Fundamental mechanisms of action of calcium antagonists in myocardial ischemia

The mammalian myocardium exhibits a spectrum of damage during an ischemic episode. After relatively short periods of ischemia the damage is reversible, but with longer periods of ischemia the number of cells that are lethally injured increases. When coronary flow is restored the lethally injured cells become overloaded with Ca++ and fail to regenerate adenosine triphosphate. The calcium antagonists provide protection under these circumstances, but only if used prophylactically. When added only upon reperfusion the calcium antagonists slow, but do not inhibit, the excessive gain in Ca++ that occurs during postischemic reperfusion. Nicotine, in a concentration equivalent to that found in the plasma of smokers (0.15 microgram/ml), exacerbates the reperfusion-induced Ca++ gain. Treatment with the long-acting calcium antagonist, anipamil, on a once-daily basis attenuates the reperfusion-induced Ca++ gain in spontaneously hypertensive rats and its exacerbation by nicotine in Sprague Dawley rats. The prolonged oral administration of at least 1 calcium antagonist, verapamil (50 mg/kg body weight/day), causes a significant (p less than 0.001) depletion of left ventricular norepinephrine.     

Mechanism of antiatherogenic action of calcium antagonists

Analysis of literature reports on the actions of calcium antagonists on the progression of experimental arterial disease has revealed a significant association between efficacy and time of commencement of administration. In the majority of studies where commencement of administration of calcium antagonists preceded or coincided with initiation of atherogenesis, suppression of atherosclerotic changes has been observed. Where administration commenced after initiation of atherogenesis, suppression of atherosclerotic changes has rarely been seen. This distribution of results strongly suggests that calcium antagonists inhibit a process occurring early in atherogenesis. We have previously shown that calcium antagonists selectively inhibit the proliferation of smooth muscle cells that occurs in response to balloon catheter injury. We report here further investigations into this effect, using nifedipine. In the rat there is a 'time window' for the antiproliferative action of nifedipine, between 8 and 30 h after balloon injury. In the rabbit, delaying nifedipine administration so that it commenced 7 days after balloon catheterisation abolished its antiproliferative effect. These results show that nifedipine inhibits smooth muscle cell proliferation at an early stage, probably during the G0/G1 phase of the cell cycle, and that after this time smooth muscle cells are refractory to its antiproliferative action. We conclude that the antiatherogenic actions of calcium antagonists in experimental models of arterial disease are the result of early inhibition of smooth muscle cell proliferation.     

Effects of calcium antagonists on lipids and atherosclerosis

The arterial accumulation of cholesterol and calcium is a hallmark of atherosclerosis. Calcium antagonists (CAs) lessen the severity of experimentally induced atherosclerosis in cholesterol-fed animals. The reduction of aortic cholesterol is one of the most striking findings. This effect is achieved without a reduction of plasma lipid or blood pressure, and is probably related to an interference of CAs with lipid metabolism in the arterial wall. To what extent these properties of CAs are due to their ability to block calcium channels still remains to be addressed. This report briefly discusses the available in vivo and in vitro evidence for the antiatherosclerotic properties of CAs, and outlines the possible mechanisms by which these compounds affect cellular lipid metabolism.     

