Differential effects of atrial natriuretic peptide infusion at physiological doses in different postures in humans

The influence of posture on the estimation of daily salt intake by the second morning urine method

The second morning urine (SMU) method was developed to evaluate daily salt intake, but the posture that should be adopted until the SMU collection remains unclear. This study investigated the influence of posture in hypertensive patients who underwent this test. The subjects were 100 patients who could collect 24-h urine samples correctly and were on a diet containing 7 g of salt per day. Their daily salt intake was estimated for three consecutive days in the recumbent, sitting, and sitting and standing positions (one posture each day). Estimated salt intake in the recumbent position (10.9+/-2.4 g day(-1)) was higher than in the sitting position (7.5+/-2.0 g day(-1)) and the sitting and standing position (6.3+/-1.7 g day(-1)). The salt intake estimated in the sitting and standing position was similar to that obtained by 24-h urine collection (6.3+/-1.6 g day(-1)) and was significantly (r=0.44, P<0.05) correlated with the 24-h urine value. The actual difference in estimated salt intake between the two methods was 0.0+/-1.7 g day(-1). There were no significant differences in estimated salt intake between the two methods in patients taking different classes of antihypertensive drugs. In conclusion, adopting the sitting and standing position until the SMU collection is important for the correct estimation of daily salt intake, and this method could replace the 24-h collection method because of its convenience, especially in outpatients.

Effect of age on renal functional and orthostatic vascular response in healthy men

1. Few studies have been reported concerning the effect of ageing on renal functional and vascular responses to various stresses during ordinary life. In the present study, we examined the effect of age on changes in renal sodium handling and renal vascular resistance (RVR) in response to standing from a supine position in subjects with normal renal function. 2. We selected 43 healthy males in the second through to the seventh decade of life and gave them a constant dietary sodium intake before the study period. Renal function was estimated by standard clearance methods with the subject in a state of euvolaemia. 3. The mean daily urinary excretion of sodium was 236 +/- 22 mEq. Standing from a supine position was associated with significant decreases (P < 0.0001) in creatinine clearance (from 125 +/- 18 to 117 +/- 19 mL/min per 1.73 m2), sodium excretion (from 178 +/- 29 to 97 +/- 23 microEq/min) and fractional excretion of sodium (from 1.02 +/- 0.19 to 0.60 +/- 0.13%). A significant increase (P < 0.0001) in the RVR index (from 0.11 +/- 0.03 to 0.14 +/- 0.04 units) was noted. Univariate analysis indicated that while the change in RVR associated with standing was significantly diminished (P < 0.05) in older subjects, orthostatic changes in other parameters associated with standing were minimally influenced by age. 4. In conclusion, although the renal vascular response is impaired in advanced age, the renal functional response to orthostasis is otherwise maintained in healthy elderly subjects under conditions of normal sodium intake and clinical euvolaemia.

Effects of ANF infusion on the renal responses to lower-body negative pressure in humans

To investigate the role of atrial natriuretic factor (ANF) in renal responses to a decrease in central blood volume, we examined the effects of ANF infusion on renal function and hormones during prolonged lower-body negative pressure (LBNP). Ten healthy volunteers participated in two experimental sequences, each comprising a 120-min baseline period followed by the application of -20 mm Hg LBNP for 90 min. During one of the two sequences, ANF was infused throughout LBNP application at the constant rate of 2.5 ng/kg/min. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured by using inulin and p-aminohippuric acid clearance techniques. LBNP induced a significant decrease in ERPF (534 +/- 28 to 457 +/- 26 ml/min; p < 0.05), GFR (120 +/- 2.5 to 112 +/- 2.5 ml/min; p < or = 0.01), in urine excretion (12 +/- 0.9 to 5.6 +/- 0.5 ml/min; p < 0.001), in sodium excretion (0.36 +/- 0.03 to 0.30 +/- 0.02 mmol/min; p < 0.05), and in plasma ANF (19 +/- 3 to 11 +/- 2 pg/ml; p = 0.001) concomitant with an increase in plasma renin activity (PRA; 0.48 +/- 0.09 to 0.87 +/- 0.16 ng/ml/h; p = 0.01) and of forearm vascular resistance (FVR; p < 0.05). The combination of ANF infusion with LBNP led to a slight increase in plasma ANF from baseline (from 20 +/- 2 to 28 +/- 3 pg/ml; p < 0.05). Compared with values obtained during LBNP with saline vehicle infusion, values obtained during LBNP with ANF infusion were similar for ERPF (463 +/- 23 vs. 457 +/- 26 ml/min), for GFR (111 +/- 2 vs. 112 +/- 2 ml/min), and for urine excretion (7 +/- 0.6 vs. 5.6 +/- 0.5 ml/min; p = 0.07), but greater for fractional excretion of sodium (2.38 +/- 0.25% vs. 1.91 +/- 0.11%; p < 0.05) and FVR (p < 0.05), and smaller for PRA (0.49 +/- 0.1 vs. 0.87 +/- 0.16 ng/ml/h; p < 0.01). These data show that ANF infusion attenuates the antinatriuretic effect of low-level LBNP and its PRA-increasing effects without altering renal hemodynamic responses to LBNP, although there is a decrease in the LBNP-induced forearm vasoconstriction. These results were obtained with plasma ANF levels slightly higher than those in baseline. They support the hypothesis that a decrease in ANF secretion might contribute to the antinatriuretic effect of LBNP.

Vasoactive peptides and uterine vessels

Endothelins and atrial natriuretic peptide (ANP) are vasoactive peptides with effects on the human uterine and umbilical arteries. Endothelin (ET) contracts the vascular smooth muscle. Both ETA- and non-ETA-non-ETB-receptors seem to be involved. Autoradiography reveals binding of ET to vascular smooth muscle. ANP counteracts the contractile effects of angiotensin II in the human uterine artery. Head-down tilt results in elevation of plasma ANP in healthy pregnant women, while the same manoeuvre induces down-regulation of the reninangiotensin-aldosterone system in non-pregnant women and patients suffering from pre-eclampsia.

Effects of head-down tilt on atrial natriuretic peptide and the renin system in pregnancy

We studied the effects of head-down tilt to 10 degrees for 30 minutes on plasma atrial natriuretic peptide and the renin-aldosterone system in 8 preeclamptic pregnant women, 8 healthy pregnant women, and 11 nonpregnant women of fertile age. Mean arterial blood pressure did not change in the pregnant groups but increased significantly in the nonpregnant control subjects. Heart rate decreased significantly in preeclamptic women but remained unchanged in both control groups. Baseline atrial natriuretic peptide concentration was significantly higher in both preeclamptic (66 +/- 4 pmol/L) and pregnant (54 +/- 6 pmol/L) control subjects compared with nonpregnant subjects (40 +/- 2 pmol/L), but the difference between the pregnant groups was not significant. Head-down tilting induced a significant increase in atrial natriuretic peptide only in healthy pregnant women. Baseline plasma renin activity and aldosterone concentrations were significantly higher in pregnant control subjects compared with both the preeclamptic and nonpregnant groups. The differences between the preeclamptic and nonpregnant control groups were nonsignificant. After head-down tilting, plasma renin activity decreased significantly only in nonpregnant control subjects, whereas aldosterone decreased significantly in preeclamptic and nonpregnant control subjects. In preeclampsia, atrial natriuretic peptide release followed blood pressure and not changes in cardiac output. When all 27 women were studied, a correlation between atrial natriuretic peptide and mean arterial pressure was found in the left lateral supine position. The results suggest that pregnant women developing preeclampsia lose their usual hemodynamic control and show reactions resembling the nonpregnant state when subjected to head-down tilt.