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Postsynaptic D1 and D2 dopamine receptors are present in rabbit renal and mesenteric arteries
Neuroscience Letters, 61 (1985) 207-211
207
Elsevier Scientific Publishers Ireland Ltd.
NSL 03600
POSTSYNAPTIC Dl AND D2 DOPAMINE RECEPTORS ARE PRESENT IN RABBIT RENAL AND MESENTERIC ARTERIES
C. MISSALE% M. PIZZI 1, M. MEMO% G.B. PICOTTI% M.O. C A R R U B A 2 and P.F. SPANO ~
llnstitute of Pharmacology and Experimental Therapeutics, School of Medicine, University of Brescia, via Valsabbina 19, 1-25124 Brescia, and 2Department of Pharmacology, Chemotherapy and Medical Toxicology, University of Milan, via Vanvitelli 32. 1-20129 Milano (Italy) (Received July 16th, 1985; Accepted July 30th, 1985)
Key words: renal artery - mesenteric artery - adenylate cyclase - SKF 82526 - bromocriptine - 6-hydroxydopamine - rabbit
Measuring adenylate cyclase activity as a biochemical index of dopamine (DA) receptors, it was found that the selective D~ DA receptor agonist, SKF 82526, was able to stimulate the cAMP formation in rabbit renal and mesenteric arteries, an effect blocked by haloperidol and by SCH 23390. The D2 DA receptor agonist, bromocriptine, elicited a concentration-dependent inhibition of adenylate cyclase activity in both arteries of either normal or 6-hydroxydopamine pretreated rabbits, this effect being prevented by ( - ) sulpiride but not by (+)-sulpiride. These data indicate that both D~ and D2 postsynaptic DA receptors, associated with stimulation or inhibition of adenylate cyclase activity, are present on the wall of rabbit renal and mesenteric arteries.
It has been generally accepted that two types of dopamine (DA) receptors are present in blood vessels. These receptors have been referred as DAI and DA2 (for review see ref. 6). The DAI receptors have been found to be located on the vascular smooth muscle, and their stimulation causes vasodilation, as physiological studies performed either in vivo on anesthetized dogs [7] or in vitro on isolated arteries (for review see ref. 2) have indicated. The DA2 receptors seem to be preferentially located on postganglionic sympathetic nerve terminals, where they mediate inhibitions of the stimulated noradrenaline (NA) release (for review see ref. 10). Adenylate cyclase (AC) is part of the transduction mechanism mediating the action of DA on its target cells. According to their relation to AC activity, central DA receptors have been characterized and classified as D1 and D2 [11]. Dl receptors are linked to the enzyme through a stimulatory N-unit [11] and D2 receptors are associated with inhibition of the cyclic adenosine monophosphate (cAMP)-generating system [14]. Recently the presence of D~ and D2 dopaminergic receptors with opposing roles in the formation of cAMP had been noticed also in a peripheral tissue, the adrenal cortex [12]. In vitro biochemical studies indicate that a [3H]spiroperidol binding is present in rabbit superior mesenteric arteries [1, 2] and a DA-stimulated AC has been found 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
208
in rabbit renal and superior mesenteric arteries [4, 5] as well as in dog renal artery
[13]. In the present study, by measuring the formation of cAMP, it has been possible to show that'both postsynaptic dopaminergic D~, associated with the stimulation of AC, and D2 receptors, linked to the inhibition of the cAMP-generating system, are present in rabbit renal and mesenteric arteries. Since DA may also interact with ~-adrenoceptors, the use of phenoxybenzamine (POB) is imperative in the analysis of dopaminergic function in most vascular preparations. This ~-adrenoceptor blocker, however, has been reported to inhibit the ligand binding to DA receptors [9] and to prevent the stimulation of the DA-sensitive AC [15]. The novel DA receptor agonist SKF 82526 lacks ~-adrenoceptor agonist activity [8], thus permitting the examination of DA recel~tors in the vasculature in the absence of POB. On this line, in the present study, the AC activity linked to DA receptors in rabbit renal and mesenteric arteries has been characterized by using either SKF 82526 or bromocriptine as specific D~ and D2 agonists and SCH 23390 or (-)-sulpiride as specific D~ and D2 antagonists, respectively. The enzyme activity was measured as previously described [12]. The data reported in Fig. 1 show the effect of the D~ agonist SKF 82526 on AC
100
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A
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~
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0 o
10.
