Vascular dopamine receptors: Demonstration and characterization by in vitro studies

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.

Dopamine D2-Subtype Receptors Outside the Blood-Brain Barrier Mediate Enhancement of Mesolimbic Dopamine Release and Conditioned Place Preference by Intravenous Dopamine

Dopamine (DA) is a cell-signaling molecule that does not readily cross the blood-brain barrier. Despite this, peripherally administered DA enhances DA levels in the nucleus accumbens and alters DA-related behaviors. This study was designed to investigate whether DA subtype-2 receptors are involved in the enhancement of nucleus accumbens (NAc) DA levels elicited by intravenous DA administration. This was accomplished by using microdialysis in the NAc and extracellular single unit recordings of putative DA neurons in the ventral tegmental area (VTA). Additionally, the reinforcing properties of intravenous DA were investigated using a place conditioning paradigm and the effects of intravenous DA on ultrasonic vocalizations were assessed. Following administration of intravenous dopamine, the firing rate of putative DA neurons in the VTA displayed a biphasic response and DA levels in the nucleus accumbens were enhanced. Pretreatment with domperidone, a peripheral-only DA D2 receptor (D2R) antagonist, reduced intravenous DA mediated increases in VTA DA neuron activity and NAc DA levels. Pretreatment with phentolamine, a peripheral α-adrenergic receptor antagonist, did not alter the effects of IV DA on mesolimbic DA neurotransmission. These results provide evidence for peripheral D2R mediation of the effects of intravenous DA on mesolimbic DA signaling.

Stuttering: A Disorder of Energy Supply to Neurons?

Stuttering is a disorder characterized by intermittent loss of volitional control of speech movements. This hypothesis and theory article focuses on the proposal that stuttering may be related to an impairment of the energy supply to neurons. Findings from electroencephalography (EEG), brain imaging, genetics, and biochemistry are reviewed: (1) Analyses of the EEG spectra at rest have repeatedly reported reduced power in the beta band, which is compatible with indications of reduced metabolism. (2) Studies of the absolute level of regional cerebral blood flow (rCBF) show conflicting findings, with two studies reporting reduced rCBF in the frontal lobe, and two studies, based on a different method, reporting no group differences. This contradiction has not yet been resolved. (3) The pattern of reduction in the studies reporting reduced rCBF corresponds to the regional pattern of the glycolytic index (GI; Vaishnavi et al., 2010). High regional GI indicates high reliance on non-oxidative metabolism, i.e., glycolysis. (4) Variants of the gene ARNT2 have been associated with stuttering. This gene is primarily expressed in the brain, with a pattern roughly corresponding to the pattern of regional GI. A central function of the ARNT2 protein is to act as one part of a sensor system indicating low levels of oxygen in brain tissue and to activate appropriate responses, including activation of glycolysis. (5) It has been established that genes related to the functions of the lysosomes are implicated in some cases of stuttering. It is possible that these gene variants result in a reduced peak rate of energy supply to neurons. (6) Lastly, there are indications of interactions between the metabolic system and the dopamine system: for example, it is known that acute hypoxia results in an elevated tonic level of dopamine in the synapses. Will mild chronic limitations of energy supply also result in elevated levels of dopamine? The indications of such interaction effects suggest that the metabolic theory of stuttering should be explored in parallel with the exploration of the dopaminergic theory.

Cardiovascular and electrocardiographic effects of the dopamine receptor agonists ropinirole, apomorphine, and PNU-142774E in conscious beagle dogs

To confirm recent in vitro findings, we examined the cardiovascular and electrocardiographic (ECG) effects of the dopamine receptor agonists ropinirole, apomorphine, and PNU-142774E in conscious dogs. Intravenous (i.v.) infusions of ropinirole totaling 20 microg/kg maximally reduced mean arterial pressure (MAP; -16 mm Hg) and the ECG PR interval (-13 milliseconds) and increased heart rate (HR; +29 b/min) and QTc length (+33 ms) at a peak plasma drug concentration (p[drug]) of 3.5 ng/ml. I.V. PNU-142774E was better tolerated through 66 microg/kg and a maximal p[drug] of 5.9 ng/ml with negligible cardiovascular changes and mild QTc reduction (13 ms). Apomorphine (25 microg/kg i.v.) was intermediate to ropinirole and PNU-142774E for emesis and peak changes in MAP (-6 mm Hg), HR (+24 b/min), and QTc (+15 milliseconds) at a mean p[drug] of 3.4 ng/ml. By comparison, the class III antiarrhythmic trecetilide (2.0 mg/kg bolus) increased QTc (+58 ms) without affecting mean arterial pressure or heart rate. This study establishes that in conscious dogs, the selective dopamine receptor agonist PNU-142774E has fewer cardiovascular and emetic effects than ropinirole and apomorphine and supports prior in vitro findings that ropinirole and apomorphine but not the PNU-142774E imidazoquinolin analog sumanirole reduces the delayed rectifier current in HERG transfected cells.

'Low-dose' dopamine worsens renal perfusion in patients with acute renal failure

'Low-dose' dopamine is frequently used in intensive care units (ICU) for its presumed renoprotective effects, but prospective and retrospective studies have so far not proven prevention or amelioration of renal injury. Data on renal perfusion following dopamine infusion are limited. In order to circumvent the problem of patient heterogeneity in the ICU setting, we used a crossover design in a prospective, double-blind randomized controlled study to investigate the effect of 'low-dose' dopamine on renal resistance indices, as determined by Doppler ultrasound. Forty patients, 10 without and 30 with acute renal failure (ARF, defined as doubling of baseline creatinine or an increase above 2 mg/dl), were included. Dopamine (2 mug/kg min) or placebo was given intravenously in alternating sequence for four subsequent periods of 60 min, starting randomly with either dopamine or placebo. Resistive (RI) and pulsatility index (PI) were closely correlated, positively related to serum creatinine values at baseline and highly reproducible during the two paired infusion periods. Dopamine reduced renal vascular resistance in patients without ARF (median RI/PI from 0.70 to 0.65/1.20 to 1.07, P<0.01) but increased resistance indices in patients with ARF (median RI/PI from 0.77 to 0.81/1.64 to 1.79, P<0.01) in the absence of effects on systemic hemodynamics. Subgroup analysis of patients with ARF revealed that dopamine induced renal vasoconstriction above 55 years (n=22) and in patients not receiving norepinephrine (n=20). In conclusion 'low-dose' dopamine can worsen renal perfusion in patients with ARF, which adds to the rationale for abandoning the routine use of 'low-dose' dopamine in critically ill patients.

