Na(+), Cl(-), Ca(2+) and Zn(2+) transport by fish gills: retrospective review and prospective synthesis

The secondary active Cl(-) secretion in seawater (SW) teleost fish gills and elasmobranch rectal gland involves basolateral Na(+),K(+)-ATPase and NKCC, apical membrane CFTR anion channels, and a paracellular Na(+)-selective conductance. In freshwater (FW) teleost gill, the mechanism of NaCl uptake is more controversial and involves apical V-type H(+)-ATPase linked to an apical Na(+) channel, apical Cl(-)-HCO-3 exchange and basolateral Na(+),K(+)-ATPase. Ca(2+) uptake (in FW and SW) is via Ca(2+) channels in the apical membrane and Ca(2+)-ATPase in the basolateral membrane. Mainly this transport occurs in mitochondria rich (MR) chloride cells, but there is a role for the pavement cells also. Future research will likely expand in two major directions, molded by methodology: first in physiological genomics of all the transporters, including their expression, trafficking, operation, and regulation at the molecular level, and second in biotelemetry to examine multivariable components in behavioral physiological ecology, thus widening the integration of physiology from the molecular to the environmental levels while deepening understanding at all levels.