In VitroEffects of Adrenal Steroid Hormones on Amino Acid Transport in Muscle1

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Abstract
The transport of A IB into isolated diaphragms of normal rats was inhibited when the muscles were incubated for 4 hr in the presence of various steroid hormones. The synthetic glucocorticoid, dexamethasone, which inhibited AIB transport at a concentration of 10-8M, had the greatest activity of the hormones tested. Cortisol, corticosterone, 11-deoxycortisol and 11-deoxycorticosterone were inhibitory at 10-7M, while 11-dehydrocorticosterone and aldosterone had some activity at 10-6M. Progesterone was inhibitory at a concentiation of 10-5M, while 17β-estradiol, Δ4- androstene-3,17-dione and cholesterol were inactive. Under the conditions employed, corticosterone (10-7M) had no effect on the concentration of amino nitrogen in the cells of the diaphragm. When incubation periods of short duration (0.5–2.0 hr) were used, corticosterone did not inhibit AIB transport when tested at concentrations ranging from 10-4 to 10-7M. However, when diaphragms were preincubated with corticosterone (10-6M) only for 3 hr, the subsequent transport of AIB during short incubation periods was inhibited. A delay in the onset of steroid-induced inhibition of amino acid transport could be demonstrated also, when dexamethasone-21-phosphate was injected into adrenalectomized rats pnor to removal of the diaphragms. The fact that there is an appreciable delay in the response of the muscle amino acid transport process to steroids both in vivo and in vitro suggests that the interaction of the steroid with the muscle cell may be at some site other than the membrane transport system. (Endocrinology76: 604, 1965)