Effects of calcium antagonists on infarcting myocardium

Numerous studies have been conducted over the past 10 years on the effects of calcium antagonists on regional myocardial ischemia and infarct size. Verapamil, for example, reduced the degree of adenosine triphosphate degradation during 15 minutes of coronary occlusion followed by reperfusion in an anesthetized canine preparation. It also preserved the ultrastructural appearance of mitochondria in myocardium subjected to 1 hour of ischemia. Using an 8-hour permanent coronary artery occlusion model in which risk zone was assessed with radioactive microspheres and infarct size determined by tetrazolium staining, verapamil, administered 1 hour after occlusion, resulted in a modest decrease in infarct size. In a reperfusion model in which anesthetized dogs were subjected to 3 hours of coronary artery occlusion followed by 3 hours of reperfusion, verapamil decreased infarct size when it was administered during the period of ischemia, but failed to decrease infarct size when administered only during the reperfusion phase, i.e., after 3 hours of ischemia. Although verapamil is known to have negative inotropic effects, the newer calcium antagonist agent nisoldipine is less negatively inotropic, and might therefore be preferable in the situation of compromised hemodynamics. In a 6-hour permanent coronary artery occlusion model, nisoldipine decreased infarct size without decreasing left ventricular contractility. Most published reports support the concept that calcium antagonists decrease infarct size in models of experimental infarction. Of 4 major clinical studies, only 1 has shown that calcium antagonists are capable of reducing infarct size in man. However, in most of these studies, drug therapy commenced relatively late--4 or more hours after symptoms. In order for these drugs to demonstrate beneficial effects, the risk zone may have to be small and the degree of collateral flow adequate, the drug may have to be given very early or even before coronary occlusion (in a prophylactic fashion) and administration of the drug may have to be coupled to early coronary reperfusion.     

Nitrates and calcium antagonists for silent myocardial ischemia

Continuous Holter monitoring of patients with coronary heart disease can show transient ischemic episodes occurring spontaneously with or without angina throughout the day. A controlled double-blind trial was conducted comparing the effects of isosorbide-5-mononitrate (IS-5-MN) and nifedipine in patients with documented transient ischemic episodes. Seventy-five percent of the ischemic episodes were not accompanied by pain. Twenty patients with documented coronary heart disease were included; 15 finished the 4-week study (1 patient had headaches, 1 thyrotoxicosis, 1 hypertensive crisis and 2 unstable angina). On a dual-channel FM-recorded electrocardiogram, ischemic episodes were counted when ST deviation was greater than 1 mm for greater than 1 minute. Patients received IS-5-MN (20 mg 3 times a day or 50 mg in a sustained-release tablet) or nifedipine (20 mg in a sustained-release tablet 3 times a day) in random order over four 1-week periods. At the end of each week, Holter monitoring was repeated and showed reductions of episodes by 67% and 67% after weeks of IS-5-MN therapy and 56% and 58% after weeks of nifedipine therapy (all p less than 0.05). Painful and painless episodes were reduced to a similar extent. Individual responses showed great variability, and in all treatment periods not more than half of the patients were completely free of ischemic episodes. One of the 12 patients did not respond to either way of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Secondary prevention after myocardial infarction: Effects of beta blocking agents and calcium antagonists

Summary Therapeutic interventions in patients with myocardial infarction, whether during the first hours after coronary occlusion or several days later, aim to reduce mortality and morbidity by several mechanisms: Prevention of fatal ventricular fibrillation, limitation of infarct size, and inhibition of platelet aggregation are some examples of such mechanisms. Results from early intervention trials with beta blocking agents, particularly from ISIS-I, suggest that 1-year mortality is significantly lower in selected patients randomized to active treatment. Late intervention studies also suggest a significant reduction in coronary mortality and morbidity with beta blockade, particularly when data are pooled.Studies with the calcium channel blockers nifedipine and verapamil were unable to demonstrate any beneficial effects of these drugs on mortality or reinfarction.In this review article, attention will be directed to the most recent information about the preventive value of beta adrenergic blocking drugs and slow calcium channel inhibitors.     

Effects of calcium antagonists on beta-receptors of cultured cardiac myocytes isolated from neonatal rat ventricle

