(Wilens, Malcolm & Vasquez, 1965), which indicates that the vasa vasorum plays a

Transcription

1 Journal of Physiology (1992), 453, pp With 7 figures Printed in Great Britain EFFETS OF AORTI PRESSURE AND VASOATIVE AGENTS ON THE VASULAR RESISTANE OF THE VASA VASORUM IN ANINE ISOLATED THORAI AORTA BY AKIYOSHI OHHIRA AND TOSHIO OHHASHI* From the First Department of Physiology, Shinshu University School of Medicine, Asahi, Matsumoto 390, Japan (Received 14 August 1991) SUMMARY 1. We have developed a new preparation for continuously measuring changes in vascular resistance of the vasa vasorum of the canine isolated thoracic aorta perfused at a constant flow rate with Krebs-bicarbonate solution. 2. An increase of more than 150 mmhg in aortic pressure caused a significant increase in the vascular resistance of the vasa vasorum Hydroxytryptamine (5-HT), noradrenaline (NA), adrenaline and dopamine caused dose-dependent increases in the vascular resistance of the vasa vasorum. The decreasing order of potency in the vasoconstrictor responses was as follows: 5-HT >> NA = adrenaline >> dopamine. The 5-HT- and adrenaline-induced vasoconstrictor responses were inhibited by methysergide and by phentolamine plus propranolol, respectively. 4. Acetylcholine (Ah), isoprenaline (ISP), histamine (His), ATP, ADP and adenosine produced dose-related decreases in the vascular resistance of aortic vasa vasorum perfused with the Krebs solution containing 10-5 M-NA. The decreasing order of potency in the response was as follows: Ah = ISP > His > > adenosine = ATP = ADP. The Ah-, ISP- and His-induced vasodilator responses were antagonized by atropine, propranolol and famotidine, respectively. 5. The results suggest that the preparation described is useful for studying the regulation of vascular resistance of aortic vasa vasorum and that aortic pressure and vasoactive compounds may directly regulate the vascular resistance of the vasa vasorum in canine isolated thoracic aorta. INTRODUTION The aorta is nourished by diffusion of nutrients from the lumen of the vessel, by diffusion from adventitial vessels, or by blood flow through vascular channels in the media called vasa vasorum (Wolinsky & Glagov, 1967). Ligation of the intercostal arteries that give rise to aortic vasa vasorum produces medial necrosis in dogs (Wilens, Malcolm & Vasquez, 1965), which indicates that the vasa vasorum plays a critical role in the nourishment of the thoracic aorta. In dog and man, the vasa can MS 9644 * To whom all reprint requests should be sent.

2 234 A. OHHIRA AND T. OHHASHI be identified in the outer portion of the media of the thoracic aorta (Wolinsky & Glagov, 1969). Electron micrographic studies (Heistad, Marcus, Law & Armstrong, 1978) indicate that aortic vasa vasorum have their own smooth muscles which are oriented in relation to the vasa, and not in relation to the aortic media. In addition, nerve fibres, most of which apparently originate in or transverse the left stellate and caudal cervical ganglia (McKibben & Getty, 1968), penetrate the aortic media near the vasa (Kienecker & Knoche, 1978). Recently, microsphere (Heistad et al. 1978) and autoradiographic (Werber & Heistad, 1985) methods have been developed to provide measurements of blood flow through vasa vasorum. The vessels are very responsive to physiological stimuli (Heistad et al. 1978), as they dilate during infusion of adenosine and contract during stimulation of sympathetic nerves (Heistad, Marcus & Martin, 1979). Using these methods, however, it is not possible to measure continuously the vascular resistance of aortic vasa vasorum and difficult to eliminate systemic neural and humoral factors which can indirectly affect blood flow through the vasa vasorum. In the present study, we have developed a new method of continuously measuring the vascular resistance of vasa vasorum of the isolated canine thoracic aorta by perfusing the vasa at a constant flow rate. In addition, by using this preparation we have investigated the direct effects of aortic pressure and of physiological vasoactive compounds on the vascular resistance of the vasa vasorum. METHODS Animal preparation One hundred and three mongrel dogs of both sexes, weighing 7-30 