The α1-adrenergic receptor not the DA1-dopaminergic receptor mediates cyclosporine–SKF38393 renovascular interaction

In this study, we investigated the effect of acute exposure to cyclosporine A (CyA) on renal vasodilations evoked by the DA1 dopaminergic agonist SKF38393 and whether dopamine DA1 receptors are directly involved in the interaction. Changes evoked by CyA in SKF38393 vasodilations were evaluated in phenylephrine-preconstricted isolated perfused rat kidneys in the absence and presence of SCH23390, a DA1 receptor antagonist. SKF38393 (3 × 10–8 to 3 × 10–6 mol) produced dose-dependent reductions in the renal perfusion pressure that were significantly attenuated in tissues pretreated with SCH23390 or CyA. Unlike SKF38393, the vasodilatory action of sodium nitroprusside, a nitrovasodilator, was not altered by CyA. The attenuating effect of CyA on SKF38393 vasodilations was preserved in preparations pretreated with SCH23390, suggesting that sites other than DA1 receptors may be involved in CyA–SKF38393 interaction. The study was then extended to investigate the possible involvement of renal α1-adrenoceptors in the interaction. Blockade of α1-adrenoceptors by prazosin (30 nmol/L) significantly reduced the vasodilatory effect of SKF38393 and virtually abolished the CyA-induced attenuation of SKF38393 responses. Further, CyA failed to alter SKF38393 vasodilations when the renal tone was raised with prostaglandin F2α (PGF2α), a vasoconstrictor whose effect is independent of α1-adenoceptors. Together, these findings support earlier reports that both DA1 and α1-receptors mediate the renal vasodilatory action of SKF38393 and suggest that CyA interacts selectively with the α1-receptor component to compromise SKF38393 responses.

Functional and autoradiographic characterization of dopamine D2-like receptors in the guinea pig heart

Dopamine receptors include the D1- (D1 and D5 subtypes) and D2-like (D2, D3, and D4 subtypes) families. D1-like receptors are positively and D2-like receptors negatively coupled to the adenylyl cyclase. Dopamine D2-like (D4 subtype) receptors have been identified in human and rat hearts. However the presence of D2 and D3 receptor subtypes is unclear. Furthermore, their role in cardiac functions is unknown. By autoradiographic studies of guinea pig hearts, we identified D3 and D4 receptors, using the selective radioligands [3H]-7-OH-DPAT and [3H]emonapride (YM-09151-2 plus raclopride). Western blot analysis confirmed D3 and D4 receptors in the right and left ventricle of the same species. Selective agonists of D3 and D4 receptors (+/-)-7-OH-DPAT and PD 168 077 (10(-9) to 10(-5) M, respectively) induced a significant negative chronotropic and inotropic effect in the isolated guinea pig heart preparation. Negative inotropic effect induced by PD 168 077 was associated with an inhibition in cyclase activity. No changes in cyclase activity were found with (+/-)-7-OH-DPAT. The aim of this study is to support the presence of D3 and D4 receptors in the heart. Although our results suggest that D3 and D4 receptors are functionally active in the heart, we need additional information with an antagonist and an agonist of improved potency and selectivity to understand the respective roles of D3 and D4 receptors in the cardiac functions.

Dopamine regulates Na-K-ATPase in alveolar epithelial cells via MAPK-ERK-dependent mechanisms

Dopamine (DA) increases lung edema clearance by regulating vectorial Na+ transport and Na-K-ATPase in the pulmonary epithelium. We studied the role of the mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) pathway in the DA regulation of Na-K-ATPase in alveolar epithelial cells (AEC). Incubation of AEC with DA resulted in a rapid stimulation of ERK activity via dopaminergic type 2 receptors. Analysis of total RNA and protein showed a 1.5-fold increase in the Na-K-ATPase beta1-subunit mRNA levels and up to a fivefold increase in beta1-subunit protein abundance after DA stimulation, which was blocked by the MAPK kinase (MEK) inhibitors PD-98059 and U-0126. Also, the DA-ERK pathway stimulated the synthesis of a green fluorescent protein reporter gene driven by the beta1-subunit promoter, which indicates that DA regulates the Na-K-ATPase beta1-subunit at the transcriptional level. The DA-mediated increase in beta1-subunit mRNA protein resulted in an increase in functional Na pumps in the basolateral membranes of alveolar type II cells. These results suggest that the MAPK-ERK pathway is an important mechanism in the regulation of Na-K-ATPase by DA in the alveolar epithelium.

Dopamine constricts porcine pial veins

Dopamine has been shown to induce pial arterial relaxation and constriction in several species. Its mode of action on pial veins, however, remains unclarified. The vasomotor effect of dopamine on porcine pial veins was, therefore, examined using an in vitro tissue bath technique. The results indicated that dopamine constricted exclusively isolated ring segments of pial veins in the presence or absence of active muscle tone. The constriction induced by dopamine was not affected by N(omega)-nitro-L-arginine (L-NNA, 2 x 10(-5) M) or indomethacin (10(-5) M). Only in few preparations was the constriction induced by maximum concentration of dopamine potentiated by L-NNA, suggesting that dopamine at high concentrations may release NO or a NO-related substance. In the presence of L-NNA (2 x 10(-5) M), dopamine-induced constriction was inhibited by phentolamine and yohimbine (but not prazosin) in a concentration-dependent manner with maximum inhibition at 10(-6) M. SKF38393 and 6-bromo-APB (selective dopamine D1 receptor agonists) and LY171555 (a selective dopamine D2 receptor agonist) also induced pial venous constriction exclusively in the presence of L-NNA. The constriction was not affected by phentolamine (10(-6) M). The order of potency for these agonists in the presence of phentolamine, propranolol, guanethidine and L-NNA was: 6-bromo-APB > SKF38393 > dopamine > LY171555. The dopamine-induced constriction in the presence of phentolamine was further inhibited by both SCH23390 (a selective dopamine D1 receptor antagonist) and sulpiride (a selective dopamine D2 receptor antagonist), but was not affected by dopamine D3 or D4 receptor antagonists. These results indicate that dopamine at low and high concentrations induces exclusively constriction of isolated porcine pial veins. The constriction is mediated by postsynaptic alpha2-adrenoceptors, and dopamine D1 and D2 receptors.

