Tahia T. Daabees

Short-term aortic barodenervation diminishes α1-adrenoceptor reactivity in rat aortic smooth muscle

Our previous studies have shown that aortic baroreceptor denervation elicits acute increases in blood pressure and significant elevations of sympathetic activity and peripheral vascular resistance. This study investigated the short-term (3 and 48 h) effect of aortic barodenervation and associated sympathetic hyperactivity on the functional activity of alpha 1-adrenoceptors in rat aortic smooth muscle. Compared with sham operation, aortic barodenervation caused acute rises in blood pressure and heart rate and reductions in baroreflex sensitivity. Blood pressure and heart rate remained elevated when measured in conscious aortic barodenervated rats 3 h after surgery but subsided to sham-operated levels at 48 h; the baroreflex sensitivity, however, remained attenuated. Hexamethonium (0.5-4 mg/kg, i.v.) elicited significantly (P < 0.05) greater depressor responses in conscious aortic barodenervated rats than in sham-operated rats at both 3 and 48 h, suggesting a higher sympathetic activity in denervated rats. Exposure of aortic rings from aortic barodenervated and sham-operated rats to cumulative addition of phenylephrine (alpha 1-adrenoceptor agonist, 3 x 10(-8)-1 x 10(-4) M) resulted in concentration related contractile responses that were similar in the two groups of rats at 3 h in contrast to significantly (P < 0.05) smaller contractions in rings from denervated rats at 48 h. The maximum contraction developed (Emax) at 48 h showed approximately 50% reduction in rings from aortic barodenervated compared with sham-operated rats (239 +/- 16 vs. 558 +/- 15 mg tension/mg tissue). The pA2 value for prazosin (alpha 1-adrenoceptor antagonist) was not altered by aortic barodenervation at 3 h but showed significant (P < 0.05) increases, compared with sham-operated values, at 48 h. It is concluded that short-term aortic barodenervation results in an elevation of sympathetic activity that coincides with reduced responsiveness of aortic smooth muscle to alpha 1-adrenoceptor activation. The aortic barodenervation-induced alpha 1-adrenoceptor desensitization is not a result of decreased receptor affinity but may involve an alteration of receptor density or in the post-receptor activation events.

Enhanced endothelial nitric oxide activity contributes to the reduced responsiveness of vascular α1-adrenoceptors following aortic barodenervation

We have recently shown that short-term aortic barodenervation diminishes constrictor responses to activation of alpha1-adrenoceptors in rat aortic smooth muscle. This study investigated the potential role of vascular endothelium and its derived vasoactive substances, nitric oxide and prostaglandins, in the reduced alpha1-adrenoceptor responsiveness after aortic barodenervation. Exposure of isolated aortic rings from aortic barodenervated and sham-operated rats 48 h after surgery to cumulative addition of phenylephrine (alpha1-adrenoceptor agonist, 3 x 10(-8) - 1 X 10(-4) M) resulted in concentration-related contractions that were significantly (P < 0.05) smaller in rings of denervated rats. Removal of the endothelium increased phenylephrine-mediated contractions in rings obtained from aortic barodenervated rats to near sham-operated levels as demonstrated by the similar contractile responses and slopes of the regression lines of the concentration-response curves. Pretreatment with indomethacin (cyclooxygenase inhibitor, 1 x 10[-5] M) had no effect on contractile responses to phenylephrine in rings from both groups of rats. In contrast, N(G)-nitro-L-arginine (nitric oxide synthase inhibitor, 3 x 10[-5] M) elevated basal vascular tone and significantly (P < 0.05) increased alpha1-adrenoceptor responsiveness, effects that were more evident in rings from denervated compared with sham-operated rats. N(G)-nitro-L-arginine produced significantly (P < 0.05) greater increases in the slopes of the regression lines (136.1 +/- 22% vs. 73.0 +/- 8.6% mg tension/mg tissue/log molar concentration) and maximum contractile response (Emax) to phenylephrine (161.2 +/- 8.2% vs. 76.7 +/- 6.1%) in rings from denervated compared with sham-operated rats suggesting an enhanced nitric oxide activity in aortas of denervated rats. This notion is further supported by the finding that cumulative i.v. administration of N(G)-nitro-L-arginine (1, 2, 4 and 8 mg/kg) elicited significantly (P < 0.05) greater pressor responses in conscious barodenervated compared with sham-operated rats. These results suggest that the endothelium plays a major role in the reduced constrictor responses to alpha1-adrenoceptor activation that occurs shortly after aortic barodenervation. This effect of the endothelium appears to involve, at least in part, enhancement of endothelial nitric oxide activity.

