Amino Acid Transport and stimulation by substrates in the absence of a Na2+ electrochemical potential gradient

Abstract

Uptake of alpha-aminoisobutyric acid (AIB) was examined in Ehrlich ascites tumor cells treated with the cation-exchange ionophore nigericin (20 microgram/ml). Membrane voltages were measured using the voltage-sensitive dye diethyloxadicarbocyanine (DOCC). In normal phosphate-buffered media, nigericin changed the distribution ratios of Na+ and K+ (the ratio of intra- to extracellular concentrations) nearly to unity, but AIB was still accumulated to a distribution ratio of approximately 9.0. When all but 40 mM Na+ in the medium was replaced by choline, nigericin resulted in K+ loss and Na+ gain and both cation distribution ratios approached 2.8-3.4, as would be expected if both ions were distributing near electrochemical equilibrium with a membrane voltage in the range of -28 to -33 mV. This conclusion was supported by the observation that the addition of 5 X 10(-7) M valinomycin to the nigericin-treated cell suspension produced no change in DOCC absorbance. In spite of the apparent zero electrochemical potential gradients for Na+ and K+, AIB was accumulated to a distribution ratio of 5.4 in the low-Na+ medium. Addition of 0.1 mM oubain or 50 microM vanadate did not alter the extent of AIB accumulation as would have been expected if a large component of the membrane voltage were due to electrogenic operation of the (Na+ + K+)-ATPase. Addition of lactate, pyruvate or glucose increased the AIB distribution ratios to 11.9, 9.4 and 15.3, respectively. The effect of glucose could be explained, at least in part, by an enhanced Na+ electrochemical potential gradient. However, neither lactate nor pyruvate produced any change either in membrane voltage or the intracellular Na+ concentration. Therefore, these results confirm the existence of a metabolic energy source which is coupled to AIB accumulation and operates in addition to the Na+ co-transport mechanism, and which is augmented by metabolic substrates such as lactate and pyruvate.