Equilibrium and steady-state models of the coupling between the amino acid gradient and the sodium electrochemical gradient in mouse ascites- tumour cells.

1. The tumour cells were incubated at 37 degrees C in Ringer solutions containing glucose, 1 mM-methionine, various concentrations of Na+ and K+ and, in some instances, ouabain or valinomycin to lower the membrane potential generated by the Na+ pump. After about 30 min, when the system had reached a steady state, the ratio [extracellular Na+]/[cellular Na+] varied from about 0.6 to 3.2 with the ionic conditions. The membrane potential, determined by means of the fluorescent probe 3,3'-dipropyloxadicarbocyanine, also varied systemically from about zero to--55mV.2. the ratio [cellular methionine]/[extra-cellular methionine] varied from about 1 to 35 in these circumstances. The Na+ electrochemical gradient, measured in the same units, varied from about 1 to 30. Its magnitude in 46 assays was almost directly proportional to, though slightly smaller than, the methionine gradient. 3. A mathematical model was used to define the relation between these two gradients, which were not in equilibrium, owing to the presence of a leak pathway for the amino acid. On the assumption that the values of [cellular Na+] were correct, the methionine gradient was about 1.8 times larger than this version of the gradient hypothesis predicted.