Skip Navigation

Cardiovascular Research 1999 41(1):237-245; doi:10.1016/S0008-6363(98)00223-5
© 1999 by European Society of Cardiology
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Scotland, R.
Right arrow Articles by Ahluwalia, A.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Scotland, R.
Right arrow Articles by Ahluwalia, A.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Copyright © 1999, European Society of Cardiology

On the regulation of tone in vasa vasorum

Ramona Scotland*, Patrick Vallance and Amrita Ahluwalia

Centre for Clinical Pharmacology, The Wolfson Institute of Biomedical Research, University College London, Rayne Institute, 5 University St, London, WC1E 6JJ, UK

* Corresponding author. Tel.: +44-171-209-6606; fax: +44-171-813-2846; e-mail: r.scotland@ucl.ac.uk

Received 29 April 1998; accepted 5 June 1998


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Drugs
 4 Data and statistics
 5 Results
 6 Discussion
 References
 
Objective: The vasa vasorum form a network of microvessels in and around the walls of large blood vessels and are thought to be necessary to deliver oxygenated blood to the outer parts of the vessel wall that are inadequately nourished by diffusion from luminal blood. This study was undertaken to investigate directly the mechanisms which control tone in the vasa vasorum. Methods: Arterial vasa vasorum were dissected from the walls of porcine or bovine thoracic aorta and mounted in a tension myograph. Concentration-response curves were constructed to vasoconstrictors; endothelin-1(ET-1), noradrenaline (NA) angiotensin II (Ang II) and thromboxane A2-mimetics (U44069 [GenBank] or U46619 [GenBank] ) or vasodilators; substance P (SP) bradykinin (BK), calcitonin gene-related peptide (CGRP) or isoprenaline. Strips of porcine aorta were mounted in 25 ml organ baths. Results: Potent concentration-dependent contraction of vasa vasorum was produced by ET-1. NA was a weak constrictor, Ang II had no effect or produced contraction that underwent tachyphylaxis and thromboxane A2-mimetics had no effect. In contrast NA, Ang II, U-44069 and ET-1 all produced potent concentration-dependent contraction of aortic strips. SP and BK produced endothelium-dependent relaxation while CGRP produced endothelium-independent relaxation of ET-1-precontracted vasa vasorum. Isoprenaline had no relaxant effect. Conclusions: We have demonstrated functional responses of arterial vasa vasorum to vasodilators and vasoconstrictors. Additionally these microvessels appear to respond to constrictors differently from the large host vessel.

KEYWORDS Microcirculation; Arteries; Vasoconstriction; Vasodilation; Porcine; Bovine


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Drugs
 4 Data and statistics
 5 Results
 6 Discussion
 References
 
The vasa vasorum form a network of microvessels in and around the walls of large blood vessels. It is thought that these small vessels deliver oxygenated blood and nutrients to the outer parts of the vessel wall that are not supplied adequately by diffusion from luminal blood. Consistent with this suggestion vasa vasorum are found most abundantly in veins and larger arteries with a wall thickness of at least 0.5 mm [1], or 29 lamellae, where lamellar units are defined as fibromuscular layers consisting of elastin and an adjacent compartment of circumferentially oriented collagen, elastin and smooth muscle [2].

Decreased flow of blood through the vasa vasorum might lead to vessel wall hypoxia and this has been implicated in the pathogenesis of certain cardiovascular disorders. Indeed removal of the adventitial vasa vasorum of the aorta results in medial necrosis in dogs [3]and disruption of flow through the vasa vasorum may enhance the development of an atheroma-like lesion in rabbits [4]. Furthermore certain portions of the human aorta contain no vasa vasorum and these areas show the greatest propensity to atherosclerosis [5]. Despite the potential pathophysiological significance of altered flow through the vasa vasorum, little is known of the mechanisms that regulate their tone.

