Mechanotransducer ion channels in chick skeletal muscle: the effects of extracellular pH.

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Abstract
The membrane of tissue-cultured chick pectoral muscle contains an ionic channel which is activated by membrane tension. With 150 mM-external K+ and 150 mM-internal Na+, the channel has a conductance of 70 pS and a reversal potential of +30 mV. With 150 mM-external Na+ and 150 mM-internal K+ (normal gradient) the channel has a conductance of 35 pS and a reversal potential of -30 mV. The ratio of K+ permeability to Na+ permeability, PK:PNa, is 4 based upon reversal potentials and is 2 based upon conductance. Kinetic analysis of single-channel records indicates that there are 1 open (0) and 3 closed (C) states. When analyzed according to a linear sequential model: C1-C2-C3O4, only the rate constant that governs the C1-C2 transition (.kappa.1,2) is found to be affected by stretch or voltage. The effects of stretch and voltage on .kappa.1,2 can be summerized as .kappa.1,2 = .**GRAPHIC**. exp (.alpha.V+.theta.P2), where .**GRAPHIC**. is the voltage and stretch-independent part of the rate constrant, .alpha. is the voltage sensitivity, V is the transmembrane potential, .THETA. is the stretch sensitivity and P is the applied suction. Increasing extracellular pH from 7.4 to 10.0 increases both .alpha. and .THETA. in a manner suggesting titration of site(s) with a pK of 9.1. A single lysine of N-terminal amino acid may be responsible for modulating both the voltage and pressure responses. Extracellular pH does not affect .**GRAPHIC**. the voltage- and stretch-independent part of .kappa.1,2 suggesting that pH in the range of 7.4-10 does not alter the local surface charge. The conductance and reversal potential of the s.a. (stretch-activated) channel are unaffected by pH, suggesting that the titrated site(s) is not close to the mouth of the channel.