Ion transport across the isolated intestinal mucosa of the winter flounder,Pseudopleuronectes americanus

Abstract

The isolated intestinal mucosa of the flounder,Pseudopleuronectes americanus, when bathed in a 20mm HCO₃-Ringer's solution bubbled with 1% CO₂ in O₂, generated a serosa-negative PD and, when short-circuited, absorbed Cl at almost 3 times the rate of Na. Reducing HCO₃ to 5mm decreased the net Cl flux by more than 60%. The following results suggest that, despite the PD, Na and Cl transport processes are nonelectrically coupled: replacing all Na with choline abolished both the PD and net Cl flux; replacing all Cl with SO₄ and mannitol abolished the PD and the net Na flux; and adding ouabain (to 0.5mm) abolished the PD and the net Cl flux. Nearly all of the unidirectional serosa-to-mucosa Cl flux (J _sm ^Cl ) seemed to be paracellular since it varied with PD and Cl concentration in a manner consistent with simple diffusion.J _sm ^Cl was only about one-fourth ofJ _sm ^Na , suggesting that the paracellular pathway is highly cationselective. The data can be explained by the following model: (i) Na and Cl uptake across the brush border are coupled 1∶1; Na is pumped into the lateral space and Cl follows passively, elevating the salt concentration there; (ii) the tight junction is permeable to Na but relatively impermeable to Cl; and (iii) resistance to Na diffusion is greater in the lateral space (considered in its entirety) than in the tight junction. If these assumptions are correct, the serosa-negative transmural PD is due mainly to a salt diffusion potential across the tight junction and, under short-circuit condition, most of the Na pumped into the lateral space diffuses back into the luminal solution, whereas most of the Cl enters the serosal solution. Morphological features of the epithelium support this interpretation: the cells are unusually long (60 μm) and narrow (3.5 μm); there is little distension of the apical 12 μm of the lateral space during active fluid absorption; and distension distal to this region is intermittently constricted by desmosomes.