Serum albumin stimulates serotonin uptake into human blood platelets

Active uptake of serotonin (5-hydroxytryptamine, 5-HT) into blood platelets from healthy donors exhibited a lower Vmax value in buffer media than in plasma. Also in plasma ultrafiltrate Vmax was reduced, but it returned to the level measured in plasma upon addition of human serum albumin (40 milligrams) containing fatty acids. Fatty-acid-free albumin was even more stimulatory and when added to platelets in a phosphate-buffered medium, it increased Vmax beyond the value observed in plasma. Km values calculated on the basis of unbound 5-HT were not affected by the media except for a slight decrease in ultrafiltrate as compared to plasma. The fraction of 5-HT (0.5 mumol/l) bound to 40 milligrams albumin was 17% with the preparation containing fatty acids and 22% with fatty-acid-free albumin, while total plasma proteins dissolved in phosphate buffer bound 24%. The uptake of 1 mumol/l 5-HT was enhanced by both albumin preparations already at 1 milligram and near-maximal effects occurred at 10 milligrams.