LOW CONCENTRATIONS OF DOPAMINE INCREASE CYTOSOLIC CALCIUM IN LACTOTROPHS

Dopamine (DA) plays a dual role in the neuroendocrine regulation of PRL secretion from the anterior pituitary gland. DA normally exerts a tonic inhibitory effect on PRL secretion. However, DA at concentrations much lower than those required for maximal inhibition of PRL secretion actually stimulates PRL secretion. In this study, the effects of stimulatory concentrations of DA on common second messenger systems were examined in pituitary cells enriched for lactotrophs in an attempt to identify the signal transduction mechanisms used in DAergic stimulation of PRL secretion. Rat pituitary cells were enriched for lactotrophs (88% lactotrophs) by differential sedimentation on a discontinuous Percoll gradient. Enriched cells in monolayer culture were responsive to both inhibitory and stimulatory doses of DA; 10 muM DA inhibited PRL secretion by 80%, and 0.1 nM DA stimulated PRL secretion by 40% after a 4-h incubation. No dose of DA (10 pM to 10 muM) had an effect on phosphatidylinositol turnover. However, 1 and 10 muM DA inhibited cAMP formation by 58% and 72%, respectively. DA at doses in the stimulatory range had no effect on cAMP formation. Single cell cytosolic calcium concentrations ([Ca2+]i) were examined by quantitative fluorescence microscopy using fura-2 as a probe. Inhibitory doses of DA (1 muM) decreased [Ca2+]i in 83% of the cells examined. Two subpopulations of cells were noted that varied in the degree of response to 1 muM DA. One subpopulation responded to DA by decreasing [Ca2+]i 50-100 nM. The other subpopulation responded to DA by decreasing [Ca2+]i 300-400 nM. A stimulatory dose of DA (0.1 nM) increased [Ca2+]i in 44% of the cells examined. This dose of DA increased [Ca2+]i by 100-400 nM. These data indicate that the second messenger mediating DAergic stimulation of PRL secretion is most likely Ca2+.