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Dynamic and kinetic differences of the vascular and myocardial effects of calcium antagonists in the rat heart.

We studied the effects of nifedipine, nimodi-pine, verapamil, D600 (gallopamil), D888 (desmethoxy-verapamil), D890 (quaternized verapamil), bepridil, and diltiazem on the coronary flow and the left ventricular pressure in the retrogradely perfused paced rat heart; in addition, we investigated the time course of onset and recovery of these effects. We found a clear difference in potency order for the vascular and cardiac effects as well as widely different kinetics of coronary flow increase and negative inotropic activity. Furthermore, positive inotropism at low doses of some calcium antagonists seemed to be related to the vascular effects of these compounds. We conclude that the rat heart contains a hydrophylic and readily accessible, vascular “dihydropyridine” site and a more hydrophobic, possibly intramembraneous or intracellular, myocardial “verapamil” site with a lower accessibility for verapamil derivatives and bepridil.

Modulation of ATP-dependent calcium extrusion and Na+/Ca2+ exchange across rat cardiac sarcolemma by calcium antagonists

The calcium antagonists verapamil, bepridil, nifedipine and nimodipine inhibited ATP-dependent 45Ca2+ uptake in purified rat ventricular sarcolemma vesicles dose dependently. This inhibition was preceded by a slight stimulation in the case of the two dihydropyridines, but not with bepridil and verapamil. In contrast, Na+/Ca2+ exchange was only inhibited by verapamil and bepridil and not affected by the dihydropyridines. The steepness of the inhibition curves was significantly different for the two processes. No stereoselectivity was found with either process for inhibition by the verapamil enantiomers. Inhibition of the exchange was not due to a decrease of the exchange velocity but to a decrease in exchange capacity. Variation of the antagonist preincubation time did not modify the inhibition of the uptake. The results indicate that two different sites, located at the inner surface of the sarcolemma are involved in the modulation of the ATP-dependent uptake and the Na+/Ca2+ exchange. However, the possibility cannot be ruled out that inhibition of the exchange process is also mediated by an extracellularly located site.

Stereoisomers of calcium antagonists discriminate between coronary vascular and myocardial sites

SummaryIn the retrogradely perfused, paced rat heart, we studied the effects of the stereoisomers of verapamil (VER), gallopamil (GAL), devapamil (DEV) and bepridil (BEP) on the coronary flow and the maximum systolic left ventricular pressure (MSLVP). In addition, the time courses of onset and recovery of these effects were measured. The verapamil analogues showed high stereoselectivity factors (sf) for MSLVP depression in favour of the (−)-enantiomers and low sfs for coronary flow increase. Bepridil showed a low sf for both parameters with the (+)-enantiomer being more potent. In a previous study we found that in the rat heart, dihydropyridine calcium antagonists clearly possess high selectivity for the vascular isochannel site as compared to the myocardial site, whereas racemic verapamil derivatives were devoid of such selectivity. In the present study the (+)-enantiomers of all verapamil congeners revealed a marked vasoselectivity. This was not found for the (−)-isomers, which surprisingly were virtually equipotent for MSLV depression and coronary flow increase, suggesting a different voltage dependence of the two isochannel verapamil sites for the enantiomers of verapamil and its congeners. Onset and offset velocities were clearly different as well. The kinetics of coronary flow increase were identical and fast for all enantiomers studied. MSLV kinetics were slower. In particular the recovery was markedly different for the enantiomers of each drug, the more potent isomer having the lower velocity. Furthermore, the differences in recovery of MSLVP depression between the verapamil type enantiomers suggest that the recovery rate may directly reflect dissociation from the myocardial isochannel verapamil site.

STEREOISOMERS OF CALCIUM-ANTAGONISTS DISTINGUISH A MYOCARDIAL AND VASCULAR MODE OF PROTECTION AGAINST CARDIAC ISCHEMIC-INJURY

Concentration-dependent effects of the enantiomers of the calcium antagonists, gallopamil, diltiazem, and bepridil have been studied in the Langendorffperfused rat heart, subjected to 30 min of global ischemia. It is shown that the time course, as well as the height of the energy deprivation-induced left ventricular diastolic contracture that develops during ischemia, can be selectively inhibited by negative inotropic concentrations of the calcium antagonist enantiomers. The time needed for recovery from the diastolic contracture during the reperfusion phase can be shortened significantly by lower, vasodilating concentrations of the drugs. In normoxically perfused hearts, stereoselectivity factors (sf) of the enantiomers of the compounds amounted to 63, 10, and 2 for the negative inotropic and 12.6, 79, and 4 for the vasodilating activities of gallopamil, cis-diltiazem, and bepridil, respectively. The sf values of negative inotropism proved to be remarkably similar to sf values of 50 and 7.9 for gallopamil and cis-diltiazem in the protection of the ischemic contracture during ischemia, whereas the sf values of coronary now increase closely paralleled the values of 7.9, 63, and 2.5 for gallopamil, cis-diltiazem, and bepridil, respectively, in protection during the reperfusion phase. The results strongly suggest that at reperfusion the vasoselective enantiomers of calcium antagonists provide protection related to improved tissue perfusion, and thereby possibly restoring the distorted ionic and energetic homeostasis, whereas the other enantiomers are more involved in a direct energy-saving activity, resulting in protection during the ischemic period.

