Pressure and temperature effects on human red cell cation transport

Summary The effects of hydrostatic pressure and temperature on the three components of K⁺ uptake in human red cells have been investigated, using ouabain and bumetanide to distinguish between the pump, passive diffusion and cotransport. The pressure sensitivity for passive diffusion has been shown to depend on the counter-ion present. The order of this effect, Cl^–>Br^–>NO₃^–>I^–, is the same as for the ionic partial modal volumes and the Hofmeister series. We have analyzed our experimental results thermodynamically, and propose a model for the activated transition-state complex of the potassium ion which involves the loss of water molecules from the secondary hydration shell, cosphere II.