Modulation by the endothelium of sympathetic vasoconstriction in an in vitro preparation of the rat tail artery

A critical review of the neurovascular nature of migraine and the main mechanisms of action of prophylactic antimigraine medications

INTRODUCTION

Migraine involves neurovascular, functional, and anatomical alterations. Migraineurs experience an intense unilateral and pulsatile headache frequently accompanied with vomiting, nausea, photophobia, etc. Although there is no ideal preventive medication, frequency in migraine days may be partially decreased by some prophylactics, including antihypertensives, antidepressants, antiepileptics, and CGRPergic inhibitors. However, the mechanisms of action involved in antimigraine prophylaxis remain elusive.

AREAS COVERED

This review recaps some of the main neurovascular phenomena related to migraine and currently available preventive medications. Moreover, it discusses the major mechanisms of action of the recommended prophylactic medications.

EXPERT OPINION

In the last three years, migraine prophylaxis has evolved from nonspecific to specific antimigraine treatments. Overall, nonspecific treatments  mainly involve neural actions, whereas specific pharmacotherapy (represented by CGRP receptor antagonists and CGRPergic monoclonal antibodies) is predominantly mediated by neurovascular mechanisms that may include, among others: (i) reduction in the cortical spreading depression (CSD)-associated events; (ii) inhibition of pain sensitization; (iii) blockade of neurogenic inflammation; and/or (iv) increase in cranial vascular tone. Accordingly, the novel antimigraine prophylaxis promises to be more effective, devoid of significant adverse effects (unlike nonspecific treatments), and more beneficial for the quality of life of migraineurs.

Sympathetic and Sensory-Motor Nerves in Peripheral Small Arteries

Small arteries, which play important roles in controlling blood flow, blood pressure, and capillary pressure, are under nervous influence. Their innervation is predominantly sympathetic and sensory motor in nature, and while some arteries are densely innervated, others are only sparsely so. Innervation of small arteries is a key mechanism in regulating vascular resistance. In the second half of the previous century, the physiology and pharmacology of this innervation were very actively investigated. In the past 10-20 yr, the activity in this field was more limited. With this review we highlight what has been learned during recent years with respect to development of small arteries and their innervation, some aspects of excitation-release coupling, interaction between sympathetic and sensory-motor nerves, cross talk between endothelium and vascular nerves, and some aspects of their role in vascular inflammation and hypertension. We also highlight what remains to be investigated to further increase our understanding of this fundamental aspect of vascular physiology.

The crosstalk between autonomic nervous system and blood vessels

The autonomic nervous system (ANS), comprised of two primary branches, sympathetic and parasympathetic nervous system, plays an essential role in the regulation of vascular wall contractility and tension. The sympathetic and parasympathetic nerves work together to balance the functions of autonomic effector organs. The neurotransmitters released from the varicosities in the ANS can regulate the vascular tone. Norepinephrine (NE), adenosine triphosphate (ATP) and Neuropeptide Y (NPY) function as vasoconstrictors, whereas acetylcholine (Ach) and calcitonin gene-related peptide (CGRP) can mediate vasodilation. On the other hand, vascular factors, such as endothelium-derived relaxing factor nitric oxide (NO), and constriction factor endothelin, play an important role in the autonomic nervous system in physiologic conditions. Endothelial dysfunction and inflammation are associated with the sympathetic nerve activity in the pathological conditions, such as hypertension, heart failure, and diabetes mellitus. The dysfunction of the autonomic nervous system could be a risk factor for vascular diseases and the overactive sympathetic nerve is detrimental to the blood vessel. In this review, we summarize findings concerning the crosstalk between ANS and blood vessels in both physiological and pathological conditions and hope to provide insight into the development of therapeutic interventions of vascular diseases.

Decompensated liver cirrhosis and neural regulation of mesenteric vascular tone in rats: role of sympathetic, nitrergic and sensory innervations

We evaluated the possible alterations produced by liver cholestasis (LC), a model of decompensated liver cirrhosis in sympathetic, sensory and nitrergic nerve function in rat superior mesenteric arteries (SMA). The vasoconstrictor response to electrical field stimulation (EFS) was greater in LC animals. Alpha-adrenoceptor antagonist phentolamine and P2 purinoceptor antagonist suramin decreased this response in LC animals more than in control animals. Both non-specific nitric oxide synthase (NOS) L-NAME and calcitonin gene related peptide (CGRP) (8-37) increased the vasoconstrictor response to EFS more strongly in LC than in control segments. Vasomotor responses to noradrenaline (NA) or CGRP were greater in LC segments, while NO analogue DEA-NO induced a similar vasodilation in both experimental groups. The release of NA was not modified, while those of ATP, nitrite and CGRP were increased in segments from LC. Alpha 1 adrenoceptor, Rho kinase (ROCK) 1 and 2 and total myosin phosphatase (MYPT) expressions were not modified, while alpha 2B adrenoceptor, nNOS expression and nNOS and MYPT phosphorylation were increased by LC. Together, these alterations might counteract the increased splanchnic vasodilation observed in the last phases of decompensated liver cirrhosis.

Spinal cord injury increases the reactivity of rat tail artery to angiotensin II

Studies in individuals with spinal cord injury (SCI) suggest the vasculature is hyperreactive to angiotensin II (Ang II). In the present study, the effects of SCI on the reactivity of the rat tail and mesenteric arteries to Ang II have been investigated. In addition, the effects of SCI on the facilitatory action of Ang II on nerve-evoked contractions of these vessels were determined. Isometric contractions of artery segments from T11 (tail artery) or T4 (mesenteric arteries) spinal cord-transected rats and sham-operated rats were compared 6–7 weeks postoperatively. In both tail and mesenteric arteries, SCI increased nerve-evoked contractions. In tail arteries, SCI also greatly increased Ang II-evoked contractions and the facilitatory effect of Ang II on nerve-evoked contractions. By contrast, SCI did not detectably change the responses of mesenteric arteries to Ang II. These findings provide the first direct evidence that SCI increases the reactivity of arterial vessels to Ang II. In addition, in tail artery, the findings indicate that Ang II may contribute to modifying their responses following SCI.

Endothelium-dependent noradrenergic hyperresponsiveness induced by thapsigargin in human saphenous veins: role of thromboxane and calcium

To further investigate the mechanisms which regulate sympathetic vascular tone, we studied the effects of the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, on the vasoconstriction induced by transmural nerve stimulation and noradrenaline in superfused human saphenous vein rings. The contractions induced by both transmural nerve stimulation and noradrenaline were potentiated by thapsigargin in endothelium-intact, but not in endothelium-denuded vessels. This potentiation was unaffected by the non-selective endothelin ET(A/B) receptor antagonist, Ro 47-0203 (4-tert-Butyyl-N-[6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-2,2'-bipyrimidin-4yl]benzene sulfonamide), or by the nitric oxide (NO) synthase inhibitor, L-NNA (N(omega)-nitro-L-arginine), but was inhibited by the thromboxane A(2) receptor antagonist, Bay u3405 (3(R)-[[(4-flurophenyl) sulphonyl]amino-1,2,3,4-tetrahydro-9H-carbazole-9-propanoic acid]) or by the thromboxane A(2) synthase inhibitor, UK 38485 (3-(1H-imidazol-1-yl-methyl)-2-methyl-1H-indole-1-propanoic acid). Moreover, the thapsigargin-induced noradrenergic hyperresponsiveness, as well as that produced by subthreshold concentrations of the thromboxane A(2) mimetic, U 46619, were blocked by the Ca(2+) channel antagonist, verapamil. In conclusion, our results indicate that thapsigargin enhances the contractions produced by sympathetic nerve stimulation in human saphenous vein rings through the endothelial release of thromboxane A(2) that potentiates the vasoconstriction induced by the noradrenergic mediator with a verapamil-sensitive mechanism.

