Salt-induced plasticity in cardiopulmonary baroreceptor reflexes in salt-resistant hypertensive patients

Post-Ganglionic Sympathetic Neurons can Directly Sense Raised Extracellular Na+ via SCN7a/Nax

The relationship between dietary NaCl intake and high blood pressure is well-established, and occurs primarily through activation of the sympathetic nervous system. Nax, a Na+-sensitive Na+ channel, plays a pivotal role in driving sympathetic excitability, which is thought to originate from central regions controlling neural outflow. We investigated whether post-ganglionic sympathetic neurons from different ganglia innervating cardiac and vasculature tissue can also directly sense extracellular Na+. Using whole-cell patch clamp recordings we demonstrate that sympathetic neurons from three sympathetic ganglia (superior cervical, stellate and superior mesenteric/coeliac) respond to elevated extracellular NaCl concentration. In sympathetic stellate ganglia neurons, we established that the effect of NaCl was dose-dependent and independent of osmolarity, Cl- and membrane Ca2+ flux, and critically dependent on extracellular Na+ concentration. We show that Nax is expressed in sympathetic stellate ganglia neurons at a transcript and protein level using single-cell RNA-sequencing and immunohistochemistry respectively. Additionally, the response to NaCl was prevented by siRNA-mediated knockdown of Nax, but not by inhibition of other membrane Na+ pathways. Together, these results demonstrate that post-ganglionic sympathetic neurons are direct sensors of extracellular Na+ via Nax, which could contribute to sympathetic driven hypertension.

Low-Sodium DASH reduces oxidative stress and improves vascular function in salt-sensitive humans

Salt induces oxidative stress in salt-sensitive (SS) animals and man. It is not known whether in SS subjects the low-sodium dietary approaches to stop hypertension (LS-DASH) reduces oxidative stress more than DASH, which is high in antioxidants. To assess the effects of DASH and LS-DASH on oxidative stress, 19 volunteers were studied after 3 weeks of a standardized usual low fruits and vegetables diet (ULFV), followed by 3 weeks on DASH (both diets approximately 120 mmol Na(+) per day), then 3 weeks on LS-DASH (60 mmol Na(+) per day). SS was defined as systolic blood pressure >or=5 mm Hg lower on LS-DASH than DASH. In SS subjects (N=9), systolic blood pressure was lower on LS-DASH (111.0+/-2.0 mm Hg) than DASH (118.0+/-2.2, P<0.01) and ULFV (122.3+/-2.7, P=0.002). In salt-resistant (SR) volunteers (N=10), systolic blood pressure was lower on DASH (113.0+/-1.6) than ULFV (119.0+/-1.8, P<0.05) but not LS-DASH (115.7+/-1.8). Urine F2-isoprostanes, a marker of oxidative stress, were lower in SS subjects on LS-DASH (1.69+/-0.24) than ULFV (3.09+/-0.50, P<0.05) and marginally lower than DASH (2.46+/-0.44, P<0.20). F2-isoprostanes were not different among the three diets in SR volunteers (2.18+/-0.29, 2.06+/-0.29, 2.27+/-0.53, respectively). Aortic augmentation index, a measure of vascular stiffness, was lower in SS subjects on LS-DASH than either DASH or ULFV, and lower on DASH than ULFV in SR volunteers. In SS but not SR subjects, LS-DASH is associated with lower values for F2-isoprostanes and the aortic augmentation index. The results suggest that LS-DASH decreases oxidative stress, improves vascular function and lowers blood pressure in SS but not SR volunteers.

Association between SNP heterozygosity and quantitative traits in the Framingham Heart Study

Associations between multilocus heterozygosity and fitness traits, also termed heterozygosity and fitness correlations (HFCs), have been reported in numerous organisms. These studies, in general, indicate a positive relationship between heterozygosity and fitness traits. We studied the association between genome-wide heterozygosity at 706 non-synonymous and synonymous SNPs and 19 quantitative traits, including morphological, biochemical and fitness traits in the Framingham Heart Study. Statistically significant association was found between heterozygosity and systolic and diastolic blood pressures as well as left ventricular diameter and wall thickness. These results suggest that heterozygosity may be associated with traits, such as blood pressure that closely track environmental variations. Balancing selection may be operating in the maintenance of heterozygosity and the major components of blood pressure and hypertension. Genome wide SNP heterozygosity may be used to understand the phenomenon of dominance as well as the evolutionary basis of many quantitative traits in humans.

