Pituitary adenylate cyclase-activating polypeptide induces the voltage-independent activation of inward membrane currents and elevation of intracellular calcium in HIT-T15 insulinoma cells

Endocrinology. 1995 Apr;136(4):1530-6. doi: 10.1210/endo.136.4.7895663.

Abstract

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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • 8-Bromo Cyclic Adenosine Monophosphate / pharmacology
  • Action Potentials / drug effects
  • Calcium / metabolism*
  • Calcium Channels / physiology
  • Colforsin / pharmacology
  • Electric Conductivity
  • Fura-2
  • Insulin / metabolism
  • Insulin Secretion
  • Insulinoma / metabolism*
  • Lanthanum / pharmacology
  • Manganese / metabolism
  • Membrane Potentials / drug effects
  • Neuropeptides / pharmacology*
  • Pancreatic Neoplasms / metabolism*
  • Pituitary Adenylate Cyclase-Activating Polypeptide
  • Sodium Channels / physiology
  • Spectrometry, Fluorescence
  • Tumor Cells, Cultured

Substances

  • Calcium Channels
  • Insulin
  • Neuropeptides
  • Pituitary Adenylate Cyclase-Activating Polypeptide
  • Sodium Channels
  • Colforsin
  • 8-Bromo Cyclic Adenosine Monophosphate
  • Manganese
  • Lanthanum
  • Calcium
  • Fura-2