Dilation Of Small Arterioles Research Articles

5-HT7 receptors mediate dilation of rat cremaster muscle arterioles in vivo.

Serotonin (5-HT) infusion in vivo causes hypotension and a fall in total peripheral resistance. However, the vascular segment and the receptors that mediate this response remain in question. We hypothesized that 5-HT7 receptors mediate arteriolar dilation to 5-HT in skeletal muscle microcirculation. Cremaster muscles of isoflurane-anesthetized male Sprague-Dawley rats were prepared for in vivo microscopy of third- and fourth-order arterioles and superfused with physiological salt solution at 34°C. Quantitative real-time PCR (RT-PCR) was applied to pooled samples of first- to third-order cremaster arterioles (2-4 rats/sample) to evaluate 5-HT7 receptor expression. Topical 5-HT (1-10 nmols) or the 5-HT1/7 receptor agonist, 5-carboxamidotryptamine (10-30 nM), dilated third- and fourth-order arterioles, responses that were abolished by 1 μM SB269970, a selective 5-HT7 receptor antagonist. In contrast, dilation induced by the muscarinic agonist, methacholine (100 nmols) was not inhibited by SB269970. Serotonin (10 nmols) failed to dilate cremaster arterioles in 5-HT7 receptor knockout rats whereas arterioles in wild-type litter mates dilated to 1 nmol 5-HT, a response blocked by 1 μM SB269970. Quantitative RT-PCR revealed that cremaster arterioles expressed mRNA for 5-HT7 receptors. 5-HT7 receptors mediate dilation of small arterioles in skeletal muscle and likely contribute to 5-HT-induced hypotension, in vivo.

5‐HT7 Receptors Mediate Dilation of Rat Cremaster Muscle Arterioles in vivo

Serotonin (5‐HT) infusion in vivo causes hypotension and a fall in total peripheral resistance (TPR) that is mediated by 5‐HT7 receptors. 5‐HT dilates small arterioles in skeletal muscle, a microcirculation that importantly contributes to TPR. However, the receptors that mediate this dilation have not been established. Therefore, we tested the hypothesis that 5‐HT7 receptors mediate the arteriolar dilation in the skeletal muscle microcirculation. Cremaster muscles of isoflurane‐anesthetized male Sprague‐Dawley rats were prepared for digital in vivo microscopy and superfused with physiological salt solution (PSS) at 34 °C. Bolus (1‐10 nmols) application of 5‐HT into the superfusate caused substantial dilation of 3rd to 5th order arterioles from 10 ± 1 to 27 ± 1 μm (n = 10 arterioles from 4 rats, p <0.0001) that recovered within 2‐3 min. Dilation to bolus application of 5‐HT was inhibited by superfusion with 1 μM SB269970, a selective 5‐HT7 receptor antagonist (diameter in SB = 9 ± 1 μm; diameter in SB + 5‐HT = 10 ± 1 μm; n = 10 arterioles from 4 rats; p = 0.8763). Similarly, steady‐state superfusion with 10‐30 nM of the 5‐HT1/7 receptor agonist 5‐carboxamidotryptamine (5‐CT) dilated 3rd‐5th‐order arterioles from 16 ± 3 to 37 ± 5 μm (n = 7 arterioles from 3 rats, p = 0.0004). This dilation was reversed (diameter in 1 μM SB269970 = 13 ± 2 μm, n = 7, p = 0.1479 vs. baseline) or blocked (diameter in 1 μM SB269970 + 10‐30 nM 5‐CT = 10 ± 2 μm, n = 7, p = 0.0005 vs. 5‐CT alone) by 1 μM SB269970. Dilation of arterioles to the muscarinic agonist, methacholine (MCH) was not inhibited by SB269970 verifying the specificity of the antagonist; arterioles dilated from 13 ± 2 μm to 25 ± 2 μm to 100 nmols MCH and dilated from 9 ± 2 μm to 32 ± 2 μm in response to MCH in the presence of 1 μM SB269970; n = 10 arterioles from 4 rats; p = 0.7232 vs. dilation in the absence of SB269970. Consistent with these data, pooled samples (4‐5 rats) of 1st and 2nd – order cremaster arterioles expressed message for 5‐HT7 receptors by quantitative real‐time PCR (CT = 36.4 ± 0.74, n = 3 pooled samples, p = 0.039 vs CT = 40). These data support our hypothesis that 5‐HT7 receptors mediate dilation of small arterioles in skeletal muscle and likely contribute to the observed hypotension when these receptors are engaged in vivo.

