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The effect of ionophore A23187 on the ability of dopamine and somatostatin to inhibit forskolin and TPA-stimulated prolactin and growth hormone release from sheep anterior pituitary cells

, : The effect of ionophore A23187 on the ability of dopamine and somatostatin to inhibit forskolin and TPA-stimulated prolactin and growth hormone release from sheep anterior pituitary cells. Journal Of Endocrinology 121(3 Suppl)

Ionophore A23187-stimulated growth hormone and prolactin release from sheep anterior pituitary cells is inhibited by somatostatin and dopamine, respectively. This effect is mediated by one or more pertussis toxin-sensitive G proteins (Boyd and Wallis, 1988a), indicating that both dopamine and somatostatin may have calcium-independent actions. TPA and forskolin-stimulated growth hormone and prolactin release are also inhibited by dopamine and somatostatin, via pertussis toxin-sensitive G protein(s) (Boyd and Wallis, 1988b). In order to investigate the role of Ca2+ in the inhibitory actions of dopamine and somatostatin, A23187 was used to elevate intracellular [Ca2+] in the presence of TPA and forskolin, and the actions of dopamine and somatostatin were investigated. Ovine pituitary cells were prepared and maintained in culture as described previously (Ray and Wallis, 1982). The ability of somatostatin (100 nM) to inhibit forskolin (10 uM) and TPA (100 nM)-stimulated growth hormone release was not significantly affected by 1 uM ionophore. At 10 uM the ionophore blocked the action of somatostatin to a small but significant extent. Dopamine (100 nM) inhibited stimulatory effects of forskolin (10 uM) and TPA (100 nM) on prolactin release and this was not significantly affected by 1 µ ionophore. 10 µ ionophore blocked the inhibitory action of dopamine on forskolinstimulated prolactin release by 50% and in some experiments partially blocked the inhibition of TPA-stimulated prolactin release due to dopamine. In conclusion, only at high ionophore concentration was a partial reversal of the inhibitory effects of somatostatin and dopamine seen. This confirms previous suggestions that dopamine and somatostatin may have Ca2±independent actions.


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