Inhibition of glucose-induced insulin secretion by calcium channel blocking drugs in-vitro but not in-vivo in the rat.

The organic calcium channel blocking drugs darodipine, nicardipine, diltiazem and verapamil inhibited glucose-induced (8.3 mmol L-1) insulin secretion by rat isolated islets in a concentration-dependent manner. The IC50 values (mumol L-1) for nicardipine, diltiazem and verapamil were 0.0025, 1.94 and 3.6, respectively. There were no significant differences between the potencies of any one drug when compared at two different glucose concentrations (27.8 and 8.3 mmol L-1). Isolated islets were also responsive to the calcium channel activating drugs BAY K 8644 and CGP 28392, which enhanced glucose-induced insulin secretion and prevented the inhibitory effect of verapamil. BAY K 8644 was more potent than CGP 28392. In the anaesthetized rat, neither verapamil (1 mg kg-1 i.v. 10 min before glucose) nor nicardipine (0.2 mg kg-1 i.v.) modified the glucose or insulin response to intravenous glucose whilst producing marked cardiovascular effects. The plasma concentrations of nicardipine (3.9 X 10(-8) M at 10 min post injection) were similar to those producing effects in-vitro whereas the plasma concentrations of verapamil (5 X 10(-7) M) were lower. It thus appears that the islet B cell calcium channel is less sensitive to calcium channel blocking drugs in-vivo than in-vitro. Moreover, in-vivo, the cardiovascular system is more sensitive to these drugs than are the islet B cells, although the potencies of the calcium channel blocking drug in isolated islets are similar to those reported for cardiac muscle and vascular smooth muscle in-vitro.