ATP-sensitive voltage- and calcium-dependent chloride channels in sarcoplasmic reticulum vesicles from rabbit skeletal muscle |
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Authors: | J. I. Kourie |
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Affiliation: | (1) Muscle Research Group, Division of Neuroscience, John Curtin School of Medical Research, ANU, PO Box 334, Canberra City, ACT, 2601 Australia, AU |
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Abstract: | Chloride channels in the sarcoplasmic reticulum (SR) are thought to play an essential role in excitation-contraction (E-C) coupling by balancing charge movement during calcium release and uptake. In this study the nucleotide-sensitivity of Cl− channels in the SR from rabbit skeletal muscle was investigated using the lipid bilayer technique. Two distinct ATP-sensitive Cl− channels that differ in their conductance and kinetic properties and in the mechanism of ATP-induced channel inhibition were observed. The first, a nonfrequent 150 pS channel was inhibited by trans (luminal) ATP, and the second, a common 75 pS small chloride (SCl) channel was inhibited by cis (cytoplasmic) ATP. In the case of the SCl channel the ATP-induced reversible decline in the values of current (maximal current amplitude, I max and integral current, I′) and kinetic parameters (frequency of opening F O , probability of the channel being open P O , mean open T O and closed T c times) show a nonspecific block of the voltage- and Ca2+-dependent SCl channel. ATP was a more potent blocker from the cytoplasmic side than from the luminal side of the channel. The SCl channel block was not due to Ca2+ chelation by ATP, nor to phosphorylation of the channel protein. The inhibitory action of ATP was mimicked by the nonhydrolyzable analogue adenylylimidodiphosphate (AMP-PNP) in the absence of Mg2+. The inhibitory potency of the adenine nucleotides was charge dependent in the following order ATP4− > ADP3− > > > AMP2−. The data suggest that ATP-induced effects are mediated via an open channel block mechanism. Modulation of the SCl channel by [ATP] cis and [Ca2+] cis indicates that (i) this channel senses the bioenergetic state of the muscle fiber and (ii) it is linked to the ATP-dependent cycling of the Ca2+ between the SR and the sarcoplasm. Received: 4 September 1996/Revised: 6 December 1996 |
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Keywords: | : ATP-sensitive — Chloride channel — Sarcoplasmic reticulum — Skeletal muscle — Counter-ion channel |
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