Effects of protein kinase C inhibitors on insulin secretory responses from rodent pancreatic islets.

The contribution of protein kinase C (PKC) to the regulation of insulin release from perifused islets was explored using staurosporine or Gö 6976 to inhibit the enzyme. Phorbol 12-myristate 13-acetate (PMA, 500 nM) addition to rat islets resulted in a slowly rising insulin secretory response. While minimally effective alone, the addition of 500 nM forskolin together with PMA resulted in a synergistic secretory response. The conventional protein-kinase-C isoform inhibitor Gö 6976 (1 microM) completely abolished PMA-induced secretion. However, the combination of forskolin plus PMA significantly enhanced secretion from Gö 6976-treated islets. Similar to previous findings made with staurosporine, Gö 6976 (1 microM) enhanced the first phase and reduced the second phase of 20 mM glucose-induced secretion from rat islets. Additional studies were conducted comparing the secretory responses of perifused rat or mouse islets to glucose. Dramatic species differences to the hexose 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 responses from rat islets averaged 115+/-28, 561+/-112 or 800+/-46 pg/islet per minute at this time point. Islet insulin stores were comparable in both species. The addition of 5 microM carbachol, 500 nM forskolin or 20 mM KCl to mouse islets together with 20 mM glucose resulted in a dramatic augmentation of insulin output. The responses to carbachol or forskolin, but not KCl, were inhibited by 50 nM staurosporine. However, staurosporine (50 nM) reduced insulin secretion from rat islets stimulated with KCl plus 20 mM glucose. Gö 6976 potentiated 20 mM glucose-induced secretion from mouse islets. These studies demonstrate that 1 microM Gö 6976 completely abolishes PMA-induced release from rat islets and has a modest inhibitory effect on 20 mM glucose-induced secretion. Gö 6976 (1 microM) 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 release when compared to Gö 6976. Finally, significant species differences to PKC inhibitors exist between mouse and rat islets.