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Induction of Na + channel voltage sensitivity in Xenopus oocytes depends on Ca 2+ mobilization
Author(s) -
Charpentier Gilles,
Kado Raymond T.
Publication year - 1999
Publication title -
journal of cellular physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.529
H-Index - 174
eISSN - 1097-4652
pISSN - 0021-9541
DOI - 10.1002/(sici)1097-4652(199902)178:2<258::aid-jcp15>3.0.co;2-d
Subject(s) - depolarization , bapta , biophysics , oocyte , xenopus , phospholipase c , chemistry , calcium , intracellular , membrane potential , reversal potential , inositol , microbiology and biotechnology , biochemistry , biology , patch clamp , enzyme , receptor , embryo , organic chemistry , gene
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 Ca 2+ chelator (BAPTA) in the oocyte, before induction of the Na + channels, prevented the appearance of the Na + current, confirming an important role for [Ca 2+ ] i . However, in oocytes perfused with Ca 2+ ‐free medium, induction of the channels could still be obtained, indicating that induction did not depend upon the entry of external Ca 2+ . Downmodulation of Ca 2+ release from inositol 1,4,5‐trisphosphate (Ins P 3 )‐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 Ca 2+ increase principally due to Ca 2+ released from Ins P 3 ‐sensitive stores is needed by the enzyme systems to produce the depolarization‐induced activation of the Na + conductance in the Xenopus oocyte. J Cell Physiol 178:258–266, 1999. © 1999 Wiley‐Liss, Inc.