Several layers of smooth muscle cells are orientated radially in relation to the vasa implying that these vessels do not simply respond passively to changes in the tension of the large vessel wall [6]. Studies in vivo using microspheres have suggested that the diameter of vasa vasorum in aorta of dogs increases in response to adenosine [7], whereas phenylephrine and serotonin decrease the diameter of vasa vasorum of coronary arteries in atherosclerotic monkeys but have no effect in normal animals [8]. However the reports of these studies acknowledge that changes in the reactivity of the smooth muscle of the host conduit vessel itself may have contributed to the changes seen in the diameter of the vasa vasorum; distinguishing a true vasa vasorum response from that of the host vessel in vivo is difficult [8, 9]. One study, taking the vasa vasorum attached to the host vessel in vitro, has suggested that vasa vasorum contract and relax to vasoactive agents but, again, it was not possible to dissociate effects of the drugs and mediators on the wall of the large vessel from direct effects on the vasa vasorum [10]. In the present study we have investigated the reactivity of isolated vasa vasorum directly using the myography technique and explored the role of the endothelium in regulating changes in tone. These results have been presented in preliminary form at the Scientific Sessions of The American Heart Association [11, 12].


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Drugs
 4 Data and statistics
 5 Results
 6 Discussion
 References
 
Bovine aortic arch or porcine aorta were collected from an abattoir and placed immediately in cold (4°C) physiological salt solution (PSS) of the following composition (mM): NaCl 119, KCl 4.7, CaCl2.2H2O 2.5, MgSO4.7H2O 1.2, NaHCO3 25, KH2PO4 1.2 and glucose 5.5. Tissues were prepared either for reactivity or histological studies. For large vessel studies the porcine thoracic aorta was cleaned of extraneous tissue and helical strips of approximately 3 mm thick and 1 cm length were cut. For small artery studies vasa vasorum at the adventitial–medial border of the aorta were dissected free from extraneous tissue. The vessels were traced from large adventitial vessels (which are easily visible) down to the point at which they penetrated the media. Vessels at the adventitial–medial border were dissected out of the large vessel wall and cleaned of surrounding tissue using micro-dissection. Blood vessels cleared of surrounding tissue were either immersed immediately in formal saline (4% paraformaldehyde in saline; histological studies), or mounted in organ baths for measurement of reactivity (functional studies).

2.1 Vasa vasorum reactivity studies
Arteries were mounted between 2 stainless steel wires (40µm in diameter) in an automated tension myograph (JP Trading, Aarhus, Denmark) for the measurement of isometric tension [13]. Vessels were bathed in PSS gassed with 5% CO2 in O2 at 37°C. Arteries were stretched in a stepwise manner to determine the relationship between passive tension and internal circumference according to Laplace's equation. From this relationship the internal diameter was determined. Vessels were then stretched to 90% of the diameter achieved when the vessel was under an effective transmural pressure of 100 mmHg. These parameters were chosen since the maximum active tension response in small arteries of this size is achieved at this resting tension and because a transmural pressure of 100 mmHg approximates to physiological conditions for resistance arteries of 100–350 µm diameter in vivo (for review see [14]). The active tension-length relationship in the vasa vasorum was studied by testing the response to high K+ (125 mM) PSS at varying basal tension and was found to be similar to that of other small arteries (data not shown). Following the normalisation procedure, vessels were constricted with high K+ (125 mM) PSS repeatedly until contractions were constant. Those vessels which did not produce active tension responses at least equivalent to that produced in response to an effective transmural pressure of 100 mmHg were rejected. In certain studies, following normalisation, the endothelium was removed by passing a single hair through the lumen of the vessel. In small vessels this procedure results in removal of the endothelium without damage to the underlying smooth muscle [15].

2.1.1 Contractile responses
Contractile concentration response curves were constructed to noradrenaline (NA, 1–10000 nM), angiotensin II (Ang II, 0.01–300 nM), endothelin-1 (ET-1, 0.01–300 nM) and the thromboxane A2 mimetics (TxA2-mimetic) U-44069 (9{alpha},11{alpha}-epoxymethano-PGH2) or U-46619 (11{alpha},9{alpha}-epoxymethano-PGH2) (1–1000 nM) in either porcine or bovine arterial vasa vasorum. To determine whether the response to ET-1 was mediated by ETA receptors some vessels were pretreated with the selective ETA receptor antagonist BQ123 (10 µM) for 30 min prior to construction of the concentration response curve to ET-1. Control experiments in the absence of the antagonist were also carried out at the same time on paired rings from the same vessel.

