Transbranchial potentials and ion fluxes across isolated, perfused gills of Uca rapax

Abstract

Transbranchial potentials (TP) and sodium or chloride fluxes were measured in an apparatus designed for the simultaneous perfusion of eight isolated gills of Uca rapax. In anterior gills perfused with U. rapax−saline (US) the TP varied almost linearly from-7.5 to +10 mV inside, and in posterior gills from +2 to-8.5 mV (inside), on exposure to salinities ranging from 8.7 through 52‰, i.e. 25 to 150% seawater (100%=34.6‰ S). Sodium influx and efflux in anterior gills exposed to US, 8.7 or 43.3‰ S (0.7 to 4.0 mmol h⁻¹ g⁻¹ dry wt) were always greater than in posterior gills (0.5 mmol h⁻¹). The chloride fluxes were slightly smaller than sodium fluxes in anterior gills, while in the posterior gills the chloride influx (2.8 to 4.6 mmol h⁻¹) was always larger than chloride efflux (0.6 to 1.1 mmol h⁻¹) or the sodium fluxes. At least three ion-transport mechanisms may be present in these gills: (1) an internal ( = basolateral), ouabain-sensitive Na⁺, K⁺ pump, restricted to anterior gills; (2) a furosemide-sensitive Na⁺, K⁺, 2Cl⁻ (plus water) transporter, apparently restricted to posterior gills, and (3) a Na⁺ exchanger (and possibly other as yet unidentified ion transporters, as suggested by large increases of the chloride influxes caused by amiloride), probably located on the apical membranes of the epithelial cells of both gill types. The differential selectivity of the gills of U. rapax for sodium or chloride may limit the transbranchial movements of either ion, without a reduction of the overall permeability of these crabs.