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.
Eberhardson
M
M
Department of Medical Cell Biology, Uppsala University, Sweden.
Grapengiesser
E
E
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology*; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D019789:Tetraethylammonium / Q000494:pharmacology*; D013873:Thionucleotides / Q000494:pharmacology; D013997:Time Factors; D014044:Tolbutamide / Q000494:pharmacology*
D015220:Calcium Channels; D004791:Enzyme Inhibitors; D007004:Hypoglycemic Agents; D013873:Thionucleotides; C015929:8-((4-chlorophenyl)thio)cyclic-3',5'-AMP; D019789:Tetraethylammonium; D014044:Tolbutamide; D000242:Cyclic AMP; D005947:Glucose; D002118:Calcium