br Introduction The discovery that the nitric

Introduction The discovery that the nitric oxide (NO), soluble guanylate cyclase (sGC), and cyclic guanosine monophosphate (cGMP) pathway mediates erection has led to the use of phosphodiesterase type 5 (PDE-5) inhibitors as the gold standard for treating erectile dysfunction (ED) through the inhibition of cGMP degradation.1, 2 Despite their high efficacy, these drugs have limitations such as the necessity of minimum NO and cGMP formation. A substantial decrease in NO bioavailability and/or sGC sensitivity for NO causes PDE-5 inhibitor resistance. This situation can be expected in pathologies accompanied by high levels of oxidative stress such as obesity, diabetes, atherosclerosis, and hypertension. The oxygen radicals scavenge NO and oxidize the heme group of sGC, making the enzyme unresponsive to NO. This leads to severe ED requiring alternative strategies that are often invasive and uncomfortable for the patient. As such, the search for new therapeutic alternatives, including sGC stimulators and activators, is needed.3, 6, 7 The NO GSK J1 sGC can present advantages as a therapeutic target for ED. Recently, the therapeutic strategy by targeting sGC and cGMP has regained attention. Several studies have shown that sGC activation, irrespective of the presence of NO, induces erectile responses.3, 8, 9, 10, 11, 12, 13 Moreover, although sGC also is a target of oxidative damage, it can be reactivated by heme-independent sGC activators, indicating that sGC could present a valid target for ED even when associated with severe oxidative stress. sGC is found in two isoforms, sGCα1β1 and sGCα2β1. Knowledge of the functional role of these two isoforms in different tissues could lead to the development of more tissue-selective therapeutics and thus minimize side effects. sGCα1β1 is the predominant isoform in the CC. Our previous studies describing NO-dependent erectile responses in corpora cavernosa (CC) isolated from a mouse model lacking functional sGCα1β1 (sGCα1−/− mice) suggested that a mechanism other than sGCα1β1 activation must be involved in erection. Whether activation of the less abundantly expressed sGCα2β1 isoform and/or an sGC-independent mechanism might be involved remains to be determined.17, 18
Aims Recently, a new mouse model expressing apo-sGC was described as lacking NO-sensitive sGC. In these sGCβ1ki/ki mice, the important histidine 105 is converted to phenylalanine in the sGCβ1 subunit. This results in uncoupling of the heme group, yielding a heterodimer that retains basal cyclase activity but fails to respond to NO owing to failure of binding the heme. Together with studies in sGCα1−/− mice, the sGCβ1ki/ki mouse model allows for the differentiation of sGC-dependent and -independent physiologic pathways involved in erectile function and enables the establishment of the relative contribution of the two sGC isoforms (sGCα1β1 and sGCα2β1). In addition, oogenesis model mimics pathologic situations in which sGC is oxidized and rendered unresponsive to NO, allowing the study of the therapeutic potential of the sGC stimulator BAY 41-2272 and the sGC activator BAY 58-2667.
Methods
Main outcome measures The present study established in vitro and in vivo erectile responses in sGCβ1ki/ki and WT mice with stimulation of NO to determine whether sGC-independent pathways could regulate CC relaxation. These results also allowed an evaluation of the role of the minor expressed sGCα2β1 isoform by comparing them with those obtained from sGCα1−/− mice. In addition, the corporal relaxing effect of an sGC stimulator and an sGC activator in mutant and WT mice was examined. Specificity of the results was assessed by studying the erectile responses of sGC-independent activators in mutant and WT mice.
Results
Discussion The main finding of the present study is that activation of sGC is the sole pathway leading to NO-induced erection and that the sGCα1β1 and sGCα2β1 isoforms are involved. In addition, our observations suggest that sGC activators such as BAY 58-2667 might be a therapeutic approach for patients with severe ED from increased oxidative stress as in diabetes, hypertension, atherosclerosis, and obesity. These conclusions are derived from experiments using sGCβ1ki/ki mice, a model in which basal sGC activity is preserved, whereas all NO-induced sGC activation is abolished as shown by the sGC assays (Figure 5).