Effects of long-term ouabain treatment on blood pressure, sodium excretion, and renal dopamine D1 receptor levels in rats

Effects of Renal Denervation on Ouabain-Induced Hypertension in Rats

Background

Ouabain, a Na+, K+-ATPase inhibitor, is elevated in hypertensive patients. Evidence suggests ouabain contributes to hypertension mainly through activation of the sympathetic nervous system (SNS). Renal nerves play a vital role in the regulation of SNS activity, so we hypothesize that renal denervation may attenuate the development of ouabain-induced hypertension.

Methods and Results

Forty Sprague-Dawley rats were divided into following groups (n = 10 each): control group (sham surgery plus intraperitoneal saline injection), RDN group (renal denervation (RDN) plus intraperitoneal saline injection), ouabain group (sham surgery plus intraperitoneal ouabain injection), and ouabain + RDN group (RDN plus intraperitoneal ouabain injection). After eight weeks, compared with the control group, rats in the ouabain group exhibited elevated blood pressure (P < 0.05), increased plasma epinephrine, norepinephrine, angiotensin II, and aldosterone levels (P < 0.05). These indexes could be significantly ameliorated by RDN. RDN also reduced the thickening of aortic tunica media and downregulated the expression of proliferating cell nuclear antigen (PCNA) in the thoracic aorta induced by ouabain. Masson staining and echocardiography showed that myocardial fibrosis and increased left ventricular mass in the ouabain group could be attenuated by RDN.

Conclusions

The present study reveals that renal nerves play an important role in the development of ouabain-induced hypertension. RDN could inhibit the pressor effect and the myocardial remodeling induced by ouabain potentially via inhibiting catecholamine release and vascular smooth muscle cell proliferation. Clinical studies are needed to explore whether RDN may exhibit better antihypertensive effects on hypertensive patients with high plasma ouabain levels as compared to those with normal plasma ouabain levels.

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

The Synergistic Roles of Cholecystokinin B and Dopamine D5 Receptors on the Regulation of Renal Sodium Excretion

Renal dopamine D₁-like receptors (D₁R and D₅R) and the gastrin receptor (CCK_BR) are involved in the maintenance of sodium homeostasis. The D₁R has been found to interact synergistically with CCK_BR in renal proximal tubule (RPT) cells to promote natriuresis and diuresis. D₅R, which has a higher affinity for dopamine than D₁R, has some constitutive activity. Hence, we sought to investigate the interaction between D₅R and CCK_BR in the regulation of renal sodium excretion. In present study, we found D₅R and CCK_BR increase each other’s expression in a concentration- and time-dependent manner in the HK-2 cell, the specificity of which was verified in HEK293 cells heterologously expressing both human D₅R and CCK_BR and in RPT cells from a male normotensive human. The specificity of D₅R in the D₅R and CCK_BR interaction was verified further using a selective D₅R antagonist, LE-PM436. Also, D₅R and CCK_BR colocalize and co-immunoprecipitate in BALB/c mouse RPTs and human RPT cells. CCK_BR protein expression in plasma membrane-enriched fractions of renal cortex (PMFs) is greater in D₅R^-/- mice than D₅R^+/+ littermates and D₅R protein expression in PMFs is also greater in CCK_BR^-/- mice than CCK_BR^+/+ littermates. High salt diet, relative to normal salt diet, increased the expression of CCK_BR and D₅R proteins in PMFs. Disruption of CCK_BR in mice caused hypertension and decreased sodium excretion. The natriuresis in salt-loaded BALB/c mice was decreased by YF476, a CCK_BR antagonist and Sch23390, a D₁R/D₅R antagonist. Furthermore, the natriuresis caused by gastrin was blocked by Sch23390 while the natriuresis caused by fenoldopam, a D₁R/D₅R agonist, was blocked by YF476. Taken together, our findings indicate that CCK_BR and D₅R synergistically interact in the kidney, which may contribute to the maintenance of normal sodium balance following an increase in sodium intake.

