389. Pituitary sensitivity to LRF and estradiol-17β receptor population during the rat estrus cycle

Abstract

This study was designed to confirm preliminary work indicating a direct effect of estradiol on the pituitary by examining the pituitary sensitivity to LRF at a series of critical time periods during the estrous cycle. Rats were bled each morning (11.00) during the 4-day estrous cycle and additionally at 14.00 18.00 and 22.00 on diestrus-2 and noon 14.00 16.00 18.00 and 20.00 on proestrus. After each bleeding LRF (50 ng) was injected and blood collected 10 60 and 120 minutes postinjection. Blood FSH response to LRF was minimal. Serum LH showed both maximal and prolonged response on proestrus; the response on the other days was acute and of short duration and the order of magnitude was estrus greater than diestrus-1 greater than diestrus-2. Pituitary and hypothalamic cytosol receptor populations measured by protamine precipitation and Scatchard plot analysis showed a significant decrease starting on proestrus followed by an increase on estrus. This pattern correlates well with plasma estradiol levels found at this time in accordance with the current concept of receptor depletion by nuclear translocation followed by subsequent cytoplasmic receptor resynthesis. The changes in pituitary receptor population correlating with its sensitivity to LRF indicates further a direct effect of estradiol on the pituitary. This finding has been confirmed by us using an isolated pituitary preparation. (FULL TEXT)