Effects of salt sensitivity on neural cardiovascular regulation in essential hypertension

Salt-sensitive hypertensive subjects, as defined by conventional categorical classification, exhibit alterations of autonomic cardiovascular control. The aim of our study was to explore whether, in hypertensive subjects, the degree of autonomic dysfunction and the level of salt sensitivity are correlated even when the latter is only mildly elevated and displays under-threshold values. Salt sensitivity of 34 essential hypertensive subjects was assessed on a continuous basis by the salt sensitivity index after low- and high-sodium diet. Beat-by-beat finger blood pressure was recorded after each diet period. Autonomic cardiovascular control was evaluated by spectral analysis of blood pressure and pulse interval and by assessment of spontaneous baroreflex sensitivity (sequence technique). Salt sensitivity and baroreflex sensitivity showed a negative relationship during low and high sodium intake, starting from low values of the salt sensitivity index. All spectral indexes of pulse interval, except the ratio between low- and high-frequency powers, were inversely related to salt sensitivity index after high sodium intake. In subjects with lower salt sensitivity, baroreflex sensitivity and pulse interval power in the high-frequency band were higher after high sodium intake than after low sodium intake. In contrast, subjects with a higher salt sensitivity index showed lower values of baroreflex sensitivity and pulse interval power in the high-frequency band, uninfluenced by salt intake. Our results provide the first demonstration of an impairment of parasympathetic cardiac control in parallel with the increase in the degree of salt sensitivity, also in subjects who were not ranked as salt-sensitive by the conventional categorical classification.

The diuretic chlorthalidone normalizes baroreceptor and Bezold-Jarisch reflexes in DOCA-salt hypertensive rats

The contributions of arterial baroreceptor and Bezold-Jarisch reflexes, and atrial natriuretic factor (ANF) to the anti-hypertensive effect of the diuretic chlorthalidone were investigated in rats with deoxycorticosterone acetate (DOCA)-salt-induced hypertension. Chlorthalidone (8 mg rat(-1)day(-1)added to food) was given to one group during all 20 days of DOCA (8 mg kg(-1)s.c. twice per week) administration (preventive regimen) and, to another group, 20 days after DOCA treatment was initiated until the 40th day (therapeutic regimen). DOCA caused a significant increase in mean arterial pressure, reduced arterial baroreflex, and increased both the Bezold-Jarisch reflex and pro-ANF converting enzyme activity. Chlorthalidone reversed or prevented the DOCA-salt-induced hypertension, which was accompanied by the normalization of both the arterial baroreflex and the Bezold-Jarisch reflex. Additionally, both preventive and therapeutic regimens with chlorthalidone did cause normalization of the plasma sodium concentration and pro-ANF converting enzyme activity in the left atrium that follows DOCA-salt hypertension. Although it is difficult to determine the relative importance of each of the above regulatory mechanisms altered by chlorthalidone treatment, these data indicate that they may account for the prevention or decrease of DOCA-salt-induced hypertension in rats.

Left ventricular mass and heart rate variability in middle-aged and elderly salt-sensitive hypertensive subjects

Previous reports have shown that in salt-sensitive hypertension a high dietary salt intake can increase sympathetic activity. We evaluated the influence of the autonomic nervous system on myocardial hypertrophy by power spectral analysis of heart rate variability in middle-aged and elderly salt-sensitive hypertensive subjects. We compared autonomic nervous system activity in 32 salt-sensitive hypertensive patients (15 subjects with mean age, 42.4+/-2.4 years and 17 subjects with mean age, 74.6+/-1.6 years) and 20 age-matched normotensive controls. Power spectral analysis detects four spectral components: total power (TP), high-frequency (HF), low-frequency (LF) and very-low-frequency (VLF) power. In the elderly subjects we found an association between the left ventricular mass index (LVMI) and the following variables: very-low frequency (P<0.0001), 24-h urinary sodium excretion (P<0.0001) and diastolic blood pressure (DBP) (P<0.0001). In contrast, in middle-aged subjects we found a significant association between the LVMI and LF (P<0.001). In middle-aged, but not in elderly salt-sensitive hypertensive subjects, increased sympathetic activity correlated with the LVMI (P<0.0001). Our findings suggest an association between sympathetic hyperactivity and the LVMI in middle-aged subjects with salt-sensitive hypertension.

