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

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 one open (O) and three closed (C) states. When analysed according to a linear sequential model: C1-C2-C3-O4, only the rate constant that governs the C1-C2 transition (k1,2) is found to be affected by stretch or voltage. The effects of stretch and voltage on k1,2 can be summarized as k1,2 = k1,2(0) exp (alpha V + theta P2), where K1,2(0) is the voltage and stretch-independent part of the rate constant, 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 be responsible for modulating both the voltage and pressure responses. Extracellular pH does not affect k1,2(0), the voltage- and stretch-independent part of k1,2, suggesting that pH in the range 7.4-10 does not alter the local surface charge. The conductance and reversal potential of the s.a. channel are unaffected by pH, suggesting that the titrated site(s) is not close to the mouth of the channel.