High salt diet down-regulates proximal tubule Na+, K(+)-ATPase activity in Dahl salt-resistant but not in Dahl salt-sensitive rats: evidence of defective dopamine regulation

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Renal amino acid transport systems and essential hypertension

Several clinical and animal studies suggest that "blood pressure goes with the kidney," that is, a normotensive recipient of a kidney genetically programmed for hypertension will develop hypertension. Intrarenal dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport. The candidate transport systems for L-DOPA, the source for dopamine, include the sodium-dependent systems B(0), B(0,+), and y(+)L, and the sodium-independent systems L (LAT1 and LAT2) and b(0,+). Renal LAT2 is overexpressed in the prehypertensive spontaneously hypertensive rat (SHR), which might contribute to enhanced L-DOPA uptake in the proximal tubule and increased dopamine production, as an attempt to overcome the defect in D1 receptor function. On the other hand, it has been recently reported that impaired arginine transport contributes to low renal nitric oxide bioavailability observed in the SHR renal medulla. Here we review the importance of renal amino acid transporters in the kidney and highlight pathophysiological changes in the expression and regulation of these transporters in essential hypertension. The study of the regulation of renal amino acid transporters may help to define the underlying mechanisms predisposing individuals to an increased risk for development of hypertension.

The sodium pump and cardiotonic steroids-induced signal transduction protein kinases and calcium-signaling microdomain in regulation of transporter trafficking

The Na/K-ATPase was discovered as an energy transducing ion pump. A major difference between the Na/K-ATPase and other P-type ATPases is its ability to bind a group of chemicals called cardiotonic steroids (CTS). The plant-derived CTS such as digoxin are valuable drugs for the management of cardiac diseases, whereas ouabain and marinobufagenin (MBG) have been identified as a new class of endogenous hormones. Recent studies have demonstrated that the endogenous CTS are important regulators of renal Na(+) excretion and blood pressure. The Na/K-ATPase is not only an ion pump, but also an important receptor that can transduce the ligand-like effect of CTS on intracellular protein kinases and Ca(2+) signaling. Significantly, these CTS-provoked signaling events are capable of reducing the surface expression of apical NHE3 (Na/H exchanger isoform 3) and basolateral Na/K-ATPase in renal proximal tubular cells. These findings suggest that endogenous CTS may play an important role in regulation of tubular Na(+) excretion under physiological conditions; conversely, a defect at either the receptor level (Na/K-ATPase) or receptor-effector coupling would reduce the ability of renal proximal tubular cells to excrete Na(+), thus culminating/resulting in salt-sensitive hypertension.

Aminopeptidase N reduces basolateral Na+ -K+ -ATPase in proximal tubule cells

Aminopeptidase N/CD13 (Anpep) is a membrane-bound protein that catalyzes the formation of natriuretic hexapeptide angiotensin IV (ANG IV) from ANG III. We previously reported that Anpep is more highly expressed in the kidneys of Dahl salt-resistant (SR/Jr) than salt-sensitive (SS/Jr) rats, Anpep maps to a quantitative trait locus for hypertension, and that the Dahl SR/Jr rat contains a functional polymorphism of the gene. This suggests that renal Anpep may be linked to salt sensitivity; however, its effect on renal Na handling has not been determined. Here, we examined regulation of basolateral Na(+)-K(+)-ATPase, a preeminent basolateral Na(+) transporter in proximal tubule cells, by Anpep in LLC-PK1 cells. Treatment of the cells with Anpep siRNA increased total cellular Na(+)-K(+)-ATPase activity and basolateral Na(+)-K(+)-ATPase abundance by approximately twofold. Conversely, Anpep overexpression reduced Na(+)-K(+)-ATPase activity and basolateral abundance by approximately 50%. Similar effects were observed after treatment with ANG IV (10 nM, x30 min and 12 h). ANG IV receptor (AGTRIV) knockdown via specific siRNA relieved the decreases in basolateral Na(+)-K(+)-ATPase levels and activity induced by Anpep overexpression. In sum, these results demonstrate that Anpep reduces basolateral Na(+)-K(+)-ATPase levels via ANG IV/AGTRIV signaling. This novel pathway may be important in renal adaptation to high salt.

