The intracellular regulation of prolactin cell function in the rainbow trout, Salmo gairdneri

The mechanisms for regulation of prolactin (PRL) secretion in the rainbow trout were investigated. The inhibitory action of dopamine on PRL release in vitro was enhanced by GTP and dopamine also reduced pituitary cAMP content. Forskolin increased both PRL release and cAMP content in vitro, but this effect was prevented by dopamine and did not occur in Ca2+-free medium. The cAMP analogue, dbcAMP increased PRL synthesis in low Ca2+ medium, though release was not significantly affected. The calcium ionophore, A23187, increased PRL release, but this effect was not seen with flunarizine, a voltage-dependent Ca2+-channel blocker. The calmodulin blocker, pimozide, increased PRL synthesis and pituitary PRL content in vivo and a second calmodulin blocker, trifluoperazine, also increased PRL synthesis, though not percentage release, in vitro. Both drugs elevated pituitary cAMP levels. These results indicate an involvement of agonist-dependent Gi proteins, Ca2+, calmodulin, and cAMP in the control of PRL cells in this teleost.