1. Ouabain (0.1 mM)-sensitive 42K influx and 24Na efflux have been measured in rabbit carotid arteries under conditions of high cellular potassium, [K]i, as well as sodium, [Na]i. About 50% of the total fluxes are ouabain-sensitive (active) under conditions of high [K]i. 2. The extracellular space, determined by 60Co-EDTA, was relatively large in comparison to cellular water. The ionic concentrations in normal solution, estimated from isotope flux components, are: [Na]i = 24; [K]i = 169; [Cl]i = 68 mmol/l cell water. 3. The ouabain=sensitive 42K influx and 24Na efflux in high-K tissues were measured at varying external concentrations of potassium, [K]o, and normal concentrations of external sodium, [Na]o. Sigmoidal kinetics were observed and fitted to a co-operative interaction model. The maximal efflux of 24Na, 0.245 muequiv/g wet weight per minute, was about 1.4 times that for 42K influx. Half-maximal stimulation was achieved at [K]0.5o of 2.4 mM for Na, and 3.4 mM for K transport. The flux ratio of Na to K approximated 1.5. 4. Increased 42K efflux was found in the presence of ouabain and the passive influx of 42K was corrected for this effect. In the absence of this correction the ouabain-sensitive 42K influx would be reduced, and the Na/K flux ratio raised to about 2. 5. The [K]o-dependence of ouabain-sensitive fluxes was measured on Na-loaded tissues. 24Na efflux exhibited saturation kinetics with a maximum of 1.18 muequiv/g wet weight per minute and [K]0.5o = 3.1 mM. The 42K influx was two thirds the active Na efflux for [K]o less than or equal to 5 mM. At high [K]o, however, the influx greatly exceeded the predicted levels. Evidence is presented for a ouabain-sensitive membrane hyperpolarization being responsible for an additional influx of 42K. 6. The ouabain-sensitive 24Na efflux showed a sigmoidal dependence on [Na]i in the presence of [K]o = 10 mM and normal [Na]o. The maximal efflux was 0.88 muequiv/g weight per minute and [Na]0.5i = 49 mmol/l cell water, which is about twice the physiological operating point. 7. It is concluded that active Na and K transport in rabbit carotid artery follow sigmoidal kinetics and the flux ratio is about 1.5. Changes in [K]o and [Na]i over the physiological range can markedly affect transport, and may regulate vascular contraction by their action on electrogenic transport.
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