European Journal o/ Pharmacology, 190 (1990) 263-267
263
Elsevier EJP20730
Shortconniption
Margaret R. MacLean A~zonomie PhysioIo~
and J.C. McGrath
Unit, Iasti~~e of Physiology, L’niversi!y of GIasgow, GIasgow Gi2 gQQ* Scorland
Received 4 September 1990, accepted 11 September 1990
Endothelin-1 and noradrenaline induced dose-dependent pressor responses in isolated in situ blood perfused mesenteric arterial beds and isolated tail arterial beds of anaesthetised spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). In the tail the sensitivity and maximum to either agonist were the same in SHR and WKY whereas in the mesenteric bed the maximum pressor responses to both agonists were increased in SHR. This effect of endothelin-1 may contribute to the greater increase in blood pressure it induces in anaesthetised SHR compared with WKY. Endothelin-1;
Spontaneously
hypertensive rats (SHR): Vasoconstriction;
1. Inhduction
Vascular endothelium-derived endothelin-1 is a potent vasoconstrictor both in vitro and in vivo (Yanaga~wa et al,, 1988). Endothe~n-1 can induce a greater maximal pressor response in anaesthetised spontaneously hypertensive rats (SHR) than in normotensive Wistar-Kyoto rats (WKY) although the dose dependency of this response was not altered (Miyauchi et al., 1989). The sensitivity to endothelin-1 can, however, be increased in isolated aortas, renal arteries and portal veins from SHR compared with those from WKY (Tomobe et al., 1988; Clozel, 1989; Kamata et al., 1990). The effect of endothehn-1 on isolated arterial or resistance vessel beds has not been shown and this prompted us to compare the pressor responses to endothelin-1 in the isolated in situ blood perfused mesenteric arterial bed and isolated tail Correspondenceto:
M.R. MacLean, Autonomic Physiology Unit, Institute of Phys~o~o~, Unive~ity of Glasgow, Glasgow G12 SQQ, Scotland. 0014-2999/90/!§03.56
Vascular beds
arterial bed of anaesthetised SHR and normotensive WKY to determine if either the sensitivity to, or the maximal response elicited by, endothelin-l were altered in the hypertensive state. 2. Materials and m&t&3
SHR (287 + 10 g) ant WKY (288 + 13 g) were obtained from Harlan O.ac Ltd. (Oxon, U.K.) and anaesthetised with 120 mg kg-’ i.p. sodium thiopentone (Evans Medical Ltd., Horsham, U.K.) prior to conducting the following experimental procedures. 2. I. The blood perfwed superior rnese~ler~~ arreriul bed preparation
The trachea was cannulated and the rats were spontaneously air-breathing. The left jugular vein was cannulated for the administration of drugs. The right common ca;otid artery was cannulated and connected to a Spc ;ranred model P23XL pressure transducer couple4 to a Grass Model 7 polygraph in order to record systemic blood pressure
Q 1990 Elsevier Science Publishers B.V. (Biomedical Division)
art rate (derived from the pressure wave by of a Grass 7P44 tachograph preamplifier). perature was maintained at 37°C by of a heating lamp. e preparation used for in situ blood perfusion of the superior mesenteric bed was a modifiescribed by Jackson and Campbell . a mid-line incision was made in domen and ligatures were placed around the omi~al aorta distal to the origin of the renal arteries and around the superior mesenteric artery shortly after its origin. Surgery was halted for 20 mm to allow haemostasis prior to the i.v. adminisi-u. kg-’ of heparin. The abdomitration of 1 nal aorta was cannulated with PP60 tubing which led in turn to a Harvard type 2903 peristaltic pump. a bubble trap (which also eliminates the pulses in the flow) and a beat exchanger to reheat the blood to 37°C. The circuit was initially filled with 0.9% saline to maintain tht Lirculating volume in the rat. The blood was returned to the animal, at 2 ml min-‘, through the superior mesenteric artery by means of an inflow cannula; l-3 ml 10% dextran (clinical grade. Sigma, Poole, U.K.) was administered to maintain blood volume if required. The input pressure was measured by means of a second Spectramed transducer placed a fixed distance from the end of the inflow cannula. Dose-response curves (n = 6) to bolus doses of endotbelin-1 (lo- I3 - 10m9 mol) (Peptide Institute, Scientific Marketing Associates, U.K.) and noradrenaline (lo- “‘-1O-‘3 mol) (Sigma, Poole, U.K.) were constructed by injection into the perfusing solution via an injection port situated prior to the heat exchanger. Changes in blood gases and pH alter the sensitivity of this preparation to endothelin-1 (MacLean et al., 1989) and so 100 ~1 samples of mesenteric blood were taken for analysis of blood gases and pH using a pH/blood gas analyser (Instrumentation Laboratory System 1302). 