Enhancement of arterial relaxation by long-term atenolol treatment in spontaneously hypertensive rats

Vasorelaxing effects of propranolol in rat aorta and mesenteric artery: a role for nitric oxide and calcium entry blockade

1. Propranolol has been prescribed successfully to patients with cardiovascular diseases, but the exact mechanisms by which it reduces peripheral vascular resistance have been poorly investigated. 2. The present study was designed to investigate the relaxing effects of propranolol in the rat isolated aorta and mesenteric artery, focusing on the contribution of the nitric oxide (NO)-cGMP pathway and calcium entry blockade. Relaxation responses to propranolol were obtained in precontracted rat aortic and mesenteric artery rings. 3. DL-Propranolol (10-100 micromol/L) produced concentration-dependent relaxations in the aorta and mesenteric artery rings with intact endothelium. The isomers D- and L-propranolol produced relaxation responses that were equipotent to the racemic mixture. 4. Metoprolol (10-100 micromol/L) produced slight relaxations, whereas atenolol (10-100 micromol/L) had no relaxant activity. 5. The NO inhibitor N(G)-nitro-L-arginine methyl ester (100 micromol/L) and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (1 micromol/L), as well as removal of the endothelium, significantly reduced the relaxation responses induced by the lower concentrations of propranolol without affecting maximal responses. In addition, DL-propranolol markedly increased cGMP levels in endothelium-intact preparations. 6. In Ca(2+)-free Krebs' solution, DL-propranolol (10-100 micromol/L) caused marked rightward shift in the concentration-response curves to CaCl(2), with a decrease of maximal responses in tissues with either intact or denuded endothelium. Nifedipine (1 micromol/L) in combination with DL-propranolol virtually abolished the CaCl(2)-induced contractile responses. 7. The relaxation responses induced by DL-propranolol were significantly reduced in aortic and mesenteric rings precontracted with phorbol-12,13-dibutyrate (1 micromol/L). 8. In conclusion, DL-propranolol relaxes arterial smooth muscle by mechanisms involving activation of the NO-cGMP pathway and calcium influx blockade, independent of beta-adrenoceptor blockade.

Long-term treatment with nebivolol improves arterial reactivity and reduces ventricular hypertrophy in spontaneously hypertensive rats

The aim of this study was to assess the effects of long-term nebivolol therapy on high blood pressure, impaired endothelial function in aorta, and damage observed in heart and conductance arteries in spontaneously hypertensive rats (SHR). For this purpose, SHR were treated for 9 weeks with nebivolol (8 mg/kg per day). Untreated SHR and Wistar Kyoto rats were used as hypertensive and normotensive controls, respectively. The left ventricle/body weight ratio was used as an index of cardiac hypertrophy, and to evaluate vascular function, responses induced by potassium chloride, noradrenaline, acetylcholine, and sodium nitroprusside were tested on aortic rings. Aortic morphometry and fibrosis were determined in parallel by a quantitative technique. Systolic blood pressure, measured by the tail-cuff method, was lower in treated SHR than in the untreated group (194 +/- 3 versus 150 +/- 4 mm Hg). The cardiac hypertrophy index was significantly reduced by the treatment. In aortic rings, treatment with nebivolol significantly reduced the maximal response to both KCl and NA in SHR. In vessels precontracted with phenylephrine relaxant, activity due to acetylcholine was higher in normotensive rats than in SHR and the treatment significantly improved this response. The effect of sodium nitroprusside on aortic rings was similar in all groups. Medial thickness and collagen content were significantly reduced in comparison with SHR. In conclusion, the chronic antihypertensive effect of nebivolol in SHR was accompanied by an improvement in vascular structure and function and in the cardiac hypertrophy index.

Losartan and atenolol on hypertension induced by adenosine receptor blockade

1. The prolonged infusion of 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, induces hypertension, an increase in plasma renin activity and morphological cardiovascular changes. 2. The aim of this work was to evaluate the effects of losartan, a selective AT1 receptor antagonist, and atenolol, a beta-adrenoceptor antagonist, on DPSPX-induced hypertension. 3. Male Wistar rats (250-300 g, n = 4-6) were treated for 1 or 4 weeks with: saline i.p.; DPSPX (90 microg kg(-1) h(-1)) i.p.; losartan (15 mg kg(-1) day(-1)) p.o.; atenolol (25 mg kg(-1) day(-1)) p.o.; DPSPX (90 microg kg(-1) h(-1)) i.p. + losartan (15 mg kg(-1) day(-1)) p.o.; DPSPX (90 microg kg(-1) h(-1)) i.p. + atenolol (25 mg kg(-1) day(-1)) p.o. Blood pressure was measured by the 'tail-cuff' method in conscious animals. Fragments of the mesenteric and tail arteries were processed for morphological study and the mean diameter of the vascular smooth muscle cells was determined. 4. DPSPX increased blood pressure. Losartan and atenolol prevented this rise but had no effect on blood pressure of control rats. DPSPX-treated groups showed hypertrophy of the vascular smooth muscle cells and proliferation of subintimal cells. Losartan but not atenolol prevented these changes. Losartan had no effect on the vascular morphology of control rats, while treatment with atenolol for 4 weeks induced hypertrophy of the vascular smooth muscle cells. 5. Both losartan and atenolol counteract the development of DPSPX-induced hypertension but only losartan prevents the alterations in vascular morphology.

