STAUROSPORINE INHIBITS PROTEIN-KINASES ACTIVATED BY CA2-AMP IN ADDITION TO INHIBITING PROTEIN-KINASE-C IN RAT ISLETS OF LANGERHANS( AND CYCLIC)

Staurosporine has been used in several studies to investigate the role of protein kinase C (PKC) in secretory responses of islets of Langerh ans to insulin secretagogues. We have assessed the effect of staurospo rine on: [i] islet PKC activity in vitro; [ii] the stimulation of insu lin secretion by nutrient secretagogues and [iii] the stimulation of p rotein phosphorylation and insulin secretion in electrically permeabil ised islets. All experiments were carried out on rat isolated islets o f Langerhans, either intact or permeabilised by high voltage discharge (3.4 kV/cm). The activity of PKC partially purified from rat islets w as inhibited by staurosporine (1.6-400 nM) in a concentration-dependen t manner. Staurosporine also inhibited insulin secretion stimulated by both glucose and glyceraldehyde, with maximal effects at 50 nM. After prolonged exposure of islets to the tumour-promoting phorbol ester, 4 beta phorbol myristate acetate (4betaPMA), a procedure which depletes islet PKC activity, staurosporine still inhibited both glucose- and gl yceraldehyde-stimulated insulin release. In electrically permeabilised islets, staurosporine inhibited both Ca2+- and cyclic AMP-stimulated protein phosphorylation and insulin secretion. These results suggest t hat staurosporine should not be used as a selective inhibitor of PKC i n rat islets.