Abstract
In freshwater-acclimated American eels (Anguilla rostrata LeSueur), ovine prolactin and grafts of the part of the pituitary gland containing the prolactin cells induced hypercalcemia. The hypercalcemia was associated with increased uptake of calcium from the water (resulting from increased influx and decreased efflux) and with enhanced high-affinity Ca2+-adenosinetriphosphatase (ATPase) activity in the gills, the putative biochemical correlate of the branchial Ca2+ pump. Kinetic analyses of ATPase-mediated Ca2+ transport in plasma membrane vesicles of branchial epithelium provided evidence that prolactin enhanced the maximum velocity of the Ca2+ pump. Prolactin treatments raised plasma cortisol levels slightly but significantly in eels. However, cortisol per se was not hypercalcemic in eels and did not stimulate the branchial Ca2+ pump. We conclude that the hypercalcemic potency of prolactin in fish relates to its stimulatory action on active Ca2+ transport in the gills.
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