Unimpeded plasma renin increase after intravenous furosemide during saline replacement

Direct vascular effects of furosemide in humans

BACKGROUND

In humans, hemodynamic changes observed within minutes after systemic administration of furosemide are often referred to as direct vasoactivity. However, these immediate changes do not per se imply a direct vascular effect. We examined the genuine direct vascular effects of furosemide on the human forearm vascular bed and dorsal hand vein.

METHODS AND RESULTS

Forearm blood flow in response to infusion of increasing dosages of furosemide into the brachial artery was recorded by venous occlusion plethysmography. Local plasma concentrations of furosemide reached a maximum of 234+/-40 microg/mL during the highest infused dose but did not significantly affect the ratio of flow in the infused/noninfused arms. Venous distensibility of a dorsal hand vein was measured with a linear variable differential transformer. During precontraction with norepinephrine, five increasing dosages of furosemide (1 to 100 microg/min) were administered locally. Additional experiments using local administration of indomethacin or N(G)-monomethyl-L-arginine (L-NMMA) were carried out to determine whether effects were dependent on local prostaglandin or nitric oxide synthesis, respectively. Also, the effects of systemic administration of furosemide were examined. Local administration of furosemide led to a dose-dependent venorelaxation of 18+/-6% at the first to 72+/-16% at the last dose. Indomethacin almost completely abolished furosemide-induced venorelaxation, whereas L-NMMA had no effect. Systemic administration of furosemide resulted in a time-dependent increase of hand vein distensibility, reaching 45+/-11% after 8 minutes.

CONCLUSIONS

Furosemide does not exert any direct arterial vasoactivity in the human forearm, even at supratherapeutic concentrations. In contrast, at concentrations estimated to be in the therapeutic range, we observed a dose-dependent direct venodilator effect on the dorsal hand vein that appears to be mediated by local vascular prostaglandin synthesis.

Inhibition of vasoconstriction by frusemide in the rat

Mesenteric blood flow was measured in anaesthetized rats with a non-cannulating electromagnetic flow probe around the superior mesenteric artery. Reductions in blood flow were produced by intravenous bolus injections of angiotensin II (1-300 ng) and noradrenaline (3-300 ng) before and after the administration of frusemide (5 mg kg-1, i.v.). Loss of volume after frusemide was prevented by either a urinary bladder-intravenous shunt or replacement of urinary output by intravenous saline. Frusemide administration caused a small increase in baseline blood pressure of 3.2 +/- 1.3 mmHg (P less than 0.05) but did not change mesenteric blood flow. This dose of frusemide inhibited the vasoconstrictor responses to both angiotensin II and noradrenaline (P less than 0.01, two way analysis of variance). Responses to angiotensin II were inhibited to a greater extent. Acute bilateral nephrectomy or treatment with indomethacin (2 mg kg-1, i.v.) completely prevented the inhibitory effect of frusemide on the responses to angiotensin II and noradrenaline. To test whether frusemide-induced increased endogenous levels of angiotensin II may be responsible for the effects of frusemide on the vasoconstrictor responses, a separate group of rats were not given frusemide but were infused with exogenous angiotensin II (12.5-25 ng kg-1 min-1). This produced a small increase in mean blood pressure (4.0 +/- 1.4 mmHg, P less than 0.05) but did not change baseline mesenteric blood flow. Unlike frusemide, the responses to bolus injections of angiotensin II and noradrenaline were not changed by the infusion of angiotensin II. 5 It is suggested that frusemide may release directly or indirectly a prostanoid from the kidney (or a substance from the kidney which leads to the formation of a prostanoid) which inhibits constrictor responses in the peripheral vasculature.

Sequential changes in plasma renin activity and plasma catecholamines in mildly hypertensive patients during acute, furosemide-induced body-fluid loss

To evaluate the role of adrenergic mechanisms in the acute response of renin to furosemide, plasma renin activity (PRA) and plasma catecholamine concentrations were measured for 3 h after i.v. administration of furosemide 1 mg/kg to 8 patients with mild essential hypertension. Furosemide induced a prompt and long-lasting increase in renin, with PRA more than doubled at all times. The increase in PRA within the first 30 min paralleled the peak increases in urinary water and sodium flow rates, and significant decreases in plasma volume and central venous pressure. There was no change in plasma catecholamine concentrations. Plasma noradrenaline was increased significantly at 60 min and adrenaline at 90 min, once furosemide had induced a marked loss of body-fluid and approximately 65% decrease in central venous pressure. Both catecholamines remained elevated until the end of the study, whereas urinary water and sodium flow rates had returned to their pre-treatment values by 150 min. Mean blood pressure was essentially unchanged throughout the study, whereas heart rate increased significantly after 90 min. The findings suggest that in mildly hypertensive patients adrenergic mechanisms are not involved in the initial renin response to furosemide, but they come into play later, probably as a result of reflex sympathetic activation triggered by marked volume depletion.