Change in lipid profile and impairment of endothelium-dependent relaxation of blood vessels in rats after bile duct ligation

Hyperlipidemia, a condition normally observed in cholestatic liver disease, is also a risk factor for the development of atherosclerosis. The relationship between the elevation of lipoproteins in cholestatic liver diseases and atherosclerosis formation has not been elucidated. In this study, we propose that the impairment of endothelium-dependent relaxation (EDR) of blood vessels in cholestatic liver diseases may lead to the development of atherosclerosis. Using bile duct ligation (BDL) in rats as a model, we examined the liver function, serum lipid profile, EDR and morphologic change of the aorta from both sham operated and BDL rats. Significant increases in liver and spleen weights, serum alanine transaminase (ALT) and aspartate transaminase (AST) activities and the bilirubin level were observed in BDL rats. Upon bile duct ligation, the total and low-density lipoprotein cholesterol levels were increased but the high-density lipoprotein cholesterol and triglyceride levels were reduced. Less contractility and lowered response to acetylcholine-induced relaxation were found in aorta segments. In addition, the acetylcholine-induced relaxation was blocked by both L-NAME and 15 mM KCl. Our results suggest that both nitric oxide and endothelium-derived hyperpolarizing factor are important elements for the impairment of the EDR in BDL rats. In addition, a mild atrophy of the media of the aorta was detected in BDL rats. We conclude that the alterations of lipid profile and the mild atrophy of the media may lead to the impairment of EDR in the aorta in BDL rats, and these factors may potentiate the development of atherosclerosis.

Male-female differences in the relative contribution of endothelial vasodilators released by rat tail artery

Several different vasodilator substances can be released by vascular endothelium in response to mechanical stimuli and vasoactive agents. The purpose of this study was to determine whether there is a male-female difference in the relative contributions of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent vasodilation. Perfusion pressure was measured in isolated tail arteries from male and female rats. Vasodilators released by mechanical shear stress were assessed by constricting the artery with methoxamine; acetylcholine was applied to induce receptor-mediated vasodilation. We used an inhibitor of NO synthase, N(G)-monomethyl-L-arginine acetate (L-NMMA), and elevated levels of K(+) (27 mM) to reveal the relative contributions of NO and EDHF, respectively. Indomethacin was present in all experiments to block prostanoid production. The results indicate that NO was the primary vasodilator released by male tail arteries in response to both mechanical stress and acetylcholine (the L-NMMA-sensitive component of the combined L-NMMA/K(+) effect was 83 +/- 8% and 101 +/- 4%, respectively). However female tail arteries appeared to utilize both NO and EDHF for vascular relaxation (e.g., L-NMMA sensitivity: 58 +/- 9%; K+-sensitivity: 42 +/- 9% in mechanical stress experiments). These findings suggest endothelial regulation differs between males and females.

Facilitatory role of NO in neural norepinephrine release in the rat kidney

We examined modulation by nitric oxide (NO) of sympathetic neurotransmitter release and vasoconstriction in the isolated pump-perfused rat kidney. Electrical renal nerve stimulation (RNS; 1 and 2 Hz) increased renal perfusion pressure and renal norepinephrine (NE) efflux. Nonselective NO synthase (NOS) inhibitors [N(omega)-nitro-L-arginine methyl ester (L-NAME) or N(omega)-nitro-L-arginine], but not a selective neuronal NO synthase inhibitor (7-nitroindazole sodium salt), suppressed the NE efflux response and enhanced the perfusion pressure response. Pretreatment with L-arginine prevented the effects of L-NAME on the RNS-induced responses. 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO), which eliminates NO by oxidizing it to NO(2), suppressed the NE efflux response, whereas the perfusion pressure response was less susceptible to carboxy-PTIO. 8-Bromoguanosine cGMP suppressed and a guanylate cyclase inhibitor [4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b)(1,4)oxazin-1-one] enhanced the RNS-induced perfusion pressure response, but neither of these drugs affected the NE efflux response. These results suggest that endogenous NO facilitates the NE release through cGMP-independent mechanisms, NO metabolites formed after NO(2) rather than NO itself counteract the vasoconstriction, and neuronal NOS does not contribute to these modulatory mechanisms in the sympathetic nervous system of the rat kidney.

Nitric oxide inhibits norepinephrine stimulated contraction of human internal thoracic artery and rat aorta

The effect of nitric oxide (NO) on norepinephrine-induced vascular contraction was evaluated using segments of rat aorta and human internal thoracic artery (ITA) and the NO donor, SNAP. NO levels were measured directly using an amperometric probe. Concentrations of NO greater than 2 nM were required to reduce vascular contraction induced by 100 nM norepinephrine (NE). Exposure of the aortic rings to SNAP prior to, or after NE addition, resulted in a similar attenuation of NE-induced contraction. In contrast, increased relaxation of ITA segments in response to SNAP was observed relative to that of rat aorta and significant development of contractile tone following NE addition was not observed. Evaluation of cytoskeletal actin demonstrated marked loss of F-actin content in smooth muscle cells following NO exposure, suggesting that NO may have direct and indirect effects on contractile tone. These data taken together suggest that vascular responsiveness to contractile agents may be significantly attenuated by prior or subsequent exposure to NO, and mechanisms in addition to vascular relaxation are likely to contribute to this effect.

Inhibition of endothelium-dependent vascular relaxation by lysophosphatidylcholine: impact of lysophosphatidylcholine on mechanisms involving endothelium-derived nitric oxide and endothelium derived hyperpolarizing factor

Hyperlipidemia has been associated with an increase in the incidence of atherosclerosis. The oxidation of low density lipoprotein (LDL) plays an important role in the initiation and progression of atherosclerosis, one of its effects being the inhibition of endothelium dependent relaxation (EDR). The elevated level of lysophosphatidylcholine (LPC) in oxidatively modified LDL has been shown to be a biochemical factor responsible for the impairment of EDR in vascular ring preparations. Several endothelium-derived modulators are thought to control vascular responsiveness. The present work examined whether acetylcholine (ACh)-induced EDR in rat aorta (pre-contracted with phenylephrine, PE) involved both endothelium-derived nitric oxide (EDNO) and endothelium-dependent hyperpolarizing factor (EDHF) and whether LPC inhibited either of these selectively. Indomethacin (10(-5) M), had no significant effect on EDR, indicating that products of cyclooxygenase, including prostacyclin, are not involved. Treatment with either N(W)-nitro-L-arginine methyl ester (L-NAME, 6.8 microM) to inhibit the production of EDNO or with elevated K+ (15 mM), to block the hyperpolarizing effect of EDHF impaired EDR considerably (each of these shifting the inhibitory dose-response relationship to ACh by almost one log unit); in muscles treated with both of these agents EDR was completely inhibited. In each of L-NAME- and K-treated muscles, the addition of LPC (20 microM) further impaired EDR. LPC did not independently raise the tone of resting- or PE-contracted aorta. We conclude that the inhibition of EDR of rat aorta by LPC involves the actions of both EDNO and EDHF.

