillation threshold of isolated heart

The hypothesis tested was that enhanced entry of calcium into cardiac cells would increase the susceptibility to ventricular fibrillation as measured by the ventricular fibrillation threshold (VFT) of the isolated perfused rat heart. Bay-K-8644 was used as a calcium-channel agonist. There was a biphasic effect with a maximal increase in left ventricular systolic pressure and oxygen uptake at a concentration of 10(-7) M. The same concentration caused a major reduction in the VFT. The bell-shaped pattern of fall of the VFT was inversely related to the effect on LV developed pressure. The changes in VFT could be dissociated from those on myocardial metabolites. Although Bay-K-8644 increased the heart rate, reduction of the VFT could also be obtained in paced hearts. The addition of ryanodine, an agent known to interrupt intracellular recycling of calcium through the sarcoplasmic reticulum, was able to abolish approximately half the effect of Bay-K-8644 on the VFT. Therefore, increased entry of calcium via the calcium channel is able to reduce VFT, acting in part through enhanced recycling of calcium through the sarcoplasmic reticulum.

Effects of therapy on renal impairment in essential hypertension

The effect of hydrochlorothiazide 50 mg, amiloride 5 mg (HCTZ-A) (Moduretic; MSD) and its combination with pindolol and nadolol on renal function were compared in patients with mild to moderate essential hypertension (mean untreated supine blood pressure 179 +/- 5.7/108 +/- 2.6 mmHg) with compromised renal function (glomerular filtration rate (GFR) < 85 ml/min). After randomisation to a beta-blocker plus diuretic, the beta-blocker was washed out (diuretic-only period) and the other beta-blocker added. Eleven patients completed the randomised cross-over comparative study, 5 receiving pindolol 15 mg plus 1 dose of HCTZ-A daily for 6 weeks, and 6 nadolol 120 mg plus 1 dose of HCTZ-A daily. The beta-blocker was then withdrawn for 4 weeks, after which each group of patients received 1 dose of HCTZ-A plus the alternative beta-blocker daily for 6 weeks. The GFR was measured using chromium-51-ethylenediamine-tetra-acetic acid at the end of each treatment phase. The mean GFR (+/- SE) fell from 69.6 +/- 5.8 to 60.6 +/- 5.1 ml/min (P < 0.01) during HCTZ-A therapy, whereas the addition of pindolol or nadolol caused no further drop in the GFR. We conclude that mild degrees of renal impairment in essential hypertension may be aggravated by HCTZ-A therapy when given as one tablet of Moduretic daily.

Endothelin release during ischaemia and reperfusion of isolated perfused rat hearts

The hypothesis tested was that release of endogenous endothelin plays a role in events associated with or leading to myocardial ischaemia and/or post-ischaemic reperfusion damage. Release of endogenous endothelin into the coronary perfusate of isolated perfused rat hearts during ischaemia and reperfusion was measured with a sensitive radioimmunoassay using a polyclonal antibody with 100% cross-reactivity for all three endothelin isomers. Basal endothelin release was 0.69 +/- 0.02 pg/min/g wet heart weight (n = 35) and was constant up to 180 min. During low-flow hypoxic ischaemia for 180 min (PO2 approximately 250 mmHg) and in the presence of 1% foetal calf serum, the release rate was reduced to below 10% of controls (P < 0.01) and increased four-fold on reperfusion (P = 0.05). The influence of endothelin on vascular and myocardial reperfusion damage was studied with exogenous endothelin-2. After 1 h of low-flow ischaemia, endothelin-2 increased the coronary perfusion pressure to a similar extent as in non-ischaemic hearts, but with a 30-times higher potency. The threshold dose for the constrictive effect was approximately 100 to 300 pg per heart, about ten times more than was recovered in the coronary effluent upon reperfusion. The influence of endothelin on myocardial reperfusion mechanical function (stunning) was assessed with 100 ng endothelin-2, a dose some 3500-fold higher than the amount released during 30 min reperfusion. This dose, given at the onset of reperfusion, improved post-ischaemic aortic output recovery during the first 20 min of reperfusion, but worsened it thereafter (up to 40 min). These data indicate that, in the isolated perfused rat heart model, (1) endothelin is released in measurable amounts into the coronary circulation, (2) the release is much reduced during ischaemia and increased on early reperfusion following prolonged ischaemia, (3) based on the amounts released and the post-ischaemic sensitization of the coronary vasculature to endothelin, the peptide could contribute to reperfusion vascular damage, and (4) endothelin is unlikely to influence stunning owing to the extremely high dose necessary to alter reperfusion mechanical function.

