Unravelling mechanisms underlying ischaemic arrhythmias - the importance of models.

A new model of regional ischaemia using blood-perfused isolated rat hearts is reported in this issue. This model has potential value in pharmacology to test the actions of drugs against the arrhythmias that arise early period (0-30 min) after induction of ischaemia. Unfortunately, the severity of arrhythmias in this new model is reduced, when compared to other models, in both the early and the late period (1-4 h) of coronary artery occlusion. This commentary compares the new model with previous models, and comments on the possible mechanisms of arrhythmias induced by ischaemia.

The antihypertensive effects of the Jamaican Cho-Cho (Sechium edule).

The experiments reported in this study constitute a preliminary investigation into the possible hypotensive effect of the Jamaican Cho-Cho (Sechium edule). Experiments were conducted in a random and blind fashion on two sub species of Sechium edule. Both the pulp and the peel were examined for hypotensive activity. Water-soluble extracts were prepared from these components of the fruit and injected into anaesthetised rats. Various cardiovascular parameters were measured including heart rate, mean arterial pressure (MAP) and several ECG intervals. We report that all extracts tested produced a fall in blood pressure with little change in ECG intervals. Extract B produced the least change in heart rate with a fall in MAP of approximately 23 mmHg. Changes in heart rate with all extracts appeared to be minimal as an ED25 value could only be determined for extract A, and ED10 values could not be evaluated for extracts C and D. The mechanism(s) by which these extracts produce their hypotensive effects could not be determined in these preliminary experiments. However, it appears not to involve direct effects on cardiac tissue. This conclusion is based on the finding that it took a minimum of 10 to 15 seconds for the hypotensive action to manifest post bolus. Future experiments will be aimed at delineating the mechanism(s) involved in decreasing MAP.

Damaging effects of the calcium paradox are reduced in isolated hearts from ethanol-dependent rats: paradoxic effects of dihydropyridine drugs.

Previous experiments showed that isolated hearts from ethanol-exposed rats show a marked increase in sensitivity to anoxic myocardial damage, and we suggested that this may be due to excess calcium entry through L-type voltage-operated calcium channels (L-VOCCs). To challenge this hypothesis, we investigated the effect of ethanol treatment ex vivo on a damaging stimulus, the "calcium paradox," which is associated with removal of calcium from the perfusate. Adult male Sprague-Dawley rats were exposed to intoxicating concentrations of ethanol for 6-10 days by inhalation. Isolated hearts from these animals were perfused with Krebs-Henseleit buffer by using a modified Langendorff technique, and the calcium paradox induced by a 10-min period of perfusion with calcium-free buffer, followed by reperfusion with calcium-containing buffer. Compared with controls, hearts from ethanol-exposed rats were significantly protected against myocardial damage, as shown by a marked reduction in release of intracellular proteins (lactate dehydrogenase, creatine phosphokinase, and myoglobin) during the reperfusion phase. These indices of myocardial damage were modified by the presence of the dihydropyridine (DHP) calcium channel antagonist nitrendipine (10(-6) M) and the DHP L-VOCC activator Bay K 8644 (10(-7) M) in the perfusate during the calcium paradox. Paradoxically, both drugs appeared to increase the damaging effects of calcium-free perfusion, with this effect being generally greater in the preparations from ethanol-exposed rats. As a result, the difference between these hearts and those from control rats was reduced, although a significant degree of protection against the calcium paradox remained. The results support the hypothesis that long-term exposure to ethanol in vivo produces marked alterations in the toxic effects of changes in myocardial calcium concentration. The increased sensitivity to DHP drugs of isolated hearts from ethanol-treated rats is consistent with previous experiments showing increased DHP radioligand-binding sites in these tissues.

Increased sensitivity to damaging effects of hypoxia and anoxia of isolated hearts from rats after prolonged exposure to ethanol: apparent protection by nitrendipine.

