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

In vitro assessment for vascular selectivity of a new dihydropyridine derivative, NB-818

The vascular selectivity of NB-818 (isopropyl methyl 2-carbamoyloxymethyl-6-methyl-4-(2,3-dichlorophenyl)-1,4-dihydropyridine -3, 5-dicarboxylate), a newly synthesized dihydropyridine derivative, was evaluated in in vitro experiments. NB-818 and nifedipine concentration dependently caused a relaxant effect in rabbit femoral arteries precontracted with 60 mM K+, a negative inotropic effect in guinea-pig papillary muscles, and a negative chronotropic effect in guinea-pig right atria. The onset of these inhibitory effects of NB-818 was much slower than that of nifedipine when compared at concentrations producing the same inhibition. The relaxant effect of NB-818 was about 10 times more potent than that of nifedipine, while the negative inotropic effect of NB-818 was about 100 times less potent than that of nifedipine. As a result, NB-818 showed about 300 times higher vascular selectivity than nifedipine. The two drugs exhibited a similar potency for the negative chronotropic effect. In a whole-cell configuration with voltage clamp, the blocking effect of NB-818 on L-type Ca2+ current (ICa) in guinea-pig ventricular cells appeared much more slowly than that of nifedipine and was hardly washed out. The potency of NB-818 to block ICa was markedly enhanced under depolarized conditions (i.e. at a holding potential of -30 mV) compared to that under polarized conditions (i.e. at a holding potential of -70 mV). Such a voltage-dependent blocking action on ICa was less pronounced for nifedipine. These results indicate that NB-818 is a slow-acting Ca2+ channel antagonist with much high vascular selectivity. Its vascular selectivity may be at least in part related to the marked voltage-dependent inhibition of ICa.

Vascular versus myocardial selectivity of dihydropyridine calcium antagonists as studied in vivo and in vitro

The use of in vitro models for the study of cardiovascular effects of drugs may not be representative for the in vivo therapeutic effects. However, drug effects in vivo are often difficult to assess because of counteracting reflexes and auto-regulatory rearrangements. To solve this dilemma, the present study presents a two-step method using both in vivo and in vitro techniques to investigate vascular versus myocardial selectivity of three dihydropyridine calcium antagonists: amlodipine, felodipine and nifedipine. The ratio between intravenous drug doses causing 25% reduction in mean arterial blood pressure (vascular potency) and in heart rate (cardiac chronotropic potency) was determined in anaesthetised spontaneously hypertensive rats during autonomic cardiac blockade. In isolated hearts from spontaneously hypertensive rats, the inotropic versus chronotropic potency ratio was determined between the two drug concentrations producing a 25% reduction in cardiac contractility (dP/dt max) and in heart rate, respectively. The vascular versus chronotropic selectivity in vivo was higher for felodipine (121) than for nifedipine (47) and amlodipine (15). The inotropic versus chronotropic potency ratios obtained from the in vitro studies were: felodipine (1), amlodipine (2) and nifedipine (20). The in vitro results were used to extrapolate the vascular versus cardiac chronotropic selectivity obtained in vivo to a vascular versus myocardial selectivity drug ratio, being 20 and 60 times higher for felodipine than for amlodipine and nifedipine, respectively.

Differential inhibition of neuronal calcium entry and [3H]-D-aspartate release by the quaternary derivatives of verapamil and emopamil

