Alterations in the Number of Ovine Luteal Receptors for LH and Progesterone Secretion Induced by Homologous Hormone1

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 [P] in serum, luteal concentrations of P 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 .+-. SE) to 327 .+-. 22 ng/ml at 10 min, then returned to control levels by 24 h. P concentrations in serum increased (P < 0.05) from 2.63 .+-. 0.29 (least squares mean .+-. SE) to 4.09 .+-. 0.29 ng/ml within 10 min after administration of LH, remained elevated for 2 h, decreased to control levels within 4 h, but never decreased below control levels. P content of the corpus luteum was correlated (r [correlation coefficient] = 0.78, P < 0.05) with P concentrations in serum. The number of unoccupied receptors decreased (P < 0.05) by 66% from control levels of 2.63 .+-. 0.48 .times. 10-12 mol/corpus luteum to 0.90 .+-. 0.48 .times. 10-12 mol/corpus luteum by 12 h. Recovery to control levels occurred between 24-48 h. Within 10 min 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 .times. 10-12 mol/corpus luteum to 5.43 .+-. 0.73 .times. 10-12 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 P 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 .times. 10-12 mol/corpus luteum) and the number of total receptors lost from 10 min to 24 h (6.75 .times. 10-12 mol/corpus luteum) after LH injection. Down-regulation of ovine luteal receptors for LH apparently involves the loss of occupied receptors, perhaps via internalization of hormone-receptor complexes. In addition, a mechanism for the replacement of receptors is proposed.