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. 1983 Jul;340:77–90. doi: 10.1113/jphysiol.1983.sp014750

Loss of 5-hydroxytryptamine from mammalian circulating labelled platelets

E E Osim 1, J H Wyllie 1
PMCID: PMC1199197  PMID: 6411907

Abstract

1. Platelets were obtained from three species of animal: rats, rabbits and dogs. They were labelled with 111In oxine to tag individual platelets and with 14C-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. 111In and 14C 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. 111In 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 111In 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 14C behaved like the 111In. However, in control rabbits the half-life for 14C was 38·0 hr which is significantly longer than for 111In (P < 0·005). 14C also disappeared more slowly than the 111In in the dogs.

3. In all species treatment with 5-HTP accelerated the disappearance of the 14C 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 14C/111In 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 greatly reduced the 14C in platelets. Under these conditions many tissues in addition to those above had a higher ratio of 14C/111In than platelets. These tissues included muscle, skin, salivary gland, kidney, heart, aorta, testis and seminal vesicle. As with platelets, the absolute counts of 14C/g of tissue decreased significantly after 5-HTP administration (platelets by 67%, brain by 56%, pancreas by 49%, lungs by 39%, liver by 37%, kidney by 29%, testis by 23% and seminal vesicle by 22%). On the other hand, there were significant rises of 93% in the skin and 21% in the muscle. Paper chromatography showed that 73-86% of the 14C in tissues still behaved like 5-HT, except in the bladder and adrenals which contained unidentified material.

7. It is concluded that under normal conditions platelets deposit 5-HT in specific tissues, notably gut, adrenals and thyroid. When unlabelled 5-HTP is administered, the labelled 5-HT is deposited in a variety of tissues, and especially in the skin.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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