Both extracellular ATP and shear stress regulate the release of nitric oxide in rat caudal artery

1. To elucidate the physiological role of nitric oxide (NO) in regulating vascular tone, the effects of NG-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, on the vasoconstrictor response to noradrenaline (NA) in rat caudal artery was examined. 2. NG-Nitro-L-arginine methyl ester significantly potentiated the NA-induced increase in perfusion pressure in the perfused caudal artery, but did not affect the NA-induced contraction in caudal artery ring preparations. In addition, an increase in perfusion pressure mechanically produced by a stepwise increase in flow rate was not affected by L-NAME. 3. Noradrenaline evoked a significant increase in the release of endogenous ATP and its metabolites from the perfused artery, whereas increased perfusion pressure as a result of increased flow rate did not evoke release of endogenous ATP. 4. In the presence of exogenously applied ATP, L-NAME significantly potentiated the increase in perfusion pressure produced by increased flow rate. 5. These results indicate that perfused vascular tone is regulated by endogenous NO and suggest that extracellular ATP may participate in the synthesis and release of NO by shear stress in endothelial cells in the rat caudal artery.

Nitric oxide mediates sympathetic vasoconstriction at supraspinal, spinal, and synaptic levels

The purposes of this study were to investigate the level of the sympathetic nervous system in which nitric oxide (NO) mediates regional sympathetic vasoconstriction and to determine whether neural mechanisms are involved in vasoconstriction after NO inhibition. Ganglionic blockade (hexamethonium), alpha1-receptor blockade (prazosin), and spinal section at T1 were used to study sympathetic involvement. NO was blocked with Nomega-nitro-L-arginine methyl ester (L-NAME). Regional blood flow in the mesenteric and renal arteries and terminal aorta was monitored by electromagnetic flowmetry in conscious rats. L-NAME (3-5 mg/kg iv) increased arterial pressure and peripheral resistance. Ganglionic blockade (25 mg/kg iv) significantly reduced the increase in resistance in the mesentery and kidney in intact and spinal-sectioned rats. Ganglionic blockade significantly decreased hindquarter resistance in intact rats but not in spinal-sectioned rats. Prazosin (200 micrograms/kg iv) significantly reduced the increased hindquarter resistance. We concluded that NO suppresses sympathetic vasoconstriction in the mesentery and kidney at the spinal level, whereas hindquarter tone is mediated at supraspinal and synaptic levels.

Endothelial modulation of neural sympathetic vascular tone in canine skeletal muscle

The effect of nitric oxide synthase (NOS) inhibition and endothelin-A (ETA)-receptor blockade on neural sympathetic control of vascular tone in the gastrocnemius muscle was examined in anesthetized dogs under conditions of constant flow. Muscle perfusion pressure (MPP) was measured before and after NOS inhibition (Nomega-nitro-L-arginine methyl ester; L-NAME) and ETA-receptor blockade [cyclo-(D-Trp-d-Asp-Pro-D-Val-Leu); BQ-123]. Zero and maximum sympathetic nerve activities were achieved by sciatic nerve cold block and stimulation, respectively. In group 1 (n = 6), MPP was measured 1) before nerve cold block, 2) during nerve cold block, and 3) during nerve stimulation. Measurements under these conditions were repeated after L-NAME and then BQ-123. The same protocol was followed in group 2 (n = 6) except that the order of L-NAME and BQ-123 was reversed. MPP and muscle vascular resistance (MVR) increased after L-NAME and then decreased to control values after BQ-123. MVR decreased after BQ-123 alone and, with the addition of L-NAME, increased to a level not different from that observed during the control period. MVR fell during nerve cold block. This response was not affected by administration of L-NAME followed by BQ-123, but it was attenuated by administration of BQ-123 before L-NAME. The constrictor response during sympathetic nerve stimulation was enhanced by L-NAME; no further effect was observed with BQ-123, nor was the response affected when BQ-123 was given first. These findings indicate that endothelin contributes to 1) basal vascular tone in skeletal muscle and 2) the increase in skeletal muscle vascular resistance after NOS inhibition. Finally, nitric oxide "buffers" the degree of constriction in skeletal muscle vasculature during maximal sympathetic stimulation.

Nitric oxide inhibits alpha2-adrenoceptor-mediated endothelium-dependent vasodilation

This study was designed to investigate the interaction between the NO/L-arginine pathway and the alpha2-adrenoceptor-mediated endothelium-dependent vasorelaxation. Reactivity of isolated resistance mesenteric arterial segments from mice lacking the gene for constitutive endothelial NO synthase (eNOS- mice, n=14) and from their wild-type controls (WT mice, n=46) was studied in isometric conditions in the presence of indomethacin (blocker of cyclooxygenase). Oxymetazoline (OXY, 0.01 to 30 micromol/L; a selective alpha2-adrenoceptor agonist) induced an endothelium-dependent relaxation of eNOS- but not WT arteries preconstricted either with phenylephrine or serotonin. In the presence of Nomega-nitro-L-arginine (l-NNA, 100 micromol/L), an inhibitor of NOS, OXY induced an endothelium-dependent relaxation of WT mesenteric arteries. l-NNA had no effect on the relaxation caused by OXY in eNOS- arterial rings. Therefore, the relaxation caused by OXY was independent of NO formation. To demonstrate the inhibitory role of NO on the alpha2-adrenoceptor-mediated relaxation, subthreshold (0.1 nmol/L) to threshold (1 nmol/L) concentrations of sodium nitroprusside (donor of NO) were added to l-NNA-treated arteries before OXY challenges: in these conditions, the alpha2-adrenoceptor-mediated relaxation of eNOS- and WT arteries was inhibited. OXY-induced relaxation was restored on readdition of methylene blue (1 micromol/L, inhibitor of guanylate cyclase), suggesting that cGMP may be the mechanism of inhibition of the alpha2-adrenergic pathway in the presence of NO. Finally, OXY-mediated relaxation was blocked by tetraethylammonium (1 mmol/L) but not glibenclamide (1 micromol/L), suggesting the involvement of an endothelium-derived hyperpolarizing factor that activates Ca2+-activated K+ channels. In conclusion, alpha2-adrenoceptor activation caused relaxation of isolated murine mesenteric arteries that was functionally blocked by NO through a mechanism that may involve activation of the soluble guanylate cyclase and cGMP formation. The endothelium-dependent alpha2-adrenoceptor-mediated relaxation is likely to be due to an endothelium-derived hyperpolarizing factor, whose release and/or production is reduced by concurrent NO formation.

