1. Na+/K+/2Cl co-transport mediates a bidirectional symport of Na+, K+ and Cl. The important properties of the co-transport system are its requirement for Na+, K+ and Cl and its inhibition by loop diuretics such as bumetanide. This co-transporter has been described in a number of animal and human tissues. However, its presence in human platelets, although inferred, has not been demonstrated directly.

2. We have studied the efflux of 86Rb+ (a marker for K+) from Rb+-loaded platelets, and have defined their response to stimulation by high concentrations of external K+.

3. KCl (30–120 mmol/l) stimulated a concentration-dependent increase in 86Rb+ efflux from the platelets. This efflux was completely inhibited by bumetanide (10 μmol/l) but was insensitive to ouabain and R(+)-[(dihydroindenyl)oxy]alkanoic acid. It also required Cl in the external medium, but did not depend on the presence of extracellular Na+.

4. These observations suggest that 86Rb+ efflux from platelets stimulated by external K+ occurs via Na+/K+/2Cl co-transport acting in a K+/K+ (K+/Rb+) exchange mode.

5. Non-stimulated efflux of 86Rb+ from the platelets (i.e. in the presence of 5 mmol/l K+) had the characteristics of Na+/K+/2Cl co-transport acting in normal mode.

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