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THE ROLE OF HISTAMINE Hr AND H2‐RECEPTORS IN THE GENERATION OF THROMBOXANE A2 IN PERFUSED GUINEA‐PIG LUNGS

1 When isolated perfused lungs from normal and ovalbumin sensitized guinea‐pigs were challenged with histamine and 2‐methylhistamine (agonists for Hj‐receptor), a release of thromboxane A2‐like substance was observed. The effect of histamine on production of thromboxane A2 (TXA2) in sensitized lungs, was more pronounced than in normal lungs (P < 0.01). 2 Specific activation of histamine H2‐receptors in normal lungs with large doses (100 jag) of dimaprit and 4‐methylhistamine, does not produce thromboxane‐like or prostaglandin‐like substances. 3 Perfusion of the lungs with pyrilamine (10 μg/ml) inhibited the release of arachidonate metabolites induced by histamine H1‐receptor stimulation, whereas cimetidine (5 μg/ml) was ineffective. 4 It is concluded that only the stimulation of histamine H1‐receptors appears to be responsible for generation of thromboxane A2 and other prostaglandin‐like substances in normal guinea‐pig lungs. In sensitized lungs, an increased ability of histamine to release TXA2 could be due to a possible interconversion of H2 into Hj‐receptors.