2.2. The perfused rat tail arterial bed The rats were killed by i.v. injection of air via the jugular cannula and the ventral surface of the tail was shaved and the skin reflected for 3-5 cm around the proximal end of the tail. The tail artery
was exposed using blunt forceps and cleaned of fat and connective tissue and cannulated with a polyethylene cannula. The tail was amputated by cutting through an intervertebral disc and placed on an elevated plastic platform allowing the arterial cannula to be connected to a perfusion circuit. The tail vasculature was perfused at a rate of 0.8 ml rnin- ’ with a modified physiological salt solution the temperature of which was maintained at 37 OC by a heat exchanger. The composition of the physiological salt solution was (in mM) NaCl 118.4, NaHCO, 25, KC1 4.7, KH,PO, 1.2, MgSO, 1.2, CaCl, 2.5, glucose 11, Na,EDTA 0.023. Ficoll (2%; molecular weight approximately 70000) was included to prevent water retention. The perfusion pressure was allowed to stabilise for 30 min before the start of any experimental procedure. Dose-response curves (n = 6) to endothelin-1 (lo-l4 10e9 mol) and noradrenaline (lo-‘* - lo-’ mol) were constructed by bolus injection of drugs into the perfusing solution via an injection port close to the point of cannulation. In both preparations the injectate volume was constant at 0.01 ~1 and perfusion pressures were allowed to recover following each noradrenaline dose whilst doses of endothelin-1 were administered in a cumulative fashion due to the long duration of the endothelin-1 response (e.g. MacLean et al., 1989). 2.3. Statistical
analysis
Data from SHR and WKY were compared using an unpaired Student’s t-test. The negative logarithm of the agonist dose required to elicit 50% of the maximum response (pD, or -log ED,,) was interpolated from the dose-response curves in the perfused tail and meaned within groups to give a measure of the sensitivity of the vascular bed to the agonists (NB: in the mesenteric bed preparations, maximum responses were not attained in the dose ranges used and therefore pD, could not be estimated).
Prior to any abdominal surgical intervention, the mean blood pressure of the anaesthetised WKY
265
was 70 f 9 rmnHg and the mean heart rate was 345 f 20 beats min- ‘. The mean blood pressure of the SHR was 145 + 19 mmHg and the mean heart rate was 400 f 15 beats min-‘. The basal perfusion pressures of the mesenteric beds from SHR and WKY were not significantly different and were 48 f 3 and 46 + 4 mmHg respectively. The mesenteric blood pH, PO2 and PCO, in SHR were 7.3 f 0.1, 130 f 10 and 35.7 _t 4 mrnHg respectively. These values were not significantly different from those in WKY which were 7.3 + 0.1, 120 + 6 and 37.4 f 5 mmHg respectively. Figure 1 shows that noradrenaline and endothehn-1 induced dose-dependent pressor responses in the mesenteric arterial beds and that endothelin-1 is some 10 times more potent than noradrenaline. The responses to noradrenaline and endothelin-1 were significantly larger than in SHR (n = 6) compared with WKY (n = 6) at individual doses. Accurate pD, values cannot be calculated since convincing ‘maxima’ were not attained (fig. 1). Nevertheless it was clear from the threshold and position of the curves that no substantial difference in ‘sensitivity’ occurred between the SHR and WKY.
-
Log dose (mol) Fig. 2. The effect of endothelin-1 and noradrenaline on the isolated tail arterial bed of anaesthetised shows the effect of endothehn-1 on beds the effect of endothelin-1 on WKY beds. shows the effect of noradrenaline on beds from SHR and (0) illustrates the effect of noradrenaline on WKY bedsThe values are given as means with the vertical lines representing S.E.M. (n = 6).