Ghrelin Preserves Ischemia-Induced Vasodilation of Male Rat Coronary Vessels Following β-Adrenergic Receptor Blockade.

Acute myocardial infarction (MI) triggers an adverse increase in cardiac sympathetic nerve activity (SNA). Whereas β-adrenergic receptor (β-AR) blockers are routinely used for the management of MI, they may also counter β-AR-mediated vasodilation of coronary vessels. We have reported that ghrelin prevents sympathetic activation following MI. Whether ghrelin modulates coronary vascular tone following MI, either through the modulation of SNA or directly as a vasoactive mediator, has never been addressed. We used synchrotron microangiography to image coronary perfusion and vessel internal diameter (ID) in anesthetized Sprague-Dawley rats, before and then again 30 minutes after induction of an MI (left coronary artery ligation). Rats were injected with either saline or ghrelin (150 µg/kg, subcutaneously), immediately following the MI or sham surgery. Coronary angiograms were also recorded following β-AR blockade (propranolol, 2 mg/kg, intravenously). Finally, wire myography was used to assess the effect of ghrelin on vascular tone in isolated human internal mammary arteries (IMAs). Acute MI enhanced coronary perfusion to nonischemicregions through dilation of small arterioles (ID 50 to 250 µm) and microvessel recruitment, irrespective of ghrelin treatment. In ghrelin-treated rats, β-AR blockade did not alter the ischemia-induced vasodilation, yet in saline-treated rats, β-AR blockade abolished the vasodilation of small arterioles. Finally, ghrelin caused a dose-dependent vasodilation of IMA rings (preconstricted with phenylephrine). In summary, this study highlights ghrelin as a promising adjunct therapy that can be used in combination with routine β-AR blockade treatment for preserving coronary blood flow and cardiac performance in patients who suffer an acute MI.

Local heat produces a shear-mediated biphasic response in the thermoregulatory microcirculation of the Pallid bat wing

Investigators report that local heat causes an increase in skin blood flow consisting of two phases. The first is solely sensory neural, and the second is nitric oxide mediated. We hypothesize that mechanisms behind these two phases are causally linked by shear stress. Because microvascular blood flow, endothelial shear stress, and vessel diameters cannot be measured in humans, bat wing arterioles (26.6 +/- 0.3, 42.0 +/- 0.4, and 58.7 +/- 2.2 microm) were visualized noninvasively on a transparent heat plate via intravital microscopy. Increasing plate temperature from 25 to 37 degrees C increased flow in all three arterial sizes (137.1 +/- 0.3, 251.9 +/- 0.5, and 184.3 +/- 0.6%) in a biphasic manner. With heat, diameter increased in large arterioles (n = 6) by 8.7 +/- 0.03% within 6 min, medium arterioles (n = 8) by 19.7 +/- 0.5% within 4 min, and small arterioles (n = 8) by 31.6 +/- 2.2% in the first minute. Lidocaine (0.2 ml, 2% wt/vol) and NG-nitro-L-arginine methyl ester (0.2 ml, 1% wt/vol) were applied topically to arterioles (approximately 40 microm) to block sensory nerves, modulate shear stress, and block nitric oxide generation. Local heat caused only a 10.4 +/- 5.5% increase in diameter with neural blockade (n = 8) and only a 7.5 +/- 4.1% increase in diameter when flow was reduced (n = 8), both significantly lower than control (P < 0.001). Diameter and flow increases were significantly reduced with NG-nitro-L-arginine methyl ester application (P < 0.05). Our novel thermoregulatory animal model illustrates 1) regulation of shear stress, 2) a nonneural component of the first phase, and 3) a shear-mediated second phase. The time course of dilation suggests that early dilation of small arterioles increases flow and enhances second-phase dilation of the large arterioles.

