Progesterone metabolism during embryonic diapause in the tammar wallaby, Macropus eugenii

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Abstract
Metabolic clearance rate (MCR), production rate (PR) and progesterone plasma protein-binding were measured during embryonic diapause and after ovariectomy in the tammar wallaby. The effect of ovine prolactin and LH [luteinizing hormone, lutropin] on progesterone [P] secretion by the quiescent corpus luteum in vitro was examined. During embryonic diapause, a low plasma P concentration of 196 .+-. 24 (s.e.m. [standard error of the mean]) pg/ml (n = 7) was associated with a low PR of 20.2 .+-. 1.4 ng/kg per min and a high MCR of 111.0 .+-. 7.6 ml/kg per min. Similar values were obtained for the 4 animals ovariectomized 9 or 12 mo. previously (plasma P = 156 .+-. 14 pg/ml; MCR = 147 .+-. 16 ml/kg per min; PR = 20.4 .+-. 2.2 ng/kg per min) although plasma P concentration was decreased by .apprx. 50% within 14 days of ovariectomy. An increased adrenal cortical secretion of P in long-term ovariectomized tammars is suggested. P was bound by plasma protein. The association constant at 20.degree. C was similar for intact (2.31 .times. 108 M-1) and ovariectomized (2.27 .times. 108 M-1) tammars while binding site concentration was lower in ovariectomized (1.41 .times. 10-7 M) than in intact (2.28 .times. 10-7 M) tammars. Cortisol and 17.alpha.-hydroxyprogesterone competed with P for binding sites while 20.alpha.-dihydroprogesterone, estradiol-17.beta. and testerone did not. P was secreted by quiescent corpora lutea in vitro but neither LH nor prolactin changed its rate of secretion.