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Antagonism of kinin effects on epithelia by Hoe 140: apparently competitive and non‐competitive interactions

1 Hoe‐140, a potent kinin receptor antagonist, was investigated for its ability to inhibit the effects of lysylbradykinin (kallidin) on a cultured colonic epithelium, HCA‐7 Colony 29, derived from a human adenocarcinoma. 2 Measurements of electrogenic chloride secretion (as short circuit current), and of intracellular Ca2+ (from Fura‐2 fluorescence) were used to assess the action of lysylbradykinin in the absence and presence of Hoe 140. 3 From short circuit current data, Hoe 140 appeared to be a competitive antagonist with a Ki value of 5 nm. However, with measurements of intracellular Ca2+ Hoe 140 was apparently a non‐competitive antagonist with a Ki of between 4–6 nm. 4 Because of the unexpected finding of non‐competitive antagonism, measurements were made with a second antagonist pair, histamine and mepyramine. Mepyramine behaved as a competitive antagonist against responses to histamine with a Ki value of ≅5 nm when short circuit current measurements were evaluated. However, when intracellular Ca2+ concentration was used as a measure mepyramine, 30 nm, produced a near parallel shift in the response curve, but at 100 nm the maximal response was depressed. 5 The reasons why the apparent type of antagonism depends upon the method of measurement is discussed, bearing in mind that the increase in intracellular Ca2+ is a signal which precedes the increase in short circuit current.