1. Verapamil and emopamil are structurally related phenylalkylamine calcium channel/5-HT2 receptor antagonists that differ in their anti-ischaemic properties in experimental studies. The quaternary ammonium derivatives of these compounds were prepared and tested in assays of neuronal voltage-sensitive calcium channel (VSCC) function to determine whether the compounds act at intra- or extracellular sites. 2. The compounds were tested in K(+)-evoked: (1) rat brain synaptosomal 45Ca2+ influx, (2) release of [3H]-D-aspartate from rat hippocampal brain slices and (3) increase of intracellular calcium in rat cortical neurones in primary culture. 3. Verapamil, emopamil and the emopamil quaternary derivative caused concentration-dependent and comparable (IC50 values approximately 30 microM) inhibition of synaptosomal 45Ca2+ influx and [3H]-D-aspartate release. The verapamil quaternary derivative was considerably less active in these assays (IC50 > 300 microM). 4. The evoked increase of intracellular calcium in cortical neurones was inhibited with the following rank order of potency (IC50 value, microM): emopamil (3.6) > verapamil (17) > emopamil quaternary derivative (38) > verapamil quaternary derivative (200). 5. The results suggest that verapamil and emopamil inhibit nerve terminal VSCC function (synaptosomal 45Ca2+ influx and [3H]-D-aspartate release) by acting at distinct intracellular and extracellular sites, respectively. Verapamil and emopamil may inhibit cell body VSCC function (evoked increase of intracellular calcium in neocortical neurones) by acting at both intracellular and extracellular sites. 6. The different 'sidedness' of action of emopamil and verapamil on nerve terminal VSCC function and/or the preferential inhibition of cell body VSCC function by emopamil may at least partially explain the relatively greater neuroprotective efficacy of emopamil in experimental models of ischaemia.

Effect of changes in aortic pressure and in coronary arterial pressure on left ventricular geometry and function Anrep vs. gardenhose effect

Sudden increases in aortic pressure (AoP, mm Hg) are associated with increases in left ventricular (LV) function which persist even after diastolic volume has returned to its initial value (Anrep effect). Likewise, increases in coronary arterial pressure (CAP, mm Hg) are associated with improved LV function (gardenhouse effect). In situ, increases in AoP are paralleled by increases in both CAP and coronary blood flow, i.e., oxygen supply. We investigated the individual contributions of AoP and CAP increases on function (peak systolic pressure: LVPmax, mm Hg; dP/dtmax, mm Hg/s; end-diastolic pressure: LVPed, mm Hg) and end-diastolic geometry (inner diameter: IDed, mm; wall thickness: WTed, mm; sonomicrometry). CAP-induced increases in coronary flow were prevented by admixing dextran to the perfusate. The experiments were performed on isolated, saline-perfused, working rabbit hearts. Increasing CAP from 60 to 80 mm Hg (n = 11) resulted in improved function: LVPmax 89 +/- 3 vs. 94 +/- 3, dP/dtmax 1160 +/- 50 vs. 1250 +/- 50, LVPed 17 +/- 1 vs. 16 +/- 1 (mean +/- SEM). IDed decreased from 9.96 +/- 0.25 to 9.64 +/- 0.33 and WTed increased from 6.02 +/- 0.16 to 6.15 +/- 0.17. In a second series, AoP was increased from 60 to 80 (n = 9). Both LVPmax, dP/dtmax and LVPed increased (90 +/- 4 vs. 97 +/- 3, 1170 +/- 70 vs. 1270 +/- 90 and 18 +/- 1 vs. 19 +/- 1). IDed increased from 9.76 +/- 0.39 to 9.99 +/- 0.37 and WTed decreased from 6.08 +/- 0.22 to 5.86 +/- 0.25. After additionally increasing CAP to 80, function further improved (LVPmax: 101 +/- 3, dP/dtmax: 1310 +/- 80) while LVPed decreased (18 +/- 1). This time, IDed decreased to 9.71 +/- 0.36 and WTed increased to 6.03 +/- 0.26. Increases in CAP improve LV function via the gardenhose effect and likely do not depend on simultaneous increases in coronary flow or oxygen supply. On the other hand, increases in AoP alone improve systolic function via the Frank-Starling mechanism. Increases in both pressures together amplify this effect. Increases in CAP and in AoP have opposing effects on IDed and WTed. In conclusion, the homeometric Anrep effect--at least in part--can be viewed as synergistic action of the Frank-Starling mechanism and the gardenhose effect for this experimental model.

