Nitroso-redox balance in control of coronary vasomotor tone

Reactive oxygen species (ROS) are essential in vascular homeostasis but may contribute to vascular dysfunction when excessively produced. Superoxide anion (O(2)(·-)) can directly affect vascular tone by reacting with K(+) channels and indirectly by reacting with nitric oxide (NO), thereby scavenging NO and causing nitroso-redox imbalance. After myocardial infarction (MI), oxidative stress increases, favoring the imbalance and resulting in coronary vasoconstriction. Consequently, we hypothesized that ROS scavenging results in coronary vasodilation, particularly after MI, and is enhanced after inhibition of NO production. Chronically instrumented swine were studied at rest and during exercise before and after scavenging of ROS with N-(2-mercaptoproprionyl)-glycine (MPG, 20 mg/kg iv) in the presence or absence of prior inhibition of endothelial NO synthase (eNOS) with N(ω)-nitro-L-arginine (L-NNA, 20 mg/kg iv). In normal swine, MPG resulted in coronary vasodilation as evidenced by an increased coronary venous O(2) tension, and trends toward increased coronary venous O(2) saturation and decreased myocardial O(2) extraction. These effects were not altered by prior inhibition of eNOS. In MI swine, MPG showed a significant vasodilator effect, which surprisingly was abolished by prior inhibition of eNOS. Moreover, eNOS dimer/monomer ratio was decreased after MI, reflecting eNOS uncoupling. In conclusion, ROS exert a small coronary vasoconstrictor influence in normal swine, which does not involve scavenging of NO. This vasoconstrictor influence of ROS is slightly enhanced after MI. Since inhibition of eNOS abolished rather than augmented the vasoconstrictor influence of ROS in swine with MI, while eNOS dimer/monomer ratio was decreased, our data imply that uncoupled eNOS may be a significant source of O(2)(·-) after MI.     

Coronary blood flow regulation in exercising swine involves parallel rather than redundant vasodilator pathways

In dogs, only combined blockade of vasodilator pathways [via adenosine receptors, nitric oxide synthase (NOS) and ATP-sensitive K+ (KATP) channels] results in impairment of metabolic vasodilation, which suggests a redundancy design of coronary flow regulation. Conversely, in swine and humans, blocking KATP channels, adenosine receptors, or NOS each impairs coronary blood flow (CBF) at rest and during exercise. Consequently, we hypothesized that these vasodilators act in parallel rather than in redundancy to regulate CBF in swine. Swine exercised on a treadmill (0-5 km/h), during control and after blockade of KATP channels (with glibenclamide), adenosine receptors [with 8-phenyltheophylline (8-PT)], and/or NOS [with Nomega-nitro-l-arginine (l-NNA)]. l-NNA, 8-PT, and glibenclamide each reduced myocardial O2 delivery and coronary venous O2 tension. These effects of l-NNA, 8-PT, and glibenclamide were not modified by simultaneous blockade of the other vasodilators. Combined blockade of KATP channels and adenosine receptors with or without NOS inhibition was associated with increased H+ production and impaired myocardial function. However, despite an increase in O2 extraction to >90% during administration of l-NNA + 8-PT + glibenclamide, vasodilator reserve could still be recruited during exercise. Thus in awake swine, loss of KATP channels, adenosine, or NO is not compensated for by increased participation of the other two vasodilator mechanisms. These findings suggest a parallel rather than a redundancy design of CBF regulation in the porcine circulation.     

NO and prostanoids blunt endothelin-mediated coronary vasoconstrictor influence in exercising swine

Withdrawal of the endothelin (ET)-mediated vasoconstrictor influence contributes to metabolic coronary vasodilation during exercise. Because production of nitric oxide (NO) and prostanoids increases with increasing shear stress and because NO and prostanoids are able to modify the release of ET, we hypothesized that the withdrawal of ET-mediated coronary vasoconstriction during exercise is mediated through NO and/or prostanoids. To test this hypothesis, 19 chronically instrumented swine were studied at rest and while running on a treadmill up to 85-90% of maximal heart rate. Blockade of ET(A)/ET(B) receptors with tezosentan resulted in an increase in coronary venous O(2) levels (i.e., in coronary vasodilation) at rest, which waned at increasing levels of exercise intensity. Inhibition of either NO synthase [N(omega)-nitro-l-arginine (l-NNA)] or cyclooxygenase (indomethacin) did not affect the response to tezosentan under resting conditions but unmasked a vasodilator response to tezosentan during exercise. The vasodilator response to tezosentan during exercise increased progressively after combined administration of l-NNA and indomethacin. These findings suggest that NO and prostanoids act synergistically to inhibit the vasoconstrictor influence of ET on the coronary circulation during exercise, thereby facilitating the exercise-induced vasodilation of coronary resistance vessels.     