The basal perfusion pressures of the tail arterial beds from SHR and WKY were not significantly different and were 57 + 4 and 48 + 6 mmHg respectively. Figure 2 shows that noradrenaline and endothehn-1 induced dose-dependent pressor responses in the tail arterial bed and that endothelin-l is some 10 times more potent than noradrenaline. The sensitivity to noradrenaline and endotheiin-1 was not significantly different in SHR (n = 6) compared with WKY (n = 6): the pD, value (log mol) for noradrenaline in WKY was - 9.5 + 0.11 and in SHR was - 9.7 f 0.11 and the pD, value (log mol) for endothelin-1 in WKY was - 10.81 +. 0.26 and in SHR was - 11.0 f 0.15. The maximum responses attained with both agonists in tails from SHR and WKY were not significantly different.
-6
Log dose (mol) Fig. 1. The effect of errdothelin-1 and noradrenaline on the blood perfused superior mesenteric bed of anaesthetised SHR ) shows the effect of endothelin-1 on beds from SHR and (0) the effect of endothelin-1 on WKY beds. ( illustrates the effect of noradrenaline on beds from SHR and (0) shows the effect of noradrenaline on WKY beds. The values are given as means with the vertical lines representing S.E.M. (n = 6). Statistical differences between the SHR and WKY groups were assessed using Student’s unpaired t-test: ** P-zO.01.
4. Discussion Miyauchi et al. (1989) showed that the maximal pressor response to endothelin-1 can be greater in anaesthetised SHR than in their age-matched WKY normotensive controls whilst the dose dependency of the response was no different with that in WKY. They showed, however, that the
superior mesenteric artery was more senisolat endothelin-I in SHR than in WKY whilst sitive e ma.ximal vasoconstriction induced by endoelin-1 was the same in both SHR and WKY. AS e resistance vasculature of the whole mesenteric bed has a more profound influence on the blood presh re than the superior mesenteric artery itself, it was of interest to determine the sensitivity of vascular bed to endcthelin-1 in both SHR and Y and to compare this with its activity in another isolated vascular bed such as the rat tail vascular bid. The effects of endothelin-1 were compared b?-ith those of noradrenaline as this is also more potent at inducing pressor responses in anaesthetised SHR than in W KY and exerts a more profound vasoconstriction in the isolated tail artery and isolated mesenteric bed in SHR than in WKY (Muir and Wardle. 1989). The results show that the mesenteric arterial bed was some 10 times more sensitive to endothelin-1 than to noradrenaline. The maximum responses obtained to the doses tested of both noradrenaline and endothelin-1 were significantly higher in the SHR than in the WKY. This greater effect of endothelin-I on SHR mesenteric resistance vessels, compared with those from WKY, may contribute substantially to its effect on blood pressure and explain the greater pressor response to endothelin-1 in SHR compared with WKY. The tail arterial bed was also some 10 times more sensitive to endothelin-1 than to noradrenaline. Unlike in the mesenteric bed, the maximum response to either agonist was the same in both SHR and WKY. In this case, the attainment of a ‘maximum’ and the superimposed position of the curves allowed the interpretation that the sensitivity of this bed to either agonist was no different in SHR than in WKY. Whilst there is evidence that endothelin-1 binding sites are increased in the brains of SHR compared with WKY, there is no evidence as yet that this is so for vascular tissue (Gu et al., 1990). Thus, the increased maxima1 response to endothelin-1 in the SHR mesenteric bed is likely to be at least partially due to the increased amount of smooth muscle present in the mesenteric resistance vessels from SHR resulting in an increase in the media-lumen ratio of these vessels (see
Mulvany et al.. 1982). This is supported by the observation here that the maximum response to noradrenaline was also increased and that this effect was not, therefore, exclusive to endothelin-1. Also, it is known that the media-lumen ratio of rat tail arteries from SHR is not increased when compared with those from WKY (Mulvany et al., 1982) and this is consistent with our finding no Jifferences in the responses to the agonists in tail arterial beds from SHR compared to those from WKY. The mesentery was perfused with blood, the tail with saline. In the perfused tail, several synergists, including endothelin-l itself, can uncover responses to certain agonists which had been absent with a simple saline perfusate (Templeton ,I al., 1989; MacLean and McGrath, in press). We cannot, therefore, exclude the possibility that a blood-borne synergist, higher in concentration or more effective in SHR, is a contributing factor to the effect seen in mesentery. However the present inter-strain difference in blood-perfused mesentety is similar in several respects to that found in the saline-perfused mesentery after angiotensin II-induced hypertension (Brown et al., 1988). In conclusion, there is a greater increase in resistance, to a given stimulus, in the mesenteric bed of SHR compared with WKY which probably contributes to the greater maximum increase in total peripheral resistance induced by endothelin-1 in SHR.
