Atrial natriuretic factor in essential hypertension

We measured circulating levels of immunoreactive atrial natriuretic factor (ANF) in 10 patients with untreated, uncomplicated mild to moderate essential hypertension and in 15 normotensive controls. ANF concentrations were significantly higher in the hypertensive group than in the control group (38.4 +/- 6.9 pg/ml versus 18.3 +/- 1.8 pg/ml, p less than 0.02). A positive correlation between ANF levels and systolic, diastolic and mean blood pressure was noted in the total study population (p less than 0.008, r = 0.52; p less than 0.005, r = 0.55; p less than 0.02, r = 0.46, respectively). Thus, plasma ANF concentrations are elevated in essential hypertension and may result from increased intraarterial pressure.     

Catecholamines and atrial natriuretic factor in Dahl and spontaneously hypertensive rats

We have previously demonstrated two different catecholaminergic patterns in genetic and experimental hypertension: a hyperdopaminergic state in spontaneously hypertensive (Okamoto) rats (SHR) and a hypernoradrenergic state in salt-sensitive Dahl rats. Plasma immunoreactive atrial natriuretic factor (IR ANF) concentrations increase in both models as a response to hypertension. To distinguish between the genetic and acquired components of these abnormalities, we measured adrenal dopamine-beta-hydroxylase (D beta H) activity and coeliac ganglionic atrial natriuretic factor (ANF) like immunoreactivity in the two animal strains. While adrenal D beta H activity was increased in Dahl S rats, it was diminished in SHR in the prehypertensive as well as in the hypertensive stages. In the hypertensive stage, the ANF-like immunoreactivity in the coeliac ganglia was lower in the Dahl S group but higher in SHR than in their respective normotensive controls; there were no changes in these animals when they were prehypertensive. Differences in D beta H activity, which determines the fine tuning of sympathoadrenomedullary catecholamine synthesis may account for the inheritance of mechanisms resulting in salt-sensitive hypertension (as in SHR) or salt-dependent hypertension (as in Dahl salt-sensitive rats). In contrast, plasma IR ANF concentrations may reflect a defense mechanism against hypertension. However ANF-like immunoreactivity in coeliac ganglia does not follow its plasma concentrations and changes in different directions in the two hypertensive strains; it may reflect a neuromodulatory function of ANF in the ganglionic neurotransmission and different implications of this role of ANF in the two hypertensive models.     

Reduction in extracellular superoxide dismutase activity in African-American patients with hypertension

Superoxide anions react with nitric oxide to form peroxynitrite and hence reduce the bioavailability of nitric oxide in the arteries. Extracellular superoxide dismutase (EC-SOD) is a major superoxide scavenger in human plasma and vascular tissues. The objective of this study is to assess whether essential hypertension is associated with an alteration in EC-SOD activity. In this report, blood samples were obtained from hypertensive (n=39) and normotensive (n=37) African-Americans. Plasma EC-SOD activity was measured using in-gel activity staining and spectrophotometric assays, EC-SOD protein level was measured using Western blotting, nitrotyrosine was measured using slot blotting, 8-isoprostane was measured with an enzyme immunoassay, and plasma copper and zinc concentrations were measured using an atomic absorption assay. Our data demonstrate that the copper, zinc, and plasma EC-SOD protein concentrations in the hypertensive and normotensive subjects are indistinguishable. Compared to normotensive controls, hypertensive patients have significantly reduced plasma EC-SOD activity. Plasma nitrotyrosine and 8-isoprostane levels are significantly higher in the hypertensive patients than in normotensive controls. Results from this study suggest that a reduction in EC-SOD activity in hypertensive patients is not due to a down-regulation of the SOD3 gene (encoding EC-SOD) or deficiency in mineral cofactors. Furthermore, the reduced EC-SOD activity might be at least partially responsible for the increased oxidative stress, as reflected by increased plasma nitrotyrosine and 8-isoprostane, in hypertensive subjects.     

Angiotensin II in essential hypertension.

Plasma concentrations of angiotensin II (PAC) were measured in a group of 146 hypertensive patients (diastolic pressure greater than 105 mm Hg) who had no apparent underlying cause for their condition and 113 randomly selected normotensive controls (diastolic pressure less than 90 mm Hg). There was no evidence of bimodality in the frequency distribution curves for plasma angiotensin II concentrations among the hypertensive patients. It was concluded that hypertension associated with low angiotensin II concentration and by implication "low-renin" hypertension is not a condition separate from essential hypertension.     

