Active NaCl absorption across posterior gills of hyperosmoregulating Chasmagnathus granulatus.

Split lamellae of posterior gills of Chasmagnathus granulatus adapted to 2.5 per thousand salinity were mounted in a modified Ussing chamber. With NaCl-saline on both sides of the preparation a transepithelial voltage (V(te)) of 4.1+/-0.5 mV (outside positive) was measured. After voltage-clamping, the negative short-circuit current (I(sc)) amounted to -142+/-21 micro A cm(-2) at a conductance (G(te)) of 44+/-5 mS cm(-2). Substitution of either chloride (by nitrate) or sodium (by choline) on both sides of split gill lamellae significantly reduced I(sc) (by 70-80%) and G(te) (by 30-50%). External CsCl (but not BaCl(2) or furosemide) inhibited the negative I(sc) without affecting G(te). Addition of ouabain, BaCl(2) or diphenylamine-2-carboxylate to the internal bath inhibited I(sc) at unchanged G(te). Internal acetazolamide did not affect I(sc) or G(te) across split gill lamellae. Unidirectional Na(+) influx across isolated and perfused posterior gills, however, was reduced by internal acetazolamide by approximately 20% at constant V(te). The results suggest that posterior gills of hyperosmoregulating C. granulatus display a high conductance epithelium that actively absorbs NaCl in a coupled way by an electrogenic mechanism similar to that seen in the thick ascending limb of Henle's loop and, to a minor degree, by an electroneutral mechanism, presumably via apical Na(+)/H(+)- and Cl(-)/HCO(3)(-)-antiports.