Acknowledgement This study was supported by a grant from The Wellcome Trust.
References Brown. W.. A.F. Lever, F. MacPherson. J.C. McGrath and V.G. Wilson, 1988, Is angiotensin Ii-induced hypertension in rats associated with the development of vascular hypertrophy?, J. Physiol. 403, 82P. Clozel. M.. 1989. Endothelin sensitivity and receptor binding in the aorta of spontaneously hypertensive rats, J. Hypert. 7, 913. Gu. X.-H., D.J. Casley, M. Cincotta and W.G. Naylor, 1990. “‘I-Endothelin binding to brain and cardiac membranes
267 from normotensive and spontaneously hypertensive rats, European J. Pharmacol. 177, 205. Jackson, E.K. and W.B. Campbell, 1980. The in situ blood perfused rat mesentery; a model for assessing modulation of adrenergic transmission, European J. Pharmacol. 66, 217. Kamata, K., N. Miyata and Y. Kasuya, 1990, Effects of endothelin on the portal vein from spontaneously hypertensive and Wistar rats, Gen. Pharmacol. 21, 127. MacLean, MR. and J.C. McGrath, Effects of pre-contraction with endothelin-1 on a,-adrenoceptor- and (endotheliumdependent) neuropeptide Y-mediated contractions in the isolated vascular rat tail. Br. J. Pharmacol. (in press). MacLean, M.R.. M.D. Randall and C.R. Hiley, 1989, Effects of moderate hypoxia, hypercapnia and acidosis on haemodynamic changes induced by endothelin-1 in the pithed rat, Br. J. Pharmacol. 98, 1055. Miyauchi, T.. T. Ishikawa, Y. Tomobe, M. Yanagisaw. S. Kimura, Y. Sugishita, I. Ito, K. Goto and T. Masaki, 1989, Characteristics of pressor response to endothelin in spontaneously hypertensive and Wistar-Kyoto rats, Hypertension 14. 427.
Muir, T.C. and K.A. Wardle, 1989, Vascular smooth muscle responses in normo- and hypertensive rats to sympathetic nerve stimulation and putative transmitters, J. Auton. Pharmacol. 9, 23. Mulvatty, M.J., H. Nilsson, N. Nyborg and E Mikkelsen, 1982, Are isolated femoral resistance vessels or tail arteries good models for the hindquarter vasculature of spontaneously hypertensive rats?, Acta Physiol. Stand. 116. 275. Templeton, A.G.B., J. Macmillan, J.C. McGrath, N.D. Storey and V.G. Wilson, 1989, Evidence for prazosin-resistant, rauwolscine-sensitive a-adrenoceptors mediating contractions in the isolated vascular bed of the rat tail, Br. J. Pharmacol. 97,563. Tomobe. Y., T. Miyauchi, A. Saito. M. Yanagisawa. S. Kimura, K. Goto and T. Masalri. 1988, Effect of endothelin on the renal artery from spontaneously hypertensive and Wistar Kyoto rats, European J. Pharmacol. 152. 373. Yanagasawa. M., S. Kurihara, Y. Kimura, M. Tomobe. Y. Kobayashi. Y. Mitsui. Y. Yazaki. K. Goto and T. Masaki, 1988, A novel potent vasoconstrictor peptide produced by vascular endothelial cells, Nature 332, 411.
