Rate-dependent changes in extracellular potassium in the rabbit atrium.

We measured levels of potassium ion in the extracellular space of isolated superfused rabbit atria continuously with double-barreled microelectrodes of which on barrel was a K+ liquid ion-exchanger microelectrode and the other a potential-sensing micropipette. Increases in heart rate resulted in transient increases in extracellular potassium ([K+]0). When the quiescent atrium was stimulated the maximal increase was 0.4 mM at rates of 60/min, 0.7 mM at 90/min, 0.9 mM at 120/min, 1.3 mM at 200/min, and 1.8 mM at 300/min. The increase was not sustained during continued stimulation but declined toward prestimulation levels. When the stimulus was terminated the extracellular potassium activity decreased below bathing solution values by 0.2 mM after 60/min, 0.5 mM after 90/min, 0.7 mM ater 120/min, 0.9 mM after 200/min, and 1.0 mM after 300/min and subsequently returned to a value equal to that of the bathing solution. The magnitude of the decline in extracellular potassium activity during prolonged stimulation was markedly decreased when the bathing solution contained either zero potassium, ouabain, LiCl, or a decreased Po2 such that an elevation in [K+]0 persisted during stimulation. Moreover, the reduction in [K+]0 that followed the cessation of stimulation also was inhibited. These results support a role of the Na-K pump in maintaining extracellular potassium activity during changes in cardiac rate.