Homotaurine: a GABAB antagonist in guinea-pig ileum

Homotaurine (3-aminopropane sulphonic acid) did not inhibit the twitch response in guinea-pig ileum longitudinal muscle whilst gamma-aminobutyric acid (GABA) and (-)-baclofen evoked dose-dependent inhibitions. The inhibitory effects of GABA and (-)-baclofen were prevented in the presence of homotaurine 2 X 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 X 10(-7) M), noradrenaline (ED50 10(-6) M) or ATP (ED50 1.5 X 10(-5) M). The inferior homologue of homotaurine, taurine 10(-3) M, did not modify the inhibitory effects of GABA and (-)-baclofen. Picrotoxin 5 X 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.