Effects of divalent cations on beta-cell electrical activity.
The effects of various divalent cations, added to the external medium, upon beta-cell action potential were studied using intracellular microelectrodes. Changes of spike peak potential, as a function of external cation concentration, indicate that Sr2+ or Ba2+ may substitute for Ca2+ as a charge carrier. Complete blockage by Mn2+ of electrical activity elicited by Sr2+, Ba2+, or Ca2+ suggests that these cations penetrate the membrane though the same Ca2+ channel. The increase of maximum rate of depolarization, dV(d)/dtmax, and decrease of maximum rate of repolarization, dV(r)/dtmax, when Sr2+ is substituted for Ca2+ suggest that Sr2+ penetrates more readily the Ca2+ channel but is less effective than Ca2+ in activating K permeability. Reversal of these effects by addition of equimolar Ca2+ to Sr2+ indicates that Ca2+ has a greater affinity than Sr2+ for the receptor site. The blockage of electrical activity by Ba2+ at a depolarized membrane level suggests that Ba2+ markedly reduces all K+ permeabilities. Analysis of dV(d)/dtmax at various Ca2+ concentrations, in the presence of nonpermeant divalent cations (Co2+, Mn2+, and Mg2+), shown that these cations bind competitively at the same receptor site with differing dissociation constants, For all of these divalent cations, the order of binding would be Co2+ greater than Mn2+ greater than Ca2+ greater than Sr2+, Mg2+.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology*; D003035:Cobalt / Q000494:pharmacology; D004553:Electric Conductivity; D007515:Islets of Langerhans / Q000187:drug effects / Q000502:physiology*; D008274:Magnesium / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D051379:Mice; D013324:Strontium / Q000494:pharmacology
D002413:Cations, Divalent; D001464:Barium; D003035:Cobalt; D008345:Manganese; D008274:Magnesium; D002118:Calcium; D013324:Strontium