PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE INDUCES THE VOLTAGE-INDEPENDENT ACTIVATION OF INWARD MEMBRANE CURRENTS AND ELEVATION OF INTRACELLULAR CALCIUM IN HIT-T15 INSULINOMA CELLS

The secretion of insulin by pancreatic beta-cells is controlled by synergistic interactions of glucose and hormones of the glucagon-related peptide family, of which pituitary adenylate cyclase-activating polypeptide (PACAP) is a member. Here we show by simultaneous recording of intracellular calcium ion ([Ca2+](i)) and membrane potential that both PACAP-27 and PACAP-38 depolarize HIT-T15 cells and raise [Ca2+](i). PACAP stimulation can result in membrane depolarization by two distinct mechanisms: 1) PACAP reduces the membrane conductance and increases membrane excitability; and 2) PACAP activates a pronounced inward current that is predominantly a Na+ current, blockable by La3+, and which exhibits a reversal potential of about -28 mV. Activation of this current does not require membrane depolarization, because the response is observed when cells are held under voltage clamp at -70 mV. This current may result from the cAMP-dependent activation of nonspecific cation channels because the current is also observed in response to forskolin or membrane-permeant analogs of cAMP. We also suggest that PACAP raises [Ca2+](i) and stimulates insulin secretion by three distinct mechanisms: 1) depolarization activates Ca2+ influx through L-type voltage-dependent calcium channels, 2) mobilization of intracellular Ca2+ stores, and 3) entry of Ca2+ via voltage-independent Ca2+ channels. These effects of PACAP may play an important role in a neuro-entero-endocrine loop regulating insulin secretion from pancreatic beta-cells during the transition period from fasting to feeding.