Kazem M. Azadzoi

A nitric oxide-like factor mediates nonadrenergic-noncholinergic neurogenic relaxation of penile corpus cavernosum smooth muscle.

This study was initiated to characterize nonadrenergic-noncholinergic (NANC) inhibitory neurotransmission in penile corpus cavernosum. Using organ baths, isometric tension measurements were made in strips of human and rabbit corpus cavernosum. In examining endothelium-mediated responses, cumulative additions of exogenous acetylcholine elicited dose-dependent relaxations which were significantly reduced or completely inhibited in tissues treated with NG-monomethyl L-arginine (L-NMMA; an inhibitor of nitric oxide synthesis), oxyhemoglobin (a nitric oxide scavenger), or methylene blue (a guanylate cyclase blocker). Tissues exposed to hypoxic conditions (PO2 = 5-10 mmHg) also did not respond to exogenous acetylcholine. Mechanical removal of the endothelium in human corporal strips or in situ treatment of rabbit corpora with detergent blocked the relaxation to acetylcholine. Transmural electrical stimulation of corporal tissue strips denuded of functional endothelium, in the presence of adrenergic blockade with bretylium and muscarinic receptor blockade with atropine, caused frequency-dependent relaxation. This neurogenic relaxation was reduced or prevented by L-NMMA, oxyhemoglobin, methylene blue, and hypoxia. The effects of L-NMMA were reversed by L-arginine and the effects of hypoxia were readily reversed by normoxic conditions. Authentic, exogenous nitric oxide relaxed corporal strips which were contracted with adrenergic agonists and this effect was significantly inhibited by oxyhemoglobin. It is concluded that (a) endothelium-mediated responses of corpus cavernosum smooth muscle are mediated by a diffusible nitric oxide-like substance; (b) NANC neurogenic inhibitory responses do not require functional endothelium, and (c) nitric oxide, or a closely related substance, may act as an inhibitory neurotransmitter in penile corpus cavernosum smooth muscle.

Endothelium-derived nitric oxide and cyclooxygenase products modulate corpus cavernosum smooth muscle tone

Relaxation of penile corpus cavernosum smooth muscle is controlled by nerve and endothelium derived substances. In this study, endothelium-dependent relaxation of corporal smooth muscle was characterized and the role of arachidonic acid products of cyclooxygenase in endothelium-dependent relaxation was examined. Endothelium removal from rabbit corpora was performed by infusion with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate and was confirmed by transmission electron microscopy. Strips of human and rabbit corporal tissues were studied in the organ chambers for isometric tension measurement. The accumulation of cyclic guanosine monophosphate (cGMP) and the release of eicosanoids from corporal tissue was measured by radioimmunoassay and correlated to smooth muscle relaxation. Our study showed that relaxation of corpus cavernosum tissue to acetylcholine, bradykinin and substance P was endothelium-dependent; potentiated by indomethacin; and inhibited by NG-monomethyl-L-arginine, methylene blue or LY83583. Relaxation to papaverine and sodium nitroprusside was endothelium-independent, and unaffected by NG-monomethyl-L-arginine. Relaxation to vasoactive intestinal polypeptide was partially endothelium-dependent; potentiated by indomethacin; attenuated by NG-monomethyl-L-arginine or methylene blue. The tissue level of cGMP was enhanced by acetylcholine and nitric oxide. Methylene blue inhibited both basal and drug-stimulated levels of cGMP. The release of eicosanoids was enhanced by acetylcholine and blocked by indomethacin. In conclusion, nitric oxide or a closely related substance accounts for the activity of endothelium-derived relaxing factor in the corporal tissue. Inhibition of the release of eicosanoids potentiates the relaxing effect of nitric oxide. Nitric oxide increases tissue cGMP which appears to modulate corporal smooth muscle relaxation.