Shear-induced modulation of vasoconstriction in the hepatic artery and portal vein by nitric oxide

The effect of shear stress on nitric oxide (NO)-mediated suppression of sympathetic nerve (2-6 Hz)- and norepinephrine (0.5 microgram.kg-1.min-1)-induced vasoconstriction in the hepatic artery (HA) and portal vein (PV) was studied using a perfusion circuit to regulate blood pressure and flow in the cat liver in situ. Holding flow constant resulted in increased shear stress during constriction; holding pressure steady prevented changes in shear stress. When shear stress was allowed to rise, the vasoconstriction (indicated by elevation in perfusion pressure) in response to nerve stimulation and norepinephrine was significantly potentiated after NO synthase blockade using NG-nitro-L-arginine methyl ester (L-NAME, 2.5 mg/kg iv) in both the HA and PV (response to nerves: HA control 28.8 +/- 6.5 mmHg, L-NAME 62.7 +/- 14.6 mmHg; PV control 1.5 +/- 0.5 mmHg, L-NAME 3.3 +/- 0.5 mmHg; response to norepinephrine: HA control 32.4 +/- 9.0 mmHg, L-NAME 60.3 +/- 8.0 mmHg; PV control 1.3 +/- 0.3 mmHg, L-NAME 3.4 +/- 0.7 mmHg). The potentiation was reversed by L-arginine (75 mg/kg). When shear stress was held constant by maintaining constant perfusion pressure, L-NAME did not cause potentiation of vasoconstriction. The data are consistent with the hypothesis that elevated shear stress in the hepatic blood vessels leads to NO-dependent postjunctional modulation of vasoconstriction.

Control of vascular tone by endogenous endothelin-1 in human pial arteries

BACKGROUND AND PURPOSE

Endothelin-1 (ET) has been shown to be involved in human pathological conditions, but its physiological function remains to be elucidated. The aim of this work was to assess whether endothelium-derived ET was involved in the overall responsiveness of freshly isolated human pial arteries.

METHODS

Samples of cerebral cortex, otherwise discarded, were obtained during tumor or epileptic lesion resections (n = 10 donors). Arterial segments were isolated and mounted on a microvessel myograph.

RESULTS

Inhibition of nitric oxide (NO) formation with N omega-nitro-L-arginine (L-NA, 100 micromol/L) increased basal tone by 7+/-1% Emax (n=5). This increase in tone was fully abolished in the presence of BQ123 (1 micromol/L; ET(A) receptor antagonist, P<.05) but potentiated by a subthreshold concentration of exogenous ET (1 nmol/L; 33+/-8% Emax; P<.05). In the presence of L-NA, serotonin (10 micromol/L)-induced tone was doubled compared with the control response (P<.05) but reduced by 90% in the presence of BQ123 (P<.05). In the absence of L-NA, BQ123 prevented serotonin-induced tone (n=3). Oxymetazoline, a selective alpha2-adrenergic receptor agonist, induced an endothelium-dependent relaxation of preconstricted human pial arteries. The relaxation was partially sensitive to NO synthase inhibition and fully prevented by the addition of ET, whereas substance P-induced relaxation was preserved. Glibenclamide (1 micromol/L), an inhibitor of ATP-sensitive K+ channels and tetraethylammonium (1 mmol/L), an inhibitor of Ca2+-activated K+ channels had no effect on oxymetazoline-induced relaxation.

CONCLUSIONS

The results of this study suggest first that ET is involved in the tonic response induced by NO synthase inhibition; second, part of the contractile response induced by serotonin is endothelium-dependent and sensitive to BQ123; and third, the data suggest that activation of alpha2-adrenergic receptors generated an endothelium-dependent relaxation that was selectively inhibited by exogenous ET.

Attenuation of neurogenic vasoconstriction by nitric oxide in hindlimb microvascular beds of the rat in vivo

There is evidence that sympathetic nerve activity leads to endothelium-derived nitric oxide release, which in turn attenuates neurogenic vasoconstriction. Here we tested in vivo (1) whether the magnitude of the vasoconstriction induced by N(G)-nitro-L-arginine methyl ester given systemically is altered when ongoing sympathetic activity is abolished by sectioning the lumbar sympathetic trunk, and (2) whether hindlimb sympathetic vasoconstriction elicited by electrical stimulation of the lumbar sympathetic trunk is enhanced after inhibition of nitric oxide synthesis. Blood flow in the microvascular beds of hairless skin and skeletal muscle of the rat hindlimb was measured with laser Doppler flowmetry. Sectioning the lumbar sympathetic trunk resulted in an increase of blood flow in both tissues, indicating that tonic neurogenic vasoconstriction was abolished. Inhibition of nitric oxide synthesis resulted in vasoconstriction in both vascular beds. This vasoconstriction was more pronounced after abolition of sympathetic activity than with intact sympathetic supply in skin but was smaller in skeletal muscle. The vasoconstriction elicited by graded electrical stimulation of the centrally sectioned lumbar sympathetic trunk with frequencies less than 5 Hz was significantly enhanced after blockade of nitric oxide in skeletal muscle but not in skin microvasculature. These findings suggest that under physiological conditions, sympathetic nerve impulses directly promote the release of nitric oxide in skeletal muscle but not in cutaneous blood vessels. Therefore, basal nitric oxide release is probably in part dependent on sympathetic activity in skeletal muscle, whereas it appears to be mainly due to flow-dependent shear stress in hairless skin microvasculature.

Alpha 1-adrenoceptor vasoconstriction in the tail artery during ageing

1. We have studied the alpha 1-adrenoceptor-mediated responses in intact tail artery rings from 3-4 and 20-22 months old Sprague-Dawley rats, focusing on possible endothelial alterations. The influence of nitric oxide released by the endothelium, the number of alpha 1-adrenoceptors and the functional receptor reserve were evaluated to determine their contribution to the contractile response mediated by this receptor. The state of the endothelial layer was assessed by confocal microscopy. 2. Noradrenaline (1 nM-100 microM) induced concentration-dependent vasoconstriction. The maximum contractions to noradrenaline (P < 0.05) and to 75 mM KCl (P < 0.01) were higher in young than in old animals. 3. The density (Bmax) of alpha 1-adrenoceptors and the dissociation constant (KD) obtained in [3H]-prazosin binding experiments were unchanged by age. 4. The apparent affinity (pKA) and the percentage of functional receptors (qx 100) remaining after phenoxybenzamine (0.03 microM) were similar in both age groups. 5. After partial alpha 1-adrenoceptor inactivation with phenoxybenzamine, NG-nitro-L-arginine methylester (30 microM) significantly potentiated the E/[A] curve to noradrenaline in young rats. However, only responses to 0.1 to 1 microM noradrenaline were significantly potentiated in old animals. In addition, 94% of the vessels from young, but only 52% from old rats were relaxed by 80-100% of the noradrenaline (0.03 microM) contraction, with 1 microM acetylcholine. 6. No modifications in the area (micron2) or in the number of endothelial nuclei (per mm2) were observed between age groups. An elongation of the nuclei of endothelial cells was observed in the old animals. 7. These data suggest that the noradrenaline-induced contraction is decreased in old rats probably due to differences in either the contractile machinary or postreceptor mechanisms. These alterations may be accompanied by an impairment of the release or production of NO from endothelial cells.

