Abstract
Exposure of freshwater fish to freshwater containing calcium and magnesium ions in concentrations found in seawater reduces prolactin cell activity to the low values characteristic for seawater fish. When applied in the same concentrations, calcium is more effective than magnesium. High sodium concentrations have only small effects. Transfer of fish to freshwater or seawater with reduced ionic calcium and magnesium levels activates the prolactin cells. In these experiments plasma ionic calcium and sodium levels were negatively correlated with prolactin cell activity, while thickness of the epidermis and density of the epidermal mucocytes were positively correlated with prolactin cell activity. It is concluded that ionic calcium concentration, and not osmolarity or sodium content, is the main environmental factor in the control of prolactin secretion in sticklebacks.
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