Maximal packet size for serotonin in storage vesicles of intact human platelets |
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Authors: | Jonathan L. Costa Helen Stark Brenda Shafer Laurence Corash Mark A. Smith Dennis L. Murphy |
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Affiliation: | 1. Clinical Neuropharmacology Branch, National Institute of Mental Health, Bethesda, Maryland 20014, USA;2. Hematology Service, Clinical Pathology Department, NIH Clinical Center, Bethesda, Maryland 20014, USA |
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Abstract: | A procedure has been developed to measure maximal packet size for serotonin in vesicles of intact, washed human platelets. Vesicles were trace-labelled with H3-5HT and subsequently permitted to accumulate larger amounts of C14-5HT from the extracellular medium. Parallel platelet aliquots were incubated with unlabelled 5HT, and total and thrombin-releasable 5HT were measured by an enzymatic assay. Prior to incubation, 5HT packet size ranged from 0.05 to 0.53 × 10?17 moles/platelet. No net addition of 5HT to vesicles occurred when this compartment contained more than 2–3 × 10?17 moles/platelet of 5HT, suggesting that maximal packet size was 2.8 × 10?17 moles/platelet. Under these conditions, a typical vesicle would contain 4 × 10?18 molesof 5 HT, at an estimated concentration of 0.6 M. Nevertheless, when vesicles were full, they continued to exchange C14-5HT from the extracellular medium with H3-5HT sequestered in vesicles. |
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