Whole Body Calcium Flux Rates in Freshwater Teleosts as A Function of Ambient Calcium and Ph Levels: A Comparison Between the Euryhaline Trout, Salmo Gairdneri and Stenohaline Bullhead, Ictalurus Nebulosus

ABSTRACT The exchange of calcium between the blood and external medium following intravascular injection of 45Ca was investigated in adult fresh water rainbowtrout (Salmogairdneri) acclimated to ambient [Ca2+] of 0.18, 0.65 and 5.0 mequivl−1, and in bull heads (Ictalurus nebulosus) at external [Ca2+] of 0.17 mequivl−1, each at near-neutral ambient pH, 11–12°C and constant Na+ and Cl− levels of 0.06 and 0.03 mequivl−1, respectively. The dispersal volume of isotope exhibited a slow exponential increase with 8–9 h required for 95% of the radiospace to be filled, irrespective ofthe acclimation medium or fish species. Equilibrium radiospaces in trout (range 1600–2000 ml kg−1) were independent of ambient [Ca2+] but 1.4 -fold higher than in bullheads. In all cases, whole body exchangeable calciumcontent was low, representing ≃3 % of total body calcium content. The whole body calcium exchange rates (mean ranges; influx or efflux) in trout of 7–22μequiv kg−1 h−1 were largely independent of ambient Ca2+ levels; lower values of 2–10μequiv kg−1 h were found in bullheads. Acute exposure (24 h) to low ambient pH 4.0-4.2 at external [Ca2+] ≃ 0.18 mequivl−1 resulted in whole body ion loss in both species. Net Ca2+ losses in trout were due to a reduction in influx whereas both increased efflux and depressed influx accounted for the response in bullheads. These changes were transitory (12 h) and minor in comparison with concomitantnet Na+ and Cl− losses which unlike Ca2+, exhibited no recovery within 24 h of acid exposure. Possible mechanisms of Ca2+ regulation in fresh water fish are discussed.