Response of the rat cremaster microcirculation to hemorrhage in vivo: differential effects of intravenous anesthetic agents.

Anesthetic agents are known to have differential effects on both the systemic circulation and the microcirculation. The aim of this study was to compare the effects of several intravenous (i.v.) agents on the microcirculatory response to hemorrhage. Male Wistar rats (n = 52) were anesthetized i.v. either with propofol and fentanyl (propofol fentanyl), ketamine, or thiopental. Cardiovascular variables were monitored. The cremaster muscle was observed by using fluorescent intravital microscopy. FITC-BSA was administered (0.25 mL/100 g, i.a.) to determine macromolecular leak, an index of vessel integrity. Animals were further allocated into control (C), 10% hemorrhage (H), or hemorrhage re-infusion (H-R, removal of 10% blood volume and then re-infusion of saline and blood) groups. When systolic arterial pressure (SAP) was maintained after hemorrhage, constriction of A3 and A4 arterioles (5-30 microm) was accompanied by no change in the diameter of A1 (80-130 microm): most frequent with ketamine (A1: -1.7 +/- 1.2; A4: -13.9 +/- 2.7%; H and H-R: n = 9/11). With lower SAP, dilation of the A3 and A4 was accompanied by constriction of the A1: most frequent with propofol/fentanyl (A1: -8.0 +/- 2.5; A4; 35.1 +/- 9.4%; H and H-R: n = 6/11). No increases in macromolecular leak occurred with any anesthetic agent or in H or H-R groups. The response of cremaster muscle microcirculation to hemorrhage differs with different i.v. anesthetic agents. Dilation of small arterioles is the predominant response with propofol/fentanyl and constriction of small arterioles with ketamine.

Diminished arteriolar responses in nitrate tolerance involve ROS and angiotensin II.

Our purpose was to evaluate hyporesponsivity to nitric oxide (NO)-induced dilation in small arterioles during nitrate tolerance. An Alza osmotic pump was implanted in the left flank of adult rats (n = 56) for continuous administration of nitroglycerin (140 microg/h) or vehicle (propylene glycol). On postoperative day 3, arcade (approximately 50-microm diameter) and terminal (approximately 20 microm) arterioles were observed in the cremaster preparation with in vivo video microscopy. Local vascular responses were obtained with micropipette-applied NO donors, with and without superoxide dismutase (SOD), Mn(III) tetrakis(4-benzoic acid) porphyrin chloride (MnTBAP), or losartan. On day 3, NO-mediated dilation was significantly attenuated in nitroglycerin-treated rats. Attenuation was greater in the terminal arterioles compared with the arcades. Control responses were restored by SOD, MnTBAP, or losartan, suggesting a role for elevated angiotensin II and reactive oxygen species (ROS) as mediators of the attenuated NO dilation (nitrate tolerance). Addition of losartan to the drinking water likewise prevented nitrate tolerance. In summary, terminal arterioles are affected by nitrates to a greater extent than the arcade arterioles that feed them, in a process dependent on angiotensin II and ROS.

Differential inhibition of functional dilation of small arterioles by indomethacin and glibenclamide.

Indomethacin or glibenclamide treatments attenuate functional dilation of larger-diameter "feed" arterioles paired with venules in hamster cremaster muscle. We tested the hypothesis that release of cyclooxygenase products from venules is important for functional dilation of third- and fourth-order arterioles. We also tested whether ATP-sensitive potassium channels are important during functional dilation of smaller arterioles. The microcirculation of hamster cremaster muscle was visualized with in vivo video microscopy. We measured diameter responses of third- and fourth-order arterioles paired and unpaired with venules in response to 2 minutes of muscle field stimulation (40 microseconds, 10 V, 1 Hz). Control diameters of vessels were 31+/-2 (n=19), 13+/-1 (n=12), 12+/-2 (n=12), and 10+/-1 (n=12) for paired and unpaired third-order and paired and unpaired fourth-order arterioles, respectively. In all groups, field stimulation resulted in increases in mean control diameter of >80%. Indomethacin (28 micromol/L) superfused on the preparation was used to inhibit cyclooxygenase metabolism, or glibenclamide (10 micromol/L) was used to block ATP-sensitive potassium channels. Indomethacin attenuated arteriolar vasodilations to electrical stimulation in paired third-order vessels only, whereas glibenclamide attenuated this vasodilation in all 4 groups. These results support a role for ATP-sensitive potassium channels in functional dilation of arterioles of all sizes regardless of whether or not they are paired with venules. Conversely, a role for cyclooxygenase products is limited to larger "feed arterioles" paired with venules. This study provides further evidence that venules may be the source of prostaglandin release during functional hyperemia.