kg, were anaesthetized with intravenous sodium pentobarbitone (25 mg/kg) and then killed by bleeding. ylindrical aortic segments (70 mm long) with the 5th and 6th intercostal arteries were dissected out and immediately placed in cold (4 Krebs-bicarbonate solution. The composition of the solution (mm) was as follows: Nal, 120-0; Kl, 5-9; NaHO3, 25-0; NaH2PO--, 1P2; al2, 2-5;Mgl2, 1-2; and glucose, 5-5. The Krebs solution was continuously aerated with 95 % 02+5 % O2 gas mixture to give a ph of 7-4. Other tissues from these animals were used in other studies. Measurement of the vascular resistance of aortic vasa vasorum A schematic illustration of the preparation used to measure continuously the vascular resistance of aortic vasa vasorum is shown in Fig. 1 A. A polyethylene catheter was inserted retrogradely into an intercostal artery (mean outer diameter, 1-89 mm, n = 1 with side branches (mean outer diameter, 0'24 mm, n = 1 which gave rise to the aortic vasa vasorum. In every preparation, an intercostal artery was ligated at the point where it originated from the adventitia of the thoracic aorta after confirming, by an injection of Evans Blue dye into the catheter, that there was a full network of small arteries in the adventitia that was served by the artery. All of the adventitial small arteries from which injected dye leaked were ligated at the leakage points. Thus, the remaining dye was confirmed morphologically to perfuse through the aortic vasa vasorum and thence to leak out of the venous vasa vasorum. The other intercostal arteries were tied off on the adventitial surface of the thoracic aorta. The cannulated intercostal artery was perfused at a constant flow rate ( ml/min in each preparation) with Krebs-bicarbonate solution by means of a peristaltic pump (ATTO, SJ121 1). The solution was continuously aerated with 95% % O2 gas mixture to give a ph of 7-4 and kept at a constant temperature of 37 T by use of a heat exchanger. A pressure transducer (Statham P22) was attached to a side branch of the inlet cannula inserted into the intercostal artery. In addition, intraluminal pressure of the isolated aortic segment was measured by a pressure transducer with a two-way stopcock attached to a cannula tied into one end of the length of aorta. The other end of the aorta was connected via a cannula to a perfusion pump (ATTO, SJ121 1). The aorta was stretched to its in vivo length and then placed in an 80 ml

3 VASULAR RESISTANE OF AORTI VASA VASORUM organ bath which was perfused with Krebs solution at ph 7-4 and 37 0 throughout the experiments. The intraluminal space of the aorta was perfused with the Krebs solution at perfusion pressures that were set according to the type of experiment (see below). Aortic pressure measurements were made when the stopcock was clamped to make a blind system. hanges in 235 A Intercostal artery annula Thoracic aorta Vacuum pump 95% % O2 Fig. 1. A, a schematic illustration of the preparation used to perfuse the aortic vasa vasorum of the canine isolated thoracic aorta. B, a schematic illustration of the experimental layout. perfusion pressure through the aorta vasa vasorum and in aortic pressure were simultaneously recorded on a direct-writing oscillograph (NE Sanei, 8K). The vascular resistance of the vasa vasorum for each preparation was calculated as the ratio of the changes in the perfusion pressure through the vasa vasorum to the perfusion volume over a constant time. A change of the perfusion pressure presumably indicated a change in the vascular resistance because the rate of the perfusion flow was kept constant during the experiment. A schematic diagram of the experimental layout is shown in Fig. IB. Histological studies To visualize the aortic vasa vasorum, Krebs solution containing 05 % Evans Blue dye or colloidal carbon was perfused through a cannula placed in an intercostal artery as described above. After perfusion for 20 min, the aorta was frozen and then cut in cross-sections (30 um in thickness)

