An impairment in blood flow, or ischaemia of the bladder, may play a role in the aetiology of bladder storage dysfunction. Recent evidence suggests that phosphodiesterase 5 (PDE5) inhibitors may have beneficial effects in relieving ischaemia and improving urinary symptoms, by relaxing the bladder vasculature and improving blood flow (1). PDE5 expression has been reported in both the vascular smooth muscle and the endothelium of blood vessels in the human bladder (2), and a recent study has demonstrated that nitrergic vasodilatory signal transmission in the bladder vasculature underlies the therapeutic actions of PDE5 inhibitors in the rat bladder vasculature (3). The aim of this study was to determine whether the phosphodiesterase-5 inhibitor tadalafil can potentiate nitric-oxide mediated vasodilation of the porcine superior vesical artery.

Porcine superior vesical arteries (from 6-month-old pigs) were obtained from a local abattoir. Sections (~4mm length, ~1mm internal diameter) were isolated and mounted between two horizontal stirrups in 10mL organ baths (GlobalTowns, CA) containing carbogen (5% CO2) gassed Krebs-bicarbonate solution at 37˚C. The endothelium of the vascular tissue was left intact. Tension developed by the circular smooth muscle was recorded to a PowerLab using LabChart software. Sympathetic vasoconstriction responses to cumulative concentrations of noradrenaline were obtained in the absence and presence of the nitric oxide synthase inhibitor L-NNA (100µM). In addition, following pre-contractions to noradrenaline (5µM) vasodilatory responses to endogenous nitric oxide release (carbachol, muscarinic receptor stimulation) and exogenous nitric oxide donors (SNAP, SIN-1, and sodium nitroprusside) were obtained in the presence and absence of the phosphodiesterase inhibitors tadalafil (100nM) or papaverine (20µM). Statistical significance of differences in responses was determined using two-tailed paired t-tests, with P<0.05 considered significant.

Sympathetic vasoconstriction of the porcine superior vesical artery elicited by noradrenaline was enhanced in the presence of the nitric oxide synthase inhibitor L-NNA (100µM), with maximal contractile responses significantly (P<0.05, n=6) increased (4.67±0.70g vs 3.21±0.44g, Figure 1A). There was no change to potency of noradrenaline. Both PDE inhibitors, the clinically used phosphodiesterase-5 inhibitor tadalafil and the non-selective phosphodiesterase inhibitor papaverine, significantly (P<0.01 and P<0.001, respectively) inhibited noradrenaline pre-contractions (2.80g±0.37 vs 2.05g±0.21 and 2.54g±0.24 vs 1.07g±0.15, respectively, Figure 1B). 
Endogenous release of nitric oxide following stimulation with carbachol caused concentration-dependent vasodilation of the superior vesical artery (Figure 2A&B). Neither of the phosphodiesterase inhibitors altered these endothelium-dependent vasodilatory responses (Figure 2A & B).
In contrast, vasodilatory responses to exogenous nitric oxide donors sodium nitroprusside (SNP), SIN-1 and SNAP were altered by both phosphodiesterase inhibitors (Figure 2). Tadalafil significantly (P<0.01, n=6) increased the potency of SNP in inducing vasodilatory responses, reducing pIC50 values from 6.47±0.15 to 6.93±0.10 (Figure 2C). Similar effects were observed with papaverine, reducing pIC50 values from 6.45±0.07 to 7.11±0.13 (P<0.05, n=4, Figure 2D). In the presence of tadalafil, vasodilatory responses to SIN-1 were significantly (P<0.001, n=5) potentiated (Figure 2E), and tadalafil reduced pIC50 values from 6.71±0.07 to 7.14±0.04. Similar effects were observed with papaverine, significantly potentiating SIN-1 responses (P<0.01, n=5, Figure 2F) with pIC50 values reduced from 6.73±0.10 to 7.40±0.11. Tadalafil also significantly (P<0.001, n=5) affected the potency of SNAP-induced vasodilation, reducing pIC50 values from 6.53±0.05 to 6.90±0.05 (Figure 2G). Papaverine also significantly (P<0.001, n=5) potentiated SNAP vasodilatory response, reducing pIC50 values from 6.53±0.05 to 6.90±0.05 (Figure 2H).

Endothelium-dependent vasodilation of the porcine superior vesical artery to muscarinic agonist carbachol evoked endogenous nitric oxide release, which was successfully inhibited by nitric oxide synthase inhibitor L-NNA. This suggests that the porcine superior vesical artery possesses endogenous nitric oxide. Neither the clinically used selective phosphodiesterase-5 isoenzyme inhibitor tadalafil nor the non-selective phosphodiesterase inhibitor papaverine potentiated the vasodilatory effects of carbachol. However, tadalafil and papaverine both significantly potentiated the potency of exogenous nitric oxide donors (SNP, SIN-1 and SNAP) in inducing vasodilation. This data suggests the presence of PDE5 in the porcine superior vesical artery, and suggests that tadalafil acts on endothelium-independent nitric-oxide-mediated vasodilatory pathways, which may contribute to its beneficial effects in improving bladder symptoms.

The porcine superior vesical artery releases nitric oxide via the endothelium and its responses to exogenous nitric oxide are potentiated by the clinically used phosphodiesterase-5 inhibitor tadalafil. This suggests that the isoenzyme phosphodiesterase-5 is present in this tissue, and that tadalafil acts on endothelium-independent nitric-oxide-mediated vasodilatory pathways, which may contribute to its beneficial effects in improving bladder symptoms.

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