Ionoregulatory Changes in the Gill Epithelia of Coho Salmon during Seawater Acclimation

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Abstract
Short‐term exposure of coho salmon smolts (Oncorhynchus kisutch) to a gradual increase in salinity over 2 d (0‰–32‰) resulted in a decrease in proton pump abundance, detected as changes in immunoreactivity with a polyclonal antibody against subunit A of bovine brain vacuolar H+‐ATPase. N‐ethylmaleimide (NEM)–sensitive H+‐ATPase activities in gill homogenates remained unchanged over 8 d to coincide with a 3.5‐fold increase in Na+/K+‐ATPase activities. A transient increase in plasma [Na+] and [Cl−] levels over the 8‐d period was preceded by a 10‐fold increase in plasma cortisol levels, which peaked after 12 h. Long‐term (1 mo) acclimation to seawater resulted in the loss of apical immunoreactivity for vH+‐ATPase and band 3–like anion exchanger in the mitochondria‐rich cells identified by high levels of Na+/K+‐ATPase immunoreactivity. The polyclonal antibody Ab597 recognized a Na+/H+ exchanger (NHE‐2)–like protein in what appears to be an accessory cell (AC) type. Populations of these ACs were found associated with Na+/K+‐ATPase rich chloride cells in both freshwater‐ and seawater‐acclimated animals.

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