Morphological analysis of growth hormone release from rat somatotrophs into blood vessels by immunogold electron microscopy.

The ultrastructure of GH release from the somatotrophs of the rat anterior pituitary was examined in vivo by immunogold electron microscopy. Under pentobarbital anesthesia, injection of GH-releasing factor clearly induced an increase in both plasma GH content and the number of exocytotic GH-immunopositive granules in the cells. The exocytotic events occurred from a part of the plasma membrane facing endocrine cells, including other somatotrophs, and other portions facing folliculo-stellate cells or the walls of blood vessels. When released from the plasma membrane, the GH-immunopositive secretory granules sometimes appeared to aggregate with each other and showed an irregular shape surrounded by a single unit membrane. The exocytotic secretory granules were released into the extracellular space, and then flowed into the sinusoids as irregularly shaped GH-immunopositive electron-dense masses. After reaching the vascular space via the intercellular spaces between the endothelial cells, the contents of each mass became diffusely dispersed into the blood stream, with concomitant disappearance of immunopositivity. The present study thus revealed the morphological aspects of the process of GH secretion from somatotrophs into the blood vessels.