Aldosterone-induced abnormal regulation of ENaC and SGK1 in Dahl salt-sensitive rat

Aldosterone plays a crucial role in controlling mineral balance in our body. The mechanism of aldosterone has been reported to elevate renal Na+ reabsorption by stimulating expression of epithelial Na+ channel (ENaC) and also activate an ENaC-regulating protein kinase, serum and glucocorticoid-regulated kinase 1 (SGK1). However, it is unknown whether aldosterone shows its stimulatory action on ENaC and SGK1 under an abnormal, salt-sensitive hypertensive condition. To clarify this point, we studied how aldosterone regulates expression of ENaC and SGK1 in Dahl salt-sensitive (DS) rat that shows hypertension with high salt diet. RNA and protein were extracted from the kidney 6 h after application of aldosterone (1.5 mg/kg body weight) subcutaneously injected into adrenalectomized DS and Dahl salt-resistant (DR) rats. Aldosterone decreased mRNA expression of beta- and gamma-ENaC in DS rat unlike DR rat, while aldosterone increased alpha-ENaC mRNA expression in DS rat similar to DR rat. Further, we found that aldosterone elevated SGK1 expression in DR rat, but not in DS rat. These observations indicate that ENaC and SGK1 are abnormally regulated by aldosterone in salt-sensitive hypertensive rats, suggesting that disturbance of the aldosterone regulation would be one of factors causing salt-sensitive hypertension.

Inhibition of Na,K-ATPase by Dopamine in Proximal Tubule Epithelial Cells

In the current report we review the results that lay grounds for the model of intracellular sodium-mediated dopamine-induced endocytosis of Na,K-ATPase. Under conditions of a high salt diet, dopamine activates PKCzeta, which phosphorylates NKA alpha1 Ser-18. The phosphorylation produces a conformational change of alpha1 NH2-terminus, which through interaction with other domains of alpha1 exposes PI3K- and AP-2-binding domains. PI3K bound to the NKA alpha1 induces the recruitment and activation of other proteins involved in endocytosis, and PI3K-generated 3-phosphoinositides affect the local cytoskeleton and modify the biophysical conditions of the membrane for development of clathrin-coated pits. Plasma membrane phosphorylated NKA is internalized to specialized intracellular compartments where the NKA will be dephosphorylated. The NKA internalization results in a reduced Na+ transport by proximal tubule epithelial cells.

Protein phosphatase 2A B56alpha during development in the spontaneously hypertensive rat

Mistargeting of the regulatory subunit of protein phosphatase 2A (PP2A), B56alpha is involved in the hyperphosphorylation and desensitization of the D1 dopamine receptor in renal proximal tubules of spontaneously hypertensive rats (SHRs). However, the renal expression of B56alpha before hypertension develops is not known. Therefore, we studied the expression of B56alpha and PP2A activity in the kidney during development in the SHR and its normotensive control, the Wistar-Kyoto (WKY) rat. PP2A B56alpha was expressed in proximal and distal tubules with no differences in the pattern of expression in WKY and SHRs at any age. In brush border membranes of renal proximal tubules, PP2A B56alpha protein was greatest in the immature rats and decreased with development. However, PP2A activity did not change with age. PP2A B56alpha protein and PP2A activity were similar in WKY and SHRs except at 2 weeks when both PP2A B56alpha protein and PP2A activity were higher in SHRs than in WKY rats. The PP2A catalytic subunit co-immunoprecipitated with the D1 receptor in renal proximal tubule cells. It is possible that the increased expression of PP2A B56alpha and increased basal PP2A activity in the young, especially in the SHRs, may serve as a compensatory mechanism in the increased phosphorylation and decreased renal D1 receptor function, including D1-receptor mediated stimulation in renal proximal tubules of SHRs.

Renal dopamine receptors and hypertension

Dopamine has been recognized as an important modulator of central as well as peripheral physiologic functions in both humans and animals. Dopamine receptors have been identified in a number of organs and tissues, which include several regions within the central nervous system, sympathetic ganglia and postganglionic nerve terminals, various vascular beds, the heart, the gastrointestinal tract, and the kidney. The peripheral dopamine receptors influence cardiovascular and renal function by decreasing afterload and vascular resistance and promoting sodium excretion. Within the kidney, dopamine receptors are present along the nephron, with highest density on proximal tubule epithelial cells. It has been reported that there is a defective dopamine receptor, especially D(1) receptor function, in the proximal tubule of various animal models of hypertension as well as in humans with essential hypertension. Recent reports have revealed the site of and the molecular mechanisms responsible for the defect in D(1) receptors in hypertension. Moreover, recent studies have also demonstrated that the disruption of various dopamine receptor subtypes and their function produces hypertension in rodents. In this review, we present evidence that dopamine and dopamine receptors play an important role in regulating renal sodium excretion and that defective renal dopamine production and/or dopamine receptor function may contribute to the development of various forms of hypertension.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Dopamine-dependent inhibition of jejunal Na+-K+-ATPase during high-salt diet in young but not in adult rats

