The theory—two histamine receptors

We are now able to return to the question we asked at the end of Section 13.2. Bearing in mind the failure of the known antihistamines to inhibit gastric acid release, why did the SK F team persevere with the antihistamine approach  

As mentioned above, conventional antihistamines failed to have any effect on gastric acid release. However, they also failed to inhibit other actions of histamine. For example, they failed to fully inhibit the dilation of blood vessels induced by histamine. The SK F scientists therefore proposed that there might be two different types of histamine receptor, analogous to the two types of acetylcholine receptor mentioned in Chapter 11. Histamine—the natural messenger—would switch both on equally effectively and would not distinguish between them. However, suitably 

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