10 -6
-5
Drug c o n c e n t r a t i o n
-4 (Log N)
-5
-4
Drug c o n c e n t r a t i o n
(Log N)
-6
Fig. 1. Effect of different concentrations of SKF 82526 on adenylate cyclase activity in rabbit renal (A) and mesenteric (B) arteries in the presence of vehicle ( 0 ) or 1/~M haloperidol (O). Values are the means of 4 independent experiments with each point measured in triplicate. The standard error for each value was less than 10%. Adenylate cyclase was assayed in a 500-#1 reaction mixture containing 80 mM TrisHC1, 16 mM MgSO4, 0.5 mM 3-isobutyl-1-methyl-xanthine, 0.6 mM ethyleneglycoltetraacetic acid, 0.02°/,; ascorbic acid (pH 7.4), 2 mM adenosine triphosphate, 5 mM phosphocreatine and 50 U/ml creatine phosphokinase. The reaction was started by adding the tissue homogenate (2 mg tissue/sample). After incubation at 3 0 C for 20 min, samples were placed in boiling water for 5 rain and then centrifuged at 2000 rpm (!000 g) for 10 rain. The cAMP present in the supernatant was measured by radioimmunoassay. The adenylate cyclase basal activity was 6.3 pmol cAMP/mg tissue/min.
209 activity in rabbit renal and mesenteric arteries. The cAMP generating system was stimulated by SKF 82526 in a dose-dependent manner (maximal stimulation 95~) with an ECs0 of 5 #M in the renal and of 6 #M in the mesenteric artery. This effect was blocked by either 1 #M haloperidol or l #M SCH 23390, thus suggesting that the AC in both arteries is associated, through a stimulatory N-unit, with a DA recognition site whose pharmacological profile is superimposable to that of Dt dopaminergic receptors present in the striatum. Furthermore, another DA recognition site associated with inhibition of AC, similar to the dopaminergic DE receptor found within the brain, was detectable in the rabbit vasculature as well. Indeed, the selective D2 agonist, bromocriptine, elicited a concentration-dependent inhibition (maximal inhibition 509/o) of AC in both renal and mesenteric arteries with IC~0s of 2 and 5 /~M, respectively. That this effect is mediated by a DE receptor is further demonstrated by the observation that 1 pM ( - ) sulpiride, but not 1 #M (+)-sulpiride, antagonized the inhibitory effect elicited by bromocriptine on the cAMP-generating system (Fig. 2). The detection of a [3H]spiroperidol binding in rabbit vasculature [1, 2] strengthens the present finding indicating the presence of dopaminergic DE receptors in those tissues. These results are at variance with previous data showing that bromocriptine does not affect AC activity in the rabbit renal artery [4]. The use in the present study of a specific and sensitive radioimmunoassay method to measure the formation of cAMP may account for the apparent discrepancy. In order to determine whether the arterial D2 receptors associated with AC in an
B
0
!
0
0
4C
Drug
concentration
(Log ~l)
Drug c o n c e n t r a t i o n (Log M) Fig. 2. Effect of different concentrations of bromocriptine on adenylate cyclase activity in rabbit renal (A) and mesenteric(B) arteries in the presenceof vehicle(O), 1 #M (+)-sulpiride (C)) or 1 #M ( -)-sulpiride (A). Values are the means of 4 independentexperimentswith each point measured in triplicate. The standard error for each value was less than 10%.
