Passive ion fluxes across toad bladder

Toad bladders mounted as sacs, with edge damage eliminated (mean spontaneous potential =101 mV) were treated with sufficient ouabain (1.89×10−3m) to eliminate active sodium transport. Fluxes of22Na,36Cl and35SO4 across the epithelium in both directions were measured at 0 mV and with potential clamped to 100 mV in various media. The results were analyzed by a set of equations, derived from previous work of others. Transport numbers are obtained from unidirectional isotope fluxes measured at 0 mV and 100 mV. Deviation from the Ussing flux ratio equation is expressed in terms of a constant ratio of bulk diffusion coefficient to tracer diffusion coefficient. Tracer flux in either direction at zero potential is the logarithmic mean of bidirectional fluxes at any potential. These equations were tested by comparing fluxes at zero potential predicted from fluxes at 100 mV with observed values, and by comparing transport numbers calculated from average net flux and current. The correspondence was within experimental error. Constant ratio of bulk diffusion coefficient to tracer diffusion coefficient for Na, Cl and SO4 ions in a given medium is more consistent with the data than constant values for exchange diffusion, independent of potential. Conductance measurements indicate that Na transport is facilitated by (or coupled with) transport of Cl and/or phosphate. Bicarbonate apparently contributes substantially to passive conductance.