ABSENCE OF COUPLING BETWEEN D-2 DOPAMINE-RECEPTORS AND CALCIUM CHANNELS IN LACTOTROPHS FROM CYCLING FEMALE RATS

Recent evidence suggests that an important mechanism underlying the in hibition of PRL secretion by dopamine in the anterior pituitary is a d irect inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled po tassium channels by D-2 receptor activation, subsequent hyperpolarizat ion of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch volt age clamp methods, we have reexamined the effect of D-2 receptor activ ation on calcium currents (I-Ca) in pituitary cells from normal cyclin g female rats and in GH4C1 pituitary tumor cells expressing cloned D-2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 mu M) failed to signif icantly inhibit I-Ca in either GH4C1 cells or normal female lactotroph s. Similarly, the D-2 agonist quinpirole (20-100 mu M) did not reduce I-Ca in lactotrophs. No responses to D-2 agonists were seen when bariu m was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracell ular macromolecules. Quinpirole also failed to inhibit I-Ca in lactotr ophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of I-Ca by activation of D-2 rece ptors. PRL secretion assessed by immunoblotting methods was dramatical ly inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calci um channels, quinpirole produced only a very modest inhibition of secr etion. We conclude that direct inhibition of I-Ca by D-2 receptor acti vation is not a major mechanism underlying the dopaminergic inhibition of PRL secretion in normal female lactotrophs.