AJP - Endocrinology and Metabolism, Vol 247, Issue 5 651-E656, Copyright © 1984 by American Physiological Society
Pituitary LH responsiveness to GnRH in vitro as related to GnRH receptor number
D. M. Baldwin, G. A. Bourne and J. C. Marshall
The objective of this in vitro study was to determine whether the increase
in the augmented phase of the biphasic luteinizing hormone (LH) response to
gonadotrophin-releasing hormone (GnRH) and its enhancement by estradiol
(E2) were associated with GnRH-stimulated increases in pituitary GnRH
receptor concentration. Pituitary glands from 72 h ovariectomized (OVX),
OVX + E2, or proestrous rats were perifused continuously with GnRH (12
ng/h). LH release was measured at 10-min intervals, and pituitary
GnRH-binding capacity (GnRH-BC) was assessed at 0, 40, 80, 120, and 240 min
after addition of GnRH. All treatment groups exhibited a biphasic pattern
of LH release; initial (20-70 min) and augmented (120-240 min) mean rates
of LH secretion (micrograms/h) were 1.78 and 3.92 (OVX), 6.40 and 16.67
(OVX + E2), and 2.79 and 18.64 (proestrus), respectively. Total LH release
was significantly greater in the OVX + E2 and proestrous groups (44.0 and
45.8 micrograms) vs. the OVX group (12.4 micrograms). Throughout the GnRH
infusion period, GnRH-BC did not change significantly in any of the
treatment groups with the exception of the OVX group in which there was a
transient small decrease at 80 min post-GnRH infusion. There were no
significant differences between treatment groups in GnRH-BC at any time
after infusion of GnRH. These results demonstrate that the acute and
augmented phases of GnRH-stimulated LH release and the enhancement of this
biphasic response by E2 occurs independent of any increase in GnRH-BC and
suggest that these events are mediated by postreceptor mechanisms.
