Effects of calmodulin antagonists on hormone release and cyclic AMP levels in GH₃pituitary cells

In GH, cells the calmodulin antagonists trifluoperazine and N-(6–aminohexyl)-I;-chloro-1–napthalene sulphonamide hydrochloride (W-7) showed a dose-dependent, biphasic effect on the release of growth hormone (GH) and prolactin (PRL). Hormone release was inhibited with 15–30, μm trifluoperazine and with 30–80, μM W-7, while stimulation was observed with 50–100, μM trifluoperazine and with 150 μM W-7. Trifluoperazine (≥ 30 μM) and W-7 (≥ 80 μM) increased the concentration of cellular cyclic AMP. Sulphoxides of trifluoperazine and chlorpromazine (≥ 150 μM) were without effect on hormone release and cellular cyclic AMP. Hydrolysis of cyclic AMP by GH, cytosol was reduced after incubation of intact GH3 cells with trifluoperazine (1540 μM). When trifluoperazine was incubated with cytosol, both the high and low affinity forms of cyclic AMP phosphodiesterase were inhibited competitively with calculated K1 of 4.5 and 56 μM, respectively. Stimulation of cyclic AMP phosphodiesterase caused by endogenous calmodulin was blocked by trifluoperazine. Particulate bound adenylyl cyclase activity was inhibited by trifluoperazine, and this effect was counteracted by endogenous calmodulin.