The effects of calcium antagonists (verapamil, diltiazem, and nicardipine) on beta-adrenergic receptors of cultured cardiac myocytes isolated from neonatal rat ventricle were studied with the hydrophilic ligand [3H]CGP-12177, which identifies cell surface-bound beta-receptors. The three calcium antagonists suppressed spontaneous beating of the myocytes, increased the number of beta-receptors, but did not alter the affinity (Kd). These effects were dose and time dependent. Verapamil (10(-6) M) increased the beta-receptor density by about 13% after 6 hours of incubation, and this increase in density reached a plateau of about 45% after 24 hours of incubation. beta-Receptor density increased by 15% with 5 x 10(-7) M and by 37% with 10(-6) M verapamil. The increased beta-receptors appeared to retain their normal function, as assessed by the increased spontaneous beating of the myocytes in response to applied isoproterenol. The increase in beta-receptors was abolished by colchicine but not by cycloheximide. When the calcium ion concentration of the medium was lowered to 0.1 mM, no significant change occurred in the density of beta-receptors compared with that in 1.8-mM Ca2+ medium. The results suggest that calcium antagonists increase beta-receptors by accelerating recycling by microtubules but not by decreasing the inward calcium current. Such effects of calcium antagonists may be clinically important and promise insight into the mechanism of the withdrawal phenomenon of calcium antagonists.     

Effects of phospholipase inhibitors and calcium antagonists on the changes in myocardial phospholipids induced by isoproterenol

Summary To investigate the change in myocardial phospholipids after the administration of isoproterenol and its prevention by pretreatment with phospholipase inhibitors and calcium antagonists, we determined the phospholipid species in the heart of female Wistar rats. Isoproterenol (40 mg/kg) was administered 24 h before excising the heart, and chlorpromazine, mepacrine (phospholipase inhibitors), nifedipine, verapamil (calcium antagonists) and propranolol (-adrenoceptor blocker) were injected intraperitoneally 30 min prior to isoproterenol administration. The phospholipid species were quantified using silica gel precoated thin-layer rods and the hydrogen flame ionization method. Isoproterenol induced significant increases in heart/body weight ratio, myocardial protein/heart weight ratio and lysophosphatidylcholine (LPC), and significant decreases in myocardial total phosphorus, creatine kinase (CK) activity, phosphatidylethanolamine and phosphatidylcholine (PC). The significant decrease in PC and increase in LPC indicated the degradation of myocardial phospholipids. Pretreatment with nifedipine (30 mg/kg), verapamil (50 mg/kg) or propranolol (20 mg/kg) completely prevented the occurrence of myocardial injury through the preservation of myocardial phospholipid composition, total phosphorus, CK activity and heart/body and protein/heart weight ratios. On the other hand, chlorpromazine (30 mg/kg) and mepacrine (50 mg/kg) partially prevented myocardial damage through the preservation of myocardial phospholipid composition, total phosphorus and CK activity. Results suggest that not only calcium influx but also phospholipase activation plays an important role in the development of myocardial injury induced by isoproterenol.     

钙离子拮抗剂等对成纤维细胞增殖的影响

目的研究硝苯啶（Ｎｉｆ）、脑益嗪（Ｃｉｎ）、Ａ２３１８７和氯丙嗪（ＣＰＺ）对３Ｔ６成纤维细胞增殖的影响．方法以含１０％小牛血清的ＲＰＭＩ１６４０培养基培育３Ｔ６细胞，分别加入３种浓度的Ｃｉｎ，Ｎｉｆ，Ａ２３１８７和ＣＰＺ，处理２４ｈ后用图像分析仪和流式细胞仪检测３Ｔ６细胞的ＤＮＡ，ＲＮＡ和总蛋白以及细胞周期．结果Ｎｉｆ剂量依赖性地延迟３Ｔ６细胞Ｇ２期向Ｍ期的进展；Ｃｉｎ，Ａ２３１８７和ＣＰＺ分别呈剂量依赖性地抑制３Ｔ６细胞Ｇ１期向Ｓ期的进程；Ｎｉｆ，Ａ２３１８７和ＣＰＺ促进细胞的ＲＮＡ合成，Ｃｉｎ抑制细胞ＤＮＡ及ＲＮＡ的合成；Ａ２３１８７和ＣＰＺ（１０μｇ）抑制细胞蛋白质的合成，Ｃｉｎ（３０μｇ）及ＣＰＺ（５μｇ）促进细胞蛋白质合成．结论Ｎｉｆ等通过对细胞周期进程的延迟及引起细胞不平衡生长，抑制３Ｔ６成纤维细胞的增殖．     

