Chronic N(G)-nitro-L-arginine methyl ester treatment does not prevent flow-induced remodeling in mesenteric feed arteries and arcading arterioles.

Although endothelium-derived NO is an important mediator in acute flow-induced changes in arterial tone, the role of NO in chronic flow-induced changes in the resistance artery and arteriolar structure remains largely unresolved. We investigated the effects of chronic inhibition of NO synthase on arterial and arteriolar remodeling in a rat mesenteric model in which flow changes were induced. Alternating first-order mesenteric arteries were ligated to shunt blood flow through the intermittent patent arteries. Animals received no treatment (NT) or a continuous infusion of N:(G)-nitro-L-arginine methyl ester (L-NAME, 25 mg/kg SC per day). After 2 weeks, local in vivo blood flow and in vitro arterial pressure-diameter relationships were assessed, as were the in situ diameters of arcading arterioles. Medial cross-sectional areas (CSAs) were measured histologically. In both groups of animals, blood flow was significantly increased in patent arteries and decreased in ligated arteries compared with control vessels. Nonetheless, in L-NAME-treated rats, patent artery flow was increased to a lesser extent, although control flow was not significantly reduced (0.18+/-0.05 versus 0.26+/-0.05 mL/min). In NT rats, the diameter of patent arteries was significantly larger and the diameter of ligated arteries was significantly smaller than that of control arteries. CSAs displayed the same pattern of change (11. 9+/-0.6 x 10(3), 6.1+/-0.7 x 10(3), and 8.2+/-1.0 x 10(3) microm(2) for patent, ligated, and control arteries, respectively). Arterioles in the NT collateral pathway (218+/-15 microm) had diameters similar to control arteriole diameters (201+/-15 microm) but had a significantly larger CSA (6.2+/-0.6 x 10(3) versus 4.2+/-0.4 x 10(3) microm(2)). In L-NAME-treated rats, the flow-induced changes of the diameter and CSA in patent arteries, ligated arteries, and arcading arterioles mimicked those in NT rats. Nonetheless, control feed arteries (430+/-21 versus 497+/-16 microm) and arcading arterioles (156+/-21 microm) were significantly narrower after L-NAME treatment. Thus, chronic blockade of NO oxide synthase (1) tended to reduce arterial blood flow and resulted in inward remodeling of mesenteric arteries and arterioles and (2) did not prevent arterial and arteriolar remodeling in response to imposed changes in blood flow. Endothelium-derived mediators other than NO can play a major role in flow-induced arterial remodeling.

Pressure but not angiotensin II-induced increases in wall mass or tone influences static and dynamic aortic mechanics.

OBJECTIVE: To distinguish between static (due to slow changes in pressure) and dynamic (due to pressure pulsatility) components of aortic compliance over a large pressure range in vivo and to examine the effects of increased vascular mass and smooth muscle tone on these components. METHODS: Using ultrasound wall tracking, aortic lumen area-pressure curves were generated in anaesthetized rats over a broad range of pressures by altering blood volume. The compliance coefficient calculated at each mean pressure was considered the dynamic compliance at that pressure; the slope of the diastolic lumen area-pressure curve represents static compliance. Experiments were performed in control rats and rats treated with angiotensin II (ANG II) acutely (500 ng/kg per min intravenously) to modify vascular tone or chronically (250 ng/kg per min subcutaneously for 2 weeks) to modify vascular mass. RESULTS: The dynamic compliance-pressure curve approximated a parabola. Maximal dynamic compliance (0.272+/-0.026 mm2/kPa in control rats) was achieved at near-normotensive pressure (+/-105 mm Hg). The diastolic lumen area-pressure curve showed an exponential relationship within a physiological range (30-130 mm Hg). ANG II-induced increases in aortic wall mass or smooth muscle tone did not modify the relationship between static or dynamic compliance and pressure. CONCLUSIONS: These findings demonstrate that static and dynamic mechanics of the rat thoracic aorta depend differently on blood pressure. Static compliance increases slightly with pressure in a physiological range, while dynamic compliance is auto-regulated around normotensive pressures. Neither static nor dynamic compliance of the rat thoracic aorta are influenced by ANG II-induced increases in aortic wall mass or smooth muscle tone.

Role of basal nitric oxide synthesis in vasoconstrictor hyporeactivity in the perfused rat hindlimb after myocardial infarction: effect of captopril.

