Metabolic Clearance and Secretion Rates of Gonadotropins at Different Stages of the Estrous Cycle in Ewes1

Abstract

Metabolic clearance rates (MCR) were determined in 5 ewes by injection of a priming dose and constant infusion of luteinizing hormone (LH)-¹²⁵I, follicle-stimulating hormone (FSH)-¹³¹I and prolactin-¹²⁵I for 120 min during the midluteal and follicular phases of the estrous cycle and on the day of estrus. Blood samples were collected at 10-min intervals during the period of infusion to quantify serum levels of gonadotropins. MCR (ml/min) was calculated as the ratio of immunoprecipitable radioiodinated hormone infused (counts infused/min) to immunoprecipitable radioiodinated hormone in the serum (cpm/ml) after equilibrium was obtained. Secretion rates (SR, ng/min) were calculated by multiplying MCR by endogenous gonadotropin concentration (ng/ml). Mean values obtained from the 5 samples collected during the last 50 min of infusion were used to calculate MCR and SR. The mass of radioiodinated hormone infused did not alter serum concentration of these gonadotropins. MCR for LH, FSH and prolactin were not significantly different during different stages of the estrous cycle. There was no evidence obtained which suggested that the functional state of the ovary influenced MCR of gonadotropins. Since the MCR did not change during the estrous cycle blood levels of these hormones are indicative of the rates of their secretion from the pituitary gland. (Endocrinology 94: 1318, 1974)