Homotaurine: a GABAB antagonist in guinea‐pig ileum

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Abstract
Homotaurine (3‐aminopropane sulphonic acid) did not inhibit the twitch response in guinea‐pig ileum longitudinal muscle whilst γ‐aminobutyric acid (GABA) and (−)‐baclofen evoked dose‐dependent inhibitions. The inhibitory effects of GABA and (−)‐baclofen were prevented in the presence of homotaurine 2 × 10−4 and 10−3 M. The log dose‐effect curves of GABA and (−)‐baclofen were shifted in a parallel manner compatible with competitive antagonism. The pA2 of homotaurine with GABA (4.22 ± 0.05) and (−)‐baclofen (4.26 ± 0.1) were the same. Homotaurine did not antagonize the inhibitory effects of morphine (ED50 4 × 10−7 M), noradrenaline (ED50 10−6 M) or ATP (ED50 1.5 × 10−5 M). The inferior homologue of homotaurine, taurine 10−3 M, did not modify the inhibitory effects of GABA and (−)‐baclofen. Picrotoxin 5 × 10−5 M antagonized GABAA receptor‐mediated contraction but did not affect GABAB receptor‐mediated inhibition. At the same concentration the drug did not influence the antagonistic action of homotaurine, thus showing no GABAA receptor‐mediated interference. It may be concluded that homotaurine is a competitive antagonist of GABAB mediated effects in the guinea‐pig ileum.