Interaction between prostanoids and nitric oxide in regulation of systemic, pulmonary, and coronary vascular tone in exercising swine

Prostacyclin and nitric oxide (NO) are produced by the endothelium in response to physical forces such as shear stress. Consequently, both NO and prostacyclin may increase during exercise and contribute to metabolic vasodilation. Conversely, NO has been hypothesized to inhibit prostacyclin production. We therefore investigated the effect of cyclooxygenase (COX) inhibition on exercise-induced vasodilation of the porcine systemic, pulmonary, and coronary beds before and after inhibition of NO production. Swine were studied at rest and during treadmill exercise at 1-5 km/h, before and after COX inhibition with indomethacin (10 mg/kg iv), and in the absence and presence of NO synthase inhibition with N(omega)-nitro-l-arginine (l-NNA; 20 mg/kg iv). COX inhibition produced systemic vasoconstriction at rest, which waned during exercise. The systemic vasoconstriction by COX inhibition was enhanced after l-NNA, particularly at rest. In the coronary circulation, COX inhibition also resulted in vasoconstriction at rest and during exercise. However, vasoconstriction was not modified by pretreatment with l-NNA. In contrast, COX inhibition had no effect on the pulmonary circulation, either at rest or during exercise. Moreover, a prostanoid influence in the pulmonary circulation could not be detected after l-NNA. In conclusion, endogenous prostanoids contribute importantly to systemic and coronary tone in awake swine at rest but are not mandatory for exercise-induced vasodilation in these beds. Endogenous prostanoids are not mandatory for the regulation of pulmonary resistance vessel tone. Finally, NO blunts the contribution of prostanoids to vascular tone regulation in the systemic but not in the coronary and pulmonary beds.     

Integrative control of coronary resistance vessel tone by endothelin and angiotensin II is altered in swine with a recent myocardial infarction

Several studies have indicated an interaction between the renin-angiotensin (ANG II) system and endothelin (ET) in the regulation of vascular tone. Previously, we have shown that both ET and ANG II exert a vasoconstrictor influence on the coronary resistance vessels of awake normal swine. Here, we investigated whether the interaction between ANG II and ET exists in the control of coronary resistance vessel tone at rest and during exercise using single and combined blockade of angiotensin type 1 (AT(1)) and ET(A)/ET(B) receptors. Since both circulating ANG II and ET levels are increased after myocardial infarction (MI), we investigated if the interaction between these systems is altered after MI. In awake healthy swine, coronary vasodilation in response to ET(A)/ET(B) receptor blockade in the presence of AT(1) blockade was similar to vasodilation produced by ET(A)/ET(B) blockade under control conditions. In awake swine with a 2- to 3-wk-old MI, coronary vasodilator responses to individual AT(1) and ET(A)/ET(B) receptor blockade were virtually abolished, despite similar coronary arteriolar AT(1) and ET(A) receptor expression compared with normal swine. Unexpectedly, in the presence of AT(1) blockade (which had no effect on circulating ET levels), ET(A)/ET(B) receptor blockade elicited a coronary vasodilator response. These findings suggest that in normal healthy swine the two vasoconstrictor systems contribute to coronary resistance vessel control in a linear additive manner, i.e., with negligible cross-talk. In contrast, in the remodeled myocardium, cross-talk between ANG II and ET emerges, resulting in nonlinear redundant control of coronary resistance vessel tone.     

Control of internal temperature threshold for active cutaneous vasodilation by dynamic exercise.

