Induction of Na+ channel voltage sensitivity in Xenopus oocytes depends on Ca2+ mobilization
✍ Scribed by Gilles Charpentier; Raymond T. Kado
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 240 KB
- Volume
- 178
- Category
- Article
- ISSN
- 0021-9541
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✦ Synopsis
An unusual inward current which is slowly elicited in the Xenopus oocyte membrane during sustained depolarization is reportedly carried by Na ϩ . It is thought that Na ϩ selective channels are in some way induced to become voltagesensitive 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 2ϩ chelator (BAPTA) in the oocyte, before induction of the Na ϩ channels, prevented the appearance of the Na ϩ current, confirming an important role for [Ca 2ϩ ] i . However, in oocytes perfused with Ca 2ϩ -free medium, induction of the channels could still be obtained, indicating that induction did not depend upon the entry of external Ca 2ϩ . Downmodulation of Ca 2ϩ release from inositol 1,4,5-trisphosphate (InsP 3 )-sensitive stores with caffeine and with a low molecular weight heparin resulted in decreased or no Na ϩ currents. The results are discussed in terms of the contributions from other endogenous calciumdependent conductances which can influence the Na ϩ current amplitudes and time courses. The results presented support the idea that intracellular Ca 2ϩ increase principally due to Ca 2ϩ released from InsP 3 -sensitive stores is needed by the enzyme systems to produce the depolarization-induced activation of the Na ϩ conductance in the Xenopus oocyte.