A VOLTAGE-DEPENDENT PROTON CURRENT IN CULTURED HUMAN SKELETAL-MUSCLE MYOTUBES

1. A voltage-dependent proton current, I(H), was studied in cultured myotubes obtained from biopsies of human muscle, using whole-cell recording with the patchclamp technique. 2. With a pH(o) of 8.0 and a calculated pH(i) of 6.3, I(H) was activated at voltages more depolarized than - 50 mV and its conductance reached its maximum value at voltages more depolarized than + 10 mV. 3. Studies of the reversal potential of I(H) during substitution of K+, Na+, Ca2+, Cl-, Cs+ and H+ in the extracellular solution indicated that protons were the major charge carriers of I(H). 4. I(H) was also activated during a voltage step to + 22 mV with a pH(o) of 7.3 and a calculated pH(i) of 7.3. 5. Acidification of the extracellular solution led to a shift towards depolarized voltages of the conductance-voltage relationship. 6. Stationary noise analysis of I(H) suggested that the elementary event underlying I(H) was very small with a conductance of less than 0-09 pS. 7. Extracellular application of various divalent cations blocked I(H). The block by divalent cations was voltage dependent, being more efficient at hyperpolarized than at depolarized voltages. For Cd2+, the Michaelis-Menten constant (K(m)) for the block was 0.6 muM at - 28 mV and 10.4 muM at + 12 mV. 8. Ca2+ was a less efficient blocker than Cd2+ but could block I(H) at physiological concentrations (the K(m) values for the block were 0.9 mM at - 38 mV and 7.3 mm at - 8 mV). 9. The voltage-dependent properties of I(H) and its ability to be affected by pH and Ca2+ suggest that I(H) might be used by skeletal muscle cells to extrude protons during action potentials. 10. A model of I(H) activation suggests that under extreme conditions, the conductance of I(H) can reach 40 % of its maximum value after less than ten action potentials.