Differential expression of Na channels in functional subpopulations of rat lactotropes.

We have investigated the voltage-dependent Na channel activity of single lactotropes in pituitary cultures from adult male rats by recording whole cell Na+ currents under voltage clamp. Cells were identified by their ability to secrete prolactin in the basal state as measured with the reverse hemolytic plaque assay. We found that cells with elevated secretory rates [large-plaque (LP) lactotropes] present relatively large Na+ currents in response to depolarization, whereas Na+ currents in cells secreting prolactin at low rates [small-plaque (SP) lactotropes] are small or are not detectable. The maximum amplitude of the inward Na+ current, normalized by cell capacitance, is about sixfold larger, on the average, in LP lactotropes than in SP lactotropes. Complete block of the Na channels with external tetrodotoxin inhibits by approximately 72% the amount of prolactin secreted from the entire cell population over a period of 1 h, an effect that depends on a drastic reduction in the proportion of LP lactotropes. The results indicate that Na channel activity promotes basal prolactin secretion in male rat lactotropes and suggest that differences in Na channel expression contribute to explain the functional heterogeneity of these pituitary cells.