Effects of External Mg2+ and Ca2+ on Branchial Osmotic Water Permeability and Prolactin Secretion in the Teleost Fish

In Sarotherodon mossambicus prolactin cell activity is related to ambient C a2 + 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 o f gill permeability, the effects o f external C a2 ~ and M g2 * 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 o f fish to high Ca2+ levels led to hypercal cemia. Both high Mg2 * and high Ca2 ~ concentrations in the ambient water reduced the osmotic water permeability o f the gills. These results represent further evidence that pro lactin 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 o f branchial permeability rather than calcium metabolism, although internal calcium may be implicated in permeability control. Prolactin secretion in euryhaline fish is generally much higher in freshwater-adapte