Functional studies of purinergic and cholinergic interactions in the rat urinary bladder - Characterization of ATP-evoked urothelial release of acetylcholine

Abstract

Functional interplays between different transmitter systems are known to affect the signalling of the micturition reflex arc. The functional implications of many of these interactions, such as between ATP and acetylcholine, have yet to be unravelled. This thesis aimed to identify and characterize the functional importance of a proposed atropine-sensitive component in purinergic responses. Furthermore, its impact on afferent signalling and changes during experimental (cyclophosphamide-induced) cystitis were to be studied in vitro and in vivo.

The existence of an atropine-sensitive part of purinergic responses important for direct detrusor contractions was proven. Furthermore, this purinergic-cholinergic link was shown to be dependent on an intact urothelium as well as caveolae. This suggests a pronounced interaction between these structures, tentatively indicating the caveolae in the urothelium to be of importance for the ATP-evoked release of acetylcholine. Blockade of neuronal transmission by tetrodotoxin did not affect this release, suggest-ing it to be non-neuronal. Cystitis altered the urothelial signalling, diminishing the purinergic atropine-sensitivity, showing this link to be important for healthy bladder signalling and to be affected during cystitis. Also, atropine significantly decreased contractions induced by the stable ATP-analogue α-β-Me-ATP, demonstrating the P2X-purinoceptors (likely P2X1 and/or P2X3) to be involved in purinergic release of acetylcholine in the healthy rat urinary bladder. Notably, in vivo the purinergic release of acetylcholine was able to activate the micturition reflex arc and trigger afferent signalling. The cholinergic afferent modulation depends on the activation of mus-carinic receptors of the M2/M4- (inhibitory) and M3-subtypes (facilitatory).

Thus, a cholinergic part of purinergic signalling exists in the rat urinary bladder, with important function both in vitro (e.g. direct contractile responses) and in vivo (e.g. modulation of afferent signalling). Cystitis hinders the purinergic-cholinergic signalling in the urothelium, presumably mediated via caveolae, showing changes in urothelial signalling to be of outmost importance in inflammatory lower urinary tract disorders. These results increase the knowledge of healthy bladder signalling and may provide new theories for future pharmacotherapies.