Possible participation of nitric oxide in the increase of norepinephrine release caused by angiotensin peptides in rat atria

In rat atria isolated with their cardioaccelerans nerves and labeled with [3H]norepinephrine, exposure to 1 x 10(-7) mol/L angiotensin II (Ang II) and 1 x 10(-7) mol/L Ang-(1-7) increased the release of radioactivity elicited by nerve stimulation (0.5-millisecond-long square-wave pulses at 2 Hz during 2 minutes) by 90% and 60%, respectively. The facilitatory effect on noradrenergic neurotransmission caused by both peptides was stereospecifically prevented by N omega-nitro-L-arginine methyl ester (1 x 10(-4) mol/L), an inhibitor of nitric oxide synthase that catalyzes the conversion of L-arginine to nitric oxide, as well as by 1 x 10(-5) mol/L methylene blue, a substance that inhibits the guanylate cyclase considered as the final target of nitric oxide action. On the other hand, the precursor of nitric oxide synthesis. L-arginine (1 x 10(-3) mol/L), reversed the prevention produced by N omega-nitro-L-arginine methyl ester on the increased release of norepinephrine caused by Ang II and Ang-(1-7). The present results suggest that nitric oxide could be involved in the neuromodulatory function elicited by both Ang II and Ang-(1-7) in rat atria. The physiological role of this observation is still under study.

Human vascular endothelium heterogeneity. A comparative study of cerebral and peripheral cultured vascular endothelial cells

BACKGROUND AND PURPOSE

Hormones, neurotransmitters, and autacoids play a key role in the regulation of vascular tone as a result of their interaction with the endothelium. The aim of this study was to compare selected properties of three human endothelial cell lines isolated from cerebral pial arteries (PEC) and two peripheral vessels, the superficial temporal (SEC) and omental (OEC) arteries.

METHODS

Intracellular free calcium concentration ([Ca2+]i) and receptor protein expression were measured in characterized primary cultures of human endothelial cells.

RESULTS

All cell lines labeled positively for factor VIII/von Willebrand factor. Growth rate and constitutive release of endothelin-1, expressed as a function of protein, were both significantly lower in cerebral cells (PEC) than in endothelial cells derived from peripheral vessels. Basal [Ca2+]i measured with the fluorescent calcium indicator fura 2-AM (2 mumol/L) did not differ in either of the three cell lines. Although PEC responded to endothelin-1 (0.1 mumol/L) and vasoactive intestinal peptide (1 mumol/L) by a twofold to threefold increase in [Ca2+]i, OEC were unresponsive to these peptides. Moreover, the calcium response to alpha-thrombin (10 nmol/L) was greater in cerebral (PEC) than in peripheral (SEC, OEC) endothelial cells, while bradykinin (100 nmol/L) increased [Ca2+]i to a similar level in all three cell types.

CONCLUSIONS

This study demonstrates that endothelial cells from different sites of the vasculature exhibit different growth rates and vary in their response to agonists.

Disruption of the rat mesenteric arterial bed endothelial function by air perfusion

The impact of air perfusion on the endothelial function of the rat mesenteric arterial bed (MAB; perfused with Krebs' bicarbonate plus indomethacin) was compared to that of the NO synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME). Air shifted the dose-response curve for the alpha-adrenoceptor agonist, norepinephrine (NE) to the left (ED50%: 2.9+/-0.7 to 0.9+/-0.7 microg, P < 0.05); maximal vasoconstriction did not change. L-NAME produced a similar increase in midrange sensitivity (ED50% 1.4+/-0.7 microg, P < 0.05) and a 20% increase in maximum (152+/-6 to 183+/-7 mmHg, P < 0.05). Electromechanical stimulation with potassium chloride (KCl) was not modified by reserpine. Neither air nor L-NAME modified midrange sensitivity to KCl. L-NAME produced a 17% increase in maximum (91+/-4 to 107+/-5 mmHg, P < 0.05); reserpine abolished the latter effect. Air and L-NAME diminished endothelium-dependent vasodilation elicited by carbachol. Air did not modify endothelium-dependent vasodilation elicited by sodium nitroprusside; this response was potentiated by L-NAME. In summary, air and L-NAME produced similar effects on receptor-dependent activation of the endothelial L-arginine nitric oxide (NO) pathway. Potentiation by L-NAME of the maximal electromechanical response suggests the existence of a tone-dependent NO system. Abolition of the latter response by reserpine suggests that this system is of sympathetic origin.

Multiple prejunctional actions of angiotensin II on noradrenergic transmission in the caudal artery of the rat

1. Angiotensin II produced concentration-dependent enhancement of both stimulation-induced (S-I) efflux of [3H]-noradrenaline and stimulation-evoked vasoconstrictor responses in isolated preparations of rat caudal artery in which the noradrenergic transmitter stores had been labelled with [3H]-noradrenaline. The threshold concentrations of angiotensin II for enhancement of S-I efflux (between 0.03 and 0.1 microM) and of the stimulation-evoked vasoconstrictor responses (about 0.3 microM) were 10-1000 times higher than those that have been found for several other vascular preparations. 2. The AT1 angiotensin II receptor antagonist losartan (0.01 and 0.1 microM), reduced or abolished the enhancement of S-I efflux by 1 and 3 microM angiotensin II and the enhancement of vasoconstrictor responses by 1 microM angiotensin II. Surprisingly, the combination of 0.01 microM losartan and 0.1 microM angiotensin II enhanced S-I efflux to a much greater extent than did 0.1 microM angiotensin II alone. Moreover, the combination of 0.01 microM losartan and 0.1 microM angiotensin II enhanced stimulation-evoked vasoconstrictor responses, in contrast to the lack of effect of 0.1 microM angiotensin II alone. 3. In a concentration of 0.01 microM, the angiotensin II AT2 receptor antagonist PD 123319 did not affect the enhancement of either S-I efflux or vasoconstrictor responses by angiotensin II. However, in a higher concentration (0.1 microM), PD 123319 antagonized the enhancement of both the S-I efflux and vasoconstrictor responses by angiotensin II. 4. In concentrations of 0.01 and 0.1 microM, PD 123319 prevented the marked enhancement of both S-I efflux and stimulation-evoked vasoconstrictor responses produced by the combination of 0.1 microM angiotensin II and 0.01 microM losartan. 5. The potentiation by losartan (0.01 microM) of the facilitatory effect of 0.1 microM angiotensin II on S-I efflux and on stimulation-evoked vasoconstriction was still observed in the presence of either the cyclooxygenase inhibitor indomethacin (3 microM), or the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM). 6. The findings confirm our previous suggestion that, in the rat caudal artery, angiotensin II receptors similar to the AT1B subtype subserve enhancement of transmitter noradrenaline release. 7. The synergistic prejunctional interaction of 0.01 microM losartan and 0.1 microM angiotensin II may be due to either the unmasking by losartan of a latent population of angiotensin II receptors also subserving facilitation of transmitter noradrenaline release, or alternatively, losartan may block an inhibitory action of angiotensin II on transmitter noradrenaline release which normally opposes its facilitatory effect.

