THE EFFECT OF PROSTAGLANDINS ON HYPOTHALAMIC L-CYSTINE ARYLAMIDASE ACTIVITY AND ON LUTEINIZING HORMONE SECRETION IN THE RAT

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Abstract
The effect of i.v. injections of several prostaglandins [PG] of L-cystine arylamidase activity in the hypothalamus of the rat was investigated. PGA1 (5 .mu.g), PGE1 (5 .mu.g), PGF2.alpha. (100 .mu.g) and PGE2 (10 .mu.g) stimulated the enzyme activity within 30 min by approximately 60 to 100%, while PGF1.alpha. was ineffective. In contrast to LH [luteinizing hormone] which stimulated hypothalamic arylamidase only in synergism with sex steroids, the enzyme activity of both intact and ovariectomized adult female rats and of immature rats was increased when the animals were treated with PGE2. LH and PGE2 at suboptimal doses were additive in their effect, whereas optimal doses failed to do so. Aspirin did not block the stimulatory action of LH on L-cystine arylamidase, indicating that this effect of LH was not mediated by the PG. L-cystine arylamidase was previously shown to inactivate LH-RH, in all probability to be involved in the short-loop feedback mechanism of LH. PG seem to act as modulators in this system since they mimic the effect of LH on this enzyme. An i.v. dose of 15 .mu.g PGE2 caused a significant increase in plasma LH within 7.5 min in intact diestrous rats, whereas in ovariectomized estrogen-progesterone treated rats between 7.5 and 30 min after the i.v. injection, the plasma LH values were significantly higher.