Reduced responsiveness of rat mesenteric resistance artery smooth muscle to phenylephrine and calcium following myocardial infarction

Cardiac and Vascular α1-Adrenoceptors in Congestive Heart Failure: A Systematic Review

As heart failure (HF) is a devastating health problem worldwide, a better understanding and the development of more effective therapeutic approaches are required. HF is characterized by sympathetic system activation which stimulates α- and β-adrenoceptors (ARs). The exposure of the cardiovascular system to the increased locally released and circulating levels of catecholamines leads to a well-described downregulation and desensitization of β-ARs. However, information on the role of α-AR is limited. We have performed a systematic literature review examining the role of both cardiac and vascular α1-ARs in HF using 5 databases for our search. All three α1-AR subtypes (α1A, α1B and α1D) are expressed in human and animal hearts and blood vessels in a tissue-dependent manner. We summarize the changes observed in HF regarding the density, signaling and responses of α1-ARs. Conflicting findings arise from different studies concerning the influence that HF has on α1-AR expression and function; in contrast to β-ARs there is no consistent evidence for down-regulation or desensitization of cardiac or vascular α1-ARs. Whether α1-ARs are a therapeutic target in HF remains a matter of debate.

Effects of Apelin on Left Ventricular-Arterial Coupling and Mechanical Efficiency in Rats with Ischemic Heart Failure

Apelin plays important roles in cardiovascular homeostasis. However, its effects on the mechanoenergetics of heart failure (HF) are unavailable. We attempted to investigate the effects of apelin on the left ventricular-arterial coupling (VAC) and mechanical efficiency in rats with HF. HF was induced in rats by the ligation of the left coronary artery. The ischemic HF rats were treated with apelin or saline for 12 weeks. The sham-operated animals served as the control. The left ventricular (LV) afterload and the systolic and diastolic functions, as well as the mechanoenergetic indices were estimated from the pressure-volume loops. Myocardial fibrosis by Masson's trichrome staining, myocardial apoptosis by TUNEL, and collagen content in the aorta as well as media area in the aorta and the mesenteric arteries were determined. Our data indicated that HF rats manifested an increased arterial load (Ea), a declined systolic function (reduced ejection fraction, +dP/dt_max, end-systolic elastance, and stroke work), an abnormal diastolic function (elevated end-diastolic pressure, τ, and declined −dP/dt_max), and decreased mechanical efficiency. Apelin treatment improved those indices. Concomitantly, increased fibrosis in the LV myocardium and the aorta and enhanced apoptosis in the LV were partially restored by apelin treatment. A declined wall-to-lumen ratio in the mesenteric arteries of the untreated HF rats was further reduced in the apelin-treated group. We concluded that the rats with ischemic HF were characterized by deteriorated LV mechanoenergetics. Apelin improved mechanical efficiency, at least in part, due to the inhibiting cardiac fibrosis and apoptosis in the LV myocardium, reducing collagen deposition in the aorta and dilating the resistant artery.

Calibration of baroreflex equilibrium diagram based on exogenous pressor agents in chronic heart failure rats

A baroreflex equilibrium diagram describes the relation between input pressure and sympathetic nerve activity (SNA) and that between SNA and arterial pressure (AP). To calibrate the SNA axis (abscissa) of the baroreflex equilibrium diagram, the AP response to intravenous bolus injections of phenylephrine (0.2-50 μg/kg) or norepinephrine (NE, 0.02-5 μg/kg) was examined in normal control rats (NC, n = 9) and rats with chronic heart failure (CHF, n = 6). The maximum slope of the dose-effect curve was significantly smaller in the CHF group than in the NC group (57.3 ± 5.2 vs 80.9 ± 6.3 mmHg/decade for phenylephrine, 60.2 ± 7.8 vs 80.4 ± 5.9 mmHg/decade for NE; P < 0.01). The CHF/NC ratio of the maximum slope was used to calibrate SNA. While the calibrated baroreflex equilibrium diagram showed increased maximum SNA and operating-point SNA in CHF rats compared with NC rats, the magnitude of increase was smaller than that expected from the excess plasma NE concentration in CHF rats. Plasma NE concentration in the CHF group could be disproportionally high relative to SNA.

