Purinergic P2X 3 heteroreceptors enhance parasympathetic motor drive in isolated porcine detrusor, a reliable model for development of P2X selective blockers for detrusor hyperactivity

Abstract

Various forms of low urinary tract symptoms (LUTS) seem dependant upon dysregulation of the purinergic pathway which produces sensory- or motor-activated incontinence. A body of evidence in human urinary bladders supports a link between up-regulation of purinergic activity and the pathogenesis of detrusor instability. This study investigated the potential role of adenosine 5′-triphosphate (ATP) in the control of detrusor motor drive in a model of porcine urinary bladder. The involvement of ATP on excitatory activity was assessed by measuring neurally-evoked [ 3H]-acetylcholine (ACh) release and smooth muscle contraction in detrusor strips. Epithelium-deprived preparations were used to minimize the influence of non-neural sources of ACh and ATP on parasympathetic neurotransmission. ACh release and smooth muscle contractility were not significantly affected by neural ATP in normal detrusor, but markedly enhanced when ATP hydrolysis was reduced by ectoATPase inhibitors, as well as by α,β-methylene-ATP (ABMA), agonist resistant to ecto-enzymes degradation. Prejunctional P2X receptors located on cholinergic nerves are involved in such potentiating effect. These purinergic heteroreceptors were characterized as P2X 3 subunits by means of the putative antagonists: NF449 (P2X 1,3 selective), NF023 (P2X 1,3 selective), PPNDS (P2X 1 selective) and A-317491 (P2X 3 selective). In porcine detrusor, P2X 3 receptors are functionally expressed at neural site facilitating neurogenic ACh release. When purine breakdown is experimentally down-regulated to mimicking the impaired purinergic pathway observed in pathological human bladders, endogenous ATP can markedly enhance detrusor contractility through activation of these receptors. Since P2X 3 blockade represents a potential therapeutic approach for diseases of the urinary tract, isolated porcine detrusor represents a reliable model for development of novel selective P2X 3 antagonists beneficial in the treatment of detrusor hyperactivity.