Oral phosphodiesterase-5 inhibitors: effect of heme oxygenase inhibition on cGMP signalling in rat cavernous tissue

Effect of Genetic Polymorphism on the Response to PDE5 Inhibitors in Patients With Erectile Dysfunction: A Systematic Review and a Critical Appraisal

INTRODUCTION

Several treatment strategies are nowadays available for erectile dysfunction (ED) patients. Currently, oral phosphodiesterase type 5 inhibitors (PDE5Is) are the first-line therapy for ED. However, they are effective in all treated cases with variable non-responsiveness. Many factors have been listed for this behavior, but the possibility of gene polymorphisms as an underlying cause has not been systematically investigated.

OBJECTIVES

This review aimed to assess the possible involvement of gene polymorphisms affecting the response to PDE5Is in men with ED.

METHODS

A systematic review was conducted based on a search of all relevant articles in various electronic sites such as PubMed, Medline Medical Subject Headings, Cochrane Library, Science Direct, Scopus, Embase, CINAHL, and Egyptian Knowledge Bank databases. Keywords used for relevant associations were sexual health, genes, variants, erectile dysfunction, polymorphisms, PDE5Is, and cavernous tissues.

RESULTS

Several studies have been carried out to determine the contribution of different encoded genes to ascertain the association between different genotypes and ED men who were non-responders for PDE5Is. 11 studies were selected for this review. In these studies, 6 investigated eNOS genetic polymorphism with variable outcomes. Only 1 study was carried out for each of the following genetic polymorphisms: phosphodiestrase 5A, G-protein β3 subunit, angiotensin converting enzyme, dimethylarginine dimethylaminohydrolase, arginase, and vascular endothelial growth factor with variable results.

CONCLUSION

Despite the relative shortage of available studies and the varied methodologies used, most of the research articles demonstrated a significant association between genetic polymorphism and the response to PDE5Is, especially for endothelial nitric oxide synthase polymorphism. The limited number of studies that investigated the possible effect of genetic polymorphism and the response to PDE5Is are challenged by many factors, particularly for the definition of responders and non-responders. This should be a motivating factor for researchers to perform further studies with a standardized methodology to address the influence of genetic variations on the response to PDE5Is. Mostafa T, Hassan A, Alghobary MF, et al. Effect of Genetic Polymorphism on the Response to PDE5 Inhibitors in Patients With Erectile Dysfunction: A Systematic Review and a Critical Appraisal. J Sex Med 2020;8:573-585.

Intranasal Tadalafil nanoemulsions: formulation, characterization and pharmacodynamic evaluation

This study aims at improving the bioavailability of a poorly soluble phosphodiesterase-5 inhibitor; tadalafil (TD) via developing intranasal (IN) nanoemulsions (NEs). Optimum NE ingredients were selected based on solubility studies, emulsification tests, and phase diagram construction. Both o/w and w/o NEs were selected based on their drug loading capacity. Optimum formulations were subjected to physicochemical characterization and were assessed for nasal toxicity through biochemical analysis of tumor necrosis factor-α (TNF-α) and caspase-3 in rat nasal tissues. Pharmacodynamic study was performed via biochemical analysis of cyclic guanosine monophosphate (cGMP) in rat penis 2-h post-treatment and compared with oral suspension of Cialis® tablets. Optimum o/w and w/o NEs were successfully prepared using different ratios of Capmul-MCM-EP, Labrasol:Transcutol-HP (1:1) and water. Optimized formulations exhibited more than 4000-fold increase in TD solubility compared with its aqueous solubility. Both formulations were optically isotropic with the majority of globules in the nanometric-size range. Nasal toxicity study revealed no significant difference in values of TNF-α and caspase-3 between the NE-treated groups and the control group. Both TD-NEs succeeded to achieve a significant enhancement in cGMP levels. Our findings suggested that IN administration of the developed TD-NEs could provide a safe and effective alternative to TD oral delivery.