2.1.2 Relaxant responses
Endothelium-intact or denuded vessels were precontracted with a submaximal concentration of ET-1 (10–30 nM) and concentration-response curves constructed to bradykinin (BK, 0.01–30 nM), substance P (SP, 0.01–30 nM) or calcitonin gene-related peptide (CGRP, 0.01–300 nM). Only one peptide was tested in any one preparation. The possible relaxant effects of NA (1–10000 nM), and the selective ß-adrenoceptor agonist isoprenaline (0.1–1000 nM), were also investigated. In a separate series of experiments concentration-response curves to the NO donor glyceryl trinitrate (GTN) were constructed in endothelium intact and endothelium-denuded vessels. Responses in endothelium-denuded vessels were compared to the responses in paired endothelium-intact vessels taken from the same segment of blood vessel.

To investigate the involvement of endogenous NO or prostanoids in the responses seen, concentration-response curves were constructed in the presence and absence of the following inhibitors:

• To determine whether the NO-guanylyl cyclase pathway was involved vessels were pretreated for 30 min with NG-nitro-L-arginine methyl ester (L-NAME, 300 µM) or ODQ (1 µM) to inhibit NO synthase and soluble guanylyl cyclase respectively [16, 17]. The effect of oxyhaemoglobin (OxyHb), an NO scavenger [18]was also investigated. Vessels were pretreated with OxyHb (10 µM) for 30 min prior to ET-1 application and then again at the time of ET-1 application to give a total concentration of 20µM.
• To determine whether generation of prostanoids was involved, vessels were pretreated with the cyclooxygenase inhibitor indomethacin (5 µM) for 30 min.
Following incubation with inhibitors, preparations were precontracted with ET-1 to give a level of tone approximately 70% of the control contractile response to K+ (i.e. equivalent to the tone used for control responses). Relaxation responses in drug-treated vasa vasorum were compared to those produced in control untreated vessels dissected from the same segment of blood vessel.

2.2 Porcine aortic strips
To compare the reactivity of the vasa vasorum with the host vessel from which they were dissected, the reactivity of porcine aortic strips was also investigated. Thoracic aortic strips (1 cm length) were mounted in a 25 ml organ bath. Basal tension was set at 2 g and the vessels left to equilibrate for 1 h in PSS bubbled with 5% CO2 in O2 at 37°C, with regular washing at 15 minute intervals. After 1 h vessels were subjected to 125 mM KCl. Following this a single concentration-response curve to NA, TxA2-mimetic, Ang II or ET-1 was constructed.

2.3 Immunohistochemistry
Sections of thoracic aorta (2 cm2) or isolated vasa vasorum were placed in 4% formal saline overnight. Vessels were then embedded in paraffin wax and longitudinal or cross-sections taken (3–4 µm) and mounted on slides. To demonstrate general structures, sections were stained with haematoxylin and eosin. Smooth muscle was detected using antibody to smooth muscle actin. For these studies the primary antibody used was a 1/100 dilution of mouse smooth muscle actin antibody followed by the standard streptavidivin–biotin technique.

The investigation conforms with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996)


   3 Drugs
Top
 Abstract
 1 Introduction
 2 Methods
 3 Drugs
4 Data and statistics
 5 Results
 6 Discussion
 References
 
Bradykinin acetate, indomethacin, noradrenaline bitartrate, L-NAME, U-44069, Ang II and SP were all purchased from Sigma Chemical Co., Poole, UK. CGRP and ET-1 were purchased from Bachem, UK and BQ123 from Clinalfa, Nottingham,UK. U-46619 was a kind gift of Upjohn, Kalamazoo, MI. Stock solutions of BK, CGRP, SP, BQ123, ET-1 and Ang II were made in sterile water and U-44069 and U-46619 in 50% DMSO. All aliquots were kept at –20°C until day of use. L-NAME and NA were made in saline on day of use. Indomethacin was dissolved in 1% sodium carbonate in saline on day of use. All dilutions were made in saline. The maximum concentration of DMSO in the bath at any time was 0.005%. Mouse smooth muscle actin monoclonal antibody was purchased from Dako Ltd, High Wycombe, UK.


