Br. J. Pharmacol. (1990),100, 153-157
(D Macmillan Press Ltd, 1990
Enhanced coronary vasoconstrictor responses to 5-hydroxytryptamine in the presence of a coronary artery stenosis in anaesthetized dogs Owen L. Woodman Department of Pharmacology, University of Melbourne, Parkville, Vic., 3052, Australia 1 In the anaesthetized dog, left circumflex coronary artery blood flow and external diameter were measured and the vascular responses to an injection of 5-hydroxytryptamine (5-HT, 0.02-2 ugkg 1) assessed, before and after a screw clamp had been placed on the artery to produce a severe stenosis. 2 In the normal coronary circulation, intra-coronary (i.c.) injection of 5-HT increased coronary blood flow (CBF) but decreased the external diameter of the large coronary artery (CD), without affecting systemic mean arterial pressure or heart rate. 3 In the normal coronary circulation, the 5-HT-induced increases in CBF were unaffected by blockade of 5-HT2-receptors with ketanserin (0.1 mg kg- i.c.) but were significantly attenuated by blockade of 5-HT1-like receptors with methysergide (0.1 mg kg 1 i.c.). 4 In the presence of a severe stenosis, the increase in CBF produced by 5-HT was markedly attenuated and a secondary decrease in CBF was revealed. The stenosis also caused a marked enhancement of the 5-HT-induced constriction of the large artery, such that the reduction of the CD was approximately doubled at all doses of 5-HT tested. The enhanced 5-HT-induced reduction in CD was similar with placement of the stenosis either proximal or distal to the site of diameter measurement. 5 In the presence of the stenosis, blockade of 5-HT2-receptors with ketanserin (0.1 mg kg- i.c.) significantly attenuated the 5-HT-induced decreases in CD and CBF. 6 These results demonstrate that, in the anaesthetized dog, placement of a severe, flow-limiting stenosis enhances 5-HT-induced constriction of the large coronary arteries and reveals a reduction in coronary blood flow. These observations support suggestions that, in the presence of coronary artery disease, 5-HT could contribute to reductions in coronary blood flow leading to myocardial ischaemia.
Introduction It has been postulated that in the presence of coronary artery disease an enhanced release of 5-hydroxytryptamine (5-HT) could contribute to coronary vasoconstriction and possibly myocardial ischaemia (Vanhoutte, 1987). 5-HT is released from aggregating platelets in sufficient amounts to cause contraction of coronary vascular smooth muscle (Cohen et al., 1983a). In addition the concentration of 5-HT is elevated in dog coronary arteries at the site of a stenosis (Ashton et al., 1986). Also, van den Berg et al. (1989) have recently shown that 5-HT is released into the coronary circulation of some patients with coronary artery disease. However, if 5-HT is going to cause myocardial ischaemia then it must decrease coronary blood flow. 5-HT constricts large coronary arteries in vivo (Haddy et al., 1957; Bove & Dewey, 1983; Lamping et al., 1985; Chu & Cobb, 1987) and in vitro (Cocks & Angus, 1983; Cohen et al., 1983b), but it may also produce endothelium-dependent relaxation of large coronary arteries in vitro (Cocks & Angus, 1983; Cohen et al., 1983b) and dilate resistance vessels in vivo (Haddy et al., 1957; Blackshear et al., 1985; Meschig et al., 1985). As the predominant determinant of resistance is the small rather than the large vessels the action of 5-HT in the normal coronary circulation is to increase rather than decrease blood flow. This suggests that if 5-HT contributes to myocardial ischaemia in the presence of coronary artery disease, the vascular response must be different from that occurring in the normal vasculature. There are potentially many factors occurring in coronary artery disease which could influence the responses to vasoactive agents. One of these is stenosis of the large coronary arteries, as an atherosclerotic plaque encroaches on the lumen resulting in dilatation of the resistance vessels and the resultant loss of vasodilator reserve. As there have been no studies in which the actions of 5-HT have been examined
under these conditions, the aim of this study was to compare the response to 5-HT in the normal coronary circulation with that occurring in the presence of a severe stenosis. Large coronary artery diameter and coronary blood flow were monitored, in order to assess the actions of 5-HT on both conduit and resistance vessels. A preliminary account of these findings has been presented to the Australian Physiological and Pharmacological Society (Woodman, 1989).
