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 Ca-45(2+) from Ca-45(2+)-preloaded rat islets in a Na+-dependent manner. This indicates that the peptide stimulates insulin secretion by Na+-dependently increasing the intrac ellular concentration of Ca2+ ([Ca2+]IC). However, whether GLP-1(7-36) amide actually affects the [Ca2+]IC in islet cells is not known. We th erefore preloaded rat islet cells with the Ca2+-fluorophor fura 2-AM a nd 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 on ly in a medium containing both Ca2+ and Na+. Also the adenylate cyclas e activator, forskolin (2.5 muM), increased the [Ca2+]IC but this acti on was evidenced also in the absence of extracellular Na+. Furthermore , the Ca2+-channel inhibitor, D-600 (50 muM), prevented the rise in [C a2+]IC after both forskolin and GLP-1(7-36)amide, whereas pertussis to xin had no effect. The results show that GLP-1(7-36)amide increases th e [Ca2+]IC in islets cells by an action that most likely is due to upt ake of extracellular Ca2+ by a Na+-dependent mechanism, whereas forsko lin increases [Ca2+]IC independently on Na+. It is hypothesized that G LP-1(7-36)amide stimulates a Na+-dependent step prior to the catalytic unit of the adenylate cyclase complex in islet cells.