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 it s effects on calcium metabolism in freshwater tilapia Oreochromis moss ambicus. Fish were fed an optimal ration of 5 % of their body mass per day. A positive correlation was found between the amount of food give n and the branchial calcium influx. In male tilapia, the mean calcium influxes were 5.80 and 11.71 mumol h-1 100 g-1 when they were fed 2 % and 5 % food, respectively. In female fish fed 5 % food, the calcium i nflux was 6.20 mumol h-1 100 g-1. Calcium influx via the gills was not affected by rtGH. However, in rtGH-treated fish, the net efflux of ca lcium 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 an d 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 i n rtGH-treated fish. A comparison of the roles of GH and prolactin (th e 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.