Synthesis, characterization and biodistribution of neutral and lipid-soluble 99mTc-bisaminoethanethiol spiperone derivatives: possible ligands for receptor imaging with SPECT

Using parts of the molecular structure of spiperone, two new ligand systems for complexation with [99mTc]technetium were prepared in order to develop potential receptor imaging agents for single photon emission computer tomography (SPECT). The bis-aminoethanethiols (BAT): 1-benzyl-4-(2-mercapto-2-methyl-4-aza-pentyl)-4-(2-mercapto-2-methyl- propylamino)-piperidine (benzylpiperidyl-BAT, BP-BAT) and 1-[3-(4-fluorobenzoyl)-propyl]-4-(2-mercapto-2-methyl-4-aza-pentyl)-4- (2-mercapto-2-methyl-propyl-amino)-piperidine (butyrophenoylpiperidyl-BAT, BUP-BAT) form stable, neutral and lipid soluble complexes with [99mTc]technetium at pH > or = 11 using SnCl2 as reducing agent in nearly quantitative radiochemical yields. Biodistribution of 99mTc-BP-BAT and 99mTc-BUP-BAT in rats showed a moderate clearance from blood and low uptake and retention in the liver, whereas brain uptake was moderate, however with prolonged brain retention. On the other hand, significant accumulations and retentions were observed in heart, kidney and lung with increasing oxygen/blood ratios up to 24 h. Within 24 h p.i. 22 and 29% of the injected dose (i.d.) of 99mTc-BP-BAT and 99mTc-BUP-BAT were eliminated by hepatobiliary excretion whereas 22% i.d. of both 99mTc-BAT complexes were excreted into the urine. Although first biodistribution studies of 99mTc-BP-BAT and 99mTc-BUP-BAT in rats showed relatively low brain uptake, the high uptake in peripheral, receptor rich organs indicates that compounds of this type may be used as a basis for further structural modification to develop agents with optimal properties for cerebral or peripheral receptor imaging with SPECT.

Increasing organ blood flow during cardiopulmonary bypass in pigs: comparison of dopamine and perfusion pressure

OBJECTIVE

To determine whether low-dose dopamine infusion (5 micrograms/kg/min) during cardiopulmonary bypass selectively increases perfusion to the kidney, splanchnic organs, and brain at low (45 mm Hg) as well as high (90 mm Hg) perfusion pressures.

DESIGN

Randomized crossover trial.

SETTING

Animal research laboratory in a university medical center.

SUBJECTS

Ten female Yorkshire pigs (weight 29.9 +/- 1.2 kg).

INTERVENTION

Anesthetized pigs were placed on normothermic cardiopulmonary bypass at a 100-mL/kg/min flow rate. After baseline measurements, the animal was subjected, in random sequence, to 15-min periods of low perfusion pressure (45 mm Hg), low perfusion pressure with dopamine (5 micrograms/kg/min), high perfusion pressure (90 mm Hg), and high perfusion pressure with dopamine. Regional perfusion (radioactive microspheres) was measured in tissue samples (2 to 10 g) from the renal cortex (outer two-third and inner one-third segments), stomach, duodenum, jejunum, ileum, colon, pancreas, and cerebral hemispheres.

MEASUREMENTS AND MAIN RESULTS

Systemic perfusion pressure was altered by adjusting pump flow rate (r2 = .61; p < .05). In the kidney, cortical perfusion pressure increased from 178 +/- 16 mL/min/100 g at the low perfusion pressure to 399 +/- 23 mL/min/100 g at the high perfusion pressure (p < .05). Perfusion pressure augmentation increased the ratio of outer/inner renal cortical blood flow from 0.9 +/- 0.1 to 1.2 +/- 0.1 (p < .05). At each perfusion pressure, low-dose dopamine had no beneficial effect on renal perfusion or flow distribution. Similar results were found in the splanchnic organs, where regional perfusion was altered by perfusion pressure but not by dopamine. In contrast, neither changing perfusion pressure nor adding low-dose dopamine altered blood flow to the cerebral cortex.

CONCLUSIONS

These data indicate that the lower autoregulatory limits of perfusion to the kidneys and splanchnic organs differ from those limits to the brain during normothermic bypass. Selective vasodilation from low-dose dopamine was not found in renal, splanchnic, or cerebral vascular beds. Increasing the perfusion pressure by pump flow, rather than by the addition of low-dose dopamine, enhanced renal and splanchnic but not cerebral blood flows during cardiopulmonary bypass.

Phenoxybenzamine mediated inhibition of the vascular dopamine D1 receptor

Cultures of rat mesenteric artery vascular smooth muscle cells express both vascular dopamine D1 receptors and beta 2-adrenoceptors but not alpha 1- or alpha 2-adrenoceptors, permitting direct investigation of the dopamine D1 receptor antagonist activity of phenoxybenzamine. After incubating cells with phenoxybenzamine (10(-5) M) for 20 min, an 80% inhibition of dopamine-induced (10(-4) M) cAMP formation was observed. Isoprenaline-induced (10(-5) M) cAMP formation remained unaffected by phenoxybenzamine. Inhibition of the dopamine response following 20 min incubation with phenoxybenzamine, was concentration-related and could not be reversed by repeated washing. Mean IC50 (95% confidence limits) = 4.68 x 10(-6) M (3.86-5.01). Exposure of cells to the selective dopamine D1 receptor partial agonist (+)-SKF 38393 (10(-6) M) prior to phenoxybenzamine incubation, resulted in protection of dopamine-induced cAMP formation. Exposure of cells to the stereo-enantiomer (-)-SKF 38393 (10(-6) M) did not produce any protective effect. The concentration-effect curve for (+)-SKF 38393 mediated protection had a mean EC50 value of 0.11 x 10(-6) M (0.10-0.11), which is comparable with the Ka apparent value (0.06 x 10(-6) M) for this compound when acting as an agonist to induce cAMP formation via the vascular dopamine D1 receptor. Previous studies of the vascular dopamine D1 receptor are likely to have been influenced by the frequent use of phenoxybenzamine, which we have shown to act as a potent antagonist at this site.