Developing essential hypertension: a syndrome involving ANF deficiency?

The pathogenesis of essential hypertension may possibly involve a deficiency in, or a decreased response to, endogenous vasodilator and natriuretic factor(s). Searching for hereditary or familial defects, it is plausible to evaluate blood pressure (BP) regulating factors in (yet) normotensive offspring of hypertensive parents (OHyp), some of whom are in fact in a stage of prehypertension. Studies by our group demonstrated that compared with healthy offspring of normotensive parents, OHyp have plasma atrial natriuretic (ANF) factor levels that are unaltered on a low salt intake but often fail to increase normally in response to a high salt intake. Plasma levels of cyclic GMP, the presumed second messenger of ANF, also may tend to be decreased in certain OHyp. On the other hand, renal excretory responses of cyclic GMP and electrolytes to ANF infused in "physiological" dose were unchanged in some OHyp tested so far. In borderline to moderate, uncomplicated essential hypertension, plasma ANF levels are often "normal." This may be inappropriately low relative to the existing BP, although the relationship of circulating ANF to atrial pressures in essential hypertension remains to be clarified. A conversion to higher plasma ANF values may occur with cardiac complications such as left ventricular hypertrophy, enlargement, dysfunction, or overt heart failure. Acute or short-term elevation of circulating ANF within the physiological and pathophysiological range by ANF infusion produces an exaggerated natriuresis and lowers BP in essential hypertensive patients. We postulate a syndrome of ANF deficiency, characterized by an impaired response of circulating ANF to high salt intake and by low cyclic GMP levels in certain yet normotensive offspring of essential hypertensive parents and by inappropriately "normal" plasma ANF in some patients with uncomplicated essential hypertension. At the stage of prehypertension, a disturbance in the ANF - cyclic GMP pathway may be expressed primarily at the circulatory rather than at the renal level. Hypertension-prone humans also tend to have an exaggerated vascular reactivity to norepinephrine. Whether the two disturbances may be interrelated is presently unknown. Both defects may potentially predispose to the development of essential hypertension. Relative ANF deficiency, an enhanced natriuretic response to ANF, and a sustained antihypertensive effect of infused ANF may represent a rational basis for treatment of essential hypertension with agents that activate the ANF system.     

Clonidine reduces elevated cerebrospinal fluid catecholamine levels in patients with essential hypertension

Cerebrospinal fluid (CSF) catecholamines were measured in normotensive patients and in patients with mild to moderate essential hypertension. CSF-norepinephrine (NE) concentrations were 50% lower in the normotensive individuals (127 ± 28 vs. 240 ± 23 pg/m1) (P<0.01). In hypertensive patients, CSF-NE was inversely related to age (r =-0.68; P<0.01) and directly related to plasma NE (r = 0.61; P<0.05). Clonidine (450 mcg/day for 2 weeks) significantly reduced CSF-NE (?40%) in hypertensive patients. In addition, it decreased blood pressure, plasma and urinary NE. Urinary VMA was not affected by clonidine. No correlation was observed between clonidine effects on BP and on plasma or CSF catecholamines. This study indicates that patients with essential hypertension have elevated levels of CSF-NE which are reduced after treatment with clonidine. The elevation of CSF-NE suggests that central (spinal?) noradrenergic activity may be increased in patients with mild to moderate essential hypertension, and that can be reduced by treatment with clonidine.     

Plasma norepinephrine and epinephrine levels in essential hypertension

Almost all comparative studies of plasma catecholamines in patients with essential hypertension (EH) and in normotensive controls have reported higher mean norepinephrine (NE) or epinephrine (E) levels in the hypertensive patients, but only about 40% of the studies have been positive, i.e., have reported statistically significant hypertensive-normotensive (H-N) differences. Virtually all studies of NE in young, consistently hypertensive patients were positive, as well as all studies of E in relatively tachycardic patients. Plasma NE increased with age in normotensive control but not EH groups. The likelihood that a study was positive with respect to NE was independent of the likelihood with respect to E. In 189 individuals with EH and 130 normotensive controls, NE increased with age in controls but not in patients with EH, so that the extent of the H-N difference in NE varied inversely with patient age. Among 41 other individuals with EH and 59 other normotensive controls, the distributions of NE and E values were shifted upward in EH. NE and E values were uncorrelated. Plasma NE levels are abnormally high in some patients with EH--especially those who are young and consistently hypertensive--and E levels are independent of age and NE. Increased sympathetic nervous system and/or sympathoadrenomedullary activity characterize a proportion of patients with EH.     

