Sodium ions: their role and mechanism of action in the control of prolactin release.

The secretion of PRL by the pituitary gland is under a tonic inhibitory control exerted by dopamine. However, the mechanisms involved in this inhibition remain to be completely defined. We have investigated the effects of sodium removal in the incubation medium on baseline PRL, on the inhibitory effects exerted by bromocriptine, haloperidol and cobalt, and on the stimulatory action of TRH and vasoactive intestinal polypeptide (VIP) on PRL release by enzymatically dispersed rat anterior pituitary cells in primary culture. The effects of ouabain, tetrodotoxin, and veratridine on baseline PRL release and on the inhibitory effects of dopaminergic agents were also investigated. Basal PRL release was slightly but significantly reduced by replacing sodium by choline in the incubation medium and was almost completely suppressed when isoosmolar concentrations of glucose were substituted for sodium. On the other hand, ouabain slightly but significantly increased basal release of the hormone. In the absence of sodium, the dose-dependent inhibitory effect of bromocriptine was greatly reduced. Of two antidopaminergic drugs, haloperidol and l-sulpiride, only the first inhibited the release of PRL at high doses (10(-5) - 10(-4) M) in the presence of sodium but became stimulatory at 10(-4) M in the absence of the ion. Ouabain, tetrodotoxin, and veratridine failed to significantly modify the inhibitory effects of either dopamine or bromocriptine. The dose-dependent inhibitory effects of cobalt and the releasing actions of TRH and VIP were not significantly influence by sodium removal from the incubation medium, although VIP appeared to be active at a lower concentration in the absence of the ion. These data indicate that, although the presence of a normal ionic charge in the incubation medium is essential for the baseline secretion of PRL, sodium ions specifically play only a minor role in a phenomenon which is essentially Ca2+-dependent. They also clearly show that sodium intervenes in dopaminergic agonist and antagonist inhibition of PRL release through a mechanism which appears independent of either dopaminergic receptor, Ca2+ or Na+ channel, or Na+/K+ pump. It is hypothesized that a normal intracellular sodium concentration is essential for dopaminergic agonist or antagonist inhibitory action.