Sodium transport inhibition, cell calcium, and hypertension. The natriuretic hormone/Na+-Ca2+ exchange/hypertension hypothesis

The vascular Na,K-ATPase: clinical implications in stroke, migraine, and hypertension

In the vascular wall, the Na,K-ATPase plays an important role in the control of arterial tone. Through cSrc signaling, it contributes to the modulation of Ca2+ sensitivity in vascular smooth muscle cells. This review focuses on the potential implication of Na,K-ATPase-dependent intracellular signaling pathways in severe vascular disorders; ischemic stroke, familial migraine, and arterial hypertension. We propose similarity in the detrimental Na,K-ATPase-dependent signaling seen in these pathological conditions. The review includes a retrospective proteomics analysis investigating temporal changes after ischemic stroke. The analysis revealed that the expression of Na,K-ATPase α isoforms is down-regulated in the days and weeks following reperfusion, while downstream Na,K-ATPase-dependent cSrc kinase is up-regulated. These results are important since previous studies have linked the Na,K-ATPase-dependent cSrc signaling to futile recanalization and vasospasm after stroke. The review also explores a link between the Na,K-ATPase and migraine with aura, as reduced expression or pharmacological inhibition of the Na,K-ATPase leads to cSrc kinase signaling up-regulation and cerebral hypoperfusion. The review discusses the role of an endogenous cardiotonic steroid-like compound, ouabain, which binds to the Na,K-ATPase and initiates the intracellular cSrc signaling, in the pathophysiology of arterial hypertension. Currently, our understanding of the precise control mechanisms governing the Na,K-ATPase/cSrc kinase regulation in the vascular wall is limited. Understanding the role of vascular Na,K-ATPase signaling is essential for developing targeted treatments for cerebrovascular disorders and hypertension, as the Na,K-ATPase is implicated in the pathogenesis of these conditions and may contribute to their comorbidity.

The Role of Na, K-Atpase Inhibitors in Hypertension and End-Stage Renal Disease

Objective

To review the role of Na, K-ATPase inhibitors in the pathogenesis of essential hypertension and hypertension associated with end-stage renal disease.

Data Sources

MEDLINE search, 1966 to 1997.

Results

There is a suggestive physiologic and epidemiologic relationship between Na, K-ATPase inhibition and hypertension. However, clearance data cannot support the hypothesis that differential metabolism of this family of compounds explains the improved hypertensive control seen in patients on peritoneal dialysis compared to those on hemodialysis.

Conclusions

As a result of the complex methodologies involved, it is unclear whether Na, K-ATPase inhibitors playa significant role in the hypertension of endstage renal disease in general and peritoneal dialysis in particular.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Inhibitory effect of bufalin and cinobufagin on steroidogenesis via the activation of ERK in human adrenocortical cells

BACKGROUND AND PURPOSE

Bufalin and cinobufagin exhibit cardiotonic and natriuretic activities. The aim of this study was to evaluate the effects of bufalin and cinobufagin on aldosterone and cortisol secretion and their mechanisms of action in human adrenocortical cells (NCI-H295).

EXPERIMENTAL APPROACH

H295 cells were incubated with bufalin or cinobufagin in the presence or absence of angiotensin II (Ang II), forskolin, 8-Br-cAMP, corticosterone or deoxycortisol. The role of ERK1/2 was studied by use of the inhibitor of MEK (U0126). The binding of transcription factor steroidogenic factor 1 (SF-1) to steroidogenic acute regulatory (StAR) gene promoter was analysed by EMSA.

KEY RESULTS

Bufalin and cinobufagin markedly inhibited basal, Ang II-, forskolin- or 8-Br-cAMP-stimulated aldosterone and cortisol secretion, and the conversions of corticosterone to aldosterone and deoxycortisol to cortisol. Bufalin and cinobufagin also inhibited StAR protein expression and SF-1 binding to StAR gene promoter. They both increased phosphorylation of ERK1/2, and U0126 fully abolished these effects on ERK1/2 in H295 cells. Furthermore, U0126 reversed the inhibitory effects of bufalin and cinobufagin on StAR protein expression and the binding of SF-1 to StAR gene promoter. However, U0126 did not completely reverse their inhibitory effects on aldosterone and cortisol release.

CONCLUSIONS AND IMPLICATIONS

The inhibitory effects of bufalin and cinobufagin on steroidogenesis of aldosterone and cortisol were associated with inhibition of aldosterone synthase and 11β-hydroxylase, as well as the suppression of StAR protein expression and SF-1 binding to StAR promoter via the phosphorylation of ERK1/2 in H295 cells.

Sodium balance in pregnancy

Sodium is the major cation in the extracellular fluid volume (ECFV) and as such, is the most important determinant of osmolality and of the volume of this fluid compartment. Hence any alteration in the control of body sodium will be reflected by changes in the ECFV, including the maternal plasma volume. There is no doubt that expansion of the plasma volume is a necessary and desirable event during pregnancy, influencing positively both maternal and fetal outcome. Therefore, studies of sodium balance in pregnancy provide important information relevant to both mother and fetus.

Effects of salt loading and various therapies on cardiac hypertrophy and fibrosis in young spontaneously hypertensive rats

We investigated the effects of salt loading on blood pressure, cardiac hypertrophy and fibrosis as well as on the effectiveness of various antihypertensive therapies in young spontaneously hypertensive rats (SHR). Twenty-five male SHR were salt-stimulated by drinking 1% NaCl from 3 to 6 months of age. Eighteen of them were treated for the last 2 weeks of salt loading with either the angiotensin-converting enzyme inhibitor captopril, the beta-adrenergic receptor blocker propranolol or the calcium-channel antagonist verapamil. Age-matched male Wistar-Kyoto (WKY) rats and SHR drinking only water served as controls. At the age of 6 months, SHR had significantly elevated blood pressure that was unchanged by salt loading. Relative heart weight was increased in SHR without (3.3) and even more so with salt intake (3.6 vs. 2.4 in WKY). Left ventricular (LV) hypertrophy was accompanied by a 17-fold increase in the expression of mRNA for atrial natriuretic factor (ANF) both in untreated and salt-loaded SHR compared to WKY (p<0.001). Collagen I and III mRNA increased 1.7-1.8-fold in SHR without and with additional salt intake (p<0.01). None of the therapies significantly reduced blood pressure or hypertrophy. Although captopril had no antihypertensive effect, it reduced ANF, collagen I and III mRNA in LV to control level. Less pronounced effects were achieved with verapamil. These findings emphasize the cardioprotective role of captopril which may not be fully expressed in the presence of elevated salt intake.

