CA2+ AND MG2+ TRANSPORT IN GILLS AND GUT OF TILAPIA, OREOCHROMIS-MOSSAMBICUS - A REVIEW

The euryhaline tilapia Oreochromis mossambicus kept in fresh water takes up calcium mainly from the water via the gills, like other freshwater fish. In the gills specialized mitochondria-rich cells, the chloride cells, are thought to mediate a transcellular Ca2+ transport. Second messenger operated calcium channels (SMOCs) in the apical membrane, regulated by the hormone stanniocalcin, allow minute-to-minute control over the entry of Ca2+. In the basolateral plasma membranes of these cells, an ATP-consuming Ca2+ transporting enzyme provides the major driving force for extrusion of Ca2+ into the blood; in addition, an Na+/Ca2+ exchanger is present in these membranes. The kinetics of the exchanger in vitro indicate that this extrusion mechanism dominates when intracellular calcium levels reach micromolar levels. In the gills, the transport of calcium appears primarily ATPase mediated and therefore largely independent of the Na+ status of the transporting cell. In enterocytes, similar mechanisms for transcellular transport of Ca2+ exist. However, in the intestinal epithelium the extrusion of Ca2+ is primarily via the Na+/Ca2+ exchanger and to a very limited extent mediated via the Ca2+-ATPase. Indeed, calcium transport over the intestinal epithelium is dependent on the Na+-status and the Na+/K+-ATPase activity of the epithelium. Prolactin and cortisol are endocrine factors determining the relative densities of calcium pumps in basolateral plasma membranes. At least 80% of the magnesium required for growth and homeostasis is absorbed from the food via the intestine. Magnesium is transported transcellularly and actively via enterocytes. The movement of Mg2+ over the apical membrane is passive, down an electrochemical gradient. The cytosolic Mg2+ concentration is kept well below its equilibrium concentration. The extrusion over the basolateral plasma membrane is mediated by an ATP-consuming enzyme. The gills contribute less than 20% to magnesium uptake, but up to 50% in tilapia fed a magnesium deficient diet. Evidence is accruing that prolactin is involved in the adaptation to low magnesium diets.