pHi responses to osmotic cell shrinkage in the presence of open-system buffers

Departments of Internal Medicine, and Physiology and Biophysics, University of Texas Medical Branch at Galveston, Galveston, Texas 77555-0876 Changes in plasma volume in vivo cause rapid changes in extracellular pH by altering the plasma bicarbonate concentration at a constant Pco 2 (Garella S, Chang BS, and Kahn SI. Kidney Int 8: 279, 1975). Few studies have examined the possibility that changes in cell volume produce comparable changes in intracellular pH (pH i ). In the present study, alveolar macrophages were exposed to hyperosmotic medium in the absence or presence of the open-system buffers CO 2 -HCO 3 , propionic acid-propionate, or NH 3 -NH 4 + . In the absence of open-system buffers, exposure to twice-normal osmolarity (2T) produced a slow cellular alkalinization [change in pH i ( pH i ) 0.38; exponential time constant ( ) 120 s]. In the presence of 5% CO 2 , 2T caused a biphasic pH i response: a rapid increase ( pH i 0.10, 15 s) followed by a slower pH i increase. Identical rapid pH i increases were produced by 2T in the presence of propionic acid (20 mM). Conversely, 2T caused a rapid pH i decrease ( pH i 0.21, 10 s) in the presence of NH 3 (20 mM). Thus osmotic cell shrinkage caused rapid pH i changes of opposite direction in the presence of a weak acid buffer (contraction alkalosis with CO 2 or propionic acid) vs. a weak base buffer (contraction acidosis with NH 3 ). Graded pH i were produced by varying extracellular osmolarity in the presence of open-system buffers; osmolarity increases of as little as 5-10% produced significant pH i . The rapid pH i responses to 2T were insensitive to inhibitors of membrane H + transport (ethylisopropylamiloride and bafilomycin A 1 ). The results are consistent with shrinkage-induced disequilibria in the total cellular buffer system (i.e., intrinsic buffers plus added weak acid-base buffer). alveolar macrophage; cell volume