Metabolic cost of sodium transport in toad urinary bladder

Summary

The metabolic cost of active sodium transport was determined in toad bladder at different gradients of transepithelial potential, Δψ, by continuous and simultaneous measurements of CO₂ production and of transepithelial electric current. Amiloride was used to block active sodium transport in order to assess the nontransport-linked, basal, production of CO₂ and the passive permeability of the tissue. From these determinations active sodium transport,J _Na, and suprabasal CO₂ production,$J_{CO_2 }^{sb}$, were calculated. Since large transients inJ _Na and$J_{CO_2 }^{sb}$ frequently accompanied any abrupt change in Δψ, steady state conditions were carefully defined.

Some 20 to 40 min were required after a change in Δψ before steady state of transport activity and of CO₂ production were achieved. The metabolic cost of sodium transport proved to be the same whether the bladder expended energy moving sodium against a transepithelial electrical potential grandient of +50 mV or whether sodium was being pulled through “the active transport pathway” by an electrical gradient of −50 mV. In both cases the value of the ratio$J_{NA} /J_{CO_2 }^{sb}$ averaged some 20 sodium ions transported per molecule of CO₂ produced.

When the Na pump was blocked by 10⁻² m ouabain, the perturbations of the transepithelial electrical potential did not elicit changes ofJ _Na nor, consequently, of$J_{CO_2 }^{sb}$.

The independence of the ratio$J_{NA} /J_{CO_2 }^{sb}$ from Δψ over the range ±50 mV indicates a high degree of coupling between active sodium transport and metabolism.