Nellans H N, Kimberg D V
Am J Physiol. 1979 Apr;236(4):E473-81. doi: 10.1152/ajpendo.1979.236.4.E473.
The mechanism of apparent calcium secretion by unstripped rat ileum in vitro has been investigated using mannitol and polyethylene glycol (mol wt 900) as markers for extracellular transepithelial flux. In the absence of electrochemical gradients between the mucosal and serosal bathing media in a modified Ussing chamber, net fluxes of both mannitol and polyethylene glycol were observed from serosa to mucosa in the presence of 11 mM D-glucose. Mucosal-to-serosal calcium flux reveals a significant cellular component, but serosal-to-mucosal calcium permeability is a linear function of mannitol permeability, suggesting an exclusively extracellular route. For the mucosal-to-serosal fluxes, inhibition of calcium flux by 1 mM N-ethylmaleimide results in a calcium-to-mannitol permeability ratio indistinguishable from that measured for serosal-to-mucosal flux. This evidence suggests that the apparent calcium secretion observed at 10 mM medium calcium is not the result of a cellular secretory mechanism. It is proposed that a hydrostatic driving force generated internal to the tissue but external to the cells results in net calcium secretion at calcium conentrations that saturate the cellular absorptive mechanism.
利用甘露醇和聚乙二醇（分子量900）作为细胞外跨上皮通量的标志物，对体外未剥离的大鼠回肠明显的钙分泌机制进行了研究。在改良的尤斯灌流室中，当黏膜和浆膜浴液之间不存在电化学梯度时，在11 mM D-葡萄糖存在的情况下，观察到甘露醇和聚乙二醇都有从浆膜到黏膜的净通量。黏膜到浆膜的钙通量显示出显著的细胞成分，但浆膜到黏膜的钙通透性是甘露醇通透性的线性函数，表明存在唯一的细胞外途径。对于黏膜到浆膜的通量，1 mM N-乙基马来酰亚胺对钙通量的抑制导致钙与甘露醇的通透性比值与浆膜到黏膜通量所测得的比值无法区分。这一证据表明，在培养基钙浓度为10 mM时观察到的明显钙分泌不是细胞分泌机制的结果。有人提出，组织内部但细胞外部产生的静水压驱动力导致在钙浓度使细胞吸收机制饱和时出现净钙分泌。
