Evidence in support of a chemiosmotic mechanism for stimulated prolactin release

In order to assess the involvement of a chemiosmotic mechanism in exocytotic prolactin (PRL) release, dispersed anterior pituitary cells from female rats were used. After primary culture they were incubated in media containing bicarbonate or isethionate ions, probenecid and carbonylcyanide p-trifluoromethoxyphenyl-hydrazone (CCTP). Cells incubated without these additives or stimulated with dibutyryladenosine 3':5'-cyclic monophosphate served as controls. Further, a crude secretory granule suspension was used to study the effects of elevated osmotic pressures. The inhibition of anion transport with probenecid always significantly reduced PRL secretion, whereas bicarbonate ions had the opposite effect. The proton ionophore CCTP only inhibited stimulated release, and the substitution of chloride ions by isethionate was ineffective. Finally, it could be observed that elevated osmotic pressures reduced the lysis of isolated secretory granules when bicarbonate ions were present in the incubation medium. The results indicate that a mechanism requiring protons and bicarbonate ions is involved in stimulated PRL release and that a different mechanism, independent from proton gradients, should be considered for basal PRL secretion.