Failure of nimodipine to prevent brain damage in a global brain ischemia model in the rat

In view of our negative results with the calcium antagonist nimodipine as a cerebroprotective agent in a cardiopulmonary resuscitation model in the rat, we examined the protective effects of nimodipine in the four-vessel (carotid and vertebral) occlusion model, a model of global brain ischemia without important cardiovascular depression. Survival and neurological status were monitored and after 72 h the hippocampus was resected and examined for histological evaluation. The animals were treated blindly and randomly with either nimodipine, its solvent or saline given subcutaneously. In two separate studies, high doses (total dose: 5 mg/kg) or low doses of nimodipine (total dose: 1.6 mg/kg) were administered. In the high dose series, the survival rates at 72 h in the nimodipine group, the saline group and the solvent group were 4% (2/44), 19% (7/37) and 20% (8/41), respectively; in the low dose series, the figures were 26% (13/50), 34% (15/44) and 39% (18/46), respectively. The differences between nimodipine, solvent and saline were not statistically significant. Likewise, no differences in neurological status or histological brain damage were found. These data suggest that nimodipine offers no cerebral protection in global brain ischemia and may even be toxic, especially when given in high doses.

Positive inotropic and negative chronotropic effects of (-)-cis-diltiazem in rat isolated atria

1. The cardiovascular effects of (-)-cis-diltiazem, an optical isomer of diltiazem, were studied in the isolated atrium and aortic strip. (-)-cis-Diltiazem (30 microM or more) increased the developed tension of the rat left atrium, while (+)-cis-diltiazem (1 microM or more) decreased it. 2. (-)-cis-Diltiazem (1 to 100 microM) decreased the rate of spontaneous beating in the right atrium as did (+)-cis-diltiazem. 3. The potency of the positive inotropic action of (-)-cis-diltiazem was almost the same as that of ouabain in the rat left atrium, but in the guinea-pig left atrium it was considerably weaker than that of ouabain. 4. In both endothelium-intact and endothelium-denuded aortic strips, (-)-cis-diltiazem relaxed the Ca(2+)-induced contraction. In the endothelium-intact rat aortic strip depolarized by 15 mM KCl, Bay K 8644, a calcium channel agonist, increased the contractile force, whereas (-)-cis-diltiazem did not. 5. These results indicate that (-)-cis-diltiazem has a positive inotropic action in isolated atria in rats and guinea-pigs, but the mode of positive inotropic action of (-)-cis-diltiazem is different from that of ouabain or Bay K 8644.

Nimodipine decreases resuscitability in a cardiopulmonary arrest model in the rat

Although calcium has been implicated in ischemia-induced brain death or dysfunction, many animal studies do not show a beneficial effect of calcium-entry blockers given after resuscitation from a cardiopulmonary arrest (CPA). This may be due to the fact that treatment was started too late; we, therefore, evaluated the effect of the calcium-entry blocker nimodipine administered at the earliest feasible postischemic moment, i.e. at the start of the resuscitation attempts. In anesthetized Wistar rats, CPA was induced by an intra-cardiac injection of KCl, and maintained for 7 min by chest restriction. At the start of the resuscitation attempts, 50 rats were blindly and randomly assigned to intravenous treatment with either nimodipine (10 micrograms/kg over 2 min, followed by 1 micrograms/kg per min for 60 min; n = 25) or saline (n = 25). In the nimodipine group, significantly less rats could be resuscitated (11/25 versus 20/25) and the survival rate at the end of the 7 days evaluation period tended to be lower (5/25 versus 11/25). In the rats surviving after 7 days, there was no difference between both groups in incidence of seizures, neurological status and histological lesions in the hippocampus. It is concluded that nimodipine, in the dose tested and given during resuscitation in this rat model, has a detrimental effect on resuscitability and no beneficial effect on the neurological outcome in the surviving animals.

The differential effect of calcium antagonists on the positive inotropic effects induced by calcium and monensin in cardiac preparations of rats and guinea-pigs

