Bibliothek

Notizen: 1. The aim of the present study was to examine the effects of long-term nitric oxide (NO) blockade on contractions of the rat ileum induced by muscarinic agonists.2. Male Wistar rats received the NO synthesis inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 20 mg/rat per day) in drinking water for 7, 15, 30 and 60 days. Concentration–responses curves to methacholine and carbachol were obtained and pEC50 values were calculated. Saturation binding assays were performed in membranes prepared from rat ileum after 60 days of l-NAME treatment and the dissociation constant (KD) and maximal number of binding sites (Bmax) were determined by Scatchard analysis.3. The NO synthase activity of the ileum was markedly reduced in all l-NAME-treated groups. At 60 days after l-NAME treatment, a significant increase in the potency of methacholine (fourfold) and carbachol (threefold) was observed. In binding studies, we found a significant increase in Bmax for [3H]-quinuclidinyl benzilate of approximately 57% in the l-NAME treated group without any significant change in KD values. The contractile response to methacholine was not modified by the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (3 µmol/L). No morphological alterations in the rat ileum were observed in l-NAME-treated rats.4. Our findings suggest that treatment with l-NAME for 60 days induces a marked increase in the potency of methacholine and carbachol, as well as an increase in receptor number in the rat ileum.

Notizen: 1. The aim of the present study was to evaluate the potency and maximal responses (Emax) to the adenosine receptor agonists N6-cyclopentyladenosine (CPA), N-ethylcarboxamidoadenosine (NECA) and N6-(3-iodobenzyl)-5′-N-methylcarbaxamidoadenosine (IB-MECA) in right atria from trained rats. We also investigated the interaction between the training bradycardia and the sensitivity of the chronotropic response mediated by adenosine receptor stimulation.2. Animals were submitted to run training for 60 min, 5 days a week, over a period of 8 weeks. Mean blood pressure and heart rate were measured in conscious animals. Right atria were isolated and concentration–response curves to CPA, NECA and IB-MECA were obtained.3. A reduction in heart rate was found in trained rats, indicating that the training programme was successful in inducing physical conditioning. The three adenosine receptor agonists induced a concentration-dependent negative chronotropic response. The rank order of potency and Emax for the three adenosine receptor agonists was CPA 〉 NECA 〉 IB-MECA.4. Dynamic exercise for 8 weeks did not alter the Emax for CPA, NECA and IB-MECA. Similarly, the potencies of CPA and NECA were not affected by run training, whereas the potency of IB-MECA was reduced (6.10 ± 0.09 vs 5.66 ± 0.10 for sedentary and trained groups, respectively).5. In conclusion, run training for 8 weeks induced a desensitization of the chronotropic response to IB-MECA without changing the potency of CPA and NECA. These findings exclude the participation of adenosine receptors in the training bradycardia.

Notizen: 1. To study the effect of acute nitric oxide (NO) inhibition on the rat heart both in vitro and in vivo, male Wistar rats received a single bolus injection of saline, Nω-nitro-L-arginine methyl ester (L-NAME; 0.5,1.5,5.0,15.0 and 45.0 mg/kg) and D-NAME (45.0mg/kg).2. Animals were killed 72 h after the bolus injection of L-NAME and the hearts were removed and studied under light microscopy. In other groups of animals, saline, L-NAME and D-NAME were administered as above and the mean arterial blood pressure (MABP/carotid) was recorded. Furthermore, L-NAME was also administered in the drinking water (20 mg/kg per day) for 72 h and animals were then killed and their hearts evaluated as described above. Hearts of control animals were perfused in vitro and coronary flow was measured following saline, L-NAME (45 μg/heart) and D-NAME (45 μ/heart).3. Areas of necrosis were observed in the left ventricle of animals that had received L-NAME at 5.0, 15.0 and 45.0 mg/kg. Also, only doses higher than 1.5 mg/kg caused an important increase in MABP. The frequency and extent of the lesions paralleled the dose of L-NAME administered and no lesions were observed in D-NAME- and saline-treated animals.4. The oral administration of L-NAME also caused myocardial lesions similar to those described above, but the frequency and extent of these lesions were more discrete compared with those observed following 5.0 mg/kg, i.v., L-NAME.5. Bolus injection of L-NAME into control rat hearts in vitro resulted in a small and transient fall in coronary flow (17.2 ± 1.4 and 12.2 ± 1.2 mL/min before and after L-NAME administration, respectively) within 30 s and this was followed 4.5 min later by a further (11.5 ± 1.6 mL/min) decrease. The administration of D-NAME to control hearts caused no change in coronary flow.6. In conclusion, the acute inhibition of NO biosynthesis by L-NAME causes myocardial necrosis. Both high levels of MABP and a small but significant reduction in coronary flow (associated or not) can be responsible for the lesions we found.

Notizen: 1. The compound BAY 41-2272 (5-cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-pyrimidin-4-ylamine) has been described as a potent, nitric oxide (NO)-independent, stimulator of soluble guanylate cyclase. In the present study, the mechanisms underlying the relaxant effect of BAY 41-2272 in endothelium-intact and -denuded precontracted rabbit aortic rings were investigated.2. Male New Zealand white rabbits were anaesthetized with pentobarbital sodium. Aortic rings were transferred to 10 mL organ baths containing oxygenated and warmed Krebs' solution. Tissues were connected to force-displacement transducers and changes in isometric force were recorded. Aortic rings were precontracted submaximally with phenylephrine (1 µmol/L).3. The addition of BAY 41-2272 (0.01–10 µmol/L) to the organ bath produced concentration-dependent relaxations of the aortic rings with a higher potency in endothelium-intact (pEC50 6.59 ± 0.05) compared with endothelium-denuded (pEC50 6.19 ± 0.04; P 〈 0.05) preparations. No differences in maximal responses were observed in either preparation. The NO synthesis inhibitor NG-nitro-l-arginine methyl ester (100 µmol/L) produced a 2.1-fold rightward shift in endothelium-intact (P 〈 0.01) rings, but had no effect in endothelium-denuded rings. The soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 1 µmol/L) caused significant rightward shifts of the concentration–response curves to BAY 41-2272 of 4.9- and 2.6-fold in endothelium-intact and -denuded rings, respectively. The phosphodiesterase-5 inhibitor sildenafil (0.1 µmol/L) significantly potentiated the relaxant effects of BAY 41-2272 in both endothelium-intact and -denuded rings.4. At 1 µmol/L, BAY 41-2272 significantly elevated the aortic cGMP content above basal levels in both endothelium-intact and -denuded rings. Furthermore, ODQ reduced BAY 41-2272-elicited increases in cGMP content by 17 and 90% in endothelium-intact and -denuded rings, respectively (P 〈 0.01).5. In conclusion, BAY 41-2272 potently relaxes endothelium-intact and -denuded rabbit aortic rings. The basal release of endothelium-derived NO enhances BAY 41-2272-induced relaxations, suggesting a synergistic effect of BAY 41-2272 and NO on soluble guanylate cyclase. In addition, the endothelium-independent relaxation involves both GMP-dependent and -independent mechanisms.