Fouad M. Sharabi

Testosterone facilitates the baroreceptor control of reflex bradycardia: role of cardiac sympathetic and parasympathetic components.

Reported clinical and experimental findings have shown that baroreflex control of heart rate is attenuated in women compared with men. This study investigated whether the sexual dimorphism in baroreflex function relates to the ability of the male hormone testosterone to facilitate baroreflex responsiveness. Relative contributions of the vagal and sympathetic autonomic components to testosterone modulation of baroreflex function were also investigated. Baroreflex curves relating changes in heart rate to increases or decreases in blood pressure evoked by phenylephrine and sodium nitroprusside, respectively, were constructed in sham-operated rats and castrated rats with and without testosterone replacement. Slope of the curves was taken as an index of baroreflex sensitivity (BRS PE and BRS NP ). Castration (for 10 days) significantly reduced plasma testosterone levels and attenuated reflex bradycardia, as indicated by significantly smaller BRS PE in castrated rats compared with values in sham-operated rats (−0.85 ± 0.07 vs. −1.51 ± 0.10 beats/min per mm Hg). Testosterone replacement in castrated rats restored plasma testosterone and BRS PE to levels similar to those of sham-operated rats. Muscarinic blockade by atropine caused 55% reduction in BRS PE in sham-operated rats, an effect that was significantly (p < 0.05) attenuated in castrated rats and restored to intact levels after testosterone supplementation. &bgr;-Adrenergic blockade by propranolol caused slight and insignificant decreases in BRS PE . Castration and testosterone supplementation had no effect on BRS NP , ruling out a modulatory effect of testosterone on reflex tachycardia. These data provide the first experimental evidence of a favorable role for testosterone in baroreceptor control of reflex bradycardia. Further, baroreflex modulation by testosterone appears to be autonomically mediated and involves an enhancement of cardiomotor vagal activity.

Testosterone depletion contributes to cyclosporine-induced chronic impairment of acetylcholine renovascular relaxations

The immunosuppressant drug cyclosporine causes nephrotoxicity mainly via alterations of renovascular reactivity. This study investigated whether this effect of cyclosporine is modulated by the male gonadal hormone testosterone. The endothelium-dependent and -independent relaxations evoked by acetylcholine and sodium nitroprusside, respectively, were evaluated in phenylephrine-preconstricted isolated perfused kidneys obtained from sham-operated, castrated, and testosterone-replaced castrated (CAS+T) male rats in the absence and presence of cyclosporine. Compared with sham-operated values, short-term (10 days) castration or cyclosporine treatment caused significant and equivalent reductions in plasma testosterone levels and vasorelaxant responses to acetylcholine. Treatment of castrated rats with cyclosporine caused no further attenuation of acetylcholine relaxations. Testosterone replacement of castrated (CAS+T) or cyclosporine-treated castrated (CAS+CyA+T) rats restored plasma testosterone and acetylcholine relaxations to near-sham-operated levels. On the other hand, castration caused significant increases in nitroprusside relaxations versus no effect for cyclosporine. The relaxant responses to nitroprusside in castrated rats were restored to sham-operated levels after testosterone replacement. Plasma urea and creatinine were not affected by castration but were significantly increased by cyclosporine. These findings suggest that testosterone exerts directionally opposite modulatory effects on endothelium-dependent and -independent renal relaxations. Further, the results demonstrate that testosterone depletion may contribute, at least partly, to the inhibitory effect of cyclosporine on renovascular endothelial function. These data are clinically important because endothelial dysfunction contributes to vascular abnormalities associating cyclosporine therapy.

Regional and endothelial differences in cyclosporine attenuation of adenosine receptor-mediated vasorelaxations.

Abstract: The present study investigated the acute effects of the immunosuppressant drug cyclosporine A on vasorelaxations evoked via activation of adenosine receptors in the phenylephrine-preconstricted rat perfused kidney and isolated aorta. The roles of endothelial relaxing factors in this interaction were also evaluated. The adenosine analogue 5′-N-ethylcarboxamidoadenosine (NECA; kidney, 6 × 10−9−1 × 10−7 mol; aorta, 1 × 10−9−1 × 10−5 M) elicited dose-dependent vasorelaxations. In the perfused kidney, NECA responses were similarly and significantly attenuated by NG-nitro-L-arginine methyl ester (L-NAME, nitric oxide synthase inhibitor) or tetraethylammonium (K+ channel blocker) versus no effect for diclophenac (cyclooxygenase inhibitor). NECA relaxations in the aorta were reduced by the three inhibitors; the reduction in the response evoked by the highest dose of NECA (1 × 10−5 M) amounted to 37.7 ± 2.0% (L-NAME), 19.8 ± 1.7% (tetraethylammonium), and 29.4 ± 1.1% (diclophenac). A combination of the three inhibitors almost abolished NECA relaxations in the two preparations. Cyclosporine (2 μM) reduced NECA relaxations in the two preparations. In the aorta, cyclosporine attenuation of NECA responses was significantly reduced after exposure to L-NAME or diclophenac but not tetraethyl-ammonium, suggesting selective involvement of nitric oxide and vasodilator prostanoids in the interaction. In contrast, the cyclosporine attenuation of NECA responses in the kidney was reduced by L-NAME or tetraethylammonium. L-arginine, a nitric oxide substrate, partially restored NECA relaxations in cyclosporine-treated preparations. These findings demonstrate that cyclosporine attenuates endothelium-dependent vasorelaxations elicited via activation of adenosine receptors and highlight the interesting possibility that the relative contribution of the endothelial relaxing factors to cyclosporine-NECA interaction is largely region dependent.