Attenuation of hypertension, cardiomyocyte hypertrophy, and myocardial fibrosis by beta-adrenoceptor blockers in rats under long-term blockade of nitric oxide synthesis

The effects of propranolol and atenolol were investigated on arterial hypertension, cardiomyocyte hypertrophy, and ventricular ischaemic lesions induced by an 8-week treatment with the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 20 mg/rat per day) in Wistar rats. Propranolol and atenolol (30 mg/rat per day each) were given in the drinking water concomitantly to L-NAME. Treatment with L-NAME induced marked arterial hypertension and cardiomyocyte hypertrophy, both of which were significantly reduced by propranolol and atenolol. A marked repairing fibrosis was also observed in L-NAME-treated rats and this was significantly attenuated in animals receiving the beta-blockers. In L-NAME group, 33% mortality was observed, whereas all the animals from the other groups survived. Our study demonstrates that propranolol and atenolol reduce arterial hypertension, cardiomyocyte hypertrophy and myocardial fibrosis induced by L-NAME, suggesting that beta-blockers are of beneficial value in treatment of vascular and cardiac alterations caused by chronic nitric oxide deficiency.

Effect of celiprolol therapy on arterial dilatation in experimental hypertension

1. It has recently been suggested that therapy with beta-adrenoceptor blockers reduces peripheral arterial resistance via enhanced vascular dilatation. Therefore, we studied the effects of celiprolol, which is a specific beta 1-antagonist that has a weak beta 2-agonist action, on arterial tone in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. 2. Two doses of celiprolol (5 and 50 mg kg-1 day-1) were administered to the SHR, while the WKY rats received only the higher dose of the drug. During the 12-week treatment period the higher dose attenuated the increase in blood pressure by approximately 20 mmHg in SHR, whereas the lower dose was without significant antihypertensive effect. Celiprolol therapy did not affect blood pressure in the normotensive WKY rats. 3. Responses of mesenteric arterial rings in vitro were examined at the end of the study. Interestingly, endothelium-mediated relaxations of noradrenaline (NA)-precontracted rings to acetylcholine (ACh) in the absence and presence of the cyclo-oxygenase inhibitor, diclofenac, were equally enhanced in both celiprolol-treated SHR groups. The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) practically abolished the relaxations to ACh in all SHR irrespective of whether they had received celiprolol, whereas in WKY rats L-NAME only attenuated the responses to ACh. However, no differences were found between the SHR groups in relaxations to ACh when hyperpolarization of smooth muscle was prevented by precontractions induced by 50 mM KCl. Vasorelaxation of NA-precontracted rings to the exogenous nitric oxide donor, nitroprusside, was also moderately augmented in both celiprolol-treated SHR groups, while the relaxation to beta-adrenoceptor agonist, isoprenaline, remained equally impaired in all SHR whether or not they had received celiprolol. No differences were observed between the two WKY groups in the responses to ACh, nitroprusside or isoprenaline. 4. Contractile sensitivity of mesenteric arterial rings to the receptor-mediated agonists, NA and 5-hydroxytryptamine, was comparable in all study groups. 5. In conclusion, SHR treatment with either the low or the higher dose of celiprolol was accompanied by enhancement of both endothelium-dependent and endothelium-independent nitric oxide-mediated arterial relaxation, possibly via a hyperpolarization mechanism. Interestingly, this effect appeared to be independent of the reduction in blood pressure.

Antihypertensive therapy and arterial function in experimental hypertension

1. Alterations in the function of the endothelium and arterial smooth muscle may be important in the establishment of hypertension. Thus, the possible favorable influences of blood pressure-lowering agents on vascular responsiveness may be important in the chronic antihypertensive actions of these compounds. 2. A number of reports have suggested that ACE inhibitors can improve arterial function in hypertension, whereas the knowledge about the vascular effects of other antihypertensive drugs, like beta-blockers, calcium channel blockers, and diuretics remains rather limited. 3. In this article, the effects of antihypertensive therapy on arterial function in human and experimental hypertension are reviewed.

