Sodium nitroprusside, a NO donor, modifies Ca2+ transport and mechanical properties in frog skeletal muscle

Recently it has been hypothesized that, in skeletal muscle, NO produced dir ectly by high-frequency stimulation could produce contraction through react ions with thiol groups on the sarcoplasmic reticulum (SR). However, a possi ble cGMP-mediated relaxing effect, similar to that seen in smooth muscle, h as also been demonstrated. We used purified SR preparations and single fibr es from frog fast muscles incubated with different concentrations of sodium nitroprusside (SNP) in this study. The results obtained from a long low-fr equency stimulation, together with those from a study on Ca2+ transport reg ulation, showed that the presence of NO precursor induced: an acceleration of the onset of fatigue in single fibres; a decreased vesicular Ca2+ conten t due to increased Ca2+ release; a shift to open status in SR Ca2+ channels ; an increase in SR Ca2+ pump activity. The data presented in this paper se em to indicate that the increased NO in the muscle fibres can influence mus cle activity in different ways, perhaps depending on the metabolic status o f the muscle and target (filaments, sarcolemma, SR) with which the NO (or i ts derivatives) acts. (C) Kluwer Academic Publishers.