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 mumole/10(11) platelets (mean +/- S.E.M.).2. The influx of potassium into platelets from citrated plasma at 37 degrees 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 mug/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 mug/ml.) or much higher (50 mug/ml.).4. The concentration of 5-HT in the platelets did not affect their potassium flux.