Systemic and pulmonary vascular effects of selective dopamine receptor blockade and stimulation in lambs

Dopamine receptor blockade improves pulmonary gas exchange but decreases exercise performance in healthy humans

Pulmonary gas exchange, as evaluated by the alveolar-arterial oxygen difference (A-aDO2), is impaired during intense exercise, and has been correlated with recruitment of intrapulmonary arteriovenous anastomoses (IPAVA) as measured by agitated saline contrast echocardiography. Previous work has shown that dopamine (DA) recruits IPAVA and increases venous admixture (Q̇s/Q̇t) at rest. As circulating DA increases during exercise, we hypothesized that A-aDO2 and IPAVA recruitment would be decreased with DA receptor blockade. Twelve healthy males (age: 25 ± 6 years, V̇O2 max : 58.6 ± 6.5 ml kg(-1) min(-1) ) performed two incremental staged cycling exercise sessions after ingestion of either placebo or a DA receptor blocker (metoclopramide 20 mg). Arterial blood gas, cardiorespiratory and IPAVA recruitment (evaluated by agitated saline contrast echocardiography) data were obtained at rest and during exercise up to 85% of V̇O2 max . On different days, participants also completed incremental exercise tests and exercise tolerance (time-to-exhaustion (TTE) at 85% of V̇O2 max ) with or without dopamine blockade. Compared to placebo, DA blockade did not change O2 consumption, CO2 production, or respiratory exchange ratio at any intensity. At 85% V̇O2 max , DA blockade decreased A-aDO2, increased arterial O2 saturation and minute ventilation, but did not reduce IPAVA recruitment, suggesting that positive saline contrast is unrelated to A-aDO2. Compared to placebo, DA blockade decreased maximal cardiac output, V̇O2 max and TTE. Despite improving pulmonary gas exchange, blocking dopamine receptors appears to be detrimental to exercise performance. These findings suggest that endogenous dopamine is important to the normal cardiopulmonary response to exercise and is necessary for optimal high-intensity exercise performance.

Effects of catecholamines on the pulmonary circulation in the ovine fetus

High levels of circulating catecholamines are found in the fetus, and fetal stress and birth induce a marked surge in catecholamine secretion. Little is known about the role of catecholamines on the fetal pulmonary circulation. To determine the effects of catecholamines on the pulmonary vascular tone, we tested the hemodynamic response to norepinephrine and dopamine infusion in chronically prepared late-gestation fetal lambs. We found that norepinephrine infusion (0.5 microg. kg(-1). min(-1)) increased pulmonary artery pressure (PAP) by 10 +/- 1% (P < 0.01), left pulmonary artery blood flow by 73 +/- 14% (P < 0.01), and decreased pulmonary vascular resistance (PVR) by 33 +/- 6% (P < 0.01). The pulmonary vasodilator effect of norepinephrine was abolished after nitric oxide synthase inhibition. Dopamine infusion at 5 microg. kg(-1). min(-1) did not significantly change PVR. Conversely, dopamine infusion at 10 microg. kg(-1). min(-1) increased PAP (P < 0.01) and progressively increased PVR by 30 +/- 14% (P < 0.01). These results indicate that catecholamines may modulate basal pulmonary vascular tone in the ovine fetus. We speculate that catecholamines may play a significant role in the maintenance of the fetal pulmonary circulation and in mediating changes in the transitional pulmonary circulation.

The effects of dopamine and epinephrine on hemodynamics and oxygen metabolism in hypoxic anesthetized piglets

Background:

The most appropriate inotropic agent for use in the newborn is uncertain. Dopamine and epinephrine are commonly used, but have unknown effects during hypoxia and pulmonary hypertension; the effects on the splanchnic circulation, in particular, are unclear.

Methods:

The effects on the systemic, pulmonary, hepatic, and mesenteric circulations of infusions of dopamine and epinephrine (adrenaline) were compared in 17 newborn piglets. Three groups [control (n = 5), dopamine (n = 6) and epinephrine (n = 6)] of fentanyl anesthetized newborn piglets were instrumented to measure cardiac index (CI), hepatic arterial and portal venous blood flow, mean systemic arterial pressure (SAP), mean pulmonary arterial pressure (PAP), and arterial, portal and mixed venous oxygen saturations. Systemic, pulmonary, and mesenteric vascular resistance indices [systemic vascular resistance index (SVRI), pulmonary vascular resistance index (PVRI), mesenteric vascular resistance index (MVRI)], and systemic and splanchnic oxygen extraction and consumption were calculated. Alveolar hypoxia was induced, with arterial oxygen saturation being maintained at 55-65%. After 1 h of stabilization during hypoxia, each animal received either dopamine or epinephrine; randomly administered doses of 2, 10, and 32 μg kg^-1 min^-1 and 0.2, 1.0, and 3.2 μg kg^-1 min^-1 respectively were infused for 1 h at each dose. Results were compared with the 1 h hypoxia values by two-way analysis of variance.

