Effects of ammonium ions on the depolarization-induced and direct activation of the contractile apparatus in mechanically skinned fast-twitch skeletal muscle fibres of the rat

Summary Mechanically skinned fibre preparations from the extensor digitorum longus muscle of the rat were used to test whether a rise in myoplasmic [NH₄⁺] in the range 2–10 mm interferes with the mechanism of excitation-contraction coupling in fast-twitch mammalian muscle. Under our conditions (pH 7.10, Mg²⁺ 1 mm, temperature 23° C), [NH₄⁺] up to 10 mm had little effect on the Ca⁺-activated force and on the peak of the t-system depolarization-induced force response. However, the duration of the depolarization-induced force response was decreased significantly at [NH₄⁺] 2 mm. From these data we conclude that the intracellular accumulation of NH₄⁺ is not likely to play a major role in fatigue. Nevertheless, the build up of NH₄⁺ during fatigue, may have a significant inhibitory effect on the force output by decreasing the duration of the t-system depolarization-induced activation of the contractile apparatus.