Role of Arachidonic Acid in the Regulation of Adrenocorticotropin Release from Rat Anterior Pituitary Cell Cultures

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Abstract
In addition to cAMP-dependent mechanisms, stimulation of pituitary ACTH secretion by various stimuli, including CRF, may involve phospholipid and arachidonic acid turnover. To determine the role of phospholipase A2 activation in corticotroph function, we studied the effect of exogenous arachidonic acid, phospholipase A2, and the phospholipase A2 activator melittin on ACTH release in cultured rat anterior pituitary cells. Incubation with 1-100 .mu.M arachidonic acid, 0.01-1 .mu.M melittin, 0.1-10 \ml phospholipase A2, and 0.1-10 nM CRF caused dose-dependent increases in ACTH release to 8.1 .+-. 1.1- (.+-.SE), 16.2 .apprx. 0.9-, 13.6 .+-. 12-, and 2.9 .+-. 0.3-fold; respectively. The participation of the major pathways of arachidonic acid metabolism in the control of ACTH release was analyzed in cells treated with nordihydroguaiaretic acid, a lipoxygenase inhibitor; indomethacin, a cycloxygenase inhibitor; and 5,8,11,14-eicosatetraynoic acid, an inhibitor of both pathways. The effects of arachidonic acid, melittin, and CRF were partially blocked by 10 .mu.M nordihydroguaiaretic acid and 5,8,11,14-eicosatetraynoic acid, but were significantly enhanced by 10 .mu.M indomethacin. These results suggest that arachidonic acid is mainly metabolized through the lipoxygenases pathway to a stimulatory metabolite and, to a lesser extent, through the cycloxygenase pathway to an inhibitory metabolite. Arachidonic acid release from anterior pituitary cells labeled with [3H]arachidonic was analyzed during cell column perifusion and stimulation by CRF and other secretagogues. Two-minute pulses of CRF (10-nM), vasopressin (10 nM) and phorbol 12-myristate 13-acetate (100 nM) caused immediate 1.5- to 2-fold increases in [3H] arachidonic acid release, and melittin (100 nM) caused a 5-fold increase in [3H] arachidonic acid release. The ability of both exogenously added and endogenously generated arachidonic acid to stimulate ACTH secretion, together with the stimulation of arachidonic acid release by ACTH secretagogues and the attenuation of stimulated ACTH release by lipoxygenase blockers, indicate that lipoxygenase products of arachidonic acid metabolism participate in the control of ACTH secretion.

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