   4 Data and statistics
Top
 Abstract
 1 Introduction
 2 Methods
 3 Drugs
 4 Data and statistics
5 Results
 6 Discussion
 References
 
For the measurement of effects of inhibitors or antagonists on concentration response curves to constrictors or dilators, the EC50 for each agonist was determined for each experiment in control and antagonist/inhibitor treated vessels. The shift was expressed as a concentration ratio (CR) of the EC50 for the agonist in control tissues and in the presence of antagonist/inhibitor. Statistical significance was calculated using analysis of variance for multiple comparisons followed by the Bonferroni test. When multiple comparisons were not required an unpaired Students t-test was used. P<0.05 was considered statistically significant.


   5 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Drugs
 4 Data and statistics
 5 Results
6 Discussion
 References
 
5.1 Histology
Fig. 1 shows typical photographs of (a) haematoxylin and eosin staining of porcine thoracic aorta showing an arterial vasa at the adventitial-medial border, (b) haematoxylin and eosin staining of isolated bovine arterial vasa vasorum showing a single layer of endothelium surrounded by several layers of smooth muscle and (c) smooth muscle actin immunoreactivity of isolated bovine arterial vasa vasorum.


Figure 1
View larger version (69K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 1 Histological studies of arterial vasa vasorum. A) haematoxylin and eosin staining of porcine thoracic aorta showing an arterial vasa at the adventitial–medial border (x400), B) haematoxylin and eosin staining of isolated bovine arterial vasa vasorum showing a single layer of endothelium surrounded by several layers of smooth muscle (x1000) and C) smooth muscle actin immunoreactivity of isolated bovine arterial vasa vasorum (x400).

 
5.2 Vasa vasorum
The mean diameters of the bovine and porcine arterial vasa vasorum vessels used in this study were 167±9.9 µm (n=52) and 174±4.4 µm (n=122) respectively.

5.3 Effects of contractile agents
ET-1 and NA caused concentration-dependent contraction of porcine arterial vasa vasorum (see Fig. 2Fig. 3A, Table 1). NA (n=5) was a weak constrictor of these vessels with an EC503000 nM and a maximum response of 15.7±7.0%. Neither TxA2-mimetic (n=5) nor Ang II (n=7) had any effect. The contractile responses of bovine vasa vasorum to the same agents are shown in Fig. 3B. NA (n=5) and ET-1 (n=4) caused concentration-dependent contraction of bovine vasa vasorum and, similar to the responses of porcine vasa vasorum, ET-1 was a potent contractile agent, whereas NA was a weak agonist with an EC503000 nM. Again, TxA2-mimetic had no effect (n=4) (see Table 1). The response to ET-1 was significantly attenuated by BQ123 (10 µM, n=4) which caused a 30-fold rightward shift of the concentration-response curve (Fig. 3B). Unlike the porcine vasa vasorum, bovine vessels responded to Ang II in a concentration-dependent manner with an EC50 of 10.5±3.5 nM (n=4). However the response to Ang II underwent a rapid desensitisation at concentrations above 30 nM.


Figure 2
View larger version (10K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 2 Typical contractile responses of porcine vasa vasorum to endothelin-1 (ET-1; panel A) and noradrenaline (NA; panel B).

 

Figure 3
View larger version (17K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 3 The effects of vasoconstrictor agents on vasa vasorum of the porcine and bovine thoracic aorta. Contractile concentration response curves of porcine vasa vasorum (panel A) and bovine vasa vasorum (panel B) to Ang II ({triangleup}, n=4), NA (bullet, n=5) thromboxane A2-mimetic ({circ}, n=3) and endothelin-1 (ET-1) in the absence ({square}, n=4) and presence ({blacksquare}, n=4) of the ETA receptor antagonist BQ123 (10 µM). All values shown are means±s.e.mean.

 

View this table:
[in this window]
[in a new window]

  		Table 1 Comparison of the potency of selected contractile agents on bovine vasa vasorum, porcine vasa vasorum and aortic strips.

