Managing Diabetes Mellitus to Prevent Erectile Dysfunction

Phosphodiesterases (PDEs) are distributed throughout various tissues; primarily in the vascular, visceral and pulmonary smooth muscles. In the penis, PDE-5 causes breakdown of cGMP, where its inhibition increases its bioavailability facilitating NO-mediated relaxation of erectile smooth muscle with sexual stimulation. (Beavo, 1995).

Conventional Phosphodiesterase 5 inhibitors (PDE5Is) include sildenafil, vardenafil, tadalafil and avanafil. New PDE5Is include udenafil and mirodenafil. PDE5Is exhibit analogous modes of action. However, they vary in their pharmacodynamics and pharmacokinetics. Sildenafil was Food and Drug Administration (FDA)-approved in 1998, vardenafil and tadalafil in 2003 and avanafil in 2012. Vardenafil is the most potent of the three in inhibiting PDE5 activity while tadalafil was reported to have extended plasma half-life up to 18 h as compared to 3–4 h in the case of sildenafil and vardenafil (Wright, 2006; Ismail and El-Sakka, 2016, 2017).

Vardenafil was effective in treating DM-induced ED with the greatest effect achieved through chronic dosing; with no additive effect measured with the final acute dose (De Young et al., 2008). Changes noted in the histology and protein expression indicate that vardenafil may have a protective effect in this disease state. Furthermore, vardenafil is a candidate therapy for the rehabilitation of patients with DM (Ferrini et al., 2006). Furthermore, recent study investigated the functional and structural changes of penile cavernous tissue after administration of low-dose tadalafil. They concluded that low-dose chronic administration of tadalafil is associated with structural and functional improvement of erection (Mostafa et al., 2013).

Rationale: due to the inability to collect human samples to investigate the effect of early versus late administration of vardenafil in DM patients with ED, this fact prompted us to investigate this effect in a rat model.

Aim of the work

To determine the possible prevention of deleterious consequences of DM on erectile function with administration of PDE5 inhibitors.

Study Objectives

Primary objective

To investigate the effect of early versus late administration of vardenafil on erectile function in a rat model of diabetes.

Secondary objective

To identify Intracavernosal pressure (ICP), histopathological and ultrastructural changes, plus eNOS, nNOS, inducible (iNOS) and TGF-β1 genes expression in a rat model of diabetes after early versus late administration of vardenafil.

Study question

Does early administration of vardenafil prevent ED in a rat model of diabetes?

Study hypothesis

Early administration of vardenafil will prevent ED in a rat model of diabetes.

Review of Literature

I. Epidemiology of erectile dysfunction

ED is a prevalent worldwide clinical condition that will be escalated up to 322 million by 2025, compared to 152 million in 1995 (McKinlay, 2000; El-Sakka, 2017). ED is prevalent in the middle-east community and reach up to 92.6% in those individuals. In addition, about 50.8% of patients with sexual disorders presented with premature ejaculation and 7.6% presented with low sexual desire. Moreover, 80% had organic and 20% had psychogenic causes of ED. Mild, moderate and severe ED was diagnosed in 10%, 40% and 5% respectively (Brock, 2000; El-Sakka, 2004).

The prevalence rate of ED was reported 63.6%, in Egypt, 57.4% in Nigeria and 80.8% in Pakistan (Seyam et al., 2003; Shaeer et al., 2003). Consequently, There have been a phenomenal improvement in the diagnosis and treatment of ED in the last three decades (Ismail and El-Sakka, 2016, 2017; El-Sakka, 2017).

II. Physiology and Pathophysiology of erectile function:

1. Physiology of Penile Erection

The cornerstone of erectile function is an incorporated process of vascular and neural functions that promote trabecular smooth muscle relaxation, leading to distention of sinusoidal spaces and blood inflow into corpora cavernosa, facilitating penile erection. This process employs mechanical compression of emissary veins integrated in the tunica albuginea causing impedance of blood flow that eventually results in penile erection (El-Sakka and Lue, 2004). Autonomic nervous system delivers sympathetic (T12–L2) and parasympathetic (S2–S4) innervation to pelvic plexus, counting the cavernous nerves. Autonomic nerve fibers are responsible for NO delivery that results in trabecular smooth muscle (SM) relaxation (Andersson and Wagner, 1995; Burnett, 1997).