Exercise induces shifts in the internal temperature threshold at which cutaneous vasodilation begins. To find whether this shift is accomplished through the vasoconstrictor system or the cutaneous active vasodilator system, two forearm sites (0.64 cm2) in each of 11 subjects were iontophoretically treated with bretylium tosylate to locally block adrenergic vasoconstrictor control. Skin blood flow was monitored by laser-Doppler flowmetry (LDF) at those sites and at two adjacent untreated sites. Mean arterial pressure (MAP) was measured noninvasively. Cutaneous vascular conductance was calculated as LDF/MAP. Forearm sweat rate was also measured in seven of the subjects by dew point hygrometry. Whole body skin temperature was raised to 38 degrees C, and supine bicycle ergometer exercise was then performed for 7-10 min. The internal temperature at which cutaneous vasodilation began was recorded for all sites, as was the temperature at which sweating began. The same subjects also participated in studies of heat stress without exercise to obtain vasodilator and sudomotor thresholds from rest. The internal temperature thresholds for cutaneous vasodilation were higher during exercise at both bretylium-treated (36.95 +/- 0.07 degrees C rest, 37.20 +/- 0.04 degrees C exercise, P less than 0.05) and untreated sites (36.95 +/- 0.06 degrees C rest, 37.23 +/- 0.05 degrees C exercise, P less than 0.05). The thresholds for cutaneous vasodilation during rest or during exercise were not statistically different between untreated and bretylium-treated sites (P greater than 0.05). The threshold for the onset of sweating was not affected by exercise (P greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of endothelin in alpha-adrenoceptor coronary vasoconstriction

It has been proposed that alpha-adrenoceptor vasoconstriction in coronary resistance vessels results not from alpha-adrenoceptors on coronary smooth muscle but from alpha-adrenoceptors on cardiac myocytes that stimulate endothelin (ET) release. The present experiments tested the hypothesis that the alpha-adrenoceptor-mediated coronary vasoconstriction that normally occurs during exercise is due to endothelin. In conscious dogs (n = 10), the endothelin ET(A)/ET(B) receptor antagonist tezosentan (1 mg/kg iv) increased coronary venous oxygen tension at rest but not during treadmill exercise. This result indicates that basal endothelin levels produce a coronary vasoconstriction at rest that is not observed during the coronary vasodilation during exercise. In contrast, the alpha-adrenoceptor antagonist phentolamine increased coronary venous oxygen tension during exercise but not at rest. The difference between the endothelin blockade and alpha-adrenoceptor blockade results indicates that alpha-adrenoceptor coronary vasoconstriction during exercise is not due to endothelin. However, in anesthetized dogs, bolus intracoronary injections of the alpha-adrenoceptor agonist phenylephrine produced reductions in coronary blood flow that were partially antagonized by endothelin receptor blockade with tezosentan. These results are best explained if alpha-adrenoceptor-induced endothelin release requires high pharmacological concentrations of catecholamines that are not reached during exercise.     

KCa+ channels contribute to exercise-induced coronary vasodilation in swine

Coronary blood flow is controlled via several vasoactive mediators that exert their effect on coronary resistance vessel tone through activation of K(+) channels in vascular smooth muscle. Because Ca(2+)-activated K(+) (K(Ca)(+)) channels are the predominant K(+) channels in the coronary vasculature, we hypothesized that K(Ca)(+) channel activation contributes to exercise-induced coronary vasodilation. In view of previous observations that ATP-sensitive K(+) (K(ATP)(+)) channels contribute, in particular, to resting coronary resistance vessel tone, we additionally investigated the integrated control of coronary tone by K(Ca)(+) and K(ATP)(+) channels. For this purpose, the effect of K(Ca)(+) blockade with tetraethylammonium (TEA, 20 mg/kg iv) on coronary vasomotor tone was assessed in the absence and presence of K(ATP)(+) channel blockade with glibenclamide (3 mg/kg iv) in chronically instrumented swine at rest and during treadmill exercise. During exercise, myocardial O(2) delivery increased commensurately with the increase in myocardial O(2) consumption, so that myocardial O(2) extraction and coronary venous Po(2) (Pcv(O(2))) were maintained constant. TEA (in a dose that had no effect on K(ATP)(+) channels) had a small effect on the myocardial O(2) balance at rest and blunted the exercise-induced increase in myocardial O(2) delivery, resulting in a progressive decrease of Pcv(O(2)) with increasing exercise intensity. Conversely, at rest glibenclamide caused a marked decrease in Pcv(O(2)) that waned at higher exercise levels. Combined K(Ca)(+) and K(ATP)(+) channel blockade resulted in coronary vasoconstriction at rest that was similar to that caused by glibenclamide alone and that was maintained during exercise, suggesting that K(Ca)(+) and K(ATP)(+) channels act in a linear additive fashion. In conclusion, K(Ca)(+) channel activation contributes to the metabolic coronary vasodilation that occurs during exercise. Furthermore, in swine K(Ca)(+) and K(ATP)(+) channels contribute to coronary resistance vessel control in a linear additive fashion.     