263
Elsevier EJP20730
Shortconniption
Margaret R. MacLean A~zonomie PhysioIo~
and J.C. McGrath
Unit, Iasti~~e of Physiology, L’niversi!y of GIasgow, GIasgow Gi2 gQQ* Scorland
Received 4 September 1990, accepted 11 September 1990
Endothelin-1 and noradrenaline induced dose-dependent pressor responses in isolated in situ blood perfused mesenteric arterial beds and isolated tail arterial beds of anaesthetised spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). In the tail the sensitivity and maximum to either agonist were the same in SHR and WKY whereas in the mesenteric bed the maximum pressor responses to both agonists were increased in SHR. This effect of endothelin-1 may contribute to the greater increase in blood pressure it induces in anaesthetised SHR compared with WKY. Endothelin-1;
Spontaneously
hypertensive rats (SHR): Vasoconstriction;
1. Inhduction
Vascular endothelium-derived endothelin-1 is a potent vasoconstrictor both in vitro and in vivo (Yanaga~wa et al,, 1988). Endothe~n-1 can induce a greater maximal pressor response in anaesthetised spontaneously hypertensive rats (SHR) than in normotensive Wistar-Kyoto rats (WKY) although the dose dependency of this response was not altered (Miyauchi et al., 1989). The sensitivity to endothelin-1 can, however, be increased in isolated aortas, renal arteries and portal veins from SHR compared with those from WKY (Tomobe et al., 1988; Clozel, 1989; Kamata et al., 1990). The effect of endothehn-1 on isolated arterial or resistance vessel beds has not been shown and this prompted us to compare the pressor responses to endothelin-1 in the isolated in situ blood perfused mesenteric arterial bed and isolated tail Correspondenceto:
M.R. MacLean, Autonomic Physiology Unit, Institute of Phys~o~o~, Unive~ity of Glasgow, Glasgow G12 SQQ, Scotland. 0014-2999/90/!§03.56
Vascular beds
arterial bed of anaesthetised SHR and normotensive WKY to determine if either the sensitivity to, or the maximal response elicited by, endothelin-l were altered in the hypertensive state. 2. Materials and m&t&3
SHR (287 + 10 g) ant WKY (288 + 13 g) were obtained from Harlan O.ac Ltd. (Oxon, U.K.) and anaesthetised with 120 mg kg-’ i.p. sodium thiopentone (Evans Medical Ltd., Horsham, U.K.) prior to conducting the following experimental procedures. 2. I. The blood perfwed superior rnese~ler~~ arreriul bed preparation
The trachea was cannulated and the rats were spontaneously air-breathing. The left jugular vein was cannulated for the administration of drugs. The right common ca;otid artery was cannulated and connected to a Spc ;ranred model P23XL pressure transducer couple4 to a Grass Model 7 polygraph in order to record systemic blood pressure
Q 1990 Elsevier Science Publishers B.V. (Biomedical Division)
art rate (derived from the pressure wave by of a Grass 7P44 tachograph preamplifier). perature was maintained at 37°C by of a heating lamp. e preparation used for in situ blood perfusion of the superior mesenteric bed was a modifiescribed by Jackson and Campbell . a mid-line incision was made in domen and ligatures were placed around the omi~al aorta distal to the origin of the renal arteries and around the superior mesenteric artery shortly after its origin. Surgery was halted for 20 mm to allow haemostasis prior to the i.v. adminisi-u. kg-’ of heparin. The abdomitration of 1 nal aorta was cannulated with PP60 tubing which led in turn to a Harvard type 2903 peristaltic pump. a bubble trap (which also eliminates the pulses in the flow) and a beat exchanger to reheat the blood to 37°C. The circuit was initially filled with 0.9% saline to maintain tht Lirculating volume in the rat. The blood was returned to the animal, at 2 ml min-‘, through the superior mesenteric artery by means of an inflow cannula; l-3 ml 10% dextran (clinical grade. Sigma, Poole, U.K.) was administered to maintain blood volume if required. The input pressure was measured by means of a second Spectramed transducer placed a fixed distance from the end of the inflow cannula. Dose-response curves (n = 6) to bolus doses of endotbelin-1 (lo- I3 - 10m9 mol) (Peptide Institute, Scientific Marketing Associates, U.K.) and noradrenaline (lo- “‘-1O-‘3 mol) (Sigma, Poole, U.K.) were constructed by injection into the perfusing solution via an injection port situated prior to the heat exchanger. Changes in blood gases and pH alter the sensitivity of this preparation to endothelin-1 (MacLean et al., 1989) and so 100 ~1 samples of mesenteric blood were taken for analysis of blood gases and pH using a pH/blood gas analyser (Instrumentation Laboratory System 1302). 2.2. The perfused rat tail arterial bed The rats were killed by i.v. injection of air via the jugular cannula and the ventral surface of the tail was shaved and the skin reflected for 3-5 cm around the proximal end of the tail. The tail artery