Pharmacological characterization of the dopamine receptor coupled to cyclic AMP formation expressed by rat mesenteric artery vascular smooth muscle cells in culture

1. Mesenteric artery vascular smooth muscle cells derived from male Wistar rats and grown in culture were prelabelled with [3H]-adenine and exposed to a range of dopamine receptor agonists and antagonists. Resultant [3H]-cyclic AMP formation was determined and concentration-effect curves constructed, in the presence of propranolol (10-6) M) and the phosphodiesterase inhibitor IBMX (5 x 10(-4) M). 2. Ka apparent values for D1/DA1 dopamine receptor agonists SKF 38393, fenoldopam, 6,7-ADTN, and dopamine were 0.06, 0.59, 4.06 and 5.77 x 10(-6) M respectively. Although fenoldopam and SKF 38393 were more potent than dopamine, they were partial agonists with efficacies, relative to dopamine of approximately 48% and 24% respectively. 6,7-ADTN, in contrast, behaved as a full agonist. 3. Dopamine-stimulated cyclic AMP formation was inhibited in a concentration-dependent manner by the D1/DA1 dopamine receptor selective antagonists, SCH 23390 and cis-flupenthixol (Ki values 0.53 and 36.1 x 10(-1) M respectively). In contrast, the D2/DA2 dopamine receptor selective antagonists, domperidone and (-)-sulpiride, were less potent (Ki values 2.06 and 5.82 x 10(-6) M respectively). Furthermore, the stereoisomers of SCH 23390 and cis-flupenthixol, SCH 23388 and trans-flupenthixol, were at least two orders of magnitude less potent (Ki values 0.14 and 13.2 x 10(-6) M respectively) indicating the stereoselective nature of this receptor. 4. Our results indicate that rat mesenteric artery vascular smooth muscle cells in culture express a dopamine receptor coupled to cyclic AMP formation, which has the pharmacological profile, characteristic of the D1 dopamine receptor subfamily.

[Dopexamine: a new dopaminergic agonist]

Dopexamine hydrochloride is a new synthetic catecholamine for intravenous use in low cardiac output states with co-existing raised systemic or pulmonary vascular resistance. Dopamine has been commonly used since several years in these situations. The drawbacks of dopamine include a vasoconstrictive effect with high infusion rates, and a marked tendency for ventricular ectopy due to the potent beta-1 adrenergic stimulation. Dopexamine hydrochloride has interesting vasodilator properties, with marked intrinsic agonist activity at beta-2 adrenoreceptors and a lesser agonist activity at dopaminergic receptors (DA1 and DA2). Its mild inotropic activity arises primarily from baroreceptor reflex stimulation with a possible contribution from direct stimulation of myocardial beta 2-adrenoreceptors. Dopexamine hydrochloride is responsible for an inhibition of neuronal re-uptake of catecholamines (uptake-1), producing an indirect stimulation of cardiac beta 1-receptors. This catecholamine has no effect at alpha 1 and alpha 2-adrenoreceptors, and only very weak and clinically insignificant beta 1-adrenoreceptor agonist activity. Dopexamine hydrochloride improves cardiac performance by a marked vasodilation and a mild inotropic activity. The specific activity at dopaminergic receptors increases cerebral, myocardial, splanchnic and renal blood flows. These haemodynamic effects are associated with an increase in diuresis and natriuresis. These benefits are achieved without side effects such as an increased myocardial oxygen consumption, although induced tachycardia may be responsible for chest pain/anginae pain in patients with ischaemic heart disease. In clinical practice, dopexamine hydrochloride is easy to use; the short plasma half-life (6 minutes in healthy volunteers and 11 minutes in patients with low cardiac output) allows a rapid return to pretreatment status at discontinuation of the infusion. Preliminary studies have shown that dopexamine hydrochloride can produce beneficial effects in patients with acute heart failure or with compromised left ventricular function following cardiac surgery. The drug has also been assessed in patients with septic shock, most often in association with dopamine or norepinephrine. In these patients, dopexamine produces a dose-related increase in cardiac index, stroke volume, heart rate and a decrease in systemic vascular resistance. Its use in this indication must be cautious, particularly in patients with hypotension or decreased venous return. Comparative therapeutic trials are clearly required to establish the efficiency and tolerance of dopexamine hydrochloride in comparison with dopamine and dobutamine, before its place in therapy can fully be defined.

Uptake and biodistribution of dopamine in bone marrow, spleen and lymph nodes of normal and tumor bearing mice

Significant labelled dopamine uptake was evident in bone marrow, spleen and lymph nodes of normal murine hosts in vivo. On the contrary animals bearing solid Ehrlich carcinoma, 3H-dopamine uptake was significantly reduced. The tumor tissue itself incorporated only insignificant amount of dopamine. Bone marrow cells, splenocytes and lymphocytes from lymph nodes showed specific uptake of this monoamine. At present the peripheral role of dopamine in the regulation of heart and kidney functions are well documented and utilized clinically for treatment of congestive heart and renal failure. The present result of specific dopamine uptake by bone marrow, spleen and lymph nodes and alterations following tumor growth where hematopoesis and immune functions are disrupted, strengthens our previous idea that dopamine might also influence the functions of these peripheral organs. Knowledge of this possible effect of DA on these peripheral organs may be of future clinical significance in the management of hematological and immune abnormalities.

Dopamine DA2-receptor activation inhibits noradrenaline release in human kidney slices

Dopamine receptor modulation of noradrenaline release from renal sympathetic nerves was investigated. Human kidney slices were incubated with 3H-noradrenaline, placed into superfusion chambers between two platinum electrodes and field-stimulated at 5 Hz. The slices accumulated radioactivity. Pretreatment of the kidney slices with 6-hydroxy-dopamine (1.2 mM) prior to the 3H-noradrenaline incubation reduced the accumulation of radioactivity. The stimulation induced (S-I) outflow of radioactivity was mainly composed of intact 3H-noradrenaline. The sodium channel blocker tetrodotoxin (1 microM), 6-hydroxy-dopamine pretreatment and omission of calcium from the superfusion solution abolished S-I outflow of radioactivity. The DA1-receptor agonist fenoldopam (SKF 82526; 0.01 and 0.1 microM) did not alter but fenoldopam (1 microM) increased S-I outflow of radioactivity. However, in the presence of either the non-selective alpha-adrenoceptor antagonist phentolamine (1 microM) or the selective alpha 2-adrenoceptor antagonist idazoxan (1 microM) fenoldopam (1 microM) had no effect. The DA2-receptor agonist quinpirole (LY 171555; 1 microM) inhibited S-I outflow of radioactivity, an effect blocked by the selective DA2-receptor antagonists S(-)-sulpiride (10 microM) and domperidone (0.3 microM) but unaltered either by the DA1-receptor antagonist SCH 23390 (1 microM) or by phentolamine (1 microM). The alpha 2-adrenoceptor agonist UK 14304 (0.1 microM) inhibited S-I outflow of radioactivity, and this effect was blocked by phentolamine (1 microM) and idazoxan (1 microM) but unaltered by S(-)-sulpiride (10 microM). Phentolamine and idazoxan, in contrast to S(-)-sulpiride, domperidone and SCH 23390, enhanced S-I outflow of radioactivity by themselves.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine and renal salvage in the critically ill patient

