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 plaques 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 that 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 the abundance of small plaque formers. The results indicate that blocking the HVA Ca channels preferentially suppresses PRL release from LP lactotropes, and suggest that the inhibited PRL secretors tend to behave functionally as SP lactotropes.