Dopamine inhibits maitotoxin-stimulated pituitary 45Ca2+ efflux and prolactin release

We examined the hypothesis that dopaminergic inhibition of prolactin release is coupled to modulation of cellular calcium flux. Dispersed female rat pituitary cells were prelabeled in 45Ca2+ and perifused to determine simultaneously fractional calcium efflux and prolactin release, as stimulated by maitotoxin, a calcium channel activator. The integrated response of each parameter to 5 ng/ml maitotoxin was obtained in individual perifusion columns in the absence or presence of various concentrations of dopamine. Maitotoxin-stimulated calcium efflux was suppressed by dopamine concentrations of 0.01 microM and greater (P less than 0.01) and achieved a maximal effect at approximately 0.1 microM, at which calcium efflux was reduced by 50%. Maitotoxin-stimulated prolactin release was inhibited by 0.03 microM dopamine and greater concentrations (P less than 0.01), and at a concentration of approximately 10.0 microM dopamine the effect became maximal at approximately 85% suppression. Haloperidol (0.1 microM) blocked the effects of 0.1 microM dopamine on both parameters. Simultaneous suppression of maitotoxin-stimulated calcium efflux and prolactin release by concentrations of dopamine within the nanomolar range suggests that dopamine receptor activation is negatively coupled to modulation of calcium flux in the physiological regulation of prolactin secretion.