Cell membrane water permeabilities and streaming currents in Ambystoma proximal tubule

Abstract

An electrophysiological approach is used to analyze the possible routes of osmotically driven water flow across the isolated perfused Ambystoma proximal tubule. The minimum hydraulic conductivities (Lp) of the cell membranes were estimated from the initial rate of change of intracellular activities of Na+ and K+ in response to a step gradient of 50 or 100 mosmol/kg sucrose. The Lp of the apical membrane is 1.30 X 10(-4) cm.s-1.osM-1 referred to the luminal epithelial surface and 2.45 X 10(-6) cm.s-1.osM-1 when corrected for amplification of the brush border (n = 8). The Lp of the basolateral membrane is 1.42 X 10(-4) cm.s-1.osM-1 referred to the basement membrane surface and 6.39 X 10(-6) cm.s-1.osM-1 when corrected for the amplification of the basal and lateral membranes (n = 5). Transepithelial water flows were generated in either direction by a unilateral step increase of osmolality with 100 mosmol sucrose. Bath-to-lumen flow increased paracellular transepithelial resistance (R3) by 48%; lumen-to-bath flow decreased R3 by only 3%. A bilateral increase in the osmolality of both solutions by 50 mosM had no significant effect on R3. Streaming potentials were observed during trans-epithelial water flow induced by unilateral gradients of sucrose; their polarity, magnitude, site of generation, and insensitivity to change of paracellular resistance are all indicative of water flow through paracellular structures, especially the lateral intercellular spaces. Contrary to earlier suggestions (J. M. Diamond, J. Membr. Biol. 51: 195-216, 1979), these potentials are not primarily diffusion potentials across anion-selective tight junctions resulting from solute polarization in the unstirred layers. Instead, a true electrokinetic basis for these streaming potentials is indicated by their continued presence after deletion of all Cl-. Thus water moves through both cellular and paracellular pathways in this epithelium.

Pseudo-streaming potentials in Necturus gallbladder epithelium. I. Paracellular origin of the transepithelial voltage changes.

“…Changes in transepithelial voltage upon osmotic challenges similar to those described here have been reported in several leaky epithelia (Pidot and Diamond, 1964;Diamond and Harrison, 1966;Frrmter and Gessner, 1974;De Mello et al, 1976;Corman, 1985;Tripathi and Boulpaep, 1988). When the tissues are initially exposed to salt solutions of physiologic composition, the polarity of the apparent streaming potentials is lumen positive in rabbit gallbladder (Pidot and Diamond, 1964;Diamond and Harrison, 1966), Necturus gallbladder (Reuss and Finn, 1977), and rat kidney proximal tubule (Frrmter and Gessner, 1974;see also De Mello et al, 1976).…”

“…In rabbit proximal convoluted tubule, the apparent streaming potential can be either lumen positive or lumen negative when the peritubular solution is rendered hyperosmotic by addition of raifinose, depending on whether the tubules were dissected from mid-cortical or juxtamedullary regions (Corman, 1985). In Ambystoma proximal tubule, sucrose addition to the luminal solution produced a lumen-negative voltage change (Tripathi and Boulpaep, 1988). In all cases, the polarity of the apparent streaming potential observed in preparations bathed with symmetric NaCl-based solutions was consistent with the overall ionic selectivity of the epithelium, assessed by transepithelial dilution potentials, i.e., by unilateral reductions in [NaCI] under isosmotic conditions (NaCl replaced with sucrose or other nonelectrolytes).…”

“…Solute-solvent coupling in leaky epithelia has been assessed by measurements of solvent drag (Hill and Hill, 1978a;Whittembury et al, 1981) and electrokinetic phenomena, in particular streaming potentials (Pidot and Diamond, 1964;Diamond and Harrison, 1966;Fr6mter and Gessner, 1974;De Mello, Lopes, and Malnic, 1976;Corman, 1985;Tripathi and Boulpaep, 1988). Streaming potentials can be generated across ion-selective pores or slits by imposing osmotic gradients across the pathway and measuring the resulting change in transmembrane voltage.…”

“…The voltage change is due to the fact that the water flow, driven by the osmotic gradient, is frictionally coupled to an ion flux in the same direction. Because the pores or slits are ion selective, they contain a solution with an excess of the more permeant species (either cation or anion), and hence the water-coupled ion flux results in transmembrane current flow or in a change in membrane voltage (House, 1974;Finkelstein, 1987;Tripathi and Boulpaep, 1988). A complication in the interpretation of such an experiment is that water and (selective) ion permeation via separate pathways would also be expected to cause a change in membrane voltage (Diamond, 1979;Barry and Diamond, 1984;Barry, 1989).…”

Water permeability of ventricular cell membrane in choroid plexus epithelium from Necturus maculosus.

“…At the surface of the epithelial cells, the initial rate of change of solution was 13-20 % s-1. The change was 90 % complete in 6-20 s. The epithelial cell layer is strongly folded with several rims and folds branching off a central rim (Carpenter, 1966 and Ambystoma proximal tubules (Tripathi & Boulpaep, 1988) where the time of change at the cell surface was about 2 s. The difference must, at least in part, originate from the different geometries of the tissues, since I recorded the same water permeability as did , when I used a Necturus gall-bladder in the chamber (see Table 2). The onset times of the changes were measured as the intercept between the baseline and the initial slopes.…”