OBJECTIVES: The contribution of vascular changes to the development of heart failure is largely unknown. In the present study, we evaluated endothelial and vascular contractile function in the rat hindlimb vascular bed after myocardial infarction (MI), including the modulatory role of basal nitric oxide (NO) production and the effects of treatment with the angiotensin converting enzyme inhibitor captopril on vascular function. METHODS: MI was induced in male Wistar rats by ligation of the left coronary artery. Acetylcholine-induced dilatations were assessed in the ex vivo perfused hindlimb at various time points. At 2 and 5 weeks post-MI, vascular contractile function in the perfused hindlimb was assessed from resistance changes induced by 35 mM and 125 mM potassium (K+) and the maximum increase in resistance (delta Rmax, 125 mM K+ and 3 mg phenylephrine). Basal NO synthesis was blocked for 2 weeks with L-nitro-arginine methylester (L-NAME) in sham and MI rats and similar contractility experiments were performed. The effect of captopril treatment from 2 to 5 weeks post-MI on vasoconstrictor responses was also tested. RESULTS: Acetylcholine-induced dilatations in the presence of 10 microM indomethacin were not different between sham and MI rats. Vasoconstrictor responses to K+ and delta Rmax were reduced at 2 weeks after MI. This reduction in vasoconstrictor ability was similar to that seen in L-NAME-treated sham rats, while chronic L-NAME treatment did not affect vasoconstrictor reactivity in MI rats. Similarly, L-NAME induced an increase in mean arterial pressure in sham rats, but not in MI rats. At 5 weeks after MI, vasoconstriction to 125 mM K+ and delta Rmax were still reduced in MI rats; this response was however partially restored after captopril treatment. CONCLUSION: The development of vascular contractile hyporeactivity in the rat hindlimb after MI may be due to reduced basal NO production. Delayed treatment with captopril improves peripheral vascular contractile function in this setting.

Time-related adaptations in plasma neurohormone levels and hemodynamics after myocardial infarction in the rat.

BACKGROUND: Neurohormonal activation plays an important role in the progression of heart failure. In this study, we investigated the progression of neurohormonal activation in conjunction with the hemodynamic status of the rat after myocardial infarction (MI). METHODS AND RESULTS: Male Wistar rats were subjected to sham or MI surgery. At 1, 3, 5, and 13 weeks after surgery, cardiac output (CO), mean arterial pressure (MAP), and total peripheral resistance (TPR), were measured. In separate groups of rats, blood was sampled at 1, 5, and 13 weeks after surgery and analyzed for various neurohormones. At 1 week after surgery, CO and TPR were not altered in MI rats, but plasma neurohormonal levels were elevated. At 3 and 5 weeks after surgery, reduced CO, increased TPR, and normal MAP were measured in MI rats compared to sham rats, but only endothelin levels were elevated. At 13 weeks after surgery, MAP was reduced in MI rats and CO and TPR were comparable between groups. Neurohormonal activation was again apparent in MI rats. CONCLUSIONS: Myocardial infarction in the rat induces early neurohormonal activation, which normalizes hemodynamic parameters. A compensatory phase follows. At 13 weeks after MI, plasma concentrations of neurohormones are again elevated, perhaps as a sign of transition to decompensation.

Effect of chronic blockade of angiotensin II-receptor subtypes on aortic compliance in rats with myocardial infarction.

This study was undertaken to investigate changes in aortic geometry and compliance after long-term blockade of angiotensin receptors type 1 (AT1) and AT2 receptors under basal conditions and after myocardial infarction (MI). Sham-operated (sham) or MI rats received either no treatment, AT1 antagonist GR138950C (GR; 2 mg/kg/day i.v.), or AT2 antagonist PD123319 (PD; 3 mg/kg/day s.c.). After 3 weeks, mean arterial blood pressure (MAP) was measured. Thoracic aorta diastolic diameter (D[dia]), compliance coefficient (CC), and distensibility coefficient (DC) were determined noninvasively in anesthetized rats by using ultrasound and wall tracking. After the rats were killed, histologic measurements were made on aortic cross sections. In sham rats, MAP was reduced by GR treatment (76 +/- 6 vs. 106 +/- 5 mm Hg), but not by PD. D(dia) was reduced in both GR-treated (1.74 +/- 0.08 vs. 2.09 +/- 0.05 mm) and PD-treated (1.83 +/- 0.05 vs. 2.09 +/- 0.05 mm) sham rats. CC and DC were not modified by either treatment. Although media cross-sectional area was not affected by either GR or PD treatment in sham rats, media thickness and media/lumen ratio were increased in both cases. Induction of MI had no effect on aortic structure, geometry, or mechanics; however, treatment with either GR or PD improved DC versus untreated MI rats. We conclude that AT1 and AT2 receptors are involved in angiotensin II-mediated effects on aortic geometry and mechanics under both basal conditions and after MI. Whereas blockade of AT1 receptors most likely influences vascular properties through a depressor mechanism, AT2 receptors induce pressure-independent remodeling.