Resibufogenin corrects hypertension in a rat model of human preeclampsia

The study of the pathogenesis of preeclampsia has been hampered by a relative dearth of animal models. We developed a rat model of preeclampsia in which the excretion of a circulating inhibitor of Na/K ATPase, marinobufagenin (MBG), is elevated. These animals develop hypertension, proteinuria, and intrauterine growth restriction. The administration of a congener of MBG, resibufogenin (RBG), reduces blood pressure to normal in these animals, as is the case when given to pregnant animals rendered hypertensive by the administration of MBG. Studies of Na/K ATPase inhibition by MBG and RBG reveal that these agents are equally effective as inhibitors of the enzyme.

Small concentrations of mercury enhances positive inotropic effects in the rat ventricular myocardium

Small concentrations of mercury reduce the force development of the rat right myocardium and inhibit the sodium pump. This inhibition might increase the amount of activator Ca(2+) in the sarcoplasmic reticulum available upon activation. We further investigated whether this action could potentiate positive inotropic effects produced by Ca(2+) increment (0.62-1.25mM) and isoproterenol (20nM). Right ventricular strips contracting isometrically were treated with 0.1 and 1μM HgCl(2). Results showed that after HgCl(2) treatment the magnitude of both inotropic interventions was increased. Considering that Hg(2+) amongst its effects induces hypertension and might be associated with cardiovascular diseases these potentiating effects might have hazardous consequences.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure; production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells; as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure;production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells;as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

Carotid and femoral intima-media thickness in relation to three candidate genes in a Caucasian population

BACKGROUND

In a Caucasian population, the prevalence and incidence of hypertension, renal function and large artery stiffness were significantly correlated with polymorphisms in the genes encoding the angiotensin-converting enzyme (ACE I/D), aldosterone synthase (-C344T) and the cytoskeleton protein alpha-adducin (Gly460Trp).

OBJECTIVE

This study investigated intima-media thickening, a precursor of atherosclerosis, in relation to these genetic polymorphisms.

METHODS

Carotid and femoral intima-media thickness were assessed with a wall-track system in 380 subjects enrolled in a population study. Subjects were genotyped for the presence of the ACE D, aldosterone synthase -344T and alpha-adducin 460Trp alleles. The statistical analysis allowed for confounders, interactions among genes, and the non-independence of the phenotypes within families.

RESULTS

The sample included 188 men (49.5%). Mean age was 39.8 years. Intima-media thickness of the carotid and femoral arteries averaged 575 and 719 microm, respectively. Intima-media thickness of the femoral-but not carotid-artery increased with the number of ACE D alleles. The effect of ACE genotype on femoral intima-media thickness was confined to carriers of the 460Trp allele and the -344T allele. Expressed as a percentage of the population mean, the mean differences between II and DD homozygotes averaged 13.4% (95% CI 5.6-21.2%) in all subjects, 21.2% (8.0-34.5%) in carriers of the 460Trp allele, 15.4% (4.1-26.8%) in carriers of the -344T allele, and 25.2% (10.7-39.7%) if the 460Trp and -344T alleles were both present.

CONCLUSION

This study shows that a relationship exists between the intima-media thickness of the large muscular femoral artery and the ACE gene. This relationship is only apparent in the presence of either the alpha-adducin 460Trp or the aldosterone synthase -344T allele. These findings may have clinical implications for the assessment of genetic cardiovascular risk.

The neurohormonal natural history of essential hypertension: towards primary or tertiary aldosteronism?

Use of the aldosterone-to-renin ratio has controversially suggested that approximately 10% of hypertensives have primary aldosteronism, and most of these individuals are thought to have idiopathic hyperaldosteronism. The usual renin-angiotensin system control is intact in these individuals and is similar to that in low renin and essential hypertensives, differing only in the degree of sensitivity. There is recent evidence suggesting that hyperaldosteronism relates to aldosterone synthase genetic polymorphism, and also that increased angiotensin II stimulation of the adrenal glands appears to paradoxically upregulate the receptors increasing angiotensin II sensitivity. Taken together, the possibility arises that, in susceptible hypertensives, hyperaldosteronism could be acquired. Indeed, it is well known that renin-driven renovascular hypertension is associated with the development of hyperaldosteronism. Hypothetically, within the wider hypertensive population, these findings set the scene that angiotensin II adrenal sensitivity increases over time until the secretion of aldosterone becomes "autonomous" and hence "tertiary" aldosteronism in a significant proportion of hypertensives.

A review of the medical treatment of primary aldosteronism

PURPOSE

Use of the aldosterone-to-renin ratio (ARR) has suggested that at least one in 10 hypertensive subjects have primary aldosteronism (PA). There is thus a timely need to review the literature for effective drug therapies and to speculate on other therapeutic options by taking into account recent advances in understanding of the PA disease pathophysiological process.

DATA SOURCE

A MEDLINE and EMBASE search of all articles published from the start of the databases until July 1999 and reviews of the bibliographies of textbooks.

STUDY SELECTION

Primary research articles on the medical treatment of PA with emphasis on diagnosis, treatment option, drug dosage, therapeutic response and adverse drug effect.

DATA EXTRACTION

Study design and quality were assessed. Relevant data on diagnostic methodology, drug usage and response were analysed and compared.

DATA SYNTHESIS

A select number of subjects with aldosterone-producing adenoma (APA) can be expected to respond well to surgical treatment For the majority of PA cases especially subjects with idiopathic hyperaldosteronism (IHA), long-term medical treatment is now safe and feasible although no randomized controlled trials have been carried out to date. The best therapeutic response is obtained by directly antagonizing aldosterone at the receptor level using medium to low dose spironolactone and this response can be predicted by a raised ARR. The response to other potassium-sparing diuretics and calcium channel blockers are modest. IHA responds better than angiotensin II-unresponsive APA to angiotensin converting enzyme inhibitors and this may also be true with angiotensin II receptor blockers. The discovery of the aldosterone synthase gene opens up the possibility for gene therapy.

