Iterative stimulation of pancreatic islets by glipizide

Iterative administrations of glipizide (2 mumol/l) for 10 min each to perifused rat pancreatic islets provoked at low glucose concentration (2.8 mmol/l) rapid and rapidly reversible decreases in 86Rb outflow, rapid and rapidly reversible increases in 45Ca outflow and modest stimulations of insulin release. In the presence of Ca2+, the reascension in 86Rb outflow was preceded by a transient fall in effluent radioactivity upon withdrawal of the sulfonylurea. At a higher concentration of D-glucose (8.3 mmol/l), glipizide provoked, each time, biphasic and rapidly reversible increases in both 86Rb and 45Ca outflow, as well as in insulin release. These results are compatible with the view that glipizide decreases K+ conductance, leading to depolarization of the B-cell plasma membrane and gating of voltage-sensitive Ca2+ channels. In the presence of 8.3 mmol/l D-glucose, however, the inhibitory effect of glipizide on effluent K+ permeability may be masked by activation of Ca(2+)-sensitive K+ channels. The present data also indicate that glipizide is able to evoke, at the occasion of iterative administrations, biphasic ionic and secretory responses, without evidence of tachyphylaxis.