ACUTE EFFECT OF EPINEPHRINE ON MUSCLE PROTEOLYSIS IN PERFUSED RAT HINDQUARTERS

An acute and direct effect of epinephrine (Epi) on muscle proteolysis was investigated using a single-pass mode of rat hindquarter perfusion . The rate of tyrosine (Tyr) release at >30 min with cycloheximide was regarded as the muscle proteolytic rate. Infusion of Epi (500 nM) to the hindquarters of fed rats led to a sharp decrease-in the Tyr releas e to 50% within 5 min, accompanied by an increase in perfusion pressur e and edema around the perfused tissues. To clarify the mechanism, alp ha- and beta-antagonists were used together with Epi. A mixture of 10 mu M prazosin and 10 mu M yohimbine (alpha-adrenergic blockade) before or after Epi infusion completely prevented the edema development and resulted in a new steady state to 80% of the initial rate. On the cont rary, 100 mu M propranolol (a beta-antagonist) with Epi did not abolis h the edema and caused fluctuation in Tyr release. Whether the above r esults are affected by changes in Tyr transport at the plasma membrane was tested by measuring Tyr efflux from the perfused muscle. Only a b eta-adrenergic blockade significantly reduced the rate constant of Tyr efflux from the intracellular pool by 13%. These results suggested th at the suppression of Tyr release by alpha-adrenergic activity was mai nly due to the effect on Tyr efflux, whereas that by beta-adrenergic a ctivity was not at the Tyr transport level but at the proteolysis leve l, demonstrating that Epi directly inhibits proteolysis of skeletal mu scle via the beta-adrenoceptor.