RESUMO
NO is a short-lived gas molecule generated by degradation of L-arg to citrulline and by the activation of enzyme NOS Ca2+/calmodulin-dependent. There are multiple NOS isoforms that strongly are expressed in skeletal muscle, suggesting the crucial role of NO in regulating muscular metabolism and function. In this study, the effect of L-arginine was examined at the neuromuscular junction of the chick biventer cervicis muscle. Biventer cervicis muscle preparations from chick's age of 3 weeks were set up in the organ bath. The organ bath had a vessel with volume of about 70 ml; it contained Tyrode solution aerated with oxygen and was kept at 37°C. NO levels was also measured in the chick biventer cervicis muscle homogenates, using spectrophotometer method for the direct detection of NO, nitrite and nitrate. Total nitrite [nitrite+nitrate] was measured by a spectrophotometer at 540 nm after the conversion of nitrate to nitrite by copperized cadmium granules. L-Arginine at 500 micro g/ml, decreased twitch response to electrical stimulation, and produced rightward shift of the dose-response curve for acetylcholine or carbachol. L-arginine at 1000 micro g/ml produced a strong shift to the right of the dose-response curve for acetylcholine or carbachol with a reduction in efficacy. The inhibitory effect of L-arginine on the twitch response was blocked by caffeine [200 micro g/ml]. NO levels were found to be significantly increased in concentrations 500 and 1000 micro g/ml of L-arginine in comparison with the control group [p < 0.001]. These findings indicate a possible role of increased NO levels in the suppressive action of L-arginie on the twitch response. In addition, the results indicate that the post-junctional antagonistic action of L-arginine is probably the result of impaired sarcoplasmic reticulum [SR] Ca2+ release