210 inhibitory way correspond to the DA2 receptors located presynaptically on sympathetic nerve terminals, a group of rabbits was treated with 6-hydroxydopamine (6O H D A ; 80 mg/kg, i.v.) that caused a chemical sympathectomy as judged by the over 95% depletion of heart N A levels (0.78 + 0.09 ng/mg protein in 6-OHDA-treated rabbits vs 14.1 + 1.3 in controls, P<0.001, n = 3 ) . Fig. 3 shows that, 6 days after 6O H D A treatment, when the sympathetic fibers impinging on the renal and mesenteric arteries were destroyed by the neurotoxin, bromocriptine was still able to inhibit the cAMP formation, suggesting that the D2 receptors negatively linked to AC and the presynaptic DA2 receptors are two distinct entities. Therefore, beside the presynaptic DA2 receptors, there appear to be dopaminergic D2 receptor sites associated with inhibition of AC located postsynaptically on the arterial wall. The observation that the inhibition of AC activity induced by bromocriptine remained unchanged after degeneration of N A fibers can also be taken as evidence that most likely the presynaptic DA2 receptors located on sympathetic fibers do not utilize the inhibition of AC as a transduction mechanism, otherwise the inhibitory effect of the DA agonist would have been drastically reduced after sympathectomy. In conclusion, these data provide direct evidence for the presence in rabbit renal and mesenteric arterial walls of postsynaptic dopaminergic D~ and D2 receptors with opposing roles in the formation of c A M P and pharmacologically indentical to those present within the central nervous system. The presence on vascular tissues of two distinct types of DA rceptors at postsynaptic level, in addition to those previously described to be present presynaptically on sympathetic fibers, indicates a more sophisticated mechanism of dopaminergic regulation than was previously envisaged.
0
5
3
sol
50
P
-6
-5
-4 DRUG CONCENTRATION
L Log N )
Fig. 3. Effect of different concentrations of bromocriptine on adenylate cyclase activity in the renal (A) and mesenteric (B) arteries from controls (O) and 6-OHDA-treated rabbits (O). Values are the means of 3 independent experiments with each point measured in triplicate. The standard error for each value was less than 10%.
211 S u p p o r t f r o m the I t a l i a n N a t i o n a l R e s e a r c h C o u n c i l ( C N R ) a n d the I t a l i a n M i n i s try o f E d u c a t i o n ( M P I ) is g r a t e f u l l y a c k n o w l e d g e d . W e w i s h t o t h a n k R a v i z z a (Italy) f o r s u p p l y i n g ( + ) - s u l p i r i d e a n d ( - ) - s u l p i r i d e , Essex (Italy) f o r s u p p l y i n g S C H 23390 a n d S m i t h K l i n e a n d F r e n c h L a b o r a t o r i e s for s u p p l y i n g S K F 82526. 1 Amenta, F., Cavallotti, C., De Rossi, M., Sancesario, G. and Gerly, R., 3H-Spiroperidol binding sites in the rabbit superior mesenteric artery, Pharmacology, 28 (1984) 104-111. 2 Berkowitz, B.A., Erikson, R., Zabko-Potavpovich, B. and Ohlstein, E.H., Vascular dopamine and dopamine receptor agonists. In G. Poste and S.T. Crooke (Eds.), Dopamine Receptor Agonists, Plenum Press, New York, 1984, pp. 195-208. 3 Brodde, D.E. and Gross, G., 3H-Spiroperidol labels dopamine receptors in membranes from rabbit mesenteric artery, Naunyn-Schmiedeberg's Arch. Pharmacol., 311 (1980) 249-254. 4 Collier, W.L., Cavallotti, C., De Rossi, M. and Amenta, F., Dopamine-sensitive adenylate cyclase in rabbit renal artery, Neurosci. Lett., 43 (1983) 197-202. 