Advances in understanding the role of cardiac glycosides in control of sodium transport in renal tubules

Cardiotonic steroids have been used for the past 200 years in the treatment of congestive heart failure. As specific inhibitors of membrane-bound Na(+)/K(+) ATPase, they enhance cardiac contractility through increasing myocardial cell calcium concentration in response to the resulting increase in intracellular Na concentration. The half-minimal concentrations of cardiotonic steroids required to inhibit Na(+)/K(+) ATPase range from nanomolar to micromolar concentrations. In contrast, the circulating levels of cardiotonic steroids under physiological conditions are in the low picomolar concentration range in healthy subjects, increasing to high picomolar levels under pathophysiological conditions including chronic kidney disease and heart failure. Little is known about the physiological function of low picomolar concentrations of cardiotonic steroids. Recent studies have indicated that physiological concentrations of cardiotonic steroids acutely stimulate the activity of Na(+)/K(+) ATPase and activate an intracellular signaling pathway that regulates a variety of intracellular functions including cell growth and hypertrophy. The effects of circulating cardiotonic steroids on renal salt handling and total body sodium homeostasis are unknown. This review will focus on the role of low picomolar concentrations of cardiotonic steroids in renal Na(+)/K(+) ATPase activity, cell signaling, and blood pressure regulation.

Role of Gα(12)- and Gα(13)-protein subunit linkage of D(3) dopamine receptors in the natriuretic effect of D(3) dopamine receptor in kidney

The D(3) dopamine receptor is the major D(2)-like receptor that regulates sodium transport in the renal proximal tubule (RPT) and helps maintain blood pressure in the normal range. In Wistar-Kyoto (WKY) rats chronically fed high-salt diet, the intrarenal arterial infusion of a D(3) receptor agonist, PD128907, increased absolute and fractional sodium excretion. We have reported that Gα(12) and Gα(13), which participate in the signal transduction of the D(5) receptor, are expressed in RPTs. As the D(3) receptor is also expressed in RPTs, we hypothesized that it may also interact with Gα(12)/Gα(13) in RPTs from WKY rats. There were co-localization and co-immunoprecipitation of D(3) receptor and Gα(12)/Gα(13) in renal brush border membranes (BBMs) and RPT cells. The intrarenal infusion of PD128907 (1 μg kg(-1) min(-1)) that increased sodium excretion also increased the co-immunoprecipitations of D(3)/Gα(12) and D(3)/Gα(13) in renal BBMs; their co-immunoprecipitation was confirmed in RPT cells. As Gα(12) and Gα(13) increase sodium pump and transporter activity (for example, Na(+)-K(+)-ATPase, NHE3), an increased association of D(3) receptors with Gα(12)/Gα(13) receptors after D(3) receptor activation may be a mechanism to prevent Gα(12)/Gα(13)-mediated stimulation of sodium transport (and thus enhance natriuresis). We conclude that a D(3) receptor interaction with Gα(12)/Gα(13) that increases sodium excretion may have a role in the regulation of blood pressure.

The neural substrates of rapid-onset Dystonia-Parkinsonism

Although dystonias are a common group of movement disorders the mechanisms by which brain dysfunction results in dystonia are not understood. Rapid-onset Dystonia-Parkinsonism is a hereditary dystonia caused by mutations in the ATP1A3 gene. Affected subjects can be symptom free for years but rapidly develop persistent dystonia and parkinsonism-like symptoms after a stressful experience. Using a mouse model here we show that an adverse interaction between the cerebellum and basal ganglia can account for the symptoms of the patients. The primary instigator of dystonia is the cerebellum whose aberrant activity alters basal ganglia function which in turn causes dystonia. This adverse interaction between the cerebellum and basal ganglia is mediated through a di-synaptic thalamic pathway which when severed is effective in alleviating dystonia. Our results provide a unifying hypothesis for the involvement of cerebellum and basal ganglia in generation of dystonia and suggest therapeutic strategies for the treatment of RDP.

