BRANCHIAL CHLORIDE CELL-PROLIFERATION IN THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS - IMPLICATIONS FOR GAS TRANSFER

The effects of branchial chloride cell proliferation on ion transport capability and gill morphometry were evaluated in the rainbow trout, Oncorhynchus mykiss, to test the hypothesis that chloride cell (CC) proliferation benefits ionic regulation at the expense of efficient gas transfer. The extent of hormone-induced CC proliferation (using ovine growth hormone (oGH), cortisol, or a combination of both) on the gill filament epithelium was assessed by determining the fractional surface area of exposed cells using scanning electron microscopy. Cortisol and oGH were equally effective in increasing CC fractional surface area (similar to 2X), owing to the enlargement of individual CCs. The combined cortisol/oGH treatment resulted in an even greater increase in CC fractional area (similar to 6X), as both the size and number of CCs increased. Sham injections were without effect on CC surface area or number. Significant increases in Na+ (J(in) Na+) and Cl- uptake (J(in) Cl-) were observed after all hormone treatments and were correlated positively with the increases in the CC fractional surface area. These findings support the view that CC proliferation enhances branchial ion transport capability. Lamellar epithelial thickness (measured by transmission electron microscopy) was increased in hormone-treated;fish, while lamellar surface area (measured using light microscopy) was unaffected. The area of the interlamellar water channels (calculated from light micrographs) was significantly reduced in hormone-treated fish. These results suggest that, in trout, a compromise is made between the efficiency of ion regulation and gas transfer in which the enlargement/proliferation of CCs may impede gas transfer.