Effects of calcium, barium and lanthanum on depolarization‐contraction coupling in skeletal muscle fibres of Rana pipiens.

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Abstract
Voltage‐clamp experiments were carried out to study the effect of Ca, Ba and La on the contractile responses of short (approximately 1.5 mm) muscle fibres. In the absence of external Ca or when 1.8 mM‐Ca was substituted by 1.8 mM‐Ba the contractile responses to short depolarizing pulses (400 ms) were not modified and only a minor shift in the tension‐voltage relationship was observed. At high concentration (76 mM) of Ba or Ca, or in the presence of 2 mM‐La, shifts in the tension‐voltage relationship of 30, 41 and 25 mV respectively were observed. In addition the steepness of the activation curve was decreased in high Ba and Ca solutions. In the presence of 76 mM‐Ba, the rate of the relaxation phase which follows membrane repolarization after a short pulse was diminished, possibly due to an intracellular action of Ba. In the absence of Ca, or when Ca was substituted by 1.8 mM‐Ba the area under the prolonged contractile responses (contractures) was reduced considerably. In the presence of 76 mM‐Ba, the normal contracture time course was altered, showing a late slower relaxation phase, occasionally with a secondary tension development. In the presence of high Ca or La, the time course of the contractures was greatly prolonged. The steady‐state inactivation curve was shifted toward more negative potentials, in the absence of external Ca or when 1.8 mM‐Ca was substituted by 1.8 mM‐Ba. In the presence of 76 mM‐Ba the steady‐state inactivation curve was not affected. In the presence of 76 mM‐Ca or 2 mM‐La the curve was shifted toward less negative potential, with a marked decrease in its steepness. Ba at a concentration as high as 1 or 2 mM did not activate tension development in chemically skinned muscle fibres. Ba at 1 or 2 mM did not appreciably alter the Ca uptake capacity of isolated sarcoplasmic reticulum vesicles. At 5 mM a decrease in the Ca uptake capacity was observed. The results indicate that Ba at low (1.8 mM) concentration is not effective in substituting for Ca. The effects of divalent cations at high concentration, and of La, on the tension‐voltage relationship and on the steady‐state inactivation curve are most probably mediated by interaction with the external surface of the fibre membrane.(ABSTRACT TRUNCATED AT 400 WORDS)