4 236 A. OHHIRA AND T. OHHASHI by use of a cryostat microtome (Bright, 503 kept at -20 '. The cryostat sections were stained with haematoxylin and eosin and examined microscopically (Olympus, BH-2, magnification x 2. Five aortae were prepared for the histological study. Effects of aortic pressure and vasoactive substances on the vascular resistance of the aortic vasa vasorum Before the start of each experiment, the preparation was allowed to equilibrate for about 90 min in the Krebs-bicarbonate solution at ph 7-4 and 37 ' during simultaneous perfusion of the intraluminal space of the aortic segment. In order to examine the effects of aortic pressure on the vascular resistance of aortic vasa vasorum, the mean perfusion pressure through the vasa vasorum was changed by adjusting the flow rate of the perfusion to low, middle and high levels before the experiments: (n = 5), (n = 6) and (n = 5) mmhg, respectively. At each level, changes in the perfusion pressure through the vasa vasorum were measured during elevation of the aortic pressure from 0 to 175 mmhg in steps of 25 mmhg. The mean perfusion pressure was calculated as the diastolic pressure plus one-third of the pulse pressure. To study the effects of vasoactive compounds on the vascular resistance of aortic vasa vasorum, the perfusion pressure through the vasa vasorum was set at about 70 mmhg before the experiment began, which was optimal for developing the maximum response to 10-5 M-noradrenaline in each preparation. The mean aortic pressure was kept at about 50 mmhg during the experiments. Measurements were made of the changes in the perfusion pressure through the vasa vasorum that were induced when the normal Krebs-bicarbonate solution was changed to a test solution containing a given concentration of a vasoactive compound; a dose-response curve for the compound was obtained by random administration of different doses. Dose-response curves for vasodilator agents were obtained in preparations with vasa vasorum perfused with the Krebs solution containing 10'- M-noradrenaline. In some experiments, the effects of antagonists on the agonist-induced responses were investigated in the same preparations. The antagonist was perfused for at least 10 min before the relevant agonist was added. Effects of vasoactive substances on isolated aortic strips After isolated thoracic aortae were cleaned of fat and connective tissue, they were cut into cylindrical strips (2 mm in width). Two fine silk threads were inserted through the aortic strip, one at either end. These were used to suspend the strip in a 10 ml organ bath which was perfused with the Krebs-bicarbonate solution at a constant rate of 4 ml/min. One of the threads was connected to a force-displacement transducer (Shinko Tushin UL-1, and the other was fixed to the bottom of the bath. The resting tension of the strip was adjusted to about 3 g, so that in each strip it was optimal for obtaining a maximum contractile response to 10-5 M-noradrenaline. All aortic strips were allowed to equilibrate for 60 min. After the equilibration period, cumulative dose-response curves for vasoactive compounds were obtained by stepwise, 10-fold increases in the concentration of the agent. When responses to vasodilator agents were observed, the strips were pre-contracted by 10-5 M-noradrenaline, which resulted in almost maximum contraction in each preparation. Drugs The following vasoactive substances were used, dissolved in the normal Krebs-bicarbonate solution; DL-noradrenaline hydrochloride (Sankyo), L-adrenaline hydrochloride (Daiichi Seiyaku), serotonin creatine sulphate (Sigma), dopamine hydrochloride (Sigma), prostaglandin F2,z (Ono), adenosine (Kowa), ADP disodium (Kyowa), ATP disodium (Kowa), L-isoprenaline hydrochloride (Nikken Kagaku), acetylcholine chloride (Daiichi Seiyaku), histamine dihydrochloride (Wako), sodium nitroprusside (Merck), propranolol hydrochloride (Sumitomo Kagaku), atropine sulphate (Tanabe Seiyaku), methysergide hydrogen maleate (Sandoz) and famotidine (Yamanouchi Seiyaku). Statistical analyses The results shown in the figures and text are expressed as the mean + standard error of the mean. In all experiments, n equals the number of preparations taken from different dogs. Statistical analyses were performed using Student's unpaired t test or one-way analysis of variance (ANOVA) followed by Dunnett's test. The differences in means were considered significant when P < 0 05.

