EFFECTS OF HIGH EXTERNAL CONCENTRATIONS OF K-86(+) EFFLUX IN HUMAN PLATELETS - EVIDENCE FOR NA+( ON RB)K+/2CL(-) COTRANSPORT/

1. Na+/K+/2Cl(-) co-transport mediates a bidirectional symport of Na+, K+ and Cl-. The important properties of the co-transport system are i ts requirement for Na+, K+ and Cl- and its inhibition by loop diuretic s such as bumetanide. This co-transporter has been described in a numb er of animal and human tissues, However, its presence in human platele ts, although inferred, has not been demonstrated directly. 2. We have studied the efflux of Rb-86(+) (a marker for K+) from Rb+-loaded plate lets, and have defined their response to stimulation by high concentra tions of external K+. 3. KCl (30-120 mmol/l) stimulated a concentratio n-dependent increase in Rb-86(+) efflux from the platelets. This efflu x was completely inhibited by bumetanide (10 mu mol/l) but was insensi tive to ouabain and R(+)-[(dihydroindenyl)oxy] alkanoic acid. It also required Cl- in the external medium, but did not depend on the presenc e of extracellular Na+. 4. These observations suggest that Rb-86(+) ef flux from platelets stimulated by external K+ occurs via Na+/K+/2Cl(-) co-transport acting in a K+/K+ (K+/Rb+) exchange mode. 5. Non-stimula ted efflux of Rb-86(+) from the platelets (i.e. in the presence of 5 m mol/l K+) had the characteristics of Na+/K+/2Cl(-) co-transport acting in normal mode. 5. Non-stimulated efflux of Rb-86(+) from the platele ts (i.e. in the presence of 5 mmol/l K+) had the characteristics of Na +/K+/2Cl(-) co-transport acting in normal mode.