Contribution of KATP+ channels to coronary vasomotor tone regulation is enhanced in exercising swine with a recent myocardial infarction

Previous studies demonstrated a decreased flow reserve in the hypertrophied myocardium early after myocardial infarction (MI). Previously, we reported that exacerbation of hemodynamic abnormalities and neurohumoral activation during exercise caused slight impairment of myocardial O(2) supply in swine with a recent MI. We hypothesized that increased metabolic coronary vasodilation [via ATP-sensitive K(+) (K(ATP)(+)) channels and adenosine] may have partially compensated for the increased extravascular compressive forces and increased vasoconstrictor neurohormones, thereby preventing a more severe impairment of myocardial O(2) balance. Chronically instrumented swine were exercised on a treadmill up to 85% of maximum heart rate. Under resting conditions, adenosine receptor blockade [8-phenyltheophylline (8-PT), 5 mg/kg i.v.] and K(ATP)(+) channel blockade (glibenclamide, 3 mg/kg i.v.) produced similar decreases in myocardial O(2) supply in normal and MI swine. However, while glibenclamide's effect waned in normal swine during exercise (P < 0.05), it was maintained in MI swine. 8-PT's effect was maintained during exercise and was not different between normal and MI swine. Finally, in normal swine combined treatment with 8-PT and glibenclamide produced a vasoconstrictor response that equaled the sum of the responses to blockade of the individual pathways. In contrast, in MI swine the vasoconstrictor response to 8-PT and glibenclamide was similar to that produced by glibenclamide alone. In conclusion, despite significant hemodynamic abnormalities in swine with a recent MI, myocardial O(2) supply and O(2) consumption in remodeled myocardium are still closely matched during exercise. This close matching is supported by increased K(ATP)(+) channel-mediated coronary vasodilation. Although the net vasodilator influence of adenosine was unchanged in remodeled myocardium, it became exclusively dependent on K(ATP)(+) channel opening.     

Feedforward sympathetic coronary vasodilation in exercising dogs.

The hypothesis that exercise-induced coronary vasodilation is a result of sympathetic activation of coronary smooth muscle beta-adrenoceptors was tested. Ten dogs were chronically instrumented with a flow transducer on the circumflex coronary artery and catheters in the aorta and coronary sinus. During treadmill exercise, coronary venous oxygen tension decreased with increasing myocardial oxygen consumption, indicating an imperfect match between myocardial blood flow and oxygen consumption. This match was improved after alpha-adrenoceptor blockade with phentolamine but was significantly worse than control after alpha + beta-adrenoceptor blockade with phentolamine plus propranolol. The response after alpha-adrenoceptor blockade included local metabolic vasodilation plus a beta-adrenoceptor vasodilator component, whereas the response after alpha + beta-adrenoceptor blockade contained only the local metabolic vasodilator component. The large difference in coronary venous oxygen tensions during exercise between alpha-adrenoceptor blockade and alpha + beta-adrenoceptor blockade indicates that there is significant feedforward beta-adrenoceptor coronary vasodilation in exercising dogs. Coronary venous and estimated myocardial interstitial adenosine concentrations did not increase during exercise before or after alpha + beta-adrenoceptor blockade, indicating that adenosine levels did not increase to compensate for the loss of feedforward beta-adrenoceptor-mediated coronary vasodilation. These results indicate a meaningful role for feedforward beta-receptor-mediated sympathetic coronary vasodilation during exercise.     