Distinct vasodilation, without reflex neurohormonal activation, induced by barnidipine in hypertensive patients

Barnidipine is a new 1,4-dihydropyridine calcium antagonist with a strong and long-lasting vasodilatory effect. In order to assess the haemodynamic profile of the antihypertensive effect of barnidipine, a randomized, double-blind study of barnidipine vs nitrendipine was performed in 24 patients with mild to moderate essential hypertension. Following an initial 4-week placebo period, patients whose sitting diastolic blood pressure (SiDBP) was between 95 and 114 mm Hg, and whose sitting systolic blood pressure was between 150 and 219 mm Hg, were randomized (2:1 ratio) to receive either barnidipine (10 mg) or nitrendipine (10 mg) once daily, for a 6-week double-blind period. Subsequently, patients with an SiDBP of less than 90 mm Hg continued for a second 6-week period with the same monotherapy, while patients with an SiDBP of 90 mm Hg or above received double the dose of antihypertensive treatment for the next 6 weeks. Two-dimensional M- and B-mode echocardiography with Doppler flowmetry was performed at the end of both the placebo and active treatment phases. Barnidipine and nitrendipine reduced blood pressure by the same degree (barnidipine: from 165 +/- 2/100 +/- 1 to 145 +/- 2/89 +/- 1 mm Hg, p < 0.01; nitrendipine: from 163 +/- 3/100 +/- 2 to 143 +/- 7/90 +/- 3 mm Hg, p < 0.01) as a result of peripheral vasodilation. This was not accompanied by reflex neurohormonal activation. Moreover, only in the group receiving barnidipine was a significant decrease in plasma noradrenaline observed, both when the patients were in the supine position (from 298 +/- 27 to 214 +/- 21 pg/ml, p < 0.05) and when they were upright (from 472 +/- 37 to 348 +/- 38 pg/ml, p < 0.05).

Dietary sodium restriction impairs endothelial effect of insulin

Hyperinsulinemia and high salt intake represent two independent cardiovascular risk factors. However, it is still unknown whether the change in dietary salt intake may affect the ability of insulin to stimulate whole-body glucose uptake and to modulate endothelial function. Regarding this latter issue, we have recently demonstrated that insulin enhances endothelial-mediated alpha2-adrenergic vasorelaxation. In overnight-fasted, freely moving Wistar-Kyoto rats (10 to 12 weeks old), we assessed whole-body glucose uptake (in milligrams per kilogram per minute) during a euglycemic-hyperinsulinemic clamp (insulin infusion rate, 3 mU x kg(-1) x min(-1)) after 3 weeks of normal (NSD, 2% NaCl), high (HSD, 6% NaCl), and low (LSD, 0.6% NaCl) sodium diet. Three days after the clamp study, rats were killed to assess alpha2-adrenergic vasorelaxation evoked by UK 14,304 (10(-9) to 10(-6) mol/L) in aortic rings in control conditions and after insulin exposure (100 microU/mL). Different sodium intakes did not modify the mean blood pressure or the insulin-stimulated whole-body glucose uptake (NSD: 14+/-1.2, n=16; HSD: 15.4+/-1.7, n=14; LSD: 14.8+/-0.8, n=14; NS). In contrast, we confirmed the ability of insulin to enhance alpha2-adrenergic vasorelaxation during NSD and HSD (delta% of maximal relaxation, NSD: from 32+/-3% to 58+/-3.4%, n=9, P<0.01; HSD: from 33+/-3.8% to 59+/-3.5%, n=8, P<0.01), but this effect was impaired during LSD (delta% maximal relaxation, from 36+/-1.5% to 36+/-3.4%, n=8, NS). In conclusion, our data demonstrate that in Wistar-Kyoto rats, changes in dietary salt intake do not modify the insulin-stimulated whole-body glucose uptake. In contrast, LSD impairs the insulin potentiation of alpha2-adrenergic vasorelaxation, thus suggesting that dietary salt restriction provokes an impairment of insulin effect on endothelial function.