Acetylcholine (ACH) is essential for ganglionic transmission (via nicotinic receptors) and vascular SM relaxation (via muscarinic receptors). Cholinergic nerves were identified within cavernous SM and neighboring penile arteries. ACH stimulates NO release from endothelial cells and potentiates direct SM relaxation during erection (Sáenz de Tejada et al., 1988)

Vasoactive intestinal polypeptide VIP is one of the neurotransmitters involved in penile erection. Immune-reactive VIP nerve fibers were identified inside the cavernous trabeculae and surrounding cavernosal arteries. Cavernosal SM relaxation is blocked by VIP antagonists (Kim et al., 1995). Other neurotransmitters include calcitonin-gene-related peptide (CGRP) (Stief et al., 1990), histidine methionine peptide (Kirkeby et al., 1992), pituitary adenylate cyclase activating polypeptide (Hedlund et al., 1994) 32 and prostaglandins (PGs) (Saenz de Tejada et al., 1989). Density of prostaglandin E1 receptor is decreased in men with ED (Aboseif et al., 1993). Vascular tone’s neural-endothelial control and neuromodulators/neurotransmitters interactions at neuromuscular junction have been also investigated (Andersson and Holmquist, 1994).

NO stimulates guanylate cyclase production of cGMP; this reduces cytosolic calcium concentration and eventually facilitates trabecular SM relaxation and increase blood flow into sinusoidal spaces (Sáenz de Tejada, 2002). This pathway is reversed by PDE5 enzymes by cGMP inactivating, resulting in cytosolic calcium concentration and smooth-muscle contraction. Sacral spinal cord provides motor nerve supply through pudendal nerve innervation. Pudendal nerve fibers join the ischio-cavernosus and bulbo-cavernosus muscles (Stief et al., 1998).

Sympathetic stimulation results in detumescence via cavernous smooth-muscle contraction, while cholinergic stimulation may facilitate erection through sympathetic inhibition in addition to endothelial NO release (Sáenz de Tejada et al., 1988). Corporeal SM relaxation is essential for normal erectile function, and evidence implicated that nNOS and eNOS are the principal mediators of SM muscle relaxation (Hurt et al., 2002). NO is synthesized from the precursor, L-arginine substrate via the catalytic activity NOS enzyme (Rajfer et al., 1992).

Penile erection is essentially a neurovascular mechanism that primarily requires initiation followed by maintenance of tumescence through corporeal SM relaxation and increased blood flow to the penis. The production and release of NO (by endothelial cells and nitrergic nerves) are widely accepted to be the pacemaker of physiological erection; depending on NO-cGMP signaling transduction pathway. If any factor involved in this cascade is affected, ED ultimately will occurs (Burnett and Musicki, 2005).

cGMP upregulation facilitates SM relaxation, through cGMP-dependent protein kinase activation. Consequently, Potassium (K+) channel activity increase and intracellular calcium (Ca2+) concentrations decreases (Bivalacqua et al., 2003).

Lack of the number of NOS-containing nerve fibers, NOS activity and endothelial SM relaxation impairment are the major contributing factor to ED in DM patients. Some studies established that eNOS and nNOS downregulation of their protein and gene expression in DM animal models are the main factors of erectile function impairment (Bivalacqua et al., 2001)

Meanwhile, these researches recognized a possible mechanism for ED development in DM patients; that is a decrease in production of NO, through reduction in nNOS and eNOS in corporeal vasculature (Akingba and Burnett, 2001).

Genetic studies have successfully implicated methods of increasing the transduction of a desired gene through gene transfer technologies. Gene transfer enhances gene expression and the functional activity of the desired gene. nNOS and eNOS gene transfer into penile tissue, vehicle on adenovirus has effectively reversed age-related ED in a rat model (Thomas R Magee et al., 2002).

