Epithelial and subepithelial resistance of rat large intestine: segmental differences, effect of stripping, time course, and action of aldosterone

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Abstract
Epithelial and subepithelial electrical resistances of rat large intestine were measured by means of a 4-electrode AC impedance technique in three segments, colon ascendens, colon descendens and rectum. (i) Epithelial resistance of colon ascendens and colon descendens was about 35 Ω · cm2 and not different between these two segments. It was, however, about 3 times higher in rectum (99 Ω · cm2). This finding is in accord with our previous observation of about 3-fold higher net fluxes of ions and water in colon ascendens and colon descendens than in rectum. It confirms the concept of a main functional difference between the terminal part of the large intestine (rectum) and the more proximal segments (colon). (ii) The acutely (within hours) varied level of aldosterone by keeping the rats for 7 h in anaesthesia caused in the rectum a more than 10-fold increase in short circuit current (ISC) and transepithelial voltage but no significant decrease in resistance. Similarly, the decline inISC, as regularly observed in the early phase of in vitro measurements on partially stripped large intestine, was paralleled by voltage changes but not by changes in resistance. We conclude that the wide range of resistance values published so far was caused to a great extent by including various portions of colon or rectum. (iii) By comparing intact (not stripped) and partially stripped preparations (muscularis propria removed) of the rectum it was shown that partial stripping did not alter the epithelial resistance but reduced the subepithelial resistance in this segment from 26 to 8 Ω · cm2, or by 68%. Subepithelial resistances of stripped rectum, colon ascendens and colon descendens were 8, 12 and 13 Ω · cm2, respectively. Based on these figures,ISC of conventional voltage clamp measurements is underestimated due to subepithelial tissue layers in intact rectum by 23% and in the partially stripped preparations of rectum, colon ascendens and colon descendens by 9%, 34%, and 37%, respectively.