Nitric oxide synthase inhibition restores vasopressor effects of norepinephrine in ovine hyperdynamic sepsis

To investigate the hypothesis that nitric oxide synthase (NOS) inhibition restores the vasopressor response to norepinephrine (NE) in ovine hyperdynamic sepsis, eight sheep were chronically instrumented. In the non-septic portion of the study, NE was titrated to achieve an increase in mean arterial pressure (MAP) by 15 mm Hg ("small dose"). Small-dose NE was repeated 1 h after administration of the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME; bolus 5 mg/kg, followed by 1 mg.kg-1.h-1). After 3 days of recovery, sepsis was induced by a continuous endotoxin infusion (Salmonella typhosa, 10 ng.kg-1.h-1). Three animals died during this period (data excluded). After 24 h, small-dose NE was given. If MAP increased less than 15 mm Hg, the NE dose was increased to achieve the targeted MAP change ("large dose"). Finally, both doses of NE were given after L-NAME administration. To increase MAP by 15 mm Hg in nonseptic animals, the rate of NE infusion was 0.18 +/- 0.03 microgram.kg-1.min-1 (small dose). During L-NAME infusion, this NE dose increased MAP by 32 +/- 8 mm Hg. In septic animals, small-dose NE increased MAP by only 9 +/- 2 mm Hg (P < 0.05 versus nonseptic state). To increase MAP by 15 mm Hg, the NE dose had to be increased to 0.34 +/- 0.06 microgram.kg-1.min-1 (large dose). During L-NAME infusion, NE administration increased MAP by 16 +/- 2 mm Hg and 28 +/- 4 mm Hg (small and large dose, respectively). Thus, L-NAME restored the vasopressor response to NE in sepsis, and increased the vasopressor response to NE in a similar fashion in healthy and septic sheep.

Nitric oxide-dependent and -independent modulation of sympathetic vasoconstriction in the human saphenous vein

The possible modulation by the endothelium of the contractile responses to sympathetic nerve stimulation was examined in isolated superfused human saphenous vein. Contractile response curves for transmural nerve stimulation and noradrenaline were higher in endothelium-denuded than in intact human saphenous vein rings. In vessels with endothelium, transmural nerve stimulation- and noradrenaline-induced contractions were unaffected by the cyclooxygenase inhibitor, indomethacin (10 microM), but were potentiated by the nitric oxide (NO) synthase inhibitor, L-N omega-nitro-L-arginine (L-NNA, 3 microM) even when combined with D-arginine (0.3 mM), but not with L-arginine (0.3 mM). As in the case of noradrenaline, contractile responses to 5-HT, but not to KCI, were enhanced by endothelium removal, L-NNA or L-NNA plus D-arginine, but were unaffected by L-NNA plus L-arginine. The guanylyl cyclase inhibitor, methylene blue (10 microM), potentiated both transmural nerve stimulation- and noradrenaline-induced contractions in endothelium intact rings, whereas it enhanced, although to a lesser degree, only the neurally evoked contractions in endothelium-denuded human saphenous vein. In the vessels without endothelium L-NNA failed to affect the vasoconstriction induced by both transmural nerve stimulation and noradrenaline. Our results suggest that at least two inhibitory factors are involved in modulating the sympathetic vasoconstriction in the human saphenous vein: (1) at a postjunctional level, NO, the release of which from endothelial cells is probably stimulated by the activation of specific receptors, and (2) at a prejunctional level, an unidentified vasodilator agent, which is unmasked by the removal of the endothelium layer and which is probably co-released along with noradrenaline, and which acts through the guanylyl cyclase pathway.

Effect of NG-nitro-L-arginine methylester (L-NAME) on functional and biochemical alpha 1-adrenoceptor-mediated responses in rat blood vessels

1. The modulation by NG-nitro-L-arginine methylester (L-NAME) of alpha 1-adrenoceptor-mediated contraction was investigated on isolated segments of rat tail artery and aorta. The influence of L-NAME on inositol phosphates accumulation by alpha 1-adrenoceptor agonists was also investigated to elucidate the intracellular mechanism responsible for this modulation. 2. In aorta but not in tail artery L-NAME (30 microM) enhanced the sensitivity (3.3 times) and the maximum contraction (Emax) induced by the full agonist, phenylephrine. 3. St-587, a partial alpha 1-adrenoceptor agonist, behaved as a weak agonist in the aorta (22.2% of phenylephrine Emax). However, when the same agonist was studied in tail artery rings a maximum contraction that was 78.4% of the phenylephrine induced Emax was reached. 4. L-NAME increased (3.3 times) the Emax for St-587 contraction in the aorta but not in the tail artery. Sensitivity to St-587 was slightly but significantly (P < 0.001) enhanced (1.9 times) by L-NAME in tail artery segments. 5. Contractile responses to phenylephrine after partial alkylation with phenoxybenzamine were analyzed by the nested hyperbolic null method. To elicit 50% of Emax for contraction only 1.1% of the receptors in the tail artery and 21% of the receptors in the aorta need to be occupied. These results indicate a higher receptor reserve for the tail artery than the aorta. 6. In the tail artery but not in the aorta, St-587 activates phosphoinositide turnover. The presence of L-NAME was without effect on inositol phosphates accumulation induced by this partial alpha 1-adrenoceptor agonist. 7. The maximum contraction induced by phenylephrine, after partial alpha-adrenoceptor alkylation, was enhanced by L-NAME in tail artery rings. However, the NO synthase inhibitor was unable to modify the phenylephrine-induced accumulation of inositol phosphates in the presence of phenoxybenzamine. 8. These results indicate that the differences in St-587-induced contraction and the modulation by L-NAME of alpha 1-adrenoceptor-mediated contraction observed between the tail artery and aorta are associated with differences in receptor reserve. In addition, our biochemical studies indicate that the potentiating effect of L-NAME is independent of intracellular calcium release via phosphatidylinositol turnover.

