The endocrine control of calcium metabolism in fish is performed by hyper- and hypocalcaemic hormones as in terrestrial vertebrates. However, the hormones involved in calcium regulation in fish, which lack parathyroid glands, differ from those in terrestrial vertebrates. The pituitary is important in hypercalcaemic regulation in fish; prolactin exerts a hypercalcaemic action in addition to its well- established hypernatraemic effect. However, alternation of plasma calcium concentration may not be the primary factor influencing prolactin secretion; changes in osmolality or sodium levels seem to be more critical for the regulation of prolactin release. Somatolactin, a putative pituitary hormone related structurally to both growth hormone and prolactin, is another possible factor responsible for hypercalcaemic regulation in fish. Exposure of rainbow trout to high-calcium environments reduces the activity of somatolactin-producing cells located in the hypophysial pars intermedia. Conversely, an increased activity of somatolactin cells is observed in low-calcium environments. Somatolactin has also been implicated in fat metabolism from comparisons of normal rainbow trout with a blue-coloured variant, which lacks most of the somatolactin cells normally present in the pituitary. Diverse functions for somatolactin, in maturation, in the stress response, in acid-base regulation and in background adaptation, have also been proposed. More detailed studies are needed to define the function of somatolactin.

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