AJP - Lung Cellular and Molecular Physiology, Vol 269, Issue 1 20-L29, Copyright © 1995 by American Physiological Society
Kinetic analysis of cytosolic pH regulation in alveolar macrophages: V-ATPase-mediated responses to a weak acid
A. Bidani and T. A. Heming
Department of Internal Medicine, University of Texas Medical Branch, Galveston 77555-0561, USA.
Three different mechanisms interact to control the cytosolic pH (pHi) of
alveolar macrophages (M phi), namely, plasmalemmal vacuolar-type
H(+)-ATPase (V-ATPase), Na+/H+ exchange, and Na(+)-independent HCO3-/Cl-
exchange. To investigate the activity of plasmalemmal V-ATPase in alveolar
M phi, we developed a nonlinear mathematical model of pHi regulation that
incorporates the biophysical determinants of pHi and the fluxes of
individual acid-base equivalents. The model was used to analyze the
acid-base responses of rabbit alveolar M phi to a weak acid (propionic
acid) under conditions that favored V-ATPase-mediated effects (presence of
1 mM amiloride and nominal absence of CO2). The pHi was measured using the
fluorescent probe, 2',7'-biscarboxethyl-5,6-carboxyfluorescein. M phi
exposure to propionic acid caused a rapid fall in pHi. Recovery of pHi
after acid loading varied directly with the magnitude of the acid load.
Mathematical analyses showed that pHi recovery was hindered by persistent
influx of propionic acid driven, in turn, by transporter-mediated H+
extrusion and propionate efflux. Eventually, a new steady state was
established in which propionate and H+ were cycled out of the M phi and
propionic acid was recycled into the cell. As a consequence, model
predictions of the rate of V-ATPase-mediated H+ efflux were almost twice
that estimated from experimental determinations of the initial rate of pHi
recovery.
