Fludrocortisone suppression of sympathetic nervous activity

Does infusion of ANG II increase muscle sympathetic nerve activity in patients with primary aldosteronism?

Patients with primary aldosteronism (PA) were shown to have suppressed muscle sympathetic nerve activity (MSNA) in our previous study. Although baroreflex inhibition probably accounts in part for this reduced MSNA in PA, we hypothesized that the lowered activity of the renin-angiotensin system in PA may also contribute to the suppressed SNA. We recorded MSNA in 9 PA and 16 age-matched normotensive controls (NC). In PA, the resting mean blood pressure (MBP) and serum sodium concentrations were increased, and MSNA was reduced. We examined the effects of infusion of a high physiological dose of ANG II (5.0 ng.kg(-1).min(-1)) on MSNA in 6 of 9 PA and 9 of 16 NC. Infusion of ANG II caused a greater pressor response in PA than NC, but, in spite of the greater increase in pressure, MSNA increased in PA, whereas it decreased in NC. Simultaneous infusion of nitroprusside and ANG II, to maintain central venous pressure at the baseline level and reduce the elevation in MBP induced by ANG II, caused significantly greater increases in MSNA in PA than in NC. Baroreflex sensitivity of heart rate, estimated during phenylephrine infusions, was reduced in PA, but baroreflex sensitivity of MSNA was unchanged in PA compared with NC. All the abnormalities in PA were eliminated following unilateral adrenalectomy. In conclusion, the suppressed SNA in PA depends in part on the low level of ANG II in these patients.

Acute effects of aldosterone on the autonomic nervous system and the baroreflex function in healthy humans

Aldosterone has been reported to impair the baroreflex response in animal models. The present study aimed to investigate the acute effects of aldosterone on the autonomic nervous system and the baroreflex control of muscle sympathetic nerve activity (MSNA) and heart rate in healthy humans. Nine healthy subjects were examined in a double-blind, placebo-controlled, cross-over study design, receiving either i.v. aldosterone (100 microg) or placebo on the experimental day. Heart rate variability (HRV) was measured at rest, whereas blood pressure, heart rate and MSNA (assessed by microneurography from the peroneal nerve) were monitored both at rest and during baroreflex tests. Baroreceptor stimulation and deactivation was induced by i.v. infusion of incremental doses of phenylephrine and sodium nitroprusside. HRV indices at rest were specifically changed by aldosterone with a significant increase in standard deviation of RR intervals and total power, and a trend towards increased time domain parameters indicating parasympathetic predominance in heart rate regulation. Basal MSNA, blood pressure and heart rate remained unaffected by aldosterone administration. Sodium nitroprusside decreased diastolic blood pressure and increased MSNA as well as heart rate in both the aldosterone and placebo experiments. However, the tachycardic response to arterial baroreceptor deactivation was more pronounced in the aldosterone experiments. By contrast, baroreflex control of MSNA and heart rate during phenylephrine infusion was not affected by aldosterone. Thus, our study demonstrates that, in healthy humans, aldosterone tends to increase cardiac vagal activity and enhances the heart rate response to nitroprusside whereas MSNA remains unaffected.

Normal sympathetic vasomotor and cardiac parasympathetic activities in patients with primary aldosteronism: assessment by spectral analysis

The role of the autonomic nervous system in hypertension due to mineralocorticoid excess remains unclear. To address this issue, we performed power spectral analysis of blood pressure (BP) and RR interval oscillations in 20 patients with primary aldosteronism (PA), 54 patients with essential hypertension (EH) and 45 normotensive (NT) subjects. Blood pressure and the degree of organ damage were similar between PA and EH groups. Age did not differ between the three groups. The Mayer wave power spectrum (MWP) of BP (approx. 0.1 Hz), an index of sympathetic vasomotor tone, was smaller in patients with PA than in patients with EH either while subjects were supine (systolic/diastolic; 3.9 +/- 3.2 (SD)/1.5 +/- 1.3 vs. 5.5 +/- 4.2/2.1 +/- 1.6 mmHg2, P < 0.05 for both) or standing (7.6 +/- 6.6/3.0 +/- 3.0 vs. 17.7 +/- 23.7/7.2 +/- 8.3 mmHg2, P < 0.05 for both). Supine respiratory-related power spectrum (RRP) of the RR interval (approx. 0.25 Hz), an index of cardiac parasympathetic tone, was greater in patients with PA than in patients with EH (545 +/- 574 vs. 302 +/- 464 ms2, P < 0.01). The MWP of BP and the RRP of the RR interval were similar between patients with PA and NT subjects. Adrenalectomy reduced the 24-h mean BP (-18 mmHg for systolic BP, P < 0.001; -12 mmHg for diastolic BP, P < 0.01) and increased the 24-h mean heart rate (+8 bpm, P < 0.001). Furthermore, the diastolic MWP increased mildly (+32%, P < 0.05) and the RRP of the RR interval decreased dramatically (-75%, P < 0.01) following adrenalectomy. These results suggest that both vascular sympathetic and cardiac parasympathetic regulatory systems have minor roles in the maintenance of hypertension in patients with PA. The autonomic nervous system contributes more to the maintenance of BP following than prior to adrenalectomy. This information may be useful for the management of hypertension still persists after removal of adrenal adenoma.

