Mechanism of isotonic water transport in glands

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.