Glucose triggers protein kinase A-dependent insulin secretion in mouse pancreatic islets through activation of the K+ATP channel-dependent pathway

Abstract

Objective: To assess the significance of protein kinase A (PKA) in glucose triggering of ATP-sensitive K+ (K+ATP) channel-dependent insulin secretion and in glucose amplification of K+ATP channel-independent insulin secretion. Methods: Insulin release from cultured perifused mouse pancreatic islets was determined by radioimmunoassay. Results: In islets cultured at 5.5 mmol/l glucose, and then perifused in physiological Krebs–Ringer medium, the PKA inhibitors, H89 (10 μmol/l) and PKI 6–22 amide (30 μmol/l) did not inhibit glucose (16.7 mmol/l)-induced insulin secretion, but inhibited stimulation by the adenylyl cyclase activator, forskolin (10 μmol/l). In the presence of 60 mmol/l K+ and 250 μmol/l diazoxide, which stimulates maximum Ca2+ influx independently of K+ATP channels, H89 (10 μmol/l) inhibited Ca2+-evoked insulin secretion, but failed to prevent glucose amplification of K+ATP channel-independent insulin secretion. In the presence of 1 mmol/l ouabain and 250 μmol/l diazoxide, which cause modest Ca2+ influx, glucose amplification of K+ATP channel-independent insulin secretion was observed without concomitant Ca2+ stimulation of PKA activity. In islets cultured at 16.7 mmol/l glucose, glucose (16.7 mmol/l)-induced insulin secretion in physiological Krebs–Ringer medium was augmented and now inhibited by H89 (10 μmol/l), implicating that culture at 16.7 mmol/l glucose may increase Ca2+-sensitive adenylyl cyclase activity and hence PKA activity. In accordance, Ca2+-evoked insulin secretion at 60 mmol/l K+ and 250 μmol/l diazoxide was improved, whereas glucose amplification of K+ATP channel-independent insulin secretion was unaffected. Conclusions: Glucose may activate PKA through triggering of the K+ATP channel-dependent pathway. Glucose amplification of K+ATP channel-independent insulin secretion, on the other hand, occurs by PKA-independent mechanisms.