Functional atropine sensitive purinergic responses in the healthy rat bladder.

While acetylcholine is regarded to be the main directly contractile transmitter substance in the urinary bladder, interactions with other transmitters likely occur. Presently, the interplay between purinergic and cholinergic signalling was investigated to unravel the involvement of the urothelium and efferent neurons in the functionally important purinergically evoked release of acetylcholine in vitro. Functional characterization of receptor subtypes involved in this interplay was also performed. In vitro organ bath experiments with electrical field stimulation (EFS) or administration of agonist were performed in the absence and presence of the neurotoxin tetrodotoxin (TTX; 5 × 10 ^-7  M) and/or receptor antagonists, in intact and urothelium-denuded full thickness rat bladder strip preparations. Interestingly, functional contractions to ATP (10 ^-6 -10 ^-3  M) remained unaffected by TTX, but were significantly lowered in the presence of the muscarinic antagonist atropine (10 ^-6  M). However, in urothelium-denuded strip preparations, this latter phenomenon was not present and the ATP response remained unaltered. To rule out purinergic interference caused by break-down of ATP, experiments were performed in which the stable ATP-analogue αβMeATP (10 ^-7 -10 ^-5  M) gave rise to functional atropine-sensitive contractions. Furthermore, contractions to ATP were not affected by P2Y6 purinoceptor blockade (by MRS2578; 10 ^-7 , 10 ^-5  M), nor were relaxatory responses to ATP sensitive to atropine, PPADS (3 × 10 ^-5  M) or αβMeATP. Lastly, relaxations to ADP (10 ^-6 -10 ^-3  M) or NECA (10 ^-8 -10 ^-5  M) were unaltered by the presence of atropine. To conclude, purinergic functional contractile, but not relaxatory, responses are supported by the cholinergic transmitter system in vitro, through non-neuronal mechanisms in the urothelium. Involved purinoceptors are of the P2X-subtype, most likely P2X1 and/or P2X3.
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