CALCIUM REGULATION IN THE FRESH-WATER-ADAPTED MUMMICHOG

In light of recent findings of an unusual pattern of ionoregulation (h igh Na+ uptake and negligible Cl- uptake) in the freshwater-adapted mu mmichog Fundulus heteroclitus, the pattern of Ca2+ regulation was exam ined. Under control conditions (water Ca2+=200 mu Eq l(-1)), unidirect ional Ca2+ influx was 11 +/- 4 nEq g(-1) h(-1). Acute variation of ext ernal Ca2+ levels revealed a saturable Ca2+ uptake system with a relat ively high affinity (K-m = 125 +/- 36 mu Eq l(-1)) and a transport cap acity (J(max) = 31 +/- 4 nEq g(-1) h(-1)) comparable to those of other teleosts. Lanthanum (equimolar to [Ca2+]) significantly blocked Ca2uptake by 67% whereas magnesium had no effect. Chronic low Ca2+ exposu re (50 mu Eq l(-1)) stimulated Ca2+ uptake almost three-fold above con trol values, whereas chronic high Ca2+ exposure (20 000 mu Eq l(-1)) h ad no effect. Lanthanum and chronic low Ca2+ treatments disturbed the normally positive Ca2+ and Na+ balances of the animals whereas acid-ba se balance and ammonia excretion were undisturbed. The results indicat e that Ca2+ regulation by the mummichog conforms to the model for fres hwater 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 ope rcular epithelia raise questions about the relative roles of branchial and opercular epithelial chloride cells in freshwater F. heteroclitus . (C) 1997 The Fisheries Society of the British Isles.