Determination of rheogenic ion transport in rat proximal colon in vivo.

A direct clamping technique is demonstrated, which allows monitoring of rapid changes of the short-circuit current (Isc) and the specific transepithelial resistance (Rm) as well as measurement of ion fluxes under short-circuit conditions in vivo. Due to the cylindrical symmetry of the colon the intraluminal electrode was devised as a centrally fixed silver rod, by which radial current injection was achieved. The geometrical arrangement of the electrodes guaranteed zero potential difference (PD) along the whole axis of the colon segment. The Isc was determined to 3.3 +/- 0.7 mueq h-1 cm-2 and Rm equal to 121 +/- 5 omega cm2. These data obtained by direct short-circuiting agree well with our earlier Rm and Isc data based on cable analysis, where the Isc was calculated from the open-circuit PD and Rm. This is considered as evidence for the reliability of the two independent in vivo techniques. Their validity was confirmed by the expected effects of drugs acting on rheogenic ion transport. Both the indirect (via Rm) as well as the direct Isc determination may be used alternatively as required; one may serve to match the other. For larger tubular structures like the rat colon the direct clamping should be preferred as the standard procedure for the Isc determination in vivo.