EXPERIMENTAL STUDY Glucose triggers protein kinase A-dependent insulin secretion in mouse pancreatic islets through activation of the KATP channel-dependent pathway

Objective: To assess the significance of protein kinase A (PKA) in glucose triggering of ATP-sensitive Kþ (KATP) channel-dependent insulin secretion and in glucose amplification of K þ 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 (10mmol/l) and PKI 6–22 amide (30mmol/l) did not inhibit glucose (16.7 mmol/l)-induced insulin secretion, but inhibited stimulation by the adenylyl cyclase activator, forskolin (10mmol/l). In the presence of 60 mmol/l Kþ and 250mmol/l diazoxide, which stimulates maximum Ca2þ influx independently of KATP channels, H89 (10mmol/l) inhibited Ca2þ-evoked insulin secretion, but failed to prevent glucose amplification of KATP channel-independent insulin secretion. In the presence of 1 mmol/l ouabain and 250mmol/l diazoxide, which cause modest Ca2þ influx, glucose amplification of KATP channel-independent insulin secretion was observed without concomitant Ca2þ 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 (10mmol/l), implicating that culture at 16.7 mmol/l glucose may increase Ca2þ-sensitive adenylyl cyclase activity and hence PKA activity. In accordance, Ca2þ-evoked insulin secretion at 60 mmol/l Kþ and 250mmol/l diazoxide was improved, whereas glucose amplification of KATP channel-independent insulin secretion was unaffected. Conclusions: Glucose may activate PKA through triggering of the KATP channel-dependent pathway. Glucose amplification of KATP channel-independent insulin secretion, on the other hand, occurs by PKA-independent mechanisms. European Journal of Endocrinology 152 671–677