Control of skeletal muscle blood flow during dynamic exercise: contribution of endothelium-derived nitric oxide

Traditional explanations for the hyperaemia which accompanies exercise have invoked the 'metabolic theory' of vasodilation, whereby contractile activity in the active muscle gives rise to metabolic by-products which dilate vessels bathed in interstitial fluid. Whilst metabolites with vasodilator properties have been identified, this theory does not adequately explain the magnitude of hyperaemia observed in active skeletal muscle, principally because large increases in flow are dependent on dilation of 'feed' arteries which lie outside the tissue parenchyma and are not subjected to changes in the interstitial milieu. Coordinated resistance vessel dilation during exercise is therefore dependent on a signal which 'ascends' from the microvessels to the feed arteries located upstream. Recent studies of ascending vasodilation have concentrated on the possible contribution of the endothelium, a monolayer of flattened squamous cells which lie at the interface between the circulating blood and vascular wall. These cells are uniquely positioned to respond to changes in rheological and humoral conditions within the cardiovascular system, and to transduce these changes into vasoactive signals which regulate blood flow, vascular tone and arterial pressure. Endothelial cells produce nitric oxide (NO), a rapidly diffusing labile substance which relaxes adjacent vascular smooth muscle. NO is released basally and contributes to the regulation of vascular tone by acting as a functional antagonist to sympathetic neural constriction. In addition, NO is spontaneously released in response to deformation of the endothelial cell membrane, indicating that changes in pulsatile flow and wall shear stress are likely physiological stimuli. Since the dilation of microvessels in response to exercise increases blood flow through the upstream feed arteries, which subsequently dilate, one explanation for ascending vasodilation is that NO release is stimulated by flow-induced shear stress. Evidence that NO contributes to ascending vasodilation is reviewed, along with studies which indicate that NO mediates exercise hyperaemia, that physical conditioning upregulates NO production and that NO controls blood flow by modifying other physiological mechanisms.

The effect of endotoxin on sympathetic responses in the rat isolated perfused mesenteric bed; involvement of nitric oxide and cyclo-oxygenase products

1. The effects of endotoxin on the vasoconstrictor responses to sympathetic nerve stimulation (SNS) were investigated in the rat isolated perfused mesenteric bed. 2. Rats received either saline (0.1 ml h-1) or endotoxin (2.5 mg kg-1 h-1) intravenously for 4 h; the mesenteric beds were then isolated, perfused with Krebs and prepared for SNS (50 V, 3 ms, 7-40 Hz). 3. SNS caused a frequency-dependent vasoconstrictor response which was abolished by either tetrodotoxin (10(-7) M), prazosin (2.4 x 10(-7) M) or guanethidine (2.4 x 10(-7) M). 4. In mesenteric vascular beds removed from rats infused with endotoxin, there were markedly impaired vasoconstrictor responses to SNS, although responses to noradrenaline were not modified. 5. Removal of the endothelium with distilled water prevented endotoxin-induced impairment of vasoconstrictor responses to SNS, without modifying these responses in preparations from control rats. 6. Pretreatment with dexamethasone (3 mg kg-1 i.p. 1h before commencing endotoxin or saline infusions) did not modify responses to SNS in control rats but prevented the effects of endotoxin. 7. Both L-NAME (10(-3) M) and indomethacin (10(-5) M) restored responses to SNS in preparations from endotoxin-treated rats without modifying these responses in control preparations. However, co-administration of L-NAME and indomethacin markedly augmented responses in both control and endotoxin-treated preparations. 8. The effects of L-NAME were reversed by addition of L-arginine (10(-3) M). 9. The data suggest that endotoxin impairs the release of noradrenaline and that this effect is secondary to increased production of nitric oxide and prostanoids, possibly by the endothelium.

Comparative effects of angiotensin II and its degradation products angiotensin III and angiotensin IV in rat aorta

1. In the present study, the contractile effects of angiotensin III (AIII) and angiotensin IV (AIV) compared with those of angiotensin II (AII) were determined in rat aortic ring preparations. 2. All three peptides caused concentration-dependent contractions with similar maximal responses. AIII proved approximately 4 times less potent than AII, whereas AIV was about 1000 times less active than AII. 3. The selective AT1-receptor antagonist, losartan (10-300 nM) caused parallel rightward shifts of the concentration-response curves (CRC) for all three peptides. The Schild plot slopes for the effect of losartan on AIII curves were significantly lower than unity (P < 0.05). The selective AT2-receptor antagonist, PD123177 did not influence the CRCs for AII and AIV. However, the AIII curves were moderately shifted leftward in the presence of PD123177 (0.1 and 1 microM). 4. Destruction of the endothelium or incubation with the NO-synthesis inhibitor NG-monomethyl-L-arginine acetate (L-NMMA) (0.1 mM) significantly enhanced the contractile responses to all three peptides. 5. Tachyphylaxis was investigated by constructing a second CRC for all three peptides, after an interval of 1 h. The presence of endothelium significantly enhanced the development of tachyphylaxis to all three peptides. However, in endothelium-denuded preparations, the Emax value of the second curve elicited by AII was about 50%, compared with the first one, whereas for AIII and AIV Emax values were as high as 90% and 100%, respectively. 6. Our results indicate that both AIII and AIV are less potent but similarly efficacious vasoconstrictor agents compared with AII. Their contractile effects are also mediated by AT1-receptors and probably modulated by endothelium. Tachyphylaxis induced by AIII and AIV proved weaker than that for AII. Tachyphylaxis appears to be enhanced by the presence of an intact endothelium.

Adenosine and the endothelium-dependent modulation of 3H-noradrenaline release in the canine pulmonary artery

This study aimed at characterizing the influence of endothelium on noradrenaline release from the canine pulmonary artery. Tritium overflow from intact or endothelium-free vessels preloaded with 0.2 mumol.l-1 3H-noradrenaline was evoked by electrical stimulation (1 Hz, during 5 min) or potassium (25-100 mmol.l-1). The fractional release of tritium evoked by electrical stimulation was increased by removing the endothelium [from 1.7 (1.2; 2.4) to 2.7(2.3; 3.2) x 10(-5).pulse-1, n = 10; P < 0.05]. Neither NG-nitro-L-arginine methyl ester (L-NAME) (up to 300 mumol.l-1) nor indomethacin (up to 30 mumol.l-1), nor endothelin-1 (up to 30 nmol.l-1), nor suramin (up to 300 mumol.l-1) changed tritium release evoked by electrical stimulation. In contrast, the selective A1-adenosine antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (3.3-33 nmol.l-1) concentration-dependently increased, and the selective A1-adenosine agonist N6-cyclopentyladenosine (CPA) (3.3-100 nmol.l-1) concentration-dependently decreased the evoked release of noradrenaline. Since the effects of DPCPX were observed in endothelium-intact tissues only, it may be concluded that adenosine secreted by the endothelium activates prejunctional release-inhibiting A1-receptors. Tetraethylammonium (TEA) (3.3-33 mmol.l-1) enhanced tritium overflow evoked by electrical stimulation more in endothelium-free than in endothelium-intact vessels, indicating that some K(+)-channel opener is involved in the inhibitory role of endothelium on noradrenaline release. Since it had been previously shown that A1-adenosine receptors are coupled to K(+)-channels, it is suggested that adenosine may inhibit noradrenaline release through the opening of K(+)-channels. In conclusion, the results show that in the canine pulmonary artery, adenosine is a good candidate for the endothelium-dependent inhibitory factor which is responsible for the reduction of noradrenaline release evoked by electrical stimulation.