Endothelial function in deoxycorticosterone-NaCl hypertension: effect of calcium supplementation

BACKGROUND

Dietary calcium intake has been suggested to correlate inversely with blood pressure in humans and experimental animals. However, the effects of calcium supplementation on hypertensive disturbances of the endothelium have not been well characterized.

METHODS AND RESULTS

Wistar-Kyoto rats made hypertensive by deoxycorticosterone (DOC)-NaCl treatment, but a concurrent increase in chow calcium content from 1.1% to 2.5% markedly attenuated the rise in blood pressure. The function of isolated mesenteric arterial rings in vitro was investigated at the close of the 10-week study. In norepinephrine-precontracted rings, the relaxations to acetylcholine (ACh) and ADP, as well as to nitroprusside, 3-morpholinosydnonimine, and isoproterenol were attenuated in hypertensive rats on 1.1% calcium supplementation. In the presence of NG-nitro-L-arginine methyl ester (L-NAME), the relaxations to ACh in hypertensive animals on normal calcium were practically absent, whereas in normotensive rats and calcium-supplemented hypertensive rats, distinct relaxations to higher concentrations of ACh were still present. These responses were reduced by 30% to 50% with apamin, a blocker of Ca2+-activated K+ channels, and were further inhibited by blockade of ATP-dependent K+ channels with glyburide. Interestingly, relaxations elicited by ACh and ADP during precontraction with 60 mmol/L KCl (preventing endothelium-dependent hyperpolarization) were not impaired in hypertensive animals. The contractile sensitivity of endothelium-intact arterial rings to 5-hydroxytryptamine and norepinephrine was higher in hypertensive rats on either normal or high-calcium diet, whereas the increase in contractile sensitivity caused by L-NAME corresponded in all groups.

CONCLUSION

High-calcium diet markedly opposed experimental DOC-NaCl hypertension, an effect associated with improved arterial relaxation, while abnormalities of vascular contractile properties remained unaffected. In particular, the hyperpolarization-related component of endothelium-dependent arterial relaxation, mediated via opening of arterial K+ channels, could be augmented by calcium supplementation in DOC-NaCl hypertension.

Nitric oxide in hypertension and renal diseases

Nitric oxide (NO) is intimately involved in the regulation of vascular tone, renal haemodynamics and sodium balance. The physiological actions of NO suggest important vascular and renal protective roles for NO. When produced in large amounts, however, NO may also mediate cytotoxic effects. Increasing evidence suggests that endothelial function, notably the NO pathway, may be compromised in hypertension. It is not known, however, whether changes in endothelial function are primary or secondary to the development of hypertension. In renal diseases evidence for both excessive and deficient activity of NO pathway has been found. Increased glomerular production of NO via inducible NO synthase (NOS) with potential cytotoxic consequences has been demonstrated in experimental acute glomerulonephritis. On the other hand, indirect evidence obtained by means of NOS inhibitors point out to an important renoprotective role for NO in renal diseases. NO may counteract disease progression in renal diseases by preventing glomerular microthrombi, maintaining renal perfusion and medullary oxygenation, and via its anti-inflammatory/antiproliferative effects. However, these beneficial effects of NO may be compromised (endothelial and/or tubular dysfunction) in chronic nephropathies resulting in an accelerated course of renal disease. In future, more specific inhibitors and activators of different NOS isoforms are needed to elucidate the role of NO in various renal diseases in detail, and for treatment strategies aimed at modifying the NO pathway.

Nitric oxide-based possibilities for pharmacotherapy

The goal of nitric oxide (NO) based pharmacotherapy is to reach proper homeostasis of NO metabolism in the target tissue where endogenous production of NO is either too weak or excessively increased. In addition to the classic NO-based therapy of cardiovascular conditions with nitrates, a variety of new therapeutic possibilities have emerged including sexual disorders, gastrointestinal system, immunology, tumour growth regulation and respiratory disorders. NO levels of target tissues can be affected directly by NO donors, or indirectly by increasing the level of L-arginine, a substrate of nitric oxide synthase (NOS). While increased production of NO by induceable NO (iNOS) by, for example, cytokines does not at present seem therapeutically meaningful, increased NO production by constitutive NOS (cNOS) may be involved in the beneficial effects of ACE inhibitors or oestrogens. NO production may be pharmacologically decreased by inhibition of expression of iNOS by glucocorticoids while both cNOS and iNOS derived NO production is inhibited by administration of false substrates, for example L-NAME. Additionally, the respiratory system and related vessels can be reached directly and more selectively by inhalation of pure NO gas. Possible problems in administering NO and perhaps some NO-donors include the toxic nature of the compound itself whereby vital enzyme systems may be inhibited and tissue damaging radicals formed. Future prospects of NO-based pharmacotherapy may feature selective ligands to different NOS isoforms and tissue selective donors that release NO in a controlled fashion.