Tracer flow, permeability, and partial conductance

Validity of the Goldman-Hodgkin-Katz equation in paracellular ionic pathways of gallbladder epithelium

The Goldman-Hodgkin-Katz equation has been extensively used to determine cationic/anionic permeability ratios in the paracellular pathways of the gallbladder epithelium. Nevertheless, new experimental evidence suggests that none of the theoretical assumptions of the equation hold for these pathways. In order to assess the experimental validity of the Goldman equation the permeability ratios were calculated from zero-current diffusion potentials by means of the Goldman equation and compared with the cationic/anionic permeability ratios measured by simultaneous determinations of cation and anion tracer fluxes in the same membranes. The results indicate that the Goldman equation is empirically valid for the tested salts (KCl and RbCl) within the experimental range of concentrations (25 to 200 mM) at an electrochemical-potential difference of zero.

Oxalate transport across the isolated rat colon. A re-examination

A net absorption of oxalate and chloride was observed when isolated, short-circuited segments of rat colon were bathed by a calcium-containing buffer. Removal of calcium promoted a two-fold decrease in transmural resistance, while the net chloride flux was reduced and the net oxalate transport abolished. It was concluded that net oxalate absorption was not observed in previous studies (employing calcium-free buffers) because calcium is reqiured to maintain the integrity of the conductive pathways across colonic epithelia.

Kinetics of bidirectional active sodium fluxes in the toad bladder

The kinetics of isotopic Na+ flows was studied in urinary bladders of toads from the Dominican Republic. Initial studies of the potential dependence of passive serosal to mucosal 22Na+ efflux demonstrated the absence of isotope interaction and/or other coupling with passive Na+ flow. The electrical current I and mucosal to serosal 22Na+ influx were then measured with transmembrane potential clamped at deltapsi=0, 25, 50, 75 or 100 mV. Subsequent elimination of active Na+ transport with mucosal amiloride permitted calculation of the rates of active Na+ transport JaNa and active and passive influx leads to JaNa and leads to JpNa. The results indicate that for Dominican toad bladders mounted in chambers only Na+ contributes significantly to transepithelial active ion transport; hence JaNa=Ja. Ja was abolished at deltapsi=E=96.3+/-1.9 (S.E.) mV. As deltapsi approached E, active efflux comes from Ja became demonstrable. At deltapsi=100 mV, comes from Ja exceeded leads to Ja, so that Ja was negative. Experimental values of leads to Ja agreed well with theoretical values predicted by a thermodynamic formulation: leads to Jaexp=0.985 leads to Jatheor (r=0.993). The dependence of leads to Ja on deltapsi is curvilinear.

Ionic permeability of epithelial tissues

The overall permeability of epithelial tissues to solutes is generally determined by analyzing net or unidirectional transepithelial fluxes in response to transepithelial differences of concentration and/or electrical potential using relations that describe diffusional movements across a single membrane. If the solute is uncharged and diffusional movements are transcellular, the overall transepithelial permeability coefficient is determined by the permeabilities of the two limiting cell membranes combinded in series. However, if the solute is charged and the pathway for transepithelial movement involves diffusional flows across at least two membranes arranged in series (i.e. transcellular transport), the value of the overall transepithelial permeability coefficient determined using relations that describe ionic diffusion across a single membrane is not an accurate measure of the permeabilities of the two limiting membranes combined in series. Further, if ionic diffusion is transcellular, permeability coefficients determined from studies of transepithelial fluxes are not only quantitatively incorrect but can also result in grossly erroneous interpretations of changes in transepithelial permeabilities and faulty inferences regarding the route of transepithelial ionic diffusion.