Potentiation of Sulfonylurea Action by an EPAC-selective cAMP Analog in INS-1 Cells: Comparison of Tolbutamide and Gliclazide and a Potential Role for EPAC Activation of a 2-APB-sensitive Ca2+ Influx

Tolbutamide and gliclazide block the K-ATP channel K(ir)6.2/Sur1, causing membrane depolarization and stimulating insulin secretion in pancreatic beta cells. We examined the ability of the EPAC-selective cAMP analog 8-pCPT-2'-O-Me-cAMP-AM to potentiate the action of these drugs and the mechanism that might account for it. Insulin secretion stimulated by both 200 mu M tolbutamide and 20 mu M gliclazide, concentrations that had equivalent effects on membrane potential, was inhibited by thapsigargin (1 mu M) or the L-type Ca2+ channel blocker nicardipine (2 mu M) and was potentiated by 8-pCPT-2'-O-Me-cAMP-AM at concentrations >= 2 mu M in INS-1 cells. Ca2+ transients stimulated by either tolbutamide or gliclazide were inhibited by thapsigargin or nicardipine and were significantly potentiated by 8-pCPT-2'-O-Me-cAMP-AM at 5 mu M but not 1 mu M. Both tolbutamide and gliclazide stimulated phospholipase C activity; however, only gliclazide did so independently of its activity at K-ATP channels, and this activity was partially inhibited by pertussis toxin. 8-pCPT-2'-O-Me-cAMP-AM alone (5 mu M) did not stimulate insulin secretion, but did increase intracellular Ca2+ concentration significantly, and this activity was inhibited by 25 mM 2-aminoethoxydiphenylborate (2-APB) or the removal of extracellular Ca2+. 8-pCPT-2'-O-Me-cAMP-AM potentiation of insulin secretion stimulated by tolbutamide was markedly inhibited by 2-APB (25 mu M) and enhanced by the PKC inhibitor bisindolylmaleimide I (1 mu M). Our data demonstrate that the actions of both tolbutamide and gliclazide are strongly potentiated by 8-pCPT-2'-O-Me-cAMP-AM, that gliclazide can stimulate phospholipase C activity via a partially pertussis toxin-sensitive mechanism, and that 8-pCPT-2'-O-Me-cAMP-AM potentiation of tolbutamide action may involve activation of a 2-APB-sensitive Ca2+ influx.