Alterations in the number of ovine luteal receptors of LH and progesterone secretion induced by homologous hormone.

Abstract

To study the events associated with down-regulation of receptors for luteinizing hormone (LH) in the ovine corpus luteum, normally cycling ewes were injected i.v. with 1 mg LH on Day 10 of the estrous cycle. Changes in concentrations of LH and progesterone in serum, luteal concen trations of progesterone and numbers of luteal receptors for LH were measured up to 72 h after LH injection. Serum levels of LH increased more than 1000-fold (P<0.01) from a control level of 298 ±23 pg/ml (mean ±SEM) to 327 ±22 ng/ml at 10 mm, then returned to control levels by 24 h. Progesterone concentrations in serum increased (P<0.05) from 2.63 ±0.29 (least squares mean ±SEM) to 4.09 ±0.29 ng/ml within 10 mm after administration of LH, remained elevated for 2 h, decreased to control levels within 4 h, but never decreased below control levels. Proges terone content of the corpus luteum was correlated (r = 0.78, P<0.05) with progesterone concen trations in serum. The number of unoccupied receptors decreased (P<0.05) by 66% from control levels of 2.63 ±0.48 X 1012 mol/corpus luteum to 0.90 ±0.48 X 1012 mol/corpus luteum by 12 h. Recovery to control levels had occurred between 24—48h. Within 10 mm the total number of receptors for LH increased 260% while the number of occupied receptors increased 13-fold from control levels of 0.41 ±0.66 X 1012 mol/corpus luteum to 5.43 ±0.73 X 1012 mol/corpus luteum. The number of receptors occupied by LH decreased rapidly until 6 h and had returned to control values by 24 h. Serum concentrations of progesterone were correlated (r = 0.80, P<0.05) with the number of occupied receptors during the first 24 h after LH injection. In addition, there was good correlation between the number of receptors occupied by LH at 10 min (5.43 X 10-12 mol/corpus luteum) and the number of total receptors lost from 10 min to 24 h (6.75 X 10-12 mol/corpus luteum) after LH injection. These data suggest that down-regulation of ovine luteal receptors forLH involves thelossofoccupiedreceptors, perhapsviainternalization ofhormone receptor complexes. In addition, a mechanism for the replacement of receptors is proposed.