Interactions of three inotropic agents, ASL-7022, dobutamine and dopamine, with alpha- and beta-adrenoceptors in vitro

Relaxation of thoracic aorta and pulmonary artery rings of marmosets (Callithrix spp.) by endothelium-derived 6-nitrodopamine

6-Nitrodopamine is a novel catecholamine released by vascular tissues, heart, and vas deferens. The aim of this study was to investigate whether 6-nitrodopamine is released from the thoracic aorta and pulmonary artery rings of marmosets (Callithrix spp.) and to evaluate the relaxing and anti-contractile actions of this catecholamine. Release of 6-nitrodopamine, dopamine, noradrenaline, and adrenaline was assessed by liquid chromatography with tandem mass spectrometry (LC-MS/MS). The relaxations induced by 6-nitrodopamine and by the selective dopamine D2 receptor antagonist L-741,626 were evaluated on U-46619 (3 nM)-pre-contracted vessels. The effects of 6-nitrodopamine and L-741,626 on the contractions induced by electric-field stimulation (EFS), dopamine, noradrenaline, and adrenaline were also investigated. Both aorta and pulmonary artery rings exhibited endothelium-dependent release of 6-nitrodopamine, which was significantly reduced by the NO synthesis inhibitor L-NAME. Addition of 6-nitrodopamine or L-741,626 caused concentration-dependent relaxations of both vascular tissues, which were almost abolished by endothelium removal, whereas L-NAME and the soluble guanylate cyclase inhibitor ODQ had no effect on 6-nitrodopamine-induced relaxations. Additionally, pre-incubation with 6-nitrodopamine antagonized the dopamine-induced contractions, without affecting the noradrenaline- and adrenaline-induced contractions. Pre-incubation with L-741,626 antagonized the contractions induced by all catecholamines. The EFS-induced contractions were significantly increased by L-NAME, but unaffected by ODQ. Immunohistochemical assays showed no immunostaining of the neural tissue markers S-100 and calretinin in either vascular tissue. The results indicated that 6-nitrodopamine is the major catecholamine released by marmoset vascular tissues, and it acts as a potent and selective antagonist of dopamine D2-like receptors. 6-nitrodopamine release may be the major mechanism by which NO causes vasodilatation.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Pharmacological and Genetic Evidence of Dopamine Receptor 3-Mediated Vasoconstriction in Isolated Mouse Aorta

Dopamine receptors (DRs) are generally considered as mediators of vasomotor functions. However, when used in pharmacological studies, dopamine and/or DR agonists may not discriminate among different DR subtypes and may even stimulate alpha1 and beta-adrenoceptors. Here, we tested the hypothesis that D2R and/or D3R may specifically induce vasoconstriction in isolated mouse aorta. Aorta, isolated from wild-type (WT) and D3R-/- mice, was mounted in a wire myograph and challenged with cumulative concentrations of phenylephrine (PE), acetylcholine (ACh), and the D3R agonist 7-hydrxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT), with or without the D2R antagonist L741,626 and the D3R antagonist SB-277011-A. The vasoconstriction to PE and the vasodilatation to ACh were not different in WT and D3R-/-; in contrast, the contractile responses to 7-OH-DPAT were significantly weaker in D3R-/-, though not abolished. L741,626 did not change the contractile response induced by 7-OH-DPAT in WT or in D3R-/-, whereas SB-277011-A significantly reduced it in WT but did not in D3R-/-. D3R mRNA (assessed by qPCR) was about 5-fold more abundant than D2R mRNA in aorta from WT and undetectable in aorta from D3R-/-. Following transduction with lentivirus (72-h incubation) delivering synthetic microRNAs to specifically inactivate D2R (LV-miR-D2) or D3R (LV-miR-D3), the contractile response to 7-OH-DPAT was unaffected by LV-miR-D2, while it was significantly reduced by LV-miR-D3. These data indicate that, at least in mouse aorta, D3R stimulation induces vasoconstriction, while D2R stimulation does not. This is consistent with the higher expression level of D3R. The residual vasoconstriction elicited by high concentration D3R agonist in D3R-/- and/or in the presence of D3R antagonist is likely to be unrelated to DRs.

