Quantitative ultrastructural evidence of alterations in prolactin secretion related to external salinity in a teleost fish (Poecilia latipinna).

Quantitative ultrastructural morphometric studies were made on the prolactin cells of Poecilia latipinna adapted to freshwater (FW), one-third seawater (1/3 SW) and full-strength seawater (SW), and at various times after transfers between 1/3 SW and FW. In fully-adapted fish the rates of prolactin (PRL) synthesis and PRL release are inversely related to environmental salinity. During adaptation to a new salinity the two rates are temporarily uncoordinated, with release increasing or decreasing more readily than synthesis. Synthesis appears to take 30 h or longer to come into balance with the increased release rate following transfer from 1/3 SW to FW, and 72 h or longer to adjust to the reduction in release rate that follows the reverse transfer. The excess PRL granules that accumulate in the latter situation appear to be removed by lysosomal digestion. As in other teleosts, in fish adapted to the external medium the size of the stored PRL granules is inversely related to external salinity, but this relationship breaks down during adaptation to a new salinity. The stellate cells which penetrate between the PRL cells are more prominent, more extensively ramified, and appear more metabolically active in FW-adapted fish than in the other groups. These cells seem to be closely related in function to the secretory activity of the PRL cells.