5 Collier, W.L., De Rossi, M. and Amenta, F., Dopamine-sensitive adenylate cyclase in rabbits superior mesenteric artery, Arch. Int. Pharmacodyn. Ther., 268 (1984) 106--112. 6 Goldberg, L.I. and Kohli, J.D., Peripheral dopamine receptors: a classification based on potency series and specific antagonism, TIPS, 4 (1983) 64-66. 7 Goldberg, L.I., Volkman, P.H. and Kohli, J.D., A comparison of the vascular dopamine receptors, Ann. Rev. Pharmacol. Toxicol., 18 (1978) 57-79. 8 Hahn, R.A., Wardell, J.R., Jr., Sarau, H.M. and Ridley, P.T., Characterization of the peripheral and central effects of SKF 82526 a novel dopamine receptor agonist, J. Pharmacol. Exp. Ther., 223 (1982) 305-313. 9 Hamblin, M.W. and Creese, I., Phenoxybenzamine treatment differentiates dopaminergic 3H-ligand binding sites in bovine caudate membranes, Mol. Pharmacol., 21 (1982) 44-51. 10 Langer, S.Z., Presynaptic regulation of the release of catecholamines, Pharmacol. Rev., 32 (1980) 337362. 11 Memo, M., Carboni, E., Uzumaki, H., Govoni, S., Carruba, M.O. Trabucchi, M. and Spano, P.F., Biochemical pharmacology of dopamine receptors. In F. Cattabeni and S. Nicosia (Eds.), Principles and Methods in Receptor Binding, Plenum Press, New York, 1984, pp. 93-11 I. 12 Missale, C., Memo, M., Liberini, P., Carruba, M.O. and Spano, P.F., Evidence for the presence of D-I and D-2 dopamine receptors in the rat adrenal cortex, Eur. J. Pharmacol., 109 (1985) 315-316. 13 Murthy, U.V., Gilbert, J.C., Goldberg, L.I. and Kuo, J.F., Dopamine-sensitive adenylate cyclase in canine renal artery, J. Pharm. Pharmacol., 28 (1976) 567-571. 14 Onali, P.L., Olianas, M.C. and Gessa, G.L., Selective blockade of dopamine D-1 receptors by SCH 23390 discloses striatal dopamine D-2 receptors mediating the inhibition of adenylate cyclase in rats, Eur. J. Pharmacol., 97 (1984) 127-128. 15 Walton, K.G., Liepman, P. and Bladessarini, R.J., Inhibition of dopamine-stimulated adenylate cyclase activity by phenoxybenzamine, Eur. J. Pharmacol., 52 (1978) 231-234.
207
Elsevier Scientific Publishers Ireland Ltd.
NSL 03600
POSTSYNAPTIC Dl AND D2 DOPAMINE RECEPTORS ARE PRESENT IN RABBIT RENAL AND MESENTERIC ARTERIES
C. MISSALE% M. PIZZI 1, M. MEMO% G.B. PICOTTI% M.O. C A R R U B A 2 and P.F. SPANO ~
llnstitute of Pharmacology and Experimental Therapeutics, School of Medicine, University of Brescia, via Valsabbina 19, 1-25124 Brescia, and 2Department of Pharmacology, Chemotherapy and Medical Toxicology, University of Milan, via Vanvitelli 32. 1-20129 Milano (Italy) (Received July 16th, 1985; Accepted July 30th, 1985)
Key words: renal artery - mesenteric artery - adenylate cyclase - SKF 82526 - bromocriptine - 6-hydroxydopamine - rabbit
Measuring adenylate cyclase activity as a biochemical index of dopamine (DA) receptors, it was found that the selective D~ DA receptor agonist, SKF 82526, was able to stimulate the cAMP formation in rabbit renal and mesenteric arteries, an effect blocked by haloperidol and by SCH 23390. The D2 DA receptor agonist, bromocriptine, elicited a concentration-dependent inhibition of adenylate cyclase activity in both arteries of either normal or 6-hydroxydopamine pretreated rabbits, this effect being prevented by ( - ) sulpiride but not by (+)-sulpiride. These data indicate that both D~ and D2 postsynaptic DA receptors, associated with stimulation or inhibition of adenylate cyclase activity, are present on the wall of rabbit renal and mesenteric arteries.
