Electrogenic sodium absorption in rabbit cecum in vitro

Abstract

Regional variations in the ion transport of the colon may have significant physiological and pathophysiological implications. However, only limited studies have been performed in cecum, which comprises 50% of the macrosurface of the rabbit colon. In vitro under short-circuit conditions, cecum actively absorbed Na and Cl (J(net)(Na) = 5.6 ± 0.3, J(net)(Cl) = 1.5 ± 0.3 μeq·cm-2·h-1) with a short-circuit current (I(sc)) of 6.29 ± 0.2 μeq·cm-2·h-1·Cl substitution with sulfate decreased both J(net)(Na) and I(sc) by 1.3 μeq/cm2·h-1·HCO3 removal decreased both J(net)(Na) and I(wsc) 3.3 μeq·cm-2·h-1. This effect was due primarily to removal of serosal HCO3. There was both a linear correlation between J(net)(Na) and I(sc) (r = 0.845) and a concentration-dependent stimulation of I(sc) by increasing [Na] in the bathing media. However, 10-4 M amiloride did not significantly alter either I(sc) or J(net)(Na). In contrast, 10-4 M phenamil, an amiloride analogue highly specific for the Na channel, significantly blocked both I(sc) and J(net)(Na). The sulfhydryl reagent PCMBS increased I(sc); this response was reversed by phenamil. Electrogenic Cl secretion was stimulated by 1 mM theophylline, 10-4 M 8BrcAMP and 10-4 M 8BrGMP. None of the secretagogues inhibited J(net)(Na). Epinephrine (5.5 μM) increased J(net)(Na) from 5.9 ± 1.3 to 7.8 ± 1.1 (P = 0.02) and J(net)(Cl) from 0.1 ± 1.2 to 2.0 ± 0.8 (P NS) μeq·cm-1·h-1. Studies of pH stat demonstrated an epinephrine-stimulated increase in J(m-s)(HCO3) without a change in J(s-m)(HCO3). Thus, cecum exhibits a distinct type of electrogenic Na electrogenic Na absorption which is partially dependent on the presence of Cl and HCO3, not blocked by amiloride but by phenamil. Because of its large surface area and its novel mechanism of electrogenic Na trnsport, the cecum exerts an important regulatory role in colonic fluid and electrolyte balance.