Norepinephrine regulation of growth hormone release from goldfish pituitary cells. II. Intracellular sites of action

Previous results suggest that norepinephrine decreases growth hormone (GH) release in goldfish by means of alpha-2 adrenoceptor activation. The intracellular mechanisms by which norepinephrine inhibits GH release were examined in the present study using dispersed goldfish pituitary cells. In 2-h static incubation experiments, norepinephrine and the alpha-2 agonist clonidine decreased basal GH release and the GH responses to stimulation by the dopamine D1 agonist SKF38393 and two native gonadotropin-releasing hormones (GnRH). Norepinephrine also reduced GH responses to the adenylate cyclase activator forskolin, two protein kinase C (PKC) activators (phorbol ester and synthetic diacylglycerol), and two Ca2+ ionophores (ionomycin and A23187). Similarly, norepinephrine applied as a 1-h pulse in cell column perifusion experiments reduced basal GH release and abolished the GH response to a 5-min pulse of arachidonic acid. In goldfish, D1-stimulated GH release is mediated by AC-, arachidonic acid-and Ca2+-dependent pathways, whereas GnRH action is coupled to PKC-and Ca2+-dependent mechanisms. These results suggest that norepinephrine activation of alpha-2 receptors inhibits ligand-induced GH secretion by actions subsequent to activation of these second messenger cascades. To further characterize norepinephrine mechanisms of action on unstimulated hormone release, the ability of norepinephrine and an alpha-2 agonist to affect activation of two second messenger cascades under basal conditions was also investigated. Static incubation with clonidine reduced cAMP production in a time-and dose-dependent manner, suggesting that norepinephrine inhibitory action can also be expressed at the level of cAMP production. Resting intracellular free calcium levels in single, identified goldfish somatotropes was unaffected by norepinephrine. However, the inhibitory effects of norepinephrine on basal GH secretion was not observed in the presence of a voltage-sensitive Ca2+ channel agonist. Whether these channels are targets for norepinephrine action on unstimulated GH release requires further investigation.