During high-salt diet endogenous dopamine (DA) reduces jejunal sodium transport in young but not in adult rats. This study was designed to evaluate whether this effect is mediated, at the cellular level, by inhibition of Na+-K+-ATPase activity. Enzyme activity was determined in isolated jejunal cells by the rate of [gamma-32P]ATP hydrolysis. Cells were obtained from weanling and adult rats fed either with high- or normal-salt diet. In 20-day-old but not in 40-day-old rats Na+-K+-ATPase activity was significantly reduced during high-salt diet. This inhibition was abolished by a blocker of DA synthesis. The decreased activity was associated with a decreased alpha1-subunit at the plasma membrane. During high-salt diet there was an increase in DA content in jejunal cells from 20-day-old rats, associated with a parallel decrease in 5-hydroxytryptamine, compared with normal-salt diet. In 40-day-old rats, however, the catecholamine level remained unchanged during high-salt diet. Incubation of isolated jejunal cells with DA resulted in a dose-dependent inhibition of Na+-K+-ATPase activity in 20- but not in 40-day-old rats. We conclude that during high-salt diet, jejunal Na+-K+-ATPase in 20-day-old rats is inhibited, and this effect is likely to be mediated by locally formed DA.

Sodium balance and jejunal ion and water absorption in Dahl salt-sensitive and salt-resistant rats

1. Apparent Na+ absorption and jejunal water, Na+, Cl- and K+ absorption in vivo was evaluated in young (prepubertal) and adult Dahl salt-sensitive (DS) and Dahl salt-resistant (DR) rats kept on a low-salt (low-salt rat chow + distilled water) or a high-salt diet (HS1 diet: NaCl-enriched rat chow + distilled water; HS2 diet: standard rat chow + 1% saline as drinking fluid). These two high-salt diets were chosen because the HS1 regimen has been shown to increase blood pressure (BP) in DS rats and the HS2 regimen decreases jejunal water and ion absorption in normotensive Wistar rats. 2. The HS1 or HS2 diet increased BP in young and adult DS rats but had no effect on the BP of young and adult DR rats. 3. Irrespective of dietary Na+ intake, no significant difference of apparent Na+ absorption (dietary Na+ intake minus faecal Na+ output) was observed between DS and DR rats both in prepuberty and in adulthood. Young DS rats kept on a low-salt diet had increased faecal Na+ output in comparison with young DR rats. This difference disappeared with increasing dietary Na+ intake. 4. There were no interstrain differences on the effect of a high-salt diet on jejunal Na+ and K+ absorption in young and adult DS and DR rats. However, high-salt diets stimulated jejunal water and Cl- absorption in young DS rats, but not in adult DS rats and young and adult DR rats. Interstrain differences of water and Cl- absorption were observed only in adulthood. Adult DR rats kept on an HS2 diet absorbed more water and Cl- than their DS counterparts. 5. Our results do not indicate any abnormalities of apparent Na+ absorption and jejunal water and electrolyte transport in DS and DR rats. We conclude that there is no relationship between intestinal Na+ absorption and sensitivity or resistance to induction of experimental salt hypertension.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Relationship between dietary Na+ intake, aldosterone and colonic amiloride-sensitive Na+ transport

The effect of changes in dietary Na+ intake on plasma aldosterone levels and electrogenic amiloride-sensitive Na+ transport (Iamil) was studied in the rat distal colon. Five groups of rats were fed on diets containing different amounts of Na+. Estimated Na+ intake ranged from about 400-80,000 mu equiv Na+/kg body weight (BW) per d. Both variables investigated, Iamil and plasma aldosterone, depended non-linearly on Na+ intake. Reduction of the daily Na+ intake increased plasma aldosterone levels and if these levels reached the value 200 pg/ml or more then Iamil was induced. The corresponding Na+ intake was 1300 mu equiv Na+/kg BW per d. Iamil was not observed at lower aldosterone levels and higher Na+ intakes. Aldosterone infusion for 7 d produced similar changes in Iamil compared with dietary Na(+)-depleted animals and made the estimation of maximum transport capacity of Iamil possible. We conclude that Iamil operates only if Na+ intake decreases below minimal Na+ requirement in growing rats and that the maximum transport capacity of this pathway is reached only after very severe Na+ deprivation.

