Influence of loop diuretics and anions on passive potassium influx into human red cells

Passive K influx into human red cells was measured with and without Cl ions, Na ions and loop diuretics. Ouabain and loop diuretics appear to inhibit specifically and respectively the Na pump and (Na+K) 'co-transport'. Inhibitors of other pathways, e.g. 4,4'-diisothiocyantostilbene-2,2'-disulphonic acid or amiloride did not inhibit passive K influx. Loop diuretics inhibited with high apparent affinity in Na-containing media and with low apparent affinity in Na-free media where there was a substantial Cl-dependent component. The Cl concentration dependence was measured using six anion substitutions for Cl. With NO3, acetate and gluconate, the curves were sigmoidal and not fully saturable at 150 mM-Cl; with iodide and thiocyanate, the curves were convex; with sulphate, there was saturation at 120 mM-Cl. The half-maximal K influx as a function of [Na]0 was 40 mM for the Cl-dependent flux component and 12 mM for the diuretic-sensitive flux.