Journal of Applied Physiology, Vol 67, Issue 5 2060-2065, Copyright © 1989 by American Physiological Society

ARTICLES

Effects of ovariectomy on energy metabolism in exercising rat muscle studied by 31P-NMR

Z. Roth, Z. Argov, J. Maris, K. K. McCully, J. S. Leigh Jr and B. Chance
Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia 19104.

The effects of ovariectomy on metabolism of high-energy phosphate compounds during and after exercise were studied in hindleg muscles of 14 rats. Sciatic nerve stimulation was used to establish different work loads, and the changes in inorganic phosphate-to-phosphocreatine ratios (Pi/PCr) were recorded by 31P nuclear magnetic resonance (NMR) in vivo. Four weeks after ovariectomy, there was evidence of significantly higher Pi/PCr during work at stimulation rates greater than 0.5 Hz. The slope for the stimulation rate-to-Pi/PCr relationship decreased from 1.98 +/- 0.15 to 1.36 +/- 0.2 Hz/Pi/PCr after ovariectomy. The normalized tension output of these muscles, tested separately using identical stimulation protocols, was not changed with ovariectomy. Thus the relationship between work (tension-time integral) and bioenergetic cost (Pi/PCr) suggested reduced maximal enzyme activity (Vmax) by 9-17% as a result of lack of ovarian sex hormones, but no change in Michaelis-Menten constant (Km) was found. Postexercise recovery was also significantly slower (3.27 +/- 0.54 PCr/Pi units per minute compared with 4.04 +/- 1.08 in controls). It is suggested that reduced levels of ovarian sex hormones decrease oxidative phosphorylation. Cytochrome oxidase activity was reduced in these muscles by 40%, but other mitochondrial enzyme systems may be affected as well. The possible significance of these data is the implication of a reduced capacity for menopausal women or amenorrheic female athletes to perform prolonged intensive exercise.