Use of afterload hemodynamic stress as a practical method for assessing cardiac performance in rats with heart failure

After myocardial infarction, the hemodynamics under basal conditions might appear to be unaltered, which makes it difficult to identify cardiac dysfunction by the usual approaches. Thus, we tested the response to sudden afterload stress in infarcted rats with apparently normal ejection function. Control (CT) and infarcted (MI) Wistar rats with various MI sizes were submitted to echocardiography 30 days after coronary occlusion, followed by assessment of hemodynamics under basal conditions and during a pharmacologically induced sudden pressure overload (phenylephrine 15-25 microg/kg, i.v.). Coronary occlusion resulted in cardiac remodeling proportional to MI size, although several functional parameters such as systolic pressure (SP), stroke volume (SV), and stroke work (SW) of all MI rats were similar to those of CT rats. However, the afterload stress that was produced led to a relative preservation of SV and an increase of SW in CT rats; MI rats exhibited a significant reduction in SV and SW generation, although global cardiac function was normal under basal conditions, as indicated by regular echocardiography and hemodynamics assessment. Thus, we propose the use of sudden pharmacologically induced afterload stress as a practical and efficient procedure for identifying impaired performance of the heart in anesthetized rats, providing an additional physiological variable to be evaluated in experimental therapeutic studies.

Myoendothelial coupling in the mesenteric arterial bed; segmental differences and interplay between nitric oxide and endothelin-1

BACKGROUND AND PURPOSE

We tested the hypothesis that activated arterial smooth muscle (ASM) stimulates endothelial vasomotor influences via gap junctions and that the significance of this myoendothelial coupling increases with decreasing arterial diameter.

EXPERIMENTAL APPROACH

From WKY rats, first-, second-, third- and fourth-order branches of the superior mesenteric artery (MA1, MA2, MA3 and MA4 respectively) were isolated and mounted in wire-myographs to record vasomotor responses to 0.16-20 micromol x L(-1) phenylephrine.

KEY RESULTS

Removal of endothelium increased the sensitivity (pEC(50)) to phenylephrine in all arteries. The nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) (100 micromol x L(-1)) did not modify pEC(50) to phenylephrine in all denuded arteries, and increased it in intact MA1, MA2 and MA3 to the same extent as denudation. However, in intact MA4, the effect of L-NAME was significantly larger (DeltapEC(50) 0.57 +/- 0.02) than the effect of endothelium removal (DeltapEC(50) 0.20 +/- 0.06). This endothelium-dependent effect of L-NAME in MA4 was inhibited by (i) steroidal and peptidergic uncouplers of gap junctions; (ii) a low concentration of the NO donor sodium nitroprusside; and (iii) by the endothelin-receptor antagonist bosentan. It was also observed during contractions induced by (i) calcium channel activation (BayK 8644, 0.001-1 micromol x L(-1)); (ii) depolarization (10-40 mmol x L(-1) K(+)); and (iii) sympathetic nerve stimulation (0.25-32 Hz).

CONCLUSIONS AND IMPLICATIONS

These pharmacological observations indicated feedback control by endothelium of ASM reactivity involving gap junctions and a balance between endothelium-derived NO and endothelin-1. This myoendothelial coupling was most prominent in distal resistance arteries.

Noradrenaline-induced increases in calcium and tension in skeletal muscle conductance and resistance arteries from rats with post-infarction heart failure