Insulin modulation of an endothelial nitric oxide component present in the alpha2- and beta-adrenergic responses in human forearm

We explored in 51 normal subjects, distributed in various series of experiments, whether endothelium nitric oxide may play a role in insulin modulation of alpha2- and beta-adrenergic- evoked vascular responses. In particular, we examined the forearm blood flow response (FBF, ml.min-1.dl-1) to intrabrachial infusion of BHT-933 (0.5, 1, and 2 microg.min-1.dl-1) or isoproterenol (1, 3, and 6 ng. min-1.dl-1) in control conditions, during intrabrachial infusion of insulin alone (0.05 mU.kg-1.min-1) and associated with l-N-monomethylarginine (L-NMMA) (0.05 microg.min-1.dl-1), a nitric oxide synthase inhibitor. In control conditions both BHT-933 and isoproterenol induced a dose-dependent vascular response. Local hyperinsulinemia (deep venous plasma insulin 68.5+/-4 microU/ml) did not change basal FBF whereas attenuated BHT-933 vasoconstriction and enhanced isoproterenol vasodilation. L-NMMA reduced basal FBF and abolished the insulin effect on BHT-933 and isoproterenol response. To clarify whether a nitric oxide component is included in alpha2- and beta-adrenergic response and may be responsible for insulin vascular effect, we further examined BHT-933 and isoproterenol responses during nitric oxide inhibition. Interestingly, L-NMMA potentiated the BHT-933 vasoconstriction and attenuated the isoproterenol vasodilation and, in these conditions, insulin was no more able to exhibit its vascular effects. Finally, to rule out the possibility that the conteracting effect of L-NMMA may not be specifically related to insulin action, dose-response curves to phenylephrine (0.5, 1, and 2 microg.min-1.dl-1) or sodium nitroprusside (1, 2, and 4 microg.min-1.dl-1) were also performed. Both insulin and L-NMMA were unable to alter the phenylephrine-induced vasoconstriction and the sodium nitroprusside vasodilation. In conclusion, our data demonstrate an endothelial nitric oxide component in the alpha2- and beta-adrenergic vascular responses which is the target of the insulin vascular action.

Intrarenal determinants of sodium retention in mild heart failure: effects of angiotensin-converting enzyme inhibition

The onset and the mechanisms leading to Na+ retention in incipient congestive heart failure (CHF) have not been systematically investigated. To investigate renal Na+ handling in the early or mild stages of CHF, Na+ balance and renal clearances were assessed in 10 asymptomatic patients with idiopathic or ischemic dilated cardiomyopathy and mild heart failure (HF) off treatment (left ventricular ejection fraction, 29.7+/-2%) and in 10 matched normal subjects during a diet containing 100 mmol/d of NaCl and after 8 days of high salt intake (250 mmol/d). Six patients were studied again after 6 weeks of treatment with enalapril (5 mg/d P.O.). At the end of the high salt diet, in patients with mild HF the cumulative Na+ balance exceeded by 110 mmol that of normal subjects (F=3.86, P<.001). During high salt intake, renal plasma flow and glomerular filtration rate were similarly increased in both normal subjects and mild HF patients. In spite of comparable increases of filtered Na+ in the two groups, fractional excretion of Na+, fractional clearance of free water, and fractional excretion of K+ (indexes of distal delivery of Na+) increased in normal subjects and were reduced in patients with mild HF. During enalapril treatment, in the mild HF patients the cumulative Na+ balance was restored to normal; furthermore, enalapril significantly attenuated the abnormalities in the distal delivery of Na+. Our results indicate that a defective adaptation of Na+ reabsorption in the proximal nephron is associated with Na+ retention in response to increased salt intake in the early or mild stages of HF. These abnormalities of renal Na+ handling are largely reversed by enalapril.

Blunted parasympathetic modulation in salt-sensitive patients with essential hypertension: evaluation by power-spectral analysis of heart-rate variability

OBJECTIVE

To evaluate autonomic nervous function by power-spectral analysis of heart-rate variability in salt-sensitive and non-salt-sensitive patients with essential hypertension under the conditions of low and high salt intakes.