CONCLUSION

The diagnosis of PA allows appropriate management with resultant blood pressure control in many hypertensive subjects who otherwise have resistant hypertension despite multiple drug therapy.

Adverse cardiac effects of salt with fludrocortisone in hypertension

The effect of salt on blood pressure (BP) is controversial. A more important question is whether salt can produce cardiac target-organ damage, irrespective of its effect on BP. We assessed the effect of salt with fludrocortisone on QT dispersion and echocardiographic left ventricular diastolic function in a prospective interventional study involving 29 hypertensive subjects with a raised aldosterone/renin ratio who were hospitalized for investigation of possible primary aldosteronism. Each subject over 4 days was given a total of 28.8 g (480 mmol) of sodium chloride and 1.5 mg of fludrocortisone with potassium supplementation. Baseline and posttreatment 12-lead ECGs and echocardiograms were obtained. There were no significant changes in body weight, pulse rate, or BP after treatment with salt and fludrocortisone. Plasma sodium was significantly increased from 141.4 (SD 2.1) to 142.6 (SD 2.4) mmol/L (P:=0.001). QT and QTc dispersion both significantly increased: +19.6 (SD 16.5) ms (95% CI, 13.4 to 25.9) (P:<0.001) and +19.8 (SD 20.9) ms (95% CI, 11.8 to 27.7) (P:<0.001), respectively. There were no significant changes in (n=15) left ventricular dimensions or systolic function, but all diastolic filling indexes, including the preload-independent index, flow propagation velocity (55.49 [SD 10.91] to 48.96 [SD 11.40] cm/s, P:=0.018) worsened, suggesting significant deterioration of left ventricular diastolic function with salt and fludrocortisone. In conclusion, a combination of salt with fludrocortisone increased QT dispersion and impaired left ventricular diastolic relaxation in hypertensive patients with high aldosterone/renin ratios. This raises the possibility that salt may have BP-independent adverse cardiac effects in susceptible hypertensive subjects.

Up-regulation of endothelial nitric oxide activity as a central strategy for prevention of ischemic stroke - just say NO to stroke!

Nitric oxide (NO) produced by the endothelium of cerebral arterioles is an important mediator of endothelium-dependent vasodilation (EDV), and also helps to prevent thrombosis and vascular remodeling. A number of risk factors for ischemic stroke are associated with impaired EDV, and this defect is usually at least partially attributable to a decrease in the production and/or stability of NO. These risk factors include hypertension, high-sodium diets, homocysteine, diabetes, visceral obesity, and aging. Conversely, many measures which may provide protection from ischemic stroke - such as ample dietary intakes of potassium, arginine, fish oil, and selenium - can have a favorable impact on EDV. Protection afforded by exercise training, estrogen replacement, statin drugs, green tea polyphenols, and cruciferous vegetables may reflect increased expression of the endothelial NO synthase. IGF-I activity stimulates endothelial NO production, and conceivably is a mediator of the protection associated with higher-protein diets in Japanese epidemiology and in hypertensive rats. These considerations prompt the conclusion that modulation of NO availability is a crucial determinant of risk for ischemic stroke. Multifactorial strategies for promoting effective cerebrovascular NO activity, complemented by measures that stabilize platelets and moderate blood viscosity, should minimize risk for ischemic stroke and help maintain vigorous cerebral perfusion into ripe old age. The possibility that such measures will also diminish risk for Alzheimer's disease, and slow the normal age-related decline in mental acuity, merits consideration. A limited amount of ecologic epidemiology suggests that both stroke and senile dementia may be extremely rare in cultures still consuming traditional unsalted whole-food diets. Other lines of evidence suggest that promotion of endothelial NO activity may decrease risk for age-related macular degeneration.

Renal sodium/calcium exchange; a vasodilator that is defective in salt-sensitive hypertension

The Na+ : Ca2+ exchanger is an important plasma membrane ion transport pathway that plays a major role in controlling [Ca2+]i. In smooth muscle cells, it may function as a Ca2+ extrusion pathway and may help lower [Ca2+]i in response to vasoconstrictor-induced increases in [Ca2+]i. It may also extrude [Ca2+]i and lead to vasodilation in response to vasodilators. Our recent studies have been performed to determine the existence and regulation of the Na+ : Ca2+ exchanger in renal contractile cells which include afferent and efferent arterioles and mesangial cells. Exchanger activity is present in all three of these contractile elements but is higher in afferent arterioles vs. efferent arterioles. We have also examined the role of altered regulation of the exchanger in the SHR and in salt-sensitive hypertension. With the establishment of high blood pressure, Na+ : Ca2+ exchanger activity is reduced in afferent but not in efferent arterioles in both models of hypertension. Other works in cultured mesangial cells and freshly dissected afferent arterioles, have shown that protein kinase C (PKC) up-regulates the Na+ : Ca2+ exchanger from Dahl/Rapp salt-resistant rats while it fails to do so in arterioles and mesangial cells from salt-sensitive rats. This defect in PKC regulation of Na+ : Ca2+ exchange is the result of a loss of PKC-mediated translocation of the exchanger to the plasma membrane in S mesangial cells. Thus, a defect in the PKC-Na+ : Ca2+ exchanger-translocation pathway may cause dysregulation of [Ca2+]i and help explain the dramatic decrease in GFR that occurs in this model of hypertension.