In vivo location and mechanism of EDHF-mediated vasodilation in canine coronary microcirculation.

Responses of epicardial coronary arterioles to ACh were measured using stroboscopic fluorescence microangiography in dogs (n = 38). ACh (0.1 and 0.5 microg. kg(-1). min(-1) ic) dilated small (<100 micron, 11 +/- 2 and 19 +/- 2%, respectively) and large (>100 micron, , 6 +/- 3 and 13 +/- 3%, respectively) arterioles at baseline. Combined administration of N(omega)-monomethyl-L-arginine (L-NMMA; 1. 0 micromol/min ic) and indomethacin (10 mg/kg iv) eliminated ACh-induced dilation in large coronary arterioles but only partially attenuated that in small arterioles. Suffusion of a buffer containing 60 mM KCl (high KCl) completely abolished cromakalim-induced dilation in arterioles and in combination with L-NMMA plus indomethacin completely blocked ACh-induced dilation in small arterioles. This indicated that the vasodilation to ACh that persists in small arterioles after administration of L-NMMA and indomethacin is mediated via a hyperpolarizing factor. The ACh-induced vasodilation remaining after L-NMMA and indomethacin was completely blocked by the large-conductance potassium-channel antagonist iberiotoxin or by epicardial suffusion of miconazole or metyrapone, inhibitors of cytochrome P-450 enzymes. These observations are consistent with the view that endothelium-derived hyperpolarizing factor (EDHF) is a product of cytochrome P-450 enzymes and produces vasodilation by the opening of large-conductance potassium channels. We conclude that ACh-induced dilation in large coronary arterioles is mediated mainly by nitric oxide (NO), whereas, in small arterioles both NO and EDHF mediate dilation to ACh. These data provide the first direct evidence for an in vivo role of EDHF in small coronary arterioles.

Release of endogenous nitric oxide mediates arteriolar dilation to endothelin in rat striated muscle.

Previous studies demonstrated that endothelin-1 is a potent vasoconstrictor in the microcirculation. In this study, we assessed the ability of endothelin-1 to induced dilation of small arterioles in the rat cremaster muscle. Responses to topical application of endothelin-1 were assessed by using intravital microscopy. Exposure to increasing concentrations of endothelin-1 (10[-13]-10[-8] M) produced a dose-dependent constriction of third-order arterioles (20 +/- 1.4 microm). Pretreatment with hydroquinone (HQ) or N omega-nitro-L-arginine methyl ester (L-NAME), antagonists of nitric oxide production, caused a significant potentiation in the reactivity of third-order arterioles to endothelin-1. In addition, we observed a significant vasodilation to low levels of endothelin-1 (10[-14]-10[-12] M) in the presence of the endothelin type-A receptor (ET-A) antagonist, BQ123. This dilation was abolished in the presence of a 10(-4) M bath concentration of L-NAME. These results indicated that endothelin-1 caused arteriolar dilation by stimulating endogenous production of nitric oxide. This effect appeared to attenuate the constrictor effects of endothelin-1 on small resistance vessels in striated muscle.

Role of ATP-sensitive potassium channels in brain stem circulation during hypotension.

