Ruthenium red effect on mechanical and electrical properties of mammalian skeletal muscle

In this work we studied the effect of ruthenium red (RR) on the mechanical and electrical properties of rat diaphragm bundles in vitro (30 degrees C). Two concentrations of RR were used: 5 and 10 microM. We measured: 1) twitches, tetanus and caffeine contracture; 2) relation between mechanical tension and resting membrane potential (Vm); 3) contraction threshold by visualization of the contraction around the stimulated area of the fiber. The main finding are the following: a) RR potentiates the twitch tension. The tetanic tension is not affected and the time course of the caffeine contracture is shortened in the RR containing solutions; b) the relationship between mechanical tension and resting membrane potential (Vm) is shifted toward more negative values of Vm in RR; c) the mechanical threshold is lowered about 7 mV in the presence of RR; d) the rates of depolarization and repolarization of the action potential are decreased in the test solutions. We suggested that the shift in the mechanical threshold and the prolongation of the action potential are the main factors involved in the potentiating effect of RR. The mechanism by which RR shifts the mechanical threshold is not known.