Effects of fatigue and altered pH on isometric force and velocity of shortening at zero load in frog muscle fibres

Abstract

Single skeletal muscle fibres ofR. temporaria (0.9–2.5° C) were stimulated to produce a 1 s isometric tetanus at regular intervals until constant mechanical responses were attained. Various degrees of force depression (‘fatigue’) were produced by decreasing the contraction interval from 30 or 15 min (control) to 120, 60, 30 and 15 s, respectively. In this way the steady-state tetanic force could be reversibly reduced to approximately 70% of the control value. The velocity of shortening at zero load,V ₀, was determined at each level of fatigue using an approach for direct measurement ofV ₀. V ₀ was not significantly affected as long as the decrease in force was less than 10%. With further reduction of the isometric tension there was a progressive decline ofV ₀ according to the following empirical relationship between percentage depression of force (ΔP ₀) and maximum speed (ΔV ₀) of shortening: ΔV ₀=0.006 ΔP ₀ ^2.48 −1.0 (correlation coefficient, 0.86). Cine photographic recording of nylon markers on the fibre surface provided evidence that fatigue developed uniformly along the fibre with no sign of failure of excitation in any segment. The change in mechanical performance during fatigue could be reproduced in the non-fatigued fibre by reducing the pH of the external medium within the range 8.0–6.6 using a bicarbonate-CO₂ buffer. A decrease in pH thus reduced both the rate of rise and the total amplitude of isometric force and prolonged the relaxation phase. Furthermore, there was a drop inV ₀ that was related to the force decline in approximately the same way as observed during fatigue. The results support the idea that fatigue involves both a reduced state of activation of the contractile system and a specific (activation independent) inhibition of crossbridge turnover. Increased intracellular H⁺ concentration is likely to contribute to the development of both these effects during fatigue.