Effects of protein kinase C inhibitors on insulin secretory responses fromrodent pancreatic islets

The contribution of protein kinase C (PKC) to the regulation of insulin rel ease from perifused islets was explored using staurosporine or Go 6976 to i nhibit the enzyme. Phorbol 12-myristate 13-acetate (PMA, 500 nM) addition t o rat islets resulted in a slowly rising insulin secretory response. While minimally effective alone, the addition of 500 nM forskolin together with P MA resulted in a synergistic secretory response. The conventional protein-k inase-C isoform inhibitor Go 6976 (1 muM) completely abolished PMA-induced secretion. However, the combination of forskolin plus PMA significantly enh anced secretion from Go 6976-treated islets. Similar to previous findings m ade with staurosporine, Go 6976 (1 muM) enhanced the first phase and reduce d the second phase of 20 mM glucose-induced secretion from rat islets. Addi tional studies were conducted comparing the secretory responses of perifuse d rat or mouse islets to glucose. Dramatic species differences to the hexos e were observed. For example, 35-40 min after the onset of stimulation with 8, 10 or 20 mM glucose insulin release rates from mouse islets averaged 32 +/- 6, 84 +/- 27 or 131 +/- 17 pg/islet per minute, respectively. The resp onses from rat islets averaged 115 +/- 28, 561 +/- 112 or 800 +/- 46 pg/isl et per minute at this time point. Islet insulin stores were comparable in b oth species. The addition of 5 muM carbachol, 500 nM forskolin or 20 mM KCl to mouse islets together with 20 mM glucose resulted in a dramatic augment ation of insulin output. The responses to carbachol or forskolin, but not K Cl, were inhibited by 50 nM staurosporine. However, staurosporine (50 nM) r educed insulin secretion from rat islets stimulated with KCl plus 20 mM glu cose. Go 6976 potentiated 20 mM glucose-induced secretion from mouse islets . These studies demonstrate that 1 muM Go 6976 completely abolishes PMA-ind uced release from rat islets and has a modest inhibitory effect on 20 mM gl ucose-induced secretion. Gd 6976 (1 muM) had no inhibitory effect on 20 mM glucose-induced release from mouse islets. These studies also confirm that staurosporine inhibits both PKC- and PKA-mediated events in islets and this lack of specificity may account for its more pronounced inhibition of rele ase when compared to Go 6976. Finally, significant species differences to P KC inhibitors exist between mouse and rat islets. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.