RESOLUTION OF OUABAIN-SENSITIVE AND EXTRACELLULAR K⁺-ACTIVATED Na⁺ EXTRUSION IN CARDIAC TISSUE

¹ Department of Physiology II, Wilhelmstrasse 31, D-5300 Bonn, F.R.G.

Intracellular sodium activity (aⁱ_Na) was measured in sheep cardiac Purkinje fibres exposed inter- mittently to calcium-free solutions containing 1·3 mM-Na⁺ with 0-16 mM-K⁺ and 0-0·2 mM ouabain. In potassium-free solution (0 K⁺) mean maximum Na+ pump activity (i.e. excluding Na⁺ influx and Na⁺-Ca²⁺ exchange) was 61% of that in the 4 mM-K⁺ control solution; in 0·2mM-ouabain-4 mM-K⁺ it was still 44% of control. This high ouabain concentration abolished Na⁺ extrusion only in 0 K⁺ while reducing it by the same absolute amount at all other extracellular K⁺ concentrations. The Na⁺ pump may therefore have two independent components in cardiac tissue.