Mineralocorticoid induced hypertension and noradrenaline spillover in man

This study examined haemodynamics and noradrenaline spillover in five normal men before and on day 7 of oral fludrocortisone treatment, 0.3 mg/day. Resting systolic (105 to 115 mm Hg, standard error of the difference +/- 2.0, p < 0.01) and diastolic (65 to 73 mm Hg, +/- 3.0, p < 0.05) blood pressure increased, as did cardiac output, from 5.0 to 5.7 L/min (+/- 0.1, p < 0.01). Calculated total peripheral resistance fell from 21.2 to 20.0 mm Hg/L/min (+/- 0.4, p < 0.05). Fludrocortisone produced a fall in plasma potassium, renin and aldosterone concentrations and haematocrit and a rise in body weight. Cold pressor responses were increased by fludrocortisone, from 7.5 to 20 mm Hg (+/- 3.0, p < 0.01), and forearm vascular resistance rose 12 arbitrary resistance units (R) before and 36 R units after treatment (+/- 5.0, p < 0.01). Total body spillover of noradrenaline was decreased from 9.48 to 7.36 ng/kg/min (+/- 0.86, p < 0.05). There were no changes in forearm noradrenaline spillover at rest or during cold pressor stimulation. It appears unlikely that the sympathetic nervous system plays a major role in the pathogenesis of mineralocorticoid hypertension in man.

Advantages and disadvantages of fludrocortisone or saline load in preventing post-spaceflight orthostatic hypotension

The purpose of this study was to compare the effectiveness of saline load to fludrocortisone (florinef) as countermeasures for reduced plasma volume and orthostatic intolerance after spaceflight. Eleven males (ages 30-50 yr) underwent a 3-day ambulatory baseline period followed by 7 days of 6 degrees head-down bedrest, during which cardiopulmonary and arterial baroreflex sensitivity and plasma volume (PV) were determined. During pre-bedrest and 2.5 h after treatment on day 8, PV was also measured and subjects underwent a 15-min unsupported stand test. Treatments consisted of 8 salt tablets (1 g NaCl per tablet) and 960 ml of water in 5 subjects and 0.6 mg (0.2 mg x 3) over 24 h in the other 6 subjects. PV decreased by 12% on day 7 of bedrest. This was restored on day 8 by florinef but not by saline load. The effect of florinef on PV was paralleled by decreases in urine volume and the urinary sodium/potassium ratio. Reduced PV was associated with greater vascular resistance for the same drop in central venous pressure, suggesting less vasoconstriction reserve after bedrest. Carotid baroreflex control of heart rate was attenuated after 7 days of bedrest. Both baroreflex functions were restored by florinef but not saline load. Only 1 of 6 subjects showed syncopal symptoms in the florinef-treated group, whereas 4 of 5 subjects did so in the saline-load group. Acute florinef treatment appears to have distinct advantages as a protective measure for post-bedrest orthostatic intolerance, not only through its salt retaining, volume-expanding mineralcorticoid effect, but possibly through its actions on baroreflex and sympathetic functions.

Effects of fludrocortisone on sympathetic nerve activity in humans

Fludrocortisone reduces plasma norepinephrine in healthy humans, but forearm vascular and pressor responses to norepinephrine are potentiated. The effects of fludrocortisone on sympathetic nerve activity in healthy humans are not known. To investigate these effects we evaluated muscle sympathetic nerve activity, heart rate, and arterial pressure in 11 healthy volunteers during three protocols: (1) before and on day 7 of fludrocortisone (0.4 mg/d) treatment with ad libitum diet (n = 6); (2) before and on day 7 of fludrocortisone (0.4 mg/d) or placebo with a 150 mmol/24 h (mEq/24 h) sodium diet (n = 7); and (3) before and on day 2 of fludrocortisone (0.4 mg/d) or placebo with a 150 mmol/24 h (mEq/24 h) sodium diet (n = 4). Placebo did not alter any parameter.(ABSTRACT TRUNCATED AT 250 WORDS)