Previous studies showed that NOS isoforms gene transfer can enhance physiological profile of penile tumescence in vivo; thereby, NOS expression improved ED associated with NOS downregulation. The effects of eNOS gene transfer into the penis of STZ-induced DM rats was being investigated; and it was concluded that erectile function could be improved if NO expression is restored (T R Magee et al., 2002). eNOS is an essential element of erectile function; it has the capability of inducing vasodilation and inhibition of vasoconstriction (Montorsi et al., 2004).

On the other hand, testosterone (T) directly affects endothelial function. NO is the pacemaker of endothelial function; its bioavailability is markedly decreased with T deprivation. Therefore, T may possess direct influence on endothelial function and migrating endothelial progenitor cells (EPC) (El-Sakka, 2017).

TGF-β1 molecule is isolated from platelets, human placenta and bovine kidney. It has a (25kD) homodimer molecular composition (Sporn and Roberts, 1992).

TGF-β1 action in tissue repair process, involves a complexed sequences of cytokine production, monocyte chemoattraction, angiogenesis and additional inflammatory mediators (Border and Ruoslahti, 1992). Moreover, TGF-β1 triggers matrix components synthesis; including tenascin, proteoglycans, collagens and fibroactin (Balza et al., 1988). In the same time, it concomitantly blocks matrix degradation by blocking protease synthesis and stimulating proteases inhibitors leading to accumulation of extracellular matrix (ECM) deposition at tissue injury site, This results into fibrosis and scarring. Moreover, TGF-β has the ability to self-control its own production; this may be the key to persistence of fibrosis and scarring (Border and Ruoslahti, 1992).

Previous studies had demonstrated the role of TGF-β in a rat model of Peyronie’s disease. they found that TGF-β2 and TGF-β3 had no remarkable protein expression, while TGF-β1 gene expression in the cytomodulin-injected rats was noted after 2 weeks post-induction (El-Sakka et al., 1997a, 1997b; El-Sakka et al., 1998).

From a similar perspective, other authors investigated the role of TGF-β1 in a canine model of prolonged penile erection; they related an association between prolonged penile erection with a relatively increased TGF-β gene expression as a moderator of fibrosis; with a resultant structural and morphological changes in a short period of time, suggesting that early therapeutic intervention with agents that counteract TFG-β action may be a desirable treatment modality (El-Sakka et al., 1998).

Collectively, TGF-β1 Gene and its expression have been found to be elevated in animal models with fibrotic conditions in various tissues (e.g. lung and liver) (Balza et al., 1988; Castilla et al., 1991; El-Sakka et al., 1997); Nevertheless, some previous studies couldn’t elicit any difference in TGF-β1 gene expression in an animal model of diabetes compared to control. However, other growth factors may play a role in that perspective (El-Sakka et al., 1999).

2. Pathophysiology of Erectile Dysfunction

There was a previous thinking referred ED to psychogenic causes only, especially in younger population. However, with the advances of ED research, it is strongly believed now that there are other organic causes contribute to ED such as hormonal, neural, pharmacological and penile factors specifically in older patients (Fazio and Brock, 2004).

Some neurologic disorders are associated with ED, including multiple sclerosis (MS), stroke, epilepsy, Alzheimer’s disease, Parkinson’s disease, spinal cord injury and radical pelvic surgeries (e.g., radical prostatectomy) (Siddiqui et al., 2012).

Liver disease, prostate disease, LUTS, pelvic surgery and renal failure lead to exposure physical inactivity. Use of recreational drugs, caffeine consumption, drug addiction and pelvic surgery are also reported as risk factors associated with the pathophysiology of ED (El-Sakka, 2012a).

III. Diabetes mellitus and erectile dysfunction:

DM is a chronic disease characterized by absolute or relative deficiency of insulin, which consequentially lead to hyperglycemia. Prolonged hyperglycemia may lead to serious complications such as nephropathy, neuropathy and retinopathy and increased cardiovascular risk (Hyttinen et al., 2003).

