STIMULATION OF THE CYCLIC AMP/GMP SIGNALLING ENHANCES THE RELAXATION OF ISOLATED HUMAN DETRUSOR SMOOTH MUSCLE ACHIEVED BY PHOSPHODIESTERASE INHIBITORS.

Phosphodiesterase (PDE) enzymes are considered being key proteins in controlling the function of smooth musculature in the human urinary tract. The use of PDE inhibitors (PDE-Is) to treat erectile dysfunction and lower urinary tract symptomatology (LUTS) secondary to benign prostatic hyperplasia (BPH) is well established. It has been shown that PDE-Is can reverse the tension induced by means of muscarinergic agents of detrusor smooth muscle and enhance the production of cyclic nucleotides. In clinical settings, the PDE1 inhibitor vinpocetine had beneficial effects in patients presenting with voiding dysfunctions. This prompted us to evaluate further the mechanism of action of PDE-Is on bladder smooth musculature. Using the tissue bath technique, relaxant responses of human detrusor smooth muscle, challenged by acetylcholine (1 µM), to vinpocetine (PDE1-I), rolipram (PDE4-I), MY 5445 and sildenafil (PDE5-Is) (0.1 µM, 1 µM, and 10 µM) were investigated with and without pre-exposure of the tissue to threshold concentrations of the NO donor drug sodium nitroprusside (SNP) or adenylyl cyclase activator forskolin (0.02 µM). The non-specific PDE-I papaverine was used as a reference compound. The cumulative addition of forskolin or SNP exerted a pronounced reversion of the tension induced by means of ACh, starting at a concentration of 1 µM (forskolin, -25,6%) and 0.1 µM (SNP, -20%), respectively. There were marginal responses of the detrusor smooth musculature to the PDE-Is, the relaxation measured ranged from -12% (vinpocetine/sildenafil) to -19% (rolipram, MY 5445). Exposure of the tissue to a threshold concentration of SNP increased the reversion of tension induced by vinpocetine (-40%), rolipram (-50%) and MY 5445 (-45%). An enhancement in the potency of the drugs was also registered. A threshold concentration of SNP did not significantly affect the maximum reversion of tension brought about by sildenafil but added positively to the in vitro potency of the PDE5-I. PDE inhibitors may tend to be more effective in systems characterized by an enhanced production of cyclic AMP/GMP (such as urogenital tissues in vivo). Our findings may explain how PDE inhibitors can affect symptoms of the overactive bladder.