Effects of dopamine on L-type Ca2+ current in single atrial and ventricular myocytes of the rat

Role of Dopamine in the Heart in Health and Disease

Dopamine has effects on the mammalian heart. These effects can include an increase in the force of contraction, and an elevation of the beating rate and the constriction of coronary arteries. Depending on the species studied, positive inotropic effects were strong, very modest, or absent, or even negative inotropic effects occurred. We can discern five dopamine receptors. In addition, the signal transduction by dopamine receptors and the regulation of the expression of cardiac dopamine receptors will be of interest to us, because this might be a tempting area of drug development. Dopamine acts in a species-dependent fashion on these cardiac dopamine receptors, but also on cardiac adrenergic receptors. We will discuss the utility of drugs that are currently available as tools to understand cardiac dopamine receptors. The molecule dopamine itself is present in the mammalian heart. Therefore, cardiac dopamine might act as an autocrine or paracrine compound in the mammalian heart. Dopamine itself might cause cardiac diseases. Moreover, the cardiac function of dopamine and the expression of dopamine receptors in the heart can be altered in diseases such as sepsis. Various drugs for cardiac and non-cardiac diseases are currently in the clinic that are, at least in part, agonists or antagonists at dopamine receptors. We define the research needs in order to understand dopamine receptors in the heart better. All in all, an update on the role of dopamine receptors in the human heart appears to be clinically relevant, and is thus presented here.

Ivabradine prevents deleterious effects of dopamine therapy in heart failure: No role for HCN4 overexpression

BACKGROUND

Exacerbations of chronic heart failure (CHF) are often treated with catecholamines to provide short term inotropic support, but this strategy is associated with long-term detrimental hemodynamic effects and increased ventricular arrhythmias (VA), possibly related to increased heart rate (HR). We hypothesized that ivabradine may prevent adverse effects of short-term dopamine treatment in CHF.

METHODS

Rats with post-myocardial infarction CHF received 2-week infusion of saline, dopamine(D), ivabradine(I) or D&I; cardiac function was assessed using echocardiography and pressure-volume loops while VA were assessed using telemetric ECG recording. Expression of HCN4, a potentially proarrhythmic channel blocked by ivabradine, was assessed in left ventricular (LV) myocardium. HCN4 expression was also assessed in human explanted normal and failing hearts and correlated with VA.

FINDINGS

Dopamine infusion had detrimental effects on hemodynamic parameters and LV remodeling and induced VA in CHF rats, while ivabradine completely prevented these effects. CHF rats demonstrated HCN4 overexpression in LV myocardium, and ivabradine and, unexpectedly, dopamine prevented this. Failing human hearts also exhibited HCN4 overexpression in LV myocardium that was unrelated to patient's sex, CHF etiology, VA severity or plasma NT-proBNP.

INTERPRETATION

HR reduction offered by ivabradine may be a feasible strategy to extract benefits of inotropic support in CHF exacerbations, avoiding detrimental effects on CHF biology or VA. Ivabradine may offer additional beneficial effects in this setting, going beyond pure HR reduction, however prevention of ventricular HCN4 overexpression is unlikely to play a major role.

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.

Cutaneous pigmentation modulates skin sensitivity via tyrosinase-dependent dopaminergic signalling

We propose a new mechanism of sensory modulation through cutaneous dopaminergic signalling. We hypothesize that dopaminergic signalling contributes to differential cutaneous sensitivity in darker versus lighter pigmented humans and mouse strains. We show that thermal and mechanical cutaneous sensitivity is pigmentation dependent. Meta-analyses in humans and mice, along with our own mouse behavioural studies, reveal higher thermal sensitivity in pigmented skin relative to less-pigmented or albino skin. We show that dopamine from melanocytes activates the D₁-like dopamine receptor on primary sensory neurons. Dopaminergic activation increases expression of the heat-sensitive TRPV1 ion channel and reduces expression of the mechanically-sensitive Piezo2 channel; thermal threshold is lower and mechanical threshold is higher in pigmented skin.