Na-dependent changes in intracellular Ca in spontaneously hypertensive rat hearts

To determine whether Na/Ca exchange is altered in primary hypertension, Na-dependent changes in intracellular Ca, ([Ca]i), were measured in isolated perfused hearts from Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Intracellular Na, (Nai, mEq/kg dry wt), and [Ca]i were measured by NMR spectroscopy. Control [Ca]i was less in WKY than SHR (176 +/- 18 vs 253 +/- 21 nmol/l; mean +/- S.E., P < 0.05), whereas Nai was not significantly different. One explanation for this is that net Na/Ca exchange flux is decreased in SHR. If this hypothesis is correct, the rate of Ca uptake in SHR should be less than WKY when Na/Ca exchange is reversed by decreasing the transmembrane Na gradient. The Na gradient was reduced by decreasing extracellular Na, ([Na]o) and/or by increasing [Na]i. To increase [Na]i, Na uptake was stimulated by acidification while Na extrusion by Na/K ATPase was inhibited by K-free perfusion. Seventeen minutes after acidification, Nai had increased but was not significantly different in SHR and WKY (18.0 +/- 2.3 to 57.4 +/- 7.6 vs 20.3 +/- 0.6 to 66.5 +/- 4.8 mEq/kg dry wt, respectively). Yet [Ca]i was greater in WKY than SHR (1768 +/- 142 vs 1201 +/- 90 nmol/l; P < 0.05). [Ca]i was also measured after decreasing [Na]o from 141 to 30 mmol/l. Fifteen minutes after reducing [Na]o, [Ca]i was greater in WKY than SHR (833 +/- 119 vs 425 +/- 94 nmol/l; P < 0.05). Thus for both protocols, decreasing the transmembrane Na gradient led to increased [Ca]i in both SHR and WKY, but less increase in SHR. The results are consistent with the hypothesis that Na/Ca exchange activity is less in SHR than WKY myocardium.

Renal arteriolar Na+/Ca2+ exchange in salt-sensitive hypertension

The present studies were performed to assess Na+/Ca2+ exchange activity in afferent and efferent arterioles from Dahl/Rapp salt-resistant (R) and salt-sensitive (S) rats. Renal arterioles were obtained by microdissection from S and R rats on either a low-salt (0.3% NaCl) or high-salt (8.0% NaCl) diet. On the high-salt diet, S rats become markedly hypertensive. Cytosolic calcium concentration ([Ca2+]i) was measured in fura 2-loaded arterioles bathed in a Ringer solution in which extracellular Na (Nae) was varied from 150 to 2 mM (Na was replaced with N-methyl-D-glucamine). Baseline [Ca2+]i was similar in afferent arterioles of R and S rats fed low- and high-salt diet. The change in [Ca2+]i (Delta[Ca2+]i) during reduction in Nae from 150 to 2 mM was 80 +/- 10 and 61 +/- 3 nM (not significant) in afferent arterioles from R rats fed the low- and high-salt diet, respectively. In afferent arterioles from S rats on a high-salt diet, Delta[Ca2+]i during reductions in Nae from 150 to 2 mM was attenuated (39 +/- 4 nM) relative to the Delta[Ca2+]i of 79 +/- 13 nM (P < 0.05) obtained in afferent arterioles from S rats on a low-salt diet. In efferent arterioles, baseline [Ca2+]i was similar in R and S rats fed low- and high-salt diets, and Delta[Ca2+]i in response to reduction in Nae was also not different in efferent arterioles from R and S rats fed low- or high-salt diets. Differences in regulation of the exchanger in afferent arterioles of S and R rats were assessed by determining the effects of protein kinase C (PKC) activation by phorbol 12-myristate 13-acetate (PMA, 100 nM) on Delta[Ca2+]i in response to reductions in Nae from 150 to 2 mM. PMA increased Delta[Ca2+]i in afferent arterioles from R rats but not from S rats. These results suggest that Na+/Ca2+ exchange activity is suppressed in afferent arterioles of S rats that are on a high-salt diet. In addition, there appears to be a defect in the PKC-Na+/Ca2+ exchange pathway that might contribute to altered [Ca2+]i regulation in this important renal vascular segment in salt-sensitive hypertension.

Role of digitalis-like substance in the hypertension of streptozotocin-induced diabetes and simulated weightlessness in rats

We have examined the role of plasma Na+-K+ pump inhibitor (SPI) in the hypertension of streptozotocin induced insulin dependent diabetes (IDDM) in reduced renal mass rats. The increase in blood pressure (BP) was associated with an increase in extracellular fluid volume (ECFV), and SPI and a decrease in myocardial Na+,K+ATPase (NKA) activity, suggesting that increased SPI, which inhibits cardiovascular muscle (CVM) cell NKA activity, may be involved in the mechanism of IDDM-hypertension. In a second study, using prolonged suspension resulted in a decrease in cardiac NKA activity, suggesting that cardiovascular deconditioning following space flight might in part result from insufficient SPI.

Hypoxia triggers release of an endogenous inhibitor of Na(+)-K(+)-ATPase from midbrain and adrenal

An endogenous inhibitor of Na(+)-K(+)-ATPase has been isolated from bovine hypothalamus and human plasma and structurally characterized as an isomer of the plant cardiac glycoside, ouabain (A. A. Tymiak, J. A. Norman, M. Bolgar, G. C. DiDonato, H. Lee, W. L. Parker, L.-C. Lo, N. Berova, K. Nakanishi, E. Haber, and G. T. Haupert, Jr. Proc.Natl.Acad.Sci. USA 90: 8189-8193, 1993; N. Zhao, L.-C. Lo, N. Berova, K. Nakanishi, J. H. Ludens, and G. T. Haupert, Jr. Biochemistry 34: 9893-9896, 1995). This hypothalamic inhibitory factor (HIF) acts on cardiovascular and renal tissues consistent with physiological regulation in vivo. Stimuli for the release of HIF from tissue are unknown. Hypoxia may be a stimulus for the elaboration of digitalis-like activity in humans, and high NaCl concentration in central nervous system stimulates ouabain-like activity in animals. We examined the ability of low O2 tension in vivo and in vitro to stimulate HIF release from midbrain and adrenal tissues in Wistar rats. In both tissues, hypoxia stimulated a remarkable release of an inhibitor cochromatographing with HIF, and this release was enhanced by 300 mM NaCl. Plasma from hypoxic rats also showed increased levels of the purified inhibitory activity. We conclude that hypoxia is a potent stimulus for the release of HIF or HIF-like activity and discuss the possibility that an Na(+)-K(+)-ATPase inhibitor could be involved in energy-conserving cellular adaptive responses to hypoxic or ischemic insult through ATP conservation.

Evaluation of changes in sympathetic nerve activity and heart rate in essential hypertensive patients induced by amlodipine and nifedipine

OBJECTIVE

To compare the effects of amlodipine and nifedipine on heart rate and parameters of sympathetic nerve activity during the acute and chronic treatment periods in order to elucidate their influence on cardiovascular outcome.