α1-adrenoceptor activity of β-adrenoceptor ligands - An expected drug property with limited clinical relevance

Many β-adrenoceptor agonists and antagonists including several clinically used drugs have been reported to also exhibit binding to α₁-adrenoceptors. Such promiscuity within the adrenoceptor family appears to occur more often than off-target effects of drugs in general. It should not be considered surprising based on the amino acid homology among the nine adrenoceptor subtypes including the counter-ions for binding the endogenous catecholamines. When β-adrenoceptor ligands also bind to α₁-adrenoceptors, they almost always act as antagonists, regardless of being agonists or antagonists at the β-adrenoceptor. The α₁-adrenoceptor affinity of β-adrenoceptor ligands in most cases is at least one, and often more log units lower than at their cognate receptor. Consistent evidence from multiple investigators indicates that β-adrenoceptor ligands relatively have the highest affinity for α_1A- and lowest for α_1B-adrenoceptors. While promiscuity among adrenoceptor subtypes causes misleading interpretation of experimental in vitro data, it is proposed based on the law of mass action that α₁-adrenoceptor binding of β-adrenoceptor ligands rarely contributes to the clinical profile of such drugs, particularly if they are agonists at the β-adrenoceptor.

Dopamine outside the brain: The eye, cardiovascular system and endocrine pancreas

Dopamine (DA) and DA receptors (DR) have been extensively studied in the central nervous system (CNS), but their role in the periphery is still poorly understood. Here we summarize data on DA and DRs in the eye, cardiovascular system and endocrine pancreas, three districts where DA and DA-related drugs have been studied and the expression of DR documented. In the eye, DA modulates ciliary blood flow and aqueous production, which impacts on intraocular pressure and glaucoma. In the cardiovascular system, DA increases blood pressure and heart activity, mostly through a stimulation of adrenoceptors, and induces vasodilatation in the renal circulation, possibly through D1R stimulation. In pancreatic islets, beta cells store DA and co-release it with insulin. D1R is mainly expressed in beta cells, where it stimulates insulin release, while D2R is expressed in both beta and delta cells (in the latter at higher level), where it inhibits, respectively, insulin and somatostatin release. The formation of D2R-somatostatin receptor 5 heteromers (documented in the CNS), might add complexity to the system. DA may exert both direct autocrine effects on beta cells, and indirect paracrine effects through delta cells and somatostatin. Bromocriptine, an FDA approved drug for diabetes, endowed with both D1R (antagonistic) and D2R (agonistic) actions, may exert complex effects, resulting from the integration of direct effects on beta cells and paracrine effects from delta cells. A full comprehension of peripheral DA signaling deserves further studies that may generate innovative therapeutic drugs to manage conditions such as glaucoma, cardiovascular diseases and diabetes.

Attenuated aortic vasodilation and sympathetic prejunctional facilitation in epinephrine-deficient mice: selective impairment of β2-adrenoceptor responses

It has been suggested that there is a link between epinephrine synthesis and the development of β2-adrenoceptor-mediated effects, but it remains to be determined whether this development is triggered by epinephrine. The aim of this study was to characterize β-adrenoceptor-mediated relaxation and facilitation of norepinephrine release in the aorta of phenylethanolamine-N-methyltransferase-knockout (Pnmt-KO) mice. Catecholamines were quantified by reverse-phase high-performance liquid chromatography-electrochemical detection. Aortic rings were mounted in a myograph to determine concentration-response curves to selective β1- or β2-adrenoceptor agonists in the absence or presence of selective β1- or β2-adrenoceptor antagonists. Aortic rings were also preincubated with [(3)H]norepinephrine to measure tritium overflow elicited by electrical stimulation in the presence of increasing concentrations of nonselective β- or selective β2-adrenoceptor agonists. β2-Adrenoceptor protein density was evaluated by Western blotting and β2-adrenoceptor localization by immunohistochemistry. Epinephrine is absent in Pnmt-KO mice. The potency and the maximal effect of the β2-adrenoceptor agonist terbutaline were lower in Pnmt-KO than in wild-type (WT) mice. The selective β2-adrenoceptor antagonist ICI 118,551 [(±)-erythro-(S*,S*)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride] antagonized the relaxation caused by terbutaline in WT but not in Pnmt-KO mice. Isoproterenol and terbutaline induced concentration-dependent increases in tritium overflow in WT mice only. β2-Adrenoceptor protein density was decreased in membrane aorta homogenates of Pnmt-KO mice, and this finding was supported by immunofluorescence confocal microscopy. In conclusion, epinephrine is crucial for β2-adrenoceptor-mediated vasodilation and facilitation of norepinephrine release. In the absence of epinephrine, β2-adrenoceptor protein density was decreased in aorta cell membranes, thus potentially hindering its functional activity.