Hypercholesterolemia Impairs Endothelium-Dependent Relaxation of Rabbit Corpus Cavernosum Smooth Muscle

The majority of cases of impotence are associated with vascular risk factors such as diabetes, hypercholesterolemia, hypertension and smoking. These factors induce impairment of endothelium-dependent relaxation of blood vessels in man and in experimental animals. In this study the effects of hypercholesterolemia on the reactivity of rabbit corpus cavernosum smooth muscle strips to endothelium-dependent and endothelium-independent agents were investigated. New Zealand White rabbits (n = 14) were randomly divided into control and treatment groups. The control group (n = 7) received a regular diet while the treatment group (n = 7) was fed a diet of 0.5% cholesterol and 4% peanut oil for 10 weeks. Animals were then sacrificed and the corporal tissue studied in organ chambers for isometric tension measurement. Tissue was contracted with phenylephrine and concentration-dependent relaxation to acetylcholine, in the presence and absence of indomethacin, and to nitroprusside were examined. Blood level of cholesterol in the cholesterol-fed group was significantly higher compared to the control group. Contractions to phenylephrine were similar in both groups. Hypercholesterolemia, however, inhibited relaxation to acetylcholine but did not alter relaxation to nitroprusside, a cyclic guanosine monophosphate (cGMP)-dependent, direct smooth muscle dilator. Indomethacin enhanced the relaxations to acetylcholine in both control and cholesterol-fed groups but did not correct the difference in the relaxation to acetylcholine between both groups. It is concluded that hypercholesterolemia impairs endothelium-mediated relaxation of rabbit corpus cavernosum smooth muscle. The mechanism for the endothelial dysfunction does not appear to involve alteration in cyclooxygenase products of arachidonate or the cGMP-dependent relaxation of corporal smooth muscle. Impairment of endothelium-dependent relaxation of corporal smooth muscle may contribute to the pathophysiology of impotence associated with hypercholesterolemia in man.

Diabetes Mellitus Impairs Neurogenic and Endothelium-Dependent Relaxation of Rabbit Corpus Cavernosum Smooth Muscle

The effect of alloxan-induced diabetes on the reactivity of corporeal nerves, endothelium and smooth muscle was studied in the New Zealand white rabbit. Fifteen rabbits were randomly divided into treated (n = 6) and control (n = 9) groups. The treated group was maintained for 6 weeks. Two control groups were studied. One control group (n = 3) was maintained for 6 weeks as littermate controls for diabetic group. The second control group (n = 6) was not maintained but was weight matched with the 6 week diabetic group. The reactivity of corpus cavernosum tissue from the diabetic animals and the control animals was studied in organ chambers. When tissue contraction was produced with phenylephrine for the study of relaxation to various stimuli, the tension induced was similar in the diabetic and the control groups. Relaxation of corpus cavernosum tissue to electrical stimulation of autonomic nerves as well as relaxation to the endothelium-dependent vasodilator acetylcholine were comparably unaffected in the weight matched and littermate control groups while significantly inhibited in the diabetic group. Treatment of the corporeal tissue with the cyclooxgenase inhibitor indomethacin enhanced the relaxation to electrical stimulation and to acetylcholine in the control and in the diabetic groups but did not improve the significant difference in relaxation between the two groups. Relaxation of corporeal tissue to endothelium-independent vasodilators, papaverine and nitroprusside was similar in the control groups and the diabetic groups. It is concluded that diabetes impairs neurogenic and endothelium-mediated relaxation of rabbit corpus cavernosum smooth muscle. These findings are comparable to those described in corpus cavernosum tissue from diabetic men, showing the validity of this experimental animal model. The mechanism for the nerve or endothelial dysfunction does not appear to involve alteration in cyclooxygenase products of arachidonate or the ability of the corporeal smooth muscle to relax via a cGMP-dependent mechanism. Since nitric oxide has been shown to act as the nonadrenergic noncholinergic neurotransmitter as well as endothelium-derived relaxing factor (EDRF) of the trabecular smooth muscle, it is possible that impairment of neurogenic and endothelium-dependent relaxation due to diabetes is mediated by alteration in the synthesis or availability of nitric oxide in corporeal tissue.

Erectile Dysfunction Due to Atherosclerotic Vascular Disease: The Development of an Animal Model

