Effects of dopamine and nitric oxide on arterial pressure and renal function in volume expansion

Prolactin, a natriuretic hormone, interacting with the renal dopamine system

BACKGROUND

Although prolactin affects sodium and water transport across the plasma membrane and interacts with dopamine in the brain, its role in the kidney is unclear. Here we examined the effect of prolactin and its possible interaction with the intrarenal natriuretic hormone dopamine, on proximal tubular Na(+), K(+)-ATPase activity in vitro and renal function in anesthetized rats.

METHODS

Na(+), K(+)-ATPase activity was measured as ouabain-sensitive adenosine triphosphate (ATP) hydrolysis in microdissected proximal tubular segments. Renal function was studied during euvolemic conditions by conventional clearance techniques.

RESULTS

Prolactin induced a dose-dependent inhibition of proximal tubular Na(+), K(+)-ATPase activity. A maximal inhibitory effect of 48% of control was observed at an in vitro prolactin concentration of 1 microg/mL. This effect was completely abolished by a dopamine D1 receptor antagonist. In tubules preincubated with inhibitors of aromatic amino acid decarboxylase (AADC), the rate-limiting enzyme in renal dopamine formation, prolactin had no effect on Na(+), K(+)-ATPase activity. In rats, prolactin infusion resulted in an increase in urinary sodium, potassium, and water excretion. These effects were also completely abolished by the D1 receptor antagonist. Prolactin had no significant effects on glomerular filtration rate (GFR) or mean arterial blood pressure.

CONCLUSION

We conclude that prolactin is a natriuretic hormone which interacts with the renal dopamine system for its effects. The natriuretic response is associated with inhibition of proximal tubular Na(+), K(+)-ATPase activity.

Vascular and renal effects of dopamine during extracellular volume expansion: Role of nitric oxide pathway

OBJECTIVE

The aim of the study was to determine the possible role of NO-system activation in vascular and renal effects of the dopaminergic system and the probable interaction between both systems during acute volume expansion in rats.

DESIGN AND METHODS

Expanded (10% bw) and non-expanded anaesthetized male Wistar rats were treated with haloperidol, a DA receptor antagonist (3 mg/kg bw, ip). Mean arterial pressure, diuresis, natriuresis, renal plasma flow, glomerular filtration rate, nitrites and nitrates excretion (NOx) were determined. NADPH diaphorase activity was measured using a histochemistry technique in kidney, aorta and renal arteries. NOS activity in kidney and aorta from expanded and non-expanded animals was determined with L-[U14C]-arginine substrate, in basal conditions and after DA (1 microM) administration.

RESULTS

The hypotensive effect of L-arg and hypertension induced by L-NAME were not modified by haloperidol. This blocker reverted the increase in diuresis, natriuresis and RPF induced by L-arg in both groups. Dopaminergic blockade induced a decrease in NOx excretion and in NADPH-diaphorase activity in glomeruli, proximal tubule and medullar collecting duct and in endothelium and vascular smooth muscle of renal arteries. DA induced an increase in NOS activity in renal medulla and cortex in both groups, but no changes in the aorta were observed.

CONCLUSIONS

Our results suggest that renal DA would be associated with the renal response induced by NO during extracellular volume expansion. NO-system activation would be one of the mechanisms involved in renal DA activity during saline load, but NO appears not to be involved in DA vascular effects.

Dopamine is metabolised by different enzymes along the rat nephron

The purpose of this study was to determine the basal levels of dopamine (DA) and to examine the enzymes involved in DA metabolism in different microdissected nephron segments from rat kidneys. Segments were incubated with DA (50 nM) or DA plus monoamine oxidase (MAO) or catechol-O-methyl transferase (COMT) inhibitors. Basal DA levels were higher in the proximal convoluted tubule (PCT, 10.8+/-3.7 pg/mm) and in the medullary collecting duct (MCD, 10.9+/-4.0 pg/mm) than in the medullary thick ascending limb of Henle's loop (MTAL, 4.9+/-0.9 pg/mm) (P<0.05). The percentage of exogenously added DA that was not metabolised was similar in both PCT (67+/-13%) and MCD (65+/-5%) and lower in MTAL (35+/-7%), suggesting that MTAL is a major site of DA metabolism. Inhibition of MAO (pargyline 1 mM) significantly increased the basal content of DA and the percentage of the added non-metabolised DA (to 95+/-10%) in PCT but had no effect on MTAL or MCD. Conversely, inhibition of COMT (nitecapone or Ro-41-0960, both 1 mM) slightly increased the basal levels of DA only in MTAL, whereas the percentage of added DA not metabolised rose to 97+/-10% in MTAL and to 91+/-15% in MCD. COMT inhibition had no effect in PCT. In conscious rats pargyline (50 mg/kg) increased urinary DA from 680+/-34 to 1,128+/-158 ng/d/100 g BW (P<0.01) while nitecapone (40 mg/kg) produced a slight non-significant increment. Our results show that DA is present all along the rat nephron and that renal DA is metabolised continuously and predominantly by MAO in proximal segments, and by COMT in the more distal ones.

