The utility of platelet catecholamine determination in assessing sympathetic nervous system activity depends on the extent to which transient fluctuations of plasma catecholamine concentration can affect platelet catecholamine content. In order to quantitate the relationship between plasma and platelet catecholamine content, the concentrations of norepinephrine, epinephrine and dopamine needed to produce an increase in platelet catecholamines in vitro were determined. The threshold for epinephrine accumulation was 10(-9)M while the thresholds for norepinephrine and dopamine were an order of magnitude higher. The uptake of catecholamines was also found to differ with regard to the time course and the amount of catecholamine accumulated. For example, after 2 h in the presence of equimolar (2 × 10(-8)M) concentrations of catecholamine the platelet content of norepinephrine doubled and epinephrine rose 10-fold and dopamine 20-fold. When platelets that had been loaded with catecholamines were suspended in catecholamine-free buffer, significant amounts of catecholamines were still retained in the platelets after 3 h, but retention of norepinephrine was greater than that of epinephrine. These results indicate the importance of the concentration of the particular catecholamine, duration of exposure of platelets to the catecholamine and the ability of platelets to retain catecholamines, in determining the relationship between plasma and platelet catecholamine content.
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