Determination of the electromotive force of active sodium transport in frog skin epithelium (Rana temporaria) from presteady-state flux ratio experiments

Abstract

The presteady-state influxes and effluxes of sodium across frog skin epithelium have been determined as a function of time while all electrophysiological parameters were maintained constant. The fluxes measured were resolved in the fractions which have passed a pathway through the cells and those that have used a paracellular pathway. The procedure is based on the theory that all presteady-state flux ratios have to be equal to the steady-state flux ratio if only one pathway is involved. The flux ratios for the transcellular route were used to calculate the electromotive force of the sodium pump. The calculation hinges on the assumptions (a) that both influx and efflux have to pass through the sodium pump and (b) that single file diffusion of sodium is not taking place anywhere along the path. The validity of both assumptions is discussed. Our calculated values for the electromotive force of the sodium pump EaNa vary between 146 and 200 mV, which is in agreement with the energy of the ATP/ADP system. There is a distinct indication that, as the electrochemical gradient for sodium opposing the transport is being increased, the emf increases towards an asymptotic value around 200 mV. The relation between the value of EaNa and the cellular phosphorylation potential for ATP is discussed.

Na+ transport in cystic fibrosis respiratory epithelia. Abnormal basal rate and response to adenylate cyclase activation.

“…The Ca-fluxes are equal. The sum of the passive ion flows for Nae and Ca-(partial ionic conductances) was -80% ofthe measured G. The disparity between the partial ionic conductances and G may represent the contribution of unmeasured solution ions (e.g., HCO3), the conductances associated with the active Na' path (apical GN.+, basolateral GK [17,18]), or both. Although there is a trend for Isc to be slightly increased in atopic polyp epithelia, no systematic differences 15 between any baseline parameters ofatopic polyp (n = 8 subjects)…”

“…We do not know the permeability of the paracellular path to mannitol in situ. However, the relatively high Pm.,,it., values (30-40 X lo-cm -s-') measured in nasal epithelia as compared with excised canine or human bronchi (15)(16)(17)(18)(19)(20) X lo-cm s-') (9,24), raise the possibility that physical damage may have occurred to the specimens during the operative excision and/or mounting procedure. Such damage would create nonselective leaks in the epithelial barrier that would serve to partially shunt the transepithelial PD.…”

Stationary and Nonstationary Ion and Water Flux Interactions in Kidney Proximal Tubule: Mathematical Analysis of Isosmotic Transport by a Minimalistic Model

“…The reversal potential of pump currents is E pump rev ¼ ÀE pump with free energy of ATP hydrolysis, ΔG ATP % À60 kJ/mol and stoichiometry of 3 Na + :2 K + :1 ATP, E pump is about 200 mV which is within the range discussed by de Weer et al (1988) and used in all our computations independently of the rate of pump flux. Unlike the reversal potential of the Na + pump of a tight epithelium, e.g., frog skin (Larsen 1973;Eskesen and Ussing 1985), the last mentioned assumption is plausible for proximal tubule cells of high density of mitochondria in remarkably close contact with pump sites that would minimize the diffusion distance between sites of dephosphorylation of ATP and rephosphorylation of ADP, respectively (Dørup and Maunsbach 1997). Following Läuger (1991), by inspection of Eq.…”

“…In an experimental study designed by Ussing of the mechanism of isosmotic transport by the small intestine, Nedergaard et al (1999) observed that sodium ions are recirculated from the serosal solution via cells back into the lateral intercellular space, which suggested a mechanism for regulating the osmolarity of the transported fluid. In the small intestine (Larsen et al 2002) and amphibian skin (Larsen et al 2009), the recirculation pathway would be the 1Na:2Cl:1K cotransporter studied by Frizzell et al (1979), Ferreira (1981), andUssing (1985), respectively. In the heterocellular proximal tubule with absorption of Na + , Cl À , and HCO 3 À , one would expect that different segments handle recirculation by different cotransporters depending on whether Cl À or HCO 3 À are transported together with Na + .…”