Sex Hormone Modification of Tissue Levels and Urinary Excretion of α-Aminoisobutyric Acid in the Rat1

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Abstract
Male rats have been found to excrete smaller amounts than females of an injected dose of the model amino acid, C14-α-aminoisobutyric acid, into the urine during 24 hr. Testosterone propionate (0.1 mg daily for 1–2 weeks) reduced by 50% the 24-hr urinary excretion of the amino acid by immature female rats, while 1 or 40 Mg daily of β- estradiol doubled its excretion over that found in control animals. These changes have been shown to be a result of changes in the renal clearance of α-aminoisobutyric acid under the conditions used; both glomerular filtration rate and renal plasma flow were unchanged by the hormones. With both steroids, the changes in retention resulted in significant changes in serum levels of the amino acid, which were reflected in alterations in tissue levels in the same direction without an influence on transport of the amino acid. Changing the serum AIB level 100-fold (by altering the dose) did not produce large changes in the distribution ratios in any of the 5 tissues examined. This behavior was interpreted to mean: 1) that the distribution ratio, in contrast to the absolute tissue level, is a valid index of AIB transport over a wide range of serum AIB levels; and 2) that the total amino acids of the serum, and not AIB alone, are important in determining AIB distribution in the body. In addition, testosterone propionate increased the distribution ratio for α-aminoisobutyric acid in skeletal muscle; and β-estradiol stimulated uptake of the amino acid by the uterus even though serum levels had been reduced by the increased urinary excretion. The uterus did not show increased amino acid uptake, however, when the rats received large or repeated doses of β- estradiol. The effects of the hormones on renal excretion, on the one hand, and on skeletal muscle or uterus on the other, appeared, therefore, to be independent, although all of them probably involve modification of transport processes.