GLP-I(7–36) Amide Augments Ba²⁺ Current Through L-Type Ca²⁺ Channel of Rat Pancreatic β-Cell in a cAMP-Dependent Manner

The whole-cell patch-clamp method was used to examine the effect of glucagon-like peptide I (GLP-I)(7–36) amide on the activation process of L-type Ca²⁺ channels of rat pancreatic β-cells. After depolarization, GLP-I (1–100 nmol/l) caused action potentials in cells exposed to a glucose-free solution for 10 min. The percentage of cells producing action potential depended on the concentration of GLP-I. In some cells, GLP-I caused action potentials without the prior depolarization of the membrane. In cells exposed to the glucose-free solution for longer than 30 min, or in cells that were deprived of ATP by a means of the conventional whole-cell configuration, GLP-I (20 nmol/l) did not cause the electrical excitation. Application of GLP-I augmented the maximum Ba²⁺ current (I_Ba) through L-type Ca²⁺ channels and shifted the current voltage curve to the left. Values of changes in the maximum I_Ba depended on GLP-I concentration. Application of dibutyryl cAMP (dbcAMP, 1 mmol/1) also augmented I_Ba. In cells pretreated with Rp-cAMP, dbcAMP did not change the magnitude of I_Ba. Also in cells pretreated with Rp-cAMP, GLP-I failed to augment I_Ba. These results suggest that in pancreatic β-cells, GLP-I, by a cAMP-dependent mechanism, increases opening of L-type Ca²⁺ channels. cAMP-dependent augmentation of Ca²⁺ entry as well as cAMP production itself by GLP-I plays a crucial role in controlling insulin secretion.