DUAL EFFECT OF GABA ON THE CONTRACTILE ACTIVITY OF THE GUINEA‐PIG ISOLATED URINARY BLADDER

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Abstract
The effects of GABA and related substances were examined in isolated detrusor strips from the dome of the guinea pig urinary bladder. GABA (0.01-1 mM) produced concentration-related phasic contractions of isolated strips from the guinea pig urinary bladder dome. This effect of GABA was mimicked by homotaurine and muscimol, selective GABAA receptor agonists but not by (.+-.)-baclofen, a selective GABAB receptor agonist. A specific cross desensitization was observed between GABA, homotaurine and muscimol but not between (.+-.)-balcofen and GABA. GABA (1 mM)-induced contractions were antagonized by picrotoxin, a selective GABAA receptor antagonist. GABA-induced contractions were almost abolished by tetrodotoxin (0.5 .mu.M, TTX) thus indicating their neurogenic origin. In addition GABA-induced contractions were partially antagonized by atropine (to about the same extent as those produced by dimethylphenylpiperazinium (DMPP), a ganglionic stimulant), but were unaffected by hexamethonium (10 .mu.M), phentolamine (0.2 .mu.M) or indomethacin (5 .mu.M). In the presence of GABA the contractile effect of both DMPP (TTX-sensitive) and acetylcholine (ACh, TTX-insensitive) were significantly reduced. Similar findings were obtained with DMPP, i.e., in preparations exposed to this ganglionic stimulant both GABA- and ACh[acetylcholine]-induced contractions were depressed. Homotaurine but not (.+-.)-baclofen mimicked the depressant effect of GABA on DMPP-induced contractions. The depressant effect of GABA on ACh-induced contractions of the guinea pig urinary bladder was neurogenic in origin, i.e., was not observed in preparations exposed to TTX. GABA apparently has a dual effect on the contractile behavior of the guinea pig isolated urinary bladder. Recently it has been proposed that endogenous GABA plays a neuromodulatory role in this organ. In the early phase of neurogenic activation of detrusor muscle (micturition reflex) GABA might transiently enhance excitatory neurotransmission followed by a more sustained inhibition of contractility.