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, blockade 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.
Leech
C A
CA
Laboratory of Molecular Endocrinology, Howard Hughes Medical Institute, Harvard Medical School, Massachusetts General Hospital, Boston 02114.
Holz
G G
GG
Habener
J F
JF
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D005576:Colforsin / Q000494:pharmacology; D004553:Electric Conductivity; D016257:Fura-2; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007340:Insulinoma / Q000378:metabolism*; D007811:Lanthanum / Q000494:pharmacology; D008345:Manganese / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D009479:Neuropeptides / Q000494:pharmacology*; D010190:Pancreatic Neoplasms / Q000378:metabolism*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D015222:Sodium Channels / Q000502:physiology; D013050:Spectrometry, Fluorescence; D014407:Tumor Cells, Cultured
D015220:Calcium Channels; D007328:Insulin; D009479:Neuropeptides; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D015222:Sodium Channels; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D008345:Manganese; D007811:Lanthanum; D002118:Calcium; D016257:Fura-2