Ion exchange activity in pulmonary artery smooth muscle cells: the response to hypoxia

The purposes of this study were to determine 1) the presence of the major i on transport activities that regulate cytoplasmic pH (pH(c)) in cat pulmona ry artery smooth muscle cells, i.e., Na+/H+ and the Na+-dependent and -inde pendent Cl-/HCO3- exchange, 2) whether pH(c) changes in cells from small (S PAs) and large (LPAs) pulmonary arteries during hypoxia, and 3) whether cha nges in pH(c) are due to changes in the balance of exchange activities. Exc hange activities as defined by physiological maneuvers rather than molecula r identity were ascertained with fluorescence microscopy to document change s in the ratio of the pH(c) indicator 2',7'-bis-(2-carboxyethyl)-5-(and-6)- carboxyfluorescein. Steady-state pH(c) was higher in LPA than in SPA normox ic smooth muscle cells. SPAs and LPAs possessed all three transport activit ies; in HCO3--containing normoxic solutions, Cl-/HCO3- exchange rather than Na+/H+ exchange set the level of pH(c);in HCO3--containing hypoxic solutio ns, pH(c) increased in SPA and decreased in LPA cells; altering the baselin e pH(c) of a cell type to that of the other did not change the direction of the pH(c) response during hypoxia. The absence of Na+ prevented hypoxia-in duced alkalinization in SPA cells; in both cell types, inhibiting the Cl-/H CO3- exchange activities reversed the normal direction of pHc changes durin g hypoxia.