Relative importance of extracellular and intracellular calcium for the two phases of glucose-stimulated insulin release: studies with theophylline.

Abstract

This study investigated, with perifused rat islets, whether theophylline (2 mM) is able to restore a biphasic release of immunoreactive insulin (IRI) upon glucose stimulation, when calcium entry into β cells is prevented by omission of extracellular Ca (<20 μM) or addition of an antagonist of Ca influx (D 600). Ca omission at the time of glucose stimulation (300 mg/100 ml) reduced IRI release to a small first phase, further diminished by ethyleneglycol-bis-(β-aminoethyl ether) N,N′-tetraacetic acid (EGTA). Ca withdrawal 5 min before glucose increase completely prevented secretion. Theophylline addition augmented the reduced rapid phase of release, but failed to restore it when it was completely suppressed; in contrast, the methylxanthine permitted reappearance of a late phase of IRI release except when EGTA was present. Theophylline introduction during glucose stimulation in the absence of Ca was followed by a rapid, although partial, restoration of IRI release. This effect was inhibited by EGTA and reduc...