Methylenedioxymethamphetamine suppresses production of the proinflammatory cytokine tumor necrosis factor-alpha independent of a beta-adrenoceptor-mediated increase in interleukin-10

Recent data suggest that 3,4-methylenedioxymethamphetamine (MDMA; "Ecstasy") has marked immunosuppressive properties. In this study, we investigate the effect of MDMA on production of the anti-inflammatory cytokine interleukin (IL)-10 in response to an in vivo challenge with bacterial lipopolysaccharide (LPS). Our data demonstrate that both acute and repeated administration of MDMA increases production of LPS-induced IL-10 in vivo, and this increase correlates inversely with the ability of MDMA to suppress the proinflammatory cytokine tumor necrosis factor (TNF)-alpha. Despite this correlation, immunoneutralization of IL-10 does not reverse the suppressive effect of MDMA on LPS-induced TNF-alpha production, indicating that suppression of this proinflammatory cytokine is not mediated by IL-10. In vitro exposure to MDMA does not mimic the immunosuppressive cytokine phenotype induced in vivo, suggesting that these immunosuppressive effects are not mediated by a direct action on monocytes per se. As MDMA activates that hypothalamic pituitary adrenal axis and sympathetic nervous system, we examined the role of glucocorticoids and catecholamines in its immunosuppressive actions. However, the immunosuppressive cytokine phenotype induced by MDMA was not altered by adrenalectomy, sympathetic denervation, or ganglionic blockade, indicating that neither glucocorticoids nor adrenal/sympathetic-derived catecholamines mediate these immunosuppressive effects of MDMA. Interestingly, beta-adrenoceptor blockade completely inhibited the increase in IL-10 induced by MDMA without altering the suppression of TNF-alpha. Taken together, these data demonstrate a role for beta-adrenoceptor activation in the ability of MDMA to increase LPS-induced IL-10 and highlight a mechanistic dissociation between the ability of MDMA to increase IL-10 and suppress production of the proinflammatory cytokine TNF-alpha.

Alpha1-adrenoceptor antagonist effect of (+/-)-dobutamine in rat isolated gastric artery preparation

(+/-)-Dobutamine at concentrations < or =10(-5) M did not evoke contractions of rat gastric artery segments. However, when the tissues were contracted with methoxamine, (+/-)-dobutamine evoked concentration-dependent relaxation. The relaxant responses were not significantly affected by propranolol. In the same preparation, propranolol competitively antagonized isoprenaline-induced relaxation with a -log K(B) value of 7.90+/-0.26. (+/-)-Dobutamine did not relax arterial ring segments precontracted with vasopressin (10(-7) M). (+/-)-Dobutamine antagonized noradrenaline-induced contractions of the gastric artery segments. The pA2 value was 6.93+/-0.20, and the slope of the Schild regression line was 1.22+/-0.14. This value (slope) was not significantly different from 1, indicating competitive antagonism. Pretreatment of gastric artery segments with dobutamine before phenoxybenzamine (PBZ) protected against inactivation of alpha1-adrenoceptors by PBZ. The dose ratio of prazosin (3x10(-9) M) and (+/-)-dobutamine (10(-5) M) in combination was close to the expected sum of their individual dose ratios minus 1, indicating interaction with a common site. It was therefore concluded that (+/-)-dobutamine evoked relaxation of rat gastric artery segments by an action not involving beta-adrenoceptor activation but by blocking alpha1-adrenoceptors.

