A comparison of metabolic clearance rates of oestradiol-17β in immature and peripubertal female pigs and possible implications for the onset of puberty

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Abstract
A continuous infusion technique of either [3H]estradiol-17.beta. (E2) or unlabeled E2 was used to investigate suggested differences in the metabolic clearance rate of estradiol-17.beta. (MCR-E2) in immature and peripubertal female pigs. Using the isotope infusion technique, the following values were obtained: 60 day old gilts; MCR-E2 = 2133 .+-. 274 (.hivin.x .+-. standard error of the mean) ml/min, MCR-E2/kg BW [body wt] = 116 .+-. 14.5 ml/min kg, and conversion ratio (CR) of E2 to estrone (E1) = 31.6 .+-. 3.7%. The respective values for 160 day old gilts were: MCR-E2 = 3027 .+-. 340, MCR-E2/kg BW = 48.5 .+-. 4.8 and CR = 15.9 .+-. 2.1. Except for a significant difference in the weight related MCR-E2 data for the 60 day old group, similar values were found following infusion of unlabeled E2. The percentage of radioactivity extractable with ether from plasma was 22.6 .+-. 3.0% and 27.2 .+-. 2.0%. Fifty-seven and 66% of total radioactivity infused was recovered in the urine within 12 h in 60 and 160 day old gilts, respectively. There was no difference in the percentage binding of E2 to plasma proteins as determined by equilibrium dialysis (80%). In addition to an activation of ovarian steroidogenesis during puberty, a gradual maturational decrease in the MCR-E2/kg BW might play a role in raising plasma E2 concentrations and thus in constituting an effective estrogen feedback signal, which results in the 1st pre-ovulatory lutropin surge.