Atrial natriuretic factor (ANF) increases urinary protein excretion in patients with essential hypertension: a possible role of ANF for renal handling of protein

Low dose iv infusion (0.01 and 0.03 micrograms/kg per min, for 30 min each) of alpha-human atrial natriuretic factor (alpha-hANF) produced a significant increase (+300%) in urinary protein excretion in patients with essential hypertension but not in normotensive controls, when their renal function was normal. The major component of excreted proteins induced by alpha-hANF infusion was presumed to be albumin on the basis of molecular weight (69,000) analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Urine output and sodium and potassium excretion rates were increased dose-dependently by alpha-hANF infusion in the hypertensive patients in a similar fashion to those in the controls. Glomerular filtration rate (GFR) remained unchanged in the controls but was slightly increased in the patients (+33%) during the infusion. These results suggest that besides its previously recognized physiological functions such as natriuresis and diuresis, ANF plays an important role in the regulation of renal handling of proteins in patients with essential hypertension.     

Nocturnal fluctuations of plasma atrial natriuretic peptide in normotensive and hypertensive men

Plasma levels of immunoreactive atrial natriuretic peptides (ANP) were measured at 10-min intervals during night-sleep in 4 normotensive and in 4 moderate, essential hypertensive subjects. The mean ANP levels ranged from 24.3 to 27.9 pg.ml-1 for the normal subjects. These mean levels were not significantly different in the hypertensive subjects (range: 26.3 to 37.2 pg.ml-1). Fluctuations, often of small amplitude, were observed around this mean, without any defined periodicity. Changes in plasma ANP were not associated with changes in heart rate. Analysis of the ANP profiles and the concomitant sleep stage patterns did not reveal any temporal relationship between ANP fluctuations and specific sleep stages or waking periods. The ANP profiles did not differ between the groups, which indicates no abnormality in ANP secretion in moderate essential hypertension.     

Increased plasma brain natriuretic peptide levels in DOCA-salt hypertensive rats: relation to blood pressure and cardiac concentration

Four experimental groups of rats treated with (1) DOCA-salt, (2) DOCA or (3) salt, and (4) controls were used to study the participation of brain natriuretic peptide (BNP) in the development of hypertension. Plasma and cardiac tissue concentrations of BNP as well as atrial natriuretic peptide (ANP) were measured in each group by using radioimmunoassays specific to rat BNP or ANP. Plasma BNP levels in DOCA-salt hypertensive group were higher than those in control (p less than 0.01), salt (p less than 0.01) and DOCA (p less than 0.01) groups. A positive correlation was observed between plasma BNP levels and blood pressure (r = 0.70, p less than 0.001) and between plasma ANP levels and blood pressure (r = 0.62, p less than 0.001). Plasma BNP/ANP ratio increased parallel with elevation of blood pressure. Plasma BNP levels correlated negatively with atrial BNP concentration (r = -0.33, p less than 0.05), but positively with ventricular BNP (r = 0.76, p less than 0.001). Compared with controls, tissue BNP-45/gamma-BNP ratio in the DOCA-salt rats was lower in atrium, but higher in ventricle. Thus, in DOCA-salt hypertension atrial BNP decreased with exhaustion of stored BNP-45, while ventricular BNP increased as BNP-45 accumulated. These results suggest that BNP is a novel cardiac hormone, synthesized, processed and secreted in response to changes in blood pressure. BNP may play different roles in controlling blood pressure than those assumed by ANP.     

