Electrolyte transport across the rabbit caecum in vitro.

Electrolyte transport across rabbit caecal epithelium was investigated in vitro using conventional short-circuiting and radioisotope techniques. In standard saline the caecum exhibited a relatively high short-circuit current (Isc = 4.4 microEq.cm-2.h-1) and conductance (6.43 mS.cm-2). Both sodium and chloride were absorbed (JNa(net) = 6.40 and JCl(net) = 3.40 microEq.cm-2.h-1) and potassium was secreted (JK(net) = -0.5 microEq.cm-2.h-1). Removal of Na+ abolished Isc and JCl(net) whereas removal of Cl- reduced JNa(net) to 2.92 microEq.cm-2.h-1 but did not alter Isc. In HCO3(-)-free salines containing 10(-4) M acetazolamide JCl(net) was abolished and JNa(net) and Isc were reduced to 2.3 and 2.5 microEq.cm-2.h-1 respectively. A positive residual ion flux (approximately 1 microEq.cm-2.h-1) was detected in standard and Cl(-)-free salines but not in Na+-free or HCO3- buffers. Mucosal amiloride (10(-3) M) decreased net Na+ and Cl- absorption but did not decrease Isc. Mucosal DIDS (10(-4) M) decreased JCl(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 increased Isc. 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-/HCO3- antiport systems in this tissue. Finally, it is proposed that up to half of the Isc is due to a Na+-dependent secretion of bicarbonate ion.