Kinetic properties of the sodium bicarbonate (carbonate) symport in monkey kidney epithelial cells (BSC-1). Interactions between Na+, HCO-3, and pH.

Abstract

BSC-1 kidney epithelial cells derived from the African green monkey are known to express a Na+HCO3- symport (Jentsch, T. J., Schill, B. S., Schwartz, P., Matthes, H., Keller, S. K., and Wiederholt, M. (1985) J. Biol. Chem. 260, 15554-15560). In the present work, 4,4-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 22Na+ uptake into confluent monolayers of BSC-1 is measured in the presence of ouabain (10(-4) M) and amiloride (10(-3) M) to define the interactions between Na+ and HCO3- binding and pH. Dependence of DIDS-sensitive 22Na+ fluxes on either Na+ or HCO3- can be described by Michaelis-Menten kinetics. External apparent Km for HCO3- decreases with increasing Na+ concentration (Km app (HCO3-) = 36 +/- 10, 18 +/- 5, and 9 +/- 3 mM at 20, 45, and 151 mM Na+o, respectively (pHo = 7.4)). Similarly, external apparent Km for Na+ decreases with increasing HCO3- concentration (Km app (Na+) = 73 +/- 22, 28 +/- 8, and 14 +/- 4 mM at 6, 17, and 56 mM HCO3o-, respectively (pHo = 7.4)). Vmax app remains constant within the experimental error. When data are replotted as a function of calculated NaCO3- concentration, they can be approximated by a single Michaelis-Menten equation. DIDS-sensitive uptake at constant Na+ and HCO3- displays a broad pH optimum in the range between 7.2 and 7.6. The data are compatible with the ion pair model in which the transported species, NaCO3-, binds to the transport site with Km = 15.3 +/- 4 microM. However, the data may also be fitted by either a random or ordered bireactant system. Sets of parameters necessary for these fits are given.