Results:

Epinephrine increased CI at all doses, with no significant effects on SAP and SVRI. Although epinephrine increased PAP at 3.2 μg kg^-1min^-1, it had no effect on PVRI. Dopamine had no effect on CI, SAP, and SVRI, but increased PAP at all doses and PVRI at 32 μg kg^-1min^-1. The SAP/PAP ratio was decreased with 32 μg kg^-1min^-1 dopamine, whereas epinephrine did not affect the ratio. In the mesenteric circulation, dopamine at 32 μg kg^-1 min^-1 increased portal venous flow and total hepatic blood flow and oxygen delivery, and decreased MVRI; epinephrine had no effect on these variables. Epinephrine increased hepatic arterial flow at 0.2 μg kg^-1 min^-1; dopamine had no effect on hepatic arterial flow at any dose. Despite these hemodynamic changes, there were no differences in systemic or splanchnic oxygen extraction or consumption at any dose of dopamine or epinephrine.

Conclusions:

Epinephrine is more effective than dopamine at increasing cardiac output during hypoxia in this model. Although epinephrine preserves the SAP/PAP ratio, dopamine shows preferential pulmonary vasoconstriction, which might be detrimental if it also occurs during the management of infants with persistent fetal circulation. Dopamine, but not epinephrine, increases portal flow and total hepatic flow during hypoxia.

Low-dose "renal" dopamine

Low dose renal dopamine continues to be infused in patients at risk for renal dysfunction or as a therapy after acute renal failure has been established. This article reviews the impact of acute renal failure on patients and reviews the history and use of dopamine therapy for patients. A discussion of the rationale, positive and equivocal evidence, side effects, and possible clinical indications for low-dose renal dopamine therapy is included.

DA1-receptor stimulation by fenoldopam in the treatment of postcardiac surgical hypertension

BACKGROUND

Besides adequate analgesia, sedation and ventilation, postcardiac surgical hypertension has to be treated frequently with vasoactive drugs to avoid possible complications. In this study the hemodynamic effects of the DA1-receptor agonist fenoldopam (F) are compared to those of the Ca-channel antagonist nifedipine (N).

METHODS

Postoperatively, 64 CABG-patients with a mean arterial pressure (MAP) of more than 105 mmHg over 10 min were investigated. Patients with compromised ventricular function, insufficient surgical repair, arrhythmia or an ECG unable to detect myocardial ischemia were excluded. The study drugs (initial dosage: F: 0.8; N: 0.3 micrograms.kg-1.min-1) were given continuously via a central venous catheter to reduce and to maintain the MAP between 80 and 95 mmHg. Hemodynamic parameters were determined using thermodilution technique.

RESULTS

A significant reduction of the MAP (F: from 121 +/- 11 to 83 +/- 4, N: from 119 +/- 8 to 82 +/- 9 mmHg) and of the calculated systemic vascular resistance (SVR) (F: 2110 +/- 500 to 970 +/- 200, N:1980 +/- 660 to 1020 +/- 300 dyn.s.cm-5) were noted in both groups, whereby in the F group the therapeutic goal could be achieved more quickly with the dosage regimen chosen. As a result, a marked increase of heart rate, cardiac index and stroke volume index could be observed, which was more pronounced due to the initially stronger decrease of SVR with F. There was also a stronger decrease of pulmonary vascular resistance in the F group, but the indices of right ventricular function did not differ between the groups.

CONCLUSION

Fenoldopam seems to be an efficient alternative to nifedipine, especially because of its more rapid onset of action.

G protein-coupled receptor signalling in the kidney

The kidney is responsible for regulation of water and electrolyte balance, filtration and absorption of plasma proteins, and control of blood volume and pressure. Homeostasis achieved by the kidney is controlled in large part by the action of hormones or proteins on specific transmembrane receptors. Conversely, many renal diseases, including that resulting from atherosclerosis, are characterised by scarring and abnormal proliferation of cellular components of the kidney, and these processes are mediated in large part by these same receptors. The G protein-coupled receptors constitute a large and diverse class of proteins, characterised by the possession of seven transmembrane-spanning domains. These receptors bind polypeptide growth factors, which function to transmit a variety of signals from the extracellular to the intracellular milieu. The receptor-associated G proteins utilised by the kidney derive their specificity not only by activating or inhibiting various second-messenger molecules, but also by their location on particular cell types. In this review, several G protein-coupled receptors will be discussed from the perspective of their importance to kidney function and to the pathogenesis of renal disease, atherosclerosis, and hypertension.