A total of 4556 patients; 1494 from Egypt, 2162 from Saudi Arabia and 900 from the United Arab Emirates were observed for ED treatment with PDE5I. DM was the most frequent diagnosis in 76% of these patients, primarily type II DM (67.5 of all patients). Symptoms of peripheral neuropathy were the most commonly reported complications of DM. Hypertension was reported in 36.3% of patients, dyslipidemia in 66.7% and coronary heart disease in 15.7% of patients (El-Sakka et al., 2011)

Type II diabetes is associated with diminished beta cell compensation of insulin resistance that eventually lead to insulin deficiency. Lifestyle modification, exercise and weight reduction may improve Insulin resistance (Solomon et al., 2008). DM, dyslipidemia, arterial hypertension, depression and coronary artery disease (CAD) are the main risk factors for the development of ED (Glina, Sharlip and Hellstrom, 2013).

Latest estimates from the International Diabetes Federation (IDF) is that in 2015 there were 415 million people with DM worldwide. By 2040, this number is anticipated to rise to 642 million. World Health Organization (WHO) and non-communicable diseases (NCDs) risk factor collaboration provided a similar estimation of 422 million cases in 2014 (Zimmet et al., 2016)

Prevalence of both diagnosed and undiagnosed DM in Egyptian population aged more than 20 years is estimated to be 9.3% (Herman et al., 1995). Recent studies estimated the prevalence of type II DM in the Egyptian population approximately 15.6% of all adults (20-79) years of age (Hegazi et al., 2015). Several studies in the Middle East disclosed a more than 40% prevalence of ED. At least 5 Arab countries were counted in the worldwide top 10 countries with a high prevalence of DM (El-Sakka, 2012a).

ED is an independent risk factor for reduced quality of life in diabetic patients. The association between DM and ED was addressed as high as 75% in some populations with an increased incidence of ED in diabetic patients by 68 cases/1000 patients per year (Malavige et al., 2014).

El-Sakka and Tayeb stated that 86.1% of Type II diabetic patients had some degrees of ED; including 7.7%, 29.4% and 49.1%. for mild, moderate and severe ED respectively. Twenty five percent of DM patients below 50 years of age had ED, escalated to 75% in those >50 years of age. Seventy percent of whom without ED were <50 years of age compared to 30% who were >50 years (El-Sakka and Tayeb, 2003).

Prolonged history of DM >10 years was associated with a triple increase in the risk of ED development, this is less likely in those with a <5 years history of DM. Poor metabolic control is associated with a 12.2 time increase in the risk of ED compared to a less likely occurrence in men with good metabolic control. Fifty-three percent of diabetic men with ED have ≥1 diabetes-related complications compared to 20.5% in those without ED (El-Sakka and Tayeb, 2003).

Penile doppler ultrasound changes were addressed in diabetic patients in the form of decreased peak systolic velocity (PSV) and poor response to intracorporeal injection (ICI) of vasoactive materials (El-Sakka, 2003).

Additional study evaluated the association between type II DM and Peyronie’s disease (PD) found an 8.1% prevalence of PD among a total of 1133 diabetic patients. Moreover, psychological disorders and dyslipidemia were associated with PD (El-Sakka and Tayeb, 2005). Another study concluded that type II DM and PD negatively affected the vascular status of erection, either solely and together (El-Sakka and Tayeb, 2009).

Pathogenesis of ED in diabetic patients is a multifactorial process associated with multiple endocrine, neuronal, hormonal, metabolic and vascular factors. Oxidative stress is also associated with endothelial and neuronal apoptosis leading to endothelial denudation and neuronal damage leading eventually to further depletion of NO (Russell et al., 2002). Moreover, superoxide employs calcium ion mobilization leading to direct vasoconstriction, possibly produce ED (Wan et al., 2010).

Dyslipidemia, obesity, metabolic syndrome, insulin resistance, hyperglycemia and other metabolic abnormalities are risk factors for development of ED with DM (Hidalgo-Tamola and Chitaley, 2009).