Possible Ameliorative Effect of Ivabradine on the Autonomic and Left Ventricular Dysfunction Induced by Doxorubicin in Male Rats.

Heart failure is a common adverse effect associated with doxorubicin treatment. The aim of this study is to investigate the effect of ivabradine treatment on doxorubicin-induced heart failure in conscious rats. Rats were treated with doxorubicin (2.5 mg/kg/d) or ivabradine (10 mg/kg/d) alone or along with doxorubicin injections. Changes in heart rate variability (HRV), baroreflex sensitivity, left ventricular (LV) function, serum cardiac troponin T, and cardiac histological features were taken as index parameters for the development of heart failure. Ivabradine significantly reduced the elevated heart rate; normalized the parameters of LV function, dP/dtmax and the relaxation time constant (Tau); reduced the elevated serum level of cardiac troponin T; and minimized the cardiac structural abnormalities in doxorubicin-treated rats. Moreover, ivabradine significantly increased the diminished time domain parameters of HRV, SDNN and rMSSD, and decreased the elevated low frequency power and the low frequency/high frequency while having no effect on the reduced high frequency power. Consistently, ivabradine significantly lowered the elevated baroreflex sensitivity measured by sodium nitroprusside. In conclusion, ivabradine ameliorated the LV dysfunction induced by doxorubicin. Moreover, ivabradine increased the overall HRV and restored the autonomic balance by reducing the sympathetic over activation. Therefore, ivabradine may have a possible therapeutic potential against doxorubicin-induced heart failure.

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 N G -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 sildenafil's action. METHODS: Measurement of intracavernosal pressure in the anesthetized rat model. KEY FINDINGS: Guanethidine (3 and 6 mg/kg) potentiated intracavernosal pressure/mean arterial pressure (ICP/MAP) rises in response to cavernous nerve stimulation by 4.375 ± 0.425 and 18.375 ± 1.085% respectively. Propranolol did the opposite. In presence of guanethidine, sildenafil (0.01, 0.1 and 1 mg/kg) potentiated ICP/MAP responses by 81.571 ± 4.918%, 147.83 ± 10.864% and 279.285 ± 23.053% at 1 Hz compared to 22.277 ± 2.139%, 123.571 ± 8.443% and 186.25 ± 13.542% respectively in the absence of guanethidine. Propranolol inhibited the effect sildenafil at all frequencies of stimulation. Verapamil exhibited a pro-erectile action and potentiated the effect of sildenafil (0.01, 0.1 and 1 mg/kg) on erectile responses corresponding to 85.25 ± 6.716%, 146 ± 11.288% and 221.571 ± 19.032% respectively compared to 26.011 ± 1.911%, 87.142 ± 8.73% and 182.2 ± 16.921% in its absence. CONCLUSIONS: This study provides functional evidence that inhibition of sympathetic tone peripherally results in enhancement of erectile function. ß-adrenergic receptors seem to play an important role in erection. The combination of sildenafil and guanethidine or verapamil could have a potential advantage on erectile function but propranolol may mask the effect of sildenafil on erectile function.

Peripheral modulation of dopaminergic receptors affects erectile responses in rats.

Clinical observations have suggested that dopaminergic mechanisms are involved in the regulation of male sexual responses, including erection. Apomorphine was initially reported to exert its erectogenic effects by modifying central dopaminergic activity. This study aimed primarily at evaluating and investigating the effect of apomorphine on erectile functions in rats and its potential effects on the cardiovascular system, as well as the possible role of dopaminergic stimulation in the peripheral control of erection. Measurement of intracavernosal pressure changes elicited by electrical stimulation of the cavernosal nerve in anaesthetized rats and mating tests were used. SCH23390, the D1 receptor antagonist, attenuated penile response to electrical stimulation. Intravenous administration of apomorphine in low dose (100 microg/kg), but not in high dose, significantly potentiated erectile responses to electrical stimulation. Intracavernosally injected apomorphine (50 microg/kg) significantly potentiated the filling rate of the corpora cavernosa 5 min. after injection, and did not induce erection in absence of electrical stimulation. In addition, apomorphine amplified the male sexual and copulatory behaviour by reducing ejaculation, mount and intromission latencies, and significantly increasing the number of ejaculations per session. However, apomorphine produced rapid and long-lasting hypotension and potentiated the hypotension and tachycardia associated with nerve-stimulated penile erection. Our results suggest that D1-dopaminergic receptors may be functionally involved in the peripheral mediation of penile erection. Apomorphine may amplify sexual and copulatory behaviour and may also, by a complementary role, amplify neurogenically mediated erections by acting in the periphery.

Inhibition of nitric oxide-guanylate cyclase-dependent and -independent signaling contributes to impairment of beta-adrenergic vasorelaxations by cyclosporine.

