A permissive role for extracellular Ca 2+ in regulation of prolactin production by 1,25-dihydroxyvitamin D 3 in GH 3 pituitary cells

Abstract

A clonal strain of rat pituitary tumor cells (GH 3) that spontaneously synthesizes and secretes prolactin (PRL) and growth hormone (GH) was used as model system to study the mechanism of action of 1,25-(OH) 2D 3. We have previously demonstrated that these cells possess specific cytosol binding proteins for 1,25-(OH) 2D 3 (Haug and Gautvik, 1985). When the GH 3 cells were incubated in a serum-free, chemically denned medium of low extracellular Ca 2+ concentration, 1,25-(OH) 2D 3 stimulated PRL production in a dose-dependent manner. The stimulation was detectable at 10 −11M, and the maximum effect (2-fold increase) was observed at 10 −9M (ED 50= 2 × 10 −11M). The dose-response curve was bell-shaped, and at 10 −6M 1,25-(OH) 2D 3 even suppressed PRL production to about 75% of controls. The stimulatory effect was first seen after 2 days and was maximal after 4 days. On a molar basis 25-OHD 3 and 1-OHD 3 were at least 100 times less potent than 1,25-(OH) 2D 3, while 24,25-(OH) 2D 3had no effect on PRL production. At an extracellular concentration of Ca 2+ as low as 4 × 10 −5M the stimulatory effect of 1,25-(OH) 2D 3 was small (1,3-fold). Increasing extracullar Ca 2+ to 1.5 × 10 −4M increased the 1,25-(OH) 2D 3-induced PRL response to 2.1-fold. In contrast to the biphasic effect of 1,25-(OH) 2D 3 on PRL production, GH production was decreased to about 60% of controls at 10 −8M and above. These findings indicate that in serum-free medium the stimulatory effect of 1,25-(OH) 2D 3 on PRL production is critically dependent on the concentration of extracellular Ca 2+.