Dopamine increases L-type calcium current more in newborn than adult rabbit cardiomyocytes via D1 and β2 receptors

Dopamine is used to treat heart failure, particularly after cardiac surgery in infants, but the mechanisms of action are unclear. We investigated differences in the effect of dopamine on L-type calcium current ( ICa) between newborn (NB, 1–4 days) and adult (AD, 3–4 mo) rabbit ventricular myocytes. Myocytes were enzymatically dissociated from NB and AD rabbit hearts. ICa was recorded by using the whole cell patch-clamp technique. mRNA levels of cardiac dopamine receptor type 1 (D1), type 2 (D2), and β-adrenergic receptors (β-ARs) were measured by real-time RT-PCR. Dopamine (100 μM) increased ICa more in NB (Emax 87 ± 10%) than in AD ventricular cells (Emax 21 ± 3%). Further investigation of this difference showed that mRNA levels of the D1 receptor were significantly higher in NB, and, with β-AR blockade, dopamine increased ICa more in NB than AD cells. Additionally, SKF-38393 (selective D1 receptor agonist) significantly increased ICa by 55 ± 4% in NB ( P < 0.05, n = 4) and by 11 ± 1% in AD ( P < 0.05, n = 6). Dopamine in the presence of SCH-23390 (D1 receptor antagonist) increased ICa in NB cells by 67 ± 5% and by 22 ± 2% in AD cells, suggesting a role for β-AR stimulation. Selective blockade of β1- or β2-receptors (with block of D1 receptors) showed that the β-AR action of dopamine in the NB was largely mediated via β2-AR activation. Dopamine produces a larger increase in ICa in NB cardiomyocytes compared with ADs. The mechanism of action is not only through β2-ARs but also due to higher expression of cardiac D1 receptor in NB.

Inotropic, chronotropic, and arrhythmogenic effects of dopamine on the isolated working heart of rabbit

Some improvements of Kodama's method for perfusing the isolated rabbit heart in its working mode were made. Increases in the right and left atrium pressure, together with an increase in the pulmonary artery pressure, were observed to occur immediately after the start of venous return, and then all of the increased pressures were found to remain at each constant level. In these stable states, the administration of dopamine (DA) into the perfusate was found to produce dose-related increases in contractile activities. In the preparations denervated with reserpine or 6-hydroxydopamine, in which tyramine (Ty) produced no response, the inotropic effectiveness of DA did not differ from that in the normal ones. On the other hand, responses to noradrenaline (NA) were found to increase significantly after the denervation. DA produced a dose-related increase in heart rates in the normal preparation, and this effect was greatly suppressed in the denervated preparations, suggesting that the primary chronotropic effect of DA is an indirect one via the release of NA from the sympathetic nerve terminals. Arrhythmogenic effects of NA, Ty or DA were also observed in these preparations. At all the doses tested, the incidence rates by NA were as high as 50% or more, the type of arrhythmia being recognized as atrial or ventricular extrasystole from the ECG analysis. On the other hand, the rates by DA were relatively low, less than 34%. From a comparison of the incidence rates between the normal and denervated preparations, this effect of DA was considered to be primarily an indirect one.

Effects of selective dopamine receptor subtype agonists on cardiac contractility and regional haemodynamics in rats

1. Activation of dopamine (DA) receptors produces cardiovascular responses such as vasodilation and hypotension. However, knowledge of the role of specific dopamine receptor subtypes (especially D3 and D4) in the cardiovascular system is limited. The objective of the present study was to characterize the haemodynamic and cardiac responses to agonists with selectivity for D1, D2, D3 and D4 receptor subtypes. 2. Inactin-anaesthetized rats were instrumented to measure regional haemodynamic and cardiac contractility responses with slow intravenous infusion of agonists. 3. Fenoldopam (a D1 receptor agonist) decreased (P < 0.05) renal vascular resistance beginning at a dose of 3 micromol/kg. Infusion of PNU-95666E (a D2 receptor agonist) produced dose-dependent decreases (P < 0.05) in mean arterial pressure (MAP), heart rate (HR) and hindquarter vascular resistance (HQVR). Administration of BP897 (a partial D3 receptor agonist) decreased (P < 0.05) MAP and HQVR at 3 micromol/kg. PD168077 (a D4 receptor agonist) caused significant increases in HQVR at 1 micromol/kg. None of the compounds tested elicited significant changes in cardiac contractility. 4. Using selective agonists of dopamine receptor subtypes, the present studies characterize distinct cardiovascular effects in anaesthetized rats. Consistent with its well-defined effects as a D1 receptor agonist, fenoldopam administration resulted in renal vasodilation. Similar to earlier studies using the non-selective D2-like receptor agonist quinpirole, selective agonism at the D2 receptor using PNU-95666E resulted in bradycardia, hindquarter vasodilation and decreases in arterial pressure. Partial agonism at the D3 receptor with BP897 had no effect on heart rate, but did produce depressor responses driven by decreases in HQVR. Conversely, agonism of the D4 receptor using PD168077 resulted in modest hindquarter vasoconstriction that was not dose dependent. Hence, by comparison, agonism of the D4 receptor has little effect in the cardiovascular system of the rat relative to the other dopamine receptor subtype agonists tested.

