Chloride cell responses to ion challenge in two tropical freshwater fish, the erythrinids Hoplias malabaricus and Hoplerythrinus unitaeniatus.

Chloride cell (CC) responses to ion challenge and plasma ion concentration were evaluated in two ecologically distinct erythrinids, Hoplias malabaricus, an exclusively water-breathing species, and Hoplerythrinus unitaeniatus, a facultative air-breathing fish, at one, two, seven, and 15 days of exposure to deionized water and to ion-rich water. H. malabaricus displayed high CC proliferation on filament and lamellar epithelium during exposure to deionized water and significant CC proliferation in the filament epithelium on the first day of exposure to water rich in NaCl and Ca2+ and in the lamellar epithelium on the first, second, and seventh day of exposure to such water. CC proliferation in H. unitaeniatus occurred only in the lamellar epithelium of fish exposed to deionized water. CC proliferation on both species was not accompanied by significant increase of CC density in contact with the external medium. The increase in the CC fractional area (CCFA) resulted from the increase of individual CC apical surface area on the first and second days of exposure to deionized water in H. malabaricus and only on the first day in H. unitaeniatus. Plasma ions in both erythrinid species showed transitory changes and, on the fifteenth day of exposure to the two types of experimental water, the plasma ion concentration was similar to the control fish. The CC responses of these erythrinid fish showed that CC proliferation depends on previous CC density in the gill and is not related solely to exposure to ion-poor water. Furthermore, CC proliferation in gill epithelium did not always involve an increase of CC density in contact with the external medium. Copyright 2003 Wiley-Liss, Inc.