A physiological and ultrastructural examination of rat jejunum

Extracellular volume expansion has been shown to decrease net absorption of sodium and water in the jejunum. It has been hypothesized that this is due to an increase in the secretory unidirectional fluxes without an equal change in the absorptive unidirectional fluxes of sodium and water. The purpose of this study was to investigate the changes in net and unidirectional fluxes of salt and water due to extracellular volume expansion correlate such changes, if possible, with any observed ultrastructural effects. It was found that: 1) volume expansion of in vivo jejunal segments resulted in several ultrastructural modifications; increased microvilli size, changes in lateral and basal spaces, formation of junctional "blisters", and increased RER cisternae width. The degree and direction of such changes was dependent on whether the preparation was perfused or nonperfused, in some cases. 2) five percent of body weight, acute volume expansion with Krebs-Ringer decreased net sodium absorption, apparently by increasing unidirectional sodium secretory flux. 3) ten percent of body weight, acute volume expansion with Krebs-Ringer had no effect on net sodium or net water fluxes. Unidirectional fluxes, both secretory and absorptive of sodium and water were increased proportionately. Absorptive site blood flow was increased. 4) eversion of jejunum results in ultrastructural changes; increase microvilli length and width, and disruption of the lateral spaces by intercellular "lakes". No changes were observed in the tight junctions. These changes, however, are in those structures thought to be involved with salt and water transport. Therefore, the suitability of the everted preparation may be limited in salt and water transport studies. No quantitative correlations were observed in comparing physiological vs. ultrastructural changes among the perfused and nonperfused studies.