Purinergic signalling in the urinary tract in health and disease

Abstract

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

Fig. 1

a Contractile responses of the guinea-pig bladder strip to intramural nerve stimulation (NS; 2 Hz, 0.2 ms pulse duration, supramaximal voltage for 20 s) and ATP (8.5 μM). Atropine (1.4 μM) and guanethidine (3.4 μM) were present throughout. b Effect of changing the Ca²⁺ concentration on the release of ATP from the guinea-pig isolated bladder strip during stimulation of intramural nerves. Upper trace: mechanical recording of changes in tension (g) during intramural NS (2 Hz, 0.2 ms pulse duration, supramaximal voltage for 20 s). Lower trace: concentration of ATP in consecutive 20-s fractions of the superfusate. The Ca²⁺ concentration in the superfusate varied as follows: (i) 2.5 mM (normal Krebs); (ii) 0.5 mM; (iii) 0.25 mM; (iv) 2.5 mM. The successive contractions were separated by 60-min intervals as indicated by the breaks in the mechanical trace. Atropine (1.4 μM) and guanethidine (3.4 μM) were present throughout. The temperature of the superfusate was between 22 °C and 23 °C. (a and b Reproduced from [113], with permission from Elsevier.) c The effect of α,β-methylene ATP (α,β-meATP) on the response of isolated guinea-pig bladder strips to NS, ATP (∆) and histamine (Hist). Upper trace: control responses; lower trace, desensitization attained by five successive applications of α,β-meATP (50 μM, filled triangle), at 4-min intervals, completely abolished nerve-mediated and ATP induced contractions, although histamine-induced contraction is only slightly reduced. (Reproduced from [356], with permission from Elsevier.) d Rabbit urinary bladder detrusor, sucrose-gap recording at 33 °C, in the presence of atropine 0.3 μM. Effect of α,β-methylene ATP (α,β-meATP) on excitatory junction potentials (EJPS) evoked by field stimulation (filled circle, 0.5 Hz, 0.3 ms, 5 V, continuously) before (left trace) and during desensitization with α,β-meATP (10 μM) (right trace). At control membrane potential, EJPS are no longer visible during desensitization with α,β-meATP. (Reproduced from [310], with permission from Elsevier.) e Fluorescent histochemical localization of quinacrine in whole-mount stretch preparation of adult rabbit urinary bladder showing a ganglion cell containing at least six fluorescent nerve cells. The nuclei (arrow) are non-fluorescent. Calibration bars = 50 μm. (Reproduced from [172], with permission from Elsevier)

Fig. 2

a Schematic of hypothesis for purinergic mechanosensory transduction in tubes (e.g., ureter, vagina, salivary and bile duct, gut) and sacs (e.g., urinary and gall bladders, and lung). It is proposed that distension leads to release of ATP from epithelium lining the tube or sac, which then acts on P2X2/3 receptors on subepithelial sensory nerves to convey sensory/nociceptive signals to the CNS. (Reproduced from [104], with permission from Blackwell Publishing.) b P2X3 receptor immunoreactivity in the mouse bladder. Immunostaining is seen on small suburoepithelial nerve fibres. Calibration bar = 50 μm. (Reproduced from [158], with permission from Nature Publishing Group)

Fig. 3

a Contractile effects of various purines on rat and dog urinary bladder smooth muscle strips. Contractions were induced by ATP (1 mM), ADP (1 mM), α,β-methylene ATP (α,β-meATP, 10 μM) and α,β-methylene ADP (α,β-meADP, 0.1 mM). In each panel, responses to ATP, ADP and α,β-meATP were obtained from the same strip, whereas those to α,β-meADP were obtained from a different strip. (Reproduced from [650], with permission from John Wiley and Sons.) b Transverse section of rat urinary bladder detrusor muscle immunostained for P2X1 receptors. Calibration bar = 100 μm. (Reproduced from [412], with permission from Elsevier)

Fig. 4

The effect of ARL 67156 in guinea-pig bladder strips on a contractions to exogenous ATP (100 μM, n = 16) and α,β-methylene ATP (α,β-meATP, 5 μM, n = 7) and b neurogenic contractions (1–8 Hz, for 20 s, n = 8). Open bars represent control responses and the hatched bars those in the presence of ARL 67156 (100 μM). **P < 0.01, ***P < 0.001. (a and b Reproduced from [722], with permission from Elsevier.) c Contractile responses of isolated strips of urinary bladder of adult and neonate (2- to 6-day-old) rabbits to nerve stimulation. Note that responses to nerve stimulation were also greater in neonatal tissue. Bars represent the mean response ± SEM for 5–7 experiments. N.S. significant difference, *P < 0.05 (Student's t-test for unpaired data). (Reproduced from [628], with permission from Elsevier)

Fig. 5

Non-cumulative concentration–response curves to α,β-methylene ATP (α,β-meATP) in longitudinal smooth muscle strips of urinary bladder from age-matched control hamsters, cold-exposed hamsters and hibernating hamsters. In cold-exposed and hibernating hamsters α,β-meATP elicited decreased contractions (ANOVA, P < 0.05; n = 6). (Reproduced from [560], with permission from Wiley)

Fig. 6

Responses to ATP, α,β-methylene ATP (MeATP) and P_l,P₆-diadenosine 5′-hexaphosphate (A6PA) in isolated human urinary bladder detrusor muscle. a Concentration–response relationships. The response curve relates contractions due to the agonists to the standard contraction to KC1 (150 mM). Points show mean ± SEM unless occluded by symbol. Curves are fitted following probit transformation and horizontal averaging. b Electrical field stimulation of the intramural nerves (NS, filled circle) evoked contractions. MeATP (0.3 μM) caused a small contraction which faded and blocked neurogenic contractions. Following washout of MeATP (W), the neurogenic responses returned. Record obtained in the presence of atropine (0.3 μM). Scale bar represents 50 mg. (Reproduced from [313], with permission from Elsevier)

Fig. 7

a and b Responses of bladder body strips from streptozotocin-diabetic rats. a Responses to nerve stimulation (supramaximal voltage, 0.5 ms pulse duration for 2 s every 2 min) of bladder strips from 2-month-old streptozotocin-diabetic (filled triangle, n = 5) and control rats (filled circle, n = 5). (b) Responses to ATP of bladder strips from 2-month-old streptozotocin-diabetic (filled triangle, n = 5) and control rats (filled circle, n = 5). For both graphs, each point represents the mean ± SEM and data are expressed as grams tension per 100 mg tissue. (a and b Reproduced from [441], with permission from ASEPT.) c Spontaneous and distension-induced activity in ureter afferent fibres. Multifibre afferent responses to rapid distension. Note that background afferent activity occurs in bursts and that ureter distension results in an initial burst of discharge (circle) followed by a phase of maintained activity (bar). d ATP can sensitise ureter afferent fibres. An example representative of distension-induced afferent activity before and following intraluminal application of increasing concentrations of ATP. (c and d Reproduced from [579], with permission from Elsevier.) e ATP concentration ([ATP]) in perfusate immediately before and after distension of the human ureter, grouped in pressure ranges. The mean [ATP] after distension is significantly greater than before distension in each pressure range (**P < 0.01; n ≥ 7 for each group of distensions, error bars represent SEM). (Reproduced from [126], with permission from Springer