The osmotic water permeability coefficient, L~, for human and dog red ceils has been measured as a function of medium osmolality, and found to depend on the osmolality of the bathing medium. In the case of human red cells L~ falls from 1.87 X 10 -11 cm3/dyne sec at 199 mOsM to 0.76 X 10 -I' cmS/dyne sec at 516 mOs~. A similar decrease was observed for dog red ceils. Moreover, L~ was independent of the direction of water movement and the nature of the solute used to provide the osmotic pressure gradient; it depended only on the final osmolality of the medium. Furthermore, L~ was not affected by pH in the range of 6 to 8 nor by the presence of drugs such as valinomycin (1 X 10 -e M) and tetrodotoxin (3.2 X 10 -6 M). The instantaneous nature of the response to changes in external osmolality suggests that the hydraulic conductivity of the membrane is controlled by a thin layer at the outer face of the membrane. I N T R O D U C T I O NIt has long been known that the permeability of single cells depends upon the osmolality of the medium in which the measurements have been made (Luck~ and McCutcheon (1)). For a given driving force, cell swelling measured under hypotonic conditions appeared to be much faster than cell shrinking measured under hypertonic conditions. Dainty and Hope (2) have pointed out that this directional effect of water movement may be only apparent due to a "sweeping away" effect associated with the presence of an unstirred layer around the cell surface. A complex dependence of permeability properties on the osmolality of the bathing solutions has been reported in systems with more than a single series barrier such as the gall bladder studied by Diamond (3). It was therefore decided to study the basic mechanism in a simple system in order to determine the direct effect of osmolality on water permeability. In m a m m alian red cells there is only a single limiting membrane and the unstirred layer has been shown by Sha'afi, et al. (4) to have a negligible effect on the hydraulic permeability coefficient, L~. The present results indicate that L~ is
Studies have been made on the cation transport system of the dog red cell, a system of particular interest because it has been shown that there is a marked dependence of cation fluxes on the cell volume. We have found that a 10 % decrease in cell volume causes a large increase in I hr uptake of 24 Na as well as a considerable inhibition of 42K uptake. This effect cannot be produced by a difference in medium osmolality but rather requires the cell volume to change. Dog red cell uptake of 2 4Na is not inhibited by iodoacetate. Phloretin inhibits 24Na uptake and lactate production, and virtually abolishes the volume effect on Na uptake. These several observations may be accounted for in terms of a working hypothesis which presupposes a cation carrier complex which pumps K into and Na out of cells of normal volume. When the cells are shrunken the carrier specificity shifts to an external Na-specific mode and there is a large increase in 24 Na uptake, driven by the inwardly directed Na electrochemical potential gradient.Some animal species, such as the cat and the dog, have red blood cells the electrolyte composition of which approaches that of the plasma (1), in contrast to human and most other mammalian red cells. Other characteristics of the electrolyte transport mechanism in dog and cat red cells also differ significantly from human and other mammalian cells. For example, there appears to be no measurable ouabain-sensitive component to the Na or K fluxes in these cells (2, 3) and membrane fragments do not show any significant ouabain-inhibitable Na-K-dependent adenosine triphosphatase (ATPase) (4).A phenomenon of particular interest in this system is the volume dependence of the Na and K fluxes initially reported by Davson (5) and subsequently studied by Parker and Hoffman (6). These investigators and others (7) demonstrated that osmotic shrinking of the dog red cell is accompanied by a dramatic increase in Na flux and a significant decrease in the flux of K. Conversely, when the cells are swollen, Na flux falls while K flux rises. This phenomenon does not occur when the cells are starved to the point at which 46
The effect of exogenous adenosine triphosphate (ATP) and other nucleotides on the transport of Na in various mammalian red cells has been studied. While they have no effect on the transport of Na in human and cat red cells, in dog red cells adenosine and its mono-, di- and triphosphorylated forms were found to increase Na-influx. Of these, ATP has the most striking effect, causing a more than 8-fold increase at a concentration of 0.6 mM and exerting this effect at a dose range of 10(-5) to 10(-3) M. The effect of ATP is rapid (less than 5 minutes) and can be reversed by washing or the addition of calcium or magnesium. In contrast to the adenosine series other phosphorylated nucleotides (GTP, CTP, UDP, GDP and cAMP) have no effect. The well known volume dependent Na-transport in these cells is reversed in the presence of 0.6 mM ATP. It is suggested that ATP acts on passive cation movements either by chelation of membrane charge or by a direct interaction with membrane proteins and may be involved in the volume regulation of cation transport in the dog erythrocyte.
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