Effect of taurine depletion on excitation-contraction coupling and C1 − conductance of rat skeletal muscle

Abstract

The pharmacological action of taurine on skeletal muscle is to stabilize sarcolemma by increasing macroscopic conductance to Cl − ( G C1), whereas a proposed physiological role for the amino acid is to modulate excitation-contraction coupling mechanism via Ca 2+ availability. To get insight in the physiological role of taurine in skeletal muscle, the effects of its depletion were evaluated on voltage threshold for mechanical activation and G C1 with the two intracellular microelectrode method in ‘point’ voltage clamp mode and current clamp mode, respectively. The experiments were performed on extensor digitorum longus muscle fibers from rats depleted of taurine by a chronic 4 week treatment with guanidinoethane sulfonate, a known inhibitor of taurine transporter. The treatment significantly modified the mechanical threshold of striated fibers; i.e. at each pulse duration they needed significantly less depolarization to contract and the fitted rheobase voltage was more negative by 10 mV with respect to untreated muscle fibers. In parallel, the treatment with guanidinoethane sulfonate produced a significant 40% lowering of G C1. In vitro application of 60 mM of taurine to such depleted muscles almost completely restored the mechanical threshold and increased G C1 even above the value of untreated control. However, in vitro application of 60 mM of either taurine or guanidinoethane sulfonate to untreated control muscles did not cause any change of the mechanical threshold but increased G C1 by 40% and 21%, respectively. Furthermore, 100 μM of the S-(−) enantiomer of 2-( p-chlorophenoxy)propionic acid almost fully blocked G C1 but did not produce any change in the mechanical threshold of normal muscle fibers. The present results show that the large amount of intracellular taurine plays a role in the excitation-contraction coupling mechanism of striated muscle fibers. This action is independent from any effect involving muscle Cl − channels, but it is likely mediated by the proposed ability of taurine to modulate Ca 2+ availability through the interaction with the Ca 2+ transporters present on sarcoplasmic reticulum.