Further studies on the mechanism of the natriuretic response to low-dose dopamine in man: effect on lithium clearance and nephrogenic cAMP formation

Dopamine increases renal oxygenation: a clinical study in post-cardiac surgery patients

BACKGROUND

Imbalance of the renal medullary oxygen supply/demand relationship can cause ischaemic acute renal failure (ARF). The use of dopamine for prevention/treatment of ischaemic ARF has been questioned. It has been suggested that dopamine may increase renal oxygen consumption (RVO(2)) due to increased solute delivery to tubular cells, which may jeopardize renal oxygenation. Information on the effects of dopamine on renal perfusion, filtration and oxygenation in man is, however, lacking. We evaluated the effects of dopamine on renal blood flow (RBF), glomerular filtration rate (GFR), RVO(2) and renal O(2) demand/supply relationship, i.e. renal oxygen extraction (RO(2)Ex).

METHODS

Twelve uncomplicated, mechanically ventilated and sedated post-cardiac surgery patients with pre-operatively normal renal function were studied. Dopamine was sequentially infused at 2 and 4 ug/kg/min. Systemic haemodynamics were evaluated by a pulmonary artery catheter. Absolute RBF was measured using two independent techniques: by the renal vein thermodilution technique and by infusion clearance of paraaminohippuric acid (PAH), with a correction for renal extraction of PAH. The filtration fraction (FF) was measured by the renal extraction of (51)Cr-EDTA.

RESULTS

Neither GFR, tubular sodium reabsorption nor RVO(2) was affected by dopamine, which increased RBF (45-55%) with both methods, decreased renal vascular resistance (30-35%), FF (21-26%) and RO(2)Ex (28-34%). The RBF/CI ratio increased with dopamine. Dopamine decreased renal PAH extraction, suggestive of a flow distribution to the medulla.

CONCLUSIONS

In post-cardiac surgery patients, dopamine increases the renal oxygenation by a pronounced renal pre-and post-glomerular vasodilation with no increases in GFR, tubular sodium reabsorption or renal oxygen consumption.

Decreased urinary dopamine excretion and disturbed dopamine/sodium relationship in type 1 diabetes mellitus

In Type 1 diabetes an increased total body sodium and an impaired ability to excrete a sodium load have been described. A possible involvement of the renal dopaminergic system in this abnormal sodium handling was evaluated through measurements of the urinary output of dopamine, sodium, the dopamine/sodium correlation, and through examining the effect of a dopamine infusion on urinary sodium excretion. Twenty-four hour urinary dopamine excretion was significantly lower in Type 1 diabetic patients as compared to normal controls. A significant correlation between urinary dopamine and sodium excretion was present in normoalbuminuric Type 1 diabetic patients and in normal controls. However, no such correlation could be found in microalbuminuric patients. The increase in fractional excretion of sodium during a 1 h low-dose dopamine (3 micrograms kg-1 min-1) infusion in Type 1 diabetic patients was negatively correlated with diabetes duration. Patients with short duration of diabetes (less than 15 years) had a comparable dopamine-induced increase in fractional excretion of sodium as normal controls. However, patients with longer duration of diabetes (more than 15 years) and microalbuminuric patients displayed no significant changes in sodium output during dopamine infusion. These findings suggest that in Type 1 diabetes mellitus a deficiency of renal dopamine production could be responsible for the impaired sodium handling. Longer duration of the disease and microalbuminuria seem to be associated with an uncoupling of the urinary dopamine/sodium relationship.

Effects of acute beta-adrenoceptor blockade with metoprolol on the renal response to dopamine in normal humans

The present study investigated the contribution of adrenergic beta 1-receptor stimulation to the cardiovascular and renal effects of low-dose dopamine in eight normal, water-loaded humans. Metoprolol (100 mg) or placebo was administered orally at 08.00 h in a randomized, double-blind fashion on two different days. Renal clearance studies were performed during a 1 h baseline period, two 1 h periods with dopamine infusion (3 micrograms kg-1 min-1), and a 1 h recovery period. Cardiac output was measured by an ultrasonic Doppler method, and lithium clearance (CLLi) was used to estimate proximal tubular outflow. Baseline values of heart rate, systolic pressure and mean arterial pressure decreased with metoprolol compared with placebo, but cardiac output, effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were not significantly changed. Metoprolol significantly decreased baseline CLLi and sodium clearance (CLNa) by 19% (P < 0.01) and 34% (P < 0.01), respectively. Metoprolol blunted the dopamine-induced increases in heart rate and systolic pressure, but cardiac output increased to the same extent on both study days by 26% (placebo, P < 0.05) and by 31% (metoprolol, P < 0.01), respectively. With and without metoprolol, dopamine did not significantly change GFR, and the percentage increases in ERPF were similar on the two study days (40% (P < 0.001) and 42% (P < 0.001), respectively). Dopamine increased CLLi and CLNa by 31% (P < 0.01) and 114% (P < 0.01), respectively, with placebo, and by 36% (P < 0.01) and 114% (P < 0.01), respectively, with metoprolol. Values during infusion remained significantly lower with metoprolol compared with placebo.(ABSTRACT TRUNCATED AT 250 WORDS)