Effects of electrical stimulation and an intracellular calcium chelator on calcium movement in suspensions of isolated myocardial muscle cells

The procedure of Haworth RA, Hunter DR and Berkoff HA (J Mol Cell Cardiol, 1980; 12:715-23) for the isolation of myocardial muscle cells from rat hearts has been modified by the addition of a step which involves centrifugation of the cells through a Percoll gradient. This increased the proportion of rod-shaped cells from 47 +/- 2.2 to 80 +/- 1.3% (mean +/- SEM, n = 7). In the absence of electrical stimulation but in the presence of 1.3 mmol . litre-1 Ca2+ less than 2% of the cells beat spontaneously. This number was not increased by addition of the Ca2+-selective ionophore A23187. A method in which isolated myocytes suspended in a cyclindrical incubation chamber are stimulated to beat by electrical impulses is described. At 1.3 mmol . litre-1 extracellular Ca2+, electrical stimulation increased by 30% the amount of 45Ca2+ exchanged in the period 0.25 to 3 min following addition of 45Ca2+. For myocytes subjected to electrical stimulation, the amount of 45Ca2+ exchanged increased as the concentration of extracellular Ca2+ increased. At 0.5 mmol . litre-1 extracellular Ca2+ verapamil reduced the amount of 45Ca2+ exchanged by 15% while La3+ reduced the amount of 45Ca2+ exchanged by 80%. Incubation of myocytes with the acetoxymethyl ester of the intracellular Ca2+ chelating agent bis (o-aminophenoxy)-ethane-N,N,N'N'-tetraacetic acid (BAPTA) for 45 min led to an inhibition of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of calcium antagonists on sympathetic nerve activity

The effect of oral treatment with calcium antagonistic drugs on the activity of sympathetic nerves was investigated by studying the uptake and release of 3H-norepinephrine and the residual 3H-content in arteries of spontaneously hypertensive rats. After three to four weeks administration of verapamil, diltiazem or nifedipine, each at two dose levels, the uptake of 3H-norepinephrine was significantly lower than in control animals. Transmural stimulation induced release of 3H-norepinephrine had a tendency to be lower in treated than in untreated rats. Residual 3H in the vessels incubated with 3H-norepinephrine was significantly lower in all but one group of animals than in controls. In previous studies we found that calcium entry blockers in vitro can release norepinephrine from tissues with sympathetic innervation. The results of present experiments suggest that oral administration of verapamil, diltiazem and nifedipine can release norepinephrine and can lead, after a few weeks, to partial depletion of norepinephrine in the vessels.     

Effects of calcium antagonists on the hypertensive kidney

Summary This review focuses on the effects of calcium antagonists on renal function in hypertensive human subjects. Specifically assessed are the acute and chronic effects of diltiazem, verapamil, amlodipine, felodipine, isradipine, nicardipine, nifedipine, and nitrendipine on glomerular filtration rate; effective renal plasma flow/renal blood flow; renal vascular resistance; and urinary protein excretion. Among the calcium antagonists, only the dihydropyridine derivatives have been demonstrated consistently to acutely increase effective renal plasma flow/renal blood flow. The acute effects on glomerular filtration rate are variable. With respect to chronic therapy, many of the calcium antagonists have been reported to produce sustained increases in the effective renal plasma flow/renal blood flow and/or the glomerular filtration rate. Renal vascular resistance is reduced. Although calcium antagonists preserve or improve renal perfusion and glomerular filtration, long-term clinical trials are required to determine their potential therapeutic benefit to modify the natural course of hypertensive renal disease.     

Modulation of the action of calcium antagonists in arteries

This paper is a review of the experimental evidence showing that specific binding sites for dihydropyridine Ca antagonists are involved in inhibition of stimulus-dependent Ca entry into arterial cells and thereby in inhibition of the contractile response. The apparent affinity of the dihydropyridine binding site is related to the proportion of a high- and a low-affinity state which is regulated by membrane potential but could also be dependent upon other factors such as G proteins. Among Ca antagonists, a subgroup of agents exhibiting voltage dependence may be identified. Their apparent pharmacological potency is highly dependent on resting membrane potential and on the duration of the depolarizing stimulus.     