Serum L-carnitine and vitamin D levels may be low among oral sildenafil citrate non-responders

This cross-sectional comparative study aimed to compare serum L-carnitine and 25(OH)D levels between men with ED non-responding for oral sildenafil citrate and healthy volunteers. Overall, 192 men, recruited from two University Hospitals, were allocated into two equal groups of matched age; healthy potent men and men with ED non-responders for oral sildenafil citrate. Oral sildenafil citrate non-responders self-reported inadequate erectile responses after four attempts using 100 mg with the manufacturer's guidelines relative to meals, associated medications, and sexual stimulation/arousal. Exclusion criteria were: diabetes, cardiovascular disorders, beta blockers treatment, morbid obesity, thyroid disorders, post-radical prostatectomy, and hepatic/renal failure. All participants were subjected to; history taking, clinical examination, validated IIEF-5 questionnaire, estimation of serum L-carnitine by calorimetric method and serum 25(OH)D by ELISA method. Compared with potent controls, ED men non-responders for oral sildenafil citrate showed significant decreases in the mean serum L-carnitine level (16.8 ± 3.6 uM/L versus 66.3 ± 11.9 uM/L, P = 0.001), the mean serum 25(OH)D level (21.2 ± 7.1 ng/ml versus 54.6 ± 7.9 ng/mL, P = 0.001) and IIEF-5 score (7.8 ± 2.6 versus 23.9 ± 1.3). Serum L-carnitine showed significant positive correlation with IIEF-5 scores (r = 0.873, P = 001), serum 25(OH)D (r = 0.796, P = 0.001) and significant negative correlation with the age (r = -0.515, P = 0.001). Serum 25(OH)D showed significant positive correlation with IIEF-5 scores (r = 0.855, P = 0.001) and significant negative correlation with the age (r = -0.223, P = 0.005). It is concluded that normal homeostasis of serum L-carnitine and 25(OH)D play a role in male sexual health being significantly decreased in ED non-responding for oral sildenafil citrate.

Gas what: NO is not the only answer to sexual function

The ability to get and keep an erection is important to men for several reasons and the inability is known as erectile dysfunction (ED). ED has started to be accepted as an early indicator of systemic endothelial dysfunction and subsequently of cardiovascular diseases. The role of NO in endothelial relaxation and erectile function is well accepted. The discovery of NO as a small signalling gasotransmitter led to the investigation of the role of other endogenously derived gases, carbon monoxide (CO) and hydrogen sulphide (H2 S) in physiological and pathophysiological conditions. The role of NO and CO in sexual function and dysfunction has been investigated more extensively and, recently, the involvement of H2 S in erectile function has also been confirmed. In this review, we focus on the role of these three sister gasotransmitters in the physiology, pharmacology and pathophysiology of sexual function in man, specifically erectile function. We have also reviewed the role of soluble guanylyl cyclase/cGMP pathway as a common target of these gasotransmitters. Several studies have proposed alternative therapies targeting different mechanisms in addition to PDE-5 inhibition for ED treatment, since some patients do not respond to these drugs. This review highlights complementary and possible coordinated roles for these mediators and treatments targeting these gasotransmitters in erectile function/ED.

Dose-dependent bioavailability indicators for curcumin and two of its novel derivatives

Novel water-soluble curcumin derivatives have been developed to overcome low in vivo bioavailability of curcumin. The aim of this work is to assess the potential utility of certain downstream targets as bioavailability indicators of systemic activity of pure curcumin and two novel water-soluble curcumin derivatives (NCD) by constructing dose-dependent response curves and to prove whether this novel curcumin derivatives retained, improved, or abolished biological activity of pure curcumin when applied in vivo. Pure curcumin (CUR), curcumin-carboxy derivative (NCD-1), and curcumin protein conjugate (NCD-2) were administered orally to rats at escalating doses: 37, 74, 148, and 296 μM/kg body weight, respectively. Plasma levels of GST activity, cavernous tissue levels of cGMP, and enzymatic activity of both HO-1 and GST were assessed one and half and 24 hours after oral administration of curcumin formulae. This study showed that there was a progressive elevation of cavernous tissue levels of cGMP and enzymatic activity of both HO-1 and GST in a dose-dependent manner that was maintained for 24 h with CUR, NCD-1, and NCD-2. Plasma GST activity was decreased by the lowest doses on the curve. The three dose-dependent bioavailability indicators as surrogates of curcumin and two of its novel derivatives are valid in the studied range of concentration and extended time. The novel curcumin derivatives still conserve with improvement the biological activity of natural curcumin when applied in vivo.

Effect of chronic low-dose tadalafil on penile cavernous tissues in diabetic rats

OBJECTIVE

To assess the effect of chronic low-dose administration of tadalafil (Td) on penile cavernous tissue in induced diabetic rats.