Decreased rates of release of atrial natriuretic peptide from isolated hearts from aging hypertensive rats

We tested the hypothesis that in genetically hypertensive New Zealand rats left ventricular hypertrophy predisposes to left atrial disease and should be associated with a decreased release of atrial natriuretic peptide (ANP) in response to acute left atrial stretch. Abrupt distention of the left atrium was achieved by the acute onset of left atrial distention in Langendorff-perfused isolated rat hearts, which resulted in a rapid release of ANP in all groups of hearts. Comparisons were made between normotensive and hypertensive hearts using rats aged 2 months, 8 months, 12 months, and 16 months. Rates of release of ANP from normotensive and hypertensive hearts were similar at 2 months. Thereafter, there was a progressive impairment of ANP release, until in rats aged 16 months, rates of release achieved in hypertensive hearts 2 min after the onset of left atrial distention were only about half of those of normotensive hearts (55%, P < .005). At all ages above 2 months, hypertensive hearts failed to increase the concentration of ANP in the coronary effluent at the onset of increased heart work. Therefore, in rats with genetic hypertension, there is an impairment of release of ANP from the isolated heart in response to left atrial distention.

Contrasting effects of cyclic AMP increase caused by beta-adrenergic stimulation or by adenylate cyclase activation on ventricular fibrillation threshold of isolated rat heart

Increased myocardial tissue cyclic AMP has been associated with both a positive inotropic and a proarrhythmic effect. We wished to determine whether two agents that increase myocardial cyclic AMP levels by different mechanisms would induce comparable changes in vulnerability of the heart to ventricular fibrillation (VF) and in the inotropic status. Using an isolated perfused rat heart model, we studied the effects of beta-adrenoceptor stimulation by isoproterenol (ISO) and direct activation of adenylate cyclase by forskolin. The ventricular fibrillation threshold (VFT) was taken as an index of the vulnerability to VF and peak left ventricular systolic pressure (LVSP) as a measure of the force of LV contraction. ISO resulted in a dose-related increase in tissue cyclic AMP with a corresponding decrease in VFT and a marked increase in LVSP. Forskolin produced a delayed but exponential increase in cyclic AMP at concentrations greater than 3 x 10(-7) M with relatively small increases in LVSP. With forskolin, the VFT decreased only at extremely high cyclic AMP levels, suggesting that the drug had increased cyclic AMP in a compartmentalized manner. The discrepant effects of ISO and forskolin on VFT could not be explained by changes in heart rate (HR). These results show that an increase in tissue cyclic AMP can have markedly different arrhythmogenic effects depending on the mechanism by which cyclic AMP is increased.

Bucindolol, a beta blocker, decreased ventricular fibrillation and maintained mechanical function in a pig model of acute myocardial ischemia

Bucindolol is a new beta blocker with marked vasodilatory properties and intrinsic sympathomimetic activity. We tested its potential effect against ventricular fibrillation (VF), in a pig model of acute myocardial ischemia. Bucindolol 6 mg/kg IV was administered in two equally divided doses, the first 30 minutes prior to, and the second 10 minutes after, ligation of the anterior descending coronary artery (CAL) in anesthetized open-chest pigs. Bucindolol decreased the incidence of VF to 1/11 versus 14/16 in the control group (p less than 0.005). Bucindolol also decreased the duration of ventricular tachycardia, 15 +/- 8 seconds versus 104 +/- 32 seconds in the control group (p less than 0.01). Bucindolol maintained LVmaxdP/dt at predrug and pre-CAL values, whereas LVmaxdP/dt was decreased by CAL in the control group. Bucindolol decreased arterial pressure and heart rate. Bucindolol increased blood flow in the peripheral ischemic zone (24.6 +/- 1.8% versus 16.2 +/- 1.7% (percent of pre-CAL value) in controls, p less than 0.002), as well as in the nonischemic zones (periischemic zone: 126.4 +/- 6.1% versus 96.7 +/- 4.8% in the control group, p less than 0.0005; remote nonischemic zone: 126.6 +/- 7.1% versus 87.1 +/- 4.3% of pre-CAL value in the control group, p less than 0.0001). Bucindolol had marked antiarrhythmic effects that were associated with beneficial effects on the mechanical function of the left ventricle and on blood flow to the ischemic myocardium.

