Leonhard-Marek Sabine, Stumpff Friederike, Brinkmann Inge, Breves Gerhard, Martens Holger
Dept. of Physiology, School of Veterinary Medicine, Bischofsholer Damm 15/102, D-30173 Hannover, Germany.
Am J Physiol Gastrointest Liver Physiol. 2005 Apr;288(4):G630-45. doi: 10.1152/ajpgi.00275.2004. Epub 2004 Nov 18.
High potassium diets lead to an inverse regulation of sodium and magnesium absorption in ruminants, suggesting some form of cross talk. Previous Ussing chamber experiments have demonstrated a divalent sensitive Na(+) conductance in the apical membrane of ruminal epithelium. Using patch-clamped ruminal epithelial cells, we could observe a divalent sensitive, nonselective cation conductance (NSCC) with K(+) permeability > Cs(+) permeability > Na(+) permeability. Conductance increased and rectification decreased when either Mg(2+) or both Ca(2+) and Mg(2+) were removed from the internal or external solution or both. The conductance could be blocked by Ba(2+), but not by tetraethylammonium (TEA). Subsequently, we studied this conductance measured as short-circuit current (I(sc)) in Ussing chambers. Forskolin, IBMX, and theophylline are known to block both I(sc) and Na transport across ruminal epithelium in the presence of divalent cations. When the NSCC was stimulated by removing mucosal calcium, an initial decrease in I(sc) was followed by a subsequent increase. The cAMP-mediated increase in I(sc) was reduced by low serosal Na(+) and serosal addition of imipramine or serosal amiloride and depended on the availability of mucosal magnesium. Luminal amiloride had no effect. Flux studies showed that low serosal Na(+) reduced (28)Mg fluxes from mucosal to serosal. The data suggest that cAMP stimulates basolateral Na(+)/Mg(2+) exchange, reducing cytosolic Mg. This increases sodium uptake through a magnesium-sensitive NSCC in the apical membrane. Likewise, the reduction in magnesium uptake that follows ingestion of high potassium fodder may facilitate sodium absorption, as observed in studies of ruminal osmoregulation. Possibly, grass tetany (hypomagnesemia) is a side effect of this useful mechanism.
高钾饮食会导致反刍动物对钠和镁的吸收出现反向调节，这表明存在某种形式的相互作用。之前的乌斯灌流小室实验已证明瘤胃上皮顶端膜存在一种对二价离子敏感的Na(+)电导。通过对瘤胃上皮细胞进行膜片钳实验，我们观察到一种对二价离子敏感的非选择性阳离子电导（NSCC），其对K(+)的通透性 > Cs(+)的通透性 > Na(+)的通透性。当从细胞内或细胞外溶液或两者中去除Mg(2+)或同时去除Ca(2+)和Mg(2+)时，电导增加，整流作用减弱。该电导可被Ba(2+)阻断，但不能被四乙铵（TEA）阻断。随后，我们在乌斯灌流小室中研究了以短路电流（I(sc)）衡量的这种电导。已知在存在二价阳离子的情况下，福斯高林、异丁基甲基黄嘌呤和茶碱会阻断I(sc)以及瘤胃上皮的Na转运。当通过去除黏膜钙刺激NSCC时，I(sc)最初会下降，随后会上升。低浆膜Na(+)、浆膜添加丙咪嗪或浆膜添加氨氯吡脒会降低cAMP介导的I(sc)增加，且这取决于黏膜镁的可用性。管腔氨氯吡脒没有作用。通量研究表明，低浆膜Na(+)会减少(28)Mg从黏膜到浆膜的通量。数据表明，cAMP刺激基底外侧Na(+)/Mg(2+)交换，降低细胞内镁含量。这会通过顶端膜中对镁敏感的NSCC增加钠的摄取。同样，如在瘤胃渗透调节研究中所观察到的，摄入高钾饲料后镁摄取的减少可能会促进钠的吸收。可能，青草搐搦（低镁血症）是这种有益机制的一种副作用。
