Parallel Bioassay as a Method of Identifying Acetylcholine

Abstract

A SURVEY of the methods used to identify the active principle(s) in extracts of nervous tissue possessing cholinomimetic activity reveals that Hosein and his group¹ have offered the only serious challenge to the generally accepted view that acetylcholine (ACh) alone accounts for such activity. As part of their criticism of earlier work, Hosein and Koh² have claimed that ACh cannot be identified by parallel assay when it is present in mixtures of “materials with the same physiological and pharmacological properties”. In support of this they report results of assays of mixtures of ACh and butyrylcholine (BuCh) which they conducted on the frog rectus abdominis muscle, the dorsal muscle of the leech and the guinea-pig ileum. The result of particular interest is that obtained from a test mixture made by adding one part of a solution containing an amount of BuCh equivalent in activity on the rectus to 1 µg of ACh, to one part of another solution containing 1 µg of ACh. When assayed on the rectus muscle the expected equivalent value of 2 µg/ml. was obtained; the leech muscle and ileum gave values of 1.8 and 1.7 µg/ml., respectively. The response of the leech is not unexpected, but because the ileum has a sensitivity to BuCh between one five-hundredth and one thousandth of its sensitivity to ACh the value of 1.7 is far too high. Hosein and Koh explain the result by suggesting that the action of BuCh on the ileum was potentiated by the ACh present.