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

Abstract

Changes in plasma volume in vivo cause rapid changes in extracellular pH by altering the plasma bicarbonate concentration at a constant Pco2 (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 (pHi). In the present study, alveolar macrophages were exposed to hyperosmotic medium in the absence or presence of the open-system buffers CO2-HCO3 −, propionic acid-propionate, or NH3-NH4 +. In the absence of open-system buffers, exposure to twice-normal osmolarity (2T) produced a slow cellular alkalinization [change in pHi(ΔpHi) ≈ 0.38; exponential time constant (τ) ≈ 120 s]. In the presence of 5% CO2, 2T caused a biphasic pHi response: a rapid increase (ΔpHi ≈ 0.10, τ ≈ 15 s) followed by a slower pHi increase. Identical rapid pHiincreases were produced by 2T in the presence of propionic acid (20 mM). Conversely, 2T caused a rapid pHi decrease (ΔpHi ≈ −0.21, τ ≈ 10 s) in the presence of NH3 (20 mM). Thus osmotic cell shrinkage caused rapid pHi changes of opposite direction in the presence of a weak acid buffer (contraction alkalosis with CO2 or propionic acid) vs. a weak base buffer (contraction acidosis with NH3). Graded ΔpHi were produced by varying extracellular osmolarity in the presence of open-system buffers; osmolarity increases of as little as 5–10% produced significant ΔpHi. The rapid pHi responses to 2T were insensitive to inhibitors of membrane H+ transport (ethylisopropylamiloride and bafilomycin A1). The results are consistent with shrinkage-induced disequilibria in the total cellular buffer system (i.e., intrinsic buffers plus added weak acid-base buffer).