We tested the hypothesis that arterial reactivity to noradrenaline is augmented in congestive heart failure (CHF), which could contribute to the deleterious changes in peripheral vascular resistance and compliance in this condition. From male Wistar rats with post-infarction CHF and sham-operated rats, skeletal muscle conductance and resistance arteries (mean lumen diameters: 514 and 186 microm) were isolated and mounted on wire myographs, and wall tension was recorded in response to cumulative application of acetylcholine and noradrenaline to the vessel segments. In a subset of experiments, wall tension and cytosolic free calcium ion concentration [Ca(2+)](i) were recorded simultaneously during noradrenaline application, using wire myography and the FURA-2 technique. No significant differences were found in the arterial baseline levels of [Ca(2+)](i) or tension between CHF and sham rats. In the resistance arteries of CHF rats, the noradrenaline-induced increases in [Ca(2+)](i) were significantly enhanced (P=0.003). Despite the augmented [Ca(2+)](i) levels, the tension responses to noradrenaline were unaltered in these arteries. In the conductance arteries, there were no significant differences in noradrenaline-induced [Ca(2+)](i) or tension responses between CHF and control rats. CHF did not alter vascular morphology or change vascular relaxations to acetylcholine in either type of artery. In conclusion, these results do not support the contention that arterial reactivity to noradrenaline is augmented in the skeletal muscle vascular bed in CHF. On the contrary, the unchanged contractile responsiveness in the resistance arteries despite the enhanced levels of [Ca(2+)](i) during noradrenaline application suggests that the contractile function of these vessels is compromised in CHF. Neither vascular remodeling, endothelial dysfunction nor changes in baseline vascular tone could be demonstrated in the skeletal muscle vascular bed of this animal model of heart failure.

Ca2+ sensitisation of force production by noradrenaline in femoral conductance and resistance arteries from rats with postinfarction congestive heart failure

In this study we tested the hypothesis that arterial myofilament Ca(2+) sensitivity and/or the Ca(2+) sensitising effect of noradrenaline (NA) is enhanced in post-infarction congestive heart failure (CHF), which could contribute to the high peripheral vascular resistance in this condition. Femoral skeletal muscle resistance and conductance arteries (mean lumen diameters of 159 and 519 microm) from rats with CHF and sham-operated control rats were used. Isometric tension development and intracellular free calcium concentration ([Ca(2+)](i)) were measured simultaneously in isolated vessel segments using wire myography and the FURA-2 fluorescence technique. In conductance and resistance arteries, the resting levels of [Ca(2+)](i) and tension in physiological saline solution (PSS) and active tension in response to single doses of 125 mM K(+) (KPSS) were unaffected by CHF. During cumulative application of extracellular Ca(2+) to arteries depolarised with 125 mM K(+) or activated with 30 microM NA, [Ca(2+)](i) and vessel wall tension were similar in CHF and control rats. However, the conductance arteries showed significantly higher calcium sensitivity than resistance arteries in these experiments. We conclude that an abnormality in the sensitivity of the contractile apparatus to Ca(2+), or in NA-induced Ca(2+) sensitisation in arterial vascular smooth muscle cells is unlikely to contribute to the ubiquitously elevated vascular resistance associated with CHF. However, our data demonstrate significant differences in vascular Ca(2+) handling, myofilament Ca(2+) sensitivity and tension development between resistance and conductance arteries, regardless of CHF.

Differences in tail vascular bed reactivity in rats with and without heart failure following myocardial infarction

Myocardial infarction (MI) was induced in rats by coronary ligation to compare changes in vascular reactivity from animals that developed heart failure (InfHF) with those that did not (Inf). Infarct size was similar in both groups. In vitro preparations of tail vascular bed were used to investigate the vascular responses to acetylcholine, sodium nitroprusside, and phenylephrine. Acetylcholine-induced relaxation was impaired in the Inf group (53 +/- 2%, n = 6) when compared with Sham (80 +/- 2%, n = 6, P < 0.05). The maximal response (E(max)) to phenylephrine increased in the Inf group (423 +/- 10 mm Hg, n = 9, P < 0.01) and decreased in InfHF (279 +/- 10 mm Hg, n = 7, P < 0.05) when compared with Sham (319 +/- 11 mm Hg, n = 8). Regardless of endothelial integrity, E(max) to phenylephrine increased in the Inf, nitro-l-arginine methyl ester, and indomethacin groups. An increased release of a prostanoid vasodilator was detected in the Inf group. Differently, the InfHF group presented a reduction of the E(max) to phenylephrine and an increment of nitric oxide release. This study demonstrates that MI without heart failure impairs endothelium-dependent relaxation and increases the reactivity to phenylephrine. This increase seems to involve a muscular component. The endothelium participates with an increased release of a vasodilator prostanoid, possibly to compensate the increased smooth muscle response. When heart failure follows MI, the reactivity to phenylephrine decreases, possibly due to an increased nitric oxide release.

