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

1. A voltage-dependent proton current, I(H), was studied in cultured m yotubes obtained from biopsies of human muscle, using whole-cell recor ding with the patchclamp technique. 2. With a pH(o) of 8.0 and a calcu lated pH(i) of 6.3, I(H) was activated at voltages more depolarized th an - 50 mV and its conductance reached its maximum value at voltages m ore 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 extr acellular solution indicated that protons were the major charge carrie rs of I(H). 4. I(H) was also activated during a voltage step to + 22 m V 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 volta ges of the conductance-voltage relationship. 6. Stationary noise analy sis of I(H) suggested that the elementary event underlying I(H) was ve ry small with a conductance of less than 0-09 pS. 7. Extracellular app lication of various divalent cations blocked I(H). The block by divale nt cations was voltage dependent, being more efficient at hyperpolariz ed than at depolarized voltages. For Cd2+, the Michaelis-Menten consta nt (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 proper ties of I(H) and its ability to be affected by pH and Ca2+ suggest tha t I(H) might be used by skeletal muscle cells to extrude protons durin g action potentials. 10. A model of I(H) activation suggests that unde r extreme conditions, the conductance of I(H) can reach 40 % of its ma ximum value after less than ten action potentials.