Metabolic Clearance Rates and Secretion Rates of Prolactin in Sheep1

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Abstract
The metabolic clearance rates (MCR) of ovine prolactin (Prl) have been estimated in lambs, and in pregnant, lactating, anestrous and ovariectomized ewes by the constant infusion to equilibrium method. The MCR were calculated using the ratio of immunoprecipitable 1-125 Prl infused to the plasma levels of immunoprecipitable 1-125 Prl. The product of MCR and endogenous plasma Prl levels was used to estimate daily Prl secretion rates (SR). The mean Prl MCR were significantly (p < 0.05) correlated (r = 0.81) with metabolic body weight (kg0-75) and with endogenous plasma Prl concentration (r = 0.69). The MCR of Prl per unit of metabolic body size was significantly (p < 0.001) higher in the lactating ewes (6.09 ± 0.41) and significantly lower (p < 0.05) in the ovariectomized (2.93 ± 0.13) ewes than in the lambs, anestrous ewes, and pregnant ewes (4.45 ± 0.19, 3.84 ± 0.41, and 4.56 ± 0.38 ml/min/metabolic bw, respectively). The daily SR(mg) per unit of metabolic body weight were significantly (p < 0.01) higher in lactating ewes (2.59 ± 0.33) than all other groups studied, and significantly (p < 0.05) higher in lambs (0.436 ± 0.065) than in ovariectomized (0.214 ± 0.027), pregnant (0.252 ± 0.048) or anestrous ewes (0.162 ± 0.059). These results suggest that differences in plasma Prl concentration between groups of animals of varying physiological conditions can be, in part, explained by differences in the MCR of Prl. It is evident that the MCR of Prl must be studied further under other physiologic conditions in the sheep. (Endocrinology92: 1414, 1973)

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