Arachidonate Metabolism in the Anterior Pituitary: Effect of Arachidonate Inhibitors on Basal and Stimulated Secretion of Prolactin, Growth Hormone and Luteinizing Hormone. II. Hormone Release from Dispersed Pituitary Cells
Authors: Bihoreau Claire , Rasolonjanahary Ramahefarizo , Gerozissis Kyriaki , Clauser Hubert , Kordon Claude 
Published: September 2006

In the accompanying study, we reported the effects of inhibitors of arachidonic acid metabolism on the regulation of prolactin, growth hormone (GH) and luteinizing hormone secretion by male hemipituitaries. The present work extends these investigations to primary cell cultures of the same origin. Arachidonic acid metabolism was inhibited by either 5, 8, 11, 14‐eicosatetraynoic acid (ETYA), a blocker of cyclooxygenase‐ and lipoxygenase‐catalysed pathways, or the cyclooxygenase inhibitors, indomethacin and aspirin.

ETYA inhibited basal GH secretion by 60%, an effect which was reversed by micromolar concentrations of exogenous arachidonic acid. ETYA was much less effective on growth hormone‐releasing factor‐induced GH release, a result which contrasts with data obtained on intact glands. Growth hormone‐releasing factor stimulation of adenylate cyclase was not affected by ETYA. Cyclooxygenase inhibitors decreased basal secretion to a more limited extent (−30%) and were ineffective on growth hormone‐releasing factor‐stimulated release.

Basal prolactin secretion was reduced by 30% in the presence of ETYA and unaffected by cyclooxygenase inhibitors. As with GH, the effect was reversed by exogenous arachidonic acid. However, in contrast to growth hormone‐releasing factor‐stimulated GH secretion, thyrotropin‐releasing hormone stimulation of prolactin release was able to overcome the inhibition by ETYA in a dose‐dependent manner. Again, the insensitivity of thyrotropin‐releasing hormone‐stimulated prolactin release to ETYA contrasts with the data obtained in intact tissue. Moreover, ETYA inhibited (−60%) prostaglandin E2 production; thyrotropin‐releasing hormone was unable to increase the prostaglandin levels in control or ETYA‐treated cells. This confirms the data obtained with cyclooxygenase inhibitors, suggesting that prostaglandins are not involved in prolactin secretion.

Intracellular accumulation of Ca2+ by the ionophore A23187 and protein kinase C stimulation by the phorbol ester 12‐O‐ tetradecanoyl phorbol acetate (TPA), strongly stimulated GH and prolactin release. Under these conditions, ETYA was no longer able to inhibit secretion of the hormones.

As with intact glands, basal and gonadotropin‐releasing hormone or TPA‐induced luteinizing hormone secretion were unaffected by any of the inhibitors used.

It is concluded that blockade of the arachidonic acid cascade interferes with a secretory pathway involved mainly with basal release of prolactin and GH, but not luteinizing hormone. Thyrotropin‐releasing hormone, a secretagogue known to trigger phospholipase C and, hence, to stimulate Ca2+ mobilization and protein kinase C, overcame ETYA inhibition of prolactin secretion. Growth hormone‐releasing factor, a secretagogue recognized by adenylate cyclase coupled receptors, did not overcome ETYA inhibition of GH secretion. However, both secretagogues strongly stimulated hormone release from their target cells in the presence of ETYA. The arachidonic acid cascade thus seems less important in neuromediator‐induced secretion coupling processes in dispersed pituitary cells, than in the intact gland. These observations suggest that eicosanoids are more likely to mediate paracrine or autocrine modulations of secretory mechanisms, rather than to function as intracellular messengers.