Action of luteinizing hormone-releasing hormone: involvement of novel arachidonic acid metabolites.

Abstract

Anterior pituitary cells were incubated in the presence of luteinizing hormone-releasing hormone and one of three inhibitors of arachidonic acid metabolism: indomethacin, an inhibitor of the cyclooxygenase system; nordihydroguaiaretic acid, an antioxidant that inhibits lipoxygenase; and icosatetraynoic acid, an acetylenic analogue of arachidonic acid that blocks all known pathways of arachidonic acid metabolism. Indomethacin was ineffective in blocking luteinizing hormone-releasing hormone-stimulated luteinizing hormone secretion. Nordihydroguaiaretic acid was only marginally capable of inhibiting luteinizing hormone-releasing hormone-stimulated luteinizing hormone secretion. Icosatetraynoic acid at 10 pM completely inhibited stimulated luteinizing hormone secretion. Addition of several epoxygenated arachidonic acid metabolites to cells in vitro resulted in secretion of luteinizing hormone equal to or greater than that induced by 10 nM luteinizing hormone-releasing hormone. The half-maximal effective dose for these compounds was approximately 50 nM. The 5,6-epoxyicosatrienoic acid was the most potent of the compounds tested. These studies suggest that luteinizing hormone-releasing hormone-stimulated luteinizing hormone release is closely coupled with the production of oxidized arachidonic acid metabolites. Moreover, one or more of the epoxygenated arachidonic acid metabolites might be a component of the cascade of reactions initiated by luteinizing hormone-releasing hormone that ultimately results in secretion of luteinizing hormone.Calcium ions, cyclic nucleotides, and combinations thereof are postulated to be the "second messenger" of luteinizing hormone-releasing hormone (LHRH) (1, 2). The action of LHRH in stimulating the release of luteinizing hormone (LH) from anterior pituitary gonadotrophs also appears to involve turnover of phosphatidylinositol (3) and release of arachidonic acid from membrane phospholipid stores (4). Previous studies have eliminated a role for prostaglandins in the action of LHRH because LHRH-stimulated LH secretion is insensitive to blockage of prostaglandin formation by indomethacin (5). At least two additional pathways of arachidonate metabolism are currently recognized: a lipoxygenase reaction leading to hydroperoxides and leukotrienes (6) and the recently reported NADPH-supported cytochrome P450-dependent epoxygenase pathway (7-9). Since arachidonic acid mobilization from cellular pools is apparently required for LHRH-stimulated LH secretion (4), we presumed that either arachidonic acid or one or more of its metabolites of the lipoxygenase or epoxygenase pathways were active in stimulated secretion of LH. The studies reported here suggest that conversion of arachidonic acid to physiologically active metabolites constitutes an essential link in the cascade of intracellular events leading to LHRH-stimulated LH release. 99.9% by gas chromatographic analysis) was added in some cultures to a final concentration of 1 ,uM. The synthetic epoxyicosatrienoic acids (EETs) were pre...

Effect of ethanol on cytochrome P450 in the rat brain.

“…the evidence that prostaglandins are the mediators of some of the effects of ethanol on the brain (34,35), increase in a pathway for the elimination of prostaglandins may influence the sensitivity of the brain to ethanol. Another pathway that could be influenced by an increase in the level of members of the P450 4A family in the brain is the release of peptides by dopamine (36). This release is thought to be mediated by epoxides of arachidonic acid that are formed through the action of P450.…”

“…The role of P450 2C7 has not been investigated nor is there any information on the levels of retinoic acid in the brain after ethanol consumption. Another catalytic activity of some members ofthe P450 2C family that may be relevant in the brain is their arachidonic acid epoxygenase activity (42), which suggests a role in intracellular signaling (36).…”

An epoxygenase metabolite of arachidonic acid mediates angiotensin II-induced rises in cytosolic calcium in rabbit proximal tubule epithelial cells.

“…It may be the major one in the proximal tubule as this area is deficient in cyclooxygenase and lipoxygenase (12)(13)(14). Interest-ingly, 5,6-EET increased [Ca2+]i in pituitary and parotid cells (15,16) and was implicated to mediate secretory responses (17)(18)(19)(20)(21). The mechanism whereby 5,6-EET increases [Ca2i] has not been investigated; other eicosanoids, such as PGF2,, PGE2, 5-hydroxyeicosatetraenoic acid (5-HETE), and leukotriene B4, are known to induce rises in [Ca2+i primarily from intracellular stores through IP3-dependent mechanisms and, to a lesser extent, through influx of extracellular Ca (22)(23)(24)(25)(26) …”

Luteinizing hormone‐releasing hormone (LHRH)‐induced arachidonic acid release in rat granulosa cells Role of calcium and protein kinase C

“…In anterior pituitary cells, recent studies have suggested that LHRH-stimulated LH release may be associated with the production of oxidized arachidonic acid metabolites [2,13,27]. One or more of the epoxygenated and/or lipoxygenated metabolites of arachidonic acid might be a component of the cascade of reactions initiated by LHRH that ultimately results in secretion of LH.…”

“…Increase in inositol lipid metabolism has been observed within minutes following the exposure of anterior pituitary cells [2,3] or ovarian cells [4- Therefore, the present study was designed to determine whether arachidonic acid release in granulosa cells could be affected by a calcium ionophore, A23187, which alters the intracellular concentrations of Ca*+, as well as by a known activator of protein kinase C, TPA.…”