DESIGN AND METHODS

The blood pressures, heart rates, and electrocardiogram R-R intervals of 20 hypertensive patients were measured at intervals of 30 min during a 24 h period using a portable recorder (TM-2425) on the last day of the high- (250 mmol NaCl/day) and low-salt (25 mmol NaCl/day) diet periods. The patients whose 24 h average mean blood pressures were increased by more than 10% by the high salt intake were defined as salt-sensitive (n = 10); the other patients were considered non-salt-sensitive (n = 10). Power-spectral analysis of R-R intervals was performed to obtain the low-frequency component (0.05-0.15 Hz) and the high-frequency component (0.15-0.40 Hz).

RESULTS

The average 24 h blood pressure in the salt-sensitive patients was increased by the high salt intake [by 19.1 +/- 2.0/9.1 +/- 0.8 mmHg (mean +/- SEM)], whereas the heart rate did not change. In contrast, the increase in 24 h blood pressure in the non-salt-sensitive patients caused by the high salt intake was not significant and the heart rate was decreased significantly by the high salt intake (by 5.9 +/- 1.4 beats/min). The high-salt diet increased significantly the high-frequency component and decreased the low-frequency:high-frequency component ratio both during the daytime and during the night-time for the non-salt-sensitive patients. In contrast, the high-frequency component and the night-time low-frequency: high-frequency component ratio of the salt-sensitive patients did not respond to dietary salt manoeuvres.

CONCLUSIONS

Responses of the parasympathetic and sympathetic nervous systems to dietary salt manoeuvres were blunted in salt-sensitive patients. These altered modulations of the autonomic nervous system may contribute to the salt sensitivity of the blood pressure in patients with essential hypertension.

Differential central modulation of the baroreflex by salt loading in normotensive and spontaneously hypertensive rats

In salt-sensitive hypertensive animal models and human subjects compared with their salt-resistant counterparts, sympathetic activity is abnormally enhanced during a high salt diet. We examined whether salt loading differentially modulates the arterial baroreceptor reflex (ABR), a major control mechanism of arterial pressure and sympathetic vasomotor activity, in young normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Six-week-old WKY and SHR were fed a normal (0.66%) or high (8.00%) salt diet for 4 weeks. After the diet regimen, baseline levels of mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), and the overall and central properties of the ABR were compared among the four groups of rats under halothane anesthesia. In WKY, a high salt diet did not affect baseline arterial pressure and RSNA but potentiated the ABR, as evidenced by an increase in the maximal slope of MAP-RSNA and MAP-heart rate relationships. In SHR, by contrast, salt loading accelerated hypertension and sympathetic overactivity and impaired the ABR. Salt-induced modulation of the ABR was associated with that of the central property, since reflex inhibition of RSNA by stimulation of the aortic depressor nerve was augmented in WKY and attenuated in SHR. These results suggest that differential modulation of the central mechanism subserving the baroreflex control of sympathetic activity at least partly accounts for the difference in salt sensitivity between WKY and SHR.

Heart rate and blood pressure variabilities in salt-sensitive hypertension

In salt-sensitive hypertension, a high sodium intake causes plasma catecholamines to rise and pulmonary baroreceptor plasticity to fall. In salt-sensitive and salt-resistant hypertensive subjects during low and high sodium intakes, we studied autonomic nervous system activity by power spectral analysis of heart rate and arterial pressure variabilities and baroreceptor sensitivity. In all subjects, high sodium intake significantly enhanced the low-frequency power of heart rate and arterial pressures at rest and after sympathetic stress. It also increased heart rate and arterial pressure variabilities. During high sodium intake, salt-sensitive hypertensive subjects had significantly higher low-frequency powers of systolic arterial pressure (7.5 mm Hg2, P < .05) and of heart rate at rest (59.2 +/- 2.4 normalized units [NU], P < .001) than salt-resistant subjects (6.6 +/- 0.3 mm Hg2, 55.0 +/- 3.2 NU) and normotensive control subjects (5.1 +/- 0.5 mm Hg2, 41.6 +/- 2.9 NU). In salt-sensitive subjects, low sodium intake significantly reduced low-frequency normalized units (P < .001) and the ratio of low- to high-power frequency (P < .001). High-sodium intake significantly increased baroreflex sensitivity in control subjects (from 10.0 +/- 0.7 to 17.5 +/- 0.7 ms/mm Hg, P < .001) and salt-resistant subjects (from 6.9 +/- 0.7 to 13.9 +/- 0.9, P < .05) but not in salt-sensitive subjects (7.4 +/- 0.3 to 7.9 +/- 0.4). In conclusion, a high sodium intake markedly enhances cardiac sympathetic activity in salt-sensitive and salt-resistant hypertension. In contrast, although reduced sodium intake lowers arterial pressure and sympathetic activity, it does so only in salt-sensitive subjects. Hence, in salt-resistant subjects, neither arterial pressure nor sympathetic activity depends on salt intake. During a high sodium intake in normotensive subjects and salt-resistant hypertensive subjects, increased sympathetic activity is probably compensated by enhanced baroreflex sensitivity.

