Facilitation of the vasorelaxant action of calcium antagonists by basal nitric oxide in depolarized artery

Abstract

We investigated the effect of NO/cyclic GMP pathway on the action of calcium antagonists (isradipine, nisoldipine, lacidipine, verapamil, diltiazem) in rat aorta exposed to 100 mM KCl. For this purpose constitutive NO synthase was blocked by using 100 microM N omega-nitro-L-arginine (L-NNA). The steady-state contractile response evoked by 100 mM KCl was enhanced when the basal NO release had been blocked. The combined effects of basal NO release and calcium antagonists resulted in an inhibition greater than additive. Concentrations of calcium antagonists producing 50% inhibition of contraction were about 3-fold lower in the presence of the basal NO release than in its absence (P < 0.01). 45Ca2+ influx stimulated by 100 mM KCl was not affected by the basal NO release, but was inhibited by isradipine and verapamil regardless of NO blockade. Thus, the facilitation of the action of calcium antagonists by NO/cyclic GMP pathway seemed not to be accompanied by a modification of their action on L-type calcium channels. To confirm this, we measured the contractile tension and the calcium signal in fura-2 loaded rings, pretreated with either verapamil or verapamil plus 8-bromo cyclic GMP (BrcGMP), and further exposed to increasing concentrations of extracellular Ca2+ ([Ca2+]o) in 100 mM KCl solution. The increase in cytosolic Ca2- ([Ca2+]cyt) evoked by increasing ([Ca2+]o) in rings pretreated with verapamil alone was not different from rings pretreated with verapamil plus BrcGMP. In contrast, the [Ca2+]o-contraction curve was significantly shifted to the right in rings pretreated with verapamil plus BrcGMP. These results show that the NO/cyclic GMP pathway facilitates the inhibitory effect of calcium antagonists on 100 mM KCl-evoked contraction. This phenomenon is not related to a modification of calcium channel blockade, but could result from the reduction of the sensitivity of contractile machinery to Ca2+ by cyclic GMP.