On the Mechanisms of Sodium Ion Transport by the Irrigated Gills of Rainbow Trout (Salmo gairdneri)

Abstract

Sodium uptake by rainbow trout gills has been investigated with a small-volume system enabling rapid, successive flux measurements in different solutions. Sodium influx obeys a Michaelis-Menten type relation, with a Km of 0.46 mM, and uptake proceeds unimpaired in the absence of penetrating counterions. This suggests a coupled cation exchange. Ammonia output is about the same as the Na+ influx when external [Na+] is 1 mM, but at higher or lower Na+ influxes, the correlation does not hold. A progressive downward shift in the pH of the irrigating medium as Na+ influx increases indicates that the exchanging cation is hydrogen. In support of this, acetazolamide, which inhibits Na+ uptake, also prevents the downward pH shift. The potential across the gill is about 10 my, body fluids positive, in NaC1 solutions up to 10 mM, and is little affected by changes in Na+ concentration below that. Finally, evidence for locating the rate-limiting step at the outer membrane of the epithelium is presented.

Determination of pH by microfluorometry: intracellular and interstitial pH regulation in developing early-stage fish embryos (Danio rerio)

“…Potentials in adult freshwater fish are found in the range of slightly negative to slightly positive values (-5 (Eddy, 1975;Kerstetter et al, 1970;Kerstetter and Kirschner, 1972). The HCO 3 -equilibrium potentials at low-level P CO 2 calculated for the present study are close to this range.…”

The role of branchial carbonic anhydrase in acid-base regulation in rainbow trout (Oncorhynchus mykiss)

“…It has long been postulated that branchial CA contributes to ionic and acid-base regulation in freshwater teleost fish by catalyzing the hydration of CO 2 within the branchial epithelium to the acid-base equivalents, H + and HCO 3 -, that are used as counter-ions for Na + and Cl -uptake, respectively (reviewed by Perry and Laurent, 1990;Goss et al, 1992Goss et al, , 1995Henry and Swenson, 2000;Claiborne et al, 2002;Perry et al, 2003;Hirose et al, 2003;Evans et al, 2005). The contribution of branchial CA to ion uptake is supported by the significant reductions in Na + or Cl -influx that have been documented in most, although not all (Kerstetter and Kirschner, 1972), studies following inhibition of branchial CA by acetazolamide in vivo (Maetz, 1956;Maetz and GarciaRomeu, 1964;Boisen et al, 2003;Chang and Hwang, 2004), in situ (Kerstetter et al, 1970) or in perfused gill preparations (Payan et al, 1975). Fewer studies have focused on the role of branchial CA in acid-base regulation.…”

Carbonic anhydrase and acid–base regulation in fish

“…Surprisingly few studies have attempted to address this question directly. More have instead focused on the question of whether branchial CA contributes to ionic regulation, finding support for such a role by using acetazolamide or other permeant CA inhibitors to demonstrate a significant reduction in Na + [goldfish, Carassius auratus (Maetz, 1956;Maetz and GarciaRomeu, 1964); rainbow trout (Kerstetter et al, 1970;Payan et al, 1975); zebrafish (Boisen et al, 2003;Esaki et al, 2007)] or Cl -uptake [goldfish (Maetz and Garcia-Romeu, 1964); tilapia (Chang and Hwang, 2004); zebrafish (Boisen et al, 2003)] when CA is inhibited (for reviews, see Perry and Laurent, 1990;Evans et al, 2005;Tresguerres et al, 2006a). Somewhat unexpectedly, elimination of cytosolic CA or CA 15a activity from HR cells using the antisense oligonucleotide morpholino approach (to selectively knockdown translation of the CA isoform of interest) increased rather than decreased Na + influx in zebrafish larvae (Lin et al, 2008).…”