The physiology of atrial natriuretic factor

Following the discovery of the natriuretic effect of atrial extract, our laboratory attempted to dissect the possible physiological role of atrial natriuretic factor. Initial micropuncture experiments demonstrated that the reduction of tubular sodium reabsorption was localized in the medullary collecting duct, a nephron site in which sodium transport was known to be inhibited after acute hypervolemia. Partial removal of the endogenous source of atrial natriuretic factor was associated with a reduced renal response to hypervolemia, confirming that the factor is causally involved in acute sodium balance. In vitro incubation of atrial tissue was used to investigate mechanisms of release of atrial natriuretic factor. It was found that agonists known to activate the intracellular polyphosphoinositide system in other tissues were effective in releasing natriuretic activity from the atria into the incubation medium. To determine whether atrial natriuretic factor might play a role in hypertension, atrial natriuretic content was measured in spontaneously hypertensive rats and their normotensive controls. Hypertension was associated with increased content. Since the renal response to exogenous factor was not impaired in these animals, we suggested that the increased content might play a compensatory role. Our early studies thus indicated that atrial natriuretic factor was a previously unrecognized hormone involved in cardiovascular regulation.     

MTHFR 677 CT/MTHFR 1298 CC genotypes are associated with increased risk of hypertension in Indians

The goals of our present study were to measure plasma homocysteine levels and determine their association with methylenetetrahydrofolate reductase (MTHFR) gene polymorphisms (C677T and A1298C) in essential hypertensive subjects. Plasma total homocysteine and folic acid levels were measured in essential hypertensive patients (n = 153) before and after oral supplementation with either 5 mg folic acid tablet/day or 5 mg placebo/day for 4 weeks and compared with age and sex matched normotensive controls (n = 133). MTHFR gene polymorphisms (C677T and A1298C) were studied by restriction fragment length polymorphism and correlated with plasma homocysteine levels. Homocysteine levels were significantly higher in hypertensive patients as compared to controls and showed a negative correlation with plasma folate levels. Folic acid supplementation (5 mg/day) for 4 weeks resulted in a significant decrease in plasma homocysteine concentrations in these patients. Patients carrying MTHFR 677T allele (OR = 1.90; 95%CI: 1.14–3.19) or MTHFR 1298C (OR = 2.6, 95%CI: 1.55–4.40) allele were at increased risk of hypertension. The frequency of co-occurrence of MTHFR 677 CT/1298 CC genotypes was significantly higher in the patients compared to controls (P < 0.05) and was associated with increased risk of hypertension (OR = 3.54, 95%CI: 0.37–4.30). Subjects with MTHFR 1298 CC genotype had significantly higher homocysteine levels compared to those with MTHFR 1298 AA genotype (P < 0.05). Our results indicate that MTHFR 677T and 1298C alleles and co-occurrence of MTHFR 677 CT/MTHFR 1298 CC genotypes are associated with increased risk of hypertension and MTHFR 1298 CC genotype is associated with higher homocysteine levels in our subjects.     

Morphological characteristics of the secretion of natriuretic factor by atrial cardiomyocytes in spontaneous hypertension in rats

The results of electron microscopic studies of the synthesis and secretion of atrial natriuretic factor (ANF) in right atrial cardiomyocytes of spontaneously hypertensive rats (SHR) and the corresponding normotensive controls are presented. Enhanced secretory activity in cardiomyocytes of SHR has been revealed. The role of enhanced ANF secretion in the origin of arterial hypertension is discussed. It is suggested that enhanced ANF secretion can be attributed to increased ANF demand in BP elevation, changes in the renal function in hypertensive subjects or genetic defect in the excretory renal function in SHR.     

Comparison of the effect of hypoxia on the secretion of the atrial natriuretic factor in spontaneously hypertensive and normotensive rats.

The effect of short lasting hypoxia on blood pressure, plasma atrial natriuretic peptide level and number of specific atrial granules were studied in 26 male spontaneously hypertensive and 24 normotensive Wistar rats. A great difference occurred in ANP secretion between hypertensive and normotensive rats. In the hypertensive animals elevated plasma ANP concentration (130 +/- 27 pg/ml) and decreased granularity in the right atria (73 +/- 2) were found on the first day of hypoxia with a slight elevation in urinary sodium content versus normotensive controls. The blood pressure also decreased although not significantly (190 +/- 14 mm Hg). In Wistar rats increased plasma ANP (130 +/- 34 pg/ml) and decreased atrial granularity versus normotensive controls (72 +/- 10 in the left and 113 +/- 16 in the right atrium) were observed only on the third day of hypoxia without changes in blood pressure and natriuresis. In SHR the rapid but short timed ANP release might be of right atrial origin and probably the consequence of a continuous and perhaps increased secretion of the peptide in normoxic conditions too. In Wistar rats the plasma ANP elevation could be secondary due to the increased plasma level of different vasoactive hormones to hypoxia. In the altered effect of ANP in hypertensive and normotensive hypoxic animals, structural and functional changes in the vascular bed may play a role.     