5 VASULAR RESISTANE OF AORTI VASA VASORUM 237 RESULTS Histological studies After injection of the colloidal carbon or Evans Blue dye into the cannulated intercostal artery, small arteries were confirmed visually to penetrate into the aortic wall from the adventitial side and then the injected carbon or dye was observed to leak out from the venous vasa vasorum. The injected substances were confirmed histologically to be in the outer layers of the media and the adventitia of the cryostat sections (Fig. 2). Effects of aortic pressure on the vascular resistance of aortic vasa vasorum Figure 3 shows the effects of increasing aortic pressure in the range mmhg on the mean perfusion pressure through aortic vasa vasorum. In these experiments the perfusion pressure for the vasa vasorum was set initially at one of three different levels, i.e (n = 5), (n = 6) and (n = 5) mmhg. At 61 mmhg, the perfusion pressure through the vasa vasorum was not affected when the aortic pressure was increased up to 75 mmhg. However, a stepwise increment of more than 125 mmhg in the aortic pressure caused a significant increase in the mean perfusion pressure through the vasa vasorum. When the aortic pressure was raised to 175 mmhg, the mean perfusion pressure through the vasa vasorum was increased up to about 100 mmhg. On the other hand, at initial perfusion pressures of 109 or 158 mmhg, a significant change in the mean perfusion pressure was not observed until the aortic pressure was raised to more than 150 mmhg. Thus, the vascular resistance of aortic vasa vasorum was increased when mean aortic pressure was elevated to more than 125 or 150 mmhg. Effects of vasoconstrictor substances administered to aortic vasa vasorum Administration of 5-hydroxytryptamine (5-HT), noradrenaline (NA), adrenaline and dopamine produced concentration-dependent increases in the perfusion pressure through aortic vasa vasorum. Four different preparations were used for each substance. The dose-response curves for these agonists are shown in Fig. 4A. 5-HT induced the most potent vasoconstrictor effect on the aortic vasa vasorum in each preparation. The pd2 values (-log ED50, where ED50 is the concentration of agonist that caused 50 % of the maximum response) for 5-HT, NA and adrenaline were , and (n = 4, in each case), respectively. The pd2 value for dopamine could not be calculated exactly, since dopamine up to a concentration of 10-4 M did not produce a maximum response. However, if a calculation is made on the assumption that 10-4 m-dopamine did induce a maximum vasoconstriction, the 'nominal' pd2 value for dopamine would be less than (n = 4). Thus, the decreasing order of potency of the vasoconstrictor responses was as follows: 5-HT > > NA = adrenaline > > dopamine. The maximum responses produced by 5-HT, NA and dopamine were respectively , and 101I % of the increment in perfusion pressure that was produced by 10-4 M- adrenaline (n = 4, in each case). The 10-5 M-adrenaline-induced constrictor response was completely blocked by

6 238 A. OHHIRA AND T. OHHASHI Fig. 2. Transverse cryostat section of a canine isolated thoracic aorta in which the aortic vasa vasorum had been perfused with Krebs-bicarbonate solution containing 0 5 % Evans Blue dye. The magnification is x 20. The dye can be observed within small arteries of the vasa vasorum in the outer layers of the media and the adventitia. a) n 200 a) c _- m E ~~~~~~~ 4* 4* *o o 50- ). * Aortic perfusion pressure (mmhg) Fig. 3. Relationship between the aortic pressure (the abscissa) and changes in the mean perfusion pressure through aortic vasa vasorum (the ordinate) in the canine isolated thoracic aortae. The mean perfusion pressure was initially set at low, middle and high levels i.e (n = 5), (n = 6) and (n = 5) mmhg, respectively. * Significantly different from the control (each initial perfusion pressure), P < pretreatment with 10-6 M-phentolamine (Fig. 5A, n = 4); instead, this same concentration of adrenaline produced a marked decrease in perfusion pressure (Fig. 5A), which was inhibited by additional treatment with 10-7 M-propranolol (n = 4).

7 VASULAR RESISTANE OF AORTI VASA VASORUM 239 Further, in the presence of 10-7 M-propranolol, treatment with 10-7 M-phentolamine caused a parallel shift to the right in the dose-response curve for adrenaline. This is shown in Fig. 5B (n = 4). The vasoconstrictor response induced by 10-6 M-5-HT was dose-dependently inhibited by pretreatment with M-methysergide (Fig. 7, A e) L- o D 0. n a 0 It 0. 6) a o Vasa vasorum._.) #-A _ B 6)._ 0 6) L 6) o o Aorta ) 6) L- O-I 7 6 -log [agonist] (M) 5 4 Fig. 4. A, dose-response curves for 5-hydroxytryptamine (, noradrenaline (@), adrenaline (A), dopamine (A) and prostaglandin F2, (PGF2a) (Eli) in the vasa vasorum preparations (n = 4, in each case). The ordinate shows the extent of the agonist-induced increase in mean perfusion pressure expressed as a percentage of the maximum response to 1o-4 M-adrenaline in each preparation. The abscissa shows the concentration of the agonist on a logarithmic scale. B, cumulative dose-response curves for the agonists in the isolated aortic strips (n = 4, in each case). The ordinate shows the extent of the agonistinduced contraction expressed as a percentage of the maximum contraction developed by 10-4 M-adrenaline. The abscissa is the same as A.

