Emptying of intracellular Ca2+ stores stimulates Ca2+ entry in mouse pancreatic beta-cells by both direct and indirect mechanisms

J Physiol. 1997 Sep 1;503 ( Pt 2)(Pt 2):387-98. doi: 10.1111/j.1469-7793.1997.387bh.x.

Abstract

1. In non-excitable cells, the depletion of intracellular Ca2+ stores triggers Ca2+ influx by a process called capacitative Ca2+ entry. In the present study, we have investigated how the emptying of these stores by thapsigargin (1 microM) influences Ca2+ influx in electrically excitable pancreatic beta-cells. The cytoplasmic Ca2+ concentration ([Ca2+]i) was monitored in clusters of mouse beta-cells or in whole islets loaded with fura-2. 2. The membrane was first held hyperpolarized by diazoxide, an opener of ATP-sensitive K+ (KATP) channels, in the presence of 4.8 mM K+. Alternating between Ca(2+)-free medium and medium containing 2.5 mM Ca2+ caused a minor rise in [Ca2+]i (approximately 14 nM) in clusters of beta-cells. A larger rise (approximately 65 nM), resistant to the blockade of voltage-dependent Ca2+ channels by D600, occurred when extracellular Ca2+ was readmitted after emptying intracellular Ca2+ stores with thapsigargin or acetylcholine. Thus there exists a small capacitative Ca2+ entry in beta-cells. 3. When the membrane potential was clamped at depolarized levels with 10, 20 or 45 mM K+ in the presence of diazoxide, [Ca2+]i increased to different plateau levels ranging between 100 and 900 nM. Thapsigargin consistently caused a further transient rise in [Ca2+]i, but had little (at 10 mM K+) or no effect on the plateau level. This confirms that the capacitative Ca2+ entry is small. 4. In clusters of cells whose membrane potential was not clamped with diazoxide, 15 mM glucose (in 4.8 mM K+) induced [Ca2+]i oscillations by promoting Ca2+ influx through voltage-dependent Ca2+ channels. The application of thapsigargin accelerated these oscillations and increased their amplitude, sometimes causing a sustained elevation of [Ca2+]i. Similar results were obtained from whole islets perifused with a medium containing > or = 6 mM glucose. The effect of thapsigargin was always much larger than expected from the capacitative Ca2+ entry, probably because of a potentiation of Ca2+ influx through voltage-dependent Ca2+ channels. 5. This potentiating effect of thapsigargin did not result from an acceleration of cell metabolism since the drug did not affect glucose-induced changes in NAD(P)H fluorescence. It is also unlikely to involve the inhibition of KATP channels because thapsigargin steadily elevated [Ca2+]i in cells in which [Ca2+]i oscillations persisted in the presence of a maximally effective concentration of tolbutamide. 6. In conclusion, the emptying of intracellular Ca2+ stores in beta-cells induces a small capacitative Ca2+ entry and activates a depolarizing current which potentiates glucose-induced Ca2+ influx through voltage-dependent Ca2+ channels.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetylcholine / pharmacology
  • Animals
  • Calcium / metabolism*
  • Calcium Channel Blockers / pharmacology
  • Calcium Channels / metabolism*
  • Calcium-Transporting ATPases / antagonists & inhibitors
  • Electric Stimulation
  • Enzyme Inhibitors / pharmacology
  • Female
  • Gallopamil / pharmacology
  • Glucose / pharmacology
  • Hypoglycemic Agents / pharmacology
  • Islets of Langerhans / metabolism*
  • Male
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Mice
  • Patch-Clamp Techniques
  • Pyridines / metabolism
  • Thapsigargin / pharmacology

Substances

  • Calcium Channel Blockers
  • Calcium Channels
  • Enzyme Inhibitors
  • Hypoglycemic Agents
  • Pyridines
  • Gallopamil
  • Thapsigargin
  • Calcium-Transporting ATPases
  • Glucose
  • Acetylcholine
  • Calcium