MUSCLE FATIGUE IN FROG SEMITENDINOSUS - ROLE OF INTRACELLULAR PH

The purpose of this study was to utilize glass microelectrodes to characterize the intracellular pH (pH(i)) before and during recovery from fatigue in the frog semitendinosus (ST) muscle. A second objective was to evaluate the relationship between pH(i) and contractile function. The frog ST muscle (22-degrees-C) was fatigued by direct electrical stimulation with 100-ms 150-Hz trains at 1/s for 5 min. Peak tetanic force (P(o)) was reduced to 8.5% of initial force and recovered in a biphasic manner, returning to the resting value by 40 min. Resting pH(i) was 7.00 +/- 0.02 (n = 37) and declined with fatigue to an average value of 6.42 at 3 min of recovery. During recovery pH(i) significantly increased and by 25 min had returned to the prefatigue value. The pH(i) recovery was highly correlated to the slow phase of P(o) recovery (r = 0.98, P < 0.001). The mean resting membrane potential was -78 + /- 1.0 mV (n = 42) and at 3 min of recovery was depolarized to -67 +/- 4 mV. Both the peak rate of twitch force development (+dP/dt) (r = 0.99, P < 0.001) and decline (-dP/dt) (r = 0.94, P < 0.0 14) were highly correlated to pH(i) during the slow phase of recovery. Contraction time (CT) and one-half relaxation time (1/2RT) increased significantly and recovered exponentially. The recovery of CT and 1/2RT were both significantly correlated to pH(i) (r = -0.93, P < 0.001 and r = -0.86, P < 0.001 for CT and 1/2RT, respectively). In conclusion, the high correlations between force, +/-dP/dt, CT, 1/2RT, and pH(i) during recovery from fatigue support the hypothesis that the slow phase of recovery is partially linked to elevated H+.