Comparative effects of sildenafil, phentolamine, yohimbine and L-arginine on the rabbit corpus cavernosum.

Penile erection involves relaxation of smooth muscle of corpus cavernosum and associated arterioles. Sildenafil, a highly selective inhibitor of phosphodiesterase type 5, is effective in the treatment of erectile dysfunction. The aim of this study is to investigate the effect of sildenafil on smooth muscle of the rabbit corpus cavernosum (RCC) and to compare its effect with those of phentolamine, yohimbine and l‐arginine. The effects of sildenafil, phentolamine, yohimbine and l‐arginine were studied on the response of the RCC to electrical field stimulation (EFS) as well as on the phenylephrine (PE, 3 × 10−6 m)‐induced tone. EFS caused transient, frequency‐dependent relaxation of the RCC that was inhibited by the nitric oxide synthase inhibitor NG‐nitro‐l‐arginine (3 × 10−5 m). Sildenafil (1 × 10−9–1 × 10−6 m) and phentolamine (1 × 10−9–1 × 10−6 m) enhanced the EFS‐induced relaxation in a concentration‐dependent manner with ED50 of 0.056 ± 0.004 and 0.572 ±0.035 μm at 8 Hz, respectively, yohimbine (3 × 10−7–3 × 10−5 m) and l‐arginine (3 × 10−6–3 × 10−4 m) did not show significant effects (ED50 at 8 Hz = 35.84 ±2.24 and 2.164 ± 0.174 μm, respectively). Sildenafil (1 × 10−9 and 1 × 10−8 m) potentiated the EFS‐induced relaxation caused by l‐arginine (3 × 10−5 m). Sildenafil, phentolamine, yohimbine and l‐arginine reduced the PE‐induced tone to different extents; the ED50 values were 0.81 ± 0.097, 0.49 ± 0.025 and 13.97 ± 1.10 μm, respectively. Maximum concentration of l‐arginine used failed to produce 50% relaxation (ED20 = 221.82 ± 15.71 μm). The muscle relaxant effects of different combinations of sildenafil and l‐arginine on PE‐induced tone did not differ significantly from the sum of the individual effects. The results demonstrate that sildenafil, when compared to other drugs used in penile erection dysfunction, shows the highest potency on the nitrergic transmission in the RCC. On the other hand, phentolamine was found to possess the highest potency in inducing relaxation of RCC proving that its action is independent on the stimulated neurogenic system. In addition, the combination of less effective doses of sildenafil and l‐arginine has a potential advantage on erectile functions. The importance of this combination remains to be solved clinically.

Effect of sildenafil on the isolated rat aortic rings

Sildenafil, a highly selective inhibitor of PDE 5, is effective in the treatment of erectile dysfunction. Penile erection involves relaxation of smooth muscle of corpus cavernosum and its associated arterioles. The objective of this study was to investigate the effect of sildenafil on nitric oxide/cyclic guanosine monophosphate (NO/cGMP)‐dependent relaxation of rat aortic rings. The contribution of sildenafil to the vasorelaxation of rat aortic rings was also investigated. Sildenafil produced significant potentiation of acetylcholine (ACh, 2 × 10−6 m)‐induced relaxation at concentration ≥1 × 10−8 m. Addition of sildenafil (1 × 10−7 m) to aortic rings failed to alter the effect of NG‐nitro‐l‐arginine (l‐NNA, 3 × 10−5 m) or methylene blue (MB, 3 × 10−5 m) on ACh response. Similarly, sildenafil (1 × 10−7 m) augmented significantly the vasorelaxation induced by sodium nitroprusside over the range of 1 × 10−9–1 × 10−8 m. When added to phenylephrine (3 × 10−6 m)‐precontracted rat aortic rings, sildenafil (1 × 10−9–1 × 10−4 m) induced concentration‐dependent relaxation reaching a maximum of 96.48 ± 1.44%. These relaxations were not significantly attenuated by previous incubation with l‐NNA (3 × 10−5 m) or MB (3 × 10−5 m). Denudation did not significantly affect the vasorelaxant effect of sildenafil. Sildenafil may act in the rat aortic rings through the amplification of NO/cGMP pathway. It may augment both basal endothelial NO function and exogenous NO‐dependent vasodilatation. However, sildenafil may act by a mechanism independent of NO/cGMP pathway and this mechanism contributes to its smooth muscle relaxant effect.

