Basolateral Na-H antiporter: Uncoupled Na transport produces an amiloride- sensitive conductance

An amiloride-inhibitable, Na⁺-H⁺ antiporter was identified in the basolateral membrane of turtle colon by measuring ²²Na⁺ fluxes across isolated tissues apically permeabilized with the pore-forming antibiotic amphotericin B. In cells shrunken by exposure to Cl^--free (gluconate) solutions and treated with ouabain to block the Na-K-ATPase, Na⁺ movement across the basolateral membrane was due entirely to the antiporter. Elevation of cytosolic Na⁺ was associated with an amiloride-inhibitable outward current across the basolateral membrane. The sensitivity of the current to various amiloride analogues paralleled that of Na⁺ exchange rather than that of the apical Na⁺ channel. Furthermore, cell volume changes altered basolateral Na⁺ exchange and basolateral Na⁺ conductance in a parallel fashion. We propose that this amiloride-sensitive basolateral Na⁺ conductance represents an altered operating mode of a basolateral Na⁺-H⁺ exchanger.