Effects of angiotensin II type 1 receptor antagonist, YM358, on cardiac hypertrophy and dysfunction after myocardial infarction in rats

This study was undertaken to investigate the effects of an angiotensin II type 1 receptor antagonist, YM358 (2,7-diethyl-5-[[2'(1H-tetrazol-5-yl)biphenyl-4-yl]methyl]-5H-pyrazolo [1,5-b] [1,2,4]triazole potassium salt monohydrate), on cardiac hypertrophy and dysfunction in rats with heart failure after myocardial infarction (MI). One day after the coronary ligation, rats were randomized, and administered YM358 or vehicle for 2, 4 or 8 weeks. In MI rats, mean blood pressure, left ventricular (LV) systolic pressure, and the first derivative of LV pressure significantly decreased, and LV end-diastolic pressure (LVEDP) markedly increased after 2 to 8 week treatment of YM358. From 2 weeks after the ligation, ratios of cardiac weight and lung weight to body weight (BW) significantly increased, which indicated the progression of cardiac hypertrophy and lung congestion in MI rats. Two weeks after the ligation, YM358 did not improve LV function, although it decreased the elevated LVEDP and cardiac weights/BW ratios 8 weeks after the ligation. These results indicated that long-term treatment with YM358 improves the reduced cardiac function and reduces cardiac hypertrophy after MI, and may be useful for the treatment of congestive heart failure.

Enhanced Ca2+-induced contractions and attenuated alpha-adrenoceptor responses in resistance arteries from rats with congestive heart failure

AIM

The aim of the present study was to examine the role of Ca2+-mediated contractile responses in isolated mesenteric resistance arteries from rats with congestive heart failure (CHF).

METHODS

Heart failure was induced by ligation of the left coronary artery. Rats exposed to the same surgical procedure except ligation served as controls (Sham). The following experiments were conducted: (1) passive increase in radial stretch (the length-tension relationship) in Ca2+-free and in depolarizing high K+-solution; (2) the contractile responses to external application of Ca2+ and high K+-solutions in the presence of nifedipine and phentolamine; and (3) a histological evaluation of CHF and Sham vessels.

RESULTS

The length-tension induced response in Ca2+-free buffer solution was significantly lower in arteries from CHF rats, starting at a very low tension (0.9+/-0.2 mN/mm for heart failure and 1.7+/-0.2 mN/mm for Sham). This difference, but at a higher degree of stretch, was also present in K+-activated vessels. The external application of Ca2+ in K+-depolarized vascular segments in the presence of phentolamine (1 microM) induced an enhanced contractile response in arteries from CHF rats compared with Sham (4.8+/-0.3 mN/mm and 3.6+/-0.6 mN/mm, respectively, P=0.059). In the absence of phentolamine the reverse response was found (4.0+/-0.4 mN/mm and 5.7+/-0.3 mN/mm for CHF vs. Sham respectively, P=0.035). Application of increasing concentrations of K+-solution induced a stronger contractile response in Sham compared with CHF arteries (Sham 4.9+/-0.4 and heart failure 4.0+/-0.3, P=0.04). Microscopic examination of vessels yielded no difference in gross morphology, media thickness or wall to lumen ratio between CHF and Sham arteries.

CONCLUSION

The results indicate an attenuation of alpha-adrenoceptors and a difference of Ca2+-mediated vascular contractility in resistance arteries of congestive heart failure rats.

Chronic hypoxia stimulates periarterial sympathetic nerve development in chicken embryo

BACKGROUND

Epidemiological findings suggest an association between low-for-age birth weight and the risk to develop coronary heart diseases in adulthood. During pregnancy, an imbalance between fetal demands and supply may result in permanent alterations of neuroendocrine development in the fetus. We evaluated whether chronic prenatal hypoxia increases arterial sympathetic innervation.

