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.

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