Effects of oxygen free radicals on isolated cardiac myocytes from guinea-pig ventricle: electrophysiological studies

Free oxygen radicals are formed during early reperfusion and are thought to contribute to some types of reperfusion abnormalities, including arrhythmias and myocardial stunning. The purpose of this study was to investigate electrophysiological effects of oxygen free radicals using voltage clamped single ventricular myocytes from guinea-pig hearts. Oxygen free radicals were produced enzymatically by the direct addition of xanthine oxidase (XOD, 0.04 U/ml) in the experimental chamber to a solution containing hypoxanthine (0.96 mM). The generation of oxygen radicals was confirmed by the formation of adrenochrome from adrenaline. Oxygen radicals caused automaticity of isolated myocytes within 20-30 min, followed by later hypercontracture. The percentage of rod-shaped cells declined sigmoidally as a function of time, with a half maximal value at 40.9 +/- 1.6 min, and a Hill slope of -0.10 +/- 0.01 (n = 26). These effects were prevented by a combination of superoxide dismutase (10(5) U/L) plus catalase (10(6) U/L). The rate at which cells underwent morphological shape changes was unchanged by ryanodine (0.5 microM) which is thought to act on the sarcoplasmic reticulum or by the Ca2+ channel blockers nisoldipine (1 microM) or Cd2+ (30 microM). Cellular automaticity and hypercontracture were delayed by variable degrees, and sometimes completely prevented, by zero (1 mM EGTA) extracellular Ca2+, MnCl2 (2 mM) and LaCl3 (50 microM), and amiloride (1 mM). On the other hand, in the presence of a low extracellular Na+ (30 mM) or caffeine (10 mM), hypercontracture occurred at a faster time scale. Whole cell voltage clamping revealed a decrease of the inward rectifying K+ current (IK1), and a decrease of the peak of the L-type Ca2+ current (ICa,L). The total ICa,L during the clamp step was increased, mainly because of an increased time constant of inactivation (47.6 +/- 4.7 ms to 72.7 +/- 15.5 ms after 30 min, n = 4, P less than 0.05). We conclude that oxygen radicals cause automaticity and hypercontracture of isolated myocytes, that these effects may be due to an increased intracellular Ca2+ concentration ([Ca2+]i), and despite an increased ICa,L, that the enhanced Ca2+ influx may occur predominantly via the Na/Ca exchange.

Potential arrhythmogenic role of cyclic adenosine monophosphate (AMP) and cytosolic calcium overload: Implications for prophylactic effects of beta-blockers in myocardial infarction and proarrhythmic effects of phosphodiesterase inhibitors

Activation of the adrenergic nervous system appears to play a crucial role in the genesis of fatal arrhythmias associated with the very early stages of acute myocardial infarction. The second messenger of beta-adrenergic catecholamine stimulation, cyclic adenosine monophosphate (AMP), has established arrhythmogenic qualities, acting by an increase in cytosolic calcium, which potentially has three adverse electrophysiologic effects. First, stimulation of the transient inward current by excess oscillations of cytosolic calcium can invoke delayed afterdepolarizations, so that triggered automaticity can develop in otherwise quiescent ventricular muscle. Second, cyclic AMP can evoke calcium-dependent slow responses in depolarized fibers, so that conditions for reentry are favored. Third, excess cytosolic calcium can cause intercellular uncoupling with conduction slowing. Focal changes in cyclic AMP and cytosolic calcium promote the development of ventricular fibrillation. Beta-adrenergic blockade can limit the formation of cyclic AMP in ischemic tissue. Furthermore, by reducing sinus tachycardia it can lessen cytosolic calcium overload. Hence, beta-adrenergic blockade helps to prevent ventricular fibrillation in the early stages of acute myocardial infarction and protects from sudden death in the postinfarction phase. In congestive heart failure, abnormalities of cytosolic calcium patterns exist with cytosolic calcium overload. It is proposed that the adverse effects of phosphodiesterase inhibitors on the mortality rate in patients with congestive heart failure can be explained by increased rates of formation of cyclic AMP and the development of calcium-dependent arrhythmias. Because calcium is the ultimate messenger of cyclic AMP-induced arrhythmias and because cytosolic calcium is increased in heart failure, it will be difficult to develop positive inotropic agents that are free of the risk of sudden death.

