Sehested J, Diernaes L, Moller P D, Skadhauge E
Department of Anatomy and Physiology, Royal Veterinary and Agricultural University, Copenhagen, Denmark.
Exp Physiol. 1996 Jan;81(1):79-94. doi: 10.1113/expphysiol.1996.sp003920.
Unidirectional transport rates of sodium (22Na+) and chloride (36Cl-) across bovine rumen epithelium were measured in vitro by the Ussing chamber technique. The active and short-chain fatty acid (SCFA)-stimulated sodium transport was shown to fit Michaelis-Menten kinetics, and was rate limited mainly by one transport system, characterized by a Km of 43 mmol l-1 Na+ and a Jmax (maximal transport rate) of 6.2 mumol cm-2 h-1 Na+. It was confirmed that the basolateral Na+,K(+)-ATPase was essential for active sodium transport, and that an apical amiloride-sensitive sodium transport system (Na(+)-H+ exchange) was involved in a minimum of 60-70% of the active sodium transport in the presence of SCFAs (butyrate). The main part of both the mucosal-serosal (MS) and serosal-mucosal (SM) sodium flux was sensitive to an applied electrical potential difference (PD). It is noteworthy that an applied PD, equal to the in vivo PD (+30 mV, lumen as reference), abolished net transport of sodium. The stimulating effect of a mixture of acetate, propionate and butyrate on active sodium transport was confirmed, and it was further shown that the stimulating effect of each of the three SCFAs was nearly equal. Analogues of naturally occurring SCFAs (isobutyrate and 2-ethyl-butyrate) did not stimulate active sodium transport, but inhibited the stimulating effect of butyrate. The stimulating effect of butyrate was clearly concentration dependent and showed a maximum at approximately 20 mmol l-1 butyrate. Above this limit active sodium transport was decreased with increasing butyrate concentration. This suggests that there was a limit to the amount of butyrate that could be handled by the epithelium. The active sodium transport was clearly correlated with the chloride concentration, and was significantly reduced, but not abolished, by replacement of chloride with gluconate. Active transport of chloride was stimulated by butyrate and reduced by the Na(+)-H+ exchange inhibitor amiloride (3 mmol l-1). There was no effect of the Cl(-)-HCO3- exchange inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid; 0.5 mmol l-1) on sodium transport. HCO3- (13 mmol l-1) and CO2 (5%) themselves had only a small and non-significant stimulating effect on sodium fluxes, however, in the presence, but not the absence of HCO3- and CO2 in the experimental solutions acetazolamide (1 mmol l-1) significantly reduced active sodium transport. It is concluded that SCFAs could stimulate the active sodium and chloride transport as a result of their metabolism. The CO2 produced could stimulate apical Na(+)-H+ and Cl(-)-HCO3- exchangers running in parallel via increased H+ and HCO3- gradients.
采用尤斯灌流小室技术在体外测定了钠（(^{22}Na^+)）和氯（(^{36}Cl^-)）跨牛瘤胃上皮的单向转运速率。结果表明，主动转运和短链脂肪酸（SCFA）刺激的钠转运符合米氏动力学，且速率主要受一个转运系统限制，其特征为米氏常数（(K_m)）为(43 mmol·l^{-1} Na^+)，最大转运速率（(J_{max})）为(6.2 μmol·cm^{-2}·h^{-1} Na^+)。已证实基底外侧的(Na^+)，(K^+)-ATP酶对主动钠转运至关重要，并且在存在SCFAs（丁酸盐）的情况下，顶端的氨氯地平敏感钠转运系统（(Na^+)-(H^+)交换）至少参与了60 - 70%的主动钠转运。黏膜 - 浆膜（MS）和浆膜 - 黏膜（SM）钠通量的主要部分对施加的电位差（PD）敏感。值得注意的是，施加的PD等于体内PD（+30 mV，以管腔为参考）时，钠的净转运被消除。证实了乙酸盐、丙酸盐和丁酸盐混合物对主动钠转运的刺激作用，并且进一步表明三种SCFAs各自的刺激作用几乎相等。天然存在的SCFAs的类似物（异丁酸盐和2 - 乙基丁酸盐）不刺激主动钠转运，但抑制丁酸盐的刺激作用。丁酸盐的刺激作用明显呈浓度依赖性，在约(20 mmol·l^{-1})丁酸盐时达到最大值。超过此限度，随着丁酸盐浓度增加，主动钠转运降低。这表明上皮细胞能够处理的丁酸盐量存在限度。主动钠转运与氯浓度明显相关，用葡萄糖酸盐替代氯可使其显著降低，但未消除。丁酸盐刺激氯的主动转运，而钠 - 氢交换抑制剂氨氯地平（(3 mmol·l^{-1})）则使其降低。氯 - 碳酸氢根交换抑制剂二硫代二苯乙烯二磺酸（DIDS；(0.5 mmol·l^{-1})）对钠转运无影响。碳酸氢根（(13 mmol·l^{-1})）和二氧化碳（5%）本身对钠通量仅有微小且无显著意义的刺激作用，然而，在实验溶液中存在（而非不存在）碳酸氢根和二氧化碳时，乙酰唑胺（(1 mmol·l^{-1})）显著降低主动钠转运。得出的结论是，SCFAs因其代谢可刺激主动钠和氯转运。产生的二氧化碳可通过增加氢离子和碳酸氢根梯度刺激并行运行的顶端(Na^+)-(H^+)和(Cl^-)-(HCO^-)交换体。