The studies described here investigate whether pathologic states that are thought to cause myocardial damage through excess calcium entry (i.e., hypoxia and anoxia) indeed cause greater damage in hearts from ethanol-exposed animals, and whether L-type voltage-operated calcium channels (L-VOCCs) are implicated. Adult male Sprague-Dawley rats were exposed to intoxicating concentrations of ethanol vapor for 6-10 days, and their isolated hearts compared with those of control animals in a Langendorff perfusion system. Hypoxia was induced by perfusion with Krebs-Henseleit buffer, which had not previously been bubbled with oxygen; anoxia was produced by perfusion with buffer bubbled with nitrogen. On reperfusion with oxygenated buffer, evidence of myocardial damage during the hypoxic/anoxic period was obtained by the release of intracellular proteins into the perfusate. After hypoxia, release of myoglobin (MYO) was significantly greater from hearts from ethanol-exposed rats than from controls; other indices of myocardial damage also were increased by hypoxia but did not differ significantly between treatment groups. After anoxic perfusion, release of lactate dehydrogenase (LDH) and creatine phosphokinase (CPK) as well as MYO were all markedly and significantly increased from ethanol-exposed hearts compared with those from control rats. The role of L-VOCCs in this damage was assessed with the calcium channel antagonist nitrendipine (10(-6) M) present in the perfusing buffer immediately before and during the anoxic stimulus. This completely reversed the situation so that preparations from ethanol-exposed rats now showed a reduced release of intracellular proteins compared with hearts from controls. Comparisons with absolute values from the previous experiments suggest that nitrendipine increased release of LDH and CPK from control hearts with little effect on these indices from ethanol-exposed hearts. However, in the case of anoxia-induced MYO release, nitrendipine markedly and significantly reduced this in hearts from ethanol-treated rats but had only a very small effect on the same parameter in controls. The results strongly suggest increased pathologic effects of hypoxia/anoxia in hearts from ethanol-exposed rats. This increased sensitivity may be at least partly a consequence of increased numbers of L-VOCCs in this tissue.

Effect of calcium, Bay K 8644, and reduced perfusion on basic indices of myocardial function in isolated hearts from rats after prolonged exposure to ethanol.

We previously reported findings consistent with a marked upregulation in functional L-type voltage-operated calcium channels (L-VOCCs) in hearts obtained from rats exposed over the long term to ethanol. These experiments were undertaken to establish whether detrimental effects on cardiac function were associated with excess calcium entry into the myocardium in these hearts. Isolated hearts from adult male Sprague-Dawley rats received intoxicating concentrations of ethanol for 6-10 days by inhalation, were perfused with Krebs-Henseleit buffer by a modified Langendorff technique, and several functional parameters were assessed continuously. In some experiments, the calcium concentration in the perfusate was first reduced from the physiologic range (1.2 mM) to 0.15 mM and then increased in a step-wise fashion to 4 mM. In other experiments, hearts were exposed to buffer containing concentrations of the L-VOCC activator, (+/-)Bay K 8644, increasing from 10(-9) to 10(-6) M. These perfusion protocols were repeated in hearts from treated animals subject to reduced coronary flow because of induction of partial left ventricular ischemia. There were some close similarities in the effects of these different stimuli. When the calcium concentration in the perfusate exceeded a physiologic level, there were signs of decreased function relative to controls in the hearts from ethanol-exposed rats. Thus R-wave amplitude and systolic pressure were lower, diastolic pressure also was reduced, but heart rate was elevated above that of controls. Similarly the presence of (+/-)Bay K 8644 in the perfusate caused a decrease in systolic and diastolic pressure and an increase in heart rate in hearts from ethanol-exposed rats. When cardiac perfusion was reduced in vitro by inflation of a balloon in the left ventricle, some of the effects of excess calcium and (+/-)Bay K 8644 were reproduced in control hearts. However, imposition of this "ischemic" stress did not appear to exacerbate the effects of prior exposure to ethanol. In general, in control hearts, indices of contractility were increased across the range of calcium concentration or by perfusing with (+/-)Bay K 8644. Hearts from ethanol-exposed rats, however, showed no further increase in these parameters once physiologic levels of calcium were exceeded, or showed inhibition of contractility in the presence of (+/-)Bay K 8644. The results are consistent with calcium entry through L-VOCCs in hearts from ethanol-exposed animals having detrimental effects on cardiac function once physiologic levels are exceeded. However, it is possible that these channels also may be involved in maintenance of cardiac function at hypocalcemic levels.