Modulation of severity of reperfusion stunning in the isolated rat heart by agents altering calcium flux at onset of reperfusion

The present study tested the hypothesis that a reduction in calcium flux across the sarcolemma or the sarcoplasmic reticulum at the onset of reperfusion could attenuate subsequent mechanical "stunning" (postischemic myocardial dysfunction). The isolated working rat heart was subjected to 20 minutes of total global ischemia, reperfused in the Langendorff mode for 5 minutes, and then made to work again for 10 minutes. During the early reperfusion period (first 2 minutes), the effects of agents thought to increase cytosolic calcium (high external calcium [modified Tyrode's solution replaced Krebs-Henseleit buffer as the perfusate], isoproterenol, forskolin, and Bay K 8644) were tested. All these interventions worsened stunning. The cardiac output (CO) of control hearts recovered to 74.7 +/- 3.4%, whereas recovery was 56.3 +/- 3.7% (p less than 0.05) for high calcium (10 mM), 53.4 +/- 3.6% (p less than 0.05) for isoproterenol, 43.4 +/- 4.1% (p less than 0.05) for Bay K 8644, and 62.7 +/- 2.4% (p less than 0.002) for forskolin. Interventions aimed at limiting calcium flux during early reperfusion, such as reperfusion with a low extracellular calcium or the addition of ryanodine (3 x 10(-9) M), nisoldipine (10(-8) M), or the inorganic blockers Mn2+ (2 mM) or Mg2+ (16 mM), were also tested. Low extracellular calcium (0.75 mM) improved CO to 91.8 +/- 0.8% (p less than 0.05). Reperfusion with ryanodine and nisoldipine gave CO recoveries of 103.6 +/- 1.8% (p less than 0.002) and 99.0 +/- 2.8% (p less than 0.002), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiarrhythmic effects of the angiotensin converting enzyme inhibitor perindoprilat in a pig model of acute regional myocardial ischemia

Previous studies on the possible antiarrhythmic effects of angiotensin converting enzyme (ACE) inhibitors during early ischemia in pigs have been inconclusive or negative; however, proof of adequate ACE inhibition was not provided. Perindoprilat, 0.06 mg/kg, i.v., was administered 30 min prior to ligation of the anterior descending coronary artery (CAL) in anesthetised open-chest pigs. Plasma ACE activity was decreased by 95.0 +/- 1.9% when measured 5 min before CAL. Within 5 min of CAL, the ventricular fibrillation threshold (VFT) in the control group was decreased from 11.8 +/- 1.9 to 7.2 +/- 1.2 mA (p less than 0.01). Perindoprilat prevented the fall in the VFT and the increase in left ventricular end-diastolic pressure caused by CAL. Perindoprilat decreased arterial pressure. Cardiac output (thermodilution) was decreased by 23 +/- 3% after CAL in the control group and by only 10 +/- 5% (p less than 0.05) in the perindoprilat group (both versus pre-CAL values). In the control group cyclic AMP was increased from 0.97 +/- 0.04 (pre-CAL) to 1.16 +/- 0.04 nmol/g (p less than 0.05) in the central ischemic zone 20 min after CAL. Perindoprilat prevented this increase in cyclic AMP. Twenty minutes after CAL blood flow (microsphere method) in the nonischemic zone of the perindoprilat group was increased, whereas blood flow in the central ischemic zone was decreased compared to the control group. However, levels of tissue metabolites (ATP, phosphocreatine, lactate) measured in drill biopsies in the same zones of the two groups were similar.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic effects of nisoldipine, a highly specific calcium antagonist, in patients with acute myocardial infarction

