Calcium channel antagonists. Part V: Second-generation agents

Pharmacologic and therapeutic differences among calcium channel antagonists: profile of mibefradil, a new calcium antagonist

Calcium antagonists are a heterogeneous group of drugs, each with a different chemical structure and cardiovascular profile. Distinguishing factors include pharmacokinetics, mode of calcium mobilization affected, class and subclass of calcium channel inhibited, state-dependent interactions, and effect of disease on the drug's activity. A new calcium antagonist, mibefradil, has a unique chemical structure and cardiovascular profile compared with those currently available, and it appears to represent a new class of calcium antagonists.

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.

Calcium channel antagonists in the management of anginal syndromes: changing concepts in relation to the role of coronary vasospasm

Despite the increasing evidence that alterations in coronary vascular tone can and do occur in patients with anginal syndromes, only in a minority of such patients with Prinzmetal's angina is there decisive evidence that the coronary vasodilation induced by calcium channel antagonists (CCAs) plays a specific therapeutic role. CCAs may also give therapeutic benefit in a number of conditions in which coronary vasoconstriction may contribute to ischemia, such as hyperventilation, cold-induced angina, or silent ischemia not caused by an increase in heart rate. Thus, the decision of whether or not to use CCAs in angina syndromes will often have to be made on grounds other than what appears to be a minor role of vasospasm in the overall spectrum of angina. There are preliminary indications that the long-term prognosis may be different among different categories of CCAs.

Time course of action of amlodipine and felodipine in the rat is most rapid in small arteries

The time course of action of amlodipine was compared to that of felodipine in rat mesenteric resistance arteries and aorta. Both amlodipine and felodipine caused a concentration-dependent relaxation of K(+)-depolarized resistance arteries: with amlodipine 3 x 10(-8) M and felodipine 10(-9) M, complete relaxation was reached after 40 min and 10 min, respectively. Furthermore, in resistance arteries, the time course of action of both drugs was shortest in vessels with the smallest diameter. In aorta, both drugs caused a marked relaxation of K(+)-induced tone, without reaching a maximal effect within 2 h. Recovery of K(+)-induced tone after both drugs was complete in resistance arteries, but not aorta, within 2 h. In resistance arteries exposed to K+ depolarization or noradrenaline, both drugs displayed the characteristics of 1,4-dihydropyridine Ca2+ channel antagonists. The results show that amlodipine was slower to have an effect than felodipine, but that both drugs acted fastest in the smallest arteries.

Effects of calcium- and ETA-receptor antagonists on endothelin-induced vasoconstriction and levels of endothelin in the human internal mammary artery

1. The effects of the ETA receptor antagonist BQ123 and dihydropyridine calcium antagonists on the vasoconstrictor effect of endothelin-1 (ET-1) were studied in human isolated internal mammary artery (IMA). The effect of the calcium antagonist, nisoldipine, on ET-1 levels has also been examined in cultured IMA endothelial cells (IMAEC). 2. The results showed that BQ123 and the calcium antagonists nisoldipine, isradipine, nitrendipine and nifedipine fully relaxed IMA precontracted with 3 nmol/L ET-1 with the EC50 values of 7.18 +/- 0.09 (-log mol/L) for BQ123, and 7.68 +/- 0.07, 7.02 +/- 0.12, 6.96 +/- 0.08 and 6.89 +/- 0.09 for the calcium antagonists, respectively. 3. Pretreatment of IMA with 10, 30, 100 and 300 nmol/L nisoldipine significantly depressed the maximal response (Max; 88.3 +/- 5.1, 75.2 +/- 4.9, 59.3 +/- 5.6 and 56.2 +/- 4.8% of maximal noradrenaline response versus 99.1 +/- 13.2% in control, P < 0.01) of IMA to ET-1 without a significant change in the EC50 values. 4. Pretreatment of IMA with 300 nmol/L BQ123 significantly increased both the EC50 (7.97 +/- 0.09 vs 8.36 +/- 0.08 in the control, P < 0.05) and the Max (138.1 +/- 10.2% vs the control, P < 0.01) of IMA to ET-1. 5. Incubation of IMAEC with nisoldipine for 7 h resulted in a dose-dependent (10(-8)-10(-5) mol/L) reduction up to 93.1% in ET levels in the conditioned media.(ABSTRACT TRUNCATED AT 250 WORDS)