Abstract
1 Thirty-three amino acids were applied separately in concentrations of 2 to 10 mM to guinea-pig uterine horns in vitro at pH 7.4. About half the acids regularly produced contractions.
2 Glycine and the straight-chain L-α-amino acids up to norleucine were active (longer ones not tested); D-isomers were less potent or inactive in these concentrations. The ω-amino acids γ-aminobutyric acid (GABA) and δ-aminovaleric, and the α,ω-diamino acids L-α,β-diaminopropionic and L-α,γ-diaminobutyric were active, whereas others of similar chain-length such as β-alanine and lysine were not. The diacidic acids, glutamic and homocysteic, were more active than the amido-amino acids, glutamine and asparagine. Histidine and phenylalanine showed little or no activity.
3 The use of appropriate blocking agents indicated that the responses to representative acids were not mediated by histamine, 5-hydroxytryptamine, acetylcholine, noradrenaline or by prostaglandins. Attempts to block the actions of glycine and GABA with strychnine, thebaine, picrotoxin, bicuculline or tetramethylenedisulphotetramine (TETS) were unsuccessful.
4 When some of the acids that were spasmogenic at 2 to 10 mM were applied at sub-spasmogenic doses, they transiently potentiated other spasmogens such as oxytocin or acetylcholine. This effect was also shown by a mixture of amino acids at approximately the normal plasma concentrations.
5 There is some similarity between the spasmogenic activities of different amino acids and their known abilities to depolarize neurones.
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Selected References
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