 
5.4 Effects of relaxant agents
SP (n=18), BK (n=19) and CGRP (n=12) caused concentration-dependent relaxation of ET-1 precontracted porcine arterial vasa vasorum and gave EC50 and maximum relaxations of 3.5±0.7 nM, 39±5.1% (SP); 3.8±0.5 nM, 71.5±3.3% (BK); 6.7±1.5 nM, 84.1±2.4% (CGRP) respectively. The response to SP underwent rapid desensitisation at concentrations above 10 nM. Neither NA or isoprenaline had any relaxant effects in precontracted tissues (n=4 in each case). All three dilators (BK, SP and CGRP) also caused concentration-dependent relaxation of bovine vasa vasorum; BK (n=5), SP (n=5) and CGRP (n=5) gave EC50 values of 1.0±0.3 nM, 0.3±0.08 nM and 199.0±70.0 nM respectively.

5.4.1 Role of endothelium-derived factors
5.4.1.1 Porcine
The responses to BK (n=3, P<0.001, Fig. 4A) and SP (n=3, P<0.05, data not shown) were abolished by removal of the endothelium whilst the response to CGRP was unaffected (Fig. 4B). L-NAME (300µM, n=5) significantly inhibited responses to SP (Fig. 5B) but had no effect on BK-induced relaxation (Fig. 5A, n=5), which was also unaffected by pretreatment of vessels with ODQ (n=8), OxyHb (n=5), or L-NAME+indomethacin (n=4). Indomethacin alone (5 µM, n=5) had no significant effect on responses to either BK or SP. The EC50 for BK was 6.1±1.27 nM (n=9) and 6.4±1.4 nM (n=9) respectively in the absence and presence of indomethacin (not significantly different).The EC50 for SP was 1.7±2.5 nM (n=5) and 1.2±3.6 nM (n=5) respectively in the absence and presence of indomethacin (not significantly different).


Figure 4
View larger version (14K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 4 The effects of endothelial-denudation on relaxation responses of porcine vasa vasorum to bradykinin (BK, panel A) and calcitonin gene-related peptide (CGRP, panel B). Values shown are means±s.e.mean of responses in endothelium-intact ({blacksquare}) and endothelium-denuded vessels ({square}). *denotes statistically significant difference P<0.05 and **P<0.01.

 

Figure 5
View larger version (12K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 5 Relaxation responses to bradykinin (BK, panel A) and substance P (SP, panel B) of endothelin-1 precontracted porcine vasa vasorum in the absence ({square}) and presence ({blacksquare}) of L-NAME (300 µM). All values shown are means±s.e.m. *denotes statistically significant difference (P<0.05).

 
5.4.1.2 Bovine
In contrast to the results in porcine vessels, the concentration-response curve to BK was shifted 2-fold to the right (n=5) by L-NAME; giving EC50 values of 1.0±0.3 nM and 1.9±0.3 nM in the absence and presence of the inhibitor (P<0.05). BK-induced relaxation of bovine vasa vasorum was unaffected by cyclooxygenase inhibition; EC50 for BK was 0.25±0.08 nM (n=4) and 0.34±0.02 nM (n=4) respectively in the absence and presence of indomethacin (not significantly different). Maximum relaxation was 91.0±3.0% (–indomethacin) and 90.0±6.0% (+indomethacin) (n=4 in each case). GTN caused a concentration-dependent relaxation of vasa vasorum that was enhanced following removal of endothelium (EC50=30.0±11.5 nM and 10.2±1.7 nM (n=3) respectively in the presence and absence of the endothelium).

5.5 Porcine aortic strips
TXA2-mimetic, NA and Ang II produced potent concentration-dependent contractions of porcine aortic strips (see Table 1). ET-1 (n=5) caused a slowly developing contraction with an EC50 of 5.8±1.3 nM and maximum contraction of 91.6±11.4%.


   6 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Drugs
 4 Data and statistics
 5 Results
 6 Discussion
References
 
The results of this study demonstrate directly that the arterial vessels of the vasa vasorum respond to vasoactive agents. The smooth muscle is sensitive to constrictor and dilator agents, and the endothelium plays an important role in mediating certain relaxant responses. The vessels we studied were the small arteries just penetrating the muscular wall of the aortic arch and the thoracic aorta. The internal diameter of the vessels used was approximately 170 µm indicating that these small arteries are likely to contribute significantly to the overall resistance of the vasa vasorum [14, 19]. The vessels were dissected free from surrounding aortic tissue and we studied the responses of isolated arterial vasa vasorum.

