Glibenclamide stimulates and glucose inhibits glucagon release induced by calcium deprivation

The effects of low extracellular calcium levels on glibenclamide- and glucose-induced A-, B-, and D-cell secretion were investigated using the perfused pancreas preparation of the rat. At normal (2.6 mM) calcium, glucagon secretion was unaffected by glibenclamide (1 microgram/ml) and transiently suppressed by glucose (6.7 and 16.7 mM). Reduction of extracellular calcium to 0.24 mM promptly and persistently enhanced glucagon secretion in the presence of 3.3 mM glucose; this effect of low calcium was, however, diminished (P less than 0.001) by 54% and 61% when the glucose concentration was increased to 6.7 and 16.7 mM, respectively. In contrast, glibenclamide enhanced the glucagon response to calcium reduction, a twofold stimulation by the drug being observed at all glucose concentrations. Reduction of calcium to 0.24 mM failed to inhibit first-phase glibenclamide or glucose-induced insulin release; second phase was, however, inhibited whether induced by glibenclamide (in the presence of 6.7 mM glucose) or by glucose 16.7 mM per se. Reduction of calcium uniformly and completely abolished glibenclamide and glucose-induced somatostatin responses. It is concluded that: (1) the glucagon response induced by calcium deprivation is inhibited by glucose but potentiated by glibenclamide, and (2) reversal of a D-cell paracrine effect could underlie the glibenclamide effect.