Effects of prolonged exposure to ethanol in vivo on functional parameters and sensitivity to nitrendipine in the isolated rat heart.

This study investigated the possibility that previously reported marked upregulation of binding sites for the dihydropyridine calcium channel antagonist nitrendipine in heart tissue during the development of ethanol dependence in the rat may represent functional L-type voltage-operated calcium channels (L-VOCCs). Isolated hearts obtained from adult Sprague-Dawley rats, which received intoxicating concentrations of ethanol for 6-10 days, by inhalation, were perfused with Krebs-Henseleit solution in the Langendorff mode. Basic measurements of cardiac function were compared with hearts from control rats not exposed to ethanol vapor. In another study, concentration-response curves were constructed for nitrendipine at concentrations in the range of 10(-10)-10(-6) M for hearts obtained from control and ethanol-exposed animals. Changes in measured cardiac parameters such as R-wave amplitude, heart rate, diastolic and systolic pressure, and (+)dP/dt(max) and coronary flow were recorded. All comparisons were made between preparations set to a similar left ventricular end-diastolic pressure. Under these conditions, there were no significant differences in R-wave amplitude, but isolated hearts from ethanol-dependent rats showed significantly greater indices of myocardial contraction than did controls. These included increased systolic and developed ventricular pressure and increased (+)dP/dt(max). Coronary flow also was significantly greater in hearts from ethanol-dependent rats compared with controls. Heart rate was higher in the alcohol-exposed group, but this difference did not achieve significance. When nitrendipine was added to the perfusate at concentrations between 10(-10) and 10(-6) M, hearts from ethanol-dependent animals showed a greater sensitivity to the effects of the drug on heart rate and systolic pressure. Effects on R-wave amplitude and (+)dP/dt(max) were less clear but also suggested a greater sensitivity to nitrendipine in hearts from ethanol-exposed rats. Effects on coronary flow were small and did not differ significantly between preparations from control and ethanol-dependent rats. The results suggest that the increase in Bmax of DHP binding previously observed in hearts from ethanol-dependent animals might represent an increase in L-VOCCs, which alters physiologic function, and pharmacologic responses in the isolated heart. These changes may represent the exposure of an adaptive mechanism designed to overcome the generally depressant effects of ethanol on cardiac function in vivo.

Time course and genetic variation in the regulation of calcium channel antagonist binding sites in rodent tissues during the induction of ethanol physical dependence and withdrawal.

Physical dependence was induced by ethanol inhalation in male Sprague-Dawley rats and, in parallel experiments, in two lines of mice (WSR and WSP) genetically selected for differential severity of ethanol withdrawal. In dependent rats [3H]nitrendipine binding sites were significantly increased in cerebral cortex, cardiac and smooth muscle (vas deferens). Cerebral cortical membranes were the first to show an increase, the Bmax for nitrendipine binding rising sharply after 3-4 days of ethanol administration, whereas binding sites in the other tissues increased after 5-6 days. Nitrendipine binding affinity in all tissues was consistently reduced immediately preceding the rise in Bmax to a new steady state, but then returned to control values. Between 6 and 10 days of ethanol exposure there was no further increase in the Bmax for nitrendipine binding, and on removal of ethanol, the numbers of nitrendipine binding sites fell precipitously to control levels within 24 h of withdrawal. In the genetically selected mice, the up-regulation of nitrendipine binding sites in cardiac membranes was significantly greater in the WSP line. This correlates with severity of physical signs of withdrawal and parallels previous results obtained in brain. The results are consistent with an increase in the synthesis and membrane insertion of dihydropyridine sensitive calcium channel proteins in several tissues during the induction of ethanol dependence and suggest that in the brain this change may play a role in the ethanol withdrawal syndrome.

