Response of Plasma Histaminase Activity to Small Doses of Heparin in Normal Subjects and Patients with Hyperlipoproteinemia

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The release of histaminase activity in plasma after small intravenous of heparin was studied in 85 normal subjects and patients. In normal subjects, plasma histaminase activity (basal level, 1.7±0.1 U/ml, mean ±SEM) increased 1.6±0.2 U/ml after 10 U of heparin/kg, 8.5±2.4 U/ml after 20 U/kg, and 33±4.9 U/ml after 75 U/kg. The extent of the increase varied widely among individuals but in a particular individual the response was constant and dose-dependent. Histaminase activity rose to peak levels within 7-15 min and then declined exponentially with a half-life of 40-120 min. This pattern of response was also observed in two patients with the histaminase-producing tumor, medullary carcinoma of the thyroid. A significantly reduced response was observed, however, in 14 patients with type I hyperlipoproteinemia, a disorder in which high plasma triglyceride levels are associated with low postheparin plasma lipolytic activity. After 10 U heparin/kg, plasma histamine activity increased 0.5±0.2 U/ml, and after 75 U heparin/kg, 10.9±5.6 U/ml. In contrast, in 27 patients with other types of hyperlipoproteinemia in whom postheparin lipolytic activity was normal, the increase (2.4±0.6 U/ml) in plasma histaminase activity after 10 U heparin/kg was not significantly different from that of normal subjects. The reduced response of the plasma histaminase activity to heparin in patients with type I hyperlipoproteinemia did not appear to be due to the presence of lipemia or to an inhibitor of the enzyme in plasma. These findings suggest that many patients with type I hyperlipoproteinemia may have deficient release of both lipolytic and histaminase activities into plasma after heparin administration.

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