Experimental and/or genetically controlled alterations of the renal microsomal cytochrome P450 epoxygenase induce hypertension in rats fed a high salt diet

Excess dietary salt induces a cytochrome P450 arachidonic acid epoxygenase isoform in rat kidneys (Capdevila, J. H., S. Wei, J. Yang, A. Karara, H. R. Jacobson, J. R. Falck, F. P. Guengerich, and R. N. Dubois. 1992. J. Biol. Chem. 267:21720-21726). Treatment of rats on a high salt diet with the epoxygenase inhibitor, clotrimazole, produces significant increases in mean arterial blood pressure (122 +/- 2 and 145 +/- 4 mmHg for salt and salt- and clotrimazole-treated rats, respectively). The salt- and clotrimazole-dependent hypertension is accompanied by reductions in the urinary excretion of epoxygenase metabolites and by a selective inhibition of the renal microsomal epoxygenase reaction. The prohypertensive effects of clotrimazole are readily reversed when either the salt or clotrimazole treatment is discontinued. The indication that a salt-inducible renal epoxygenase protects against hypertension, are supported by studies with the Dahl rat model of genetic salt-sensitive hypertension. Dahl resistant animals responded to excess dietary salt by inducing the activity of their kidney microsomal epoxygenase(s) (0.102 +/- 0.01 and 0.240 +/- 0.04 nmol of products formed/min per mg of microsomal protein for control and salt-treated rats, respectively). Despite severe hypertension during excess dietary salt intake (200 +/- 20 mmHg), Dahl salt-sensitive rats demonstrated no increase in renal epoxygenase activity. These studies indicate that acquired or inherited abnormalities in renal epoxygenase activities and/or regulation can be related to salt-sensitive hypertension in rodents. Studies on the human renal epoxygenase and its relationship to salt hypertension may prove useful.

Dopamine regulation of renal Na+,K(+)-ATPase activity is lacking in Dahl salt-sensitive rats

Dopamine is a natriuretic hormone that acts by inhibiting tubular Na+, K(+)-ATPase activity by activation of the dopamine-1 receptor (the thick ascending limb [TAL] of Henle) or by a synergistic effect of dopamine-1 and dopamine-2 receptors (the proximal tubule). The dopamine-1 receptor is coupled to adenylate cyclase. In this article we show that prehypertensive Dahl salt-sensitive (DS) rats have a blunted natriuretic response to dopamine determined during euvolemic conditions compared with Dahl salt-resistant (DR) rats. Furthermore, we have examined the renal tubular effects of dopamine in DS and DR rats. Basal Na+,K(+)-ATPase activity was similar in DS and DR rats. In proximal tubule, dopamine (10(-5) M) inhibited Na+,K(+)-ATPase activity in DR but not in DS rats. The dopamine-2 agonist LY171555 (10(-5) M) together with dibutyryl cyclic AMP (10(-6) M) inhibited proximal tubule Na+,K(+)-ATPase activity in both DS and DR rats. LY171555 alone had no effect. In TAL, the dopamine-1 agonist fenoldopam (10(-5) M) inhibited Na+,K(+)-ATPase activity in DR but not in DS rats. Dibutyryl cyclic AMP (10(-5) M) inhibited TAL Na+,K(+)-ATPase activity in both DS and DR rats. In cell suspensions from the cortex and the medulla, activation of the dopamine-1 receptor significantly increased cyclic AMP content in DR but not in DS rats. The results indicate that DS rats lack the capacity to inhibit tubular Na+,K(+)-ATPase activity because of a defective dopamine-1 receptor adenylate cyclase coupling. This defect may contribute to the impaired natriuretic capacity in DS rats.

The influence of high salt intake on intestinal Na,K-ATPase in Wistar and Dahl rats

The Na,K-ATPase of intestinal mucosa was compared in Wistar (W), salt-sensitive (DS) and salt-resistant (DR) Dahl rats fed a low-salt (LS) and high-salt (HS) diet. ATPase activity and the kinetics of its activation by Na+ were determined in three intestinal segments (jejunum, ileum, distal colon). There were demonstrated only small differences in the affinity for Na+ among the strains studied but we found a considerable profile of Na,K-ATPase activity along the intestine in all strains; the activity was higher in jejunum and lower in ileum and distal colon. Chronic salt loading decreased the affinity of Na,K-ATPase for Na+ but had no effect on Vmax. The changes in salt intake were accompanied by different response of plasma aldosterone in particular strains. According to the stimulation of aldosterone level by LS diet the sensitivity of the strains was DR > W > DS. HS diet suppressed aldosterone level to similar values in all strains. The data indicate that the kinetics of intestinal Na,K-ATPase and its response to HS intake is independent of the genotype of the rats.