DESIGN

A randomized and single-blind study.

METHODS

We performed 24 h ambulatory electrocardiography and blood pressure monitoring of 45 essential hypertensive inpatients. Plasma and urinary catecholamine levels were measured during the control (pretreatment) period, on the first day (acute period) and after 4 weeks (chronic period) of administration of amlodipine and of short-acting nifedipine or its slow-releasing formulation. The low-frequency and high-frequency power spectral densities and low-frequency: high-frequency ratio were obtained by heart rate power spectral analysis.

RESULTS

Blood pressure was significantly and similarly reduced by administrations of amlodipine, short-acting nifedipine and slow-releasing nifedipine during the chronic period. The total QRS count per 24 h, which remained constant during the chronic period of administration of slow-releasing nifedipine and was increased by administration of nifedipine, was decreased by 2.8% by administration of amlodipine. Administration of amlodipine decreased the plasma and urinary norepinephrine levels during the chronic period, whereas the levels were significantly increased by administration of short-acting nifedipine and not changed by administration of slow-release nifedipine. Although low-frequency: high-frequency ratio was increased significantly by administration of short-acting nifedipine and slightly by administration of slow-releasing nifedipine, administration of amlodipine reduced it during the acute and chronic periods.

CONCLUSIONS

Administration of amlodipine did not induce an increase in sympathetic nerve activity in essential hypertensive patients during the chronic period, suggesting that beneficial effects on essential hypertension can be expected after its long-term administration. Administration of slow-releasing nifedipine induces milder reflex sympathetic activation than does that of short-acting nifedipine.

Nonesterified fatty acids in the pathogenesis of hypertension: theory and evidence

This paper approaches the hypothesis that fatty acids contribute to hypertension by examining possible interactions of nonesterified fatty acids with renal pressure-natriuresis, peripheral vascular resistance, and the central nervous barostat, three loci where long-term regulation of blood pressure is probably controlled. By inhibiting aldosterone secretion, nonesterified fatty acids may lower blood pressure by facilitating pressure-natriuresis. Oxygenated metabolites of fatty acids appear to stimulate aldosterone secretion. In different experimental situations, fatty acids either constrict or dilate arteries. There is no evidence of an effect of fatty acids on the central nervous barostat, but they do sensitize peripheral vessels to alpha-adrenergic stimuli. Obesity and diabetes are marked by increased incidence of hypertension, and elevated levels of fatty acids or their P450 oxygenated metabolites may contribute to this association. Drugs that influence plasma fatty acids, like heparin, do not have reproducible effects on blood pressure. Experimental evidence suggests but does not prove that nonesterified fatty acids can affect the long-term set-point of blood pressure.

Sodium pump and Na+/H+ activities in uremic erythrocytes. A microcalorimetric and pH-metric study

The sodium pump and Na+/H+ antiport activities in red blood cells from uremic hemodialyzed patients were measured concomitantly. The patients selected (n = 35) were normotensive and free of intercurrent illness known to affect Na transport. The Na pump activity of intact red blood cells in suspension in their own plasma was measured by flow microcalorimetry. The Na+/H+ antiport activity of the erythrocytes from the same patients was determined by a titrimetric technique. The mean global value of the sodium pump was lower in uremics than in controls (13.3 +/- 0.6 vs. 11.3 +/- 0.8 mW/l cells, P < 0.05). The Na+/H+ antiport maximal activity was decreased in uremics (2.9 +/- 0.3 vs. 4.6 +/- 0.5 mmol H+/l cells/h, P < 0.05). Our results thus confirm that uremia per se can affect sodium transport. Moreover it has been shown that a decrease in Na+/H+ antiport activity is able to counteract an impairment of sodium pump. The decrease found in this study could thus explain, at least in part, the absence of hypertension in the patients studied despite their decreased sodium pump activity.

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

Altered sodium-calcium exchange in afferent arterioles of the spontaneously hypertensive rat

Studies were performed to determine if there is a derangement in Na-Ca exchange activity in afferent (AA) and efferent (EA) arterioles from 3- and 9-week-old spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Cytosolic calcium concentration ([Ca2+]i) was assessed using microscope-based photometry in fura-2 loaded arterioles bathed in a Ringer's solution. Baseline [Ca2+]i was similar in the AA of 3- and 9-week-old WKY and SHR. In AA from 3-week-old rats, [Ca2+]i increased by 89 +/- 15 nM in WKY and by 73 +/- 13 nM in SHR during decreases in bath sodium concentration ([Na+]e) from 150 to 2 mM (Na+ replaced with n-methyl-D-glucamine). In 9-week-old hypertensive SHR (SBP = 150 mm Hg), increases in [Ca2+]i were attenuated (24 +/- 3 nM) relative to 3-week-old WKY and SHR, and 9-week-old WKY (90 +/- 9 nM; P < 0.05). Likewise, the rate of removal of Ca2+ in the continued presence of 2 mM Nae (Ca2+ sequestration and/or extrusion) was markedly reduced in AA of 9-week-old SHR (-0.15 +/- 0.03 nM/second) versus 3-week-old SHR (-0.72 +/- 0.12 nM/second) and 3- and 9-week-old WKY (-0.49 +/- 0.10 and -0.67 +/- 0.14 nM/second). In other experiments, AAs were preincubated in 1 mM ouabain to increase intracellular [Na+]. This maneuver augmented the increase in [Ca2+]i obtained with removal of Na+e; however, the responses obtained in 9-week-old SHR arterioles were still attenuated compared to those obtained in arterioles for 3- and 9-week-old WKY and 3-week-old SHR. These results suggest that exchanger number and/or sensitivity to the transmembrane Na gradient was reduced in the SHR AA. In EA, baseline [Ca2+]i was similar in 3- and 9-week-old WKY and SHR. In contrast to AA, the magnitude of Na-dependent and Na-independent changes in [Ca2+]i was not different in the EA of 3- and 9-week-old WKY and SHR. These results indicate that regulation of Na-Ca exchange activity may differ between AA and EA segments. Furthermore, diminished Na-Ca exchange and Na-independent Ca2+ sequestering/extrusion mechanisms could contribute to altered AA [Ca2+]i in the SHR.