The basilar artery and its branch arterioles dilate actively in response to a marked decrease in blood pressure and maintain cerebral blood flow (CBF) to the brain stem. We tested the hypothesis that ATP-sensitive potassium (KATP) channels play a role in the autoregulatory responses of the brain stem circulation in vivo. In anesthetized Sprague-Dawley rats, local CBF to the brain stem was determined with laser-Doppler flowmetry, and diameters of the basilar artery and branch arterioles were measured through a cranial window during stepwise hemorrhagic hypotension. During topical application of 10(-6) and 10(-5) mol/l of glibenclamide, a selective KATP-channel blocker, the lower limit of CBF autoregulation shifted upward to 60-75 from 30-45 mmHg in the vehicle group. Glibenclamide significantly impaired the dilator response of small arterioles (baseline diameter 45 +/- 2 microns) throughout hypotension (P < 0.03) but did not impair the dilatation of the basilar artery (247 +/- 3 microns) or large arterioles (99 +/- 4 microns). Thus KATP channels appear to play an important role in the regulation of CBF to the the brain stem during hypotension by mediating the compensatory dilatation of small arterioles. In contrast, these channels may not be a major regulator of the vascular tone of larger arteries during hypotension.

Elevation of plasma viscosity induces sustained NO-mediated dilation in the hamster cremaster microcirculation in vivo.

We studied whether a flow-independent increase of luminal wall shear stress (WSS) could dilate hamster arterioles in vivo and which endothelial mediators are potentially involved. To this end the plasma viscosity was elevated by exchanging blood for dextran-erythrocyte solution thereby augmenting WSS. Diameters of small and large arterioles as well as red blood cell velocities were measured before and after exchange of blood for solutions of identical haematocrit containing either high- (HMWD) or low-molecular weight dextran (LMWD). The potential role of endothelial autacoids was investigated by local application of the NO-synthase inhibitor NG-nitro-L-arginine (L-NNA), the inhibitor of cyclooxygenase, indomethacin (3 microM), or the K+-channel blocker, tetrabutylammonium (TBA, 0.1 mM) to assess the potential effects of EDHF. HMWD (n = 11 animals) increased plasma viscosity by 64 +/- 3% and dilated arterioles of all branching orders (A1-A4) significantly [by 24 +/- 3% (A1-A2) and 32 +/- 3% (A3-A4)]. This dilation compensated fully for the calculated initial increase of WSS. LMWD (n = 6) did not affect plasma viscosity or arteriolar diameters. Tissue treatment with L-NNA (30-300 microM, n = 12) substantially diminished the HMWD-induced dilation in small arterioles (A3-A4; to 13 +/- 3%; P<<0.05) and virtually abolished it in large ones (A1-A2). Consequently, the calculated WSS increased significantly in these arterioles (by 31 +/- 5%). TBA combined with L-NNA (n = 4) did not reduce further the remaining dilation. Indomethacin (n = 6) had no effect on HMWD-induced dilation. We conclude that an increase of WSS induces a mainly NO-mediated arteriolar dilation. This dilation occurs in all arteriolar branching orders and is of sufficient magnitude to compensate for the initial WSS-increase. Thus, any elevations of WSS fulfil the requirement for a signal to change diameter along the arteriolar tree in a coordinated manner. The fully compensating dilation which we observed indicates that WSS is a controlled variable. It does, however, raise questions as to its role as a continuous endothelial stimulus.

Altered microvascular responses to C5a in rat striated muscle during two-kidney, one clip renovascular hypertension

To determine how two-kidney, one clip (2-K,1C) renovascular hypertension alters microvascular responses in rat striated muscle to complement C5a, one of the most important inflammatory mediators. 2-K,1C hypertension was induced in male Sprague-Dawley rats. Under anesthesia with pentobarbital (50 mg/kg, intraperitoneally) the cremaster muscle microcirculatory preparation with intact neurovascular connections was studied in vivo by closed-circuit videomicroscopy. Recombinant human C5a was applied topically in the tissue bath at concentrations of 10(-12), 10(-10) and 10(-8) mol/l, consecutively. Changes in the microvessel diameters in small arterioles, large arterioles and venules were measured. In normotensive rats complement C5a induces a significant dilation in small arterioles at low bath concentrations (10(-12) or 10(-10) mol/l), but the dilation is attenuated at a higher concentration (10(-8) mol/l). In contrast, in 2-K,1C hypertensive rats C5a constricts small arterioles at low concentrations (< 10(-10) mol/l) but dilates them at a higher concentration (10(-8) mol/l). Large arterioles and venules have minimal responses to C5a in either normotensive or 2-K,1C hypertensive rats. 2-K,1C hypertension dramatically alters C5a-induced microvascular responses in small arterioles. The alteration might be attributable to the enhanced vasoconstrictor mechanisms and impaired vasodilator mechanisms during 2-K,1C renovascular hypertension.

