The secretion and metabolic clearance rates of oestradiol in the rat

The secretion rate (SR) of oestradiol in rats at different stages of the oestrous cycle was determined by measuring the concentration of this hormone in ovarian vein plasma by a competitive protein binding method utilizing rabbit uterine cytosol. The plasma metabolic clearance rate (MCRP) for oestradiol was determined in groups of rats of the same strain and housed under the same conditions by giving a single intravenous injection of tritiated oestradiol-17beta and measuring the rate of disappearance of labelled hormone from the plasma. From the relationship SR equals MCRP times plasma concentration, predicted values for the plasma oestradiol concentration were obtained and shown to be close to those obtained by direct analysis of peripheral plasma. The MCRP for oestradiol was not greatly influenced by the stage of the oestrous cycle, the dose of oestradiol-17beta injected, ovariectomy 18 h previously or by a sham-operation immediately before the injection of labelled hormone. The anaesthetic used did affect the MCRP ; the value for adult female rats injected under ether anaesthesia was about 970 ml/h but under Avertin anaesthesia clearance rates of about 1500 ml/h were obtained. Ligation of the bile duct acutely reduced the MCRP but functional nephrectomy had no immediate effect. Sub-total hepatectomy 18 h previously reduced the MCRP considerably. A small decrease in MCRP was observed when the labelled hormone was administered in third trimester human pregnancy plasma and the MCRP was lower in rats on Day 14 of pregnancy than in rats examined during the oestrous cycle.