Dopamine is a catecholamine used widely in critically ill patients and those undergoing major surgery, often as a 'renal protective' agent. Direct renal vasodilatation with 'low-dose' dopamine is the widely accepted basis for its use--hence the term 'renal dose' dopamine. However, recent evidence has revealed that the renal effects of this agent are far more complex. Moreover, some of these effects may be undesirable in the 'at-risk' kidney. The increased renal blood flow (RBF) of dopamine may be largely attributable to its inotropic (myocardial) action, even with low doses (i.e. less than 5 micrograms/kg/min). Similar increases in RBF can also be demonstrated with other (non-dopaminergic) inotropes. The early evidence for direct renal vasodilatation in response to dopamine has been brought into question by more recent research. The diuresis and natriuresis commonly seen following dopamine administration is now known to be due to a direct renal tubular (or 'diuretic') action. Furthermore, increasing knowledge regarding the pathophysiology of acute (ischaemic) renal failure, including RBF and the concept of 'oxygen supply and demand' in relation to tubular function, suggests that dopamine may mask important signs of renal ischaemia. Whether or not dopamine is truly beneficial to renal function currently remains unanswered. As it stands however, there is sufficient evidence to question its routine use in the setting of renal dysfunction in the critically ill patient.

Endothelium-dependent and -independent relaxation by dopamine in the rabbit pulmonary artery

1. The relaxing response of dopamine (DA) was studied in the rabbit pulmonary artery. DA caused concentration-related relaxation in helically cut strips of the artery contracted with prostaglandin F2 alpha in the presence of prazosin. 2. The DA-induced relaxation in endothelium-denuded strips was reduced to about 40% compared with that in endothelium-intact strips. 3. Methylene blue and haemoglobin, inhibitors of endothelium-dependent relaxation, reduced the DA-induced relaxations in endothelium-intact strips to the level of endothelium-denuded strips. These results indicate that the DA-induced relaxation is partially mediated or modified by the release of endothelium-derived relaxing factor (EDRF). 4. Apomorphine, as a DA agonist, caused concentration-dependent relaxation in endothelium-intact strips. Bromocriptine, a DA2 agonist, produced only a little relaxation at higher concentration. 5. In endothelium-intact strips, haloperidol, a DA antagonist, and the DA1 antagonists, fluphenazine and SCH 23390 inhibited DA-induced relaxations. On the other hand spiperone and domperidone, DA2 antagonists, were inactive. 6. In endothelium-denuded strips, fluphenazine and SCH 23390 inhibited DA-induced relaxations, but domperidone was inactive. 7. These results indicate that the DA-induced relaxation is mediated by DA receptors, and that DA1 receptors are involved in both endothelium-dependent and -independent relaxation in the rabbit pulmonary artery.

Enhancement of dopaminergic agonist bromocriptine of gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine in Wistar rats

The effects of the dopamine agonist 2-bromo-alpha-ergocryptine methanesulfonate (bromocriptine) on the incidence, number and histology of gastric cancer induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were investigated in Wistar rats. Rats were given 1 or 2 mg kg-1 body weight of bromocriptine subcutaneously every other day in depot form after 25 weeks of oral treatment with MNNG. Prolonged administration of bromocriptine at both dosages every other day resulted in a significant increase in the incidence and number of gastric cancers of the glandular stomach by week 52. Bromocriptine treatment did not influence the histological type of gastric cancer, but caused a significant increase in the labelling index of epithelial cells of the antrum. These findings indicate that the dopamine agonist bromocriptine promotes gastric carcinogenesis, and that this effect may be related to its effect in increasing proliferation of epithelial cells in the antral mucosa.

Dopaminergic receptors linked to adenylate cyclase in human cerebromicrovascular endothelium

Cultured endothelium derived from three fractions of human cerebral microvessels was used to characterize dopamine (DA) receptors linked to adenylate cyclase activity. DA or D1 agonist, (+/-)-SKF-82958 hydrobromide, stimulated endothelial cyclic AMP formation in a dose-dependent manner. The selective D1 antagonist, (+/-)SCH-23390, inhibited in a dose-dependent manner the production of cyclic AMP induced by DA. The affinity for the D1 receptor appeared to be greater in endothelium derived from large and small microvessels than from capillaries. Cholera toxin ADP-ribosylation of Gs proteins abolished the DA stimulatory effect on endothelial adenylate cyclase, whereas pertussis toxin ADP-ribosylation enhanced the DA-inducible formation, indicating the presence of both D1 and D2 receptors. Agonists of alpha 1-adrenergic receptors (phenylephrine, 6-fluoronorepinephrine) or serotonin (5-HT), which stimulated the production of cyclic AMP, had no additive effect on DA-stimulated cyclic AMP formation. Incubation of these agents with DA produced the same or lower levels of cyclic AMP as compared to that formed by DA alone. The effect of alpha 1-adrenergic agonists or 5-HT on DA production of cyclic AMP was partially prevented by the D2 antagonist, S(-)-sulpiride, or ketanserin (5-HT2 greater than alpha 1 greater than H1 antagonists), respectively. These findings represent the first demonstration of D1- (stimulatory) and D2- (inhibitory) receptors linked to adenylate cyclase in microvascular endothelium derived from human brain. The data also indicate that dopaminergic receptors can interact with either alpha 1-adrenergic or or 5-HT receptors in endothelium on the adenylate cyclase level.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiology and pharmacology of cardiovascular catecholamine receptors: implications for treatment of chronic heart failure