An animal model was developed to study the pathophysiology of erectile dysfunction due to atherosclerotic vascular disease. Thirty one New Zealand white male rabbits were divided into control (n = 5) and treatment groups (n = 26). The control group was placed on a regular diet while the treatment group underwent balloon de-endothelialization of the aorto-iliac arteries and received 1.6% cholesterol and 4% triglyceride diet for eight weeks. After eight weeks in the control animals (n = 5), blood levels of cholesterol, triglycerides and low density lipoproteins, radiologic studies as well as hemodynamic parameters of erectile function were all normal. In the surviving treatment animals (n = 21) after the same time period, a significant increase in blood levels of cholesterol, triglyceride and low density lipoprotein were observed. In addition, 62% of these animals developed hypertension which was not observed in the control group. Angiographically, 10 animals (48%) demonstrated severe atherosclerotic lesions (75% to 100% occlusion of common or internal iliac arteries on one side and over 50% occlusion of the opposite side), five (24%) had moderate lesions (50 to 75% luminal occlusion of right and left common iliac or internal iliac arteries) and 6 revealed minimal lesions (less than 50% occlusion of the right and left common iliac or internal iliac arteries). Of the 15 animals with 50% or greater luminal occlusion of the iliohypogastric arteries, erectile dysfunction was found in 93% of cases. Due to the development of erectile dysfunction in 33% of animals with minimal occlusive lesions, it appears that factors, other than large vessel luminal occlusion, may exist in this animal model which adversely influence erectile function. This model may therefore be of further benefit in the study of other factors associated with atherosclerosis and impotence, such as the possible concomitant hypercholesterolemic and atherosclerotic-induced alterations in the local reactivity of corpus cavernosum smooth muscle and lacunar space endothelial cells.

Study of Etiologic Relationship of Arterial Atherosclerosis to Corporal Veno-Occlusive Dysfunction in the Rabbit

PURPOSEnThe aim of this study was to explore the possible etiologic relationship of hypercholesterolemia and atherosclerosis to corporal veno-occlusive dysfunction.nnnMATERIALS AND METHODSnIn the New Zealand White rabbit, the competence of the corporal veno-occlusive mechanism was examined at various intervals after exposure to control diet, high cholesterol diet, or aortoiliac atherosclerosis.nnnRESULTSnInitially, all animals showed normal erectile function and corporal veno-occlusion. After 8 weeks and 16 weeks, the control animals preserved normal erection and corporal veno-occlusion, while most of the hypercholesterolemic and atherosclerotic animals developed corporal veno-occlusive dysfunction. The incidence of corporal veno-occlusive dysfunction in the hypercholesterolemia and atherosclerotic animals increased with time.nnnCONCLUSIONSnThis study suggests that a close relationship exists between prolonged atherosclerotic occlusion of major penile arteries and the development of corporal veno-occlusive dysfunction. Ischemia-induced corporal veno-occlusive dysfunction may be the result of alterations in corporal smooth muscle relaxation or changes in the structure and fibroelastic properties of erectile tissue.

Engineering analysis of penile hemodynamic and structural-dynamic relationships: Part II—Clinical implications of penile buckling

Purpose: Penile buckling force was analytically described in terms of its constituents. In addition, theoretically-derived buckling force data were compared to clinically measured data and the influence of each constituent on penile buckling force data was assessed.Methods: Using engineering buckling theory for a column, a mathematically-derived penile buckling model was developed which incorporated geometric and hemodynamic data obtained by dynamic infusion pharmacocavernosometry studies in 21 impotent patients (age 43, range 24–62u2005y) as well as penile tissue mechanical characteristics previously developed (Part I).Results: In 17 of 21 patients the mean difference between theoretically derived and clinically measured buckling force data was 0.33±0.25u2005kg (r=0.96). Factors which increased penile buckling forces were: high intracavernosal pressure values (rigidity was related to pressure in an exponential-like fashion); high penile aspect ratio (D/L) values (relatively large diameter/short length penile geometry) and high flaccid diameter; and high cavernosal expandability values (a measure of the ability of the corpora to approach its erect volume with relatively low intracavernosal pressures).Conclusions: Pressure-volume data (pressure, geometry and tissue characteristics) obtained during erectile function testing have been shown, for the first time, to theoretically predict the magnitude of clinically-measured penile buckling forces.

