ATP-sensitive voltage- and calcium-dependent chloride channels in sarcoplasmic reticulum vesicles from rabbit skeletal muscle

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 SC1 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 ATP(4-) > ADP(3-) > > > AMP(2-). 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.