Methods Greyhound dogs of either sex and weighing 25-38kg were anaesthetized with thiopentone (25-30mgkg-1 i.v.) followed by a-chloralose (70mgkg-1 i.v., supplemented as necessary). The dogs were ventilated with room air plus additional oxygen as necessary to maintain arterial blood Po2, Pco2 and pH within the normal ranges (Po2: 85- 10 mmHg; Pco2: 3035mmHg; pH: 7.25-7.35). Pressure in the thoracic aorta was measured by passing a catheter from the right femoral artery and connecting the catheter to a Druck pressure transducer. A continuous measurement of heart rate was obtained by a cardiotachometer coupler triggered by the ECG. A thoracotomy was performed at the left fifth intercostal space and the pericardium was opened. A pair of 7 MHz piezoelectric transducers were sutured to opposing surfaces of the left circumflex coronary artery. External diameter of the artery was measured with an ultrasonic transit-time dimension gauge (Triton Technology Inc.). Blood flow through the artery was measured by an electromagnetic flow probe (Statham) placed on the artery. A needle tipped catheter was placed in the artery proximal to the transducers for the administration of drugs. Care was taken during the placement of the transducers to limit dissection and damage of any visible nerves.
154
O.L. WOODMAN
Experimental protocol 5-HT (0.02-2upgkg-') was injected as a bolus of less than 0.5ml directly into the left circumflex coronary artery. After control responses to 5-HT had been obtained, a screw clamp was placed on the circumflex coronary artery and tightened until there was no reactive hyperaemia in response to a 10s occlusion of the artery. This occlusion had no effect on cardiac rhythm. In ten dogs the stenosis was placed proximal to the diameter transducers, whilst in a further four dogs the stenosis was distal to the transducers. The 5-HT injections were then repeated in the presence of the stenosis. Ten of these dogs were then randomly allocated to one of two groups. In the first group the stenosis was removed and the responses to 5-HT (0.1-2 pg kg- l i.c.) re-examined. In the second group the stenosis was maintained and the responses to 5-HT were assessed after blockade of 5-HT2-receptors with ketanserin (0.1 mgkg- ') injected into the coronary artery every 15 min. In a separate group of five dogs without coronary stenosis, the effect of 5-HT (0.02-2 pg kg'- i.c.) on coronary blood flow was assessed before and after blockade of 5-HT2-receptors with ketanserin (0.1 mg kg-' i.c.) and combined blockade of 5-HT1-like and 5-HT2-receptors with methysergide (0.1 mg kg- i.c.) plus ketanserin.
1 OOr
80
T0T
0-_o
601
60
401 U-
o-
m 20
a
--T-
o
T-
0
0
0.02
0.05
0.5
0.2
0.1
1.0
2.0
5-HT (pug kg-') 1 Figure The effect of the intra-coronary injection of 5hydroxytryptamine (5-HT) on left circumflex coronary artery blood flow (CBF) in the absence (0) and presence (El *) of a coronary artery stenosis. The values shown are the mean of 14 experiments; vertical lines indicate s.e.mean. In the normal coronary circulation 5-HT produced only increases in blood flow whereas in the presence of the stenosis the initial small increase in flow (E) was followed by a secondary vasoconstriction (U). Analysis of variance showed that the stenosis caused a significant (P < 0.01) attenuation of the vasodilator responses to 5-HT.
Drugs
0.02
5-Hydroxytryptamine creatinine sulphate (Sigma), ketanserin tartrate (Janssen) and methysergide maleate (Sandoz) were dissolved in distilled water with further dilutions in saline.
0.05
5-HT (~tg kg-') 0.1 0.2 0.5
-5
2.0
i
'I'
T
1.0
1"I
0
\
-
Statistics Baseline values of the haemodynamic variables measured before and after placement of the stenosis were compared by Student's t test for paired data. Statistical comparison of the responses to 5-HT recorded after stenosis or antagonism of 5-HT receptors was achieved by a two-way analysis of variance, the two factors being the dose of 5-HT and the treatment. This test treated all doses as a group to reveal whether a treatment had any significant effect on a response. When there were 3 treatment groups, e.g. control, ketanserin and ketanserin plus methysergide, a post hoc comparison was performed by use of Scheffe's test (Wallenstein et al., 1980), to compare each treatment group against the two others.
Results
u 0
-10
-15L
Figure 2 Percentage changes in left circumflex coronary artery diameter (CD) produced by the intra-coronary injection of 5hydroxytryptamine (5-HT) in the absence (0) and presence (0) of a coronary artery stenosis. The values shown are the mean of 14 experiments; vertical lines indicate s.e.mean. Placement of the stenosis proximal or distal to the site of diameter measurement caused a similar enhancement of the response to 5-HT so the data were pooled for presentation in this graph. Analysis of variance showed that the stenosis caused a significant (P < 0.01) enhancement of the 5-HT-induced large artery constriction.
5-Hydroxytryptamine 5-HT (0.02-2pgkg-') injected into the left circumflex coronary artery caused dose-dependent increases in coronary blood flow (CBF) (Figure 1) but decreases in coronary artery diameter (CD) (Figure 2). The maximum reduction in CD was 6 + 1% of the resting diameter or 170 + 40 pm at the highest dose tested (Figure 2). There was no significant change in systemic arterial pressure (AP) or heart rate (HR) at any dose of 5-HT injected into the coronary artery.
140r 1 20~
Ic
100
In five dogs the effect of selective antagonists of '5-HT1-like' and 5-HT2-receptors on the 5-HT-induced increases in coronary blood flow were assessed. Ketanserin (0.1 mg kg-1 i.c.), a selective 5-HT2 receptor antagonist, had no significant effect on resting CBF (control = 32 + 5 ml min- ', ketanserin treated = 27 + 5 ml min- 1) or on the vasodilator responses to 5-HT (Figure 3). Methysergide (0.1 mg kg-' i.c.), given after the administration of ketanserin, had no significant effect on CBF resting (methysergide-treated = 29 + 5 ml min- '). Methysergide, in the presence of ketanserin, caused a signifi-
~ I
_ 80 E 60 mL o 40
20
Effect of ketanserin and methysergide on 5-HT-induced increases in coronary bloodflow
0
T 0
0.02
0a /w I 1- I 0.05
0.1
0.2
I _
0.5
1.0
2.0
5-HT(pg kg-1) Figure 3 Increases in coronary blood flow (CBF) produced by the intra-arterial injection of 5-hydroxytryptamine (5-HT). Values shown are the mean of 5 experiments; vertical lines indicate s.e.mean. The control responses to 5-HT (0) are compared to those observed after treatment with the 5-HT2-receptor antagonist ketanserin (0.1 mg kgi.c., *) or the combined antagonims of 5-HT2- and 5-HT1-like receptors with ketanserin plus methysergide (0.1 mgkg'- i.c., *). Analysis of variance followed by Scheffe's test demonstrated that the dilator responses to 5-HT were not different in the control and ketanserintreated groups, but that the addition of methysergide caused a significant (P < 0.001) attenuation of those responses.
5-HT-INDUCED CORONARY VASCOCONSTRICTION Table 1 Baseline levels of the haemodynamic variables measured in anaesthetized dogs before and after placement of a stenosis on the left circumflex coronary artery
Mean arterial pressure (mmHg) Mean external coronary artery diameter (mm) Mean coronary blood flow (ml min -1) Heart rate (beats min- 1)
Control
Stenosis
116+4 3.1 + 0.1
119+3 3.1 + 0.2
63+8 127+6
49+7* 121 +8
Values are presented as the mean + s.e.mean of data obtained from 14 animals. *P < 0.01 compared to control, Student's t test for paired comparisons.
cant attenuation of the 5-HT-induced increase in CBF (Figure
3).
Effect of coronary artery stenosis Tightening of the screw clamp, to stenose the left circumflex coronary artery until there was no reactive hyperaemia to a ten second total occlusion of the artery, resulted in a small, but significant, reduction in CBF. The stenosis had no significant effect on CD, AP or HR (Table 1). In the presence of a stenosis the 5-HT-induced increase in CBF was markedly attenuated and, in addition, at the higher doses of 5-HT (0.5-2ygkg-1) a secondary decrease in CBF was revealed (Figure 1). The constriction of the large artery was enhanced by the stenosis at all doses of 5-HT tested. The enhancement of the large artery constriction was independent of the placement of the stenosis. On average there was a doubling of the degree of constriction observed in the presence of the stenosis at all doses tested. The degree of enhancement was similar with both proximal (2.0 + 0.4 fold enhancement) and distal stenoses (2.5 + 0.7 fold enhancement). The data for proximal and distal stenoses have been pooled and are illustrated in Figure 2.
155
5-HT (jig kg-') 0.5
0.1
1.0
,;- 0
2.0
-a----.
C I
E -5
I
1-
-10 m-1
< 15~ -
-20L Figure 5 Decreases in coronary blood flow (CBF) produced by the intra-coronary injection of 5-hydroxytryptamine (5-HT) in the presence of a stenosis (0, n = 10) are compared to responses to 5-HT after treatment with ketanserin (0.1 mg kg-1 i.c.) whilst the stenosis is maintained (E, n = 5) or after removing the stenosis (*, n = 5). The values shown are the mean and vertical lines indicate s.e.mean. The constrictor response to 5-HT was significantly (P < 0.05, analysis of variance) attenuated by both ketanserin treatment and the removal of the stenosis.
presence of the stenosis, but of similar magnitude to the control responses (Figure 6).
Effect of ketanserin on the response to S-HT in the presence of the stenosis In five dogs with stenosed coronary arteries, treatment with the 5-HT2-receptor antagonist ketanserin (0.1mgkg-1 i.c.) had no significant effect on the baseline levels of CD (3.1 + 0.2mm before ketanserin; 3.0 + 0.1 mm after ketanserin) or CBF (30 + 8 ml min- before ketanserin; 25 + 6 ml min- 1 after ketanserin). Ketanserin attenuated the constriction of the large coronary artery produced by 5-HT (1igure 6) and in addition abolished the decrease in CBF (Figure 5).
Response to 5-HT after removing the stenosis
Discussion
In five dogs the response to 5-HT was examined after the removal of the stenosis. After the stenosis was removed 5-HT injection caused an increase in CBF (Figure 4) of similar magnitude to the control response and there was no significant secondary decrease in flow (Figure 5). The 5-HT-induced constriction of the large artery was less than that observed in the
This study has demonstrated that the injection of 5-HT into the left circumflex coronary artery of the anaesthetized dog constricts the proximal portion of that artery, but simultaneously increases blood flow in that vascular bed. As there were no accompanying changes in arterial pressure or heart rate, the increase in flow indicated that 5-HT dilates the coronary resistance vessels. The response to 5-HT was then reexamined in the presence of a coronary artery stenosis, which 5-HT (pLg
100 -
kg-') 1.0
0.5
0.1
2.0
80-
E 60-
.I
T
--~--' ---
*
-
1-_
I
I
-
-5
m- 40
-
U
0
< 2c
(D Macmillan Press Ltd, 1990
Enhanced coronary vasoconstrictor responses to 5-hydroxytryptamine in the presence of a coronary artery stenosis in anaesthetized dogs Owen L. Woodman Department of Pharmacology, University of Melbourne, Parkville, Vic., 3052, Australia 1 In the anaesthetized dog, left circumflex coronary artery blood flow and external diameter were measured and the vascular responses to an injection of 5-hydroxytryptamine (5-HT, 0.02-2 ugkg 1) assessed, before and after a screw clamp had been placed on the artery to produce a severe stenosis. 2 In the normal coronary circulation, intra-coronary (i.c.) injection of 5-HT increased coronary blood flow (CBF) but decreased the external diameter of the large coronary artery (CD), without affecting systemic mean arterial pressure or heart rate. 3 In the normal coronary circulation, the 5-HT-induced increases in CBF were unaffected by blockade of 5-HT2-receptors with ketanserin (0.1 mg kg- i.c.) but were significantly attenuated by blockade of 5-HT1-like receptors with methysergide (0.1 mg kg 1 i.c.). 4 In the presence of a severe stenosis, the increase in CBF produced by 5-HT was markedly attenuated and a secondary decrease in CBF was revealed. The stenosis also caused a marked enhancement of the 5-HT-induced constriction of the large artery, such that the reduction of the CD was approximately doubled at all doses of 5-HT tested. The enhanced 5-HT-induced reduction in CD was similar with placement of the stenosis either proximal or distal to the site of diameter measurement. 5 In the presence of the stenosis, blockade of 5-HT2-receptors with ketanserin (0.1 mg kg- i.c.) significantly attenuated the 5-HT-induced decreases in CD and CBF. 6 These results demonstrate that, in the anaesthetized dog, placement of a severe, flow-limiting stenosis enhances 5-HT-induced constriction of the large coronary arteries and reveals a reduction in coronary blood flow. These observations support suggestions that, in the presence of coronary artery disease, 5-HT could contribute to reductions in coronary blood flow leading to myocardial ischaemia.
Introduction It has been postulated that in the presence of coronary artery disease an enhanced release of 5-hydroxytryptamine (5-HT) could contribute to coronary vasoconstriction and possibly myocardial ischaemia (Vanhoutte, 1987). 5-HT is released from aggregating platelets in sufficient amounts to cause contraction of coronary vascular smooth muscle (Cohen et al., 1983a). In addition the concentration of 5-HT is elevated in dog coronary arteries at the site of a stenosis (Ashton et al., 1986). Also, van den Berg et al. (1989) have recently shown that 5-HT is released into the coronary circulation of some patients with coronary artery disease. However, if 5-HT is going to cause myocardial ischaemia then it must decrease coronary blood flow. 5-HT constricts large coronary arteries in vivo (Haddy et al., 1957; Bove & Dewey, 1983; Lamping et al., 1985; Chu & Cobb, 1987) and in vitro (Cocks & Angus, 1983; Cohen et al., 1983b), but it may also produce endothelium-dependent relaxation of large coronary arteries in vitro (Cocks & Angus, 1983; Cohen et al., 1983b) and dilate resistance vessels in vivo (Haddy et al., 1957; Blackshear et al., 1985; Meschig et al., 1985). As the predominant determinant of resistance is the small rather than the large vessels the action of 5-HT in the normal coronary circulation is to increase rather than decrease blood flow. This suggests that if 5-HT contributes to myocardial ischaemia in the presence of coronary artery disease, the vascular response must be different from that occurring in the normal vasculature. There are potentially many factors occurring in coronary artery disease which could influence the responses to vasoactive agents. One of these is stenosis of the large coronary arteries, as an atherosclerotic plaque encroaches on the lumen resulting in dilatation of the resistance vessels and the resultant loss of vasodilator reserve. As there have been no studies in which the actions of 5-HT have been examined
under these conditions, the aim of this study was to compare the response to 5-HT in the normal coronary circulation with that occurring in the presence of a severe stenosis. Large coronary artery diameter and coronary blood flow were monitored, in order to assess the actions of 5-HT on both conduit and resistance vessels. A preliminary account of these findings has been presented to the Australian Physiological and Pharmacological Society (Woodman, 1989).
Methods Greyhound dogs of either sex and weighing 25-38kg were anaesthetized with thiopentone (25-30mgkg-1 i.v.) followed by a-chloralose (70mgkg-1 i.v., supplemented as necessary). The dogs were ventilated with room air plus additional oxygen as necessary to maintain arterial blood Po2, Pco2 and pH within the normal ranges (Po2: 85- 10 mmHg; Pco2: 3035mmHg; pH: 7.25-7.35). Pressure in the thoracic aorta was measured by passing a catheter from the right femoral artery and connecting the catheter to a Druck pressure transducer. A continuous measurement of heart rate was obtained by a cardiotachometer coupler triggered by the ECG. A thoracotomy was performed at the left fifth intercostal space and the pericardium was opened. A pair of 7 MHz piezoelectric transducers were sutured to opposing surfaces of the left circumflex coronary artery. External diameter of the artery was measured with an ultrasonic transit-time dimension gauge (Triton Technology Inc.). Blood flow through the artery was measured by an electromagnetic flow probe (Statham) placed on the artery. A needle tipped catheter was placed in the artery proximal to the transducers for the administration of drugs. Care was taken during the placement of the transducers to limit dissection and damage of any visible nerves.
154
O.L. WOODMAN
Experimental protocol 5-HT (0.02-2upgkg-') was injected as a bolus of less than 0.5ml directly into the left circumflex coronary artery. After control responses to 5-HT had been obtained, a screw clamp was placed on the circumflex coronary artery and tightened until there was no reactive hyperaemia in response to a 10s occlusion of the artery. This occlusion had no effect on cardiac rhythm. In ten dogs the stenosis was placed proximal to the diameter transducers, whilst in a further four dogs the stenosis was distal to the transducers. The 5-HT injections were then repeated in the presence of the stenosis. Ten of these dogs were then randomly allocated to one of two groups. In the first group the stenosis was removed and the responses to 5-HT (0.1-2 pg kg- l i.c.) re-examined. In the second group the stenosis was maintained and the responses to 5-HT were assessed after blockade of 5-HT2-receptors with ketanserin (0.1 mgkg- ') injected into the coronary artery every 15 min. In a separate group of five dogs without coronary stenosis, the effect of 5-HT (0.02-2 pg kg'- i.c.) on coronary blood flow was assessed before and after blockade of 5-HT2-receptors with ketanserin (0.1 mg kg-' i.c.) and combined blockade of 5-HT1-like and 5-HT2-receptors with methysergide (0.1 mg kg- i.c.) plus ketanserin.
1 OOr
80
T0T
0-_o
601
60
401 U-
o-
m 20
a
--T-
o
T-
0
0
0.02
0.05
0.5
0.2
0.1
1.0
2.0
5-HT (pug kg-') 1 Figure The effect of the intra-coronary injection of 5hydroxytryptamine (5-HT) on left circumflex coronary artery blood flow (CBF) in the absence (0) and presence (El *) of a coronary artery stenosis. The values shown are the mean of 14 experiments; vertical lines indicate s.e.mean. In the normal coronary circulation 5-HT produced only increases in blood flow whereas in the presence of the stenosis the initial small increase in flow (E) was followed by a secondary vasoconstriction (U). Analysis of variance showed that the stenosis caused a significant (P < 0.01) attenuation of the vasodilator responses to 5-HT.
Drugs
0.02
5-Hydroxytryptamine creatinine sulphate (Sigma), ketanserin tartrate (Janssen) and methysergide maleate (Sandoz) were dissolved in distilled water with further dilutions in saline.
0.05
5-HT (~tg kg-') 0.1 0.2 0.5
-5
2.0
i
'I'
T
1.0
1"I
0
\
-
Statistics Baseline values of the haemodynamic variables measured before and after placement of the stenosis were compared by Student's t test for paired data. Statistical comparison of the responses to 5-HT recorded after stenosis or antagonism of 5-HT receptors was achieved by a two-way analysis of variance, the two factors being the dose of 5-HT and the treatment. This test treated all doses as a group to reveal whether a treatment had any significant effect on a response. When there were 3 treatment groups, e.g. control, ketanserin and ketanserin plus methysergide, a post hoc comparison was performed by use of Scheffe's test (Wallenstein et al., 1980), to compare each treatment group against the two others.
Results
u 0
-10
-15L
Figure 2 Percentage changes in left circumflex coronary artery diameter (CD) produced by the intra-coronary injection of 5hydroxytryptamine (5-HT) in the absence (0) and presence (0) of a coronary artery stenosis. The values shown are the mean of 14 experiments; vertical lines indicate s.e.mean. Placement of the stenosis proximal or distal to the site of diameter measurement caused a similar enhancement of the response to 5-HT so the data were pooled for presentation in this graph. Analysis of variance showed that the stenosis caused a significant (P < 0.01) enhancement of the 5-HT-induced large artery constriction.
5-Hydroxytryptamine 5-HT (0.02-2pgkg-') injected into the left circumflex coronary artery caused dose-dependent increases in coronary blood flow (CBF) (Figure 1) but decreases in coronary artery diameter (CD) (Figure 2). The maximum reduction in CD was 6 + 1% of the resting diameter or 170 + 40 pm at the highest dose tested (Figure 2). There was no significant change in systemic arterial pressure (AP) or heart rate (HR) at any dose of 5-HT injected into the coronary artery.
140r 1 20~
Ic
100
In five dogs the effect of selective antagonists of '5-HT1-like' and 5-HT2-receptors on the 5-HT-induced increases in coronary blood flow were assessed. Ketanserin (0.1 mg kg-1 i.c.), a selective 5-HT2 receptor antagonist, had no significant effect on resting CBF (control = 32 + 5 ml min- ', ketanserin treated = 27 + 5 ml min- 1) or on the vasodilator responses to 5-HT (Figure 3). Methysergide (0.1 mg kg-' i.c.), given after the administration of ketanserin, had no significant effect on CBF resting (methysergide-treated = 29 + 5 ml min- '). Methysergide, in the presence of ketanserin, caused a signifi-
~ I
_ 80 E 60 mL o 40
20
Effect of ketanserin and methysergide on 5-HT-induced increases in coronary bloodflow
0
T 0
0.02
0a /w I 1- I 0.05
0.1
0.2
I _
0.5
1.0
2.0
5-HT(pg kg-1) Figure 3 Increases in coronary blood flow (CBF) produced by the intra-arterial injection of 5-hydroxytryptamine (5-HT). Values shown are the mean of 5 experiments; vertical lines indicate s.e.mean. The control responses to 5-HT (0) are compared to those observed after treatment with the 5-HT2-receptor antagonist ketanserin (0.1 mg kgi.c., *) or the combined antagonims of 5-HT2- and 5-HT1-like receptors with ketanserin plus methysergide (0.1 mgkg'- i.c., *). Analysis of variance followed by Scheffe's test demonstrated that the dilator responses to 5-HT were not different in the control and ketanserintreated groups, but that the addition of methysergide caused a significant (P < 0.001) attenuation of those responses.
5-HT-INDUCED CORONARY VASCOCONSTRICTION Table 1 Baseline levels of the haemodynamic variables measured in anaesthetized dogs before and after placement of a stenosis on the left circumflex coronary artery
Mean arterial pressure (mmHg) Mean external coronary artery diameter (mm) Mean coronary blood flow (ml min -1) Heart rate (beats min- 1)
Control
Stenosis
116+4 3.1 + 0.1
119+3 3.1 + 0.2
63+8 127+6
49+7* 121 +8
Values are presented as the mean + s.e.mean of data obtained from 14 animals. *P < 0.01 compared to control, Student's t test for paired comparisons.
cant attenuation of the 5-HT-induced increase in CBF (Figure
3).
Effect of coronary artery stenosis Tightening of the screw clamp, to stenose the left circumflex coronary artery until there was no reactive hyperaemia to a ten second total occlusion of the artery, resulted in a small, but significant, reduction in CBF. The stenosis had no significant effect on CD, AP or HR (Table 1). In the presence of a stenosis the 5-HT-induced increase in CBF was markedly attenuated and, in addition, at the higher doses of 5-HT (0.5-2ygkg-1) a secondary decrease in CBF was revealed (Figure 1). The constriction of the large artery was enhanced by the stenosis at all doses of 5-HT tested. The enhancement of the large artery constriction was independent of the placement of the stenosis. On average there was a doubling of the degree of constriction observed in the presence of the stenosis at all doses tested. The degree of enhancement was similar with both proximal (2.0 + 0.4 fold enhancement) and distal stenoses (2.5 + 0.7 fold enhancement). The data for proximal and distal stenoses have been pooled and are illustrated in Figure 2.
155
5-HT (jig kg-') 0.5
0.1
1.0
,;- 0
2.0
-a----.
C I
E -5
I
1-
-10 m-1
< 15~ -
-20L Figure 5 Decreases in coronary blood flow (CBF) produced by the intra-coronary injection of 5-hydroxytryptamine (5-HT) in the presence of a stenosis (0, n = 10) are compared to responses to 5-HT after treatment with ketanserin (0.1 mg kg-1 i.c.) whilst the stenosis is maintained (E, n = 5) or after removing the stenosis (*, n = 5). The values shown are the mean and vertical lines indicate s.e.mean. The constrictor response to 5-HT was significantly (P < 0.05, analysis of variance) attenuated by both ketanserin treatment and the removal of the stenosis.
presence of the stenosis, but of similar magnitude to the control responses (Figure 6).
Effect of ketanserin on the response to S-HT in the presence of the stenosis In five dogs with stenosed coronary arteries, treatment with the 5-HT2-receptor antagonist ketanserin (0.1mgkg-1 i.c.) had no significant effect on the baseline levels of CD (3.1 + 0.2mm before ketanserin; 3.0 + 0.1 mm after ketanserin) or CBF (30 + 8 ml min- before ketanserin; 25 + 6 ml min- 1 after ketanserin). Ketanserin attenuated the constriction of the large coronary artery produced by 5-HT (1igure 6) and in addition abolished the decrease in CBF (Figure 5).
Response to 5-HT after removing the stenosis
Discussion
In five dogs the response to 5-HT was examined after the removal of the stenosis. After the stenosis was removed 5-HT injection caused an increase in CBF (Figure 4) of similar magnitude to the control response and there was no significant secondary decrease in flow (Figure 5). The 5-HT-induced constriction of the large artery was less than that observed in the
This study has demonstrated that the injection of 5-HT into the left circumflex coronary artery of the anaesthetized dog constricts the proximal portion of that artery, but simultaneously increases blood flow in that vascular bed. As there were no accompanying changes in arterial pressure or heart rate, the increase in flow indicated that 5-HT dilates the coronary resistance vessels. The response to 5-HT was then reexamined in the presence of a coronary artery stenosis, which 5-HT (pLg
100 -
kg-') 1.0
0.5
0.1
2.0
80-
E 60-
.I
T
--~--' ---
*
-
1-_
I
I
-
-5
m- 40
-
U
0
< 2c