Dobutamine as selective beta(1)-adrenoceptor agonist in in vivo studies on human thermogenesis and lipid utilization

The use of dobutamine as selective beta(1)-adrenoceptor agonist in in vivo studies on human thermogenesis and lipid utilization was investigated in 20 men. At 2.5, 5, and 10 microg x kg(-1) x min(-1), dobutamine induced significant increases in energy expenditure, lipid oxidation, and lipolysis. The beta(1)-adrenoceptor antagonist atenolol (bolus: 42.5 microg/kg, infusion: 1.02 microg x kg(-1) x min(-1)) blocked all dobutamine-induced effects on thermogenesis and lipid utilization. All parameters remained at levels comparable to those during saline infusion. The dose of atenolol used did not inhibit beta(2)-adrenoceptor-specific changes in energy expenditure, lipid oxidation, and lipolysis during salbutamol infusion (85 ng x kg(-1) x min(-1)). This indicates that atenolol was specific for beta(1)-adrenoceptors and did not camouflage concomitant beta(2)-adrenoceptor stimulation during dobutamine infusion. Therefore, we conclude that dobutamine can be used as a selective beta(1)-adrenoceptor agonist at dosages </=10 microg x kg(-1) x min(-1) in in vivo studies on human thermogenesis and lipid utilization.

Pharmacological characterization of beta-adrenoceptors mediating relaxation of the rat urinary bladder in vitro

1. Isoproterenol relaxed KCl-precontracted rat bladder strips with a pD2 of 7.21 leaving a residual contractile response of 3.2% after 30 microM. The selective beta1-agonist, T-0509 (pD2 : 6.24, 10.1% residual contraction after 100 microM), beta2-agonist, terbutaline (pD2 : 5.43, 13.7% residual contraction after 100 microM), and beta3-agonists, BRL 37344A (pD2 : 6.60, 17.3% residual contraction after 100 microM), and SR 58611A (pD2 : 5.15, 34.0% residual contraction after 100 microM), also relaxed bladder strips. 2. The relaxant response to isoproterenol was weakly but significantly antagonized by 1 microM propranolol which produced a 3 fold shift of the concentration-response curve to the right, and significantly antagonized by the beta1-selective antagonist, metoprolol (10 microM, 3 fold shift), and the beta2-selective antagonist, butoxamine (100 microM, 6 fold shift). A combination of 10 microM metoprolol and 100 microM butoxamine caused a 15 fold shift of the concentration-response curve for isoproterenol to the right. Incubation with the beta3-antagonist, SR 59230A (1 microM), caused a 6 fold shift of the concentration response curve for isoproterenol to the right. 3. The non-conventional partial agonist, CGP 12177A, weakly relaxed KCl-precontracted bladder strips (pD2 : 3.31, 51.3% residual contraction after 300 microM); the relaxation was resistant to blockade by 1 or 10 microM propranolol. 4. In the presence of 200 microM propranolol, CGP 12177A (20 microM) or SR 59230A (10 microM) antagonized surmountably the relaxant effects of BRL 37344A. 5. The data suggest that rat urinary bladder body contains beta1, beta2, and beta3-adrenoceptors, all of which mediate relaxation.

Functional and molecular biological evidence for a possible beta3-adrenoceptor in the human detrusor muscle

The possible existence of a beta3-adrenergic receptor (beta3-AR) in the human detrusor muscle was investigated by in vitro functional studies and analysis of mRNA expression. Isoprenaline, noradrenaline and adrenaline each produced a concentration-dependent relaxation of the human detrusor. The rank order for their relaxing potencies was isoprenaline (pD2 6.37+/-0.07) > or = noradrenaline (pD2 6.07+/-0.12) > or = adrenaline (pD2 5.88< or =0.11). Neither dobutamine (beta1- and beta2-AR agonist) nor procaterol (beta2-AR agonist) produced any significant relaxation at concentrations up to 10(-5) M. BRL37344A, CL316243 and CGP-12177A (beta3-AR agonists), relaxed the preparations significantly at concentrations higher than 10(-6) M. The pD2 values for BRL37344A, CL316243 and CGP-12177A were 6.42+/-0.25, 5.53+/-0.09 and 5.74+/-0.14, respectively. CGP-20712A (10(-7) - 10(-5) M), a beta1-AR antagonist, did not affect the isoprenaline-induced relaxation. On the other hand, ICI-118,551, a beta2-AR antagonist, produced a rightward parallel shift of the concentration-relaxation curve for isoprenaline only at the highest concentration used (10(-5) > M) and its pKB value was 5.71+/-0.19. Moreover, SR58894A (10(-7) - 10(-5) M), a beta3-AR antagonist, caused a rightward shift of the concentration-relaxation curve for isoprenaline in a concentration-dependent manner. The pA2 value and slope obtained from Schild plots were 6.24+/-0.20 and 0.68+/-0.31. The beta1-, beta2- and beta3-AR mRNAs were all positively expressed in detrusor smooth muscle preparations in a reverse transcription polymerase chain reaction assay. In conclusion, the present results provide the first evidence for the existence of the beta3-AR subtype in the human detrusor. They also suggest that the relaxation induced by adrenergic stimulation of the human detrusor is mediated mainly through beta3-AR activation.