In the sympathetic nervous system the physiologic effects of the endogenous catecholamines noradrenaline (NA) and adrenaline (A) are mediated by alpha- and beta-adrenoreceptors (ARs). Both AR-types can be subdivided into two major subtypes: alpha-ARs into alpha-1 (predominant effect: vasoconstriction) and alpha-2 (presynaptic: inhibition of NA-release; postsynaptic: vasoconstriction), beta-ARs into beta-1 (cardiac effects, renal renin release, and lipolysis) and beta-2 (presynaptic: facilitation of NA-release; postsynaptic: vascular, bronchial, and uterine smooth muscle relaxation, glycogenolysis and possibly part of the A-mediated cardiac effects). During the last 30 years growing evidence has accumulated that dopamine (DA), the third endogenous catecholamine and the immediate precursor of NA, may also cause peripheral effects through stimulation of specific DA-receptors, in addition to its known action at alpha- and beta-ARs. It is now well accepted that at least two different DA-receptors are present in many peripheral tissues (DA1 and DA2), including those of the cardiovascular and autonomic nervous system. They seem to be involved in dilation of certain vascular beds, inhibition of NA-release during nerve stimulation, natriuresis, and aldosterone release. In chronic heart failure cardiac beta-AR function decreases (presumably due to endogenous "down-regulation" by the elevated catecholamines), and this decrease is related to the severity of heart failure (judged clinically by New York Heart Association functional class). The human heart contains both functional beta-1 and beta-2 ARs; cardiac beta-1 and beta-2 ARs seem to be differentially affected by different kinds of heart failure; in end-stage dilated cardiomyopathy beta-1 ARs are selectively reduced, whereas beta-2 ARs are nearly normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Subclassification of peripheral dopamine receptors

1. There is convincing evidence to suggest that two DA-receptor subtypes exist in the periphery, that can at best be differentiated by the potency of the antagonists SCH 23390 and domperidone: at DA-1 receptors the order of potency is SCH 23390 much greater than domperidone, at DA-2 receptors it is domperidone much greater than SCH 23390. 2. As in the adrenergic, muscarinic or 5-HT-receptor field, future studies employing both molecular biology and pharmacological approaches will show whether such a subclassification is sufficient to explain all peripheral effects of DA or whether DA-1 and DA-2 receptors have to be subdivided into further subtypes.

Cardiogenic hepatorenal syndrome

A variant of hepatorenal syndrome occurring in patients with chronic congestive heart failure following an episode of cardiogenic pulmonary edema, and in the absence of hypotension, is described. This was observed in 13 patients during an eleven-year period. The clinical picture is characterized by hepatic injury and functional renal impairment. Increase of serum glutamic oxaloacetic transaminase levels as high as 2100 IU; prolongation of prothrombin time; elevation of serum bilirubin, creatinine, blood urea nitrogen, and potassium levels; decrease in urinary sodium excretion; and a normal urinary sediment are the salient laboratory abnormalities of this entity. Treated with conventional medication, the patients' course was fatal in 4 cases. When the splanchnic vasodilator dopamine was added to the patients' management, 5 of 9 patients recovered. Cardiogenic hepatorenal syndrome is a severe but potentially reversible complication of heart failure. The apparently beneficial effect of low-dose dopamine needs further evaluation.

Cardiovascular effects of bromocriptine in rats: role of peripheral adrenergic and dopaminergic receptors

1. Experiments were designed to study the involvement of alpha-adrenoceptors and dopamine receptors in the hypotensive and bradycardic actions of bromocriptine in rats. 2. Intravenous administration of bromocriptine reduced blood pressure and heart rate which was inhibited by ganglionic blocking agents or by pithing. 3. The fall in blood pressure produced by bromocriptine was not modified by atropine, atenolol, prazosin, yohimbine, bilateral vagotomy or carotid ligation, but was blocked by sulpiride, domperidone and haloperidol. 4. The bradycardia produced by bromocriptine in intact rats was assumed to be mediated by the autonomic nervous system since it was partly reduced by bilateral vagotomy or atenolol, and entirely prevented by pithing. Furthermore, sulpiride but not yohimbine antagonized this effect. 5. In pithed rats, bromocriptine decreased both the pressor response (above 10 micrograms kg-1) and the tachycardia (above 50 micrograms kg-1) elicited by electrical stimulation of spinal cord outflow. Both effects were inhibited by sulpiride or yohimbine. 6. In pithed rats, bromocriptine did not affect the hypertension due to exogenous noradrenaline, phenylephrine, B-HT 920, nor the bradycardia evoked by stimulation of the cardiac muscarinic receptors by carbachol. 7. These results suggest that, in rats, bromocriptine produces hypotension via an action on presynaptic and/or ganglionic dopamine receptors, and causes bradycardia by activation of central dopamine receptors.

The effects of quinpirole and fenoldopam on the potassium-evoked overflow of endogenous dopamine and noradrenaline in dog mesenteric arteries

(1) Dopamine and noradrenaline overflow from the main trunk of the dog mesenteric artery and its proximal branches, elicited by K+ (52 mmol/l), was measured by high pressure liquid chromatography with electrochemical detection. (2) Quinpirole (0.1, 1 and 10 nmol/l) produced a concentration dependent reduction of dopamine and noradrenaline overflow in both segments of the mesenteric artery. The inhibitory effect of quinpirole (10 nmol/l) on amine overflow was antagonized by sulpiride (1 mumol/l) but not by phentolamine (0.2 mumol/l) or the selective dopamine (DA1), antagonist SK&F 83566 (1 mumol/l). (3) Fenoldopam (0.1 and 1 mumol/l) did not alter dopamine and noradrenaline overflow from both segments of the mesenteric artery; only 10 mumol/l fenoldopam was found to increase the overflow of dopamine and noradrenaline in both segments of the mesenteric artery. This effect of fenoldopam on amine overflow was not altered by the addition to the perifusion fluid of SK&F 83566 (1 mumol/l). (4) Clonidine (100 nmol/l) significantly reduced amine overflow from both segments of the mesenteric artery and this effect was antagonized by fenoldopam (10 mumol/l). (5) These results suggest that quinpirole inhibits sympathetic neurotransmission through the activation of prejunctional dopamine receptors of the DA2 subtype. The facilitatory effect of fenoldopam (10 mumol/l) on amine release appears to be mediated through the blockade of prejunctional alpha 2-adrenoceptors.

The action of dopamine and vascular dopamine (DA1) receptor agonists on human isolated subcutaneous and omental small arteries

1. Human small arteries were obtained from surgical specimens and studied in vitro by use of a myograph technique. Following induction of tone with a potassium depolarizing solution, dopamine in the presence of beta-adrenoceptor and catecholamine uptake blockade relaxed isolated omental and subcutaneous arteries. Preincubation of tissues with phentolamine increased the maximum relaxation in response to dopamine. 2. The selective vascular dopamine receptor agonists, fenoldopam and SKF 38393 also relaxed isolated subcutaneous and omental arteries in a concentration-dependent manner. The order of potency for agonists was dopamine greater than fenoldopam greater than SKF 38393. 3. Dopamine-induced relaxation was competitively antagonized by SCH 23390, (R)- and (S)-sulpiride, and fenoldopam induced relaxation by SCH 23390 and (+)- but not (-)-butaclamol. 4. These results indicate the presence of vascular dopamine receptors (DA1 subtype) on human isolated resistance arteries from omental and subcutaneous sites.