was exposed using blunt forceps and cleaned of fat and connective tissue and cannulated with a polyethylene cannula. The tail was amputated by cutting through an intervertebral disc and placed on an elevated plastic platform allowing the arterial cannula to be connected to a perfusion circuit. The tail vasculature was perfused at a rate of 0.8 ml rnin- ’ with a modified physiological salt solution the temperature of which was maintained at 37 OC by a heat exchanger. The composition of the physiological salt solution was (in mM) NaCl 118.4, NaHCO, 25, KC1 4.7, KH,PO, 1.2, MgSO, 1.2, CaCl, 2.5, glucose 11, Na,EDTA 0.023. Ficoll (2%; molecular weight approximately 70000) was included to prevent water retention. The perfusion pressure was allowed to stabilise for 30 min before the start of any experimental procedure. Dose-response curves (n = 6) to endothelin-1 (lo-l4 10e9 mol) and noradrenaline (lo-‘* - lo-’ mol) were constructed by bolus injection of drugs into the perfusing solution via an injection port close to the point of cannulation. In both preparations the injectate volume was constant at 0.01 ~1 and perfusion pressures were allowed to recover following each noradrenaline dose whilst doses of endothelin-1 were administered in a cumulative fashion due to the long duration of the endothelin-1 response (e.g. MacLean et al., 1989). 2.3. Statistical
analysis
Data from SHR and WKY were compared using an unpaired Student’s t-test. The negative logarithm of the agonist dose required to elicit 50% of the maximum response (pD, or -log ED,,) was interpolated from the dose-response curves in the perfused tail and meaned within groups to give a measure of the sensitivity of the vascular bed to the agonists (NB: in the mesenteric bed preparations, maximum responses were not attained in the dose ranges used and therefore pD, could not be estimated).
Prior to any abdominal surgical intervention, the mean blood pressure of the anaesthetised WKY
265
was 70 f 9 rmnHg and the mean heart rate was 345 f 20 beats min- ‘. The mean blood pressure of the SHR was 145 + 19 mmHg and the mean heart rate was 400 f 15 beats min-‘. The basal perfusion pressures of the mesenteric beds from SHR and WKY were not significantly different and were 48 f 3 and 46 + 4 mmHg respectively. The mesenteric blood pH, PO2 and PCO, in SHR were 7.3 f 0.1, 130 f 10 and 35.7 _t 4 mrnHg respectively. These values were not significantly different from those in WKY which were 7.3 + 0.1, 120 + 6 and 37.4 f 5 mmHg respectively. Figure 1 shows that noradrenaline and endothehn-1 induced dose-dependent pressor responses in the mesenteric arterial beds and that endothelin-1 is some 10 times more potent than noradrenaline. The responses to noradrenaline and endothelin-1 were significantly larger than in SHR (n = 6) compared with WKY (n = 6) at individual doses. Accurate pD, values cannot be calculated since convincing ‘maxima’ were not attained (fig. 1). Nevertheless it was clear from the threshold and position of the curves that no substantial difference in ‘sensitivity’ occurred between the SHR and WKY.
-
Log dose (mol) Fig. 2. The effect of endothelin-1 and noradrenaline on the isolated tail arterial bed of anaesthetised shows the effect of endothehn-1 on beds the effect of endothelin-1 on WKY beds. shows the effect of noradrenaline on beds from SHR and (0) illustrates the effect of noradrenaline on WKY bedsThe values are given as means with the vertical lines representing S.E.M. (n = 6).
The basal perfusion pressures of the tail arterial beds from SHR and WKY were not significantly different and were 57 + 4 and 48 + 6 mmHg respectively. Figure 2 shows that noradrenaline and endothehn-1 induced dose-dependent pressor responses in the tail arterial bed and that endothelin-l is some 10 times more potent than noradrenaline. The sensitivity to noradrenaline and endotheiin-1 was not significantly different in SHR (n = 6) compared with WKY (n = 6): the pD, value (log mol) for noradrenaline in WKY was - 9.5 + 0.11 and in SHR was - 9.7 f 0.11 and the pD, value (log mol) for endothelin-1 in WKY was - 10.81 +. 0.26 and in SHR was - 11.0 f 0.15. The maximum responses attained with both agonists in tails from SHR and WKY were not significantly different.