Angiotensin II induces media hypertrophy and hyperreactivity in mesenteric but not epigastric small arteries of the rat.

We examined effects of a 2-week infusion of angiotensin II (AII, 250 ng x kg[-1] x min[-1]) on properties of mesenteric resistance arteries (MrA) and superior epigastric arteries (SEA) of male Wistar rats. Histochemistry and pharmacological tools showed that MrA are densely innervated, whereas SEA are only sparsely innervated. AII infusion resulted in a significant elevation in mean arterial pressure and in plasma AII and noradrenaline levels. Organ chamber studies and morphometry were used to determine arterial contractile reactivity and structure. After AII infusion, in MrA (i) maximal contractile responses to 125 mM K+, noradrenaline, serotonin and adrenergic nerve stimulation were significantly increased, without modification of the sensitivity to these stimuli and (ii) a significant increase in media cross-sectional area and media thickness was observed without alterations in lumen diameter. The observed increase in vascular reactivity could fully be attributed to the observed increase in wall mass since no alterations in maximal active wall stress were noted. In SEA, no significant changes in responsiveness to vasoconstrictor stimuli or in wall structure were observed. These findings suggest that perivascular nerves are involved in the hypertrophy and subsequent hyperreactivity of small arteries in rats exposed for 2 weeks to a low dose of AII.

The influence of angiotensin II-induced increase in aortic wall mass on compliance in rats in vivo.

OBJECTIVE: This study was undertaken to examine the effects of angiotensin II-induced structural changes in the aortic wall on the dynamic mechanical properties of the vessel in the rat. METHODS: Wistar rats were infused s.c. with 250 ng/kg/min angiotensin II (Ang II) for 14 days (ANG). Both ANG and control rats (CON) were equipped with an arterial catheter for measurement of arterial blood pressure. Thoracic aorta diameter, compliance coefficient (CC), and distensibility coefficient (DC) were determined non-invasively in pentobarbital-anesthetized animals using a B-mode imager attached to a vessel wall tracking system. After sacrifice, medial cross-sectional area (CSA), and elastin and collagen densities were determined by morphometry on cross-sections. Media thickness (Mt) and wall-to-lumen ratio (W/L) were subsequently calculated. RESULTS: Ang II infusion significantly increased mean arterial blood pressure in conscious rats (122 +/- 3 mmHg CON vs. 157 +/- 4 mmHg ANG). This was normalized when the rats were anesthetized, thus making it possible to determine CC and DC under isobaric conditions where the diastolic diameters were also similar. Two-week infusion of Ang II induced a significant increase in CSA from 0.48 +/- 0.02 mm2 in CON to 0.61 +/- 0.03 mm2. Mt and W/L were likewise increased, but collagen and elastin densities remained unchanged. CC and DC were not effected by this increase in aortic wall mass (CC: 0.143 +/- 0.009 CON, 0.147 +/- 0.014 mm2/kPa ANG; DC: 0.052 +/- 0.005 CON, 0.051 +/- 0.004 kPa-1 ANG). CONCLUSIONS: An increase in aortic wall mass resulting from chronic infusion of angiotensin II does not alter the dynamic compliance of the vessel under isobaric conditions.

Effect of nitric oxide synthase inhibition on the acetylcholine response in the perfused hind limb of rats.

The effect of nitric oxide (NO) synthase inhibition on acetylcholine-induced vasodilation in the perfused rat hind limb was investigated to find the contribution of NO to such relaxation. Although NO synthase inhibition with 150 micrograms L-nitro-arginine increased vascular resistance considerably, from 7.28 +/- 0.29 to 10.83 +/- 0.44 mm Hg.min/ml (n = 7), the acetylcholine responses were not attenuated. Acetylcholine (10 micrograms) induced a peak relaxation of 66 +/- 4% before, and 63 +/- 7% after L-nitro-arginine. The duration of the peak and total responses, examined in separate sets of animals (n = 12), was similar in both circumstances (61 +/- 4 s before vs. 53 +/- 5 s after, and 7.45 +/- 0.61 min before vs. 7.48 +/- 0.64 min after respectively). These results suggest that a non-NO factor is responsible for acetylcholine-stimulated relaxation in the rat hind limb vascular bed.