The effect of 5‐hydroxytryptamine on the potassium exchange of human platelets

1. The concentration of potassium in platelets freshly isolated from nine persons was 74 ± 4·8 μmole/10 11 platelets (mean ± S.E.M. ). 2. The influx of potassium into platelets from citrated plasma at 37° C, estimated with the use of 42 K, was 18·3 ± 1·6 n‐mole/sec. 10 11 platelets. 3. When plasma contained 5‐hydroxytryptamine (5‐HT) at a concentration of about 5 μg/ml., the influx of potassium into platelets was increased for as long as they took up 5‐HT. This effect was absent when the concentration of 5‐HT was either lower (2 μg/ml.) or much higher (50 μg/ml.). 4. The concentration of 5‐HT in the platelets did not affect their potassium flux.