Mechanism of isotonic water transport in glands

Abstract

Since water and electrolytes pass cell membranes via separate channels, there can be no interactions in the membranes, and osmotic interactions between water and solutes can be expressed as the product of solute flux, frictional coefficient of solute, and length of pathway. It becomes clear that isotonic transport via a cell is impossible. In glands, where cation-selective junctions impede anion flux between the cells, isotonic water transport is only possible if sodium, after having passed the junction, is reabsorbed in the acinus and returned to the serosal side. Thus it can be recycled via the cation-selective junction and exert its drag on water more than once. This hypothesis was tested on frog skin glands. Skins were mounted in flux chambers with identical Ringer solutions on both sides. Na channels of the principal cells were closed with amiloride in the outside solution, and secretion stimulated with noradrenaline in the inside solution. Influx and efflux of Na, K and Br (used as tracer for Cl) were measured on paired half-skins during the constant-secretion phase. Flux ratios for both Na and K were higher than expected for electrodiffusion, indicating outgoing solvent drag. Flux ratios for K were much higher than those for Na. This is an agreement with the concept that Na is reabsorbed in the acinus and K is not. Two independent expressions for the degree of sodium recycling are developed. Under all experimental conditions these expressions give values for the recycling which are in good agreement.

Membrane Mechanisms in Volume Regulation in Vertebrate Cells and Epithelia

“…Full lines, the pathways for Na + and Cl-; dashed loops, the recycling paths for Na +. h I> Distance from the inside solution to the junction; h2 , distance from the junction to the outside bath Experimental evidence for this recycling of Na + [317] strongly supports this hypothesis. Furthermore, it is experimentally possible to vary the relative amounts of N a + and Cl-which pass through the glands.…”

“…By a reformulation of Fick's law it is easily seen that the water/solute interactions can be expressed in terms of (a) forward and backward solute fluxes (b) free diffusion coefficients of the solutes, and (c) length of the transport path. An analysis in terms of this principle shows that isotonic water transport through a cell is impossible, even if the cell membranes were infinitely permeable to water [317]. The reason is that the friction of water against the stagnant solutes in the cell (for instance, proteins) forces the solvent (water) to lag behind the driving (transported) solute.…”

“…It has been suggested [317] that the processes of cell volume regulation are involved in the regulatory processes leading to isotonicity. As a background for discussing this problem it is necessary to realize that there is a problem.…”

Mechanism of fluid transport across corneal endothelium and other epithelial layers: a possible explanation based on cyclic cell volume regulatory changes

“…Later on, as the relatively high osmotic permeability of some epithelial cell membranes became apparent,17 simple transcellular local osmosis was proposed as an explanation 17 ; however, theoretical treatments 18–20 eventually showed that local osmosis could produce only hypertonic but not isotonic fluid transport. This led to an alternative proposal based on electro-osmotic coupling of a recirculating electrolyte,20 but such a mechanism could not account for some experimental results 21. This review proposes a novel idea—epithelial layers that transport fluid might do so via a cell volume driven pulsatile mechanism that would be essentially a particular case of cell volume regulation.…”

The regulation of epithelial cells volume in norm and pathology

“…Изображения заимствованы из [3] и модифицированы Изменения объема могут быть задействованы также в процессе изоосмотического транспорта, при котором ýпителий сохраняет изоосмотич-ность транспортируемой жидкости по отноше-нию к внеклеточным растворам [7]. Объяснение данному явлению было предложено H. Ussing и K. Eskesen и заключалось в том, что регулируе-мый кругооборот активно выбрасываемого иона натрия контролирует тоничность транспортиру-емой жидкости [67]. Позже H. Ussing дополнил ýту гипотезу, объясняя изоосмотичность транс-портируемой жидкости следующим образом: «ре-циркулирование натрия регулируется осмоляр-ностью секетируемой жидкости за счет действия на объем клетки и, как следствие, на объем-чув-ствительные транспортеры и каналы натрия» [68].…”