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Effects of Low Ambient Calcium Levels on Wholebody Ca2+ Flux Rates and Internal Calcium Pools in the Freshwater Cichlid Teleost, Oreochromis Mossambicus
1 Department of Zoology II, University of Nijmegen Toernooiveld 25, 6525 ED Nijmegen, The Netherlands
2 Department of Biology, University of Ottawa, Ottawa, Ontario, Canada KIN 6N5
3 Department of Radiochemistry, Interuniversity Reactor Institute, Mekelweg 15, 2629 JB Delft, The Netherlands
4 Laboratoire Jean Maetz, CEA Departement de Biologie, BP 68, F, 06230 Villefranche-sur-Mer, France
Calcium fluxes and internal calcium pools were measured in fed, rapidly growing, male tilapia, Oreochromis mossambicus, acclimated to 0.8 mmol l-1 (FW) and 0.2mmoll-1 (LFW) Ca2+. Plasma calcium levels were slightly and significantly higher in the LFW tilapia, but muscle calcium concentrations were independent of ambient Ca2+. At the time of the experiments, the LFW fish were growing and accumulated calcium, although the calcium content of their hard tissues was reduced. The LFW fish had higher Ca2+ influx and efflux rates than the FW fish. The increase in the influx of Ca2+ in LFW fish was, however, substantially greater than the increase in the efflux of Ca2+, giving these fish a more than four-fold increase in net Ca2+ influx from the water: for a 20-g tilapia net uptake rates of Ca2+ from the water were 390 and 1620 nmol Ca2+ h-1 for FW- and LFW-adapted fish, respectively. These values were calculated to represent at least 69% of the total calcium accumulated by these growing fish. This indicates that even in low-calcium water, tilapia absorb a significant amount of their calcium requirement directly from the water.
The pool of readily exchangeable calcium in the bone of FW fish was estimated to be about 7 % of the total hard tissue calcium. In the fish acclimated to LFW, this percentage increased to about 15 % as total hard tissue mineralization decreased. This may indicate that tilapia can increase the mobility of their hard tissue calcium during periods of low calcium stress.
Key words: teleost, bidirectional Ca2+ fluxes, low ambient calcium, internal calcium stores
Accepted on October 1, 1985
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