8 240 A. OHHIRA AND T. OHHASHI n = 4). In contrast to these agonists, prostaglandin F2<, ranging from 10-8 to 10-4 M produced no increase of the perfusion pressure through the vasa vasorum (n = 4). Effects of vasoconstrictor substances on the isolated aortic strips oncentration-dependent contractions of the isolated aortic strips were also induced by 5-HT, NA, adrenaline and dopamine. Four different strips were used for Vasa vasorum perfusion Phentolamine pressure (mmhg) (10-6 M) 100] 50- Adrenaline (10-5 M) 20 min B 15- E E E 10, g / / at+ 4 6 * log [adrenaline] (M) Fig. 5. A, a typical recording of effects of 10-5 M-adrenaline on the perfusion pressure through the aortic vasa vasorum in the absence and presence of 10-6 M-phentolamine. B, effect of 10-7 M-phentolamine on dose-response curves for adrenaline in the presence of 10- M-propranolol in the vasa vasorum preparations (n = 4, in each case). 0, control; *, 10- M-phentolamine. The ordinate shows the extent of the adrenaline-induced increase in the absolute value of the mean perfusion pressure (AP) through the aortic vasa vasorum. The abscissa shows the concentration of adrenaline on a logarithmic scale. * Significantly different from the control, P < each substance. Dose-response curves for these agonists are shown in Fig. 4B. The pd2 value for the 5-HT was (n = 4). The nominal pd2 values (as defined above) for NA, adrenaline and dopamine were less than , and (n = 4, in each case), respectively. The decreasing order of potency of the contractile responses was as follows: 5-HT >> NA = adrenaline >> dopamine. The maximum responses produced by 5-HT, NA and dopamine were , and % (n = 4, in each case) of the 10-4 M-adrenaline-induced contraction in each strip, respectively. Up to 10-4 M-prostaglandin F2. caused no contraction in the isolated aortic strips (n = 4).

9 VASULAR RESISTANE OF AORTI VASA VASORUM 241 Effects of vasodilator substances administered to the aortic vasa vasorum Acetylcholine (Ah), histamine (His), isoprenaline (ISP), ATP, ADP, adenosine and sodium nitroprusside (SNP) caused dose-dependent decreases in the perfusion pressure through the aortic vasa vasorum perfused with the Krebs solution e am A a) 01) 0._ cl a) 0 01) log [agonist] (M) Vasa vasorum B c 0-0. c o n am I.. 'a ~0 co 0X Aorta Fig. 6. A, dose-response curves for sodium nitroprusside (O), acetylcholine (-), histamine (A), isoprenaline (A), ATP (IZ), ADP (A) and adenosine (x) in the vasa vasorum preparations (n = 4, in each case). The ordinate shows the extent of the agonistinduced decrease in the mean perfusion pressure through the vasa vasorum expressed as a percentage of the increased value of the perfusion pressure produced by 10-5 M- noradrenaline in each preparation. The abscissa is the same as Fig. 4A. B, cumulative dose-response curves for the vasodilator agonists in the isolated aortic strips precontracted by 10-5 M-noradrenaline (n 4, in each case). The ordinate shows the = extent of the agonist-induced relaxation expressed as a percentage of the precontraction produced by 10-5 M-noradrenaline. The abscissa is the same as Fig. 4A. containing 10- m-na (see Fig. 6A). Four different preparations were used for each substance. The pd2 values for Ah and His were 6X47 and 5 70 (n = 4, in each case), respectively. By contrast, dose-response curves for ISP, ATP, ADP and adenosine did not show a plateau in concentrations ranging from 1O-7 to 1O-4 M. Thus, the

10 242 A. OHHIRA AND T. OHHASHI nominal pd2 values (as defined above) for ISP, ATP, ADP and adenosine were calculated to be less than 6-26, 4-92, 4-84 and 5-20 (n = 4, in each case), respectively. The decreasing order of potency in the vasodilator responses was as follows: Ah = ISP > His >> adenosine = ATP = ADP. The maximum responses produced by 5-HT (10-6 M) ISP (10-5 M) His (10-5M) Ah (10-5M) 100 ** **.0* 50 L Methysergide Propranolol Famotidine Atropine -log [antagonist] (M) Fig. 7. Effects of methysergide ( M), propranolol ( M), famotidine ( M) and atropine ( M) on the 10-6 M-5-hydroxytryptamine (5-HT), 10-5 M-isoprenaline (ISP), 10-5 M-histamine (His) and 10-5 M-acetylcholine (Ah)-induced vasoactive responses