REGULATION OF INSULIN-SECRETION VIA ATP-SENSITIVE K- ROLE OF PHOSPHOLIPASE-C( CHANNEL INDEPENDENT MECHANISMS )

Groups of rat or mouse islets were isolated and perifused with 20 mM g lucose plus 200 mu M diazoxide. The further addition of 30 mM K+ resul ted in a rapid and sustained biphasic insulin secretory response. The onset of secretion in response to the addition of K+ was comparable in both species, but the magnitude of the response was significantly gre ater from rat islets. After the labeling of islet phosphoinositide poo ls with 2-[H-3]inositol, the accumulation of labeled inositol phosphat es (IP) in response to 30 mM K+ addition in the simultaneous presence of 20 mM glucose plus diazoxide was assessed. The addition of 30 mM K significantly increased IP accumulation similar to 300% in rat islets , whereas only an insignificant 25-30% increase was observed in mouse islets. The protein kinase C inhibitor staurosporine (50 nM) dramatica lly reduced the sustained secretory response from rat islets in the pr esence of 30 mM K+, 20 mM glucose, and diazoxide. Its effect was minim al on mouse islets and a significant inhibitory effect on insulin secr etion was observed only during the final 5 min of the perifusion. The further addition of carbachol, an agonist that activates an isozyme of phospholipase C distinct from that activated by glucose, together wit h K+, 20 mM glucose, plus diazoxide resulted in a sustained amplificat ion of insulin secretion from mouse but not rat islets. K+ (30 mM)-ind uced insulin secretion in the presence of 3 mM glucose was similar fro m perifused rat or mouse islets, a finding that would seem to preclude the activation of voltage-regulated Ca2+ channels as the pertinent di fference. These results confirm previous observations with these speci es and document another anomaly that exists between the responses of r at islets compared with mouse islets. The inability to activate a nutr ient- and calcium-regulated phospholipase C isozyme in mouse islets to the same extent as in rat islets appears to account, at least in part , for these different insulin secretory responses under these unique c onditions.