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.

Evaluation of l-arginine on kidney function and vascular reactivity following ischemic injury in rats: Protective effects and potential interactions

BACKGROUND There is an interaction between many cell types involved in the pathophysiology of ischemic acute renal failure. Nitric oxide (NO) precursors, especially l-arginine, may have protective effects on tissue ischemia/reperfusion injury (IRI); however, their molecular mechanisms are unclear. In the present study, the interaction between l-arginine, cyclo-oxygenase (COX)-2 and reactive oxygen species (ROS) in the pathogenesis of ischemic acute renal failure was investigated. METHODS Ischemia/reperfusion injury model in rats was used and various biochemical parameters examined. The rat isolated aortic rings served as model for hypoxia/reoxygenation where endothelium dependent and independent relaxations were exerted. RESULTS Pre-treatment of rats subjected to IRI with l-arginine (125mg/kg) significantly reduced kidney MDA levels, elevated kidney SOD activity, GSH level and total NO levels at 24 and 48h after reperfusion. Kidney COX-2 level was only different in the l-arginine-treated group 48h after reperfusion compared to the IRI group. Pre-treatment with l-arginine (10(-2)M) alone or in combination with celecoxib significantly potentiated the acetylcholine (Ach)-induced relaxations in control and hypoxic rings. The effect of the combination was synergistic only in hypoxic rings. Addition of ascorbic acid to the celecoxib-arginine combination did not produce further potentiation. Sodium nitroprusside-induced relaxations in control and hypoxic rings were potentiated by l-arginine or celecoxib-arginine combination but not by ascorbic acid. CONCLUSIONS The protective effect of l-arginine may result from the interaction between NO and ROS and increased NO bioavailability. The protective effects of combined celecoxib and l-arginine against IRI could be attributed to their antioxidant activity which exceeded that of ascorbic acid.

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.

Injectable and oral contraceptive steroids in relation to some neurotransmitters in the rat brain

Abstract The influence of both injectable and oral contraceptive steroids on brain acetylcholine and serotonin was investigated. The effects of such steroids on some brain amino adds such as tryptophan, γ-aminobutyric acid (GABA), and glutamate (Glu) levels were determined. The steroids used in the present study were the long-acting injectable medroxyprogesterone acetate (MPA) and the orally active steroids ethinyloestradiol (EE) and norethisterone acetate (NEA). Acetylcholine content of the rat brain was not altered significantly following treatment with any of the steroids used. In contrast, all types of treatment caused a significant rise in brain serotonin levels. Tryptophan concentration was similarly increased following MPA injection, administration of NEA alone, or its combination with EE. No changes were noted, however, in brain tryptophan content following the administration of EE alone. MPA injection was found to be devoid of any effect on the balance between Glu, the excitatory amino acid, and GABA, the inhibitory one. On the other hand, all the orally active steroid regimens used significantly increased the GABA content of the rat brain. Only NEA pretreatment resulted in a significant decrease in brain Glu content. Changes in these central chemical transmitters and their possible relations to both antifertility and to the untoward effects of such contraceptive measures are discussed.

Effect of oral contraceptives (Lyndiol®) on rat brain gamma aminobutyric acid system

Abstract (1) Virgin female albino rats were treated with the estrogenic substance mestranol, the progestogenic substance lynestrenol or with a combination of these two compounds. These two drugs are the components of the oral contraceptive pill Lyndiol®. (2) Mestranol pretreatment caused a significant increase in brain GABA level, lynestrenol pretreatment caused a significant decrease in brain glutamic acid level and pretreatment with a combination of the two drugs caused a significant rise in brain GABA content. Other changes were statistically insignificant. All three types of treatment, however, caused an identical and significant decrease in the glutamic acid: GABA ratio in rat brains. (3) The changes produced by oral contraceptive steroids on two brain enzymes, namely glutamate decarboxylase and aminobutyrate aminotransferase could not satisfactorily justify the assumption that the brain GABA concentration is controlled solely by the activities of such enzymes.

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.

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.