It has been generally accepted that two types of dopamine (DA) receptors are present in blood vessels. These receptors have been referred as DAI and DA2 (for review see ref. 6). The DAI receptors have been found to be located on the vascular smooth muscle, and their stimulation causes vasodilation, as physiological studies performed either in vivo on anesthetized dogs [7] or in vitro on isolated arteries (for review see ref. 2) have indicated. The DA2 receptors seem to be preferentially located on postganglionic sympathetic nerve terminals, where they mediate inhibitions of the stimulated noradrenaline (NA) release (for review see ref. 10). Adenylate cyclase (AC) is part of the transduction mechanism mediating the action of DA on its target cells. According to their relation to AC activity, central DA receptors have been characterized and classified as D1 and D2 [11]. Dl receptors are linked to the enzyme through a stimulatory N-unit [11] and D2 receptors are associated with inhibition of the cyclic adenosine monophosphate (cAMP)-generating system [14]. Recently the presence of D~ and D2 dopaminergic receptors with opposing roles in the formation of cAMP had been noticed also in a peripheral tissue, the adrenal cortex [12]. In vitro biochemical studies indicate that a [3H]spiroperidol binding is present in rabbit superior mesenteric arteries [1, 2] and a DA-stimulated AC has been found 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
208
in rabbit renal and superior mesenteric arteries [4, 5] as well as in dog renal artery
[13]. In the present study, by measuring the formation of cAMP, it has been possible to show that'both postsynaptic dopaminergic D~, associated with the stimulation of AC, and D2 receptors, linked to the inhibition of the cAMP-generating system, are present in rabbit renal and mesenteric arteries. Since DA may also interact with ~-adrenoceptors, the use of phenoxybenzamine (POB) is imperative in the analysis of dopaminergic function in most vascular preparations. This ~-adrenoceptor blocker, however, has been reported to inhibit the ligand binding to DA receptors [9] and to prevent the stimulation of the DA-sensitive AC [15]. The novel DA receptor agonist SKF 82526 lacks ~-adrenoceptor agonist activity [8], thus permitting the examination of DA recel~tors in the vasculature in the absence of POB. On this line, in the present study, the AC activity linked to DA receptors in rabbit renal and mesenteric arteries has been characterized by using either SKF 82526 or bromocriptine as specific D~ and D2 agonists and SCH 23390 or (-)-sulpiride as specific D~ and D2 antagonists, respectively. The enzyme activity was measured as previously described [12]. The data reported in Fig. 1 show the effect of the D~ agonist SKF 82526 on AC
100
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B
A
4J > >
+
~J ~A (u c
~
m
0 o
10.
10 -6
-5
Drug c o n c e n t r a t i o n
-4 (Log N)
-5
-4
Drug c o n c e n t r a t i o n
(Log N)
-6
Fig. 1. Effect of different concentrations of SKF 82526 on adenylate cyclase activity in rabbit renal (A) and mesenteric (B) arteries in the presence of vehicle ( 0 ) or 1/~M haloperidol (O). Values are the means of 4 independent experiments with each point measured in triplicate. The standard error for each value was less than 10%. Adenylate cyclase was assayed in a 500-#1 reaction mixture containing 80 mM TrisHC1, 16 mM MgSO4, 0.5 mM 3-isobutyl-1-methyl-xanthine, 0.6 mM ethyleneglycoltetraacetic acid, 0.02°/,; ascorbic acid (pH 7.4), 2 mM adenosine triphosphate, 5 mM phosphocreatine and 50 U/ml creatine phosphokinase. The reaction was started by adding the tissue homogenate (2 mg tissue/sample). After incubation at 3 0 C for 20 min, samples were placed in boiling water for 5 rain and then centrifuged at 2000 rpm (!000 g) for 10 rain. The cAMP present in the supernatant was measured by radioimmunoassay. The adenylate cyclase basal activity was 6.3 pmol cAMP/mg tissue/min.
209 activity in rabbit renal and mesenteric arteries. The cAMP generating system was stimulated by SKF 82526 in a dose-dependent manner (maximal stimulation 95~) with an ECs0 of 5 #M in the renal and of 6 #M in the mesenteric artery. This effect was blocked by either 1 #M haloperidol or l #M SCH 23390, thus suggesting that the AC in both arteries is associated, through a stimulatory N-unit, with a DA recognition site whose pharmacological profile is superimposable to that of Dt dopaminergic receptors present in the striatum. Furthermore, another DA recognition site associated with inhibition of AC, similar to the dopaminergic DE receptor found within the brain, was detectable in the rabbit vasculature as well. Indeed, the selective D2 agonist, bromocriptine, elicited a concentration-dependent inhibition (maximal inhibition 509/o) of AC in both renal and mesenteric arteries with IC~0s of 2 and 5 /~M, respectively. That this effect is mediated by a DE receptor is further demonstrated by the observation that 1 pM ( - ) sulpiride, but not 1 #M (+)-sulpiride, antagonized the inhibitory effect elicited by bromocriptine on the cAMP-generating system (Fig. 2). The detection of a [3H]spiroperidol binding in rabbit vasculature [1, 2] strengthens the present finding indicating the presence of dopaminergic DE receptors in those tissues. These results are at variance with previous data showing that bromocriptine does not affect AC activity in the rabbit renal artery [4]. The use in the present study of a specific and sensitive radioimmunoassay method to measure the formation of cAMP may account for the apparent discrepancy. In order to determine whether the arterial D2 receptors associated with AC in an
B
0
!
0
0
4C
Drug
concentration
(Log ~l)
Drug c o n c e n t r a t i o n (Log M) Fig. 2. Effect of different concentrations of bromocriptine on adenylate cyclase activity in rabbit renal (A) and mesenteric(B) arteries in the presenceof vehicle(O), 1 #M (+)-sulpiride (C)) or 1 #M ( -)-sulpiride (A). Values are the means of 4 independentexperimentswith each point measured in triplicate. The standard error for each value was less than 10%.
210 inhibitory way correspond to the DA2 receptors located presynaptically on sympathetic nerve terminals, a group of rabbits was treated with 6-hydroxydopamine (6O H D A ; 80 mg/kg, i.v.) that caused a chemical sympathectomy as judged by the over 95% depletion of heart N A levels (0.78 + 0.09 ng/mg protein in 6-OHDA-treated rabbits vs 14.1 + 1.3 in controls, P<0.001, n = 3 ) . Fig. 3 shows that, 6 days after 6O H D A treatment, when the sympathetic fibers impinging on the renal and mesenteric arteries were destroyed by the neurotoxin, bromocriptine was still able to inhibit the cAMP formation, suggesting that the D2 receptors negatively linked to AC and the presynaptic DA2 receptors are two distinct entities. Therefore, beside the presynaptic DA2 receptors, there appear to be dopaminergic D2 receptor sites associated with inhibition of AC located postsynaptically on the arterial wall. The observation that the inhibition of AC activity induced by bromocriptine remained unchanged after degeneration of N A fibers can also be taken as evidence that most likely the presynaptic DA2 receptors located on sympathetic fibers do not utilize the inhibition of AC as a transduction mechanism, otherwise the inhibitory effect of the DA agonist would have been drastically reduced after sympathectomy. In conclusion, these data provide direct evidence for the presence in rabbit renal and mesenteric arterial walls of postsynaptic dopaminergic D~ and D2 receptors with opposing roles in the formation of c A M P and pharmacologically indentical to those present within the central nervous system. The presence on vascular tissues of two distinct types of DA rceptors at postsynaptic level, in addition to those previously described to be present presynaptically on sympathetic fibers, indicates a more sophisticated mechanism of dopaminergic regulation than was previously envisaged.
0
5
3
sol
50
P
-6
-5
-4 DRUG CONCENTRATION
L Log N )
Fig. 3. Effect of different concentrations of bromocriptine on adenylate cyclase activity in the renal (A) and mesenteric (B) arteries from controls (O) and 6-OHDA-treated rabbits (O). Values are the means of 3 independent experiments with each point measured in triplicate. The standard error for each value was less than 10%.
211 S u p p o r t f r o m the I t a l i a n N a t i o n a l R e s e a r c h C o u n c i l ( C N R ) a n d the I t a l i a n M i n i s try o f E d u c a t i o n ( M P I ) is g r a t e f u l l y a c k n o w l e d g e d . W e w i s h t o t h a n k R a v i z z a (Italy) f o r s u p p l y i n g ( + ) - s u l p i r i d e a n d ( - ) - s u l p i r i d e , Essex (Italy) f o r s u p p l y i n g S C H 23390 a n d S m i t h K l i n e a n d F r e n c h L a b o r a t o r i e s for s u p p l y i n g S K F 82526. 1 Amenta, F., Cavallotti, C., De Rossi, M., Sancesario, G. and Gerly, R., 3H-Spiroperidol binding sites in the rabbit superior mesenteric artery, Pharmacology, 28 (1984) 104-111. 2 Berkowitz, B.A., Erikson, R., Zabko-Potavpovich, B. and Ohlstein, E.H., Vascular dopamine and dopamine receptor agonists. In G. Poste and S.T. Crooke (Eds.), Dopamine Receptor Agonists, Plenum Press, New York, 1984, pp. 195-208. 3 Brodde, D.E. and Gross, G., 3H-Spiroperidol labels dopamine receptors in membranes from rabbit mesenteric artery, Naunyn-Schmiedeberg's Arch. Pharmacol., 311 (1980) 249-254. 4 Collier, W.L., Cavallotti, C., De Rossi, M. and Amenta, F., Dopamine-sensitive adenylate cyclase in rabbit renal artery, Neurosci. Lett., 43 (1983) 197-202. 5 Collier, W.L., De Rossi, M. and Amenta, F., Dopamine-sensitive adenylate cyclase in rabbits superior mesenteric artery, Arch. Int. Pharmacodyn. Ther., 268 (1984) 106--112. 6 Goldberg, L.I. and Kohli, J.D., Peripheral dopamine receptors: a classification based on potency series and specific antagonism, TIPS, 4 (1983) 64-66. 7 Goldberg, L.I., Volkman, P.H. and Kohli, J.D., A comparison of the vascular dopamine receptors, Ann. Rev. Pharmacol. Toxicol., 18 (1978) 57-79. 8 Hahn, R.A., Wardell, J.R., Jr., Sarau, H.M. and Ridley, P.T., Characterization of the peripheral and central effects of SKF 82526 a novel dopamine receptor agonist, J. Pharmacol. Exp. Ther., 223 (1982) 305-313. 9 Hamblin, M.W. and Creese, I., Phenoxybenzamine treatment differentiates dopaminergic 3H-ligand binding sites in bovine caudate membranes, Mol. Pharmacol., 21 (1982) 44-51. 10 Langer, S.Z., Presynaptic regulation of the release of catecholamines, Pharmacol. Rev., 32 (1980) 337362. 11 Memo, M., Carboni, E., Uzumaki, H., Govoni, S., Carruba, M.O. Trabucchi, M. and Spano, P.F., Biochemical pharmacology of dopamine receptors. In F. Cattabeni and S. Nicosia (Eds.), Principles and Methods in Receptor Binding, Plenum Press, New York, 1984, pp. 93-11 I. 12 Missale, C., Memo, M., Liberini, P., Carruba, M.O. and Spano, P.F., Evidence for the presence of D-I and D-2 dopamine receptors in the rat adrenal cortex, Eur. J. Pharmacol., 109 (1985) 315-316. 13 Murthy, U.V., Gilbert, J.C., Goldberg, L.I. and Kuo, J.F., Dopamine-sensitive adenylate cyclase in canine renal artery, J. Pharm. Pharmacol., 28 (1976) 567-571. 14 Onali, P.L., Olianas, M.C. and Gessa, G.L., Selective blockade of dopamine D-1 receptors by SCH 23390 discloses striatal dopamine D-2 receptors mediating the inhibition of adenylate cyclase in rats, Eur. J. Pharmacol., 97 (1984) 127-128. 15 Walton, K.G., Liepman, P. and Bladessarini, R.J., Inhibition of dopamine-stimulated adenylate cyclase activity by phenoxybenzamine, Eur. J. Pharmacol., 52 (1978) 231-234.