K(ATP)(+) channels, nitric oxide, and adenosine are not required for local metabolic coronary vasodilation

The role of ATP-sensitive K(+) (K(ATP)(+)) channels, nitric oxide, and adenosine in coronary exercise hyperemia was investigated. Dogs (n = 10) were chronically instrumented with catheters in the aorta and coronary sinus and instrumented with a flow transducer on the circumflex coronary artery. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous plasma concentrations using a previously tested mathematical model. Experiments were conducted at rest and during graded treadmill exercise with and without combined inhibition of K(ATP)(+) channels (glibenclamide, 1 mg/kg iv), nitric oxide synthesis (N(omega)-nitro-L-arginine, 35 mg/kg iv), and adenosine receptors (8-phenyltheophylline, 3 mg/kg iv). During control exercise, myocardial oxygen consumption increased ~2.9-fold, coronary blood flow increased ~2.6-fold, and coronary venous oxygen tension decreased from 19.9 +/- 0.4 to 13.7 +/- 0.6 mmHg. Triple blockade did not significantly change the myocardial oxygen consumption or coronary blood flow response during exercise but lowered the resting coronary venous oxygen tension to 10.0 +/- 0.4 mmHg and during exercise to 6.2 +/- 0.5 mmHg. Cardiac adenosine levels did not increase sufficiently to overcome the adenosine receptor blockade. These results indicate that combined inhibition of K(ATP)(+) channels, nitric oxide synthesis, and adenosine receptors lowers the balance between total oxygen supply and consumption at rest but that these factors are not required for local metabolic coronary vasodilation during exercise.     

Coronary vasoconstrictor influence of angiotensin II is reduced in remodeled myocardium after myocardial infarction

The renin-angiotensin system plays an important role in cardiovascular homeostasis by contributing to the regulation of blood volume, blood pressure, and vascular tone. Because AT(1) receptors have been described in the coronary microcirculation, we investigated whether ANG II contributes to the regulation of coronary vascular tone and whether its contribution is altered during exercise. Since the renin-angiotensin system is activated after myocardial infarction, resulting in an increase in circulating ANG II, we also investigated whether the contribution of ANG II to the regulation of vasomotor tone is altered after infarction. Twenty-six chronically instrumented swine were studied at rest and while running on a treadmill at 1-4 km/h. In 13 swine, myocardial infarction was induced by ligation of the left circumflex coronary artery. Blockade of AT(1) receptors (irbesartan, 1 mg/kg iv) had no effect on myocardial O(2) consumption but resulted in an increase in coronary venous O(2) tension and saturation both at rest and during exercise, reflecting coronary vasodilation. Despite increased plasma levels of ANG II after infarction and maintained coronary arteriolar AT(1) receptor levels, the vasodilation evoked by irbesartan was significantly reduced both at rest and during exercise. In conclusion, despite elevated plasma levels, the vasoconstrictor influence of ANG II on the coronary circulation in vivo is reduced after myocardial infarction. This reduction in ANG II-induced coronary vasoconstriction may serve to maintain perfusion of the remodeled myocardium.     

Contribution of endothelin to coronary vasomotor tone is abolished after myocardial infarction

Left ventricular dysfunction in swine with a recent myocardial infarction (MI) is associated with neurohumoral activation, including increased catecholamines and endothelin (ET). Although the increase in ET may serve to maintain blood pressure and, hence, perfusion of essential organs such as the heart and brain, it could also compromise myocardial perfusion by evoking coronary vasoconstriction. In the present study, we tested the hypothesis that endogenous ET contributes to perturbations in myocardial O2 balance during exercise in remodeled myocardium of swine with a recent MI. For this purpose, 26 chronically instrumented swine (10 with and 16 without MI) were studied at rest and while running on a treadmill at 1-4 km/h. After MI, plasma ET increased from 3.2 +/- 0.4 to 4.9 +/- 0.3 pM (P < 0.05). In normal swine, blockade of ETA (by EMD-122946) or ETA-ETB (by tezosentan) receptors resulted in an increase in coronary venous PO2, i.e., coronary vasodilation at rest, which decreased during exercise. In contrast, neither ETA nor ETA-ETB receptor blockade resulted in coronary vasodilation in swine with MI. Coronary vasoconstriction to intravenous ET-1 infusion in awake resting swine was blunted after MI. To investigate whether factors released by cardiac myocytes contributed to decreased vascular responsiveness to ET, we performed ET-1 dose-response curves in isolated coronary arterioles (70-200 microm). Vasoconstriction to ET-1 in isolated arterioles from MI swine was enhanced. In conclusion, the vasoconstrictor influence of endogenous as well as exogenous ET on coronary circulation in vivo is reduced. Because the response of isolated coronary arterioles to ET is increased after MI, the reduced vasoconstrictor influence in vivo suggests modulation of ET receptor sensitivity by cardiac myocytes, which may serve to maintain adequate myocardial perfusion.     