Activation of beta2-adrenergic receptors hastens relaxation and mediates phosphorylation of phospholamban, troponin I, and C-protein in ventricular myocardium from patients with terminal heart failure

BACKGROUND

Catecholamines hasten cardiac relaxation through beta-adrenergic receptors, presumably by phosphorylation of several proteins, but it is unknown which receptor subtypes are involved in human ventricle. We assessed the role of beta1- and beta2-adrenergic receptors in phosphorylating proteins implicated in ventricular relaxation.

METHODS AND RESULTS

Right ventricular trabeculae, obtained from freshly explanted hearts of patients with dilated cardiomyopathy (n=5) or ischemic cardiomyopathy (n=5), were paced at 60 bpm. After measurement of the contractile and relaxant effects of epinephrine (10 micromol/L) or zinterol (10 micromol/L), mediated through beta2-adrenergic receptors, and of norepinephrine (10 micromol/L), mediated through beta1-adrenergic receptors, tissues were freeze clamped. We assessed phosphorylation of phospholamban, troponin I, and C-protein, as well as specific phosphorylation of phospholamban at serine 16 and threonine 17. Data did not differ between the 2 disease groups and were therefore pooled. Epinephrine, zinterol, and norepinephrine increased contractile force to approximately the same extent, hastened the onset of relaxation by 15+/-3%, 5+/-2%, and 20+/-3%, respectively, and reduced the time to half-relaxation by 26+/-3%, 21+/-3%, and 37+/-3%. These effects of epinephrine, zinterol, and norepinephrine were associated with phosphorylation (pmol phosphate/mg protein) of phospholamban 14+/-3, 12+/-4, and 12+/-3; troponin I 40+/-7, 33+/-7, and 31+/-6; and C-protein 7.2+/-1.9, 9.3+/-1.4, and 7.5+/-2.0. Phosphorylation of phospholamban occurred at both Ser16 and Thr17 residues through both beta1- and beta2-adrenergic receptors.

CONCLUSIONS

Norepinephrine and epinephrine hasten human ventricular relaxation and promote phosphorylation of implicated proteins through both beta1- and beta2-adrenergic receptors, thereby potentially improving diastolic function.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Neurotransmitter depletion compromises the ability of indirect-acting amines to provide inotropic support in the failing human heart

To test the hypothesis that cardiac norepinephrine depletion related to heart failure alters contractile responses to beta-adrenergic agonists with a component of "indirect" action (acting by release of neuronal norepinephrine), we examined the inotropic potential of several pharmacologically distinct beta-agonists. Contractile responses to the nonselective beta-agonist isoproterenol, the beta 2-selective agonist zinterol, and the direct- and indirect-acting agonists dopamine and dopexamine were compared in isolated right ventricular trabeculae removed from failing, nonfailing innervated, and previously transplanted and, therefore, denervated nonfailing human hearts. In failing hearts, the contractile response to isoproterenol was significantly lower (41%) than that in nonfailing innervated hearts. The responses to the mixed agonists dopamine and dopexamine were even more attenuated in failing hearts, to a level 76-90% lower than those of nonfailing innervated hearts. In denervated, previously transplanted, nonfailing hearts, the contractile responses to the mixed agonists dopamine and dopexamine were 66-72% lower than those in the nonfailing innervated group, but the response to isoproterenol was not significantly different. The response to zinterol was not significantly different among the three groups. In subjects with severe heart failure, in vivo hemodynamic responses to dopexamine were compared with those of the direct-acting beta-agonist dobutamine. Responses to dopexamine and dobutamine were measured before and after prolonged continuous infusions of each drug. The response to dopexamine, but not to dobutamine, diminished over time. We conclude that a large component of the inotropic response to dopamine and dopexamine in human hearts is due to the ability of these agonists to promote the release of neuronal norepinephrine; when neuronal norepinephrine is depleted, indirect-acting agonists are less able to produce an inotropic response.