in the vasa vasorum preparations, respectively (n = 4, in each case). The ordinate shows the extent of inhibition produced by the antagonists expressed as a percentage of the maximum vasoconstrictor response induced by 10-6 M-5-HT or the maximum vasodilator response developed by 10-5 M-ISP, 10-5 M-His or 10-5 M-Ah, respectively. * Significantly different from the control (without pretreatment with each antagonist), P < Ah and His were 16-0 and 29-4 % of the 1O- M-NA-induced increment of the perfusion pressure through the vasa vasorum, respectively (n = 4, in each case). SNP produced the most potent vasodilator effect on the perfusion pressure in all preparations. The 1O-5 M-Ah-, 1O- M-ISP- and 10-5 M-His-induced vasodilator responses were reduced in a dose-dependent fashion by pretreatment with atropine (10-'-10-7 M), propranolol (10-s-10-6 M) and famotidine ( M), respectively (n = 4, in each case) (Fig. 7). Effects of vasodilator substances on the isolated aortic strips The vasodilator agents also caused dose-related relaxations of isolated aortic strips that were precontracted by 10-5 M-NA (see Fig. 6B). Four different strips were used for each substance. The pd2 values for Ah and His were 6-50 and 4-75 (n = 4, in each case), respectively. The nominal pd2 values (as defined above) for ISP, ATP, ADP and adenosine were calculated to be less than 5 49, 5 44, 5.75 and 4 50 (n = 4, in each case), respectively. The decreasing order of potency in the relaxant responses was as follows: Ah > > ADP = ISP = ATP > > His = adenosine. SNP had the most potent vasodilator action on the isolated aortic strips.

11 VASULAR RESISTANE OF AORTI VASA VASOR UM 243 DISUSSION Measurement of the vascular resistance of aortic vasa vasorum The vasa vasorum may play a role in disease states. Insufficient blood flow through aortic vasa vasorum may contribute to medial necrosis of the aorta (Wilens et al. 1965) and to aortic atherosclerosis (Geiringer, 1951). It has been suggested also that the changes in aortic vasa vasorum seen in hypertension and other vascular diseases merit further study (Heistad & Marcus, 1979). For the investigation of physiological and pathophysiological roles in the haemodynamic regulation of aortic vasa vasorum, a method for evaluating the flow rate through the vasa vasorum must be developed first. We have described a new method for continuously measuring the vascular resistance of the vasa vasorum of isolated canine thoracic aortae. These preparations also allowed us to study the direct effects of physiological stimuli on the smooth muscles in the walls of the vasa vasorum. Our histological findings suggested that colloidal carbon or Evans Blue dye injected into the cannulated intercostal artery could be clearly observed in outer layers of the media in the thoracic aorta (Fig. 2). These findings are compatible with the evidence that aortic vasa vasorum provide a considerable blood flow to the outer layers of canine thoracic aorta (Heistad et al. 1978). They also suggest that the preparation we have developed may be suitable for continuously recording the vascular resistance of aortic vasa vasorum. However, in such preparations, there is some difficulty in deciding which component provides the major vascular resistance in the aortic vasa vasorum. Recently, we have modified the preparation by inserting a catheter into the venous vasa vasorum in the isolated thoracic aorta (Ohhashi & Yoshinaka, 199. The vascular resistance of the vasa vasorum in this modified preparation was almost the same as that obtained in the preparation used in the present study. Thus, it seems that the major contribution to the vascular resistance of the vasa vasorum is provided by the arterial and arteriolar vasa vasorum in the aortic walls. This conclusion is supported by the finding that Evans Blue dye injected into the cannulated intercostal artery remained within the arterioles or small arteries in the media of the aortic wall (Fig. 2). An electron microscopic study showing that arterioles or small arteries distributed from aortic vasa vasorum have their own smooth muscles which are oriented in relation to the vasa, and