Homologous Recombinant Growth Hormone and Calcium Metabolism in the Tilapia, Oreochromis Mossambicus, Adapted to Fresh Water

Homologous recombinant tilapia growth hormone (rtGH) was tested for its effects on calcium metabolism in freshwater tilapia Oreochromis mossambicus . Fish were fed an optimal ration of 5% of their body mass per day. A positive correlation was found between the amount of food given and the branchial calcium influx. In male tilapia, the mean calcium influxes were 5.80 and 11.71 mmol h21 100 g21 when they were fed 2 % and 5% food, respectively. In female fish fed 5% food, the calcium influx was 6.20 mmol h21 100 g21. Calcium influx via the gills was not affected by rtGH. However, in rtGH-treated fish, the net efflux of calcium was lower than in the controls. Apparently, the calcium taken up from the water was more efficiently stored in the body. GH increased the hepatosomatic index and had mild growth-promoting effects (mass and length increases); it increased the total body calcium pool without affecting bone or scale calcium density. The chloride cell density in the opercular epithelium almost doubled after GH treatment. GH did not influence plasma ion composition. Plasma cortisol levels were lower in rtGH-treated fish. A comparison of the roles of GH and prolactin (the products of the prolactin gene family) in calcium regulation of the tilapia led us to conclude that GH has specific calcitropic effects on freshwater tilapia that differ from those of prolactin.