Concentrationo-dependence of nonelectrolyte permeability of toad bladder

Summary A theoretical formulation was derived for the dependence of bulk solute permeability,P, defined as net flux :- concentration gradient, c, across any membrane in which solute concentration is controlling for net flux,. According to this formulation, is stimulated by increments in trans concentration,c₂, in the rangec₂/c₁=0.0–0.1. Net flux of urea across toad bladder down concentration gradients was shown to be stimulated threefold by small increments in trans urea concentration. The theory also predicts that, in the absence of concentration gradients, tracer permeability,P^*, defined as tracer flux :- tracer concentration, will be independent ofc provided thatP=P^*, but will diminish with increasingc ifP/P^*<1.P/P^* was not significantly different from unity for urea, and bothP andP^* were independent ofc in the absence of concentration gradients. However,P/P^* was significantly less than unity (0.90 and 0.85) for thiourea and mannitol, respectively. In conformity with theory,P^* (and alsoP) of these two solutes, measured asc was increased by 3–4 orders of magnitude, diminished progressively. These effects are more consistent with this formulation than with transport via a saturable carrier.