Nitric oxide (NO)/cyclic GMP (cGMP)-mediated mechanisms have a pivotal function in reducing the tone of the penile smooth musculature during normal erectile responses. The cyclic AMP (cAMP) signaling pathway is also involved in the adjustment of smooth muscle contractility, and suggestions for interactions between cGMP- and cAMP-mediated mechanisms have been presented. Using activators of the cGMP- or the cAMP-pathway, as well as inhibitors of protein kinase A (PKA; cAMP-dependent kinase) and protein kinase G (PKG; cGMP-dependent kinase), the present study was undertaken to further delineate the functional relation between these pathways in the penis. In addition, the distribution of PKA and some cAMP-binding phosphodiesterases (cAMP-PDEs) were investigated in human erectile tissue. Functional experiments were performed on isolated human corpus cavernosum (HCC). The effects of an inhibitor of the PKA, Rp-8CPT-cAMPS (10 microM), or the PKG, Rp-8-pCPT-cGMPS (10 microM), on relaxation induced by the cumulative administration of sodium nitroprusside (SNP), forskolin, sildenafil or tadalafil (IC351) were studied in preparations of HCC precontracted with 1 microM norepinephrine (NE). Using immunohistochemical procedures, the presence of immunoreactivity for cAMP-PDEs PDE3, PDE4, and PDE4A, as well as for PKA was investigated in specimens of HCC from which preparations were also used in the functional experiments. Forskolin, SNP, sildenafil, and IC 351 dose-dependently reversed NE-induced tension of isolated HCC preparations. The relaxing effects of SNP were significantly attenuated by Rp-8-pCPT-cGMPS, but not by Rp-8CPT-cAMPS. In contrast, relaxation induced by forskolin, sildenafil and tadalafil were significantly reversed by both Rp-8-pCPT-cGMPS and Rp-8CPT-cAMPS. Abundant immunoreactivity for PDE3 and PKA was observed in the corpus cavernosum smooth muscle cells. Immunoreactivity for PDE4 was also detected in the smooth musculature and in the cytoplasm of endothelial cells lining the cavernous sinusoids, as well as in nerve fibres interspersing the trabecular stroma. The present results support the hypothesis of interactions between cGMP- and cAMP-mediated signals in the HCC, and suggest that the effects of inhibitors of PDE5 on isolated erectile tissue may also partly or indirectly include actions of the cAMP second messenger system. The exact mechanism by which such an interaction occurs is not clear, but it may involve altered activity of the cGMP-inhibited PDE3 brought about by a change in the intracellular levels of cGMP by the inhibition of PDE5. This will in turn lead to increasing levels of cAMP, facilitating the interaction of cAMP with the PKA. The immunoreactivity specific for PDE3, PDE4, PDE4A and PKA registered in HCC section is also in support of an important role for the cAMP/PKA-system for penile smooth muscle function.