6.1 Contraction of vasa vasorum smooth muscle
It is clear that the vasa vasorum have sufficient radial smooth muscle to generate considerable tension. The immunohistochemical studies demonstrate the presence of several layers of smooth muscle around the small arteries and the tension response to endothelin, which was significantly attenuated by ETA receptor antagonism, was similar to that reported in other resistance vessels. However, unlike the contractile responses in other small arteries, or indeed large vessels such as the host aorta from which the vessels were taken, the vasa vasorum did not contract to TXA2-mimetics, NA produced only a low level of contraction even at high concentrations (EC50 >3000 nM), and Ang II had no effect or produced a response that suffered rapid desensitisation. This profile of contractile reactivity was evident in both porcine and bovine vasa vasorum suggesting that under certain conditions decreased sensitivity to constrictor agents may be characteristic of vasa vasorum.

The physiological significance of the differences in contractile responses between the vasa vasorum and the large vessel and whether these differences exist in pressurised vessels in vivo are not known. However, it seems reasonable to speculate that it might be important for the vasa vasorum to be resistant to certain mediators that control tone in the large host artery itself. For example, our results indicate that NA released during increased sympathetic activity might constrict the major vessel, but would not affect the tone of vasa vasorum. This would ensure a patent system for the nutrition and oxygenation of the vessel wall even during sustained sympathetic activation and vasoconstriction. Similarly, the failure to respond to thromboxane A2-mimetics might protect the vessel wall from the constrictor effects of activated platelets.

Unlike the responses to the other constrictors the vasa vasorum showed potent concentration-dependent contractions to ET-1 with an EC50 as low as that reported for the effects of ET-1 in large vessels and indeed very similar to that seen in the parent aorta from which the vasa vasorum were dissected (Table 1). Immunohistochemistry of the vasa vasorum of human vessels has demonstrated considerable ET-1 binding [20]and together these findings raise the possibility that local generation of endothelin might be a major determinant of the regulation of flow through vasa vasorum in health or disease.

6.2 Relaxation of vasa vasorum smooth muscle
The vasa vasorum is also sensitive to vasodilators and we have shown that precontracted vessels relax to BK, SP, CGRP and GTN. These relaxant responses to BK and SP were endothelium-dependent whereas the responses to CGRP and GTN were not. Endothelial denudation resulted in selective removal of the endothelium without damage to the underlying smooth muscle; thus contractile responses of endothelium-intact vessels, to either ET-1 or K+, were not significantly different from responses in endothelium-denuded vessels (data not shown). The responses to SP appeared to be mediated by endothelium-derived NO since the relaxations were attenuated by inhibition of NO synthase. However, in contrast to their effects on the response to SP, neither L-NAME nor indomethacin, either alone or in combination, significantly altered the responses to BK in porcine vessels. Thus the response to BK was endothelium-dependent but appeared to be independent of NO or prostanoids. It is unlikely that the lack of effect of L-NAME was due to insufficient block of NOS since the concentrations used have previously been shown to provide almost total inhibition of enzyme activity [21]and indeed were effective in blocking the response to SP. The suggestion that the endothelium-dependent response to BK was independent of NOS activity was supported by the lack of effect of ODQ (inhibitor of guanylyl cyclase) or OxyHb (binding of free NO). It is possible that endothelium-dependent hyperpolarisation is involved in the responses seen and, consistent with this possibility, our preliminary studies suggest that raising the concentration of extracellular K+ concentration (50 mM) diminishes the response to BK.

In contrast to the responses in porcine vasa vasorum, the response to BK in bovine vasa vasorum was partly dependent on NO since the concentration-response curve was shifted 2-fold to the right by L-NAME. However, even in these vessels it seems that a major component of the response to BK occurs independently of NOS activity. Thus in the vasa vasorum BK evokes an endothelium-dependent response which is not fully accounted for by generation of NO and is independent of prostanoids.

