Thiopental-induced insulin secretion via activation of IP3-sensitive calcium stores in rat pancreatic β-cells

Abstract

While glucose-stimulated insulin secretion depends on Ca²⁺ influx through voltage-gated Ca²⁺ channels in the cell membrane of the pancreatic β-cell, there is also ample evidence for an important role of intracellular Ca²⁺ stores in insulin secretion, particularly in relation to drug stimuli. We report here that thiopental, a common anesthetic agent, triggers insulin secretion from the intact pancreas and primary cultured rat pancreatic β-cells. We investigated the underlying mechanisms by measurements of whole cell K⁺ and Ca²⁺ currents, membrane potential, cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i), and membrane capacitance. Thiopental-induced insulin secretion was first detected by enzyme-linked immunoassay, then further assessed by membrane capacitance measurement, which revealed kinetics distinct from glucose-induced insulin secretion. The thiopental-induced secretion was independent of cell membrane depolarization and closure of ATP-sensitive potassium (K_ATP) channels. However, accompanied by the insulin secretion stimulated by thiopental, we recorded a significant intracellular [Ca²⁺] increase that was not from Ca²⁺ influx across the cell membrane, but from intracellular Ca²⁺ stores. The thiopental-induced [Ca²⁺]_i rise in β-cells was sensitive to thapsigargin, a blocker of the endoplasmic reticulum Ca²⁺ pump, as well as to heparin (0.1 mg/ml) and 2-aminoethoxydiphenyl borate (2-APB; 100 μM), drugs that inhibit inositol 1,4,5-trisphosphate (IP₃) binding to the IP₃ receptor, and to U-73122, a phospholipase C inhibitor, but insensitive to ryanodine. Thapsigargin also diminished thiopental-induced insulin secretion. Thus, we conclude that thiopental-induced insulin secretion is mediated by activation of the intracellular IP₃-sensitive Ca²⁺ store.