Presynaptic dopamine DA2-receptors in rabbit jejunal arteries. An electrophysiological study

Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mumol/l, SKF 38393 10 mumol/l, dopamine 10 mumol/l and clonidine 0.1 mumol/l depressed all e.j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mumol/l, domperidone 1 mumol/l, SCH 23390 1 mumol/l and rauwolscine 1 mumol/l did not change, or even depressed the first e.j.ps. Of these compounds only S- and R-sulpiride, 10 mumol/l and rauwolscine 1 mumol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 mumol/l had the largest effect. S- and R-sulpiride, 10 mumol/l, as well as domperidone 1 mumol/l antagonized the action of LY 171555 1 mumol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mumol/l and rauwolscine 1 mumol/l blunted the effect of SKF 38393 10 mumol/l. Rauwolscine 1 mumol/l slightly reduced the inhibition by dopamine 10 mumol/l; S-sulpiride 10 mumol/l was antagonistic only in the presence of rauwolscine 1 mumol/l. When rauwolscine 1 mumol/l, prazosin 0.1 mumol/l, propranolol 1 mumol/l and cocaine 10 mumol/l was added to the medium, dopamine 10 mumol/l continued to produce the same depression of e.j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mumol/l also counteracted dopamine 10 mumol/l. Rauwolscine 1 mumol/l prevented the effect of clonidine 0.1 mumol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA2- and postsynaptic DA1-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e.j.ps. Dopamine interferes with neuroeffector transmission due to alpha 2-adrenoceptor agonist properties; its DA2-effect is unmasked only after alpha 2-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine.

Effect of dopamine, ibopamine, and epinine on alpha- and beta-adrenoceptors in canine pulmonary circulation

Dopamine has been widely utilized in the treatment of acute congestive heart failure, ibopamine, the diisobutyrate ester of N-methyldopamine (epinine), is a novel inotropic agent that, unlike-dopamine, is orally active. In clinical studies at doses that produce favorable hemodynamic responses, ibopamine and dopamine can evoke a slight and transient increase in pulmonary artery pressure and pulmonary capillary wedge pressure, an effect that is no longer apparent 1 h after administration. We have previously demonstrated in anesthetized dogs that this effect is due to stimulation of alpha-adrenoceptors in the pulmonary circulation by dopamine and ibopamine, as well as by the active form of ibopamine, epinine. The aim of the present investigation was to determine how dopamine, ibopamine, and epinine interact with beta-adrenoceptors in the canine pulmonary circulation, since this activity may serve to offset the alpha-adrenoceptor-mediated pulmonary vasoconstrictor responses. Intraarterial injection of dopamine, ibopamine, and epinine resulted in dose-dependent pulmonary vasoconstrictor responses with a maximum increase of approximately 50-60% above resting pulmonary vascular tone. When animals were pretreated with propranolol (1 mg/kg iv) to block beta-adrenoceptors, pulmonary vasoconstrictor responses to dopamine were unchanged, whereas pulmonary vasopressor responses to ibopamine and epinine were significantly potentiated, especially for epinine. Upon intraduodenal administration of a therapeutically effective dose of ibopamine (i.e. 36 mg/kg) to normal dogs, virtually no pulmonary pressor response was observed. However, administration of this same dose of ibopamine to dogs pretreated with propranolol (1 mg/kg iv) resulted in a marked pulmonary pressor response. These data indicate that epinine, and therefore the parent compound ibopamine, have the capacity to stimulate beta 2-adrenoceptors in the pulmonary circulation to a far greater degree than dopamine, and that this activity serves to offset, at least in part, the alpha-adrenoceptor-mediated pulmonary vasoconstriction that occurs in response to ibopamine.

Relaxant effect of dopamine on the isolated rat uterus

The effect of dopamine was studied on the isolated uterus of diethylstilboestrol-treated rats. Dopamine, at concentrations (10(7)-10(-4) M) produced a concentration-dependent relaxation in the K+-depolarized rat uterus. On a molar basis, dopamine was about 500 times less potent than adrenaline in relaxing the uterus, the maximum degree of relaxation obtained with both drugs was the same. Pretreatment of the rats with reserpine (5 mg/kg) did not produce any modification of the dose-response curve to dopamine. Similarly, cocaine (3 x 10(-6) M) failed to modify the relaxant effect of dopamine. The dopamine induced relaxation was inhibited by propranolol (10(-9)-10(-7) M) in a dose-dependent manner. Prazosin (10(-7) M), SCH 23390 (10(-7) M) and sulpiride (10(-7) M) did not affect the dopamine dose-response curve. In the isolated rat uterus which was not preconstricted by KCl neither dopamine nor adrenaline produced any effect when added to the organ bath. This lack of response to both catecholamines was present even in tissues pretreated with propranolol or sulpiride. It is concluded that dopamine produced a concentration-dependent relaxation of the uterus from diethylstilboestrol-treated rats by direct activation of beta-adrenoceptors. There was no evidence for indirect action (catecholamine release and neuronal uptake mechanisms) and specific dopamine receptor mediated relaxation and alpha-adrenoceptor mediated contractions have not been found in this preparation.

