Human blood platelets were incubated with 5-hydroxytryptamine and with tryptamine and the uptake of each amine measured. The uptake of tryptamine, unlike that of 5-hydroxytryptamine, was linearly related to the concentration of the amine in the surrounding fluid, was similar in amount at 0°and 370 C and varied directly with the pH of the solution. When both amines were present together the uptake of 5-hydroxytryptamine was depressed. The antagonism of tryptamine was found to be competitive, and the possible site of its action is discussed. The effect of a number of other substances on the uptake of 5-hydroxytryptamine by platelets was examined; of these imipramine, cocaine and chlorpromazine were more potent and dihydroergotamine and lysergic acid diethylamide somewhat less potent than tryptamine in inhibiting uptake.

No abstract

It has been shown by Humphrey & Jaques (1954) and by Udenfriend & Weissbach (1954) that 5-hydroxytryptamine (HT) in the blood is carried almost entirely by platelets. This suggested that light might be thrown on the physiological function of HT by an inquiry into its concentration and distribution in blood disorders, especially those in which platelet formation and function are abnormal. Some preliminary work showed that there is, in fact, a very marked reduction in both the whole blood and platelet content of this substance in certain diseases. The work reported here, a preliminary account of part of which has already appeared (Hardisty & Stacey, 1955), was carried out in order to establish a reliable method for estimating HT in blood, to determinine standards for normal human subjects and to develop a method for evaluating the ability of platelets to absorb HT. METHODS Platelet-rich plasma. Blood obtained by clean venepuncture with a siliconed syringe and needle was transferred to a siliconed centrifuge tube containing either disodium ethylene-diamine tetraacetate (1-0 g/100 ml. 0.9% sodium chloride) or sodium citrate (3-1 g/100 ml.) in the proportion of 1 to 9 vol. blood. After mixing by gentle inversion, the blood was centrifuged for 10-20 min at 100 g, the platelet-rich plasma separated and platelet counts made on it and on the whole blood from which it was derived, using the method of Baar (1948

In normal platelets a proportionality was found between the amount of adenosine triphosphate (ATP) and amount of 5-hydroxytryptamine (5-HT) in platelets both before and after incubating them in plasma to which 5-HT had been added. In patients receiving reserpine and in others with myeloid leukaemia, the amount of 5-HT in the platelets and the uptake of 5-HT by them were depressed, while the amount of ATP was normal. The possibility that ATP is involved in the accumulation of 5-HT by platelets is discussed.Hardisty and Stacey (1957) showed that, in two substances are plotted against one another. myeloid leukaemia and some other diseases of Although there is considerable scatter, there is a the blood, the amount of 5-hydroxytryptamine rough proportionality between the amounts of (5-HT) in the platelets is reduced. In some cases 5-HT (y) and ATP (x) in the platelets which is this was associated with an impaired capacity of expressed by the equation y=0.01 +0.023x (r= the platelets to take up 5-HT when they were 0.67, P<0.01). incubated in plasma to which 5-HT had been Fig. 2 shows that after platelets from the same added. Further, it has been shown (Born, Ingram, people had been incubated with 5-HT the proand Stacey, 1956; Born and Gillson, 1957) that portionality became closer. The corresponding the amount of 5-HT in platelets is proportional equation is y=0.035+0.094x (r=0.78, P<0.001). to the amount of adenosine triphosphate (ATP) The mean ratios of the number of ATP molecules which they contain. This paper reports observa-to the number of 5-HT molecules in the platelets tions on the amounts of 5-HT and ATP in plate-before and after incubation with 5-HT are given in lets obtained from normal people, from patients Table I

No abstract