Regulation of calcium and magnesium handling in fishes

Abstract

Divalent ions, such as Ca2+ and Mg2+ are carefully regulated in cells and body fluids, but also have wide influence on other processes in osmoregulation. For example, Ca2+ alters membrane permeability and electrical properties of epithelia, as well as the control of some ion transporters. Intracellular free Ca2+ levels are submicromolar, and extracellular fluid Ca2+ concentrations in fish are around 1.25 mM. Calcium influx, and the large inward gradient for Ca2+ must be carefully controlled, while export of Ca2+ from cells is an up-hill, energy-requiring process. The control of Ca2+ homeostasis can be considered in terms of hypercalcemic regulation and hypocalcemic regulation, dealing with control above and below the normal set point. This chapter reviews the roles of the hormones involved in hypercalcemic regulation including, prolactin, parathyroid hormone related protein (PTHrP), parathyroid hormone (PTH), and calcitriol. The roles of stanniocalcin and calcitonin in hypocalcemic regulation are also described. Knowledge gaps are identified, and potential new feedback loops in the endocrine control of Ca2+ regulation are discussed.