Longitudinal differences in vascular control mechanisms in isolated resistance arteries of the rat cremaster muscle

Skeletal muscle arteriolar function following myocardial infarction: Analysis of branch-order effects

Diminished bioavailability of nitric oxide (NO) may impair skeletal muscle arteriolar function after myocardial infarction (MI). We tested the hypotheses that chronic MI induced would diminish 1) endothelial function in large (resting diameter ~75μm) feed arterioles, and 2) functional dilation in feed arterioles, but not smaller arcade (~25μm) or transverse (~15μm) arterioles, in the spinotrapezius muscle of female Sprague-Dawley rats. Additionally, we hypothesized that blockade of NO production with N(G)-nitro-l-arginine methyl ester (l-NAME; 30mg/kg i.v.) would have a greater blunting effect on control rats than MI rats. Endothelial function of the feed arterioles was assessed with an infusion of acetylcholine (1.5μg i.v.) after pretreatment with indomethacin (5mg/kgi.p.). MI blunted the response to acetylcholine in feed arterioles (p=0.037), but did not affect resting or post-contraction diameter at any branching order. l-NAME had similar effects on MI and SHAM rats; the response to acetylcholine was blunted in feed arterioles (p=0.003), resting diameter was diminished in arcade arterioles (p=0.003), and post-contraction diameter was diminished in both arcade arterioles (p=0.03) and transverse arterioles (p=0.05). In conclusion, despite endothelial dysfunction in feed arterioles, functional dilation was not affected by MI in any branching order studied. l-NAME had similar effects on MI and SHAM rats that were branch order-dependent. These branch-order effects should be considered in future studies of the control of blood flow.

Chronic intermittent hypoxia alters NE reactivity and mechanics of skeletal muscle resistance arteries

Although arterial dilator reactivity is severely impaired during exposure of animals to chronic intermittent hypoxia (CIH), few studies have characterized vasoconstrictor responsiveness in resistance arteries of this model of sleep-disordered breathing. Sprague-Dawley rats were exposed to CIH (10% inspired O2 fraction for 1 min at 4-min intervals; 12 h/day) for 14 days. Control rats were housed under normoxic conditions. Diameters of isolated gracilis muscle resistance arteries (GA; 120–150 μm) were measured by television microscopy before and during exposure to norepinephrine (NE) and angiotensin II (ANG II) and at various intraluminal pressures between 20 and 140 mmHg in normal and Ca2+-free physiological salt solution. There was no difference in the ability of GA to constrict in response to ANG II ( P = 0.42; not significant; 10−10–10−7 M). However, resting tone, myogenic activation, and vasoconstrictor responses to NE ( P < 0.001; 10−9–10−6 M) were reduced in CIH vs. controls. Treatment of rats with the superoxide scavenger 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (tempol; 1 mM) in the drinking water restored myogenic responses and NE-induced constrictions of CIH rats, suggesting that elevated superoxide production during exposure to CIH attenuates vasoconstrictor responsiveness to NE and myogenic activation in skeletal muscle resistance arteries. CIH also leads to an increased stiffness and reduced vessel wall distensibility that were not correctable with oral tempol treatment.

Endotoxin Releases a Substance from the Aorta that Dilates an Isolated Arteriole by Up-Regulating INOS

BACKGROUND

Loss of vascular tone in resistance arterioles has been implicated as the cause of hypotension in septic shock. It is believed that the overproduction of nitric oxide (NO) by the inducible isoform of nitric oxide synthase (iNOS) results in the vasodilatation seen in septic shock. However, we have shown that endotoxin has no effect on vascular tone of an isolated resistance vessel unless the endotoxin flows over a segment of aorta or vena cava upstream in the superfusion line. The aim of this study was to determine if the subsequent vasodilation was due to the release of a direct vasodilator or production of NO in the arteriole and if its source was iNOS by using its selective inhibitor, aminoguanidine.

MATERIALS AND METHODS

First-order rat cremaster arterioles (n = 36) were isolated and cannulated onto micropipettes, superfused with physiological buffer at 34 degrees C, pressurized to 70 mm Hg, and allowed to gain spontaneous tone over 90 min. A segment of abdominal aorta was then placed in series with the arteriole so that the superfusate passed over the aorta and then into the tissue bath containing the isolated arteriole. The vessels were allowed to equilibrate over 60 min. During this interval, the arteriole was exposed to l-NAME (100 mum), aminoguanidine (100 mum), or buffer. The aorta and arteriole were then superfused with endotoxin (Salmonella enteritidis 2.5 mug/ml). Internal diameters of cannulated arterioles were measured and recorded with videomicroscopy and videocalipers at a resolution of +/-1 mum every 15 min for 1 h. Six groups were created with n = 6 for each group: Group 1, endotoxin; Group 2, control; Group 3, l-NAME and endotoxin; Group 4, l-NAME; Group 5, aminoguanidine and endotoxin; and Group 6, aminoguanidine.

RESULTS

After the 60-min equilibration period, there was no significant difference in resting tone among the six groups. At t = 120, the percentage of tone in the control group was 42.7 +/- 0.4% (mean +/- SEM) and this was not changed by treatment with aminoguanidine (42.2 +/- 0.7%). However, exposure to l-NAME alone resulted in vasoconstriction with a gain in tone to 49.5 +/- 1.6% (P > 0.05). Endotoxin alone caused arteriolar tone to fall to 33.5 +/- 1.2% (P < 0.05). Arterioles treated with aminoguanidine did not lose tone (42.6 +/- 1.7%) when exposed to endotoxin and arterioles treated with l-NAME retained their elevated tone (46.0 +/- 2.2%) after treatment with endotoxin.

CONCLUSIONS

This study demonstrates that the aorta exposed to endotoxin releases a substance that vasodilates resistance arterioles through the up-regulation of iNOS. Aminoguanidine prevented the fall in tone following exposure to endotoxin, while use of the nonselective NOS inhibitor, l-NAME, not only blocked the fall due to endotoxin but increased basal tone by blocking the constitutively active eNOS.