CYTOSOLIC [H+] DURING SEQUENTIAL, FATIGUING CONTRACTILE PERIODS IN ISOLATED SINGLE XENOPUS SKELETAL MUSCLE FIBERS

Abstract

The purpose of the present study was to test the hypothesis that a preceding contractile period in isolated single skeletal muscle fibers would attenuate the rise in cytosolic [H+] ([H+]cyt) during a subsequent contractile period, thereby reducing the rate of fatigue. Intact single skeletal muscle fibers (n = 9) were isolated from Xenopus lumbrical muscle, and incubated with the [H+]cyt indicator BCECF AM for 30 min. Following incubation, fibers were mounted in a glass chamber on the stage of an inverted microscope configured for epl-illumination. Two identical contractile periods were performed in each fiber, separated by a 1hr recovery period. Force and [H+cyt were measured simultaneously while fibers were stimulated (tetanic contractions of 350 ms trains with 70 Hz stimuli at 9 V) at progressively increasing frequencies (0.25, 0.33, 0.5 and 1 contraction/sec) until significant fatigue developed (50% initial force). No significant difference (P < 0.05) was observed between the first and second contractile periods in initial force development or in resting [H+]cyt. Similarly, no significant difference was observed in the time to fatigue between the first (341 ± 36 sec) and second (333 ± 38 sec) contractile periods. However, the increase in [H+]cyt from rest to fatigue was significantly greater (P < 0.05) during the first contractile period (175 ± 36 nmol) when compared to the second (70 ± 18 nmol). The results of the present study demonstrated in contracting single skeletal muscle fibers that when preceded by an initial fatiguing contractile period, the subsequent increase in [H+]cyt during a second contractile period was reduced. However, this did not attenuate the fatigue process in the second contractile period, suggesting that factors other than H+ ion accumulation compromised force development under these conditions. Supported by NIH Grant No. AR-40155