Plasma levels of human atrial natriuretic peptide in patients with hypertensive diseases

Three types of antihuman atrial natriuretic peptide antiserum were obtained. From the study of cross-reactivity to human atrial natriuretic peptide fragments, it was suggested that antisera-1, -2, and -3 are mostly specific to 1-28, 5-25, and the ring structure, respectively. The estimated values of this hormone were significantly lower in the order of antisera-1, -2, and -3. Moreover, high performance liquid chromatographic study showed that various types of fragments of atrial natriuretic peptide exist in human plasma. These findings suggested that the highly specific antiserum to 1-28 human atrial natriuretic peptide such as antiserum-1 should be used to estimate the 1-28 human atrial natriuretic peptide levels in human plasma. From the study by using antiserum-1, it was concluded that the plasma human atrial natriuretic peptide increased in essential hypertensives, and in patients with primary aldosteronism, chronic renal failure, and malignant hypertension. Regarding the pathophysiological significance of increased plasma atrial natriuretic peptide, it is unlikely that this plays an important role in the etiology of essential hypertension or other hypertensive diseases, because the plasma level of this hormone is elevated in these patients. The increase of plasma atrial natriuretic peptide level in these patients should be considered to be a secondary or compensatory reaction to high blood pressure.     

An endogenous digitalis-like compound extracted from human urine: biochemical and chemical studies

Plasma and urine levels of an endogenous digitalis-like compound (EDLC) are increased in low renin Na+-dependent experimental hypertension, in some normotensive offspring of hypertensive patients and in some essential hypertensive patients. Urine-drived EDLC was purified from 550 L of urine from essential hypertensive patients (n = 8) and from normotensive subjects with a family history of hypertension (n = 27), using flash chromatography on C18 reversed-phase, anion exchange chromatography and various reversed-phase high performance liquid chromatographies. The mechanism of Na+-K+ ATPase inhibition and the related effects of semipurified urine-derived EDLC were studied and compared with those of ouabain. Its action was similar to that of ouabain in 8 out of 10 of the tests applied. The main effects of such a compound were the depression of Na+-K+ pump activity of human erythrocytes, the inhibition of 5-hydroxytryptamine reuptake by human platelets, and the induction of natriuresis in urethanized rats. Therefore, EDLC may be considered as one of the natriuretic hormones whose mechanism of action closely resembles that of ouabain.     

Angiotensins in plasma of hypertensive rats and human

The plasma levels of des-aspartate-angiotensin I (DAA-I) in three models of hypertensive rats and hypertensive subjects were determined and compared with their normotensive controls. The rationale for the study was based on our earlier findings showing that DAA-I is a physiological angiotensin peptide that is involved in the pathophysiology of the cardiovascular system. The determination was carried out by the technique of capillary electrophoresis. Plasma level of angiotensin I, angiotensin II, and angiotensin III was also determined as a measurement of the status of the renin-angiotensin system in the different models of hypertension. DAA-I was found to be significantly lower in the spontaneously hypertensive rats (SHR) (46.6 +/- 2.5 pmol/l compared to 66.1 +/- 3.4 pmol/l for the normotensive control Wistar Kyoto rats), renal hypertensive rats (54.2 +/- 5.1 pmol/l compared to 72 +/- 2.5 pmol/l for the normotensive control Sprague-Dawley rats), and essential human hypertensive subjects (15.2 +/- 0.9 pmol/l compared to 19.5 +/- 2.5 pmol/l for the normotensive adult), whilst plasma concentration of angiotensin I and angiotensin II is reflective of the state of the renin-angiotensin system in the particular model of hypertension. When the SHR and human hypertensive subjects were treated with an angiotensin converting enzyme (ACE) inhibitor, the plasma level of DAA-I increased significantly. These findings suggest that the low plasma level of DAA-I could be a characteristic defect of the renin-angiotensin system in the two genetic models of hypertension (SHR and human essential hypertensive subjects). The increase of the nonapeptide following ACE inhibitor treatment could be an important hitherto unrecorded contributory factor to the effectiveness of ACE inhibitors in combating heart pathology.     

