Radioligand and functional estimates of the interaction of the 1,4-dihydropyridines, isradipine and lacidipine, with calcium channels in smooth muscle

1. The present experiments were undertaken in order to characterize further the apparently irreversible inhibition of the contraction of depolarized rat aorta caused by lacidipine, a 1,4-dihydropyridine calcium antagonist. 2. We studied the effect of lacidipine on contraction evoked by 100 mM KCl solution in rat aorta, treated by N omega-nitro-L-arginine (0.1 mM), an inhibitor of nitric oxide (NO) synthesis. We compared the effect of prolonged depolarization on lacidipine and (+)-isradipine inhibition and the reversal of this inhibition after washout in the absence of dihydropyridines. Assuming that the onset of lacidipine-evoked inhibition was a pseudo-first order association kinetics, we estimated the dissociation rate constant (k-1 = 0.031 min-1), the association rate constant (k1 = 2.70 x 10(8) M-1 min-1) and the dissociation constant (KD = k-1/k1 = 115 pM) which was close to the IC50 value in steady-state conditions (160 pM). 3. The inhibitory effects of lacidipine and (+)-isradipine on rat aorta contraction were reversibly enhanced after preincubation with the drug in a 40 mM KCl-solution. Washout with drug-free 40 mM K(+)-depolarizing solution reversed inhibition in the (+)-isradipine-treated preparations, but not in the lacidipine-treated ones. 4. Radioligand binding studies were performed with [3H]-lacidipine and [3H]-isradipine in microsomes from rat aorta and rat ileum.(ABSTRACT TRUNCATED AT 250 WORDS)