Endothelium-Dependent Microvascular Responses to Activated Complement

Infusion of Escherichia coli bacteria to cause high cardiac output bacteremia produces a differential microvascular response with constriction of large arterioles and dilation of small arterioles in skeletal muscle of rats. An important component to host-defense mechanisms during bacteremia is activation of the complement system. One part of this study explored the possibility that microvascular responses to bacteremia could be mediated by activation of the alternative complement cascade to alter skeletal muscle blood flow during sepsis. Complement activation by iv zymosan into unanesthetized (decerebrate) Sprague-Dawley rats caused constriction of large arterioles and dilation of small arterioles in cremaster muscle, while cardiac output stayed normal or was elevated. These microvascular responses mimic those during bacteremia, suggesting that components of the complement system reediate skeletal muscle microcirculatory responses to live E. coli sepsis. The vasodilation response of small arterioles in skeletal muscle during bacteremia is endothelium-dependent and is mediated at least partially by endothelial-derived relaxing factor (EDRF). Complement activation gives products which interact with endothelial cells. Thus, a second part of this study explored the role of EDRF in the vasodilation of skeletal muscle small ar terioles during activation of the alternate complement pathway. Blockade of EDRF action by hydroquinone totally abolished small arteriole dilation and large arteriole constriction responses to complement activation by zymosan infusion.

Microcirculatory responses to complement activation are blunted by hypertension.

Vasodilation of small arterioles in skeletal muscle during sepsis is an endothelium-dependent response. Renovascular hypertension significantly attenuates this response. Complement activation by zymosan infusion causes small arteriole dilation in skeletal muscle similar to that seen during sepsis. This study was conducted to show whether renovascular hypertension alters the skeletal muscle microcirculatory responses in normotensive rats to systemic activation of the alternative complement system. We found that hypertension abolished the constriction of large A1 arterioles (+3 +/- 2% change at 45 min) and dilation of small A4 arterioles (-2 +/- 2% change at 45 min) in skeletal muscle. Hypertension attenuated but did not abolish (A4: +70 +/- 13% change in hypertensive vs +111 +/- 18% in normotensive rats) the ability of small arterioles to dilate to nitroprusside, an endothelium-independent vasodilator. This suggest that hypertension modifies some type of receptor-level mechanism to reduce small arteriolar dilation during complement activation. We next used hydroquinone to topically block endothelium-derived relaxing factor (EDRF) in hypertensive animals whose complement systems were activated. Hydroquinone did not change the response of large A1 arterioles (+5 +/- 4% hydroquinone change vs +3 +/- 2% non-hydroquinone at 45 min) to complement activation. However, hydroquinone allowed construction of small A4 arterioles (-12 +/- 5% hydroquinone change vs -2 +/- 2%, non-hydroquinone at 45 min) during complement activation in hypertensive rats. We conclude that during hypertension the release of EDRF in small arterioles after complement activation is counterbalanced by a constrictor-producing mechanism.

The role of ATP-sensitive potassium channels in regulating coronary microcirculation.

The ATP-sensitive potassium channel (K+ATP channel) is known to exist in blood vessels and to regulate vascular tone. We examined the role of this channel in coronary arteriolar vasomotion during coronary autoregulation, ischemia, reactive hyperemia and endothelium-dependent response by acetylcholine in vivo. Experiments were performed with anesthetized open-chest dogs. Coronary arterioles were directly observed in situ by means of a floating objective system or a stroboscopic epi-illumination system synchronized with cardiac motion. Small arterioles less than 100 microns in internal diameter dilated in response to reduction in perfusion pressure (perfusion pressure: 60, 40, 25 mm Hg). Glibenclamide, a selective blocker of the K+ATP channel, reversed the dilation. Reactive hyperemia produced by 20-second occlusion of the left anterior descending coronary artery resulted in arteriolar dilation, the magnitude of which was greater in smaller arterioles than in larger ones. Glibenclamide significantly inhibited the dilation in both large and small arterioles. Acetylcholine (ACh) produced dilation in arterioles of all sizes. NG-monomethyl L-arginine, a competitive inhibitor of nitric oxide synthesis, abolished the dilation of large arterioles, but failed to abolish the dilation in small arterioles. Glibenclamide, however, did not have any additional inhibitory effect on ACh-induced arteriolar dilation. Thus, we conclude that the K+ATP channel plays an important role in coronary microvascular vasomotion during autoregulation, ischemia and reactive hyperemia, but not during endothelium-dependent vasodilation induced by ACh in vivo.