Dopamine inhibits prostaglandin F2 alpha-induced depolarization of rabbit jejunal arteries via activation of DA1-receptors

In rabbit jejunal arteries, the membrane potential of single smooth muscle cells decreased on the application of noradrenaline 3 mumol/l. LY 171555 1 mumol/l did not change, whereas SKF 38393 10 mumol/l reversed the effect of noradrenaline. When prostaglandin F2 alpha (PGF2 alpha) was used to evoke depolarization in the presence of prazosin 0.1 mumol/l, rauwolscine 1 mumol/l and propranolol 1 mumol/l, both SKF 38393 10 mumol/l and dopamine 10 mumol/l repolarized the membrane. SCH 23390 1 mumol/l antagonized the effects of SKF 38393 10 mumol/l and dopamine 10 mumol/l. Thus, the change in membrane potential is mediated by a DA1-receptor.

Specific binding of 125I SCH 23982, a selective dopamine (D1) receptor ligand to plasma membranes derived from human kidney cortex

Binding of the selective D-1 dopamine receptor ligand 125I SCH 23982 was studied using crude plasma membranes derived from human renal cortex. 125I SCH 23982 bound saturably to a single high affinity site (Kd = 650 pM, Bmax = 19 fmol/mg protein). Binding at 37 degrees was rapid and reversible with forward and reverse rate constants of 5.79 x 10(8) min-1 m-1 and 0.156 min-1 respectively. Antagonist and agonist competition for 125I SCH 23982 binding was also consistent with the existence of a single site possessing pharmacological characteristics similar to a D-1 dopamine receptor. It is suggested that this site may represent a D-1 (or DA1) dopamine receptor present in human renal cortex.

Cultured mesenteric vascular smooth muscle cells express dopamine DA1-receptors

Incubation of cultured mesenteric vascular smooth muscle cells with dopamine, in the presence of propranolol, caused an increase in cyclic AMP formation in a concentration-dependent manner (Ka apparent 6.8 +/- 0.5 microM). This effect of dopamine was inhibited by the DA1-receptor antagonist SCH 23390 (Ki = 1 nM). These results suggest that cultured mesenteric vascular smooth muscle cells express DA1-receptors linked to adenylate cyclase.

Adrenergic, serotoninergic, histaminergic, and imipramine binding sites in post-mortal human cerebral microvessel preparations

Cerebral microvessels were prepared from fresh and frozen human brain samples obtained from autopsy cases. Structural integrity and purity of the microvessels were confirmed by light and electron microscopy, and by measurement of the enzymatic marker gamma-glutamyltranspeptidase. Similar morphological and enzymatic characteristics were found for the microvessels prepared from fresh and frozen brain samples. Radioligand binding experiments indicated the presence both in the "fresh" and "frozen" microvessel preparations of specific alpha 1-, alpha 2-, and beta-adrenergic, histamine H1, serotonin S1 and imipramine binding sites, although the density of beta-adrenergic and histamine H1 specific binding sites were lower in the frozen samples than in the fresh samples. Low levels of specific binding to muscarinic, GABAergic and serotonin S2 sites (with respect to the specific binding densities in the crude homogenates) were found in the microvessel preparations.

Dopamine receptors in the developing sheep kidney

These studies were designed to characterize dopamine receptor density and affinity in kidneys removed from sheep of varying ages (fetal, newborn, and adult) using radioligand binding methods. Three different radioligands were used: the specific dopamine-1 antagonist 3H-SCH 23390, the dopamine-1/dopamine-2 antagonist 3H-haloperidol, and the dopamine-2 antagonist 3H-spiroperidol. The specific binding of 3H-haloperidol and 3H-spiroperidol was saturable with time and ligand concentration, being indicative of dopamine receptors. The specific binding of the dopamine-1 selective radioligand 3H-SCH 23390 was also saturable with time but displayed several points of saturation with increasing ligand concentration. The specific binding of 3H-haloperidol, which had a low affinity and is indicative of dopamine-1 receptors, showed no age-related changes in maximum receptor density or affinity. On the other hand, the maximum receptor density of dopamine-2 receptors measured by 3H-spiroperidol decreased with age. The observations that renal dopamine-1 receptor density or affinity do not change with maturation are in agreement with our previous studies that showed no age-related changes in dopamine-receptor-mediated renal vasodilatation in sheep. The significance of the decrease in renal dopamine-2 receptor density with age remains to be determined.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

Dobutamine: positive inotropy by nonselective adrenoceptor agonism in isolated guinea pig and human myocardium

Positive inotropic responses to dobutamine have been examined using isolated myocardium from guinea pigs und humans. The potency (EC50) of dobutamine was 1.5 X 10(-6) mol/l on guinea pig papillary muscles, 1.8 X 10(-6) mol/l on guinea pig left atria and 2.5 X 10(-6) mol/l on human papillary muscle strips. In guinea pig cardiac muscles, Schild plots for the beta 1-selective antagonist, 1-practolol, using dobutamine as agonist, had slopes of less than unity. This suggested the involvement of other receptors in the inotropic response to dobutamine. The beta 2-selective antagonist, ICI 118,551, but not the alpha 1-selective antagonist, prazosin, attenuated the dobutamine response in guinea pig papillary muscles. Both ICI 118,551 and prazosin shifted the dobutamine concentration-response curve in guinea pig left atria. The positive inotropic response to dobutamine in human papillary muscles was antagonised by l-practolol and ICI 118,551 but not by prazosin. The maximal inotropic response to dobutamine was 90% that of calcium measured in the same guinea pig papillary muscles but only 37% that of calcium in human papillary muscle strips. This reduced maximal effect of dobutamine in human myocardium is probably a disease-induced change but species variations cannot be excluded.

Relaxant effect of dopamine on isolated rabbit pulmonary artery

In rabbit pulmonary artery, dopamine (10(-11)-10(-5) M) produced a concentration-dependent relaxation of the arterial strips contracted with prostaglandin F2 alpha (PGF2 alpha) in the presence of prazosin (10(-6) M), yohimbine (10(-6) M), propranolol (10(-6) M), and methysergide (10(-6) M). SKF38393, an agonist for D1 or DA1 dopamine receptor, mimicked partially the concentration-response curve for dopamine, whereas LY171555 and apomorphine did not. The order of potency of dopamine antagonists on the inhibitory effect was: cis-flupenthixol greater than bulbo-capnine greater than metoclopramide greater than haloperidol. Sulpiride was inactive. Cis-flupenthixol did not block the relaxation induced by acetylcholine, adenosine, and papaverine. In the arterial strips of the rabbits pretreated with 6-hydroxydopamine, the concentration-response curve for dopamine was similar to that in non-treated rabbits. Thus it is concluded that a specific dopamine receptor is located on the postsynaptic muscle membrane of the rabbit pulmonary artery.