6.3 Limitations of the study
The myograph allows measurement of the reactivity of small vessels of a minimum diameter of 100 µm. It is clear from previous work that the diameter of the vasa vasorum penetrating the media is often as small as 50 µm. Whether it is these medial vessels or the vessels we studied that are the most important with regards to regulation of perfusion of the large vessel wall is not known. However we studied vessels of a size that are considered to contribute significantly to vascular resistance and it seems likely that modulation of the tone of adventitial–medial vasa vasorum would have an impact on blood flow in the walls of large blood vessels.

6.4 Clinical significance
The finding that the vasa vasorum respond to vasoactive agents and that the endothelium modulates responses may have implications for the pathophysiology of disease affecting the host vessel. The endothelium of resistance vessels is important to co-ordinate the distribution of blood flow to tissues [22]. If endothelial function of vasa vasorum is altered by disease or following physical manoeuvres, such as balloon angioplasty of the large host vessel, this would affect blood flow in the vessel wall. Several studies suggest that the vasa vasorum may be affected by angioplasty of the host vessel [23–25]. The number of microvessels in the aortic wall increases following angioplasty in a canine disease model [23]and it would be interesting to determine whether the reactivity of the vessels is also affected. Furthermore in a balloon-injury model in the rat carotid artery it has been shown that there is an initial decrease in neuronal CGRP and SP-immunoreactivity around the vasa vasorum of the injured vessel and a compensatory increase in the control contralateral vessel [26]. The functional consequences of these changes are not known but our study shows that these mediators may alter the tone of the vasa vasorum.

In conclusion we have demonstrated functional responses of the arterial vasa vasorum to vasodilators and vasoconstrictors and our results suggest that this network responds differently from large vessels including the host vessel. We have studied two species and found similar results in each. This suggests that our results may be typical for vasa vasorum at the adventitial–medial border. The finding that the endothelium is important in mediating the responses may have implications for understanding the mechanisms of hypoxia or altered nutrient supply within the vessel wall in disease states or after vessel injury.

Time for primary review 23 days.


   Acknowledgements
 
AA is the recipient of an intermediate BHF Fellowship and RS is funded by an MRC studentship.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Drugs
 4 Data and statistics
 5 Results
 6 Discussion
 References
 

