Schreiber S, Scheid P
Institut für Physiologie, Ruhr-Universität Bochum, Germany.
Am J Physiol. 1997 Jan;272(1 Pt 1):G63-70. doi: 10.1152/ajpgi.1997.272.1.G63.
Proton transport with the gastric mucus was investigated in the guinea pig in vitro by use of three experimental series. In series I, pH profiles were obtained in the mucus and mucosa of a gastric explant with fine-tipped double-barreled microelectrodes. With a luminal pH of 1.8, pH increased across this layer to approximately 6 at the epithelial surface. Thickness of the gastric mucous gel layer increased continuously by 170 +/- 100 microns/h in the unstimulated and by 450 +/- 120 microns/h in the histamine-stimulated preparation (means +/- SD). In series II, fresh guinea pig gastric mucus was obtained from the gastric mucosa and titrated at 10 degrees C from pH 6.5 to 0.7, followed by an incubation period of 30 min at 37 degrees C. During this incubation period, a spontaneous acidic shift was observed, corresponding to a proton release from the mucus of 130 +/- 19 mM. This proton release could be blocked by the pepsin inhibitor pepstatin and was not observed when titrating down to only pH 3. Buffer values calculated as the mean slope of the titration curves in the pH range of 7 to 3 averaged 40 mM/pH unit. In series III, when titration was repeated with purified porcine mucin, no proton release was observed during incubation at pH 1.0, unless pepsinogen (375 U/ml) had been added before titration. Proton release during incubation at pH 1.0 and 37 degrees C in the presence of pepsinogen averaged 50 mM. The data suggest that protons secreted by the gastric mucosa are buffered by the continuously secreted mucus and transported, bound to the proteins of the mucus, toward the gastric lumen. During this transport, pepsinogen is converted within the mucus to pepsin. Pepsin modifies the buffering properties of the mucus, whereby protons are released from the protein binding. Thus the mucus forms a vehicle for proton transport toward the gastric lumen while, at the same time, constituting a diffusion barrier to prevent proton backdiffusion toward the gastric epithelium.
通过三个实验系列，在体外对豚鼠胃黏液中的质子转运进行了研究。在系列I中，使用尖细的双管微电极获取胃外植体黏液和黏膜中的pH分布图。管腔pH为1.8时，该层的pH值在上皮表面处升高至约6。在未受刺激的情况下，胃黏液凝胶层的厚度以170±100微米/小时的速度持续增加，在组胺刺激的制剂中以450±120微米/小时的速度增加（平均值±标准差）。在系列II中，从胃黏膜获取新鲜的豚鼠胃黏液，并在10℃下从pH 6.5滴定至0.7，随后在37℃下孵育30分钟。在此孵育期间，观察到自发的酸性变化，相当于从黏液中释放出130±19 mM的质子。这种质子释放可被胃蛋白酶抑制剂胃蛋白酶抑素阻断，当滴定至仅pH 3时未观察到这种情况。在pH 7至3范围内，滴定曲线的平均斜率计算得出的缓冲值平均为40 mM/pH单位。在系列III中，用纯化的猪黏蛋白重复滴定时，在pH 1.0孵育期间未观察到质子释放，除非在滴定前加入胃蛋白酶原（375 U/ml）。在pH 1.0和37℃下，在胃蛋白酶原存在的情况下孵育期间的质子释放平均为50 mM。数据表明，胃黏膜分泌的质子被持续分泌的黏液缓冲，并与黏液中的蛋白质结合，向胃腔转运。在此转运过程中，胃蛋白酶原在黏液中转化为胃蛋白酶。胃蛋白酶改变黏液的缓冲特性，从而使质子从蛋白质结合中释放出来。因此，黏液形成了质子向胃腔转运的载体，同时构成了防止质子向胃上皮反向扩散的扩散屏障。