Role of beta 2-adrenergic receptors on coronary resistance during exercise

The effects of regional alpha- and specific beta 2-adrenergic receptor blockade on measurements of late diastolic coronary resistance (LDCR) and mean coronary blood flow velocity (CBFV) during exercise were examined in 14 conscious adult mongrel dogs. Specific beta 2-adrenergic receptor blockade (ICI 118.551) significantly decreased CBFV and increased LDCR by blockade of beta 2-vasodilator tone independent of alpha-adrenergic receptor-mediated tone and independent of altering myocardial metabolism. alpha-Adrenergic receptor blockade (phentolamine, 1 mg) significantly increased CBFV and decreased LDCR by blocking sympathetically mediated vasoconstrictor tone. There was no significant difference in the magnitude of response between alpha- and beta 2-adrenergic receptor blockade. These results demonstrate that alpha- and beta 2-adrenergic receptors have a significant and evidently equal influence on CBFV and LDCR during exercise. Four weeks of daily exercise and left stellate ganglionectomy (LSGx) prevented phentolamine-induced vasodilation but not ICI 118.551-induced vasoconstriction. This suggests that daily exercise and LSGx significantly decreased the alpha-adrenergic receptor-mediated vasoconstrictor tone on the coronary circulation, resulting in an apparently greater role for the coronary vascular beta 2-adrenergic receptor on the control of CBFV and LDCR during exercise.     

The involvement of norepinephrine, neuropeptide Y, and nitric oxide in the cutaneous vasodilator response to local heating in humans.

Presynaptic blockade of cutaneous vasoconstrictor nerves (VCN) abolishes the axon reflex (AR) during slow local heating (SLH) and reduces the vasodilator response. In a two-part study, forearm sites were instrumented with microdialysis fibers, local heaters, and laser-Doppler flow probes. Sites were locally heated from 33 to 40 degrees C over 70 min. In part 1, we tested whether this effect of VCN acted via nitric oxide synthase (NOS). In five subjects, treatments were as follows: 1) untreated; 2) bretylium, preventing neurotransmitter release; 3) N(G)-nitro-L-arginine methyl ester (L-NAME) to inhibit NOS; and 4) combined bretylium + L-NAME. At treated sites, the AR was absent, and there was an attenuation of the ultimate vasodilation (P < 0.05), which was not different among those sites (P > 0.05). In part 2, we tested whether norepinephrine and/or neuropeptide Y is involved in the cutaneous vasodilator response to SLH. In seven subjects, treatments were as follows: 1) untreated; 2) propranolol and yohimbine to antagonize alpha- and beta-receptors; 3) BIBP-3226 to antagonize Y(1) receptors; and 4) combined propranolol + yohimbine + BIBP-3226. Treatment with propranolol + yohimbine or BIBP-3226 significantly increased the temperature at which AR occurred (n = 4) or abolished it (n = 3). The combination treatment consistently eliminated it. Importantly, ultimate vasodilation with SLH at the treated sites was significantly (P < 0.05) less than at the control. These data suggest that norepinephrine and neuropeptide Y are important in the initiation of the AR and for achieving a complete vasodilator response. Since VCN and NOS blockade in combination do not have an inhibition greater than either alone, these data suggest that VCN promote heat-induced vasodilation via a nitric oxide-dependent mechanism.     

Prostanoids suppress the coronary vasoconstrictor influence of endothelin after myocardial infarction