METHODS

The study investigaged 48 adult male albino rats, comprising a control group, sham controls, streptozotocin-induced diabetic rats, and induced diabetic rats that received Td low-dose daily (0.09 mg/200 g weight) for 2 months. The rats were euthanized 1 day after the last dose. Cavernous tissues were subjected to histologic, immunohistochemical, morphometric studies, and measurement of intracavernosal pressure and mean arterial pressure in anesthetized rats.

RESULTS

Diabetic rats demonstrated dilated cavernous spaces, smooth muscles with heterochromatic nuclei, degenerated mitochondria, vacuolated cytoplasm, and negative smooth muscle immunoreactivity. Nerve fibers demonstrated a thick myelin sheath and intra-axonal edema, where blood capillaries exhibited thick basement membrane. Diabetic rats on Td showed improved cavernous organization with significant morphometric increases in the area percentage of smooth muscles and elastic tissue and a significant decrease of fibrous tissue. The Td-treated group showed enhanced erectile function (intracavernosal pressure/mean arterial pressure) at 0.3, 0.5, 1, 3, and 5 Hz compared with diabetic group values at the respective frequencies (P <.05) that approached control values.

CONCLUSION

Chronic low-dose administration of Td in diabetic rats is associated with substantial improvement of the structure of penile cavernous tissue, with increased smooth muscles and elastic tissue, decreased fibrous tissue, and functional enhancement of the erectile function. This raises the idea that the change in penile architecture with Td treatment improves erectile function beyond its half-life and its direct pharmacologic action on phosphodiesterase type 5.

Effects of a water-soluble curcumin protein conjugate vs. pure curcumin in a diabetic model of erectile dysfunction

INTRODUCTION

Curcumin is involved in erectile signaling via elevation of cyclic guanosine monophosphate (cGMP).

AIM

Assessment of the effects of water-soluble curcumin in erectile dysfunction (ED).

METHODS

One hundred twenty male white albino rats were divided into: 1st and 2nd control groups with or without administration of Zinc protoporphyrin (ZnPP), 3rd and 4th diabetic groups with or without ZnPP, 5th diabetic group on single oral dose of pure curcumin, 6th diabetic group on pure curcumin administered daily for 12 weeks, 7th and 8th diabetic groups on single dose of water-soluble curcumin administered with or without ZnPP, 9th and 10th diabetic groups on water-soluble curcumin administered daily for 12 weeks with or without ZnPP. All curcumin dosage schedules were administered after induction of diabetes.

MAIN OUTCOME MEASURES

Quantitative gene expression of endothelial nitric oxide synthase (eNOS), neuronal NOS (nNOS), inducible NOS (iNOS), heme oxygenase-1 (HO-1), nuclear transcription factor-erythroid2 (Nrf2), NF-Кβ, and p38. Cavernous tissue levels of HO and NOS enzyme activities, cGMP and intracavernosal pressure (ICP).

RESULTS

Twelve weeks after induction of diabetes, ED was confirmed by the significant decrease in ICP. There was a significant decrease in cGMP, NOS, HO enzymes, a significant decrease in eNOS, nNOS, HO-1 genes and a significant elevation of NF-Кβ, p38, iNOS genes. Administration of pure curcumin or its water-soluble conjugate led to a significant elevation in ICP, cGMP levels, a significant increase in HO-1 and NOS enzymes, a significant increase in eNOS, nNOS, HO-1, and Nrf2 genes, and a significant decrease in NF-Кβ, p38, and iNOS genes. Water-soluble curcumin showed significant superiority and more prolonged duration of action. Repeated doses regimens were superior to single dose regimen. Administration of ZnPP significantly reduced HO enzyme, cGMP, ICP/ mean arterial pressure (MAP), HO-1 genes in diabetic groups.

CONCLUSION

Water-soluble curcumin could enhance erectile function with more effectiveness and with more prolonged duration of action.

Novel water-soluble curcumin derivative mediating erectile signaling

INTRODUCTION

Curcumin is an inducer of heme oxygenase enzyme-1 (HO-1) that is involved in erectile signaling via elevating cyclic guanosine monophosphate (cGMP)levels.

AIM

To assess the effect of oral administration of a water-soluble long-acting curcumin derivative on erectile signaling.

