THE ACTIVITY OF AN LH-RH-DEGRADING ENZYME IN THE ANTERIOR PITUITARY DURING THE RAT OESTRUS CYCLE AND ITS ALTERATION BY INJECTIONS OF SEX HORMONES

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Abstract
The basal activity of an LH-RH[luteinizing hormone-releasing hormone]-degrading enzyme system, L-cystine arylamidase, was determined in the pituitary gland of female rats at 4 h intervals throughout the 4-day estrus cycle. The activity of the enzyme was fluctuating during the 4 stages of the cycle in a circadian rhythm with the highest values occurring at night and the lowest values at noon. The maximal activity for the whole cycle was measured at 04.00 h on the day of metestrus, and the minimal activity between 12.00 h and 16.00 h on the day of proestrus. The i.v. injection of various doses of estradiol caused only a slight change in enzyme activity of the pituitary during estrus, but considerable increases were observed during the other 3 stages. The stimulation of enzyme activity by progesterone was much more pronounced during diestrus and proestrus as compared with metestrus and estrus. Similarly, the reaction to the injection with LH and prostaglandin E2, respectively, were not very pronounced during estrus and metestrus, whereas the enzyme activity rose by +50% and +100%, respectively, during diestrus and proestrus. The LH-RH-degrading enzyme system in the pituitary seems to be involved in the control of the tonic LH release, and in the maintenance and regulation of the sensitivity of the gonadotrophs to alterations in LH-RH release from the median eminence.