AJP - Endocrinology and Metabolism, Vol 270, Issue 6 E961-E967, Copyright © 1996 by American Physiological Society
Acute effect of epinephrine on muscle proteolysis in perfused rat hindquarters
M. Kadowaki, T. Kamata and T. Noguchi
Department of Applied Biological Chemistry, Faculty of Agriculture, Niigata University, Japan.
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 release to 50%
within 5 min, accompanied by an increase in perfusion pressure and edema
around the perfused tissues. To clarify the mechanism, alpha- and
beta-antagonists were used together with Epi. A mixture of 10 microM
prazosin and 10 microM 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 contrary, 100
microM propranolol (a beta-antagonist) with Epi did not abolish the edema
and caused fluctuation in Tyr release. Whether the above results are
affected by changes in Tyr transport at the plasma membrane was tested by
measuring Tyr efflux from the perfused muscle. Only a beta-adrenergic
blockade significantly reduced the rate constant of Tyr efflux from the
intracellular pool by 13%. These results suggested that the suppression of
Tyr release by alpha-adrenergic activity was mainly due to the effect on
Tyr efflux, whereas that by beta-adrenergic activity was not at the Tyr
transport level but at the proteolysis level, demonstrating that Epi
directly inhibits proteolysis of skeletal muscle via the beta-adrenoceptor.