METHODS

Two hundred and thirty six male white albino rats were divided into four groups; group 1 (N = 20) includes control. Group 2 (N = 72) was equally divided into four subgroups; subgroup 1 received pure curcumin (10 mg/kg), subgroup 2 received the long-acting curcumin derivative (2 mg/kg), subgroup 3 received the long-acting curcumin derivative (10 mg/kg), and subgroup 4 received sildenafil (4 mg/kg). Subgroups were sacrificed after the first, second, and third hour. Group 3 (N = 72) was equally divided into the same four subgroups already mentioned and were sacrificed after 24 hours, 48 hours, and 1 week. Group 4 (N = 72) was subjected to intracavernosal pressure (ICP) measurements 1 hour following oral administration of the same previous doses in the same rat subgroups.

MAIN OUTCOME MEASURE

Cavernous tissue HO enzyme activity, cGMP, and ICP.

RESULTS

In group 2, there was a significant progressive maintained elevation of HO activity and cGMP tissue levels starting from the first hour in subgroups 3 and 4, whereas, the rise in HO activity and cGMP started from second hour regarding the other rat subgroups. Sildenafil effect decreased after 3 hours. In group 3, there was a significant maintained elevation of HO activity and cGMP tissue levels extended to 1 week as compared to controls for all rat subgroups that received both forms of curcumin. In group 4, long-acting curcumin derivative exhibited more significant potentiation of intracavernosal pressure as compared to control and to the pure curcumin.

CONCLUSION

Water-soluble long-acting curcumin derivative could mediate erectile function via upregulating cavernous tissue cGMP.

Effects of losartan, HO-1 inducers or HO-1 inhibitors on erectile signaling in diabetic rats

INTRODUCTION

Activation of the renin-angiotensin system which is common in diabetes mellitus might affect heme oxygenase (HO-1) gene expression.

AIM

Assessment of the effects of administration of angiotensin II (Ang II) receptor antagonist (losartan) with HO-1 inducer or inhibitor on erectile signaling in diabetic rats.

MATERIALS AND METHODS

Seventy male rats were divided equally into seven groups; healthy controls, streptozotocin-induced diabetic rats, rats on citrate buffer, diabetic rats on losartan, diabetic rats on HO-1 inducer (cobalt protoporphyrin [CoPP]), diabetic rats on losartan and CoPP, and diabetic rats on losartan and HO-1 inhibitor (stannus mesoporphyrin [SnMP]).

MAIN OUTCOME MEASURE

HO enzyme activity, HO-1 gene expression, cyclic guanosine monophosphate (cGMP) assay, intracavernosal pressure (ICP), and cavernous tissue sinusoids surface area.

RESULTS

HO-1 gene expression, HO enzymatic activity, and cGMP were significantly decreased in the cavernous tissue of diabetic rats. These parameters were significantly elevated with the use of CoPP that restored the normal control levels of HO enzyme activity. Administration of losartan exhibited a significant enhancing effect on these parameters compared with the diabetic group, but not restored to the control levels, whereas administration of CoPP combined with losartan led to the restoration of their normal levels. ICP demonstrated significant decline in diabetic rats. The use of CoPP and/or losartan led to its significant improvement compared with diabetic rats. Administration of either losartan and/or CoPP led to a significant increase in the cavernous sinusoids surface area of diabetic rats. Administration of losartan with SnMP significantly decreased the enhancing effect of losartan on the studied parameters.

CONCLUSION

The decline in erectile function in diabetes mellitus could be attributed to the downregulation of HO-1 gene expression. HO-1 induction added to Ang II receptor antagonist could improve erectile function.

Effect of HO-1 cDNA-liposome complex transfer on erectile signalling of aged rats

