Diamond J M, Bossert W H
Department of Physiology, University of California at Los Angeles School of Medicine, Los Angeles, California 90024, USA.
J Cell Biol. 1968 Jun;37(3):694-702. doi: 10.1083/jcb.37.3.694.
Many fluid-transporting epithelia possess dead-end, long, and narrow channels opening in the direction to which fluid is being transported (basal infoldings, lateral intercellular spaces, etc.). These channels have been thought to possess geometrical significance as standing-gradient flow systems, in which active solute transport into the channel makes the channel contents hypertonic and permits water-to-solute coupling. However, some secretory epithelia (choroid plexus, Malpighian tubule, rectal gland, etc.) have "backwards" channels opening in the direction from which fluid is being transported. It is shown that these backwards channels can function as standing-gradient flow systems in which solute transport out of the channel makes the channel contents hypotonic and results in coupled water flow into the channel mouth. The dependence of the transported osmolarity (isotonic or hypertonic) on channel radius, length, and other parameters is calculated for backwards channels for values of these parameters in the physiological range. In addition to backwards channels' being hypotonic rather than hypertonic, they are predicted to differ from "forwards" channels in that some restrictions are imposed by the problem of solute exhaustion, and in the presence of a sweeping-in effect on other solutes which limits the solutes that may be transported.
许多液体运输上皮细胞具有盲端、长且窄的通道，这些通道朝着液体运输的方向开口（基底内褶、细胞间侧隙等）。这些通道被认为作为驻梯度流系统具有几何意义，在该系统中，溶质主动运输到通道中使通道内容物变为高渗，并允许水与溶质耦合。然而，一些分泌上皮细胞（脉络丛、马尔皮基氏小管、直肠腺等）具有“反向”通道，这些通道朝着液体运输的相反方向开口。研究表明，这些反向通道可以作为驻梯度流系统发挥作用，在该系统中，溶质从通道中运输出去使通道内容物变为低渗，并导致耦合水流向通道口。对于生理范围内这些参数的值，计算了反向通道中运输的渗透压（等渗或高渗）对通道半径、长度和其他参数的依赖性。除了反向通道是低渗而非高渗之外，预计它们与“正向”通道的不同之处在于，溶质耗尽问题会施加一些限制，并且对其他溶质存在一种扫入效应，这限制了可能运输的溶质。
