Activation of somatolactin cells in the pituitary of the rainbow trout Oncorhynchus mykiss by low environmental calcium.

The effects of changes in environmental calcium on the activity of hypophysial somatolactin (SL) cells were examined in the rainbow trout. In fish 10 and 21 days after transfer from fresh water (FW) to FW containing 10 mM calcium (Ca-rich FW) or to 80% seawater (80% SW) containing 8 mM calcium, sectional nuclear areas of SL cells were reduced. The levels of SL-mRNA, determined by in situ hybridization, were lower 10 days after transfer to Ca-rich FW. When the fish were transferred from Ca-rich FW to FW, nuclear areas of SL cells increased along the significant increases in SL-mRNA and plasma SL levels. The increased activity of SL cells in the fish in low calcium environment suggests a hypercalcemic action as one of the functions of SL. On the other hand, a significant reduction in prolactin cell activity occurred after transfer to 80% SW, but not after transfer to Ca-rich FW. Growth hormone cells were activated 21 days after acclimation to 80% SW, in accord with its described SW-adapting action in salmonids.