cAMP-independent decrease of ATP-sensitive K+ channel activity by GLP-1 inrat pancreatic beta-cells

Using the patch-clamp method, we studied the mechanism of depolarization of rat pancreatic beta-cells induced by glucagon-like peptide 1 (7-36) amide (GLP-1). GLP-1 caused depolarization in a concentration-dependent manner (0 .2-100 nM). Exendin (9-39) amide, a GLP-1 receptor antagonist, prevented th e GLP-1-induced depolarization. GLP-I reduced tolbutamide-sensitive membran e currents evoked by voltage ramps from -90 to -50 mV, recorded in the perf orated whole-cell configuration, suggesting that GLP-1 decreased the activi ty of the ATP-sensitive K+ channel (K-ATP). This GLP-1 effect was prevented by exendin (9-39) amide. In cells treated with Rp-cAMPS, an inhibitor of t he cAMP-dependent protein kinase (PKA), GLP-1 still caused depolarization a nd reduced the whole-cell membrane current through K-ATP. Examined in the c ell-attached configuration, 20 nM GLP-1, applied out of the patch, had litt le effect on K-ATP activity. In the inside-out configuration, the open time probability and the single-channel conductance of K-ATP in the absence of ATP inside the membrane were unaffected by the presence of 20 nM GLP-1 in t he pipette. In both conditions, application of ATP to the inside of the mem brane reduced K-ATP activity. The half-maximal concentrations (k(i)) of ATP were 11.6 mu M without and 5.6 mu M with 20 nM GLP-1 in the pipette (P<0.0 5). The values of the Hill coefficient (h) were 1.03 without and 1.01 with GLP-1. We conclude that GLP-1 reduces KATP activity by elevating the sensit ivity of KATP to ATP, resulting in depolarization of pancreatic beta-cells. This GLP-1 action is independent of the cAMP signalling pathway.