Serotonin-induced dilation of small arterioles is not mediated via endothelium-derived relaxing factor in skeletal muscle

Serotonin (5-HT) dilates precapillary arterioles in skeletal muscle. The purpose of this study was to determine if 5-HT release endothelium-derived relaxing factor (EDRF) in this tissue. Diameters of third-order arterioles (A3) in the cremaster muscle of pentobarbital-anesthetized rats were measured via videomicroscopy. Concentration-response curves for acetylcholine, nitroprusside and 5-HT were obtained before and after the application of either hydroquinone (5 × 10 −4 M) or N G-nitro-L-arginine (10 −4 M). The involvement of prostaglandins was eliminated by ibuprofen (10 −4 M). In one group, 5-HT (20 μg/kg) and N G-nitro-L-arginine were given i.v. (30 mg/kg). The non-EDRF-dependent vasodilator papaverine (10 −5 M) was applied at the end of the protocol to determine the maximal resting diameter. When applied topically, both hydroquinone and N G-nitro-L-arginine significantly inhibited the dilation induced by acetylcholine, but neither agent affected the dilation to nitroprusside or 5-HT. N G-Nitro-L-arginine (i.v.) attenuated acetylcholine-induced dilation but not the dilation to intravenous 5-HT. These data suggest that 5-HT-induced dilation of small arterioles in skeletal muscle is EDRF-independent.

Coronary microvascular response to endothelin is dependent on vessel diameter and route of administration.

Endothelin is a 21-amino acid peptide originally isolated from vascular endothelial cells. In the present study, we examined the effect of topical and intracoronary administration of endothelin-1 on the coronary microcirculation and the effect of inhibition of cyclooxygenase on the microvascular response to intracoronary endothelin. In anesthetized dogs (n = 39), the coronary microcirculation was visualized using stroboscopic epi-illumination synchronized to the cardiac cycle. Topical application of endothelin [(5 x 10(-9) to 10(-8) M] constricted all arteries and arterioles with the degree of constriction inversely related to vessel size. Coronary veins and venules did not constrict to endothelin. Topical application of EDTA (10 mg/ml) reversed the constriction to endothelin in arterioles of all sizes. In contrast, intracoronary administration of endothelin (10(-8) to 10(-7) M) produced dilation of small arterioles (less than 130 microns) and no response of large arterioles (greater than 130 microns). The response of small arterioles to intracoronary endothelin was not altered by inhibition of cyclooxygenase with indomethacin (5 mg/kg); however, large arterioles constricted. Thus the coronary microvascular response to endothelin is dependent on the route of administration. Constriction of arteries and arterioles of all sizes to endothelin is dependent on extracellular calcium. Vasodilator prostaglandins may be released in response to intracoronary administration of endothelin predominantly in larger vessels. Thus the differential response to endothelin with topical and intracoronary administration may reflect a diffusional barrier of the endothelium or release of endothelium-derived relaxing factor and prostaglandins in response to endothelin.

EDRF as a possible mediator of sepsis-induced arteriolar dilation in skeletal muscle.

