Effects of glucagon like peptide-1(7-36) amide on the cytoplasmic Ca(2+)-concentration in rat islet cells.

Glucagon like peptide-1(7-36) amide (GLP-1(7-36)amide) stimulates both insulin secretion and the efflux of 45Ca2+ from 45Ca(2+)-preloaded rat islets in a Na(+)-dependent manner. This indicates that the peptide stimulates insulin secretion by Na(+)-dependently increasing the intracellular concentration of Ca2+ ([Ca2+]IC). However, whether GLP-1(7-36)amide actually affects the [Ca2+]IC in islet cells is not known. We therefore preloaded rat islet cells with the Ca(2+)-fluorophor fura 2-AM and examined the [Ca2+]IC in spectrophotofluorometry. We found that GLP-1(7-36)amide increased [Ca2+]IC both at 3.3 and 8.3 mM glucose but only in a medium containing both Ca2+ and Na+. Also the adenylate cyclase activator, forskolin (2.5 microM), increased the [Ca2+]IC but this action was evidenced also in the absence of extracellular Na+. Furthermore, the Ca(2+)-channel inhibitor, D-600 (50 microM), prevented the rise in [Ca2+]IC after both forskolin and GLP-1(7-36)amide, whereas pertussis toxin had no effect. The results show that GLP-1(7-36)amide increases the [Ca2+]IC in islets cells by an action that most likely is due to uptake of extracellular Ca2+ by a Na(+)-dependent mechanism, whereas forskolin increases [Ca2+]IC independently on Na+. It is hypothesized that GLP-1(7-36)amide stimulates a Na(+)-dependent step prior to the catalytic unit of the adenylate cyclase complex in islet cells.