Engineering analysis of penile hemodynamic and structural- dynamic relationships: Part I—Clinical implications of penile tissue mechanical properties

Purpose: The least investigated physical determinant of penile rigidity has been penile tissue material properties. The goals in this study (Part I) were to define two penile mechanical parameters, cavernosal expandability X and tunical distensibility VE/VF, determine their magnitudes in humans and develop an analytical expression for penile volume as a function of these two tissue characteristics and intracavernosal pressure.Methods: Dynamic infusion pharmacocavernosometry was performed in 21 impotent patients (age 43±19u2005y) to provide human geometric, hemodynamic and structural data. A mathematically derived model of hemodynamic and structural-dynamic characteristics of penile erection was developed (Parts I, II, III) incorporating penile tissue mechanical qualities.Results: Cavernosal expandability X provided a measure of the ability to approach maximum volume at relatively low intracavernosal pressures. Tunical distensibility VE/VF denoted the maximal erect to flaccid penile volume ratio. The magnitudes of X and VE/VF in the study population were 0.04–0.17u2005mmHg−1 and 1.7–5.0 respectively.Conclusions: Enabling penile volume to be derived as a function of tissue mechanical characteristics and pressure, allows for penile rigidity to be expressed (in Part II) as a function of pressure, geometry and tissue qualities.

Engineering analysis of penile hemodynamic and structural-dynamic relationships: Part III—Clinical considerations of penile hemodynamic and rigidity erectile responses

Purpose: The extent to which hemodynamic erectile responses predict penile buckling forces has not previously been analytically investigated. An engineering study was performed to compare hemodynamic data with penile buckling force values.Methods: Dynamic infusion pharmacocavernosometry studies in 21 impotent patients (age 43, range 24–62u2005y) were accomplished to obtain information during penile erection concerning hemodynamic values, penile buckling forces and their determinants: intracavernosal pressure, erectile tissue mechanical properties and penile geometry.Results: In the 21 patients, discrepancies existed in several patients who demonstrated normal hemodynamic values (low flow-to-maintain and high equilibrium intracavernosal pressures) but elevated cavernosal compliance values and diminished penile buckling forces. There was poor correlation between cavernosal compliance and equilibrium intracavernosal pressure (r=−0.36); better correlation between compliance and expandability (r=−0.72) and best correlation between dimensionless compliance and the dimensionless product of expandability with equilibrium pressure (r=−0.88). These data implied that cavernosal compliance was dependent on multiple factors, not only equilibrium intracavernosal pressure.Conclusions: Hemodynamic indices which correlate with intracavernosal pressure alone do not predict penile buckling forces since the latter are dependent not only on intracavernosal pressure but also on penile geometry and erectile tissue properties. The most relevant tissue property in predicting adequate penile buckling forces is cavernosal expandability. A new impotence classification system and diagnostic algorithm based on the determinants of penile rigidity and not exclusively on hemodynamic responses is proposed.

Systemic administration of apomorphine improves the hemodynamic mechanism of clitoral and vaginal engorgement in the rabbit.

Clitoral and vaginal engorgement during sexual stimulation depend in part on the increase of arterial inflow. It has been shown that apomorphine (APO), a non-selective dopamine receptor agonist, produces penile erection by activating dopaminergic receptors in the central nervous system. Our aim was to study whether systemic administration of APO improves the hemodynamic mechanism of clitoral and vaginal engorgement in the rabbit.Female New Zealand white rabbits (3.5–4u2005kg, n=6) were anesthetized. To examine sexual arousal function, the vaginal/clitoral branch of the pelvic nerve was stimulated electrically and maximal increases in clitoral intracavernosal and vaginal wall blood flows and pressures were recorded. After this APO was injected intravenously in a dose–response manner (0.05, 0.1, 0.2, 0.3 and 0.4u2005mg/kg) and nerve stimulation was performed after each dose. Changes in nerve-stimulated increase of clitoral intracavernosal and vaginal blood flows and pressures after APO was compared to those recorded before APO.Electrical stimulation of the vaginal/clitoral branch of the pelvic nerve significantly increased clitoral intracavernosal and vaginal wall blood flows in the rabbit. Intravenous administration of APO caused concentration dependent increase in nerve stimulation-induced peak clitoral intracavernosal and vaginal wall blood flows reaching to statistically significant at the concentration of 0.1 and 0.2u2005mg/kg. Inravenous administration of APO greater than 0.2u2005mg/kg (0.3 and 0.4u2005mg/kg) were less effective or produced adverse effects on clitoral intracavernosal and vaginal wall blood flows. Intravenous APO also tended to increase nerve-stimulated increase of clitoral intracavernosal and vaginal wall pressures, but the effect was not statistically significant.In conclusion, our studies suggest that systemic administration of APO may improve clitoral and vaginal engorgement by increasing clitoral intracavernosal and vaginal wall arterial inflow.