Glucose and tolbutamide trigger transients of Ca2+ in single pancreatic beta-cells exposed to tetraethylammonium.

Isolated pancreatic beta-cells respond to glucose stimulation with increase of the cytoplasmic Ca2+ concentration ([Ca2+]i) in terms of membrane-derived slow oscillations (0.2-0.5/min) with superimposed transient of intracellular origin. To evaluate under which conditions transients may result also from entry of extracellular Ca2+, the cytoplasmic concentration of the ion was measured with dual wavelength fluorometry and fura-2 in individual mouse beta-cells exposed to the K+ channel blocker tetraethylammonium (TEA). In the presence of 20 mM TEA, the beta-cells responded to closure of the KATP channels (increase of the glucose concentration to 11 mM or addition of 1 mM tolbutamide) with pronounced transients of [Ca2+]i. However, there were no transients when the beta-cells were depolarized by raising extracellular K+ to 30 mM in the presence of 20 mM TEA. The glucose-induced [Ca2+]i transients became more pronounced after thapsigargin inhibition of the endoplasmic reticulum Ca(2+)-ATPase. The tolbutamide-induced transients were amplified when promoting the entry of Ca2+ (rise of extracellular Ca2+ to 10 mM or addition of BAY K 8644), unaffected in the presence of thapsigargin and the Na+ channel blocker tetrodotoxin and slightly reduced by glucagon. Blockage of voltage-dependent Ca2+ channels with methoxyverapamil resulted in a prompt disappearance of the transients induced by glucose or tolbutamide. The observations indicate that closure of the KATP channels can precipitate pronounced transients of [Ca2+]i when other K+ conductances are suppressed.