Acute angiotensin II increases plasma F2-isoprostanes in salt-replete human hypertensives

Angiotensin (Ang) II induces oxidative stress in vitro and in animal models of hypertension. We tested the hypothesis that Ang II increases oxidative stress in human hypertension, as assessed by plasma F2-isoprostane concentrations. Plasma F2-isoprostanes, hemodynamic and endocrine parameters were measured at baseline and following a 55 min infusion of 3 ng/kg/min Ang II in 13 normotensive and 13 hypertensive volunteers ingesting a high- (200 mmol/d) or low- (10 mmol/d) sodium diet. Mean arterial pressure (MAP) and body mass index were higher in hypertensive subjects. Ang II infusion increased MAP (p<.001) and plasma aldosterone concentrations (p<.001) and decreased plasma renin activity (p<.001) and renal plasma flow (p<.001) to a similar extent in both groups. Plasma F2-isoprostane concentrations were similar at baseline. There was no effect of Ang II on F2-isoprostane concentrations during low-salt intake in either group (normotensive 51.7 +/- 7.1 to 53.7 +/- 6.5 pg/ml and hypertensive 52.2 +/- 8.2 to 56.2 +/- 10.0 pg/ml; mean +/- SE). During high-salt intake, Ang II increased F2-isoprostane concentrations in the hypertensive group (52.3 +/- 7.2 to 63.2 +/- 10.4 pg/ml, p=0.010) but not in the normotensive group (54.2 +/- 4.4 to 58.9 +/- 6.6 pg/ml, p=0.83). Acute Ang II infusion increases oxidative stress in vivo in hypertensive humans. The renin-angiotensin system may contribute to oxidative stress in human cardiovascular disease.     

Effect of plasma from hypertensive patients on contractile response of vascular smooth muscle from normotensive rat

This study examines whether incubation with plasma from essential hypertensive patients increases the contractile activity of vascular smooth muscle from rats in response to noradrenaline (NA) and potassium (K+). Plasma samples were obtained from age- and sex-matched essential hypertensive patients and normotensive people. Vascular strips were prepared from aorta and portal veins of normotensive rats and placed in physiological solution in muscle baths for measurement of mechanical response. Aortic strips exposed to hypertensive plasma showed increased responsiveness to NA compared with normotensive plasma, but K+ caused an opposite effect. Portal vein exposed to normotensive or hypertensive plasma did not produce any response to NA, but the responsiveness produced in the presence of normotensive plasma to K+ was higher than that of hypertensive plasma. Portal vein exposed to normotensive plasma or hypertensive plasma showed a dose-dependent increase in the spontaneous activity up to 50% concentration of the plasma samples, but further increase in the concentration of plasma inhibited the spontaneous activity. Spontaneous activity at any given concentration of hypertensive plasma was significantly higher than that of normotensive plasma. The spontaneous activity in the presence of heated or unheated normotensive plasma or unheated normotensive serum was not significantly different from each other. These results indicate that the plasma factor from hypertensive patients, which alters the reactivity of vascular smooth muscle from normotensive rat, is present in the serum fraction and is not heat sensitive.     

Evidence for a raised concentration of a circulating sodium transport inhibitor in essential hypertension.

A cytochemical technique that measures the ability of plasma to stimulate guinea-pig renal glucose-6-phosphate dehydrogenase (G6PD) activity in vitro, which is a marker of its ability to inhibit Na+-K+-adenosine-triphosphatase (Na+-K+-ATPase), was used in 19 patients with essential hypertension and 23 normotensive, healthy subjects. The ability of plasma to stimulate G6PD was significantly greater in the hypertensive patients when they were taking their normal sodium diet than in the normotensive subjects, and was significantly correlated with blood pressure. The ability of plasma to stimulate G6PD was inversely correlated with plasma renin activity in the hypertensive patients and increased with age and sodium intake in the normotensive subjects. These results support the hypothesis that essential hypertension, and also perhaps the increase in blood pressure with age in communities that consume large quantities of salt, is in part due to an increase in a circulating concentration of an inhibitor of Na+-N+-ATPase.