pH(i) responses to osmotic cell shrinkage in the presence of open-system buffers

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) (DeltapH(i)) approximately 0.38; exponential time constant (tau) approximately 120 s]. In the presence of 5% CO(2), 2T caused a biphasic pH(i) response: a rapid increase (DeltapH(i) approximately 0.10, tau approximately 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 (DeltapH(i) approximately -0.21, tau approximately 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 DeltapH(i) were produced by varying extracellular osmolarity in the presence of open-system buffers; osmolarity increases of as little as 5-10% produced significant DeltapH(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).