Effects of single ingestion of arginine on mTORC1 activation in rat fast- and slow-twitch muscles / 体力科学

This study aimed to examine whether a single ingestion of arginine activates the mammalian target of rapamycin complex 1 (mTORC1) in rat fast- and slow-twitch skeletal muscles. In the first experiment, the rats were orally administered arginine (3 or 10 mmol/kg body weight) in water. The plantaris, gastrocnemius, and soleus muscles were excised 1 h after the administration. Immunoblot analysis showed that the administration with a higher dose (10 mmol/kg) resulted in increased phosphorylation of ribosomal S6 kinase (S6K) and ribosomal protein S6 only in the soleus muscles. The amounts of cellular arginine sensor for mTORC1 subunit 1 (CASTOR1) expressed were similar in these three muscles. In the second experiment, the plantaris and soleus muscles were excised 1 h after the administration of 10 mmol/kg of arginine. The binding of CASTOR1 to the GATOR2 complex was not detected in either muscle in co-immunoprecipitation and immunoblot analyses, irrespective of arginine administration. In the third experiment, a role of nitric oxide (NO) was elucidated. Treatment with an inhibitor of NO synthase blocked the arginine-induced increase in S6K phosphorylation. These results indicate that a single ingestion of arginine is capable of activating mTORC1 only in slow-twitch muscles and suggest that the activation may be mediated via NO, but not via the CASTOR1-GATOR2 complex pathway.

EFFECTS OF ECCENTRIC CONTRACTIONS ON IN VITRO Na⁺-K⁺-ATPase ACTIVITY AND SARCOPLASMIC RETICULUM Ca²⁺-SEQUESTERING IN RAT SKELETAL MUSCLE / 体力科学

The purpose of this study was to examine the effects of eccentric contractions (Ecc) on cation (i.e., K⁺, Na⁺ and Ca²⁺) regulation in skeletal muscle. The left anterior crural muscles of male Wistar rats were subjected <i>in vivo</i> to either Ecc or isometric contrations (Iso) for 200 cycles. The extensor digitorum longus and tibialis anterior muscles were removed immediately after and 2, 4 and 6 days following contractions and used for measures of force output and biochemical analyses, respectively. Ecc led to a 75% decrease in maximal tetanic force. Decreased force output did not revert to pre-exercise levels during 6 days of recovery. Sarcoplasmic reticulum (SR) Ca²⁺-ATPase activity was reduced by 52 and 60% 4 and 6 days after Ecc, respectively. The reduction in catalytic activity after 6 days was accompanied by a 63% decrease in SR Ca²⁺-ATPase protein and an approximately 3.5-fold increase in calpain activity. Na⁺-K⁺-ATPase acticity was decreased by 23% immediately after Ecc and restored during 2 days of recovery. These alterations were specific for Ecc and not observed for Iso. These results suggest that disturbances in cation regulation may account, at least partly, for Ecc-induced decreases in force and power which can take a number of days to recover and that the decrease in SR Ca²⁺-ATPase activity would result from the degradation of the enzyme.