Abnormal tubular handling of sodium and water induced by atrial natriuretic peptide in essential hypertension

Pressure-dependent, enhanced natriuretic response to low-dose, atrial natriuretic peptide infusion in essential hypertension

OBJECTIVE

To examine whether the effect of atrial natriuretic peptide (ANP) on renal glomerular and tubular segmental handling of sodium in patients with essential hypertension is pressure dependent.

DESIGN

Part 1. The renal effects of a low-dose continuous infusion (10 ng kg-1 min-1) with ANP for 1 h were compared in 10 untreated essential hypertensives (EH) and 13 normotensive control subjects (CS). Part 2. The hypertensives were studied on another day with ANP infusion during preceding acute BP reduction with sodium nitroprusside infusion (NP). The results were compared with those obtained during infusion with ANP+placebo (Part 1).

METHODS

Lithium clearance was used to estimate the proximal tubular reabsorption of sodium.

RESULTS

Part 1. Atrial natriuretic peptide caused an exaggerated increase in urinary sodium excretion (+102 vs. +38%: P < 0.05), fractional excretion of sodium (+80 vs. +37%: P < 0.05), and urinary output (+56 vs. +8.3%; P < 0.05) in EH compared with CS. Glomerular filtration rate and filtration fraction increased to the same degree in both groups. Absolute lithium clearance (CLi) increased and FELi tended to increase (P = 0.061) in EH, but these were unchanged in CS. The increase in plasma cyclic guanosine 5'-phosphate (cGMP) and urinary excretion of cGMP and the decrease in plasma aldosterone during ANP infusion were the same in the two groups. Part 2. During NP infusion the natriuresis caused by ANP in EH was reduced (+51 vs. +99%; P < 0.05). The relative changes in GFR, CLi, and FELi during ANP infusion were not affected by the preceding BP reduction with NP. Mean arterial pressure was reduced from 122 to 101 mmHg during NP infusion. The relative increase in sodium excretion in EH was significantly correlated to mean arterial pressure.

CONCLUSIONS

Low-dose ANP infusion causes an exaggerated natriuresis in untreated essential hypertensives due to a more pronounced reduction in tubular reabsorption. After BP reduction, the natriuresis induced by ANP in essential hypertensives is decreased, probably due to a less pronounced reduction in tubular reabsorption beyond the proximal tubules. We suggest that the enhanced natriuretic response to ANP in EH in secondary in some degree to the elevated systemic pressure.

Is ANP responsible for the hemoconcentration in preeclampsia?

A hypothesis is presented that high plasma concentration of alpha-human Atrial Natriuretic Peptide (ANP) might be responsible for hemoconcentration of preeclampsia (PE), mediated by increased diuresis and shift of fluid to the interstitium. ANP is known as a peptide regulating blood volume by diuresis and natriuresis, and perhaps through increased permeability in vessels. Moreover ANP is an antagonist to the renin-angiotensin-aldosterone (RAA) system, especially when this system is activated quantitatively as in pregnancy or qualitatively as in PE.