Uptake of 22Na+ by cultured dog kidney cells (MDCK)

22Na+ uptake measurements were conducted on the dog kidney cell line, MDCK, to determine the mechanism of ouabain-insensitive sodium transport. The radioisotope was found to be taken up into monolayer cultures via an ATP-independent, saturable process (Km = 40 mM). The presence of sodium on the opposite side of the membrane gave rise to a transstimulation of the 22Na+ flux. Studies utilizing potassium and valinomycin suggested that the transport system was insensitive to changes in the membrane potential. Replacement of chloride in the assay buffer with other anions did not decrease the rate of 22Na+ uptake at 14 mMNa+, but bicarbonate and acetate were stimulatory. Potassium and rubidium increased the rate of 22Na+ influx (Ka = 13mM with 14 mM NaCL in the medium). Lithium (Ki = 7.5mM) and amiloride (Ki = 1.7 x 10(-5) M) were competitive and partially (or mixed) competitive inhibitors, respectively. The data are consistent with a mechanism of sodium uptake that includes a carrier(s) capable of catalyzing net sodium uptake and sodium-sodium exchange.