Activation of muscarinic receptors inhibits neurogenic nitric oxide in the corpus cavernosum.

The functional role of cholinergic transmission in erection is still far from being fully elucidated. This work aims to further elucidate the modulatory role of neostigmine on NO in the corpus cavernosum and to highlight whether cholinergic transmission in the penis modulates sildenafil action. The isolated rabbit corpus cavernosum and measurement of intracavernosal pressure in the anesthetized rat model were used. Neostigmine (0.02 mg/kg) reduced increase of intracavernosal pressure/mean arterial pressure (ICP/MAP) next to cavernous nerve stimulation. Higher doses (0.06 and 0.4 mg/kg) potentiated ICP/MAP rise and atropine (1.5 and 10 mg/kg) did the opposite. In vitro, neostigmine (10⁻⁵ and 10⁻⁴ M) potentiated neurogenic relaxations and this effect was significantly inhibited by hexamethonium (10⁻⁴ M) or N(ω)-propyl-L-arginine (3 × 10⁻⁵ M) and partially but significantly reduced in the presence of atropine. Lower dose neostigmine (10⁻⁷ M), inhibited electrically induced relaxation over the range of 1-4 Hz, atropine (10⁻⁶ M) almost abolished this inhibitory effect as well as N(G)-nitro-L-arginine (10⁻⁵ M). It was also significantly reduced by selective nNOS inhibitor N(ω)-propyl-L-arginine (3 × 10⁻⁵ M). Nicotine (10⁻⁴ M) significantly potentiated electrically induced relaxations amounting to 84.625 ± 8.06% at 1 Hz and potentiated the effect of sildenafil synergistically. Hexamthonium did the opposite. The potentiatory effect of sildenafil on neurogenic erection was significantly reduced by low dose neostigmine both in vitro and in vivo. This study provides evidence that muscarinic receptors may modulate NO synthesis in nitrergic nerves by inhibiting nNOS and high level of cholinergic stimulation may activate nicotinic receptors to promote erection probably by potentiating NO synthesis in nitrergic nerves.

Prazosin-induced blockade of extraneuronal uptake facilitates dopaminergic modulation of muscle twitches in rat vas deferens

Preliminary findings in our laboratory have shown that prazosin augmented the inhibitory effects of dopamine on the electrically‐evoked muscle twitches in rat vas deferens. In this study, we opted to investigate the underlying mechanism and whether a prazosin‐induced blockade of extraneuronal uptake process may be involved.

Neuronal Voltage Gated Potassium Channels May Modulate Nitric Oxide Synthesis in Corpus Cavernosum

Potassium channels (K+Ch) in corpus cavernosum play an important role in the regulation of erection. Nitric oxide (NO) acts through opening of K+Ch leading to hyperpolarization and relaxation. Aim : This study aims to update knowledge about the role of voltage-gated K+Ch (KV) channels in erectile machinery and investigate their role in the control of NO action &/or synthesis in the corpus cavernosum. Methods : Tension studies using isolated rabbit corpus cavernosum (CC) strips and rat anococcygeus muscle were conducted. Results are expressed as mean ± SEM. Results : Electric field stimulation (EFS, 2–16 Hz) evoked frequency-dependent relaxations of the PE (phenylephrine)-precontracted CC strips. At 2 Hz, EFS-induced relaxation amounted to 73.17 ± 2.55% in presence 4-AP (10−3 M) compared to 41.98 ± 1.45% as control. None of the other selective K+Ch blockers tested inhibited EFS-induced relaxation. 4-AP (10−3M) significantly attenuated ACh-induced relaxation of rabbit CC where dose-response curve was clearly shifted upward, and attenuated SNP- induced relaxation, for example, to 49.28 ± 4.52% compared to 65.53 ± 3.01% as control at 10−6 M SNP. The potentiatory effect of 4-AP on EFS was abolished or reversed in presence of NG-nitro-L-arginine (L-NNA, non-selective nitric oxide synthase inhibitor, 10−5M, and 2 × 10−4M). Same results were observed in rat anococcygeus muscle which is a part of the erectile machinery in rats. Conclusion : This study provides evidence for the presence of prejunctional voltage-gated K+Ch in CC, the blockade of which may increase the neuronal synthesis of NO.

Combined effect of sildenafil and guanethidine, propranolol or verapamil on erectile function in rats.

Objectives  This study aims to further elucidate the role of adrenergic transmission in erection and to highlight whether adrenergic transmission in the penis modulates sildenafils action.