Effect of low doses of clozapine on behaviour of isolated and group-housed male mice in the elevated plus-maze test

Clozapine is effective over a low and narrow dose range in the treatment of the negative symptoms of schizophrenia. Its efficacy is said to reflect an anxiolytic property of the neuroleptic. This study examined the effects of a low dose range of clozapine (0.1, 0.2 and 0.4 mg/kg) administered to isolated (7 days) and group-housed male mice on their behaviour in the elevated plus-maze. The results using this model of anxiety showed no dose-dependent effect on behaviour after treatment with low doses of clozapine. Some doses (0.1 and 0.4 mg/kg), however, reduced the duration and frequency of open arm entries compared to control-treated mice in the group-housed animals. This might indicate a slight anxiogenic effect. Support for this view is provided by some of the ethological measures, particularly head-dipping (HD). Seven days of isolation did not alter the main anxiety indices in controls. Nevertheless, some measures (e.g. closed entries and duration in the closed arm) were modified by the clozapine treatment (0.2 mg/kg) in a manner suggesting a mild anxiogenic action. The effects of the drug in individually and group-housed male mice were certainly different.

Dopamine stimulation of cardiac beta-adrenoceptors: the involvement of sympathetic amine transporters and the effect of SKF38393

1. Mechanisms underlying beta-adrenoceptor stimulation by dopamine were examined on guinea-pig Langendorff-perfused hearts and isolated cells from the right atrium, by using the chronotropic effects and the enhancement of L-type Ca2+ current (ICa,L) in the presence of prazosin as indicators of beta-adrenoceptor stimulation. Dopamine-induced overflow of noradrenaline (NA) concentrations was measured by high-performance liquid chromatography. 2. Dopamine caused positive chronotropic effects with an EC50 of 2.5 microM and induced NA overflow with a similar EC50 (1.3 microM). The chronotropic effect of dopamine was abolished by bisoprolol (1 microM). 3. The effects of dopamine were maintained during prolonged application, whereas the effects of tyramine faded with time. Dopamine (3 microM) restored the chronotropic effects and the NA release suppressed by pretreatment with tyramine, suggesting a de novo synthesis of NA during the exposure to dopamine. 4. Dopamine (3 microM)-induced NA release was not affected by tetrodotoxin, omega-conotoxin, rauwolscine, ICI118551 or sulpiride, but was inhibited by desipramine, a NA uptake inhibitor (IC50 approximately 1 microM). It was also not affected by GBR12909 and bupropion, dopamine uptake inhibitors in the central nervous system. 5. SKF38393, a D1 receptor partial agonist, potently inhibited the 3 microM dopamine-induced release of NA (IC50 approximately 0.1 microM). D1 receptors are not involved in the DA-induced release of NA, since SCH23390 (3 microM), a potent D1 antagonist, inhibited the NA release only slightly, and dihydrexidine (1 microM) and chloro-APB (1 microM), full D1 agonists, caused no significant NA release. 6. SKF38393 inhibited tyramine-induced overflow of NA, and potentiated the field stimulation-induced NA release. SKF38393 and desipramine retarded the decay of the stimulation-induced tachycardia in a similar manner. These results indicate that SKF38393 is a potent monoamine transport inhibitor and a useful tool for the functional evaluation of indirectly-acting sympathomimetic agonists in the heart. In the presence of SKF38393 (10 microM), dopamine at 1 microM showed no chronotropic effect. 7. Voltage clamp experiments with isolated atrial cells revealed that dopamine is a weak partial agonist. The EC50 for ICa,L stimulation by dopamine was high (13 microM). As a result, dopamine at 1 microM did not affect ICa,L. Bisoprolol abolished the stimulation of ICa,L by dopamine (30 microM), and dihydrexidine (1 microM) did not affect ICa,L. 8. It was concluded that the cardiac effects of dopamine at clinically relevant concentrations (< 1 microM) result almost exclusively from the indirect effect of beta adrenoceptor stimulation, involving the release of NA from sympathetic nerve terminals. The roles of the direct stimulation of beta adrenoceptors by dopamine at these concentrations and the stimulation of postjunctional D1 receptors seem negligible. The desipramine- and SKF38393-sensitive monoamine transporter mediates the release of NA.