An intestinal hormone glucagon-like-peptide-1 (GLP-1) is a prominent c
andidate for incretin. In vitro experiment showed (Fridolf and Ahren,
Mol. Cell. Endocrinol., 96 (1993) 85-90) that GLP-1 increased both ins
ulin secretion and the efflux of Ca-45(2+) in a Na+-dependent manner.
Further, GLP-1 depolarizes the pancreatic beta cells in the presence o
f high concentration of glucose. Here, we report the effect of GLP-1 o
n the membrane potential with a physiological concentration of glucose
in perforated patch clamp of primary cultured rat beta cells. 10 nM G
LP-1 depolarized the beta cell, which was completely reversed by repla
cing Na+ with the impermeant molecule N-methyl-D-glucamine (NMDG). The
Ca2+ channel blocker, Co2+ suppressed the Ca2+ spikes without hyperpo
larizing the cell. GLP-1-induced insulin secretion in perifused islets
was also suppressed by a prior replacement of Na+ with NMDG. In addit
ion, GLP-1 slightly augmented the long-lasting Ba2+ current, which was
reverted to the control level by a selective inhibitor of protein kin
ase A, H-89. These results indicate: (i) GLP-1 depolarizes the beta ce
ll by activating the membrane Na+ permeability; (ii) GLP-1 slightly mo
dulates the L-type Ca2+ channel probably through protein kinase A; and
(iii) at least in part, these mechanisms may be involved in the insul
in secretion induced by GLP-1.