Insulin modulation of beta-adrenergic vasodilator pathway in human forearm

BACKGROUND

Insulin modulates sympathetic vasoconstriction, but the mechanisms underlying this effect are not completely elucidated. We have recently investigated the insulin effect on the alpha 1- and alpha 2-adrenergic vasoconstriction pathway, where it is still conflicting with the possible insulin influence on the beta-adrenergic vasodilator pathway. The aim of the present study was to investigate this issue.

METHODS AND RESULTS

The study was performed on the forearm of healthy humans, and all test substances were infused into the brachial artery at systemically ineffective rates. In five subjects, we evaluated isoproterenol-induced vasodilation (1, 3, 6, and 9 ng. kg-1. min-1) both under control conditions and during insulin infusion (0.05 mU. kg-1. min-1). In another group of five subjects, we tested whether the vasorelaxant effect of sodium nitroprusside (1, 2, 4, and 8 ng . kg-1 . min-1) was modified by insulin. Moreover, to explore whether the interaction between insulin and forearm beta-adrenergic pathway participates in insulin modulation of sympathetic-evoked vasoconstriction, we measured in six normal subjects the forearm vascular response to lower-body negative pressure under control conditions and during intrabrachial infusion of insulin alone and in combination with a selective beta-adrenergic blocking agent (propranolol 10 micrograms/100 mL per minute). Finally, to verify whether insulin interaction with the beta-adrenergic pathway may also account for insulin modulation of alpha 2-adrenergic vasoconstriction, we assessed the vascular response to a selective alpha 2-adrenergic agonist before and after propranolol administration. Insulin exposure potentiated the vascular responsiveness to isoproterenol but did not affect the vasodilator response to sodium nitroprusside. Furthermore, the insulin-induced attenuation of sympathetic vasoconstriction was partially corrected by propranolol. In contrast, the insulin modulation of alpha 2-adrenergic vasoconstriction was not influenced by beta-adrenergic blockade.

CONCLUSIONS

Taken together, our results suggest that insulin modulation of sympathetic-induced vasoconstriction is carried out through an interaction of the hormone with the pathways of both alpha 2-and beta -adrenergic receptors.

Insulin blunts sympathetic vasoconstriction through the alpha 2-adrenergic pathway in humans

We investigated the mechanisms underlying the insulin-induced attenuation of sympathetic forearm vasoconstriction in healthy humans. In 5 subjects, we applied 20 mm Hg lower body negative pressure for 30 minutes in control conditions and during a 60-minute infusion of insulin (0.05 mU/kg per minute) in the brachial artery and measured forearm norepinephrine kinetics and hemodynamics. In 11 subjects, we applied graded lower body negative pressure at 5, 10, 15, and 20 mm Hg for 5 minutes each in control conditions and during the simultaneous intrabrachial administration of insulin (0.05 mU/kg per minute) (5 subjects) or insulin plus ouabain (3.5 micrograms/min per liter) (6 subjects) to investigate whether insulin acts through a potentiation of the vascular smooth muscle Na+,K(+)-ATPase. To assess a possible effect of insulin on a specific adrenergic receptor pathway, in a further study group we evaluated (1) the forearm vascular response to intrabrachial infusion of the alpha 1-adrenergic receptor agonist phenylephrine (0.5, 1, and 2 micrograms/kg per minute; n = 7) and of the alpha 2-adrenergic receptor agonist BHT-933 (0.5, 1, 2, and 4 micrograms/kg per minute; n = 9), and (2) the effects of intra-arterial infusion of prazosin (0.5 microgram/100 mL per minute) alone or combined with insulin on the forearm vascular response to graded lower body negative pressure (7 subjects). Insulin blunted the peak increase in forearm vascular resistance (from 13 +/- 2 to 6 +/- 2 U, P < .05) but not the rise in forearm norepinephrine spillover induced by 20 mm Hg lower body negative pressure (from 8.3 +/- 1.8 to 11.1 +/- 3.5 pmol/min per liter, P = NS). Ouabain administration did not prevent the insulin-induced attenuation of the forearm vasoconstrictive response to graded lower body negative pressure. Insulin infusion in the brachial artery did not modify the forearm vasoconstriction induced by intra-arterial infusion of phenylephrine but significantly reduced the increase in forearm vascular resistance induced by BHT-933 (F = 6.111, P < .001). Finally, intra-arterial infusion of prazosin significantly attenuated the forearm vasoconstriction induced by graded lower body negative pressure. The residual vasoconstrictive response was abolished by insulin infusion. Taken together, these findings suggest that insulin interacts with the sympathetic nervous system at the vascular level predominantly through the alpha 2-adrenergic vasoconstrictive pathway.

