INVITRO GOLDFISH GROWTH-HORMONE RESPONSES TO GONADOTROPIN-RELEASING-HORMONE - POSSIBLE ROLES OF EXTRACELLULAR CALCIUM AND ARACHIDONIC-ACID METABOLISM

Two hours of incubation of primary static cultures of dispersed goldfish pituitary cells with 0.01 nM to 1 μM [Trp7,Leu8]-gonadotropin-releasing hormone (sGnRH) increased growth hormone (GH) secretion in a dose-dependent manner with an ED50 estimate of 0.13 ± 0.04 nM. Addition of calcium ionophores, 1 to 100 μM A23187 and 5 to 100 μM ionomycin, significantly elevated GH release with ED50s of 0.84 ± 0.38 and 4.34 ± 1.02 μM, respectively. Replacement of normal calcium-containing media with calcium-deficient media (prepared without the addition of calcium salts) significantly depressed basal GH secretion, attenuated the A23187- and ionomycin-stimulated GH release, and completely abolished the GH response to sGnRH. Arachidonic acid (AA) at 1 to 50 μM also enhanced GH secretion with an ED50 of 4.72 ± 1.52 μM. Coincubation with 1 and 10 μM of a lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA), 10 μM of the cyclooxygenase inhibitor, indomethacin, and 10 μM of eicosatetraynoic acid, an enzyme blocker with mixed activities on both the lipoxygenase and cyclooxygenase pathways, did not alter basal, AA-, and sGnRH-induced GH release. However, at 100 μM concentration, NDGA increased AA- and sGnRH-stimulated, as well as basal GH, responses. These results confirm the direct stimulatory action of GnRH on goldfish somatotropes and indicate the importance of extracellular calcium in mediating basal and GnRH-induced GH responses. Although AA stimulates GH secretion, its lipoxygenase and cyclooxygenase metabolites probably do not mediate sGnRH action on somatotropes. © 1990.