  1. Geiringer E. Intimal vascularization and atherosclerosis. J Path Bact (1951) 63:201–211.[CrossRef][Medline]
  2. Wolinsky H., Glasgov S. Nature of species differences in the medial distribution of aortic vasa vasorum in mammals. Circ Res (1967) 20:409–442.[Abstract/Free Full Text]
  3. Wilens S.L., Malcolm J.A., Vasquez J.M. Experimental infarction (medial necrosis) of the dog's aorta. Am J Pathol (1965) 47:695–771.[ISI][Medline]
  4. Booth R.F.G., Martin J.F., Honey A.C., et al. Rapid development of atherosclerotic lesions in the rabbit carotid artery induced by perivascular manipulation. Atherosclerosis (1989) 79:257–268.[CrossRef][ISI][Medline]
  5. Wolinsky H., Glagov S. Comparison of abdominal and thoracic aortic medial structure in mammals. Circ Res (1969) 25:677–686.[Abstract/Free Full Text]
  6. Heistad D.D., Marcus M.L. Role of vasa vasorum in nourishment of the aorta. Blood Vessels (1979) 16:225–238.[ISI][Medline]
  7. Koudy Williams J., Armstrong M.L., Heistad D.D. Blood flow through new microvessels: factors that affect regrowth of the vasa vasorum. Am J Physiol (1988) 254:H126–H132.[ISI][Medline]
  8. Koudy Williams J., Armstrong M.L., Heistad D.D. Vasa vasorum in atherosclerotic coronary arteries: responses to vasoactive stimuli and regression of atherosclerosis. Circ Res (1988) 62:515–523.[Abstract/Free Full Text]
  9. Heistad D.D., Marcus M.L., Martins J.B. Effects of neural stimuli on blood flow through vasa vasorum in dogs. Circ Res (1979) 45:615–620.[Abstract/Free Full Text]
  10. Ohhira A., Ohhashi T. Effects of aortic pressure and vasoactive agents on the vascular resistance of the vasa vasorum in canine isolated thoracic aorta. J Physiol (1992) 453:233–245.[Abstract/Free Full Text]
  11. Ahluwalia A., Vallance P.J. Reactivity of the vasa vasorum (Abstract). Circulation (1996) 94:I–588.
  12. Scotland R.S., Vallance P.J., Ahluwalia A. Comparison of reactivity of aorta and its vasa vasorum (Abstract). Circulation (1997) 96:I–165.
  13. Mulvany M.J., Halpern W. Contractile properties of small arterial resistance vessels in spontaneously hypertensive and normotensive rats. Circ Res (1977) 41:19–26.[Free Full Text]
  14. Mulvany M.J., Aalkjaer C. Structure and function of small arteries. Physiol Rev (1990) 70:921–961.[Abstract/Free Full Text]
  15. Kakuyama M., Vallance P., Ahluwalia A. Endothelium-dependent sensory NANC vasodilatation: involvement of ATP, CGRP and a possible NO store. Br J Pharmacol (1998) 123:310–316.[CrossRef][ISI][Medline]
  16. Garthwaite J., Southam E., Boulton C.L., et al. Potent and selective inhibition of nitric oxide-sensitive guanylyl cyclase by 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one. Mol Pharmacol (1995) 48:184–188.[Abstract]
  17. Moro M.A., Russell R.J., Cellek S., et al. cGMP mediates the vascular and platelet actions of nitric oxide: confirmation using an inhibitor of the soluble guanylyl cyclase. Proc Natl Acad Sci USA (1996) 93:1480–1485.[Abstract/Free Full Text]
  18. Martin W., Smith J.A., White D.G. The mechanisms by which haemoglobin inhibits the relaxation of rabbit aorta induced by nitrovasodilators, nitric oxide or bovine retractor penis inhibitory factor. Br J Pharmacol (1986) 89:563–571.[ISI][Medline]
  19. Johnson P.C., Hanson K.M. Effect of arterial pressure on arterial and venous resistance of intestine. J Appl Physiol (1962) 17:503–508.[Abstract/Free Full Text]
  20. Dashwood M.R., Barker S.G.E., Muddle J.R., Yacoub M.H., Martin J.F. [125I]-Endothelin-1 binding to vasa vasorum and regions of neovascularization in human and porcine blood vessels: a possible role for endothelin in intimal hyperplasia and atherosclerosis. J Cardiovasc Pharmacol (1993) 22(S8):S343–S347.[ISI][Medline]
  21. Rees D.D., Palmer R.M.J., Schulz R., Hodson H.F., Moncada S. Characterization of three inhibitors of endothelial nitric oxide synthase in vitro and in vivo. Br J Pharmacol (1990) 101:746–752.[ISI][Medline]
  22. Griffith T.M., Edwards D.H., Davies R.L., Harrison T.J., Evans K.T. EDRF coordinates the behaviour of vascular resistance vessels. Nature (1987) 329:442–445.[CrossRef][Medline]
  23. Pisco J.M., Correia M., Esperance-Pina J.A., Aires de Sousa L. Changes in the vasa vasorum following percutaneous transluminal angioplasty in a canine model of aortic stenosis. J Vasc Interven Radiol (1994) 5:561–566.[ISI][Medline]
  24. Zollikofer C.L., Redha F.H., Bruhlmann W.F., et al. Acute and long-term effects of massive balloon dilation on the aortic wall and vasa vasorum. Radiology (1987) 164:145–149.[Abstract/Free Full Text]
  25. Cragg A.H., Einzig S., Rysavy J.A., et al. The vasa vasorum and angioplasty. Radiology (1983) 148:75–80.[Abstract/Free Full Text]
  26. Milner P., Crowe R., Loesch A., et al. Neurocompensatory responses to balloon-catheter-induced injury of the rat carotid artery. J Vasc Res (1997) 34:31–40.[ISI][Medline]

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?



This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Scotland, R.
Right arrow Articles by Ahluwalia, A.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Scotland, R.
Right arrow Articles by Ahluwalia, A.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?