Role of arachidonic acid and calmodulin in mediating dopamine D1- and GnRH-stimulated growth hormone release in goldfish pituitary cells

Abstract

In goldfish, growth hormone (GH) release is stimulated by dopamine via D1 receptors and cAMP-dependent mechanisms and by gonadotropin-releasing hormone (GnRH) through a protein kinase C (PKC) pathway; in addition, both D1 and GnRH actions require extracellular Ca2+. In this study, the involvement of arachidonic acid (AA) and calmodulin (CaM) in mediating the GH responses to D1 and GnRH stimulation was examined using primary cultures of dispersed goldfish pituitary cells. In 2-hr static incubation experiments, the phospholipase A2 inhibitor bromophenacylbromide (BPB; 50 μM) decreased the GH responses to the D1 agonist SKF38393 (1 μM), the adenylate cyclase activator forskolin (10 μM), and the cAMP analog 8Br-cAMP (1 mM), but not the responses to salmon (s)GnRH (100 nM), chicken (c)GnRH-II (100 nM), and AA (50 μM). Similarly, the phospholipase A2 inhibitor quinacrine (50 μM) and an inhibitor of AA metabolism, nordihydroguaiaretic acid (NDGA; 50 μM), reduced the GH responses to SKF38393, forskolin, and 8Br-cAMP. The response to the dopamine agonist apomorphine (1 μM) was also decreased by NDGA. The GH responses to AA did not add to those of forskolin or SKF38393, but were additive to responses to sGnRH and the PKC activator tetradecanoyl phorbol acetate (TPA; 100 nM). In perifusion experiments, treatment with BPB reduced the acute GH response to 1 μM SKF38393, 10 μM forskolin, or 1 mM 8Br-cAMP. Taken together, these results suggest that mobilization and metabolism of AA mediate both acute and prolonged GH responses to D1, but not GnRH. The involvement of AA probably occurs distal to D1-induced cAMP generation. Two- hour static incubation with 10 nM to 10 μM KN62, a CaM-dependent kinase II inhibitor, decreased the GH response to 100 nM sGnRH or cGnRH-II. KN62 (1 μM) similarly decreased the GH response to 1 μM SKF38393, 10 μM forskolin, 1 mM 8Br-cAMP, or 100 nM TPA. In perifusion studies, KN62 (1 μM) also reduced the acute GH response to 5-min pulses of 100 nM sGnRH, 100 nM cGnRH- II, or 1 μM SKF38393. These results indicate that CaM mediates the acute, as well as the prolonged, GH responses to GnRH and dopamine. The involvement of CaM likely occurs distal to cAMP and PKC.