LACTOTROPE SUBTYPES ARE DIFFERENTIALLY RESPONSIVE TO CALCIUM-CHANNEL BLOCKERS

Pituitary cultures from adult rats contain two subtypes of prolactin ( PRL) cells, small-plaque (SP) and large-plaque (LP) lactotropes, which exhibit distinct rates of basal secretion and thereby form PRL plaque s of different sizes in reverse hemolytic plaque assay experiments. In the present study, we have used plaque assays to examine the effects of omega-conotoxin (omega-CgTx) and nifedipine, which block Ca2+ entry through high voltage-activated (HVA) channels in the plasma membrane, on basal PRL secretion from single male rat lactotropes. We found tha t omega-CgTx, like nifedipine, is a potent inhibitor of PRL secretion. In addition, we observed that both drugs decrease the number of cells forming large PRL plaques, while promoting a comparable increase in t he abundance of small plaque formers. The results indicate that blocki ng the HVA Ca channels preferentially suppresses PRL release from LP l actotropes, and suggest that the inhibited PRL secretors tend to behav e functionally as SP lactotropes.