The importance of the subepithelial resistance for the electrical properties of the rat jejunum in vitro.

The aim of the study was to evaluate full-thickness and partially stripped jejunum as a model for neurogenic control of electrogenic ion transport. The electrical properties of full-thickness and partially stripped segments were studied in Ussing chambers. Using square-pulse analysis, subepithelial and epithelial resistances (Rs and Rp) were determined, and by compensating for the potential fall across Rs, the current generated by the epithelium could be measured. In full-thickness tissue, Rs was approximately 80% of total tissue resistance, and the current measured during short-circuiting of the whole tissue (SCC) was therefore only 20-25% of the current generated by the epithelium (Im). Surgical stripping of the tissue decreased Rs by 10-20%. This means that in full thickness as well as in stripped tissue, 70-80% of the potential difference across the epithelial layer remains after traditional 'short circuiting'. Over a 25-min period, none of the electrical parameters changed significantly in the full-thickness tissues. In the stripped group PD, SCC and Im fell significantly, and in parallel during the same period of time. Neither glucose, noradrenaline, met-enkephalin or carbachol had any significant effect on Rs, Rp or the Rs/Rp ratio. The relative effects of these agents on Im and SCC were therefore similar. Substance P and VIP increased the Rs/Rp ratio significantly and, therefore, the effect of these drugs on Im was significantly more pronounced than the effect on SCC. The results show that the subepithelial resistance must be taken into account when the electrogenic activity in the epithelium is to be determined correctly. Conventionally measured SCC reflects the electrogenic effect of the tested putative neurotransmitters, but the magnitude of the responses is grossly underestimated, particularly for substance P and VIP.