Evidence from fluorescence microscopy and comparative studies that rat, ovine and bovine colonic crypts are absorptive.

Abstract

SUMMARY1. To test whether colonic crypts are secretory or absorptive interstitial [Na+] in rat descending colonic mucosa is determined using video-enhanced imaging of the impermeant acid form of the fluorescent Na+ probe SBFI (Molecular Probes) and intracellular [Na'] (FS). FS accumulates rapidly within crypt lumens of control tissues to a 2-fold higher concentration than in the external bathing solution, but FS does not accumulate in crypts of tissues treated with azide (2 mM). The increment in FS accumulation within the crypt lumen above the bulk solution decreases by 80 % within 1 min following exposure to theophylline (10 mM), indicating that fluid absorption into crypts is reduced. Estimates of the total fluid influx from the rate and extent of FS concentration polarization within crypts indicate that it is sufficient to account for the entire transcolonic fluid absorption.3. Comparative studies of isolated bovine and ovine colon were also undertaken to investigate the failure of bovine colon to generate a hypertonic absorbate and hence its incapacity to produce hard faeces. The interstitial fluid surrounding ovine colonic crypts is hypertonic to the bulk solution, whereas the interstitial fluid surrounding bovine colonic crypts is nearly isotonic with the bathing solution. Additionally, fluorescein disulphonate accumulates within ovine colonic crypt lumens by concentration polarization, whereas no concentration of FS occurs within bovine colonic crypt lumens. This corroborates the view that a hypertonic interstitial fluid is absent from bovine colon mainly because of a high rate of transepithelial leakage of low molecular weight solutes via paracellular routes. MS 9602 18PHY 460

Effects on fluid and Na+ flux of varying luminal hydraulic resistance in rat colon in vivo.

“…X in all cases for ileal fluid n = 13 and for colonic fluid n = 17; for 2 5 % agarose gels with ileum n = 22 and with colon n = 25. dehydrate faeces or a loose hydrogel, such as used in this study, the osmotic pressure has to be transduced to a hydrostatic suction pressure which can be propagated some distance from the epithelial membrane surface into the semi-solid luminal contents of the colon. The mechanism whereby the colon transduces the large osmotic pressure of the pericryptal fluid surrounding colonic crypts into a large hydraulic suction pressure without preventing fluid reflux via wide paracellular channels has been discussed previously (Bleakman & Naftalin, 1990;McKie et al 1991;Pedley & Naftalin, 1993). The results in this paper show that the mechanism proposed applies also to the in vivo condition and for the first time quantifies the colonic dehydrating force and power.…”

“…rats, rabbits, sheep and humans, do so by transporting fluid against the high hydraulic resistance imposed by the solid faecal mass (McKie, Powrie & . The ability to move fluid against this hydraulic resistance is dependent upon the colon's capacity to generate a hypertonic absorbate (Bleakman & Naftalin, 1990;McKie, Goecke & Naftalin, 1991;Pedley & Naftalin, 1993). The hypertonic absorbate generates an osmotic gradient across the crypt wall which creates a high fluid tension within the crypt lumen.…”

Electrolyte Transport in the Mammalian Colon: Mechanisms and Implications for Disease

“…As outlined below, absorption can be electrogenic via the ENaC or is electroneutral via parallel Na ϩ /H ϩ and Cl Ϫ /HCO 3 Ϫ exchange. According to the work of some groups, the high osmotic pressure gradient that is necessary to absorb water from the intestinal lumen and to generate the mammalian feces can only be created by a trapping mechanism for ions located in the crypts rather than the surface epithelium (434,435,457,571). These interesting results are based on fluorescence shifts using fluorescent dextrans.…”

Fluid absorption in isolated perfused colonic crypts.

“…However, recent studies in rat colon (5) and jejunum (6) suggest that secretory mechanisms also may be present in surface and villous cells. To date, there has been no direct evidence that absorptive processes are located in crypt epithelium, although it has been suggested, based on theoretical considerations and indirect observations, that crypts absorb fluid (7,8).…”