GABA‐RELATED ACTIVITIES OF AMINO PHOSPHONIC ACIDS ON GUINEA‐PIG ILEUM LONGITUDINAL MUSCLE

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Abstract
The effects of phosphonic analogues of GABA, β‐alanine and glycine on guinea‐pig ileum longitudinal muscle were measured. Aminomethylphosphonic acid (AMPh) and 2‐aminoethylphosphonic acid (2‐AEPh) were devoid of any effect both in non‐stimulated preparations and in electrically‐stimulated preparations. The phosphonic analogue of GABA, 3‐aminopropylphosphonic acid (3‐APPh) possessed a GABAB agonistic effect (relaxation and inhibition of twitch response) at doses of 10−3 M. No agonistic effect on GABAa receptors was observed. 3‐APPh at doses tested (2 × 10−4 M and 10−3 M) also displayed antagonistic action on the effects of GABAB agonists producing a parallel shift of the log dose‐effect curves of GABA‐ and (‐)‐baclofeninhibition of twitch responses. In contrast 3‐APPh did not antagonize the inhibitory effect of morphine and noradrenaline. The contractile effect of GABA, mediated via GABAA receptors, was unaffected by 3‐APPh (10−3 M). It is concluded that 3‐APPh is a partial agonist at the GABAB site in guinea‐pig ileum.