Vascular endothelial cells influence microvessel diameters in vivo and in vitro and participate in host-defense mechanisms during sepsis. We examined whether small arteriole dilation in skeletal muscle during high cardiac output bacteremia (HOB) and low cardiac output live Escherichia coli sepsis (LOS) is mediated by an endothelium-derived relaxing factor (EDRF). Local chemical blockade of EDRF by hydroquinone (HQ) substantially blunted acetylcholine-induced dilation of small arterioles. HQ also prevented large arteriole (55-135 microns) constriction and small arteriole (6-22 microns) dilation in the cremaster muscle of rats during HOB. In LOS, small arteriole dilation was also prevented by HQ but only during the early period when blood pressure was unchanged from baseline. HQ did not alter large arteriole constriction during LOS. We conclude that small arteriole vasodilation in skeletal muscle is mediated at least in part by EDRF during bacteremia. Because EDRF cannot mediate large arteriole constriction and because HQ blunted large arteriole constriction during HOB, we now suspect that HQ also interferes at least in part with some large arteriole vasoconstrictor mechanism, possibly leukotrienes or an endothelium-derived constricting factor, which mediates large arteriole constriction during HOB. Our data also suggest that large arteriole constriction during LOS is partly mediated by factors that are unaffected by HQ. The endothelium appears to play an important role in the microcirculatory responses of skeletal muscle to live E. coli sepsis through more than one mechanism.

Prostaglandin-related Microvascular Dilation in Pentobarbital- and Etomidate-anesthetized Rats

Etomidate is characterized by minimal systemic cardiovascular effects, but its effect on the microvasculature has not been assessed. We compared the microvasculature of etomidate-anesthetized animals to that of animals anesthetized with pentobarbital, since its effects on the microvasculature are known. Male Sprague-Dawley rats were anesthetized with etomidate or pentobarbital. The cremaster muscle was prepared for microscopic viewing, leaving the neural and vascular supply intact. Small arterioles were near their maximal diameters in etomidate-anesthetized rats, whereas the pentobarbital group had a large dilator capacity (maximal diameter-basal diameter/basal diameter). The effect on resting arteriolar diameters of endothelium-derived relaxing factor (EDRF) and prostaglandin synthesis inhibitors was tested. Dilator capacity was not affected by the EDRF inhibitor nitro-L-arginine, but it was significantly increased by mefenamic acid and ibuprofen in etomidate-anesthetized animals. To test whether dilator and constrictor mechanisms were normal, serotonin concentration-response curves were obtained in pentobarbital and etomidate-anesthetized animals with and without mefenamate or ibuprofen present. The dilation of small arterioles to serotonin in the etomidate groups with mefenamate or ibuprofen was not significantly different from that of the pentobarbital groups. Serotonin produced a comparable constriction of large arterioles in both anesthetic groups. The topical application of etomidate to the cremaster muscle did not affect arteriolar diameters. Thus, etomidate appears to trigger the release of dilator prostaglandins in striated muscle through a central or indirect mechanism.

Effect of an arginine analogue on acetylcholine-induced coronary microvascular dilatation in dogs

The purpose of this study was to elucidate the contribution of endothelium-derived relaxing factor (EDRF) derived from arginine to acetylcholine (ACh)-induced coronary arteriolar vasodilatation in vivo. Experiments were performed in 62 open-chest anesthetized dogs. Internal diameters of small arterioles (less than 120 microns) and large arterioles (greater than 120 microns) were measured using an intravital microscope and stroboscopic epiillumination synchronized to the cardiac cycle. Topically administered NG-monomethyl-L-arginine (L-NMMA, 3 x 10(-4) M) constricted small arterioles (-10.7 +/- 3.1% from control diameter, P less than 0.05), but L-NMMA did not produce vasoconstriction in large arterioles. ACh, in the absence of L-NMMA, caused a dose-dependent vasodilatation in both small and large arterioles. In large arterioles, L-NMMA completely abolished the ACh-induced vasodilatation (10(-5) M topical ACh: from 13.3 +/- 3.0 to -2.0 +/- 1.5%, P less than 0.05; 10(-4) M ACh: from 20.9 +/- 3.9 to -3.0 +/- 1.9%, P less than 0.01). In small arterioles, L-NMMA only partially inhibited the vasodilatation (10(-5) M ACh: from 35.4 +/- 4.0 to 19.0 +/- 2.7%, P less than 0.05; 10(-4) M ACh: from 42.5 +/- 4.8 to 22.6 +/- 3.1%, P less than 0.05). L-Arginine (10(-3) M topically) reversed L-NMMA inhibition of ACh-induced vasodilatation. Persistent dilatation of small arterioles also occurred when NG-nitro-L-arginine rather than L-NMMA was administered.(ABSTRACT TRUNCATED AT 250 WORDS)