METHODS AND RESULTS

Chicken embryos were maintained from 0.3 to 0.9 of the 21-day incubation period under normoxic (21% O(2)) or hypoxic conditions (15% O(2)). At 0.9 incubation, the degree of sympathetic innervation of the embryonic femoral artery was determined by biochemical, histological, and functional (in vitro contractile reactivity) techniques. Chronic hypoxia increased embryonic mortality (32% versus 13%), reduced body weight (21.9+/-0.4 versus 25.4+/-0.6 g), increased femoral artery norepinephrine (NE) content (78.4+/-9.4 versus 57.5+/-5.0 pg/mm vessel length), and increased the density of periarterial sympathetic nerve fibers (14.4+/-0.7 versus 12.5+/-0.6 counts/10(4) microm(2)). Arteries from hypoxic embryos were less sensitive to NE (pD(2), 5.99+/-0.04 versus 6. 21+/-0.10). In the presence of cocaine, however, differences in sensitivity were no longer present. In the embryonic heart, NE content (156.9+/-11.0 versus 108.1+/-14.7 pg/mg wet wt) was also increased after chronic hypoxia.

CONCLUSIONS

In the chicken embryo, chronic moderate hypoxia leads to sympathetic hyperinnervation of the arterial system. In humans, an analogous mechanism may increase the risk for cardiovascular disease in adult life.

Vascular hyporesponsiveness of the renal circulation during endotoxemia in anesthetized pigs

OBJECTIVE

To compare the vascular reactivity of the renal circulation in control and septic conditions.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Anesthetized pigs (n = 17).

INTERVENTIONS

Ten pigs received a continuous intravenous infusion of endotoxin from Escherichia coli (160 ng x kg(-1) x hr(-1)) during 18 hrs, whereas seven control animals received a saline infusion. To test the vascular reactivity, norepinephrine (NE) (1 microg x kg(-1)), acetylcholine (10 microg x kg(-1)), and sodium nitroprusside (10 microg x kg(-1)) were intravenously injected for 20 secs and changes of mean arterial pressure and renal blood flow were observed during the 200 secs after the drug administration. To compare the evolution of the vascular reactivity over time, three tests were performed 5 hrs, 11 hrs, and 17 hrs after initial endotoxin or saline administration.

MEASUREMENTS AND MAIN RESULTS

Endotoxin infusion induced a hypotensive and hypokinetic syndrome with renal hypoperfusion. The mean arterial pressure increase after NE injection and the mean arterial pressure decrease after acetylcholine and nitroprusside were lower in endotoxin than in control pigs. In the renal circulation, the increase of resistance after NE injection and the decrease of renal resistance after acetylcholine and nitroprusside injections were lower in endotoxin than in control pigs.

CONCLUSIONS

This study shows a hyporesponsiveness of the renal circulation to vasoactive agents during endotoxemia. Vasoconstriction to NE, endothelium-dependent as well as endothelium-independent relaxations are altered during endotoxemia but not abolished, and despite the continuous infusion of endotoxin for 18 hrs, no recovery was observed over time.

Alterations in vascular reactivity in isolated vessel segments from dogs with naturally occurring heart failure

To investigate if functional vascular reactivity is altered in heart failure, the reactivity of isolated canine saphenous vein (SV) and femoral artery (FA) rings, from control dogs and dogs with naturally occurring heart failure was examined. In both vessels, relaxation responses to the endothelium-dependent vasodilator, acetylcholine were unaffected by heart failure. In the FA, in heart failure, there was a significant reduction in the potency of the agonist noradrenaline (pEC(50)6.05+/-0.07 (N = 8) and 5.54 +/- 0.13 (N = 7) for control and heart failure respectively). There was no significant alteration in potency in the SV. In addition, in the FA the maximum responses to both noradrenaline (control 3.64 +/- 0.31 KPa, (N = 8); failure 5.11 +/- 0.35 KPa, (N = 7) P = 0.004) and potassium chloride (control 2.18 +/- 0.26 KPa, (N = 8); failure 4.46 +/- 0.25 KPa, (N = 7) P = 0.001) were significantly increased in heart failure. It is suggested that enhanced agonist induced responses, in the femoral artery, in dogs with heart failure, may limit blood flow to exercising skeletal muscle and subsequently reduce exercise tolerance.

