Loss of 5-hydroxytryptamine from mammalian circulating labelled platelets

• Published in: The Journal of physiology 1983-07, Vol.340 (1), p.77-90
• Main Authors: Osim, E. E., Wyllie, J. H.
• Format: Article
• Language: English
• Subjects: 5-Hydroxytryptophan - pharmacology; Animals; Blood Platelets - metabolism; Carbon Radioisotopes; Dogs; Female; Half-Life; Indium; Kinetics; Male; Organometallic Compounds; Oxyquinoline - analogs & derivatives; Rabbits; Rats; Rats, Inbred Strains; Serotonin - blood; Serotonin - metabolism; Time Factors; Tissue Distribution
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.1983.sp014750

1. Platelets were obtained from three species of animal: rats, rabbits and dogs. They were labelled with 111 In oxine to tag individual platelets and with 14 C-labelled 5-hydroxytryptamine (5-HT). Doubly labelled platelets from rabbits and dogs were returned to their donors; in the case of rats the platelets were injected intravenously into other, identical rats. At time intervals from 2 to 64 hr, blood samples were drawn and platelets were collected. 111 In and 14 C were separately counted. In some experiments animals received the 5-HT precursor, 5-hydroxytryptophan (5-HTP) I.P. (for rats and rabbits) or subcutaneously (for dogs) in a dose of 20 mg/kg daily to accelerate synthesis of 5-HT. 2. 111 In disappeared in approximately an exponential fashion in all experiments and the rate of disappearance was not affected by treatment with 5-HTP. The half-life for 111 In in four control rats was 18·7 hr and in five rats treated with 5-HTP was 17·8 hr. In rabbits the half-life was 20·4 hr for eight control and 21·2 hr for seven treated with 5-HTP. In the dogs the half-life was 21·0 hr for control and 27·7 hr for experiments with 5-HTP. In control rats, the 14 C behaved like the 111 In. However, in control rabbits the half-life for 14 C was 38·0 hr which is significantly longer than for 111 In ( P < 0·005). 14 C also disappeared more slowly than the 111 In in the dogs. 3. In all species treatment with 5-HTP accelerated the disappearance of the 14 C approximately three-fold. This was not a reserpine-like effect because the platelets contained more, not less 5-HT than usual. 4. In an attempt to discover the fate of 5-HT disappearing from circulating platelets, experiments were made in which platelets from one rat were doubly labelled, and were then injected into two other rats from the identical strain; one of the recipients received daily I.P. injections of 20 mg/kg of 5-HTP. The other rat in each pair acted as a control. 5. Results from twelve control rats showed that the 14 C/ 111 In ratio in several tissues deviated from that found in platelets. Deviations occurred in both directions; in the spleen, liver and kidney the ratio was significantly lower than in the platelets ( P < 0·01), whereas in the adrenals, thyroid, bladder and gut the ratio was significantly higher ( P < 0·05 for thyroid, < 0·01 for others). In the gut, however, the ratio was significantly raised ( P < 0·01) only at 5 hr. 6. Administration of unlabelled 5-HTP to another twelve rats