Electron-microscopic cytochemical studies on the secretory process in rat prolactin cells in primary culture.

Three aspects of the secretory process in male rat prolactin (PRL) cells grown in primary cultures for 7--14 days have been investigated by cytochemical methods. The subcellular localization of prolactin has been studied using preembedding or postembedding immunocytochemical methods after various fixatives. With the postembedding method, PRL is localized essentially in secretory granules. The maximum intensity of staining is obtained with PAF fixative. With the preembedding method, subcellular localization of the staining varies depending on the fixative. After PAF-fixation, positive staining is observed on secretory granules, ground cytoplasm, the outer face of some RER cisternae and, in a few cells, on the innermost Golgi cisternae, as well as on masses of condensing secretory material. After Ohtsuki's hypotonic fixative followed by saponin permeabilization, PRL is visualized within the totality of RER cisternae, including the perinuclear cisternae and the peripheral saccules on the cis-Golgi face. Secretory granules are unstained. Membrane traffic was investigated using the Con A-HRP indirect method as a tracer of surface saccharides. Plasma membrane, coated with Con A-HRP at 4 degrees C, is slowly internalized at 37 degrees C. This involves both randomly distributed invaginations and capping. The final step of endocytosis (1--2 hours) is located in the Golgi zone, where very few smooth membranes are stained. In contrast, a conspicuous deposit is found around the dense content of secretory granules. This suggests a recycling of internalized membrane and a transfer of Con A-HRP from the inner face of smooth cisternae to the secretory material. The internalization of Con A-HRP-coated membrane leads to an inhibition of PRL release starting after 30 minutes. This is accompanied by a marked increase of acid phosphatase activity, mostly around forming and mature secretory granules.