The kinetics of ouabain‐sensitive ionic transport in the rabbit carotid artery.

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Abstract
Ouabain (0.1 mM)-sensitive 42K influx and 24Na efflux were measured in rabbit carotid arteries under conditions of high cellular K, [K]i, and Na, [Na]i. About 50% of the total fluxes are ouabain-sensitive (active) under conditions of high [K]i. The extracelluar 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. The ouabain-sensitive 42K influx and 24Na efflux in high-K tissues were measured at varying external K concentrations, [K]o, and normal concentrations of external Na, [Na]o. Sigmoidal kinetics were observed and fitted to a co-operative interaction model. The maximal efflux of 24Na, 0.245 .mu.eq/g wet wt/min, was about 1.4 times that for 42K influx. Half-maximal stimulation was achieved at .**GRAPHIC**. of 2.4 mM for Na, and 3.4 mM for K transport. The flux ratio of Na to K approximated 1.5. 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. 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 .mu.eq/g wet wt/min and .**GRAPHIC**. = 3.1 mM. The 42K influx was 2/3 the active Na efflux for [K]o .ltoreq. 5 mM. At high [K]o, the influx greatly exceeded predicted levels. Evidence is presented for a ouabain-sensitive membrane hyperpolarization being responsible for an additional influx of 42K. 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 .mu.eq/g wt/min and .**GRAPHIC**. = 49 mmol/l cell water, which is about twice the physiological operating point. 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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