The pharmacology of dobutamine

Dobutamine is a sympathomimetic amine that was designed as an inotropic agent for use in congestive heart failure. Clinically, dobutamine increases cardiac output by selectively augmenting stroke volume, and this is associated with a decrease in total peripheral vascular resistance that is mediated, in part, by reflex withdrawal of sympathetic tone to the vasculature. This hemodynamic profile of dobutamine makes the drug of value in the management of low output cardiac failure. The inotropic activity of dobutamine has previously been attributed to selective stimulation of myocardial beta 1-adrenoceptors. However, recent studies from a number of laboratories indicate that the mechanism of action of dobutamine is substantially more complex. Dobutamine has the capacity to stimulate beta 1-, beta 2-, and alpha 1-adrenoceptors in the cardiovascular system at doses that approximate those used clinically. It has recently been suggested that the inotropic activity of dobutamine results from combined beta 1- and alpha 1-adrenoceptor stimulation in the myocardium, and that this activity could explain, at least in part, the inotropic selectivity of the compound. Furthermore, in the vasculature, the beta 2-adrenoceptor-mediated vasodilatory effect of dobutamine is exactly offset by the alpha 1-adrenoceptor-mediated vasoconstrictor activity, such that net changes in blood pressure are minimal following the administration of dobutamine. It is concluded, therefore, that the hemodynamic profile of dobutamine in patients with congestive heart failure is derived from a unique and complex series of interactions with alpha- and beta-adrenoceptors in the cardiovascular system.

Evidence for and against heterogeneity of alpha 1-adrenoceptors

Recent experimental evidence has suggested that the alpha 1 adrenoceptor may need to be further subdivided. It can no longer be stated categorically that alpha 1-adrenoceptors are present only at postjunctional sites, in view of several reports of alpha 1-mediated modulation of adrenergic and cholinergic neurotransmission. Furthermore, comparison of the pharmacologic characteristics of the alpha 1-adrenoceptor in different species and/or tissues can show clear differences in sensitivity to selective agonists and antagonists, and differences in the degree of dependence on extracellular calcium. However, in other cases, alpha 1-adrenoceptors at diverse sites have been found to have identical characteristics. Furthermore, the subcategories identified by the various selective agents do not fall into the same discrete groups, in contrast to division of alpha-adrenoceptors into alpha 1 and alpha 2-adrenoceptors. Therefore, at this time it seems premature to subdivide the alpha 1-adrenoceptor further.

Dopaminergic modulation of hippocampal noradrenaline release. Evidence for alpha 2-antagonistic effects of some dopamine receptor agonists and antagonists

3H-Noradrenaline release in the rabbit hippocampus and its possible modulation via presynaptic dopamine receptors was studied. Hippocampal slices were preincubated with 3H-noradrenaline, continuously superfused in the presence of cocaine (30 mumol/l) and subjected to electrical field stimulation. The electrically evoked tritium overflow from the slices was reduced by 0.1 and 1 mumol/l dopamine and apomorphine, but significantly enhanced by 10 mumol/l apomorphine or by 0.1 and 1 mumol/l bromocriptine. If the alpha 2-adrenoceptor antagonist yohimbine (0.1 mumol/l) was present throughout superfusion, the inhibitory effects of dopamine and apomorphine were more pronounced and even 10 mumol/l apomorphine and 1 mumol/l bromocriptine inhibited noradrenaline release. Qualitatively similar observations were made in the presence of another alpha 2-antagonist, idazoxane (0.1 mumol/l). In the presence of the D2-receptor antagonist domperidone (0.1 mumol/l) the inhibitory effects of dopamine were almost abolished, whereas both apomorphine (greater than 1 mumol/l) and bromocriptine (greater than 0.01 mumol/l) greatly facilitated noradrenaline release. The D2-receptor agonist LY 171555 (0.1 and 1 mumol/l) significantly reduced the evoked noradrenaline release whereas the D1-selective agonist SK & F 38393 was ineffective at similar concentrations. The effects of LY 171555 were abolished in the presence of domperidone (0.1 mumol/l) but remained unchanged in the presence of yohimbine or idazoxane (0.1 mumol/l, each). At 1 mumol/l the D2-receptor antagonists domperidone and (-)sulpiride significantly increased the evoked noradrenaline release by about 10%. However, at this concentration, domperidone (but not (-)sulpiride) affected also basal tritium outflow. Bulbocapnine and the preferential D1-receptor antagonists SCH 23390 enhanced the evoked noradrenaline release already at 0.1 mumol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of the enantiomers of 3-O-methyldobutamine with alpha- and beta-adrenoceptors in vitro

