Induction of Na+ channel voltage sensitivity in Xenopus oocytes depends on Ca2+ mobilization.
An unusual inward current which is slowly elicited in the Xenopus oocyte membrane during sustained depolarization is reportedly carried by Na+. It is thought that Na+ selective channels are in some way induced to become voltage-sensitive by the depolarization. Earlier studies report that the induction process involves a phospholipase C and a protein kinase C as well as calcium ions. The present work investigated the origins of this calcium in the oocyte. We show that injection of the powerful Ca2+ chelator (BAPTA) in the oocyte, before induction of the Na+ channels, prevented the appearance of the Na+ current, confirming an important role for [Ca2+]i. However, in oocytes perfused with Ca2+ -free medium, induction of the channels could still be obtained, indicating that induction did not depend upon the entry of external Ca2+. Downmodulation of Ca2+ release from inositol 1,4,5-trisphosphate (InsP3)-sensitive stores with caffeine and with a low molecular weight heparin resulted in decreased or no Na+ currents. The results are discussed in terms of the contributions from other endogenous calcium-dependent conductances which can influence the Na+ current amplitudes and time courses. The results presented support the idea that intracellular Ca2+ increase principally due to Ca2+ released from InsP3-sensitive stores is needed by the enzyme systems to produce the depolarization-induced activation of the Na+ conductance in the Xenopus oocyte.
Laboratoire de Neurobiologie Cellulaire, Université de Picardie, Faculté des Sciences, Amiens, France. Gilles.Charpentier@u-Picardie.fr
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D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D005260:Female; D006493:Heparin / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D007424:Intracellular Fluid / Q000378:metabolism; D008564:Membrane Potentials; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism*; D015222:Sodium Channels / Q000378:metabolism*; D014982:Xenopus laevis
D002614:Chelating Agents; D015222:Sodium Channels; D002110:Caffeine; D004533:Egtazic Acid; D015544:Inositol 1,4,5-Trisphosphate; D006493:Heparin; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium