Experimental and laboratory reports Dopamine effects on the intestinal circulation Wieslaw Pawlik, M.D. David Mailman, Ph.D. Linda L. Shanbour, Ph.D. Eugene D. Jacobson, M.D. Houston, Texas
Many investigators have been concerned with responses of the normal cardiovascular system to the administration of dopamine (3,4-dihydroxphenylethylamine).1-7 Effects of dopamine on the peripheral circulation are variable and species dependent. 4'~' In dogs the effect of low doses of dopamine on total peripheral resistance appears to be the result of a balance between vasoconstriction in some peripheral vascular beds and vasodilatation in others, including the superior mesenteric and celiac arteries. 1' '~ At higher doses dopamine causes constriction of peripheral and visceral vascular beds. Effects of dopamine on intestinal blood flow have been studied by several investigators, but reports are contradictory.'. 6. Previous studies from our laboratory indicated that dopamine at higher doses consistently decreased mesenteric blood flow in dogs, 6 not only under normal conditions but also in various forms of shock. 8 The finding that dopamine constricted the mesenteric circulation poses the possibility t h a t the drug not only decreases total intestinal blood flow via contraction of mesenteric arteriolar smooth muscle, but also that dopamine acts on the precapillary sphincters to reduce perfusion of the vessels from which oxygen is absorbed. Thus, dopamine could depress intestinal oxygen consumption and induce not only intestinal ischemia From the Program in Physiology, The University of Texas Medical School at Houston and the Biology Department. University of Houston, Houston, Texas. These investigations were supported in part by United States Public Health Service Grants No. AM 15997 and ONR contract No. N00014A-0001 and U.S.P.H.S. Grant No. AM/HL 18629. Received for publication Nov. 20, 1974. Reprint requests: Eugene D. Jacobson. M.D.. Program in Physiology, The University of Texas Medical School at Houston, Texas Medical Center, 6400 W. Cullen St., Houston, Texas 77025.
March, 1976, Vol. 91, No. 3, pp. 325-331
but also hypoxia, which could lead to necrosis of the gut. The current investigation also had another purpose, which was to characterize the pharmacological nature of the receptors mediating the action of dopamine on the smooth muscle of intestinal microcirculatory structures. Methods
Experiments were performed on 27 mongrel dogs of both sexes, weighing 15 to 23 kilograms. Animals were fasted for 24 hours before the experiment and anesthetized with sodium pentobarbital (30 mg. per kilogram) administered intravenously and supplemented as needed. Large-bore catheters were inserted into both femoral arteries and veins. After a midline laparotomy, a distal trunk of the superior mesenteric artery supplying a segment of terminal ileum was carefully exposed. The ends of the intestinal segment were ligated to prevent perfusion from collateral vessels. The mean weight of the segment of intestine was 81 _+ 28 (S.E.) Gm. An electromagnetic flow transducer {Micron, I.D., 2.0 mm.) was positioned around the mesenteric artery and connected to a blood flow amplifier {Micron}. Zero flow was obtained by occluding the artery downstream from the flow probe. The first side branch of the superior mesenteric artery distal to the flow probe was isolated and cannulated with a catheter (PE 160) for intra-arterial drug infusion. After hemostasis, an initial dose of 100 U. of heparin per kilogram of body weight was given intravenously and supplements of 50 U. per kilogram were given at hourly intervals thereafter. A side branch of the mesenteric vein draining the intestinal segment was also cannulated with a
American Heart Journal
325
P a w l i k et al.
Fig. 1. Results of a single experimentin which three doses of dopamine were infusedinto the mesentericartery for 10 minutes. MBF decreased greatly and A-VhO~ increased slightly; hence, calculated Vo~ decreased. Arterial pressure was unaffected. catheter (PE 190) for venous blood sampling and to obtain venous blood for arteriovenous oxygen content, determinations. A constant-flow pump (Cole-Parmer, Model 7595) withdrew arterial blood from a femoral arterial catheter and venous blood from the mesenteric vein and passed the blood through the arterial and venous cuvettes of a photometric arteriovenous oxygen difference analyzer (Oxford Instrument Company). This instrument provided a continuous direct record of the arteriovenous oxygen difference (A-V~ 02). Venous and arterial effluent from the apparatus was then pumped back to the circulation via a femoral vein. At the beginning of each experiment, arterial blood was passed through both the arterial and the venous cuvettes to determine zero difference. Intestinal oxygen consumption (Vo2) was calculated from the A-V~ O2 and total intestinal segment blood flow (MBF) and was expressed as milliliters of 02 per 100 Gm. of intestine per minute. Systemic arterial pressure was monitored with a pressure transducer (Hewlett-Packard, 1280C) from a femoral artery. MBF, A-Va 02, and arterial pressure were monitored in all 27 dogs on a direct-writing recorder (HewlettPackard model 7759A). In 13 of the animals effective capillary surface area was estimated from the clearance of ~Rb. 8~RbC1 (Amersham Searle) was diluted in 0.9 per cent saline and was infused intra-arterially by a syringe-type constant flow p u m p (Harvard Instruments) at a rate of 10,000 c.p.m, through
326
the mesenteric arterial catheter. Six minutes after beginning the infusion three 7 ml. samples of. mesenteric venous blood were collected in tared gamma counting tubes. Preliminary experiments had shown that the ~Rb concentration in mesenteric vein blood stabilized within 4 minutes. Collections were obtained during a control period and after 5 minutes of dopamine infusion before and after haloperidol or propranolol. Collection time for the three samples ranged from 1.5 to 6 minutes, depending on the flow rate. The a m o u n t of 86Rb infused was determined by timed collection of the infusion solution directly from the arterial cannula and then counting the ~6Rb in 7 ml. of blood obtained before any isotope had been infused. Samples of femoral arterial blood were obtained at the end of each period to determine the concentration of ~Rb due to isotope which had recirculated through the gut and into the general circulation. Samples were usually counted to at least 10,000 counts above background in a gamma counter (Nuclear Chicago). The concentration of 8~Rb in arterial blood was calculated from the a m o u n t of ~Rb infused per minute and the blood flow at t h a t time determined by the flowmeter. T h e product of capillary permeability and surface area (PS product) was calculated from the blood flow and the ~Rb concentration in the mesenteric vein and artery and was corrected for the small (less t h a n 1 per cent) ~Rb concentration in the general circulation? -1~ The equation used was PS = - Q
March, 1976, Vol. 91, No. 3
Dopamine and intestinal circulation
DOPAMINE INFUSION
DOPAMINE INFUSION20ug/kg/min
I
t.
0" MESENTERIC BLOOD FLOW (% CONTROL)
4
+75]
+25'
+50 t
D + H
+25
-25"
MESENTERIC BLOOD FLOW_ 0t (% CONTROL) -25
--50"
-75
O---O1/~g/kg/min 5/~g/kg/min =- -- 20pg/kg/min
+25"
D+ H+ P
-50
D
75 +25'
INTESTINAL 0, OXYGEN -25CONSUMPTION
INTESTINAL 0.=.~ ~ OXYGEN 2 5 CONSUMPTION (% CONTROL) -50-
(% CONTROL)
-50-
--75
I , I
0
I
2
I
I
I
I
4 6 TIME (rain)
I
I
8
I
D+ H D+H+P D
~
I
10
-75TIME (min)
Fig. 2. Effects of three doses of dopamine on MBF and ~'o,. Each value represents the mean _+ S.E. I n ( l - E ) , where PS represents the p r o d u c t of capillary permeability and surface area, Q represents blood flow, and E the arterial-venous extraction ratio. During the measuring period all values were relatively constant. Since infusion of ~Rb lasted no longer t h a n 12 minutes, no corrections were made for ~'Rb a c c u m u l a t i o n in the gut tissue nor in the red blood cells. T h e infusion c a t h e t e r was c o n n e c t e d to the constant-infusion p u m p ( H a r v a r d Apparatus). Following a postsurgical interval when M B F , arterial pressure, and A-V~ O2 had been stable for 20 to 30 minutes, intra-arterial infusion of dopamine was begun. D o p a m i n e (Sigma) was dissolved in 0.9 per cent saline and infusions were carried out for 10 minutes. In separate experiments three doses of dopamine were used: 1.0/~g/Kg.-min. (seven dogs), 5,.0 /~g/Kg.-min. (seven dogs), and 20/~g/Kg.-min. (13 dogs). After cessation of d o p a m i n e infusion, observations were recorded for 10 minutes in seven of the animals following m e a s u r e m e n t s of arterial pressure, MBF, and A-V~ O2 before, during, a n d after infusion of the 20/~g/Kg.-min. dose. Dopamine receptor blockade was p e r f o r m e d with haloperidol (2.0/~g per kilogram, McNeil) in
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Fig. 3. Effects of dopamine (20 ttg/Kg.-min.) on MBF and ~'0,_, before and after blockade with haloperidol and propranolol. D = dopamine alone; D + H = dopamine after haloperidol; D + H + P = dopamine after combined haloperidol and propranolol. saline, administered over a 5 m i n u t e period directly into the mesenteric arterial ~atheter. T e n minutes after haloperidol was administered, dopamine (20 ~g/Kg.-min.) was infused again. Half an h o u r after haloperidol administration, beta-adrenergic receptor blockade was c o n d u c t e d with 0.5 mg. of propranolol (Ayerst) infused over a 5 m i n u t e period into the mesenteric artery. Infusion of the same dose of d o p a m i n e was repeated 10 minutes following the propranolol injection. D a t a were expressed in terms of m e a n _ standard error and analyzed for statistical significance by the t test for paired values. Results Mesenteric
blood
flow
(MBF).
The
initial
control value of M B F was 31.5 _+ 6.3 ml. per minute per 100 Gm. of intestinal segment. T h e response to intra-arterial infusion of d o p a m i n e was a consistent reduction of MBF, indicative of local vasoconstriction, since arterial blood pressure was unaffected (Figs. 1 and 2). T h e decrease
327
P a w l i k et al.
PS-PRODUCT
PS-PRODUCT
+20-
+20
.,J
O er
.J
O n-
-20"
z
8
z O
-40"
-40
-60* -80 -
DOPAMINE lug/kg/min ~ ' ~ DOPAM IN E 5#g/kg/min DOPAMINE 20/~g/kg/min
Fig. 4; Effects of three doses of dopamine on ~';Rb clearance (PS product). An asterisk indicates a significant change.
in MBF started within 30 seconds following the onset of drug infusion. When dopamine was infused in its lowest and intermediate doses, postinfusion hyperemia was observed after cessation of the drug; after cessation of the highest dose, MBF slowly returned to the control value. At the lowest dose (1 #g/Kg.-min.) dopamine caused an initial fall in MBF in the first minute of infusion followed by a transient, small increase in flow over the next minute, after which MBF declined steadily throughout the remainder of the 10 minutes infusion period (Fig. 2). MBF was significantly (p < 0.05) decreased below control values by 7 to 10 minutes after the infusion was begun. The mean decrease of MBF at the end of the 10 minute infusion period was 10 per cent (p < 0.05). The intermediate dose of dopamine (5/zg/Kg.min.) decreased MBF more abruptly than the lesser dose {Fig. 2). The decrease at 10 minute of infusion was 33 per cent (p < 0.001). The highest dose of dopamine caused an early decline in MBF, with some autoregulatory escape subsequently {Fig, 2). MBF was decreased 56 per cent (p < 0.001) by the 20/~g/Kg.-min. dose at the end of the infusion.
328
-20
-60
DOPAMINE ALONE 1772l DOPAMIN E AFTER HALOPERIDOL DOPAMINE AFTER HALOPERIDOL & PROPRANOLOL
Fig. 5. Effects of dopamine on PS product before and after blockade with haloperidol and propranolol. An asterisk indicates a significantchange.
Following dopamine receptor blockade with haloperidol, the 20/~g/Kg.-min. dose of dopamine caused an initial decline in MBF in the first minute of infusion similar to the response before haloperidol {Fig. 3). This initial decline in MBF was followed by a rapid increase to a value 50 per cent greater than control (p < 0.001). After dopamine receptor blockade, beta-adrenergic blockade with propranolol was performed, which restored, subsequent responses to dopamine to a predominantly constrictor effect. Thus, after combined haloperidol and propranolol treatment, dopamine evoked a 25 per cent reduction (p < 0.001) in MBF. Intestinal oxygen consumption (~o2). Control value for "~o~ was 2.7 _ 0.6 m]. per m i n u t e per 100
Gm. ~o2 was significantly decreased 10 per cent (p < 0.05) with the lowest dose of dopamine (Fig. 2). Dopamine in its intermediate dose decreased Vo~ 18 per cent (p < 0.001). When the dopamine infusion was stopped, Vo2 promptly returned to the control level and oscillated for a few minutes above that level. The highest dose of dopamine
March, 1976, Vol. 91, No. 3
Dopamine and intestinal circulation
INTESTINAL MICROCIRCULATION
=SMOOTH MUSCLE
ARTERIOLE PCS
=PRECAPILLARY SPHINCTER
NON NUTRIENT CIRCULATION
PCS
NUTRIENT CIRCULATION
OPEN AND CLOSED CAPILLARIES AND CELLS
VENULE
Fig. 6. Schematic depiction of the intestinal microcirculation and the smooth muscle elements responsible for its regulation. Contraction of the smooth muscle wall of the arteriole would lead to a diminished MBF and an increase in the resistance to blood flow, but would not necessarily cause a decrease in capillary perfusion, since blood could shunt from the non-nutrient to the nutrient circulation. A decrease in capillary perfusion would occur if the smooth muscle Of the precapillary sphincters contracted, thereby closing open capillaries. Dopamine decreased MBF, "~o2and PS product, indicating that the agent constricts both arteriolar and PCS smooth muscle, causing both ischemia and hypoxia of the gut.
rapidly decreased Vo_~; at the end of 10 m i n u t e s ' infusion, Vo~ was reduced 45 per cent (p < 0.001). Following d o p a m i n e r e c e p t o r b l o c k a d e with haloperidol, d o p a m i n e increased Vo~ 10 per cent (p < 0.025) after an initial decline (Fig. 3). T h e addition of beta-adrenergic blockade with p r o p r a nolol restored the d o p a m i n e responses to their pre-haloperidol c h a r a c t e r ; Vo~ was decreased 25 per cent (p < 0:001). 86Rb c l e a r a n c e (PS product). T h e lowest dose of d o p a m i n e did not a l t e r P S p r o d u c t significantly, b u t the two higher doses of d o p a m i n e reduced PS p r o d u c t significantly (Fig. 4). W i t h 5/~g/Kg.-min. of dopamine, PS p r o d u c t decreased 40 per cent (p < 0.003); it declined 55 per cent (p < 0.001) with the 20 ~g/Kg.-min. dose. Following d o p a m ine receptor blockade, our highest dose of d o p a m i n e did not change PS product, b u t a f t e r addition of beta-adrenergic b l o c k a d e P S p r o d u c t
American Heart Journal
was reduced 47 per cent (p < 0.001). T h e s e results a p p e a r in Fig. 5. Systemic arterial pressure. T h e m e a n c o n t r o l value of arterial pressure was 135 _+ 15 m m . Hg. I n t r a - a r t e r i a l infusions of t h e three d o s e s of dopamine, either before or a f t e r the blocking drug, did not significantly change the m e a n systemic arterial pressure (Fig. 1). Discussion
Our results indicate t h a t d o p a m i n e is a p o t e n t vasoactive drug in the m e s e n t e r i c circulation, producing significant d i m i n u t i o n in blood flow t h r o u g h the gut in doses as low as 1 t~g/Kg.-min. infused directly into the artery. Since s y s t e m i c arterial pressure was unaffected b y i n t r a - a r t e r i a l infusions of the drug, d o p a m i n e has to be considered a direct v a s o c o n s t r i c t o r of the m e s e n t e r i c circulation. O t h e r a u t h o r s h a v e concluded t h a t d o p a m i n e is a vasodilator a g e n t in this circula-
329
Pawlik et al. tion; however, the doses employed in those reports were small and were injected as a bolus. 1' 7 In our experiments the lowest dose (1 ~g/Kg.min.) produced a small early increase in blood flow which was transient and gave way to a subsequent decrease in MBF. We also found that dopamine evoked decreases in intestinal uptake of oxygen. The declines of Vo~ paralleled the decreases in MBF in response to the different doses of dopamine. Hence, dopamine induced both ischemia and hypoxia in the intestine. In this respect, dopamine acts upon the mesenteric Circulation in a manner similar to most other constrictor agents we have studied, except epinephrine, which decreases MBF without also diminishing Vo~ and PS product. TM 1-~ The microcirculatory sites at which constrictor drugs act in the mesenteric vasculators are the arteriolar smooth muscle, which regulates resistance to the total flow of blood through the gut, and the smooth muscle of the precapillary sphincter, which regulates the flow of blood through the capillaries. These sites are depicted schematically in Fig. 6. Since significant oxygen exchanges occur only across the capillary endothelium, a reduced uptake of oxygen ensues when blood flow through the capillaries is reduced. An accepted measurement of capillary perfusion (the nutrient circulation) is obtained from the clearance of the non-metabolized marker 8~Rb (PS product). In our studies dopamine decreased MBF and PS product comparably a t the three doses of the drug. This suggests that dopamine induces indiscriminate contraction of arteriolar and precapillary sphincteric smooth muscle, thereby reducing total blood flow by constricting arterioles and reducing the nutrient circulation by closing capillaries. A specific receptor for the major local circulatory action of dopamine has been predicated, 4This receptor is blocked by haloperidol. 4 In the current investigation, the mesente~c vasoconstrictor response to dopamine was prevented by haloperidol, as was the diminished uptake of oxygen and the decrease in the nutrient circulation. After haloperidol blockade dopamine induced vasodilator responses, accompanied by increased Vow. These responses, in t u r n , were prevented by betaadrenergic blockade. Our findings suggest t h a t dopamine activates at least two receptors in the mesenteric circulation; the predominant receptor, which constricts arteriolar and precapillary
330
sphincteric smooth muscle, evoking dose-related decreases in MBF, Vow, and PS product; and the occult receptor, which relaxes mesenteric vascular smooth muscle, thereby increasing MBF and ~'o2. The second receptor appears to be a peripheral beta-adrenergic receptor. The dominant receptor may be a unique dopaminergic receptor or it may be the alpha-adrenergic receptor. In another report the mesenteric vasoconstrictor response to dopamine was prevented by phenoxybenzamine. 6 Our findings do not, however, answer the questions about the physiological role of dopamine in the mesenteric vasculature.
Summary The effects of intra-arterial infusion of dopamine on superior mesenteric artery blood flow, intestinal oxygen consumption, and capillary density were studied in anesthetized dogs before and after blockade of dopamine receptors with haloperidol and after beta-adrenergic receptor blockade with propranolol. Mesenteric blood flow to a distal segment of the small intestine was measured with an electromagnetic blood flowmeter and intestinal oxygen consumption was calculated from the measured arteriovenous oxygen difference across the intestine and total blood flow. Intestinal capillary density was estimated from the clearance of ~Rb. In normal animals prior to dopaminergic or beta-adrenergic blockade, dopamine caused a dose-related decrease in mesenteric blood flow, intestinal oxygen consumption, and 8~Rb clearance. Only the lowest dose of the drug, 1 ~g/Kg.-min., did not significantly change the intestinal capillary density. In dogs pretreated with the dopamine receptor, antogonist~ haloperidol, dopamine (20 ~g/Kg.min.) caused a significant increase in blood flow and oxygen consumption and did not significantly alter the number of perfused intestinal capillaries. These increases in haloperidol-blocked animals administered dopamine were reversed by propranolol. Our results indicate t h a t dopamine caused smooth muscle contraction in mesenteric arterioles and precapillary sphincters, thereby producing intestinal ischemia and hypoxia. These findings with haloperidol and propranolol indicate that dopamine stimulates at least two different receptors in the canine mesenteric vascular bed: a constrictor receptor blocked by haloperidol and a dilator receptor blocked by propranolol.
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Dopamine and intestinal circulation
REFERENCES
1. Goldberg, L . I.: Cardiovascular and renal actions of dopamine: potential clinical applications, Pharmacol. Rev: 24:1, 1972. 2..Maxwell, C. M., Rowe, G. C., and Castillo, C. A.: The effect of dopamine (3-hydroxytyramine) upon the systemic, pulmonary, and cardiac hemodynamics and metabolism of intact dog, Arch. Int. Pharmac0dyn. Ther. 129:62, 1960. 3. McDonald, R. H., a n d Goldberg, L. I.: Analysis of the cardiovascular effects of dopamine in the dog, J. Pharmacol. Exp. Ther. 140:60, 1963. 4. Yeh, B. U:, McNay, J: L., and Goldberg, L. I.: Attenuation of dopamine renal and mesenteric vasodilation by haloperido]: Evidence for a specific dopamine receptor, J. Pharmacol. Exp. Ther. 168:303, 1969. 5. Burn, J. H., and Rand, M. J:: The depressor action of dopamine and adrenaline, Br. J. Pharmacol. Chemother. 13:471, 1958. 6. Shanbour, L. L., and Parker, D.: Effects of dopamine and other catecholamines on the splanchnic circulation, Can. J: Physiol. Pharmacol. 50:599, 1972.
American Heart Journal
7.
8. 9. 10. 11. 12.
Higgins, C. B., Millar, R. W., and Braunwald, E.: Effects and mechanisms of action o f dopamine on regional hemodynamics in the conscious dog, Am. J. Physiol. 225:432, 1973. Ulano, H. B., Treat, E., Shanbour, L. L., and Jacobson, E. D.: Selective dilation of the constricted superior mesenteric artery, Gastroenterology 62:39, 1972. Renken, E. M.: Transport of potassium ~ from blood to tissue in isolated mammalian skeletal muscle, Am. J. Physiol. 197:1205, 1959. Dresel, P., Folkow, B., and WaHentin, L.: Rubidium ~'* clearance during neurogenic redistribution of intestinal blood flow, Acta Physiol. Scand. 67:173, 1966. Jacobson, E. D., Mailman, D., Shepherd, A. P., and Pawlik, W.: Intestinal ischemia and hypoxia: vasopressin vs. epinephrine, Clin. Res. 22:63A, 1974. Pawlik, W., Shepherd, A. P., and Jacobson, E. D.: Effects of vasoactive agents on intestinal oxygen consumption and blood flow, J. Clin. Invest. 56:484, 1975.
331
Many investigators have been concerned with responses of the normal cardiovascular system to the administration of dopamine (3,4-dihydroxphenylethylamine).1-7 Effects of dopamine on the peripheral circulation are variable and species dependent. 4'~' In dogs the effect of low doses of dopamine on total peripheral resistance appears to be the result of a balance between vasoconstriction in some peripheral vascular beds and vasodilatation in others, including the superior mesenteric and celiac arteries. 1' '~ At higher doses dopamine causes constriction of peripheral and visceral vascular beds. Effects of dopamine on intestinal blood flow have been studied by several investigators, but reports are contradictory.'. 6. Previous studies from our laboratory indicated that dopamine at higher doses consistently decreased mesenteric blood flow in dogs, 6 not only under normal conditions but also in various forms of shock. 8 The finding that dopamine constricted the mesenteric circulation poses the possibility t h a t the drug not only decreases total intestinal blood flow via contraction of mesenteric arteriolar smooth muscle, but also that dopamine acts on the precapillary sphincters to reduce perfusion of the vessels from which oxygen is absorbed. Thus, dopamine could depress intestinal oxygen consumption and induce not only intestinal ischemia From the Program in Physiology, The University of Texas Medical School at Houston and the Biology Department. University of Houston, Houston, Texas. These investigations were supported in part by United States Public Health Service Grants No. AM 15997 and ONR contract No. N00014A-0001 and U.S.P.H.S. Grant No. AM/HL 18629. Received for publication Nov. 20, 1974. Reprint requests: Eugene D. Jacobson. M.D.. Program in Physiology, The University of Texas Medical School at Houston, Texas Medical Center, 6400 W. Cullen St., Houston, Texas 77025.
March, 1976, Vol. 91, No. 3, pp. 325-331
but also hypoxia, which could lead to necrosis of the gut. The current investigation also had another purpose, which was to characterize the pharmacological nature of the receptors mediating the action of dopamine on the smooth muscle of intestinal microcirculatory structures. Methods
Experiments were performed on 27 mongrel dogs of both sexes, weighing 15 to 23 kilograms. Animals were fasted for 24 hours before the experiment and anesthetized with sodium pentobarbital (30 mg. per kilogram) administered intravenously and supplemented as needed. Large-bore catheters were inserted into both femoral arteries and veins. After a midline laparotomy, a distal trunk of the superior mesenteric artery supplying a segment of terminal ileum was carefully exposed. The ends of the intestinal segment were ligated to prevent perfusion from collateral vessels. The mean weight of the segment of intestine was 81 _+ 28 (S.E.) Gm. An electromagnetic flow transducer {Micron, I.D., 2.0 mm.) was positioned around the mesenteric artery and connected to a blood flow amplifier {Micron}. Zero flow was obtained by occluding the artery downstream from the flow probe. The first side branch of the superior mesenteric artery distal to the flow probe was isolated and cannulated with a catheter (PE 160) for intra-arterial drug infusion. After hemostasis, an initial dose of 100 U. of heparin per kilogram of body weight was given intravenously and supplements of 50 U. per kilogram were given at hourly intervals thereafter. A side branch of the mesenteric vein draining the intestinal segment was also cannulated with a
American Heart Journal
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P a w l i k et al.
Fig. 1. Results of a single experimentin which three doses of dopamine were infusedinto the mesentericartery for 10 minutes. MBF decreased greatly and A-VhO~ increased slightly; hence, calculated Vo~ decreased. Arterial pressure was unaffected. catheter (PE 190) for venous blood sampling and to obtain venous blood for arteriovenous oxygen content, determinations. A constant-flow pump (Cole-Parmer, Model 7595) withdrew arterial blood from a femoral arterial catheter and venous blood from the mesenteric vein and passed the blood through the arterial and venous cuvettes of a photometric arteriovenous oxygen difference analyzer (Oxford Instrument Company). This instrument provided a continuous direct record of the arteriovenous oxygen difference (A-V~ 02). Venous and arterial effluent from the apparatus was then pumped back to the circulation via a femoral vein. At the beginning of each experiment, arterial blood was passed through both the arterial and the venous cuvettes to determine zero difference. Intestinal oxygen consumption (Vo2) was calculated from the A-V~ O2 and total intestinal segment blood flow (MBF) and was expressed as milliliters of 02 per 100 Gm. of intestine per minute. Systemic arterial pressure was monitored with a pressure transducer (Hewlett-Packard, 1280C) from a femoral artery. MBF, A-Va 02, and arterial pressure were monitored in all 27 dogs on a direct-writing recorder (HewlettPackard model 7759A). In 13 of the animals effective capillary surface area was estimated from the clearance of ~Rb. 8~RbC1 (Amersham Searle) was diluted in 0.9 per cent saline and was infused intra-arterially by a syringe-type constant flow p u m p (Harvard Instruments) at a rate of 10,000 c.p.m, through
326
the mesenteric arterial catheter. Six minutes after beginning the infusion three 7 ml. samples of. mesenteric venous blood were collected in tared gamma counting tubes. Preliminary experiments had shown that the ~Rb concentration in mesenteric vein blood stabilized within 4 minutes. Collections were obtained during a control period and after 5 minutes of dopamine infusion before and after haloperidol or propranolol. Collection time for the three samples ranged from 1.5 to 6 minutes, depending on the flow rate. The a m o u n t of 86Rb infused was determined by timed collection of the infusion solution directly from the arterial cannula and then counting the ~6Rb in 7 ml. of blood obtained before any isotope had been infused. Samples of femoral arterial blood were obtained at the end of each period to determine the concentration of ~Rb due to isotope which had recirculated through the gut and into the general circulation. Samples were usually counted to at least 10,000 counts above background in a gamma counter (Nuclear Chicago). The concentration of 8~Rb in arterial blood was calculated from the a m o u n t of ~Rb infused per minute and the blood flow at t h a t time determined by the flowmeter. T h e product of capillary permeability and surface area (PS product) was calculated from the blood flow and the ~Rb concentration in the mesenteric vein and artery and was corrected for the small (less t h a n 1 per cent) ~Rb concentration in the general circulation? -1~ The equation used was PS = - Q
March, 1976, Vol. 91, No. 3
Dopamine and intestinal circulation
DOPAMINE INFUSION
DOPAMINE INFUSION20ug/kg/min
I
t.
0" MESENTERIC BLOOD FLOW (% CONTROL)
4
+75]
+25'
+50 t
D + H
+25
-25"
MESENTERIC BLOOD FLOW_ 0t (% CONTROL) -25
--50"
-75
O---O1/~g/kg/min 5/~g/kg/min =- -- 20pg/kg/min
+25"
D+ H+ P
-50
D
75 +25'
INTESTINAL 0, OXYGEN -25CONSUMPTION
INTESTINAL 0.=.~ ~ OXYGEN 2 5 CONSUMPTION (% CONTROL) -50-
(% CONTROL)
-50-
--75
I , I
0
I
2
I
I
I
I
4 6 TIME (rain)
I
I
8
I
D+ H D+H+P D
~
I
10
-75TIME (min)
Fig. 2. Effects of three doses of dopamine on MBF and ~'o,. Each value represents the mean _+ S.E. I n ( l - E ) , where PS represents the p r o d u c t of capillary permeability and surface area, Q represents blood flow, and E the arterial-venous extraction ratio. During the measuring period all values were relatively constant. Since infusion of ~Rb lasted no longer t h a n 12 minutes, no corrections were made for ~'Rb a c c u m u l a t i o n in the gut tissue nor in the red blood cells. T h e infusion c a t h e t e r was c o n n e c t e d to the constant-infusion p u m p ( H a r v a r d Apparatus). Following a postsurgical interval when M B F , arterial pressure, and A-V~ O2 had been stable for 20 to 30 minutes, intra-arterial infusion of dopamine was begun. D o p a m i n e (Sigma) was dissolved in 0.9 per cent saline and infusions were carried out for 10 minutes. In separate experiments three doses of dopamine were used: 1.0/~g/Kg.-min. (seven dogs), 5,.0 /~g/Kg.-min. (seven dogs), and 20/~g/Kg.-min. (13 dogs). After cessation of d o p a m i n e infusion, observations were recorded for 10 minutes in seven of the animals following m e a s u r e m e n t s of arterial pressure, MBF, and A-V~ O2 before, during, a n d after infusion of the 20/~g/Kg.-min. dose. Dopamine receptor blockade was p e r f o r m e d with haloperidol (2.0/~g per kilogram, McNeil) in
American Heart Journal
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Fig. 3. Effects of dopamine (20 ttg/Kg.-min.) on MBF and ~'0,_, before and after blockade with haloperidol and propranolol. D = dopamine alone; D + H = dopamine after haloperidol; D + H + P = dopamine after combined haloperidol and propranolol. saline, administered over a 5 m i n u t e period directly into the mesenteric arterial ~atheter. T e n minutes after haloperidol was administered, dopamine (20 ~g/Kg.-min.) was infused again. Half an h o u r after haloperidol administration, beta-adrenergic receptor blockade was c o n d u c t e d with 0.5 mg. of propranolol (Ayerst) infused over a 5 m i n u t e period into the mesenteric artery. Infusion of the same dose of d o p a m i n e was repeated 10 minutes following the propranolol injection. D a t a were expressed in terms of m e a n _ standard error and analyzed for statistical significance by the t test for paired values. Results Mesenteric
blood
flow
(MBF).
The
initial
control value of M B F was 31.5 _+ 6.3 ml. per minute per 100 Gm. of intestinal segment. T h e response to intra-arterial infusion of d o p a m i n e was a consistent reduction of MBF, indicative of local vasoconstriction, since arterial blood pressure was unaffected (Figs. 1 and 2). T h e decrease
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P a w l i k et al.
PS-PRODUCT
PS-PRODUCT
+20-
+20
.,J
O er
.J
O n-
-20"
z
8
z O
-40"
-40
-60* -80 -
DOPAMINE lug/kg/min ~ ' ~ DOPAM IN E 5#g/kg/min DOPAMINE 20/~g/kg/min
Fig. 4; Effects of three doses of dopamine on ~';Rb clearance (PS product). An asterisk indicates a significant change.
in MBF started within 30 seconds following the onset of drug infusion. When dopamine was infused in its lowest and intermediate doses, postinfusion hyperemia was observed after cessation of the drug; after cessation of the highest dose, MBF slowly returned to the control value. At the lowest dose (1 #g/Kg.-min.) dopamine caused an initial fall in MBF in the first minute of infusion followed by a transient, small increase in flow over the next minute, after which MBF declined steadily throughout the remainder of the 10 minutes infusion period (Fig. 2). MBF was significantly (p < 0.05) decreased below control values by 7 to 10 minutes after the infusion was begun. The mean decrease of MBF at the end of the 10 minute infusion period was 10 per cent (p < 0.05). The intermediate dose of dopamine (5/zg/Kg.min.) decreased MBF more abruptly than the lesser dose {Fig. 2). The decrease at 10 minute of infusion was 33 per cent (p < 0.001). The highest dose of dopamine caused an early decline in MBF, with some autoregulatory escape subsequently {Fig, 2). MBF was decreased 56 per cent (p < 0.001) by the 20/~g/Kg.-min. dose at the end of the infusion.
328
-20
-60
DOPAMINE ALONE 1772l DOPAMIN E AFTER HALOPERIDOL DOPAMINE AFTER HALOPERIDOL & PROPRANOLOL
Fig. 5. Effects of dopamine on PS product before and after blockade with haloperidol and propranolol. An asterisk indicates a significantchange.
Following dopamine receptor blockade with haloperidol, the 20/~g/Kg.-min. dose of dopamine caused an initial decline in MBF in the first minute of infusion similar to the response before haloperidol {Fig. 3). This initial decline in MBF was followed by a rapid increase to a value 50 per cent greater than control (p < 0.001). After dopamine receptor blockade, beta-adrenergic blockade with propranolol was performed, which restored, subsequent responses to dopamine to a predominantly constrictor effect. Thus, after combined haloperidol and propranolol treatment, dopamine evoked a 25 per cent reduction (p < 0.001) in MBF. Intestinal oxygen consumption (~o2). Control value for "~o~ was 2.7 _ 0.6 m]. per m i n u t e per 100
Gm. ~o2 was significantly decreased 10 per cent (p < 0.05) with the lowest dose of dopamine (Fig. 2). Dopamine in its intermediate dose decreased Vo~ 18 per cent (p < 0.001). When the dopamine infusion was stopped, Vo2 promptly returned to the control level and oscillated for a few minutes above that level. The highest dose of dopamine
March, 1976, Vol. 91, No. 3
Dopamine and intestinal circulation
INTESTINAL MICROCIRCULATION
=SMOOTH MUSCLE
ARTERIOLE PCS
=PRECAPILLARY SPHINCTER
NON NUTRIENT CIRCULATION
PCS
NUTRIENT CIRCULATION
OPEN AND CLOSED CAPILLARIES AND CELLS
VENULE
Fig. 6. Schematic depiction of the intestinal microcirculation and the smooth muscle elements responsible for its regulation. Contraction of the smooth muscle wall of the arteriole would lead to a diminished MBF and an increase in the resistance to blood flow, but would not necessarily cause a decrease in capillary perfusion, since blood could shunt from the non-nutrient to the nutrient circulation. A decrease in capillary perfusion would occur if the smooth muscle Of the precapillary sphincters contracted, thereby closing open capillaries. Dopamine decreased MBF, "~o2and PS product, indicating that the agent constricts both arteriolar and PCS smooth muscle, causing both ischemia and hypoxia of the gut.
rapidly decreased Vo_~; at the end of 10 m i n u t e s ' infusion, Vo~ was reduced 45 per cent (p < 0.001). Following d o p a m i n e r e c e p t o r b l o c k a d e with haloperidol, d o p a m i n e increased Vo~ 10 per cent (p < 0.025) after an initial decline (Fig. 3). T h e addition of beta-adrenergic blockade with p r o p r a nolol restored the d o p a m i n e responses to their pre-haloperidol c h a r a c t e r ; Vo~ was decreased 25 per cent (p < 0:001). 86Rb c l e a r a n c e (PS product). T h e lowest dose of d o p a m i n e did not a l t e r P S p r o d u c t significantly, b u t the two higher doses of d o p a m i n e reduced PS p r o d u c t significantly (Fig. 4). W i t h 5/~g/Kg.-min. of dopamine, PS p r o d u c t decreased 40 per cent (p < 0.003); it declined 55 per cent (p < 0.001) with the 20 ~g/Kg.-min. dose. Following d o p a m ine receptor blockade, our highest dose of d o p a m i n e did not change PS product, b u t a f t e r addition of beta-adrenergic b l o c k a d e P S p r o d u c t
American Heart Journal
was reduced 47 per cent (p < 0.001). T h e s e results a p p e a r in Fig. 5. Systemic arterial pressure. T h e m e a n c o n t r o l value of arterial pressure was 135 _+ 15 m m . Hg. I n t r a - a r t e r i a l infusions of t h e three d o s e s of dopamine, either before or a f t e r the blocking drug, did not significantly change the m e a n systemic arterial pressure (Fig. 1). Discussion
Our results indicate t h a t d o p a m i n e is a p o t e n t vasoactive drug in the m e s e n t e r i c circulation, producing significant d i m i n u t i o n in blood flow t h r o u g h the gut in doses as low as 1 t~g/Kg.-min. infused directly into the artery. Since s y s t e m i c arterial pressure was unaffected b y i n t r a - a r t e r i a l infusions of the drug, d o p a m i n e has to be considered a direct v a s o c o n s t r i c t o r of the m e s e n t e r i c circulation. O t h e r a u t h o r s h a v e concluded t h a t d o p a m i n e is a vasodilator a g e n t in this circula-
329
Pawlik et al. tion; however, the doses employed in those reports were small and were injected as a bolus. 1' 7 In our experiments the lowest dose (1 ~g/Kg.min.) produced a small early increase in blood flow which was transient and gave way to a subsequent decrease in MBF. We also found that dopamine evoked decreases in intestinal uptake of oxygen. The declines of Vo~ paralleled the decreases in MBF in response to the different doses of dopamine. Hence, dopamine induced both ischemia and hypoxia in the intestine. In this respect, dopamine acts upon the mesenteric Circulation in a manner similar to most other constrictor agents we have studied, except epinephrine, which decreases MBF without also diminishing Vo~ and PS product. TM 1-~ The microcirculatory sites at which constrictor drugs act in the mesenteric vasculators are the arteriolar smooth muscle, which regulates resistance to the total flow of blood through the gut, and the smooth muscle of the precapillary sphincter, which regulates the flow of blood through the capillaries. These sites are depicted schematically in Fig. 6. Since significant oxygen exchanges occur only across the capillary endothelium, a reduced uptake of oxygen ensues when blood flow through the capillaries is reduced. An accepted measurement of capillary perfusion (the nutrient circulation) is obtained from the clearance of the non-metabolized marker 8~Rb (PS product). In our studies dopamine decreased MBF and PS product comparably a t the three doses of the drug. This suggests that dopamine induces indiscriminate contraction of arteriolar and precapillary sphincteric smooth muscle, thereby reducing total blood flow by constricting arterioles and reducing the nutrient circulation by closing capillaries. A specific receptor for the major local circulatory action of dopamine has been predicated, 4This receptor is blocked by haloperidol. 4 In the current investigation, the mesente~c vasoconstrictor response to dopamine was prevented by haloperidol, as was the diminished uptake of oxygen and the decrease in the nutrient circulation. After haloperidol blockade dopamine induced vasodilator responses, accompanied by increased Vow. These responses, in t u r n , were prevented by betaadrenergic blockade. Our findings suggest t h a t dopamine activates at least two receptors in the mesenteric circulation; the predominant receptor, which constricts arteriolar and precapillary
330
sphincteric smooth muscle, evoking dose-related decreases in MBF, Vow, and PS product; and the occult receptor, which relaxes mesenteric vascular smooth muscle, thereby increasing MBF and ~'o2. The second receptor appears to be a peripheral beta-adrenergic receptor. The dominant receptor may be a unique dopaminergic receptor or it may be the alpha-adrenergic receptor. In another report the mesenteric vasoconstrictor response to dopamine was prevented by phenoxybenzamine. 6 Our findings do not, however, answer the questions about the physiological role of dopamine in the mesenteric vasculature.
Summary The effects of intra-arterial infusion of dopamine on superior mesenteric artery blood flow, intestinal oxygen consumption, and capillary density were studied in anesthetized dogs before and after blockade of dopamine receptors with haloperidol and after beta-adrenergic receptor blockade with propranolol. Mesenteric blood flow to a distal segment of the small intestine was measured with an electromagnetic blood flowmeter and intestinal oxygen consumption was calculated from the measured arteriovenous oxygen difference across the intestine and total blood flow. Intestinal capillary density was estimated from the clearance of ~Rb. In normal animals prior to dopaminergic or beta-adrenergic blockade, dopamine caused a dose-related decrease in mesenteric blood flow, intestinal oxygen consumption, and 8~Rb clearance. Only the lowest dose of the drug, 1 ~g/Kg.-min., did not significantly change the intestinal capillary density. In dogs pretreated with the dopamine receptor, antogonist~ haloperidol, dopamine (20 ~g/Kg.min.) caused a significant increase in blood flow and oxygen consumption and did not significantly alter the number of perfused intestinal capillaries. These increases in haloperidol-blocked animals administered dopamine were reversed by propranolol. Our results indicate t h a t dopamine caused smooth muscle contraction in mesenteric arterioles and precapillary sphincters, thereby producing intestinal ischemia and hypoxia. These findings with haloperidol and propranolol indicate that dopamine stimulates at least two different receptors in the canine mesenteric vascular bed: a constrictor receptor blocked by haloperidol and a dilator receptor blocked by propranolol.
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Dopamine and intestinal circulation
REFERENCES
1. Goldberg, L . I.: Cardiovascular and renal actions of dopamine: potential clinical applications, Pharmacol. Rev: 24:1, 1972. 2..Maxwell, C. M., Rowe, G. C., and Castillo, C. A.: The effect of dopamine (3-hydroxytyramine) upon the systemic, pulmonary, and cardiac hemodynamics and metabolism of intact dog, Arch. Int. Pharmac0dyn. Ther. 129:62, 1960. 3. McDonald, R. H., a n d Goldberg, L. I.: Analysis of the cardiovascular effects of dopamine in the dog, J. Pharmacol. Exp. Ther. 140:60, 1963. 4. Yeh, B. U:, McNay, J: L., and Goldberg, L. I.: Attenuation of dopamine renal and mesenteric vasodilation by haloperido]: Evidence for a specific dopamine receptor, J. Pharmacol. Exp. Ther. 168:303, 1969. 5. Burn, J. H., and Rand, M. J:: The depressor action of dopamine and adrenaline, Br. J. Pharmacol. Chemother. 13:471, 1958. 6. Shanbour, L. L., and Parker, D.: Effects of dopamine and other catecholamines on the splanchnic circulation, Can. J: Physiol. Pharmacol. 50:599, 1972.
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7.
8. 9. 10. 11. 12.
Higgins, C. B., Millar, R. W., and Braunwald, E.: Effects and mechanisms of action o f dopamine on regional hemodynamics in the conscious dog, Am. J. Physiol. 225:432, 1973. Ulano, H. B., Treat, E., Shanbour, L. L., and Jacobson, E. D.: Selective dilation of the constricted superior mesenteric artery, Gastroenterology 62:39, 1972. Renken, E. M.: Transport of potassium ~ from blood to tissue in isolated mammalian skeletal muscle, Am. J. Physiol. 197:1205, 1959. Dresel, P., Folkow, B., and WaHentin, L.: Rubidium ~'* clearance during neurogenic redistribution of intestinal blood flow, Acta Physiol. Scand. 67:173, 1966. Jacobson, E. D., Mailman, D., Shepherd, A. P., and Pawlik, W.: Intestinal ischemia and hypoxia: vasopressin vs. epinephrine, Clin. Res. 22:63A, 1974. Pawlik, W., Shepherd, A. P., and Jacobson, E. D.: Effects of vasoactive agents on intestinal oxygen consumption and blood flow, J. Clin. Invest. 56:484, 1975.
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