The enantiomers of 3-O-methyldobutamine, a metabolite of dobutamine, were evaluated for their alpha- and beta-adrenoceptor mediated effects in vitro in a variety of isolated organs and in radioligand binding studies. Neither enantiomer of 3-O-methyldobutamine possessed alpha 1-adrenoceptor agonist activity in isolated guinea pig aorta. However, both enantiomers of 3-O-methyldobutamine were competitive alpha 1-adrenoceptor antagonists, with the (+)-enantiomer being approximately 10-fold more potent than the (-)-enantiomer as assessed either in guinea pig aorta or by displacement of 3H-prazosin binding from alpha 1-adrenoceptors in rat cerebral cortex. The alpha 1-adrenoceptor blocking activity of (+)-3-O-methyldobutamine was relatively potent and corresponded to a pA2 of 7.33 in guinea pig aorta and a -log Ki of 7.72 in radioligand binding studies. Neither enantiomer of 3-O-methyldobutamine possessed alpha 2-adrenoceptor agonist activity in field-stimulated guinea pig ileum. Although (+)-3-O-methyldobutamine weakly inhibited the twitch response in field-stimulated guinea pig ileum, the response was not blocked by the selective alpha 2-adrenoceptor antagonist, yohimbine, and was found to result from weak anticholinergic activity (pA2 = 5.06). Neither enantiomer of 3-O-methyldobutamine possessed beta 1-adrenoceptor agonist activity in guinea pig atria, however the (+)-enantiomer was a weak noncompetitive antagonist at beta 1-adrenoceptors. In contrast, both enantiomers of 3-O-methyldobutamine were weak beta 2-adrenoceptor agonists in rat uterus, however these weak effects were not highly stereoselective, which was also confirmed in radioligand binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Systemic hemodynamic effects of dopamine, (+/-)-dobutamine and the (+)-and (-)-enantiomers of dobutamine in anesthetized normotensive rats

The hemodynamic effects of dopamine, (+/-)-dobutamine (racemic mixture) and the (+)- and (-)-enantiomers of dobutamine were evaluated in anesthetized normotensive rats. Dopamine and (+/-)-dobutamine produced hemodynamic effects in anesthetized rat that were qualitatively similar to those reported for these compounds in man. The increase in cardiac output produced by dopamine and (+/-)-dobutamine was due mainly to an increase in stroke volume, with heart rate being only minimally affected. Dopamine produced a large increase in mean arterial pressure and slightly increased total peripheral vascular resistance, whereas (+/-)-dobutamine only modestly increased blood pressure and significantly reduced total peripheral resistance. The (-)-enantiomer of dobutamine, which possesses mainly alpha 1-adrenoceptor agonist activity, produced marked increases in cardiac output, stroke volume, total peripheral resistance and mean arterial pressure, but did not significantly increase heart rate. In contrast, (+)-dobutamine, which possesses predominantly beta 1-and beta 2-adrenoceptor agonist activity, elicited only a modest increase in cardiac output which was due entirely to increased heart rate since stroke volume was not increased. Total peripheral vascular resistance and mean arterial blood pressure were both reduced by (+)-dobutamine, characteristic of a beta-adrenoceptor agonist. The increase in cardiac output and blood pressure produced by (+/-)-dobutamine, but not the positive chronotropic effect, were significantly inhibited by alpha 1-adrenoceptor blockade with prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)