Treatment of diabetic orthostatic hypotension with pindolol

The hemodynamic variables, plasma noradrenaline and plasma renin concentrations were studied in a 57-year-old female with insulin-dependent diabetes of long-standing and orthostatic hypotension. For 6 months she had been bedridden because of severe orthostatic symptoms. Reflex responses in the heart rate and blood pressure during the Valsalva manoeuvre as well as beat-to-beat variations in heart rate were absent. Plasma noradrenaline concentrations were subnormal both in the supine and upright positions. After treatment with Pindolol 15 mg/day the patient was able to walk around and lead an almost normal life. The orthostatic symptoms recurred after withdrawal of therapy and disappeared on resumption of therapy. Arterial blood pressure measured intra-arterially decreased less in the up-right position during treatment with pindolol compared to that in the untreated condition. The heart rate did not change during treatment with Pindolol. Our findings suggest that Pindolol may be an important drug in the treatment of diabetic orthostatic hypotension.

Lack of adrenergic influence on renin release after furosemide in normal man

The role of the sympathetic nervous system in furosemide-induced renin release was investigated in six normal subjects. After intravenous administration of furosemide, plasma renin concentrations increased more than two-fold within 15 min. Neither replacement of urinary fluid loss by intravenous infusion of saline nor pharmacological beta-blockade with d,1-propranolol changed the renin response to furosemide. The activity of the sympathetic nervous system, as estimated by measurement of plasma catecholamine concentrations, remained at the reference level after furosemide. It is concluded that the sympathetic nervous system is not involved in renin release after intravenous administration of furosemide.

Stimulation of renin by acute selective chloride depletion in the rat

To determine whether acute chloride depletion per se stimulates renin, we produced selective chloride depletion without sodium depletion in rats by peritoneal dialysis (PD) against 0.15 M NaHCO3 or 0.15 M NaNO3. Control rats were dialyzed against 0.15 M NaCl. Plasma renin activity (PRA) was measured before (PRA1) and 105 minutes after (PRA2) PD. Plasma volume was expanded after PD by infusion of salt-free albumin and was measured immediately after PRA2 by [131I]albumin. In experiment 1, rats were prepared on a normal diet. PRA2 (7.0 +/- 1.0 ng/ml per hr, mean +/- SEM) was increased (P less than 0.05) over PRA1 (4.7 +/- 0.7 ng/ml per hr) in Cl-depleted but not in control rats (PRA1 = 5.3 +/- 0.7, PRA2 = 6.1 +/- 0.7, P = NS). In experiment 2, to produce greater chloride depletion, all rats were prepared for 2 weeks on a low salt diet. PRA2 (47 +/- 5 ng/ml per hr) was increased as compared to PRA1 (24 +/- 2 ng/ml per hr, P less than 0.005) in the Cl-depleted group but not in the control group (PRA1 = 24 +/- 3, PRA2 = 27 +/- 6 ng/ml per hr, P = NS). Serum potassium and final plasma volume were slightly but not significantly lower than controls in these Cl-depleted rats. To exclude an additive effect of these two stimuli for renin, in experiment 2a we infused chloride-depleted rats with three times as much albumin as controls and with KHCO3, 100 mEq/liter. Despite volume expansion and potassium loading, PRA2 (41 +/- 6 ng/ml per hr) was significantly elevated as compared to PRA1 (25 +/- 4 ng/ml per hr, P less than 0.01). Since acute metabolic alkalosis also was present in all Cl-depleted renin-stimulated rats, an additional group (2b) was dialyzed against 0.15 M NaNO3; final plasma arterial pH (7.43) was not different from controls (7.42). Nevertheless, PRA2 levels again were higher (36 +/- 6 ng/ml per hr, P less than 0.05) as compared to PRA1 (23 +/- 4 ng/ml per hr). In all experiments, arterial blood pressure, glomerular filtration rate, and filtered sodium load were not different. Free water reabsorption was lower in Cl-depleted than in control rats. We conclude that acute selective chloride depletion per se is a potent stimulus for renin release.