Silver ion-induced tension development and membrane depolarization in frog skeletal muscle fibres

Abstract

Silver ions elicit dose-dependently a transient contracture in single fibres of bull-frog toe muscle placed in 0-Ca²⁺, Cl⁻-free MOPS solution containing 3 mM Mg²⁺ and NO ⁻₃ . To elucidate the mechanisms involved, changes in membrane potential and in tension development were continuously measured following exposure to Ag⁺. The effect of Ag⁺ on contraction in fibres in which the membrane had been depolarized by elevating the external K⁺ concentration was also examined. The major findings of this investigation are as follows. (1) The mechanical threshold was shifted towards more negative potentials by 5 mV (−51 to −56 mV), when Ca²⁺ and Cl⁻ in the Ringer's solution were replaced with Mg²⁺ and NO ⁻₃ , respectively. (2) On the exposure of the fibres to 5 μM Ag⁺, the membrane potential decreased by 1.6 mV from −87.8 mV and tension was developed. (3) In fibres soaked in a solution containing 10 mM K⁺ (corresponding to a membrane potential of −69.5 mV), 5 μM Ag⁺ produced a large contracture similar to that seen in the control solution. (4) The Ag⁺-induced contracture was inactivated when more than 20 mM K⁺ was used. (5) The membrane depolarization evoked by either 20 or 50 μM Hg²⁺ did not produce contraction. (6) Muscle fibres which had been exposed to 20 μM Hg²⁺ for 5 min responded to 5 μM Ag⁺ by a transient tension development. These findings strongly suggest that Ag⁺-induced tension development is not associated with depolarization of the surface membrane but rather is caused by specific actions of Ag⁺ on membrane proteins in the T-tubules.