Effects of anipamil, a long acting analog of verapamil, in pigs subjected to myocardial ischemia.

In this study we examined the cardiovascular and possible antiarrhythmic actions of anipamil, a long acting analog of verapamil. Initial dose-response studies for anipamil (0.25-6.0 mg/kg, i.v.) in pentobarbitone-anesthetized pigs (n = 4) were conducted to determine the effects of the drug on EKG and hemodynamic measures. In this initial study anipamil was found to produce a dose-dependent reduction in blood pressure, left-ventricular pressure and its derivative (dP/dtmax), cardiac output, and increase in heart rate. These results were used as a basis from which to choose doses for a second study to assess antiarrhythmic actions of anipamil against arrhythmias induced by regional myocardial ischemia. The antiarrhythmic effects of the two doses were compared with verapamil when the latter was given at a dose producing cardiovascular effects mid-way between those produced by the two doses of anipamil. Anesthetized pigs were randomly assigned to receive one of three drug treatments, or vehicle control, prior to occlusion of the left-anterior descending coronary artery. Antiarrhythmic effectiveness of low (1.0 mg/kg + 0.10 mg/kg/min infusion, n = 8) and high (5.0 mg/kg + 0.50 mg/kg/min infusion, n = 12) dose anipamil was compared to that of verapamil (0.5 mg/kg + 0.60 mg/kg/min infusion, n = 8) in a vehicle controlled study (n = 15). Arrhythmic events (VPB, VT and VF incidence) were monitored and grouped according to their time of occurrence after occlusion. Thus phase 1a arrhythmias occurred 0-5 min after initiation of occlusion, phase 1b, 5-30 min, and phase 2, 0.5-4 hr after occlusion. This study showed that during phase 1a there was a low incidence of arrhythmias in all groups except the one receiving 5 mg/kg anipamil where the group incidence of VT was 58% as compared to 20% in controls (n = 15). Most ventricular arrhythmias occurred in all groups during phase 1b. In this phase verapamil abolished VF and reduced VT, as compared with controls. Anipamil (high and low doses) tended to reduce VT but not VF. In the period 0.5 to 4 hours post occlusion (phase 2) all three drug treatments were associated with fewer arrhythmias but this only reached statistical significance with verapamil. Thus verapamil was more efficacious than anipamil at providing antiarrhythmic protection against both early and late onset arrhythmias. Anipamil may have been proarrhythmic in the early phase of arrhythmias and only moderately antiarrhythmic, if at all, in the later phase.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding characteristics of the calcium channel antagonist [3H]-nitrendipine in tissues from ethanol-dependent rats.

Male Sprague-Dawley rats were made physically dependent on ethanol by exposure to ethanol vapour in inhalation chambers for 6-10 days. Animals were sacrificed immediately after removal from the chambers. After sacrifice, membrane preparations from cerebral cortex, heart, vas deferens and thigh skeletal muscle were prepared and studied to determine the characteristics of [3H]-nitrendipine binding. In membranes from cerebral cortex, the number of binding sites (Bmax) was increased 40% in preparations from ethanol-dependent rats compared to controls. Similar results were obtained in heart (Bmax increased 125%), vas deferens (Bmax increased 50%) and skeletal muscle (Bmax increased 33%). Binding affinity (Kd) in all these tissues was unchanged. Membranes from cortex and heart were studied more extensively. Displacement studies using other dihydropyridines and the effects of ionic composition on [3H]-nitrendipine binding provided no evidence to suppose that the binding sites induced by the development of ethanol dependence were from a population different from those in control tissues. The results suggest a generalised 'up-regulation' of the dihydropyridine-sensitive subclass of voltage operated calcium channels in excitable tissues from ethanol-dependent rats.