Effect of short- and long-term heart failure on small artery morphology and endothelial function in the rat

Chronic heart failure (HF) is associated with hemodynamic changes and activation of several neurohormonal systems, which are able both to inhibit and to facilitate arterial growth or remodeling and also to influence endothelial function. As these vascular changes may depend on the duration of HF, we evaluated morphologic and endothelial functional alterations in a rat model of HF after a short and long duration of HF. Rats with coronary ligation and sham-operated controls were investigated either 8 or 26 weeks after the operation with measurements of hemodynamics and isolated mesenteric small artery morphology and endothelial function. The effect of HF and duration of HF were examined by using two-way analysis of variance (ANOVA). HF rats had altered hemodynamics with reductions in cardiac output, left ventricular systolic pressure, and mean blood pressure, whereas left ventricular diastolic pressure was increased. HF caused remodeling of anatomically well-defined mesenteric small arteries with a reduction in media thickness and media-to-lumen ratio, but without change in the media cross-sectional area. Neither HF nor time had any influence on sensitivity or maximal relaxation to acetylcholine in the presence of indomethacin, but HF reduced vasoconstriction due to nitric oxide synthase blockade with N(G)-nitro-L-arginine independent of time. Our results indicate that HF, induced by coronary ligation in the rat, has a remodeling effect on mesenteric small arteries. However, the remodeling is moderate compared with that observed in hypertension. Furthermore, our results suggest that HF reduces basal release of NO.

Estrogen restores role of basal nitric oxide in control of vascular tone in rats with chronic heart failure

This study examined the cardiovascular effects of 17beta-estradiol in ovariectomized rats with heart failure. Two groups (50-60 days old) were implanted with 60-day-release pellets containing 17beta-estradiol (25 microg/day) or vehicle at 7 days before ligation of the left coronary artery. Another group was sham operated and given vehicle pellets. After 7 wk, they were studied under pentobarbital anesthesia. Relative to sham-operated rats, ligated rats had reduced mean arterial pressure (MAP, -24 +/- 6 mmHg), cardiac output (-27 +/- 4 ml/min), left ventricular (LV) end-systolic pressure (-29 +/- 8 mmHg), depressor responses to ACh (-6 +/- 4 mmHg at 7.2 microg/kg) and sodium nitroprusside (SNP, -22 +/- 6 mmHg at 9 microg/kg), and pressor responses to NG-nitro-L-arginine methyl ester (L-NAME, -14 +/- 6 mmHg at 8 mg/kg) and increased LV end-diastolic pressure (LVEDP, 10.3 +/- 0.8 mmHg) but no change in total peripheral resistance (TPR). Treatment of ligated rats with 17beta-estradiol reduced TPR (-0.19 +/- 0.06 mmHg . min . ml-1), LVEDP (-3.6 +/- 1 mmHg), and responses to ACh (-16 +/- 4 mmHg) and augmented responses to L-NAME (14 +/- 3 mmHg) but did not alter other variables. Therefore, 17beta-estradiol reduces preload and afterload and restores the vasodilator role of basal nitric oxide in ovariectomized rats with chronic heart failure.

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.

Vasomotor responses in chronically hyperperfused and hypoperfused rat mesenteric arteries

We evaluated the reactivity of small arteries after remodeling induced by elevated or reduced blood flow. In 6-wk-old rats, every other first-order side branch of the superior mesenteric artery was ligated near the bifurcation of second-order branches. Four weeks after surgery, vessels that had been exposed to high flow (HF) or low flow (LF) were isolated and mounted in a pressure myograph at 100 mmHg and were compared with vessels from sham-operated rats (Sham). In HF: 1) basal lumen diameter was increased; 2) sensitivity to norepinephrine, arginine vasopressin, and perivascular nerve stimulation was not modified; 3) maximal constrictor responses (delta diameter) to these stimuli and 125 mM K+ were increased; and 4) sensitivity and maximal dilator responses to sodium nitroprusside, acetylcholine, and flow were not modified. In LF: 1) basal diameter was reduced; 2) sensitivity to constrictor stimuli was not altered; 3) maximal responses to all vasoconstrictors except arginine vasopressin were reduced; and 4) sensitivity but not maximal dilator responses to sodium nitroprusside and acetylcholine was reduced. During acute flow-induced dilatations, lower shear stress was maintained in HF (48 +/- 7 dyn/cm2) than in Sham (63 +/- 10 dyn/cm2), but no shear stress regulation was observed in LF. These observations indicate that arterial structural responses to altered blood flow are accompanied by modified reactivity of the arterial smooth muscle, which entails changes in responsiveness to neurogenic and endothelium-dependent stimuli.