THE RELEASE OF 5-HYDROXYTRYPTAMINE, POTASSIUM AND AMINO ACIDS FROM PLATELETS

¹ Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins University School of Medicine, Baltimore, Maryland

Differential centrifugation of homogenates of human platelets revealed that potassium (K) and amino acids (AA) were largely free, whereas 5-hydroxytryptamine (5-HT) was bound within granules.

Washed platelets treated at 37°C with 5 µg/ml of trypsin or 1 NIH unit/ml of thrombin liberated 80 to 85% of the 5-HT and 30 to 45% of the K in 10 minutes. However, incubation for as long as 1 hour did not cause any loss of AA. This observation obviated the hypothesis that trypsin and thrombin release 5-HT as a result of a marked increase in platelet permeability.

All concentrations of trypsin which released 5-HT also liberated K. However, the ratio of 5-HT loss to K loss was not constant. Furthermore, ouabain (1 x 10^-4 M) released 43% of the K in 1 hour with only a negligible loss of 5-HT.

Platelets subjected to 1 x 10^-4 M p-chloromercuribenzoic acid (PCMB) showed a lag in the liberation of 5-HT, although K and AA were lost quickly. The possibility that 5-HT release might be secondary to the loss of K and AA and the entry of sodium and water was tested by treating platelets with PCMB in isotonic sucrose. The liberation of 5-HT was retarded but not prevented; K release occurred at the same rate as in saline.

The release of 5-HT by 1 x 10^-2 M fluoride or 1 x 10^-2 M iodoacetic acid was accompanied by a loss of both K and AA.

Reserpine (1 µg/ml) liberated 30% of the 5-HT in 1 hour without any release of K or AA.

Amphetamine and tryptamine released 5-HT at a more rapid rate than maximally effective concentrations of reserpine. However, the high concentrations of the amines needed to demonstrate this difference caused some loss of AA and K. Epinephrine was weaker than amphetamine or tryptamine in its ability to liberate 5-HT. Platelets caused to lose 60% of their 5-HT by treatment with 100 µg/ml of amphetamine showed no defect in concentrating exogenous 5-HT following removal of the drug by washing. At a concentration of 1000 µg/ml amphetamine resembled reserpine in inhibiting irreversibly the 5-HT transport system.

Platelets kept at 4°C for 1 hour lost 15% of their K but no 5-HT or AA. The K was rapidly regained at 37°C. All of the 5-HT releasing agents were practically inactive at 4°C. The liberation of AA, and often K, was also less at the lower temperature.

The discussion revolved about 5-HT and possible mechanisms for its release.

This article has been cited by other articles:

				H. W. Strauss, R. B. Smith, P. Polimeni, A. C. Schenker, V. J. Schenker, and J. H. Stuckey Plasma Serotonin Levels in Stored Human Blood Angiology, September 1, 1967; 18(9): 535 - 546. [PDF]