Salt sensitivity in hypertension. Renal and cardiovascular implications

The mechanisms responsible for the increase in blood pressure response to high salt intake in salt-sensitive patients with essential hypertension are complex and only partially understood. A complex interaction between neuroendocrine factors and the kidney may underlie the propensity for such patients to retain salt and develop salt-dependent hypertension. The possible role of vasodilator and natriuretic agents, such as the prostaglandins, endothelium-derived relaxing factor, atrial natriuretic factor, and kinin-kallikrein system, requires further investigation. An association between salt sensitivity and a greater propensity to develop renal failure has been described in certain groups of hypertensive patients, such as blacks, the elderly, and those with diabetes mellitus. Salt-sensitive patients with essential hypertension manifest a deranged renal hemodynamic adaptation to a high dietary salt intake. During a low salt diet, salt-sensitive and salt-resistant patients have similar mean arterial pressure, glomerular filtration rate, effective renal plasma flow, and filtration fraction. On the other hand, during a high salt intake glomerular filtration rate does not change in either group, and effective renal blood flow increases in salt-resistant but decreases in salt-sensitive patients; filtration fraction and glomerular capillary pressure decrease in salt-resistant but increase in salt-sensitive patients. Salt-sensitive patients are also more likely than salt-resistant patients to manifest left ventricular hypertrophy, microalbuminuria, and metabolic abnormalities that may predispose them to cardiovascular diseases. In conclusion, salt sensitivity in hypertension is associated with substantial renal, hemodynamic, and metabolic abnormalities that may enhance the risk of cardiovascular and renal morbidity.

Abnormalities of sodium handling and of cardiovascular adaptations during high salt diet in patients with mild heart failure

BACKGROUND

Sodium retention and hormonal activation are fundamental hallmarks in congestive heart failure. The present study was designed to assess the ability of patients with asymptomatic to mildly symptomatic heart failure and no signs or symptoms of congestion to excrete ingested sodium and to identify possible early abnormalities of hormonal and hemodynamic mechanisms related to sodium handling.

METHODS AND RESULTS

The effects of a high salt diet (250 mEq/day for 6 days) on hemodynamics, salt-regulating hormones, and renal excretory response were investigated in a balanced study in 12 untreated patients with idiopathic or ischemic dilated cardiomyopathy and mild heart failure (NYHA class I-II, ejection fraction < 50%) (HF) and in 12 normal subjects, who had been previously maintained a 100 mEq/day NaCl diet. In normal subjects, high salt diet was associated with significant increases of echocardiographically measured left ventricular end-diastolic volume, ejection fraction, and stroke volume (all P < .001) and with a reduction of total peripheral resistance (P < .001). In addition, plasma atrial natriuretic factor (ANF) levels increased (P < .05), and plasma renin activity and aldosterone concentrations fell (both P < .001) in normals in response to salt excess. In HF patients, both left ventricular end-diastolic and end-systolic volumes increased in response to high salt diet, whereas ejection fraction and stroke volume failed to increase, and total peripheral resistance did not change during high salt diet. In addition, plasma ANF levels did not rise in HF in response to salt loading, whereas plasma renin activity and aldosterone concentrations were as much suppressed as in normals. Although urinary sodium excretions were not significantly different in the two groups, there was a small but systematic reduction of daily sodium excretion in HF, which resulted in a significantly higher cumulative sodium balance in HF than in normals during the high salt diet period (P < .001).

