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 &mu;Eq l-1), unidirectional Ca2+ influx was 11&plusmn;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&plusmn;36 &mu;Eq l-1) and a transport capacity (Jmax=31&plusmn;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 &mu;Eq l-1) stimulated Ca2+ uptake almost three-fold above control values, whereas chronic high Ca2+ exposure (20 000 &mu;Eq 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
Patrick
ML
Department of Biology, McMaster University, 1280 Main St. West, Hamilton, Ontario, L8S 4K1, Canada
Wood
CM
Marshall
WS