The relationship between prolactin cell activity and plasma calcium, sodium, and osmolarity was studied in stanniectomized sticklebacks. In freshwater fish removal of the Stannius bodies led to a threefold increase of plasma calcium and a slight reduction of plasma sodium, while osmolarity was unaffected. Under these conditions the prolactin cells atrophied. It is concluded that plasma calcium, and not sodium or osmolarity as is generally assumed, is inversely related to prolactin cell activity. In seawater fish, normally characterized by low prolactin secretion, stanniectomy induced the same changes in plasma composition, but the prolactin cells remained unaltered. If seawater fish were transferred to fresh water, the expected activation of the cells occurred in the controls only. The lack of response in the stanniectomized fish is attributed to the high internal calcium level. Adaptation of freshwater fish to low-calcium or calcium-enriched fresh water showed that in the controls prolactin cell activity was inversely related to internal as well as external calcium concentration. In the stanniectomized fish in low-calcium fresh water prolactin cell activity was higher than expected on the basis of their high internal calcium concentration. This discrepancy indicates that internal and external calcium concentrations influence prolactin secretion by different mechanisms. © 1978.