This study investigated the role of endothelium- and smooth muscle-dependent mechanisms in the interaction of cyclosporine (CyA), an immunosuppressant drug, with beta-adrenoceptor (isoprenaline)-mediated relaxations in isolated rat aortas precontracted with phenylephrine. CyA effects were assessed in the absence and presence of NG-nitro-L-arginine methyl ester (L-NAME, nitric oxide synthase inhibitor), methylene blue (guanylate cyclase inhibitor), or propranolol (beta-adrenoceptor antagonist). In aortas with intact endothelium (E+), pretreatment with L-NAME or methylene blue significantly reduced isoprenaline (1 x 10(-9) to 1 x 10(-7)M) relaxations in contrast to no effect for tetraethylammonium (K+ channel blocker), or diclophenac (cyclooxygenase inhibitor), suggesting a major role for the nitric oxide-guanylate cyclase (NO-GC) pathway, but not endothelial hyperpolarizing factor or vasodilator prostanoids, in isoprenaline responses. Isoprenaline relaxations were still evident, though significantly attenuated, in endothelium-denuded aortas (E-) and were resistant to L-NAME or methylene blue. Acute exposure to CyA (2 microM) caused propranolol-sensitive reductions in isoprenaline responses in E+ and E- aortas. The CyA-induced attenuation of isoprenaline responses in E+ aortas largely disappeared in L-NAME-treated aortas and after supplementation with L-arginine, the substrate of nitric oxide. CyA also reduced the endothelium-independent, GC-dependent aortic relaxations evoked by sodium nitroprusside, an effect that was virtually abolished by methylene blue. We conclude that: (i) endothelial and smooth muscle mechanisms contribute to aortic beta-adrenoceptor relaxations and both components are negatively influenced by CyA, and (ii) NO-GC signaling plays an integral role in the vascular CyA-beta-adrenoceptor interaction. The clinical relevance of the present study is warranted given the established role of impaired vascular function in CyA toxicity.

The alpha1-adrenergic receptor not the DA(1)-dopaminergic receptor mediates cyclosporine-SKF38393 renovascular interaction.

In this study, we investigated the effect of acute exposure to cyclosporine A (CyA) on renal vasodilations evoked by the DA(1) dopaminergic agonist SKF38393 and whether dopamine DA(1) receptors are directly involved in the interaction. Changes evoked by CyA in SKF38393 vasodilations were evaluated in phenylephrine-preconstricted isolated perfused rat kidneys in the absence and presence of SCH23390, a DA(1) receptor antagonist. SKF38393 (3 x 10(-8) to 3 x 10(-6) mol) produced dose-dependent reductions in the renal perfusion pressure that were significantly attenuated in tissues pretreated with SCH23390 or CyA. Unlike SKF38393, the vasodilatory action of sodium nitroprusside, a nitrovasodilator, was not altered by CyA. The attenuating effect of CyA on SKF38393 vasodilations was preserved in preparations pretreated with SCH23390, suggesting that sites other than DA(1) receptors may be involved in CyA-SKF38393 interaction. The study was then extended to investigate the possible involvement of renal alpha1-adrenoceptors in the interaction. Blockade of alpha(1)-adrenoceptors by prazosin (30 nmol/L) significantly reduced the vasodilatory effect of SKF38393 and virtually abolished the CyA-induced attenuation of SKF38393 responses. Further, CyA failed to alter SKF38393 vasodilations when the renal tone was raised with prostaglandin F2alpha (PGF2alpha), a vasoconstrictor whose effect is independent of alpha(1)-adenoceptors. Together, these findings support earlier reports that both DA(1) and alpha(1)-receptors mediate the renal vasodilatory action of SKF38393 and suggest that CyA interacts selectively with the alpha(1)-receptor component to compromise SKF38393 responses.

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

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 x 10(-9)-1 x 10(-7) mol; aorta, 1 x 10(-9)-1 x 10(-5) M) elicited dose-dependent vasorelaxations. In the perfused kidney, NECA responses were similarly and significantly attenuated by N-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 x 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 microM) 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.

Relative roles of endothelial relaxing factors in cyclosporine-induced impairment of cholinergic and beta-adrenergic renal vasodilations.

Vascular toxicity is a major adverse effect for the immunosuppressant drug cyclosporine A. The present study sought to characterize the relative roles of the endothelium-derived relaxing factors (nitric oxide, endothelium-derived hyperpolarizing factor [EDHF], and prostaglandins) in the cyclosporine-induced impairment of renovascular responsiveness to acetylcholine receptor or beta-adrenoceptor activation. Changes evoked by cyclosporine in the responses to either vasorelaxant were evaluated in phenylephrine-preconstricted isolated perfused rat kidneys in the absence and presence of N(G)-nitro-l-arginine methyl ester (l-NAME, nitric oxide synthase inhibitor), tetraethylammonium (K(+) channel blocker), or diclophenac (cyclooxygenase inhibitor). Acetylcholine (0.03-2 nmol) vasodilations were significantly inhibited by prior treatment with l-NAME, tetraethylammonium, or diclophenac, suggesting a role for nitric oxide, EDHF, and prostaglandins in acetylcholine vasodilations. Isoprenaline (0.125-4 micromol) vasodilations were inhibited by l-NAME and tetraethylammonium versus no effect for diclophenac. Cyclosporine (1-4 microM) produced a concentration-related inhibition of vasodilations relaxations produced by either vasodilator. Cyclosporine-induced inhibition of acetylcholine vasodilations was attenuated in tissues pretreated with l-NAME or tetraethylammonium but not diclophenac, implicating nitric oxide and EDHF in cyclosporine-acetylcholine interaction. On the other hand, the inhibition of isoprenaline vasodilations by cyclosporine was virtually abolished by l-NAME. In cyclosporine-treated kidneys, exposure to l-arginine, the substrate of nitric oxide synthesis, fully restored isoprenaline vasodilations to control levels and significantly increased acetylcholine vasodilations. It is concluded that the identity and relative contributions of endothelial factors to renal vasodilatory responses as well as to the inhibition of these responses by cyclosporine largely depend on the vasodilator stimulus.

