Extraintestinal calcium influxes were measured in the killifish, Fundulus heteroclitus, in solutions with different calcium concentrations, from distilled water level (near 0) to seawater level (approximately 12 mM). The extraintestinal influx is modified by the concentration of calcium in the medium during the adaptive period. In freshwater-adapted fish, calcium depletion resulted in an increase in calcium uptake. Such an adaptation was not observed in calcium-depleted fish in artificial calcium-deficient seawater. Calcium depletion in either medium seems to increase the calcium permeability. No correlation was found between Ca-ATPase activity in the gill tissue and calcium uptake.

Enriched plasma membrane preparations of the branchial epithelium of freshwater-adapted eels were used to study adenosine triphosphatase (ATPase) activities insensitive to ouabain and responding to Ca2+ and Mg2+. Ca2+ induced ATP hydrolysis; two kinetics were observed in the presence or absence of chelators, one with a high-affinity site (0.3 microM) and one with a lower affinity site (10-20 microM). The high-affinity Ca2+ site or enzyme had a prerequisite for Mg2+ (endogenous Mg2+ being sufficient to satisfy the Mg2+ need) but was inhibited by exogenous Mg2+ (Ki0.5 less than 10 microM Mg2+). The low-affinity site or enzyme appears to have kinetic parameters comparable to those found for Mg2+-induced ATP hydrolysis. In the absence of Ca2+ ligands and with no exogenous Mg2+, the two Ca2+ sites or enzymes can be considered stimulated. The results are discussed in relation to the branchial ion environment and transport ion capacities.

The intracellular localization of a Cl--HCO3--ATPase, which is inhibited by SCN-, was studied in the gills of the rainbow trout, Salmo gairdneri. This activity can be measured in the absence of contamination by mitochondria (i.e., in the absence of succinate dehydrogenase or cytochrome c oxidase activities). The distribution of the 5'-nucleotidase and of the ATPase stimulated by Cl- and HCO3- after sucrose density gradient centrifugation of the microsomal fraction was compared. Because those activities cannot be separated, it is postulated that the anion-stimulated ATPase is located in the plasma membrane. The activation of this microsomal anion ATPase by chloride has been studied extensively. The possible role of the Cl--HCO3--ATPase of trout gills in the Cl-/HCO3- exchange and in the regulation of the internal acid-base balance is discussed.

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