Role of nitric oxide pathway in hypotensive and renal effects of furosemide during extracellular volume expansion

OBJECTIVE

In previous studies we demonstrated that the administration of furosemide associated with L-arginine contributes to enhanced hypotension and induces greater water than electrolyte excretion, in both normal and expansion conditions. The aim of the present study was to elucidate the interaction between furosemide and the nitric oxide (NO) system in renal and vascular responses during extracellular volume expansion.

DESIGN AND METHODS

Expanded [10% body weight (bw)] and non-expanded anaesthetized male Wistar rats were treated with furosemide (7.5 mg/kg bw). Mean arterial pressure, nitrite and nitrate excretion (NOx) were determined. NADPH-diaphorase activity, a marker of nitric oxide synthase (NOS) activity, was measured histochemically in different segments of the nephron, aorta and renal arteries. NOS activity was determined using an L-[U14C]-arginine substrate in the kidney and aorta of expanded and non-expanded rats, in basal conditions and after furosemide (10 micromol/l).

RESULTS

The hypotensive effect of furosemide was enhanced when NO production was stimulated in expanded and non-expanded animals. The diuretic treatment induced a significant increase in NOx excretion, in NADPH-diaphorase activity in the thick ascending limb of Henle, renal arteries and aorta, and in NOS activity in aorta and kidney in both groups.

CONCLUSIONS

Our results suggest that the hypotensive effect of furosemide may be attributed to NO-mediated vasodilation. The enhanced NOS activity, observed in the renal artery of furosemide-treated rats, could explain the increased renal plasma flow induced by furosemide. In addition, NO-pathway stimulation in the kidney could be one of the mechanisms by which furosemide exerts its diuretic and natriuretic effects, in control and in expansion conditions.

Renal and Vascular Nitric Oxide System in Reduced Renal Mass Saline Hypertension

BACKGROUND

Reduction in renal mass is associated with several structural and functional adaptations including compensatory renal growth and hemodynamic changes. The mediators of the renal hemodynamic adaptations have not been definitively identified. Several investigators have postulated that nitric oxide (NO) is involved this physiological mechanisms. The purpose of this study was to evaluate the role of vascular and renal NO pathway in the model of subtotal nephrectomy-salt load hypertension.

MATERIALS AND METHODS

Wistar rats with 75% renal mass reduction (RMR) and saline load were studied during 4 weeks. Weekly, indirect systolic blood pressure (SBP) were measured. One week after nephrectomy, animals were divided in two groups, hypertensive (SBP > 140 mm Hg) and normotensive (SBP < 140 mm Hg). Urinary excretion of nitrates and nitrites (NOx), urinary chemioluminiscence levels and NOS activity in the left kidney and in the thoracic aorta artery were determined at the fourth week after subtotal nephrectomy.

RESULTS

Urinary excretion of sodium was higher in normotensive rats than hypertensive rats and in both groups this parameter was higher than in sham rats. NOx excretion and NOS activity in the different nephron segments were higher in normotensive rats than in the hypertensive ones. In contrast, NOS activity in aorta sections and urinary chemiluminescence levels in hypertensive animals were enhanced compared with normotensive rats. These parameters were higher in both groups of nephrectomized rats than in sham ones.

CONCLUSION

This study provides evidence to support the fact that the activation of the renal NO system is an important mechanism whereby the remnant kidney regulates sodium and water balance, contributing to control the arterial blood pressure in the renal mass reduction and saline load model.