This work aimed to assess the efficacy of haeme oxygenase-1 (HO-1) cDNA-liposome complex transfer as a mediator of erectile signalling in aged rats. One hundred and fifty aged white albino rats were equally divided into five groups: controls, rats receiving lipofectamine, rats receiving intracorporeal HO-1 cDNA-lipsome complex, rats receiving HO-1 cDNA-liposome complex plus nitric oxide synthase (NOS) inhibitor, and rats receiving HO-1 cDNA-liposome complex plus HO inhibitor. Six rats were killed from each group after 12, 24 and 48 h, and after1 and 2 weeks. In dissected cavernous tissues, the following were assessed: HO-1 gene expression, Western blot for HO-1, HO enzyme activity, cGMP and histopathology. The results showed that HO-1 cDNA-liposome complex transfer led to a significant increase in cavernous tissue HO-1 protein, HO-1 gene expression, HO enzyme activity and cGMP up to 1 week. NOS inhibition exhibited no effect on HO-1 gene enhancement of cavernous tissue HO enzyme activity or cGMP, whereas inhibition of HO significantly decreased these parameters. Histopathology of cavernous tissue demonstrated a significant dilatation of helicine arteries in HO-1 cDNA-liposome complex treated group after 48 h compared with the controls. It is concluded that HO-1 cDNA-liposome complex transfer augments cavernous tissue cGMP with subsequent sinusoidal relaxation.

Putative role of carbon monoxide signaling pathway in penile erectile function

INTRODUCTION

Erectile response depends on nitric oxide (NO) generated by NO synthase (NOS) enzyme of the nerves and vascular endothelium in the cavernous tissue. NO activates soluble guanylate cyclase (sGC), leading to the production of cyclic guanosine monophosphate (cGMP). cGMP activates cGMP-dependent protein kinase that activates Ca(2+)/ATPase pump that activates Ca(2+)/K efflux pump extruding Ca(2+) across the plasma membrane with consequent smooth muscle cell relaxation. A role similar to that of NOS/NO signaling has been postulated for carbon monoxide (CO) produced in mammals from heme catabolism by heme oxygenase (HO) enzyme.

AIM

To assess CO signaling pathway for erectile function by reviewing published studies.

METHODS

A systematic review of published studies on this affair based on Pubmed and Medical Subject Heading databases, with search for all concerned articles.

MAIN OUTCOME MEASURES

Documentation of positive as well as negative criteria of CO/HO signaling focused on penile tissue.

RESULTS

The concept that HO-derived CO could play a role in mediating erectile function acting in synergism with, or as a potentiator for, NOS/NO signaling pathway is gaining momentum. CO/HO signaling pathway has been shown to partially mediate the actions of oral phosphodiesterase type 5 inhibitors. In addition, it was shown that the use of CO releasing molecules potentiated cavernous cGMP levels. However, increased CO production or release was reported to be associated, in some studies, with vasoconstriction.

CONCLUSION

This review sheds a light on the significance of cavernous tissue CO signaling pathway that may pave the way for creation of therapeutic modalities based on this pathway.

Oral phosphodiesterase type 5 inhibitors: nonerectogenic beneficial uses

INTRODUCTION

Phosphodiesterase type 5 (PDE5) hydrolyses cyclic guanylate monophosphate (cGMP) specifically to 5' GMP. PDE5 inhibitors were a breakthrough medication that addressed a previously unfulfilled medical need. They promoted vascular relaxation in the corpora cavernosa and penile erection during sexual stimulation. Sildenafil, vardenafil, and tadalafil were approved then introduced as effective treatments for male erectile dysfunction. This impact has stimulated academic, clinical, and industrial research.

AIM

To highlight the nonerectogenic beneficial uses of oral PDE5 inhibitors.

METHOD

A systematic review of published studies in this affair based on a Pubmed and medical subject heading databases search of all concerned articles.

MAIN OUTCOME MEASURES

Demonstrated beneficial as well as applicable uses of oral PDE5 inhibitors.

RESULTS

As chemical molecules, these drugs were shown to exert potential nonerectogenic beneficial effects. They showed efficacy as a useful adjunct in the management of pulmonary hypertension. Additional uses were extended to different utilities: essential hypertension, benign prostatic hyperplasia, gastrointestinal disorders, endothelial dysfunction, female sexual dysfunction, genital blood flow, exercise capacity, Raynaud's phenomenon, sperm motility, etc.

CONCLUSION

Exploring PDE5 inhibitors for their possible medical applications in diverse specialties seems to be beneficial in making use of these molecules for the welfare of humanity.