Myocardial infarction (MI) is associated with endothelial dysfunction resulting in an imbalance in endothelium-derived vasodilators and vasoconstrictors. We have previously shown that despite increased endothelin (ET) plasma levels, the coronary vasoconstrictor effect of endogenous ET is abolished after MI. In normal swine, nitric oxide (NO) and prostanoids modulate the vasoconstrictor effect of ET. In light of the interaction among NO, prostanoids, and ET combined with endothelial dysfunction present after MI, we investigated this interaction in control of coronary vasomotor tone in the remote noninfarcted myocardium after MI. Studies were performed in chronically instrumented swine (18 normal swine; 13 swine with MI) at rest and during treadmill exercise. Furthermore, endothelial nitric oxide synthase (eNOS) and cyclooxygenase protein levels were measured in the anterior (noninfarcted) wall of six normal and six swine with MI. eNOS inhibition with N(ω)-nitro-L-arginine (L-NNA) and cyclooxygenase inhibition with indomethacin each resulted in coronary vasoconstriction at rest and during exercise, as evidenced by a decrease in coronary venous oxygen levels. The effect of l-NNA was slightly decreased in swine with MI, although eNOS expression was not altered. Conversely, in accordance with the unaltered expression of cyclooxygenase-1 after MI, the effect of indomethacin was similar in normal and MI swine. L-NNA enhanced the vasodilator effect of the ET(A/B) receptor blocker tezosentan but exclusively during exercise in both normal and MI swine. Interestingly, this effect of L-NNA was blunted in MI compared with normal swine. In contrast, whereas indomethacin increased the vasodilator effect of tezosentan only during exercise in normal swine, indomethacin unmasked a coronary vasodilator effect of tezosentan in MI swine both at rest and during exercise. In conclusion, the present study shows that endothelial control of the coronary vasculature is altered in post-MI remodeled myocardium. Thus the overall vasodilator influences of NO as well as its inhibition of the vasoconstrictor influence of ET on the coronary resistance vessels were reduced after MI. In contrast, while the overall prostanoid vasodilator influence was maintained, its inhibition of ET vasoconstrictor influences was enhanced in post-MI remote myocardium.     

Nitric oxide production is maintained in exercising swine with chronic left ventricular dysfunction

Left ventricular (LV) dysfunction caused by myocardial infarction (MI) is accompanied by endothelial dysfunction, most notably a loss of nitric oxide (NO) availability. We tested the hypothesis that endothelial dysfunction contributes to impaired tissue perfusion during increased metabolic demands as produced by exercise, and we determined the contribution of NO to regulation of regional systemic, pulmonary, and coronary vasomotor tone in exercising swine with LV dysfunction produced by a 2- to 3-wk-old MI. LV dysfunction resulted in blunted systemic and coronary vasodilator responses to ATP, whereas the responses to nitroprusside were maintained. Exercise resulted in blunted systemic and pulmonary vasodilator responses in MI that resembled the vasodilator responses in normal (N) swine following blockade of NO synthase with N(omega)-nitro-L-arginine (L-NNA, 20 mg/kg iv). However, L-NNA resulted in similar decreases in systemic (43 +/- 3% in N swine and 49 +/- 4% in MI swine), pulmonary (45 +/- 5% in N swine and 49 +/- 4% in MI swine), and coronary (28 +/- 4% in N and 35 +/- 3% in MI) vascular conductances in N and MI swine under resting conditions; similar effects were observed during treadmill exercise. Selective inhibition of inducible NO synthase with aminoguanidine (20 mg/kg iv) had no effect on vascular tone in MI. These findings indicate that while agonist-induced vasodilation is already blunted early after myocardial infarction, the contribution of endothelial NO synthase-derived NO to regulation of vascular tone under basal conditions and during exercise is maintained.     

Acute adaptations of the coronary circulation to exercise

Coronary blood flow is tightly coupled to myocardial oxygen consumption to maintain a consistently high level of myocardial oxygen extraction. This tight coupling has been proposed to depend on periarteriolar, oxygen tension, signals released from cardiomyocytes (adenosine acting on K _ATP ⁺ channels), and/or the endothelium (prostanoids, nitric oxide, endothelin [ET]) and autonomic influences (catecholamines), but the contribution of each of these regulatory pathways and their interactions are still incompletely understood. Until recently, experimental studies into the regulation of coronary blood flow during exercise were principally performed in the dog. We have performed several studies on the regulation of vasomotor tone in coronary resistance vessels in chronically instrumented exercising swine. These studies have shown that the coronary resistance vessels in swine lack significant α-adrenergic control, but that these vessels are subject to β-adrenergic feed-forward control during exercise, which is aided by a parasympathetic withdrawal. In addition, withdrawal of an ET-mediated vasoconstrictor influence also contributes to exercise-induced coronary vasodilation. Coronary blood flow regulation by endothelial and metabolic vasodilator pathways contributes to resting vasomotor tone regulation but does not appear to contribute to the exercise-induced coronary vasodilation. Furthermore, blockade of one vasodilator pathway is not compensated by an increased contribution of the other vasodilator mechanisms, suggesting that porcine coronary vasomotor control by endothelial and metabolic factors occurs in a linear additive rather than a nonlinear synergistic fashion.     