Electrolyte content of serum, erythrocyte, kidney and heart tissue in salt induced hypertensive rats

The effect of salt load on the systolic blood pressure (SBP) and electrolyte levels of serum, erythrocyte, kidney and heart tissue was studied in rats. NaCl treatment increased sodium (5.69 +/- 0.4 mmol/L p < 0.001, 149.8 +/- 4.0 mEq/L, p < 0.001) and decreased potassium (112.6 +/- 2.4 mmol/L p < 0.001, 5.0 +/- 0.2 mEq/L, p < 0.001) in red cell and serum respectively. A decreased level of serum magnesium (1.4 +/- 0.3 mEq/L, p < 0.005) was observed. Sodium content was increased in both heart (39.93 +/- 2.9 mumol/g, p = n.s) and kidney tissues (44.39 +/- 0.5 mumol/g, p < 0.001). A pronounced increase in intracellular calcium (2.54 +/- 0.2 mumol/g, p < 0.001) and a decrease of magnesium content (6.05 +/- 0.8 mumol/g, p < 0.001) was observed in kidney tissue after treatment. The results suggested that marked changes in electrolyte levels of erythrocytes, serum, heart and kidney tissues in NaCl loaded rats may play a definite role in the development of salt induced hypertension.

Intracellular cations and hypertension in blacks

Considerable attention has been focused in recent years on the role of intracellular ions in the pathophysiology of hypertension in African Americans. Following the identification of marked differences in red cell sodium content and sodium-lithium counter-transport between blacks and whites, the hypothesis emerged that cation metabolism at the cellular level might account for part of the ethnic difference in susceptibility to hypertension. Unfortunately, findings in the red cell have not significantly increased our understanding of the physiologic pathways and may prove to be anomalous. Interest has recently shifted to calcium metabolism, because of its greater physiologic relevance, and a new series of questions are being defined. With the development of fluoroprobes to measure sodium and calcium simultaneously, wider application of pharmacologic agonists and the use of single cell techniques have provided a new direction for this field. Whether the ethnic contrasts observed for sodium in the red cell can be reproduced in platelets remains to be seen. It may be that the earlier red cell differences were epiphenomena, unrelated to the control of blood pressure, and that the more closely one approaches basic physiologic mechanisms, the greater the cross-ethnic group similarity. Rather than drawing attention to unusual phenotypic characteristics, the study of hypertension in blacks may be more instructive for the insight it provides into the basic physiology of this disorder, common to all ethnic groups.

Possible dietary measures in the prevention of pre-eclampsia and eclampsia

The recurring theme when considering the various dietary manipulations that have been advocated in the prevention of pre-eclampsia is of dietary deficiencies and excesses that appear to be associated with the disease. However, whether these deficiencies and excesses cause, or arise as a consequence of, pre-eclampsia has frequently not been established. Advocates of dietary theories of the aetiology of the disease have tended to base their claims upon uncontrolled clinical trails and inadequate research investigations. With very few exceptions, the better-designed studies have failed to show any effect of dietary supplementation or restriction on the incidence of pre-eclampsia. While certain manipulations, for example calcium and n-3 fatty acid supplementation seem intriguing and merit further study, potentially harmful effects of other manipulations, namely weight restriction (Campbell Brown, 1983), salt restriction (Robinson, 1958) and magnesium supplementation (Lamm et al, 1988), have been demonstrated. The persistent tendency for the medical profession to provide dogmatic and often conflicting advice, and to advocate rigorous intervention, is thus culpable. Until the appropriate research is performed in a less haphazard fashion, with all interventions performed in the context of controlled trials, no dietary intervention can be advocated.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Normal sensitivity of Na+/K(+)-ATPase isolated from brain and kidney of spontaneously hypertensive rats to sodium, ouabain or mercury

Genetically hypertensive rats are excellent animal models for investigating putative Na+/K(+)-ATPase alterations associated with the disease. Highly purified Na+/K(+)-ATPase preparations from these animals have not yet been examined. Na+/K(+)-ATPases of two strains of spontaneously hypertensive rats, the Milan hypertensive strain (MHS) and the spontaneously hypertensive rat (SHR) were characterized in comparison with enzymes isolated from their matched normotensive controls; the sensitivity to Na ions as well as the shape and span of the inhibition curves for ouabain and mercury of the isolated Na+/K(+)-ATPases were compared. No functional changes between the purified 'normotensive' and 'hypertensive' Na+/K(+)-ATPases from brain and kidney were detected ruling out drastic structural alterations of the transport system in these two organs of diseased animals.

Interaction of hypothalamic Na,K-ATPase inhibitor with isolated human peripheral blood mononuclear cells

A ligand for the digitalis receptor located on the membrane-embedded Na,K-ATPase (NKA; EC 3.6.1.37) has been isolated from bovine hypothalamus (hypothalamic inhibitory factor; HIF) and identified as isomeric ouabain (Tymiak et al., 1993, Proc. Natl. Acad. Sci. 90: 8189-8193). In analogy to cardioactive steroids (CS) derived from plants or from toad, HIF inhibits the Na/K-exchange process and the ATPase activity of isolated Na,K-ATPase although by a different molecular action mechanism. In the present work we show that, as plant-derived ouabain, HIF inhibits 86Rb-uptake by isolated human lymphocytes with an IC50 of about 20 nM; above this concentration HIF reduces cell viability in contrast to ouabain. The decrease in cell viability by excess HIF is accompanied by discrete morphological alterations (mitochondrial swelling) visible by transmission electron microscopy of ultra-thin sectioned peripheral blood mononuclear cells. Taken together the results show that the hypothalamic NKA inhibitor blocks NKA of isolated human lymphocytes with high potency at nanomolar concentrations without toxicity; concentrations exceeding the ones required to block 86Rb-uptake reduce cell viability, probably due to leak formation across the NKA molecule. Thus, lymphocytes constitute a potential target for HIF action and by their altered NKA status a possible messenger between the nervous and the immune system.