Participation of calcium ions in the molecular mechanism of cardioprotective action of exogenous phosphocreatine

Summary The purpose of this study was to find out whether Ca²⁺ is necessary for the protective effect of phosphocreatine (PCr) on ischemic myocardium. Isolated Langendorff-perfused rat hearts were used in the study. When ischemic arrest was induced in Ca²⁺-free buffer, PCr did not exert a protective effect on ischemic myocardium. PCr improved postischemic contractile recovery of hearts subjected to ischemia in perfusion media containing 0.5 and 1.2 mmol/l Ca²⁺.Phosphoarginine, a structural analogue of PCr which possesses Ca²⁺-binding property similar to that of PCr did not exert any protective effect on ischemic myocardium. The effects of PCr and Ca²⁺ on lipid order of sarcolemmal vesicles from canine heart were studied by using ESR spectroscopy. PCr made membrane phospholipids more tightly packed at mildly acidic and neutral pH, but did not at pH 8.5. Although Ca²⁺ itself did not influence the membrane structure at pH 5.5, it potentiated the effect of PCr on sarcolemmal phospholipids. Thus, the protective effect of PCr on ischemic myocardium is not attributed to its Ca²⁺ binding properly, but Ca²⁺ is a necessary component of the mechanism of protective effect of PCr on ischemic myocardium.     

Effects of CCK-receptor antagonists on CCK-stimulated pepsinogen secretion and calcium increase in isolated guinea pig gastric chief cells

The effects of CCK-receptor antagonists such as dibutyryl cyclic GMP (dbcGMP), L-364,718, and CR1409 on COOH-terminal octapeptide of CCK (CCK-8) -stimulated chief cell responses were examined using isolated guinea pig gastric chief cells. L-364,718 and CR1409 inhibited CCK-8-stimulated pepsinogen secretion over the same concentration range as they inhibited CCK-8-stimulated increases in intracellular Ca²⁺ concentration ([Ca²⁺]i), respectively. Schild analysis of the CCK dose-response curve indicates that L-364,718 and CR1409 exert their inhibitory effects on CCK-8-stimulated chief cell responses in a competitive manner. By contrast, dbcGMP inhibited not only CCK-8-but also carbachol-stimulated pepsinogen secretion. Furthermore, dbcGMP inhibited CCK-8-stimulated pepsinogen secretion more potently than the increases in [Ca²⁺]i. These results suggest that L-364,718 and CR1409 act as CCK-receptor antagonists, but dbcGMP has another inhibitory effect on pepsinogen secretion in addition to the effect as a CCK-receptor antagonist in guinea pig gastric chief cells.     

Effect of calcium antagonists on aortae isolated from normotensive and hypertensive rats: relaxation and calcium influx blockade

Verapamil and diltiazem were equally potent (ie, similar EC50s) in relaxing potassium-contracted aortas of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. The mechanical EC50s produced approximately 50% calcium influx blockade, suggesting a causal link between relaxation and calcium influx blockade. Nitrendipine was about 250 times more potent in relaxing aortic smooth muscle in SHR than in WKY rats (EC50s in -log [M] were 14.10 +/- 0.30 and 11.70 +/- 0.54, respectively). This difference was not affected by endothelial denudation, and was present when nitrendipine was used by preincubation rather than during established potassium chloride contractions. In spite of the different relaxant potency of nitrendipine in SHR and WKY rats, both strains showed similar EC50s for calcium influx blockade for this compound (9.21 +/- 0.36 in SHR and 8.75 +/- 0.26 in WKY). The dissociation between aortic smooth muscle relaxation and calcium influx blockade after nitrendipine was more pronounced in the SHR strain. This suggests that mechanisms other than or in addition to calcium influx blockade play a role in the relaxation of potassium-contracted vascular smooth muscle with dihydropyridine compounds, but not with other calcium antagonists.