-6
Log dose (mol) Fig. 1. The effect of errdothelin-1 and noradrenaline on the blood perfused superior mesenteric bed of anaesthetised SHR ) shows the effect of endothelin-1 on beds from SHR and (0) the effect of endothelin-1 on WKY beds. ( illustrates the effect of noradrenaline on beds from SHR and (0) shows the effect of noradrenaline on WKY beds. The values are given as means with the vertical lines representing S.E.M. (n = 6). Statistical differences between the SHR and WKY groups were assessed using Student’s unpaired t-test: ** P-zO.01.
4. Discussion Miyauchi et al. (1989) showed that the maximal pressor response to endothelin-1 can be greater in anaesthetised SHR than in their age-matched WKY normotensive controls whilst the dose dependency of the response was no different with that in WKY. They showed, however, that the
superior mesenteric artery was more senisolat endothelin-I in SHR than in WKY whilst sitive e ma.ximal vasoconstriction induced by endoelin-1 was the same in both SHR and WKY. AS e resistance vasculature of the whole mesenteric bed has a more profound influence on the blood presh re than the superior mesenteric artery itself, it was of interest to determine the sensitivity of vascular bed to endcthelin-1 in both SHR and Y and to compare this with its activity in another isolated vascular bed such as the rat tail vascular bid. The effects of endothelin-1 were compared b?-ith those of noradrenaline as this is also more potent at inducing pressor responses in anaesthetised SHR than in W KY and exerts a more profound vasoconstriction in the isolated tail artery and isolated mesenteric bed in SHR than in WKY (Muir and Wardle. 1989). The results show that the mesenteric arterial bed was some 10 times more sensitive to endothelin-1 than to noradrenaline. The maximum responses obtained to the doses tested of both noradrenaline and endothelin-1 were significantly higher in the SHR than in the WKY. This greater effect of endothelin-I on SHR mesenteric resistance vessels, compared with those from WKY, may contribute substantially to its effect on blood pressure and explain the greater pressor response to endothelin-1 in SHR compared with WKY. The tail arterial bed was also some 10 times more sensitive to endothelin-1 than to noradrenaline. Unlike in the mesenteric bed, the maximum response to either agonist was the same in both SHR and WKY. In this case, the attainment of a ‘maximum’ and the superimposed position of the curves allowed the interpretation that the sensitivity of this bed to either agonist was no different in SHR than in WKY. Whilst there is evidence that endothelin-1 binding sites are increased in the brains of SHR compared with WKY, there is no evidence as yet that this is so for vascular tissue (Gu et al., 1990). Thus, the increased maxima1 response to endothelin-1 in the SHR mesenteric bed is likely to be at least partially due to the increased amount of smooth muscle present in the mesenteric resistance vessels from SHR resulting in an increase in the media-lumen ratio of these vessels (see
Mulvany et al.. 1982). This is supported by the observation here that the maximum response to noradrenaline was also increased and that this effect was not, therefore, exclusive to endothelin-1. Also, it is known that the media-lumen ratio of rat tail arteries from SHR is not increased when compared with those from WKY (Mulvany et al., 1982) and this is consistent with our finding no Jifferences in the responses to the agonists in tail arterial beds from SHR compared to those from WKY. The mesentery was perfused with blood, the tail with saline. In the perfused tail, several synergists, including endothelin-l itself, can uncover responses to certain agonists which had been absent with a simple saline perfusate (Templeton ,I al., 1989; MacLean and McGrath, in press). We cannot, therefore, exclude the possibility that a blood-borne synergist, higher in concentration or more effective in SHR, is a contributing factor to the effect seen in mesentery. However the present inter-strain difference in blood-perfused mesentety is similar in several respects to that found in the saline-perfused mesentery after angiotensin II-induced hypertension (Brown et al., 1988). In conclusion, there is a greater increase in resistance, to a given stimulus, in the mesenteric bed of SHR compared with WKY which probably contributes to the greater maximum increase in total peripheral resistance induced by endothelin-1 in SHR.
Acknowledgement This study was supported by a grant from The Wellcome Trust.
References Brown. W.. A.F. Lever, F. MacPherson. J.C. McGrath and V.G. Wilson, 1988, Is angiotensin Ii-induced hypertension in rats associated with the development of vascular hypertrophy?, J. Physiol. 403, 82P. Clozel. M.. 1989. Endothelin sensitivity and receptor binding in the aorta of spontaneously hypertensive rats, J. Hypert. 7, 913. Gu. X.-H., D.J. Casley, M. Cincotta and W.G. Naylor, 1990. “‘I-Endothelin binding to brain and cardiac membranes
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