Electrolyte transport across the rabbit caecum in vitro

Abstract

Electrolyte transport across rabbit caecal epithelium was investigated in vitro using conventional shortcircuiting and radioisotope techniques. In standard saline the caecum exhibited a relatively high short-circuit current (I _sc=4.4 μEq · cm⁻² · h⁻¹) and conductance (6.43 mS · cm⁻²). Both sodium and chloride were absorbed (J ^Na_net =6.40 andJ ^Cl_net =3.40 μEq · cm⁻² · h⁻¹) and potassium was secreted (J ^K_net =−0.5 μEq · cm⁻² · h⁻¹). Removal of Na⁺ abolishedI _sc andJ ^Cl_net whereas removal of Cl⁻ reducedJ ^Na_net to 2.92 μEq · cm⁻² · h⁻¹ but did not alterI _sc. In HCO ⁻₃ free salines containing 10⁻⁴ M acetazolamideJ ^Cl_net was abolished andJ ^Na_net andI _sc were reduced to 2.3 and 2.5 μEq · cm⁻² · h⁻¹ respectively. A positive residual ion flux (∼ 1 μEq · cm⁻² · h⁻¹) was detected in standard and Cl⁻-free salines but not in Na⁺-free or HCO ⁻₃ buffers. Mucosal amiloride (10⁻³ M) decreased net Na⁺ and Cl⁻ absorption but did not decreaseI _sc. Mucosal DIDS (10⁻⁴ M) decreasedJ ^Cl_net while mucosal bumetanide (10⁻⁴ M) did not affect any of the measured parameters. Finally, addition of theophylline (8 mM) stimulated Cl⁻ secretion and increasedI _sc. It is concluded that net sodium absorption by caecal epithelia occurs by both electrogenic and electroneutral mechanisms whereas net chloride absorption occurs only by an electroneutral process. Coupling of the absorptive fluxes of Na⁺ and Cl⁻ may result from Na⁺/H⁺ and Cl⁻/HCO ⁻₃ antiport systems in this tissue. Finally, it is proposed that up to half of theI _sc is due to a Na⁺-dependent secretion of bicarbonate ion.