Cyclic AMP-specific and cyclic GMP-specific phosphodiesterase isoenzymes in human cavernous arteries—immunohistochemical distribution and functional significance

Objectives: It has been well established that male erectile dysfunction is frequently associated with vascular diseases. The normal function of cavernous arteries is considered a prerequisite to maintain sufficient blood flow to the trabecular spaces in order to enable penile erection. Contractility of cavernous arteries is regulated by the peripheral autonomic nervous system and endogenous factors released from the endothelial cell layer. A significant increase of blood flow in the central cavernous arteries is the initial event leading to penile tumescence and rigidity. Besides the significance of the nitric oxide/cyclic guanosine monophosphate (cGMP)-mediated mechanisms, the cyclic AMP (cAMP) signalling pathway is also involved in the regulation of tone of the erectile tissue, and interactions between cGMP- and cAMP-mediated mechanisms have been demonstrated. The aim of the present study was to investigate by means of immunohistochemistry the presence of the phosphodiesterase (PDE) isoenzymes 3, 4 (cAMP-specific PDEs) and 5 (cGMP-specific PDE) in thin sections of human central cavernous arteries (HCA) and their functional significance in the mechanism of vessel tone regulation. Methods: Functional experiments were performed using circular segments of HCA and strip preparations of the human corpus cavernosum (HCC). Relaxant effects induced by the cumulative addition of the PDE inhibitors milrinone (PDE3 inhibitor), rolipram (PDE4 inhibitor) and sildenafil (PDE5 inhibitor; 0.01, 0.1, 1 and 10 M) were studied in preparations of HCA and HCC challenged by 1 M norepinephrine (NE). Moreover, immunohistochemistry was carried out in order to evaluate the expression of PDE3, PDE4 and PDE5 in thin sections of HCA. Results: Milrinone, rolipram and sildenafil dose-dependently reversed the NE-induced tension of the isolated vascular segments and HCC strips with sildenafil being the most effective drug. Neither rolipram nor milrinone reached an EC₅₀ value. Abundant immunoreactivities specific for PDE3, PDE4 and PDE5 were observed in the entire smooth musculature of the wall of HCA and resistance arteries. In addition, immunoreactivity for PDE4 was also detected in the cytoplasm of endothelial cells lining the cavernous arteries. Conclusions: The cGMP-dependent relaxation of cavernous arteries is not only dependent on the normal function of the peripheral autonomic nervous system but also on the functional integrity of the vascular endothelium. The expression of the cGMP-specific PDE5 and the ability of the PDE5 inhibitor sildenafil to reverse the adrenergic tension of isolated segments of HCA underline the important role of the NO/cGMP pathway in the control of smooth muscle tone of human trabecular smooth musculature and penile cavernous arteries. Our results also suggest a significance of the cAMP-dependent signaling mechanisms in the regulation of tension of central HCAs. The present findings are also in support of the hypothesis of interactions between the cGMP- and cAMP-mediated signaling pathways in HCAs. Further investigations are indicated in order to outline potential differences between central HCAs and helicine resistance arteries. This may help to understand better the relations between structural and functional changes in the penile erectile tissue in patients with cardiovascular diseases and endothelial dysfunction.