Positive inotropic effects of calcium channel antagonists are not necessarily caused by partial calcium channel agonism

SummaryRecently it has been reported that some dihydropyridine calcium channel antagonists (nifedipine, nimodipine, nitrendipine) are able to produce positive inotropic effects in isolated perfused guinea pig hearts. We studied the effects of nifedipine in isolated perfused paced rat hearts under constant pressure and constant flow perfusion conditions. We found that nifedipine is able to produce a positive inotropic effect under constant pressure conditions but not under constant flow conditions. We conclude that nifedipine does not have partial calcium channel agonistic properties and that the positive inotropic effect seen under constant pressure conditions is a result of the vasodilating properties of the drug. Positive inotropic effects caused by vasodilatation can be explained by the “garden-hoseeffect”.

Protection by verapamil and nifedipine against ischaemia-induced loss of [3H]-(+)-PN 200-110 binding sites in the rat heart

SummaryWe have studied the effects of 60 min global ischaemia and 30 min of subsequent reperfusion on the binding of [3H]-(+)-PN 200−110 and [3H]-(−)-devapamil (desmethoxyverapamil or D888) in rat heart membranes. The hearts were perfused in the Langendorff-mode and pretreated with 1 μmol/l verapamil, 30 nmol/l and 1 μmol/l nifedipine. After 60 min of global ischaemia in the absence of drugs, we found a reduction of [3H]-(+)-PN 200-110 binding sites, without changes in the equilibrium dissociation binding constant (Kd). After the subsequent reperfusion maximum specific binding (Bmax) was further reduced, whereas the Kd remained constant. [3H]-devapamil binding sites were influenced to a lower extend and showed only a decrease in Bmax at reperfusion. Pretreatment with 1 μmol/l verapamil completely prevented the changes which were observed for [3H]-(+)-PN 200-110. Pretreatment with a low, vasodilating concentration (30 nmol/l) of nifedipine displayed selective protection against the extra reduction in Bmax which was observed during reperfusion. It is concluded that calcium antagonists show protection against the ischaemia-induced loss of dihydropyridine binding sites in relation to their negative inotropic, energy-saving activity. Furthermore, nifedipine at low, vasodilating but not negative inotropic concentrations protects against further reperfusion-induced injury, which protection may be related to an improved flow during reperfusion.

Calcium Agonists and Antagonists Modulate Ischemic Injury via Vascular and Myocardial Mechanisms

Global ischemia has far-reaching consequences for the beating heart. With cessation of the coronary flow, an immediate lack in O2 and glucose occurs, leading to a reduction of CP and ATP. Furthermore, a deterioration of intracellular Ca2+-resequestration as well as an increase of extracellular potassium may induce increased intracellular Ca2+ levels that activate Ca2+-ATPases and accelerate the reduction of ATP levels. In the rat heart, ATP depletion results after about 10 minutes in a highly strengthened interaction between actin and myosin, causing an elevation of the end diastolic pressure.’-’ We have investigated the concentration-dependent effects of the Ca antagonists nifedipine (NIF), ( + )and ( )-gallopamil (GAL) and the agonist ( )-BAY K 8644 (BAY) on the end diastolic left ventricular pressure (EDP) in the Langendorffperfused rat heart subjected to global ischemia. Without drugs the EDP increased from zero to a maximum of 6.18 2 0.12 kPa after 15.1 2 1.2 minutes of ischemia, with a concomitant cessation of contractility within one to two minutes. After 30 minutes of ischemia the reperfusion was started, resulting in a quick rise of EDP, followed by a slow decrease to control level. Recovery of EDP with 90% (tR90) takes 90 f 12 minutes. Contractility usually returned within 10 minutes. NIF, (-)and (+)-GAL at negative inotropic concentrations (PEG, values: 6.7, 7.4, and 5.6, respectively) were found to (a) delay the time at which EDP was maximal (tEDPm), (b) decrease EDPm itself, and (c) shorten the time of recovery during reperfusion (tR90, FIG. 1). On the other hand, NIF and (+ )-GAL at low, vasodilating concentrations (pECx, values 8.4 and 6.7, respectively), were selectively active in reducing the time of recovery at reperfusion, almost without affecting tEDPm and EDPm. The data suggest a close relation between ( 1) the negative inotropic (energy saving) activity of the drugs and the protection against the rise in EDP during ischemia and (2) the vasodilating activity and the reduction of tR90 during reperfusion. This was confirmed by the observation that GAL showed a high stereoselectivity factor (sf) toward tEDPm and EDPm, but not toward tR90, which parallels a high sf for the inotropic and a low sf for the vasodilating activity of the drug in normoxic hearts? BAY showed