Renal effects of SK&F 82526 in anesthetized rats

Dopamine affects renal hemodynamics, renal tubular functions, and the secretion of renin. We have studied the renal effects of SK&F 82526 (an agonist which is selective for the DA1 subclass of dopamine receptors) in anesthetized rats. Infused intravenously at 0.005 mumol/min/kg, this drug increased renal plasma flow and the clearances of PAH and insulin, effects which are consistent with decreased renovascular resistance. Concomitantly, urine flow and K excretion increased, and Na excretion tended to increase. All these effects of SK&F 82526 were antagonized by intravenous metoclopramide (1 mumol/min/kg). Despite its diuretic effect and despite its lack of effect on arterial blood pressure, SK&F 82526 increased arterial plasma renin concentration, suggesting a stimulatory effect on renin secretory rate. Taken together, our results demonstrate that the renal effects of SK&F 82526 mimic those of dopamine.

Alpha-adrenoceptor blocking activity of fenoldopam (SK&F 82526), a selective DA1 agonist

The alpha-adrenoceptor blocking activity of fenoldopam (SK&F 82526), a selective dopamine vascular receptor (DA1) agonist, was evaluated in isolated segmental preparations of rabbit aorta, dog mesenteric and rabbit splenic arteries. Fenoldopam, in concentrations ranging from 10(-6) to 10(-4) M, produced parallel, dextral shifts of concentration-contractile response curves to noradrenaline. Slopes of the Schild regression lines were not significantly different from unity in the three vessels. pA2 values for fenoldopam in the rabbit aorta, dog mesenteric and rabbit splenic arteries were 5.48 +/- 0.08, 5.78 +/- 0.05, and 5.20 +/- 0.05, respectively. In experiments where the drug was added to the bathing medium before exposing the vascular segments to the irreversible alpha-adrenoceptor antagonist, phenoxybenzamine, fenoldopam provided nearly complete protection against alpha-adrenoceptor blockade. These results demonstrate that fenoldopam possesses alpha-adrenoceptor antagonist activity and competes with alpha-phenoxybenzamine, for occupancy at the same receptor site.

Effects of dopaminomimetics on the secretion of VIP-like immunoreactivity in conscious dogs

The effects of some dopaminomimetics on VIP levels in peripheral venous blood of conscious dogs were analysed with a radioimmunoassay. The dopamine D2 agonist pergolide, like apomorphine and bromocriptine, increased VIP levels. The putative DA autoreceptor agonist 3PPP, as well as the D1 agonist SK&F 38393 were devoid of action. The D1 antagonist SCH 23390 did not abolish the effect of apomorphine. It is suggested that monitoring of VIP levels could be an interesting screening test for activity at D2 receptors. Amphetamine did not modify VIP levels suggesting that DA neurons are not involved in the mechanism leading to a release of VIP. The VIP response to apomorphine was not suppressed by an infusion of somatostatin. Decreasing blood pressure with nitroglycerin or with the adrenergic antagonist prazosin did not release VIP. The mechanism by which administration of dopaminomimetics lead to a release of VIP is further discussed.

Dopamine-induced potentiation of the contractile response to arachidonic acid in guinea-pig aorta

In guinea-pig aorta, the potentiating effect of dopamine on the response to arachidonic acid was examined. The potentiation by dopamine was completely abolished by nordihydroguaiaretic acid, but not by indomethacin, aspirin, corticosterone, haloperidol, SK & F 83566, propranolol, PGI2 or imidazole. Dopamine did not have any significant effect on the response to PGF2 alpha U-46619, KCl, 5-hydroxytryptamine or histamine.

Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.

Dopamine receptors in the guinea-pig heart. A binding study

The binding of dopaminergic agonists and antagonists to guinea-pig myocardial membrane preparations was studied using 3H-dopamine and 3H-spiperone as radioligand. 3H-Dopamine bound specifically to heart membranes while 3H-spiperone did not. A Scatchard analysis of 3H-dopamine binding showed a curvilinear plot indicating the presence of two dopamine receptor populations that we have termed high- (Kd = 1.2 nM, Bmx = 52.9 fmol/mg prot.) and low- (Kd = 11.8 nM, Bmx = 267.3 fmol/mg prot.) affinity binding sites, respectively. The characterisation of the high-affinity component of 3H-dopamine binding indicated that the binding is rapid, saturable, stereospecific, pH- and temperature-dependent, and displaced by dopaminergic agonists and antagonists known to act similarly in vivo. The finding that pretreatment with dibenamine (which has been described as an alpha-adrenoceptor irreversible blocker) did not affect the binding of dopamine to cardiac membrane preparations suggests that alpha-adrenoceptors and dopamine receptors have separate recognition sites in the heart. We conclude that 3H-dopamine binds to specific dopamine receptors in the heart of guinea-pigs.

Dopamine receptors and hypertension. Physiologic and pharmacologic implications

The endogenous catecholamine dopamine lowers blood pressure by acting on two receptor subtypes: dopamine 1 and dopamine 2. Dopamine 1 receptors subserve vasodilation, especially in the renal, coronary, mesenteric, and cerebral vascular beds. Dopamine 2 receptors have been located at the endings of postganglionic sympathetic nerves and, when activated, inhibit norepinephrine release. Inhibition of emesis and inhibition of prolactin release also appear to be dopamine 2-mediated phenomena. The receptor subtypes have been classified by differences in chemical structure of agonists and by specific antagonists. Dopamine also acts on beta 1 receptors to stimulate the heart and alpha 1 and alpha 2 receptors to cause vasoconstriction. Alpha adrenergic activity and lack of oral availability limit the use of dopamine in the treatment of hypertension. However, studies with the selective dopamine 1 agonist, fenoldopam, and dopamine 2 agonists such as LY 141865 and bromocriptine, indicate that agonists of both receptor subtypes can lower blood pressure in experimental animals and in hypertensive patients. Initial use of dopamine agonists in the treatment of hypertension and its possible involvement in the etiology and maintenance of hypertension are discussed.