Effects of external Mg2+ and Ca2+ on branchial osmotic water permeability and prolactin secretion in the teleost fish Sarotherodon mossambicus.

In Sarotherodon mossambicus prolactin cell activity is related to ambient Ca2+ levels, and prolactin has hypercalcemic activity in this species. To study whether prolactin has a direct action on calcium metabolism, or whether prolactin's relationship with calcium is indirect and connected with control of gill permeability, the effects of external Ca2+ and Mg2+ on prolactin secretion and gill permeability were compared. It appeared that high external Mg2+ was associated with reduced prolactin secretion, even though high Mg2+ resulted in a marked hypocalcemia. Exposure of fish to high Ca2+ levels led to hypercalcemia. Both high Mg2+ and high Ca2+ concentrations in the ambient water reduced the osmotic water permeability of the gills. These results represent further evidence that prolactin secretion in S. mossambicus may be affected by any external factor that interferes with branchial permeability. It is concluded that prolactin's main function in this species is connected with control of branchial permeability rather than calcium metabolism, although internal calcium may be implicated in permeability control.