McKie A T, Goecke I A, Naftalin R J
Division of Biomedical Sciences (Physiology Group), King's College London, Strand, United Kingdom.
Am J Physiol. 1991 Sep;261(3 Pt 1):G433-42. doi: 10.1152/ajpgi.1991.261.3.G433.
Although bovine and ovine descending colon absorbed solute and sodium at approximately the same rate in vitro, water absorption by bovine colon was faster (15.0 +/- 2.0 microliters.h-1.cm-2, n = 8) than by ovine colon (8.6 +/- 1.3 microliters.h-1.cm-2, n = 9; P less than 0.01). Consequently, the observed osmolality of cattle absorbate was lower (364 +/- 13 mosmol/kg, n = 8) than with sheep (807 +/- 135 mosmol/kg, n = 9; P less than 0.01). Paracellular permselectivity was examined to elucidate this difference; the permeability of bovine descending colon to [3H]polyethylene glycol 400 and 4000 was higher than in sheep (P less than 0.001). A paracellular solvent drag was observed in bovine but not in ovine colon, and the electrical resistance of bovine colon was lower (16.0 +/- 1.4 omega.cm2) than ovine colon (28.0 +/- 2.4 omega.cm2; P less than 0.001). Pore radii of 2.5 nm for ovine and 5 nm for bovine colonic paracellular route were estimated from these data. It is concluded that an increased hydraulic conductance of the "active" route for solute absorption combined with raised solute reflux via the wider paracellular pathway may account for the failure of cattle to form a hypertonic absorbate and, consequently, hard feces.
尽管牛和羊的降结肠在体外吸收溶质和钠的速率大致相同，但牛结肠的水吸收速度（15.0±2.0微升·小时⁻¹·厘米⁻²，n = 8）比羊结肠（8.6±1.3微升·小时⁻¹·厘米⁻²，n = 9；P<0.01）更快。因此，观察到的牛吸收液的渗透压较低（364±13毫摩尔/千克，n = 8），低于羊（807±135毫摩尔/千克，n = 9；P<0.01）。通过检查细胞旁渗透选择性来阐明这种差异；牛降结肠对[³H]聚乙二醇400和4000的通透性高于羊（P<0.001）。在牛结肠中观察到细胞旁溶剂拖曳现象，而在羊结肠中未观察到，并且牛结肠的电阻（16.0±1.4欧姆·厘米²）低于羊结肠（28.0±2.4欧姆·厘米²；P<0.001）。根据这些数据估计，羊结肠细胞旁途径的孔半径为2.5纳米，牛结肠为5纳米。得出的结论是，溶质吸收的“主动”途径的水力传导增加，加上通过更宽的细胞旁途径的溶质反流增加，可能是牛无法形成高渗吸收液并因此导致硬粪的原因。