The aim of the study was to investigate the hemodynamic effects of a short-acting, potent, highly specific calcium antagonist, nisoldipine, in patients with acute myocardial infarction. Twenty-four patients were selected on the basis of an elevated wedge pressure and/or elevated blood pressure, less than 12 hours after the onset of symptoms. Patients were randomized to receive either placebo or low-dose nisoldipine (2 micrograms/kg) as a single intravenous injection over a 3-minute period. hemodynamic effects were monitored for 20 minutes, and thereafter patients were crossed over to the other agent after the preserved parameters had returned to baseline. An open-label study using double the dose of nisoldipine in 20 patients who had not reacted adversely to low-dose nisoldipine followed. Standard hemodynamic monitoring showed that peak effects of nisoldipine were reached at 5 minutes, with some residual effect at 20 minutes, and it took up to 60 minutes to return to baseline. Both doses of nisoldipine had similar effects: a fall in the systemic vascular resistance by about 600 units, variable tachycardia, little or no change in the wedge pressure, a decrease in the arterial pressure, an unchanged rate-pressure product, and an increase in ejection fraction. Tachycardia of more than 15 beats/min resulted in 5 of 24 patients with low-dose nisoldipine and 6 of 20 patients with high-dose nisoldipine. In view of the risk of tachycardia, nisoldipine seems unsuitable for use in the acute phase of myocardial infarction.

Should calcium antagonists be used after myocardial infarction? Ischemia selectivity versus vascular selectivity

The use of calcium antagonists for postinfarct cardioprotection remains controversial. Several major trials have failed to show benefit, despite positive expectations based on promising experimental data. A clue to the problem with the calcium antagonists was provided by the diltiazem trial, in which an adverse effect in the presence of congestive heart failure masked a benefit in those without heart failure. Accordingly, the most recent trial, DAVIT-II, was carried out in patients in whom preexisting left ventricular failure had been excluded. One of the interesting byproducts of that study was the possibility that verapamil prevented postinfarct sudden death, which implies a potential antiarrhythmic mechanism. It is proposed that cytosolic calcium overload could play a role in ischemic ventricular fibrillation. Experimentally, calcium antagonists are most effective antifibrillatory agents when catecholamine stimulation is combined with acute ischemia, as would be the situation in the acute phase of myocardial infarction. This potential benefit of calcium antagonists may be offset in the presence of congestive heart failure because left ventricular dilation is directly arrhythmogenic. The ideal calcium antagonist, aimed at preventing postinfarct ischemic arrhythmias, but without a significant negative inotropic effect, could be based on 1 of 2 principles. First, the agent could be highly selective for the ischemic but not the nonischemic zone of the myocardium (ischemic-selective agent). Second, the agent could be highly vascular selective, so that left ventricular dilation would be avoided. A comparative study of these two types of calcium antagonists should be undertaken in postinfarct patients.

Choosing the correct drug for the individual hypertensive patient

With the availability of a wide selection of antihypertensive drugs acting by different mechanisms, it should be possible to match the requirement of individual patients with the pharmacological and clinical properties of an appropriate agent. Although the concept of stepped-care therapy is now largely outdated, therapy must be initiated with one agent. Diuretics remain a first-choice option in the elderly and in Black patients, as do calcium antagonists. In patients with ischaemic heart disease or enhanced adrenergic drive, beta-blockers are preferred. Calcium antagonists or ACE inhibitors are finding increasing use as initial therapy when quality of life is important and metabolic neutrality is required. The choice of antihypertensive agent may be limited by adverse effects, e.g. pedal oedema with nifedipine, constipation with verapamil, and cough with ACE inhibitors. Certain advantages are evident for both calcium antagonists and ACE inhibitors. Calcium antagonists are more likely to be effective first-line therapy than ACE inhibitors in Black patients, in those with a high salt intake, in patients with Raynaud's disease, and when angina pectoris is present. ACE inhibitors are preferred for use in combination with diuretic agents, and in the presence of congestive heart failure or low salt intake. Combination therapy between these 2 drug classes is finding increasing acceptance because of its many theoretical advantages, and may provide a means of maximising benefit.