Selective role of dopamine in the natriuresis produced by iso-osmotic saline infusion

Effects of carbidopa and of intravenous saline infusion into normal and hypertensive subjects on urinary free and conjugated dopamine

OBJECTIVE

To investigate the possible role played by endogenous dopamine as a modulator of renal sodium (Na+) reabsorption after a combined Na+ and volume load.

DESIGN

A randomized placebo-controlled study.

METHODS

Ten healthy volunteers and four hypertensive patients were subjected to intravenous infusions of 21 0.9% saline (308 mmol Na+) administered from 1000 to 1300 h after oral administration of placebo or of carbidopa, a dopamine decarboxylase inhibitor.

RESULTS

Studies on control subjects after placebo showed that natriuresis occurred during the 6 h after commencement of the saline infusion, with falls in plasma albumin concentration, plasma renin activity and plasma aldosterone concentration; in comparison with results of mock infusion (6 mmol Na+) there was no change in the urinary excretion of dopamine and noradrenaline (In their free or conjugated forms). There was, however, a marked surge in excretion of urinary conjugated dopamine and in the dopamine: noradrenaline ratio from 1300 to 1600 h, after either type of infusion. Administration of carbidopa before the saline infusion resulted in a marked decrease in excretion of urinary free dopamine, but had no effect on the surge in excretion of urinary conjugated dopamine. Saline infusion decreased proximal fractional Na+ reabsorption. Administration of carbidopa delayed but did not prevent this decrease. The effects of saline infusion and of carbidopa on the urinary excretion of dopamine and noradrenaline from hypertensive patients were similar to those observed with the healthy volunteers.

CONCLUSIONS

These findings indicate that volume expansion by intravenous saline infusion has no appreciable effect on the urinary free dopamine excretion from normal or hypertensive humans; with any apparent increase, it is important to exclude the possibility of conversion of conjugates to free dopamine in vitro. Furthermore, that carbidopa administration did not inhibit the afternoon surge of conjugated dopamine suggests that administration of carbidopa is deficient as a tool to investigate the functional role of the renal dopamine system.

The pathophysiological role of renal dopamine and kallikrein in deoxycorticosterone acetate (DOCA)-salt treated rats

To elucidate the pathophysiological role of the renal dopamine (DA) and kallikrein-kinin systems in volume dependent hypertension under excess mineralocorticoid, the urinary excretions of DA and kallikrein (KAL) were investigated in DOCA-salt treated rats (5 week-old Sprague-Dawley rats, DOCA 100mg pellet, s.c., 1% saline ingestion for 4 weeks, n = 7) for comparison with those of control rats (vehicle, 1% saline ingestion for 4 weeks, n = 10). In vehicle, systolic blood pressure (SBP), urinary excretion of free DA (UDA) and kallikrein (UKAL) did not change through the study. In DOCA treated rats, marked natriuresis was observed with sodium load. SBP and UKAL significantly increased at the 4th week and at the 2nd week, respectively. UDA increased significantly to the peak level at the 1st week, and then decreased gradually. At the 4th week, UDA was significantly lower in DOCA-treated rats than that of vehicle rats. A significant inverse correlation was found between UDA and SBP, and UKAL correlated positively with SBP and urinary excretion of sodium and negatively with UDA in DOCA treated rats. These results suggest that the augmentation of renal dopaminergic activity which may in part cause sodium escape appears at the early stage of DOCA-salt treatment, but this augmentation is subsequently blunted; volume and sodium retention may be associated with the blood pressure elevation at the late stage of DOCA-salt treatment, and that the augmentation of renal kallikrein-kinin system may be a compensatory response to sodium and volume retention in DOCA-salt treated rats.