Plasma histamine and clinical tolerance to infused histamine in normal, atopic and urticarial subjects

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Abstract
Five subjects with a recent history of urticaria (U), five atopic (A) subjects and a non-atopic (NA) control group were given intravenous infusions of histamine starting at 0.05 μg/kg/min, increasing by 0.05 μg/kg/min every 30 minutes to a maximum of 0.35 μg/kg/min. Plasma histamine levels were monitored every 15 minutes. The infusion was stopped when an objective clinical endpoint was reached, involving either evidence of peripheral vasodilatation (rise of skin temperature by at least 1 °C) or a 20% fall of peak expiratory flow rate. There were no significant differences in resting plasma histamine in the three groups. Those with urticaria reached the clinical endpoint at a lower infusion rate than non-atopic subjects (U 0.22±0.02 μg/kg/min; A 0.26±0.02 μg/kg/min; NA 0.32±0.2 μg/kg/min.ppp=0.05). After the histamine infusion was stopped, the fall in the blood level of histamine was slower in urticarial subjects than in the other two groups, with a half-life of 6.2±1.3 min (A 3.0±1.2 min, NA 4.0±0.7 min,p<0.02). There were thus differences in the metabolism of histamine in our non-atopic urticarial subjects and increased histamine sensitivity in atopic subjects which require further study.