not in relation to the aortic media (Heistad et al. 1978), is consistent with the conclusion. Effects of aortic pressure on the vascular resistance of aortic vasa vasorum Marcus, Heistad, Armstrong & Abboud (1985) suggested that the decrease in conductance of aortic vasa vasorum during hypertension may be a result of distension of the aorta and distortion of the vasa vasorum. Thus, we have attempted to evaluate directly the effects of aortic pressure on the vascular resistance of the vasa vasorum in our preparation. The present results suggest that an increase in mean aortic pressure to more than 125 or 150 mmhg causes a significant increase of the vascular resistance of the aortic vasa vasorum. These findings strongly support Heistad et al.'s conclusion (1978), obtained with the microsphere technique, that deformation or stretch of the thoracic aortic wall may compress the vasa vasorum

12 244 A. OHHIRA AND T. OHHASHI during acute hypertension. In addition, our results clearly demonstrated that the regulatory action of aortic pressure on the vascular resistance of the vasa vasorum was more pronounced when the perfusion pressure through the vasa vasorum was lower. This tendency may be explained, in part, by the evidence that an acute increase in aortic pressure (hypertension) elicits an increase in the circumferential tension in the aortic walls (Simon, Kobayashi, Wiederhielm & Strandness, 1973). This increase in tension would be transmitted to the aortic smooth muscles as well as to the elastic and collagen fibres in the wall, and may distort or compress the aortic vasa vasorum, resulting in an increase in the vascular resistance of the vasa vasorum. The increase in the vascular resistance produced by the augmentation of the circumferential tension may be inhibited reciprocally by an increase of the perfusion pressure through the vasa vasorum. It is well known that aortic circumferential tension is not homogenous in the wall; it has been calculated on theoretical grounds to be lower in the outer layers of the wall (Sato, 1977). Further, the localization of aortic vasa vasorum is restricted to the outer two-thirds of the media in canine thoracic aorta (Heistad et al. 1978). Thus, there is an avascular zone in the inner layers of the media in the aorta. It is therefore reasonable to propose that the localization of the aortic vasa vasorum may be related, in part, to the gradient of circumferential tension in the aortic wall which is determined by intra- and extraluminal pressure, and the inner and outer radii (Azuma & Oka, 1971). Effects of vasoactive substances on the vascular resistance of aortic vasa vasorum We investigated the effects of vasoconstrictor and vasodilator compounds on the vascular resistance of aortic vasa vasorum, and compared them with the mechanical responses of the same aortic strips to the vasoactive compounds. The vascular resistance of the aortic vasa vasorum was raised by the stimulation of serotonergic receptors or x-adrenoceptors. In the aortic vasa vasorum, 5-HT was the most potent vasoconstrictor compound. The agonist-induced responses, however, may be caused either by: (1) a direct effect of the agonist on the smooth muscles of the vasa vasorum or (2) an indirect effect of the agonist such that a change in the mechanical activity of aortic smooth muscles in the media, which changes the vascular resistance of the vasa vasorum. However, whilst the pd2 values for NA and adrenaline in the vasa vasorum preparations are almost the same as those obtained for the aortic strips, the pd2 value for 5-HT was higher for the vasa vasorum, suggesting that 5-HT may have a direct vasoconstrictor effect on the smooth muscles of the vasa vasorum. On the other hand, NA and adrenaline may actually regulate the vascular resistance through aortic vasa vasorum in physiological and pathophysiological states, even if the mechanisms of the a-agonist-induced responses include direct as well as indirect actions on the smooth muscles of the vasa vasorum. This suggestion is compatible with the experimental evidence that electrical stimulation of the stellate ganglion reduces blood flow through aortic vasa vasorum in canine thoracic aorta (Heistad et al. 1979), and that an increase in the release of endogenous catecholamines, NA and adrenaline may contribute to ischaemia of the aortic wall, initiating aortic vascular diseases (Wilens et al. 1965). On the other hand, the vascular resistance of aortic vasa vasorum was lowered by the stimulation of muscarinic receptors, H2-histaminergic receptors or

