Electrolyte transport across rabbit late proximal colon in vitro.

The second part of rabbit proximal colon was investigated in vitro under short circuit conditions. Unidirectional sodium and chloride fluxes were measured during the soft faeces period and during the hard faeces period. Rabbit late proximal colon has a potential difference (psi mS) of 4 mV, a tissue conductance (GT) of 10-11 mS/cm2 and a short circuit current (Isc) of 1.5 mueq/cm2 X hr. Under control conditions sodium (2.65 mueq/cm2 X hr) and chloride (0.67 mueq/cm2 X hr) are absorbed. Ouabain abolished psi ms,Isc and the net sodium flux totally, whereas 0.1 mM amiloride only slightly decreased the net sodium flux. No differences in electrical properties and Na,Cl-fluxes were found between the faeces periods. Removal of sodium abolished psi ms and Isc totally, and a high potassium solution depolarized the preparation (psi ms = 0). A linear current-voltage relation characterizes the tissue as an ohmic resistor between -40 and +50 mV, and reveals a slope conductance of 14 mS/cm2 under KCl conditions. We conclude that the transport functions under in vitro conditions differ markedly from the in vivo situation, and that the diurnal differences of electrolyte transport in vivo occur mainly by the involvement of ionic gradients.