3-Aminopropylphosphinic acid - A potent, selective GABA(B) receptor agonist in the guinea-pig ileum and rat anococcygeus muscle

1. 3-Aminopropylphosphinic acid, a γ-aminobutyric acid (GABA) analogue, was tested for activity on guinea-pig isolated ileum and rat isolated anococcygeus muscle preparations. The effects of 3-aminopropylphosphinic acid were compared with those of GABA and baclofen. 2. In the electrically stimulated ileum, 3-aminopropylphosphinic acid, like GABA and baclofen, caused a concentration-dependent inhibition of the cholinergic twitch contraction, the IC₅₀ value being 1.84 ± 0.23 μM (n = 12). Unlike GABA, but like baclofen, 3-aminopropylphosphinic acid did not produce an initial contraction. 3. The inhibitory effect of 3-aminopropylphosphinic acid and baclofen in the guinea-pig ileum were not significantly antagonized by bicuculline (10 μM), phentolamine plus propranolol (both 1 μM), yohimbine (1 μM), naloxone (1 μM), impromidine (1 μM) or 8-phenyltheophylline (10 μM). The inhibitory effects of 3-aminopropylphosphinic acid, but not of baclofen, were however antagonized by phaclofen (500 μM). In addition the effects of 3-aminopropylphosphinic acid were abolished by baclofen desensitization in the guinea-pig ileum. 4. 3-Aminopropylphosphinic acid, GABA and baclofen reduced the twitch contraction evoked by electrical field stimulation in the rat anococcygeus muscle. The IC₅₀ for 3-aminopropylphosphinic acid inhibition of the anococcygeus contraction was 0.89 ± 0.15 μM (n = 8). 5. It is concluded that 3-aminopropylphosphinic acid is a potent, selective GABA(B) agonist, being seven times more potent than baclofen in the guinea-pig ileum and five times more potent than baclofen in the rat anococcygeus muscle preparations.