Improvement of cardiovascular effects of metoprolol by replacement of common salt with a potassium- and magnesium-enriched salt alternative

1. The influence of sodium chloride (NaCl)-enrichment of the diet (6% of the dry weight) and that of a novel sodium-reduced, potassium-, magnesium-, and L-lysine-enriched salt alternative on the cardiovascular effects of the beta 1-adrenoceptor blocking drug, metoprolol, was studied in stroke-prone spontaneously hypertensive rats. 2. Increased dietary sodium chloride intake produced a marked rise in blood pressure and induced left ventricular and renal hypertrophy. By contrast, the salt alternative did not increase blood pressure and caused remarkably less cardiac and renal hypertrophy than did sodium chloride. 3. Metoprolol treatment at a daily dose of 250 mg kg-1 lowered blood pressure and decreased left ventricular hypertrophy index during the control diet. Sodium chloride-enrichment blocked the antihypertensive effect of metoprolol, while a partial protective effect on left ventricular and renal hypertrophy persisted. In the presence of the salt alternative-enrichment both at the level of 6% and 10.5% (corresponding to a NaCl level of 6%), metoprolol was fully able to exert its beneficial cardiovascular and renal effects. 4. Both salt supplementations, irrespective of metoprolol treatment, induced a 3 to 4 fold increase in the urinary excretion of calcium. There was a linear correlation between the urinary excretions of sodium and calcium. The urinary excretion of magnesium rose by 90% and that of potassium by 110% in the salt alternative group. 6. Our findings suggest that replacement of common salt by a potassium-, and magnesium-enriched salt alternative in the diet produces beneficial cardiovascular effects and improves the antihypertensive efficacy of metoprolol in stroke-prone spontaneously hypertensive rats. Increased intake of potassium and/or magnesium and L-lysine from the salt alternative is involved in the beneficial effects of the salt alternative. The NaCl-induced myocardial and renal hypertrophies appear to be partially mediated by Beta-adrenoceptor activation.

Intracellular calcium concentration and activation of the Na+/H+ exchanger in essential hypertension

To investigate the relationship between changes in intracellular calcium concentration ([Ca2+]i) and agonist-induced activation of the Na+/H+ exchanger in essential hypertension (EH), platelet [Ca2+]i and pHi were measured in 24 patients with EH (14 males) aged 48 +/- 2 years and 23 matched normotensive controls (NT) (12 males) aged 45 +/- 3 years. Measurements were done with spectrofluorimetry using the dyes Fura-2 for [Ca2+]i and BCECF for pHi. [Ca2+]i and pHi were measured in the resting condition and after stimulation in vitro with 0.1 U/ml human thrombin. The thrombin-induced rise in pHi was Na+ dependent and amiloride sensitive, indicating that it was mediated by the Na+/H+ exchanger. Unstimulated [Ca2+]i was higher in patients with EH than in NT (60 +/- 3 vs. 48 +/- 1 nmol/liter, P < 0.005), but there were no differences in resting pHi between both groups (7.16 +/- 0.01 vs. 7.16 +/- 0.008). In the presence of 1 mmol/liter external calcium (Ca2+o), thrombin-induced increment in [Ca2+]i was significantly greater in patients with EH than in NT (281 +/- 21 vs. 206 +/- 19; P < 0.05) as was the pHi increment (EH: 0.137 +/- 0.01; NT: 0.095 +/- 0.01; P < 0.05). Both agonist-induced increments in [Ca2+]i and in pHi were correlated with mean arterial pressure (MAP) only in the EH group (r = 0.58, P < 0.005 and r = 0.59, P < 0.005, respectively). The agonist-induced rise in pHi was positively correlated with the rise in [Ca2+]i both in the EH group (r = 0.65, P < 0.001) and in the NT (r = 0.55, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte Na+,K(+)-ATPase properties and adenylate energy charge in normotensives and in essential hypertensives

The activity and some kinetic properties of RBC Na+,K(+)-ATPase (EC 3.6.1.37) were investigated in essential hypertensives (EH; 40 subjects) and normotensives (NT; 20 subjects). A decrease in ouabain-sensitive 86Rb uptake as well as ouabain-sensitive ATPase activity was found in EH. [Na+]i and [K+]i of EH did not show any statistical difference from NT. Na+,K(+)-ATPase showed a reduced Mg2+ activation and the apparent Km value for Mg2+ was 2-fold increased in the EH group. The influence of temperature on the Na+,K(+)-ATPase showed a reduced modulation and a minor activity peak at 37 degrees C in the patients, consequently the calculated activation energy of the enzyme was increased at temperatures lower than 40 degrees C. Increased RBC adenylate energy charge (EC) was observed in EH when compared with NT. A negative correlation between EC and total Na+,K(+)-ATPase activity was found when all subjects were compared and also in both groups, showing a possible pump involvement in the regulation of the RBC metabolic flux in EH. These data provide evidence about some modifications in active Na+,K+ transport and in EC in RBC which allows a further characterization of membrane cation fluxes in EH.

A Na pump inhibitor from bovine posterior pituitary: purification, structure determination and its cardiovascular effect in rat

We examined the hypothesis that hypothalamo-hypophysial tissue contains an endogenous Na pump inhibitor. From bovine posterior pituitary, we purified a substance which inhibits Rb uptake by human erythrocytes. This inhibitory activity was found in the eluate of 10% acetonitrile from a C18 flash column and purified by subsequent three steps of reversed-phase high-performance liquid chromatography (HPLC). Sequence analysis revealed that this substance was identical to joining peptide, one of the major products of proopiomelanocortin (POMC). This peptide had hypertensive and tachycardiac effects in spontaneously hypertensive rats (SHR) after central administration, with weak Na,K-ATPase inhibitory activity (IC50 = 0.5 mM).

Effects of ouabain on muscle tension and intracellular Ca2+ level in guinea-pig aorta

The effects of ouabain on muscle tension and the intracellular Ca2+ level ([Ca2+]i) were examined in guinea-pig aorta loaded with fura-2. Ouabain caused a gradual and sustained increase in both [Ca2+]i and muscle tension. There was a positive correlation between these two parameters. In Ca(2+)-free solution, ouabain did not affect either [Ca2+]i or muscle tension, suggesting that the ouabain-induced increase in [Ca2+]i was not due to Ca2+ release from storage sites. The ouabain-induced increase in [Ca2+]i and muscle tension was inhibited by Ni2+, which inhibits the Na+/Ca2+ exchanger, but not by verapamil. Furthermore, anionic and cationic amphiphiles were used as modulators of the Na+/Ca2+ exchanger. Sodium dodecyl sulfate accelerated the responses to ouabain, whereas dodecyltrimethylammonium bromide inhibited them. These results suggest that in the guinea-pig aorta, ouabain induces contraction by increasing the Ca2+ influx through the Na+/Ca2+ exchanger on the plasma membrane, but not through verapamil-sensitive Ca2+ channels.