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.

Cyclosporine attenuates the autonomic modulation of reflex chronotropic responses in conscious rats.

Cyclosporine A (CyA), an immunosuppressant drug, has been shown to attenuate the baroreflex control of heart rate (HR). This study investigated whether or not the CyA-induced baroreflex dysfunction is due to alterations in the autonomic (sympathetic and parasympathetic) control of the heart. We evaluated the effect of muscarinic or beta-adrenergic blockade by atropine and propranolol, respectively, on reflex HR responses in conscious rats treated with CyA (20 mg x kg(-1) x day(-1) dissolved in sesame oil) for 11-13 days or the vehicle. Baroreflex curves relating changes in HR to increases or decreases in blood pressure (BP) evoked by phenylephrine (PE) and sodium nitroprusside (NP), respectively, were constructed and the slopes of the curves were taken as a measure of baroreflex sensitivity (BRS(PE) and BRS(NP)). Intravenous administration of PE and NP produced dose-related increases and decreases in BP, respectively, that were associated with reciprocal changes in HR. CyA caused significant (P < 0.05) reductions in reflex HR responses as indicated by the smaller BRS(PE) (-0.97 +/- 0.07 versus -1.47 +/- 0.10 beats x min(-1) x mmHg(-1) (1 mmHg = 133.322 Pa)) and BRS(NP) (-2.49 +/- 0.29 versus -5.23 +/- 0.42 beats x min(-1) x mmHg(-1)) in CyA-treated versus control rats. Vagal withdrawal evoked by muscarinic blockade elicited significantly lesser attenuation of BRS(PE) in CyA compared with control rats (40.2 +/- 8.0 versus 57.7 +/- 4.4%) and abolished the BRS(PE) difference between the two groups, suggesting that CyA reduces vagal activity. CyA also appears to impair cardiac sympathetic control because blockade of beta-adrenergic receptors by propranolol was less effective in reducing reflex tachycardic responses in CyA compared with control rats (41.6 +/- 4.2 versus 59.5 +/- 4.5%). These findings confirm earlier reports that CyA attenuates the baroreceptor control of HR. More importantly, the study provides the first pharmacological evidence that CyA attenuates reflex chronotropic responses via impairment of the autonomic modulation of the baroreceptor neural pathways.

Cyclosporine adversely affects baroreflexes via inhibition of testosterone modulation of cardiac vagal control.

Previous studies have shown that the immunosuppressant drug cyclosporine A attenuates arterial baroreceptor function. This study investigated whether the modulatory effect of cyclosporine on baroreceptor function involves inhibition of the baroreflex-facilitatory effect of testosterone. The role of cardiac autonomic control in cyclosporine-testosterone baroreflex interaction was also investigated. Baroreflex curves relating bradycardic responses to increments in blood pressure evoked by phenylephrine were constructed in conscious, sham-operated, castrated rats and in testosterone-replaced castrated (CAS + T) rats in the absence and presence of cyclosporine. The slopes of the curves were taken as an index of the baroreflex sensitivity (BRS). Short-term (11-13 days) cyclosporine treatment or castration reduced plasma testosterone levels and caused similar attenuation of the reflex bradycardia, as indicated by the significantly smaller BRS compared with sham-operated values (-0.97 +/- 0.07, -0.86 +/- 0.06, and -1.47 +/- 0.10 beats/min/mm Hg, respectively). The notion that androgens facilitate baroreflexes is further confirmed by the observation that testosterone replacement of castrated rats restored plasma testosterone and BRS to sham-operated levels. Cyclosporine had no effect on BRS in castrated rats but caused a significant reduction in CAS + T rats. Muscarinic blockade by atropine caused approximately 60% reduction in the BRS in sham-operated rats, an effect that was significantly and similarly diminished by castration, cyclosporine, or their combination. beta-Adrenergic blockade by propranolol caused no significant changes in BRS. These findings suggest that cyclosporine attenuates baroreflex responsiveness via, at least partly, inhibition of the testosterone-induced facilitation of cardiomotor vagal control.