Heterogeneity in vascular smooth muscle responsiveness to angiotensin II. Role of endothelin

We compared the role of endothelium and of endothelin in mediating the vasoconstrictor responses to angiotensin II (Ang II) in three vascular smooth muscle preparations--aorta, mesenteric artery, and tail artery--isolated from adult male Sprague-Dawley rats. The vasoconstrictor potency for Ang II in blood vessels with endothelium varied in the following rank order: aorta > mesenteric artery > tail artery. Although the maximal tension responses to Ang II were similar for mesenteric and tail arteries, it was significantly lower in aorta. Endothelium removal led to a leftward shift in the concentration-response curves to Ang II in the aorta but a rightward shift in the mesenteric artery. Strikingly, Ang II failed to evoke tension responses in tail artery in the absence of endothelium. The endothelin-A (ETA)-selective antagonist BQ-123 blocked the responses to Ang II in a noncompetitive manner, with partial and complete attenuation of responses in the endothelium-intact mesenteric and tail artery preparations, respectively. In contrast, BQ-123 did not affect the responses to Ang II in the aorta. BQ-123 also failed to affect the responses to Ang II in endothelium-denuded mesenteric artery rings. The Ang II type 1 (AT1) receptor-selective antagonist losartan competitively blocked the responses to Ang II in the three tissues (pA2, 8.3 to 8.7) when endothelium was present. These data suggest that there are endothelium-dependent regional variations in vascular tissue sensitivity to Ang II.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of phosphoramidon on contractile responses to angiotensin II in rat blood vessels

1. Cumulative concentration-tension response (C-R) curves to angiotensin II (AII), big endothelin-1 (big ET-1), ET-1 and arginine vasopressin (AVP) were determined in endothelium intact-ring preparations of aorta, mesenteric artery and tail artery isolated from adult male Sprague-Dawley rats in the presence or absence of the neutral metalloprotease inhibitor, phosphoramidon. 2. The order of sensitivity of the three rat vascular smooth muscle preparations to AII, big ET-1 and ET-1 was aorta > mesenteric artery > tail artery whereas that for AVP was reversed, namely, tail artery > mesenteric artery > aorta. 3. Phosphoramidon blocked the responses to AII in a concentration-dependent manner, whereas even very high concentrations of phosphoramidon (100 microM) failed to affect the tension responses evoked by ET-1 and AVP in all three preparations. Low concentrations of phosphoramidon (10 microM) produced significant increases in EC50 values for AII in tail artery (P < 0.01) and mesenteric artery (P < 0.05) but not in aorta. The rank order of sensitivity to the inhibition by phosphoramidon was tail artery > mesenteric artery > aorta. Phosphoramidon-evoked rightward shifts in the C-R curves to AII were much higher than those to big ET-1 in both mesenteric artery and tail artery. 4. In endothelium-denuded preparations, AII failed to evoke any increases in tension in tail artery while the responsiveness of the mesenteric artery to AII was reduced significantly relative to endothelium-intact tissues with a rightward shift in the C-R curve and a decrease in the maximal response. On the other hand, the C-R curve to AII was shifted to the left in aorta following removal of the endothelium.Importantly, ET-1 and AVP evoked vasoconstrictor responses were unaffected by the inclusion of a high concentration of phosphoramidon (100 microM) in endothelium-denuded aorta and mesenteric artery.5. The results suggest that AII-evoked tension responses of blood vessels such as tail artery are completely endothelium-dependent; in relatively larger blood vessels such as mesenteric artery they are partially endothelium-dependent while in much bigger conduit type blood vessel such as aorta, they are endothelium-independent. It is concluded that the vasoconstrictor responses to AII in mesenteric artery and tail artery may be mediated by the release of endothelins from the endothelium by increased formation from big ET, an effect that is blocked by phosphoramidon.

Inhibition of sympathetic vasoconstriction is a major principle of vasodilation by nitric oxide in vivo

The objective of this study was to determine whether vasodilator effects of nitric oxide (NO) can be explained by the inhibition of vasoconstriction caused by peripheral sympathetic nerve activity (SNA) in vivo. For this purpose, we studied the effects of systemic inhibition of NO synthesis during experimental variation of SNA in anesthetized cats. Intravenous infusion of NG-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg) in baroreceptor-intact animals (n = 6) caused increases in mean arterial blood pressure (MAP) from 105.8 +/- 3.4 to 192.0 +/- 4.3 mm Hg that were associated with slight decreases in preganglionic SNA recorded from the white ramus of the third thoracic segment. Higher SNA appeared in completely baroreceptor-denervated cats (n = 10) than in the intact cats, but no changes in nerve activity occurred after the subsequent administration of L-NAME. In contrast, MAP increased from 123.3 +/- 4.0 to 245.8 +/- 5.1 mm Hg. In baroreceptor-denervated cats, reversible suppression of peripheral SNA produced by cooling of the ventral surface of the rostral ventrolateral medulla oblongata (RVLM) caused significant hypotension (61.1 +/- 2.6 mm Hg) and almost completely reversed the hypertension caused by L-NAME (76.0 +/- 3.7 mm Hg). Intravenous administration of the alpha 1-adrenergic receptor antagonist prazosin after L-NAME reduced MAP to a similar extent. In contrast, hypertension induced by angiotensin II could not be reversed by RVLM cooling. The pressor effects of intravenously administered noradrenaline during RVLM cooling were markedly potentiated by L-NAME and attenuated by the NO-donor compound S-nitroso-N-acetylpenicillamine (SNAP).(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathetic neurotransmission in the tail artery of aging rats

1. Age-related changes in noradrenergic neurotransmission in the tail arteries of three rat strains: outbred Wistar (WI/Ico), inbred Wistar (WAG/Rij) and inbred Fischer (F344) have been compared in the present study. 2. The arterial noradrenaline content varied from 5 to 10 ng mg-1 wet weight amongst young (3 to 6-month old) representatives of each strain, but did not change with age. As protein content increased in senescent rats (24-month old) by 30-40%, arterial tissue growth would not appear to receive a concomitant increase in sympathetic growth leading to relative, age-related, structural sympathectomy in all strains. 3. The vasoconstrictor response to transmural electrical stimulation was diminished in adult and senescent rats of all strains. 4. As far as could be judged from the increase in noradrenaline release following perfusion with the alpha-adrenoceptor antagonist, phentolamine (1 microM), the presynaptic alpha 2-adrenoceptor-mediated inhibition of noradrenaline release was intact in old representatives of all strains. 5. With blockade of the two main systems which control noradrenaline release in the rat tail artery, viz, neuronal reuptake with cocaine (4 microM) and presynaptic alpha 2-adrenoceptors with phentolamine (1 microM), stimulation-evoked release of noradrenaline was similar at all ages and in all strains. This suggests that in the rat tail artery the basic mechanism of neuronal release of noradrenaline is not functionally modified by aging. 6 We conclude that as sympathetic nerve terminals are apparently intact in all three strains of senescent rats used, the age-associated deficit of alpha-adrenergic control of vascular function is postsynaptic in nature.