It was the aim of the present study to gain more insight into the role of extracellular calcium and of calcium from intracellular sources in the development of contractile force in the mammalian heart. In rat Langendorff hearts the effect of nifedipine, verapamil, diltiazem, bepridil and lidoflazine as well as of the intracellularly acting calcium antagonists ryanodine and TMB-8 on the increase of the left ventricular pressure induced by calcium and the sodium ionophore monensin, respectively, was studied. In rat and guinea-pig papillary muscles the influence of nifedipine, ryanodine and lidoflazine on the effect of monensin on the force of contraction was evaluated. Calcium and monensin concentration-dependently increased the left ventricular pressure in rat Langendorff hearts. The calcium-induced effect was characterized by a sharp initial rise of the left ventricular pressure which stabilized at a lower level while monensin elicited a gradual rise of the left ventricular pressure. Nifedipine, verapamil and diltiazem, applied at the EC50 and the EC80 for the reduction of the left ventricular pressure under control conditions, shifted the concentration-response curves for calcium and monensin into the right. Ryanodine, TMB-8, lidoflazine and bepridil, applied at the EC50, displaced the concentration-response curves for calcium and monensin to the right but reduced the maximal increase of the left ventricular pressure. At the EC80, these drugs almost completely abolished the positive inotropic effects elicited by calcium and monensin, respectively. In rat papillary muscles monensin did not influence the basal force of contraction. A clear positive inotropic effect was only observed in the presence of nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of verapamil on ischaemia-induced impairment of ATP-dependent calcium extrusion in rat heart sarcolemma

1. The effects of ischaemia and reperfusion were studied on adenosine 5'-triphosphate (ATP)-dependent 45Ca2+-transport in rat heart sarcolemma vesicles. 2. The effect of verapamil, 1 mumol l-1, was studied by pretreatment of the hearts during Langendorff-perfusion and in vitro by adding the drug after isolation of the vesicles. 3. Without drug pretreatment the Ca2+-uptake appeared to be strongly reduced after 30 and after 60 min of global ischaemia, whereas after 30 min of reperfusion it was restored to slightly above the control level. 4. Verapamil pretreatment during the Langendorff perfusion significantly increased Ca2+-uptake in sarcolemma vesicles both before the onset of ischaemia and after 30 min of reperfusion, whereas no beneficial effect was found on the impaired uptake activity during the ischaemic period. 5. When tested in vitro after the isolation of the sarcolemma vesicles, verapamil only inhibited the Ca2+-uptake activity with an IC50 of 112 mumol l-1, which was increased to 250 mumol l-1 after ischaemia and reperfusion. 6. The present study indicates that pretreatment with verapamil, 1 mumol l-1, of the intact rat heart activates an ATP-dependent Ca2+ extrusion process that may contribute to decrease cellular calcium levels in control and, more importantly, in a reperfusion situation. In contrast, in vitro only a less potent inhibition of the extrusion process was found, indicating that physiological regulatory mechanisms may be altered in the vesicles.

Bepridil, myocardial accumulation kinetics and dynamic effects in the isolated rabbit heart

Both myocardial uptake and disposition of bepridil in the isolated rabbit heart showed two-compartment characteristics which possibly reflects the existence of superficial and deep binding sites. Terminal accumulation and disposition half-lives were 218 and 196 min., respectively. The half-times of the initial distributory processes were about 33 min. At a drug concentration in the perfusion liquid of 0.54 micrograms ml-1 (1.27 microM) the average concentration of bepridil in the myocardium at steady state was about 489 micrograms g-1 (1161 microM) with 43% referable to the deepest, presumably intracellular compartment. Increasing bepridil concentrations from 3 to 2333 ng ml-1 (7-5542 nM) in the perfusion liquid caused a terminal decrease in coronary flowrate to 58% of the mean control flowrate. Amplitude and velocity of myocardial contraction both decreased in a biphasic way to about 28.6 and 13.6%, respectively. Apparent dynamic steady states developed within about 20 min. Inhibitory Em-values related to the first phase were 39.8 and 53.2%, and to the second phase 97.7 and 98.5%, respectively. Heart beating frequency also decreased biphasically to 53.9% and showed inhibitory Em-values of 17.2 and 47.5% related to the two phases. Myocardial oxygen consumption decreased to 55.6%. The electrocardiographic PQ- and QRS-intervals increased to 147 and 133%, respectively. The frequency-corrected QT-interval also increased significantly from 100 to 123%. Our findings demonstrate a slow and very pronounced accumulation of bepridil in the rabbit heart. Biphasic and very marked negative inotropic and chronotropic effects and a less than proportional decrease in oxygen consumption developed much faster.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Recently 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-hose-effect".