Effects of dehydrouramil on protein phosphorylation and insulin secretion in rat islets of Langerhans

Dehydrouramil hydrate hydrochloride (DHU), a stable analogue of alloxan, inhibited the phosphorylation of an endogenous protein of Mr 53,000 catalysed by a Ca2+-calmodulin-dependent protein kinase in extracts of islets of Langerhans. The concentration of DHU required for 50% inhibition was 0.09 mM. DHU did not inhibit islet cyclic AMP-dependent protein kinase and caused only slight inhibition of Ca2+-phospholipid-dependent protein kinase. Inhibition of Ca2+-calmodulin-dependent protein kinase was neither prevented nor reversed by dithiothreitol. DHU did not affect the ability of calmodulin to activate cyclic AMP phosphodiesterase. In intact islets, pre-exposure to DHU impaired the insulin-secretory response to glucose and blocked the potentiatory effect on insulin secretion of forskolin, an activator of adenylate cyclase, and of tetradecanoylphorbol acetate (TPA), an activator of Ca2+-phospholipid-dependent protein kinase. The increase in islet cyclic AMP elicited by forskolin was not affected by DHU. The data are consistent with the hypothesis that protein phosphorylation catalysed by a Ca2+-calmodulin-dependent protein kinase may play a central role in the regulation of insulin secretion.