Elevated concentrations of endogenous ouabain in patients with congestive heart failure

BACKGROUND

An endogenous digitalis-like compound in mammals has long been postulated, but only recently has a substance indistinguishable from ouabain been identified in human plasma. Because of the potential significance of such a substance in patients with congestive heart failure, we sought to evaluate the pathophysiology of endogenous ouabain in these individuals.

METHODS AND RESULTS

Using an immunoassay, we determined plasma ouabain concentrations in 51 patients with heart failure and in 19 control subjects. Plasma ouabain concentrations in control subjects ranged from 0.16 to 0.77 nM (mean, 0.44 +/- 0.20 nM). In 19 matched heart failure patients receiving digoxin, the mean ouabain was significantly elevated at 1.59 +/- 2.2 nM (range, 0.17-8.76 nM, p less than 0.05 versus control subjects). The ouabain concentration correlated inversely with both cardiac index (r = -0.62, p less than 0.005) and mean arterial pressure (r = -0.51, p less than 0.05). However, there was no correlation between ouabain and left ventricular filling (r = 0.19, NS) or right atrial pressures (r = 0.20, NS). In 16 heart failure patients not receiving digoxin, the mean ouabain was 1.52 +/- 2.58 nM. No relation between renal function and ouabain was detected.

CONCLUSIONS

The unanticipated lack of correlation of ouabain with atrial pressures indicates that volume is not the chief determinant of ouabain concentration in patients with congestive heart failure. However, the significant relations of plasma ouabain concentration with cardiac index and mean arterial pressure imply that endogenous ouabain may be an important homeostatic factor in humans.

Enhancement of vascular contractility by plasma substances obtained from pregnancy-induced hypertension

1. Augmentation of norepinephrine (NE)-induced contraction of isolated dog mesenteric arteries by ouabain was significantly enhanced under 4 mM potassium ([K+]) medium condition compared with usual medium condition at 6 mM [K+]. 2. Vascular contractility to NE was significantly enhanced by plasma substances obtained from pregnancy-induced hypertension (PIH) patients compared with that from normotensive pregnant women under 4 mM [K+] medium condition despite of no difference between them under 6 mM [K+] medium condition. 3. These results suggest the possible involvement of the ouabain-like substance in development of hypertension in PIH patients.

Mild exercise decreases plasma endogenous digitalislike substance in hypertensive individuals

Changes in a plasma endogenous digitalislike substance were investigated in relation to the antihypertensive mechanism of mild exercise. Fifteen women with mild essential hypertension and seven normotensive female volunteers were divided into exercised hypertensive (n = 10), nonexercised hypertensive (n = 5), and nonexercised normotensive (n = 7) groups. A 4-week general clinical observation period preceded the study period of 10 weeks. The exercised hypertensive individuals were treated with a lactate threshold exercise that corresponded to approximately 50% of the maximum oxygen consumption three times a week, whereas the nonexercised groups were observed at the outpatient clinic as control groups. In the exercised group, systolic blood pressure fell by 7 mm Hg (p = 0.05), diastolic by 6 mm Hg (p less than 0.01), and mean blood pressure by 7 mm Hg (p less than 0.01) after 10 weeks. The reduction in the plasma endogenous digitalislike substance was significant after 7 (-1.02 ng/ml, p less than 0.05) and 10 (-1.04 ng/ml, p less than 0.05) weeks in this group. It positively correlated with the reduction in diastolic (r = 0.70, p less than 0.05) or mean (r = 0.66, p less than 0.05) blood pressure and with changes in plasma norepinephrine (r = 0.76, p less than 0.05). The mean corpuscular volume of erythrocytes decreased (-1.7 fl, p less than 0.01) after 10 weeks of exercise, and the plasma volume index tended to decrease (-108 ml/m2, p = 0.28). In the control groups, significant changes in blood pressure and plasma endogenous digitalislike substance were not observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of angiotensin-converting enzyme inhibitor on erythrocyte sodium ion transport in essential hypertension

The acute effects of angiotensin-converting enzyme inhibitor, captopril, on sodium ion transport systems were investigated in essential hypertensive and normotensive subjects. The passive sodium efflux through the erythrocyte membrane was significantly higher and erythrocyte sodium-potassium cotransport was lower in patients with essential hypertension when compared with normal subjects. However, sodium-potassium pump activity and sodium-lithium countertransport did not differ significantly between the hypertensive patients and the normal subjects. Immediately after captopril administration, erythrocyte passive sodium efflux and sodium-potassium cotransport returned to normal levels in the hypertensive subjects. Although the plasma renin activity and plasma aldosterone concentration were altered by captopril, they did not correlate with changes in any sodium transport system. These results suggest that the changes in sodium transport systems which occur immediately after captopril administration may contribute, at least in part, to its antihypertensive action.

Role of sarcolemmal membrane sodium-calcium exchange in vascular smooth muscle tension

A body of information obtained by experiments with intact tissues, isolated cells, and sarcolemmal vesicles indicates, beyond a reasonable doubt, that a specific Na(+)-Ca2+ exchange system exists in vascular smooth muscle. However, its role in the regulation of cytosolic free-Ca2+ concentration and cell tension under physiological conditions remains unclear. Under pharmacological conditions in which the Na(+)-K+ pump is inhibited either by digitalis glycosides or K(+)-free medium, Na(+)-Ca2+ exchange may be modulated by increases in cytosolic free Na+ to increase the cytosolic free-Ca2+ concentration and cell tension. Under pathological conditions in which the cytosolic Na+ concentration is increased as a result of inhibition of the Na(+)-K+ pump by endogenous ouabain or a digitalis-like factor, or activation of the Na(+)-H+ exchange or passive permeability of Na+, the Na(+)-Ca2+ exchange activity of vascular smooth muscle and the nerve terminal may play an important role in the development and/or maintenance of hypertension. These and other premises remain to be confirmed or discounted.