Calcium regulation in the freshwater-adapted mummichog

In light of recent findings of an unusual pattern of ionoregulation (high Na+ uptake and negligible Cl- uptake) in the freshwater-adapted mummichog Fundulus heteroclitus, the pattern of Ca2+ regulation was examined. Under control conditions (water Ca2+=200 μEq l-1), unidirectional Ca2+ influx was 11±4 nEq g-1 h-1. Acute variation of external Ca2+ levels revealed a saturable Ca2+ uptake system with a relatively high affinity (Km=125±36 μEq l-1) and a transport capacity (Jmax=31±4 nEq g-1 h-1) comparable to those of other teleosts. Lanthanum (equimolar to [Ca2+]) significantly blocked Ca2+ uptake by 67% whereas mag-nesium had no effect. Chronic low Ca2+ exposure (50 μEq l-1) stimulated Ca2+ uptake almost three-fold above control values, whereas chronic high Ca2+ exposure (20 000 μEq l-1) had no effect. Lanthanum and chronic low Ca2+ treatments disturbed the normally positive Ca2+ and Na+ balances of the animals whereas acid-base balance and ammonia excretion were undisturbed. The results indicate that Ca2+ regulation by the mummichog conforms to the model for freshwater Ca2+ transport whereby chloride cells on the gills take up Ca2+ actively from the water. However, the absence of extra-intestinal Cl- uptake and the recent demonstration of significant Ca2+ uptake by opercular epithelia raise questions about the relative roles of branchial and opercular epithelial chloride cells in freshwater F. heteroclitus. 1997 The Fisheries Society of the British Isles