Exercise preserves endothelium-dependent relaxation in coronary arteries of hypercholesterolemic male pigs.

We tested the hypothesis that exercise training (Ex) attenuates hypercholesterolemia-induced impairment of endothelium-dependent relaxation (EDR) in male porcine coronary arteries [left anterior descending coronary arteries (LAD)] by increasing nitric oxide (NO) release [due to increased endothelial NO synthase (NOS) expression] and/or increased bioactivity of NO. Adult male pigs were fed a normal-fat (NF) or high-fat (HF) diet for 20-24 wk. Pigs were Ex or remained sedentary (Sed) for 16-20 wk, beginning after 4 wk on diet. Four groups of pigs were used: NF-Sed, NF-Ex, HF-Sed, and HF-Ex. HF enhanced LAD contractions induced by KCl, aggregating platelets (AP), and serotonin (5-HT). AP and 5-HT produced EDR after blockade of cyclooxygenase with indomethacin (Indo) and smooth-muscle 5-HT(2) receptors with ketanserin. HF impaired EDR induced by AP, 5-HT, and bradykinin. Results indicate a decreased contribution of NO to EDR in HF-Sed LADs, because the percentage of bradykinin-induced EDR inhibited by N(G)-nitro-L-arginine methyl ester was 27% in NF-Sed and 34% in NF-Ex but only 17% in HF-Sed. Also, N(G)-nitro-L-arginine methyl ester + Indo results indicate that release of an Indo-sensitive vasoconstrictor contributes to blunted EDR in HF-Sed LAD. Immunoblot and immunohistochemistry results indicate the following: 1) LAD endothelial NOS protein content was similar among groups; 2) HF decreased LAD superoxide dismutase (SOD) but increased caveolin-1 content; and 3) Ex increased SOD content of HF LADs. We conclude that HF impairs EDR by impairing the contribution of NO released from NOS (due to decreased SOD and increased caveolin-1 protein content) and by production of an Indo-sensitive vasoconstrictor. Ex preserves EDR in HF LADs by decreasing the production of the constrictor and increasing NO-release by NOS and/or NO bioactivity and bioavailability.     

Vasomotor control in mice overexpressing human endothelial nitric oxide synthase

Nitric oxide (NO) plays a key role in regulating vascular tone. Mice overexpressing endothelial NO synthase [eNOS-transgenic (Tg)] have a 20% lower systemic vascular resistance (SVR) than wild-type (WT) mice. However, because eNOS enzyme activity is 10 times higher in tissue homogenates from eNOS-Tg mice, this in vivo effect is relatively small. We hypothesized that the effect of eNOS overexpression is attenuated by alterations in NO signaling and/or altered contribution of other vasoregulatory pathways. In isoflurane-anesthetized open-chest mice, eNOS inhibition produced a significantly greater increase in SVR in eNOS-Tg mice compared with WT mice, consistent with increased NO synthesis. Vasodilation to sodium nitroprusside (SNP) was reduced, whereas the vasodilator responses to phosphodiesterase-5 blockade and 8-bromo-cGMP (8-Br-cGMP) were maintained in eNOS-Tg compared with WT mice, indicating blunted responsiveness of guanylyl cyclase to NO, which was supported by reduced guanylyl cyclase activity. There was no evidence of eNOS uncoupling, because scavenging of reactive oxygen species (ROS) produced even less vasodilation in eNOS-Tg mice, whereas after eNOS inhibition the vasodilator response to ROS scavenging was similar in WT and eNOS-Tg mice. Interestingly, inhibition of other modulators of vascular tone [including cyclooxygenase, cytochrome P-450 2C9, endothelin, adenosine, and Ca-activated K(+) channels] did not significantly affect SVR in either eNOS-Tg or WT mice, whereas the marked vasoconstrictor responses to ATP-sensitive K(+) and voltage-dependent K(+) channel blockade were similar in WT and eNOS-Tg mice. In conclusion, the vasodilator effects of eNOS overexpression are attenuated by a blunted NO responsiveness, likely at the level of guanylyl cyclase, without evidence of eNOS uncoupling or adaptations in other vasoregulatory pathways.