13 VASULAR RESISTANE OF AORTI VASA VASORUM fl-adrenoceptors. Ah was the most potent vasodilator compound in the aortic vasa vasorum. Furthermore, by comparison with pd2 values obtained with the vasa vasorum preparations and the aortic strips, it seems that His and ISP were more effective on the smooth muscles of vasa vasorum. This suggests that H2-histaminergic receptors and fl-adrenoceptors are present on the smooth muscles of the vasa vasorum. The pd2 value for adenosine in the vasa vasorum was also higher than that obtained with the aortic strips. This finding agrees with the experimental evidence obtained by the microsphere method that adenosine increased blood flow to aortic vasa vasorum in the outer media of canine thoracic aorta (Heistad & Marcus, 1979). The potency of adenosine as a vasodilator compound was, however, much lower than that of His and ISP. Further studies will obviously be needed to evaluate the physiological significance of the changes in the vascular resistance of the aortic vasa vasorum that can be produced by vasoactive compounds and to investigate the modes of actions of Ah and histamine with special reference to the role of the endothelium in the vasa vasorum. This study was supported by Grant-in-Aid for Scientific Research ( , ) from the Japanese Ministry of Education, Science and ulture, and Research Grant for ardiovascular Diseases from the Ministry of Health and Welfare (1-3). The authors are indebted to Mrs Sachiyo Fukushima for her technical assistance. REFERENES AZUMA, T. & OKA, S. (1971). Mechanical equilibrium of blood vessel walls. American Journal of Physiology 221, GEIRINGER, E. (1951). Intimal vascularisation and atherosclerosis. Journal of Pathology and Bacteriology 63, HEISTAD, D. D. & MARUS, M. L. (1979). Role of vasa vasorum in nourishment of the aorta. Blood Vessels 16, HEISTAD, D. D., MARUS, M. L., LAW, E. G. & ARMSTRONG, M. L. (1978). Regulation of blood flow to the aortic media in dogs. Journal of linical Investigation 62, HEISTAD, D. D., MARUS, M. L. & MARTIN, J. B. (1979). Effects of neural stimuli on blood flow through vasa vasorum in dogs. irculation Research 45, KIENEKER, E. W. & KNOHE, H. (1978). Sympathetic innervation of the pulmonary artery, ascending aorta, and coronar glomera of the rabbit. ell and Tissue Research 188, MKIBBEN, J. S. & GETTY, E. (1968). A comparative morphologic study of the cardiac innervation of domestic animals. I. The canine. American Journal of Anatomy 122, MARUS, M. L., HEISTAD, D. D., ARMSTRONG, M. L. & ABBOUD, F. M. (1985). Effects of chronic hypertension on vasa vasorum in the thoracic aorta. ardiovascular Research 19, OHHASHI, T. & YOSHINAKA, Y. (199. Physiological roles of vasa vasorum on micro- and macromolecular transport through aortic walls with special reference to the topography of atherosclerotic plaques. Annals of the New York Academy of Sciences 598, SATO, M. (1977). Deformation and hydraulic resistance of the arterial vasa vasorum (in Japanese). Japanese Journal of Medical Engineering 15 (15), SIMON, B. R., KOBAYASHI, A. S., WIEDERHIELM,. A. & STRANDNESS, D. E. (1973). Deformation of the arterial vasa vasorum at normal and hypertensive arterial pressure. Journal of Biomechanics 6, WERBER, A. H. & HEISTAD, D. D. (1985). Diffusional support of arteries. American Journal of Physiology 248, H WILENS, S. L., MALOLM, J. A. & VASQUEZ, J. M. (1965). Experimental infarction (medial necrosis) of the dog's aorta. American Journal of Pathology 47, WOLINSKY, H. & GLAGOV, S. (1967). Nature of species differences in the medial distribution of aortic vasa vasorum in mammals. irculation Research 20, WOLINSKY, H. & GLAGOV, S. (1969). omparison of abdominal and thoracic aortic medial structure in mammals. irculation Research 25,