Blocking actions of Ca2+ antagonists on the Ca2+ channels in the smooth muscle cell membrane of rabbit small intestine.

Actions of Ca2+ antagonists, verapamil, nicardipine and diltiazem, were investigated on the Ca2+ inward current in the fragmented smooth muscle cell membrane (smooth muscle ball; SMB) obtained from the longitudinal muscle layer of the rabbit ileum, by enzymatic dispersion. All Ca2+ antagonists inhibited the inward current, in a dose-dependent manner. The ID50 value on the maximum amplitude of the inward current of nicardipine was 24 nM, and this value was roughly 50 times lower than values obtained with verapamil and diltiazem, when the inward current was provoked by 0 mV command pulse from the holding potential of -60 mV. Lowering the holding potential to -80 mV shifted the dose-response curve to the right. When depolarizing pulses (100 ms, stepped up to 0 mV from -60 mV or -80 mV) were applied every 20 s, the peak amplitude of the inward current remained unchanged, but nicardipine immediately, and diltiazem and verapamil slowly reduced the peak amplitude. These slow inhibitions by the latter two drugs depended on the frequency or number of stimulations. Nicardipine but not diltiazem and verapamil shifted the voltage-dependent inactivation curve to the left (3 s duration of the conditioning pulse). However, with a longer conditioning pulse (10 s) verapamil and diltiazem shifted the voltage-dependent inactivation curves to the left. Therefore, the inhibitory actions of these Ca2+ antagonists differ. Namely, diltiazem and verapamil inhibit the Ca2+ channels, mainly in a frequency- or use-dependent manner while nicardipine does so in a voltage-dependent manner.