Inhibitory effect of sulfonylureas on protein phosphatase activity in rat pancreatic islets

We have measured protein phosphatase (PP) activity in crude homogenates as well as in the total 105,000×g supernatant and precipitate fractions from normal rat pancreatic islets. On the basis of the inhibition produced by either 1 nM or 1 µM okadaic acid, both PP₁ and PP₂A activity were present in crude islet homogenates in equivalent proportions (53% and 47%, respectively); PP₁ was the main activity present in the precipitate, whereas in the supernatant it was PP₂A. Tolbutamide, glybenclamide and glyclazide significantly decreased PP activity in islet homogenates in a dose-dependent manner, with a K _i0.5 value that in the case of glybenclamide correlated with its K _d for binding site, its EC₅₀ on K_ATP channel, and its EC₅₀ on insulin release. These data indicate that PPs play a role in the control of insulin secretion and suggest a further possible target for sulfonylureas within their overall action as insulin secretagogues.