Heme oxygenase enzyme activity in human seminal plasma of fertile and infertile males

This work aimed to assess heme oxygenase (HO) enzyme activity relationship with different human semen parameters. One hundred and twenty men were divided according to their sperm count and clinical examination into: obstructive azoospermia (n = 20), nonobstructive azoospermia (NOA) (n = 25), oligozoospermia (n = 35) and normozoospermia (n = 40). Semen analysis, western blot for HO-1 and HO-2, and estimation of seminal plasma HO enzyme activity chemically in the form of bilirubin concentration were carried out. Seminal plasma HO enzyme activity was very low in OA specimens, low in NOA, moderate in oligozoospermia while higher in normozoospermia (mean +/- SD; 6.26 +/- 2.2, 81.4 +/- 35.5, 283.8 +/- 90.1, 657.4 +/- 227.6 pmol ml(-1) min(-1)) with significant differences. Western blot analysis demonstrated HO-2 expression in all studied groups whereas HO-1 was highly expressed in fertile normozoospermic group compared with other groups. There was positive correlation between seminal plasma HO enzyme activity and sperm concentration, sperm motility percentage, motile spermatozoa ml(-1) and sperm normal morphology per cent. It is concluded that HO enzyme activity in the human seminal plasma is related to spermatogenesis and sperm-motility processes.

The role of PDE5 inhibitors in heme oxygenase-cGMP relationship in rat cavernous tissues

INTRODUCTION

Heme oxygenase (HO) enzyme catalyzes oxidative degradation of heme to biliverdin and carbon monoxide (CO). CO shares many properties with nitric oxide (NO) including the activation of soluble guanyl cyclase.

AIM

To assess cavernous tissue HO activity and cyclic guanosine monophosphate (cGMP) levels in response to oral phosphodiesterase type 5 (PDE5) inhibitors.

METHODS

Seven hundred twenty male Sprague-Dawley rats, divided into six groups, were investigated. Group 1, controls; group 2 received sildenafil citrate orally; group 3 received vardenafil hydrochloride; and group 4 received tadalafil. Group 5 was subdivided into three equal subgroups, received the same dose of each drug added to the HO inhibitor, Zn protoporphyrin. Group 6 was subdivided into three equal subgroups, received the same dose of each drug added to the NO inhibitor, L-nitroarginine methylester. Eight rats from each group/subgroup were sacrificed at 0.5, 1, 2, 3, 4, 6, 18, 24, and 36 hours, respectively.

MAIN OUTCOME MEASURES

HO enzyme activity assay and cGMP tissue levels in dissected rat cavernous tissues.

RESULTS

Both cavernous tissue HO enzyme activity and cGMP levels were increased significantly in sildenafil-, vardenafil-, and tadalafil-treated rats compared with the controls, with significant decreases after either HO or NO inhibition. Cavernous tissue HO enzyme activity and cGMP showed a positive significant correlation (r = 0.854, P < 0.001).

CONCLUSION

The effects of PDE5 inhibitors in cavernous tissue are partly mediated through HO enzyme activity.

Effect of hemin and carbon monoxide releasing molecule (CORM-3) on cGMP in rat penile tissue

INTRODUCTION

Cyclic guanosine monophosphate (cGMP) levels can be regulated by heme oxygenase-1 and 2 (HO-1 and HO-2)-derived carbon monoxide (CO).

AIMS

Assessment of the effect of upregulating CO in rat corpora cavernosa (CC) on cavernous cGMP.

METHODS

Three experimental groups were studied: first group (N = 40), short-term HO induction over 2 weeks by injection of intraperitoneal increasing doses of hemin; the second group (N = 40) was subjected to intracavernosal injection of CO donor, CORM-3, or its inactive form (iCORM-3) over 2 weeks; the third group (N = 60) was subdivided into three subgroups: the first one received a combined hemin and CORM-3, the second one received hemin and its inhibitor stannus mesoporphyrin (SnMP), and third one received a combined hemin, CORM-3, and SnMP.

MAIN OUTCOME MEASURES

In CC, HO-1 and HO-2 gene expression, Northern blot and Western blot, cGMP levels, and HO enzyme activity.

RESULTS

In the first group, maximum induction of HO-1 gene expression, HO enzyme activity, and cGMP occurred with 4-mg hemin dose with a successive increase over 2 weeks. In the second group, CORM-3 increased cGMP by twofold compared with iCORM-3, and also increased HO-1 protein. In the third group, SnMP inhibited the enhancing effect of CORM-3 and HO on erectile signaling molecules; i.e., HO-1 gene, enzyme activity, and cGMP.

CONCLUSIONS

CORM-3- or hemin-mediated CO release could increase cavernous tissue cGMP.