CONCLUSIONS

These results show a reduced ability to excrete a sodium load and early abnormalities of cardiac and hemodynamic adaptations to salt excess in patients with mild heart failure and no signs or symptoms of congestion.

Insulin reduces reflex forearm sympathetic vasoconstriction in healthy humans

Previous in vitro studies indicate that insulin modifies vascular reactivity to different agents. We have previously demonstrated that in normotensive humans physiological hyperinsulinemia is associated with an increase of forearm norepinephrine release but does not modify vascular resistance. To explore whether insulin modulates peripheral vasoconstriction induced by reflex sympathetic activation, we studied its effects on forearm hemodynamics (strain-gauge plethysmography) during graded levels of lower body negative pressure (-5, -10, -15, and -20 mm Hg, each for 5 minutes) in normotensive subjects. For this purpose, eight subjects received an intrabrachial artery infusion of regular insulin at a systemically ineffective rate (0.05 milliunits/kg per minute) so that deep-venous insulin levels increased in the experimental forearm from 16.5 +/- 2.9 to 379.6 +/- 30 pmol/L (p < 0.01), whereas arterial insulin levels remained unchanged (from 40.9 +/- 8.6 to 43.1 +/- 7.9 pmol/L, NS). In the control arm, forearm vascular resistance (units) increased from 52.3 +/- 3 to a peak of 78.4 +/- 5 (p < 0.001) during lower body negative pressure. In the insulin-exposed forearm, vascular resistance (46.4 +/- 2 at baseline) remained unchanged during insulin infusion (45.8 +/- 3, NS) and rose to a peak of 54.8 +/- 6 (p < 0.05) during lower body negative pressure. The response of forearm vascular resistance to lower body negative pressure was different in the two forearms (F = 4.506, p < 0.01, repeated-measures analysis of variance with grouping factor). Our results demonstrate that in normotensive subjects local physiological hyperinsulinemia reduces the forearm vasoconstrictive response to reflex sympathetic activation.

Angiotensin converting enzyme inhibition restores cardiac and hormonal responses to volume overload in patients with dilated cardiomyopathy and mild heart failure

BACKGROUND

Angiotensin converting enzyme (ACE) inhibition exerts a favorable effect on the response to exercise in heart failure. This study was planned to define the influence of ACE inhibition on the adaptation to volume overload.

METHODS AND RESULTS

We studied the hemodynamic, hormonal, and renal responses to acute volume expansion (sodium chloride, 0.9%, 0.25 ml.kg-1.min-1 for 2 hours) in patients with idiopathic or ischemic dilated cardiomyopathy and mild heart failure (New York Heart Association class I or II, ejection fraction < or = 50%). The patients were studied without any pretreatment (n = 14) or after 1 week of treatment with the oral ACE inhibitor quinapril at a dosage of 10 mg/day (n = 11). Seven patients were studied during constant intravenous infusion with nitroglycerin (0.1 micrograms.kg-1.min-1). The study groups had similar hemodynamic and clinical characteristics and hormonal profile at baseline evaluation. In the untreated patients, volume expansion did not increase left ventricular end-diastolic volume measured by echocardiography and was associated with a reduction in ejection fraction (p < 0.05) and with a paradoxical increase in forearm vascular resistance (p < 0.05) estimated by plethysmography. In addition, plasma atrial natriuretic factor did not change, and plasma norepinephrine was increased by saline loading. In contrast, in the patients treated with quinapril, volume expansion induced an increase of both left ventricular volumes (p < 0.001) without changing ejection fraction and reduced forearm vascular resistance (p < 0.05). In addition, in this group, plasma atrial natriuretic factor levels increased (p < 0.05) and plasma norepinephrine did not change during volume overload. During nitroglycerin infusion, volume expansion was associated with peripheral vasodilatation, increases of left ventricular volumes, and no change in ejection fraction. In this group, however, plasma atrial natriuretic factor levels did not change in response to volume overload.

CONCLUSIONS

We conclude that pretreatment with the ACE inhibitor quinapril significantly improves compromised responses to acute isotonic volume overload in patients with dilated cardiomyopathy and mild heart failure. The favorable influence of ACE inhibition on cardiovascular and hormonal responses to volume expansion seems to be related to the cardiac unloading produced by this treatment.