No abstract

Various applications of pAx measurements are discussed based on the hypothesis that drugs and drug antagonists compete for receptors according to the mass law. Examples are given illustrating the use of pAx measurements to identify agonists which act on the same receptors and to compare the receptors of different tissues. Tests of competitive and noncompetitive antagonism are considered in relation to the antagonisms acetylcholine‐atropine, histamine‐atropine and acetylcholine‐cinchonidine. A new measure, pAh, is introduced to express the activity of unsurmountable antagonists.

Best showed in 1929 that extracts of animal tissues destroy histamine and named the enzyme system concerned in this reaction histaminase. In 1938 Zeller described an enzyme, diamine oxidase, which destroys the diamines putrescine and cadaverine (Zeller, 1938a), and subsequently concluded that histaminase and diamine oxidase were identical (Zeller, 1938b). This conclusion was based on the finding of a strict parallelism between destruction of histamine and cadaverine by diamine oxidase preparations from different sources; competitive inhibition ofdiamine oxidase by histamine and cadaverine; and inhibition of the destruction of both substances by antagonists of diamine oxidase. More recently, however, Kapeller-Adler (1949, 1951 has questioned the identity of the two enzymes.All carbonyl reagents so far examined inhibit diamine oxidase, often in very low concentrations (Zeller, 1942). Zeller has shown that semicarbazide, thiosemicarbazide and hydroxylamine produce this effect, and has suggested that diamine oxidase itself contains carbonyl groups which interact with diamines and are blocked by carbonyl reagents. Various other compounds also inhibit diamine oxidase, though less actively, e.g. guanidine and iminazole and their derivatives. One of the derivatives of guanidine, however, aminoguanidine, is a powerful inhibitor of diamine oxidase (Schuler, 1952), and it is probably relevant that this compound possesses a reactive amino group like the typical carbonyl reagents.The present experiments started with the observation that in the presence of semicarbazide the action of histamine on guinea-pig intestine was markedly potentiated. Then it was found that other effects of histamine were also potentiated and that other histaminase inhibitors acted like semicarbazide. Experiments dealing with the potentiation of histamine by histaminase inhibitors are described in the first part of this paper. The experiments described in the second part were designed to test whether the potentiating effects of * British Council Scholar.