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

A continuous infusion technique of either [3H]oestradiol-17 beta (E2) or unlabelled E2 was used to investigate suggested differences in the metabolic clearance rate of oestradiol-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 (mean +/- SEM) ml/min, MCR-E2/kg BW = 116 +/- 14.5 ml/min kg, and conversion ratio (CR) of E2 to oestrone (E1) = 31.6 +/- 3.7%. The respective values for 160 day old gilts were: MCR-E2 = 3027 +/- 340, MCR-E2/kg BW = 48.5 +/- 4.82, and CR = 15.9 +/- 2.12. 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 unlabelled 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%). It is concluded that in addition to an activation of ovarian steroido-genesis during puberty, a gradual maturational decrease in the MCR-E2/kg VW might play a role in raising plasma E2 concentrations and thus in constituting an effective oestrogen feedback signal, which results in the first pre-ovulatory LH-surge.