Evidence for contribution by increased cytoplasmic Na+ to the insulinotropic action of PACAP38 in HIT-T15 cells.
Pituitary adenylate cyclase-activating polypeptide (PACAP) is localized to pancreatic nerve terminals and stimulates insulin secretion. The insulinotropic effect of PACAP38 in insulin-producing HIT-T15 cells is accompanied by increases in cellular cAMP and cytoplasmic Ca2+ ([Ca2+]cyt). As also intracellular Na+ is important for insulin secretion after glucose and other cAMP forming peptides, we examined the Na+ dependence of the insulinotropic effect of PACAP38 in HIT-T15 cells. We found that PACAP38 (100 nM)-induced insulin secretion was diminished by approximately 50% by removal of extracellular Na+ (replaced by equimolar N-methyl-D-glucamine). In contrast, removal of Na+ did not diminish the formation of cellular cAMP (measured by radioimmunoassay) or the increase in [Ca2+]cyt (measured in FURA-2AM-loaded cell suspensions) induced by PACAP38. Furthermore, PACAP-38 increased the cytoplasmic Na+ ([Na+]cyt) in single HIT-T15 cells as measured by the fluorophore sodium-binding benzofran isophthalate. This increase was reduced by removal of extracellular Na+ and by inhibition of protein kinase A by H-89. We conclude that the insulinotropic action of PACAP38 is Na+-dependent. We propose that PACAP38 opens plasma membrane Na+ channels by an action partially mediated by cAMP and protein kinase A, and the subsequent raise in [Na+]cyt elicits insulin secretion by an as yet unsolved mechanism.
Filipsson
K
K
Department of Medicine, Lund University, S-205 02 Malmö, Sweden. karin.filipsson@medforsk.mas.lu.se
Karlsson
S
S
Ahrén
B
B
D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D003593:Cytoplasm / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000378:metabolism*; D009479:Neuropeptides / Q000494:pharmacology*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D015398:Signal Transduction / Q000187:drug effects*; D012964:Sodium / Q000378:metabolism*
D007328:Insulin; D009479:Neuropeptides; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D012964:Sodium