Abstract
The osmotic water permeability coefficient, L(p), for human and dog red cells 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(p) falls from 1.87 x 10(-11) cm(3)/dyne sec at 199 mOSM to 0.76 x 10(-11) cm(3)/dyne sec at 516 mOSM. A similar decrease was observed for dog red cells. Moreover, L(p) 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(p) was not affected by pH in the range of 6 to 8 nor by the presence of drugs such as valinomycin (1 x 10(-6)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.