Comparison of rubidium-86 and potassium-42 fluxes in rat aorta

Potassium transport has frequently been assessed by measurement of 86Rb. However, recent reports indicated that the K ion channels are selective to K over Rb. Therefore, the purpose of this study was to evaluate whether the basal and stimulated fluxes of Rb and K were equivalent in rat aorta in the absence of endothelium. The ouabain-sensitive (active) uptake of 86Rb and 42K was similar. However, the basal 86Rb efflux was only 80% of the 42K efflux. Norepinephrine and KCl depolarization stimulated 86Rb and 42K effluxes via a calcium-dependent process. The stimulated 86Rb efflux ranged from 56 to 74% of the 42K efflux. Diltiazem reduced the KCl-stimulated 86Rb and 42K effluxes. The 86Rb efflux was 82% of the 42K efflux in the presence of KCl plus diltiazem, similar to that under basal conditions. Substitution of Rb for K in the incubation solution was associated with a marked increase in spontaneous contractile activity. There was no change in the norepinephrine concentration required for a 50% stimulation of contraction or 86Rb efflux from Rb-loaded tissues. We conclude from these studies that the basal and calcium-activated potassium channels are selective for K over Rb and therefore 86Rb fluxes quantitatively underestimate that of 42K. However, 86Rb is an appropriate substitute for the measurement of active K transport.