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A double-blind, placebo-controlled, cross-over study in ten normal volunteers assessed the effects of the subcutaneous administration of adrenaline on platelet MAO activity, in an attempt to replicate previous findings of increased MAO activity following adrenaline. Platelet counts, cortisol plasma concentrations, and uptake of metaraminol by the platelets were also determined to elucidate possible mechanisms of action. Blood pressure and heart rate were used as indices of the concentration of adrenaline effectively in circulation. MAO activity towards benzylamine was significantly increased by adrenaline. Non-significant increases were noted in the deamination of tyramine and tryptamine, in platelet counts, cortisol concentrations and metaraminol uptake. Increases in MAO activity were also noted early in the experimental sessions suggesting that other experimental variables such as venepuncture may affect platelet MAO activity. Possible mechanisms for this adrenaline effect are discussed and it is suggested that variables such as "arousal" and "stress" must be taken into account when interpreting clinical data on platelet MAO.

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