Inhibition of renal caveolin-1 reduces natriuresis and produces hypertension in sodium-loaded rats

Renal dopamine receptor function and ion transport inhibition are impaired in essential hypertension. We recently reported that caveolin-1 (CAV1) and lipid rafts are necessary for normal D(1)-like receptor-dependent internalization of Na-K-ATPase in human proximal tubule cells. We now hypothesize that CAV1 is necessary for the regulation of urine sodium (Na(+)) excretion (U(Na)V) and mean arterial blood pressure (MAP) in vivo. Acute renal interstitial (RI) infusion into Sprague-Dawley rats of 1 μg·kg⁻¹·min⁻¹ fenoldopam (FEN; D(1)-like receptor agonist) caused a 0.46 ± 0.15-μmol/min increase in U(Na)V (over baseline of 0.29 ± 0.04 μmol/min; P < 0.01). This increase was seen in Na(+)-loaded rats, but not in those under a normal-sodium load. Coinfusion with β-methyl cyclodextrin (βMCD; lipid raft disrupter, 200 μg·kg⁻¹·min⁻¹) completely blocked this FEN-induced natriuresis (P < 0.001). Long-term (3 day) lipid raft disruption via continuous RI infusion of 80 μg·kg⁻¹·min⁻¹ βMCD decreased renal cortical CAV1 expression (47.3 ± 6.4%; P < 0.01) and increased MAP (32.4 ± 6.6 mmHg; P < 0.001) compared with vehicle-infused animals. To determine whether the MAP rise was due to a CAV1-dependent lipid raft-mediated disruption, Na(+)-loaded rats were given a bolus RI infusion of CAV1 siRNA. Two days postinfusion, cortical CAV1 expression was decreased by 73.6 ± 8.2% (P < 0.001) and the animals showed an increase in MAP by 17.4 ± 2.9 mmHg (P < 0.01) compared with animals receiving scrambled control siRNA. In summary, acute kidney-specific lipid raft disruption decreases CAV1 expression and blocks D(1)-like receptor-induced natriuresis. Furthermore, chronic disruption of lipid rafts or CAV1 protein expression in the kidney induces hypertension.

HK-2 human renal proximal tubule cells as a model for G protein-coupled receptor kinase type 4-mediated dopamine 1 receptor uncoupling

HK-2 human renal proximal tubule cells (RPTC) are commonly used in the in vitro study of "normal" RPTCs. We discovered recently that HK-2 cells are uncoupled from dopamine 1 receptor (D(1)R) adenylyl cyclase (AC) stimulation. We hypothesized that G protein-coupled receptor kinase type 4 (GRK4) single nucleotide polymorphisms may be responsible for the D(1)R/AC uncoupling in HK-2. This hypothesis was tested by genotyping GRK4 single nucleotide polymorphisms, measuring D(1)-like receptor agonist (fenoldopam)-stimulated cAMP accumulation, quantifying D(1)R inhibition of sodium transport, and testing the ability of GRK4 small interfering RNA to reverse the D(1)R/AC uncoupling. We compared HK-2 with 2 normally coupled human RPTC cell lines and 2 uncoupled RPTC cell lines. The HK-2 cell line was found to have 4 of 6 potential GRK4 single nucleotide polymorphisms known to uncouple the D(1)R from AC (namely, R65L, A142V, and A486V). AC response to fenoldopam stimulation was increased in the 2 normally coupled human RPTC cell lines (FEN: 2.02+/-0.05-fold and 2.33+/-0.19-fold over control; P<0.001; n=4) but not in the 2 uncoupled or HK-2 cell lines. GRK4 small interfering RNA rescued the fenoldopam-mediated AC stimulation in the uncoupled cells, including HK-2. The expected fenoldopam-mediated inhibition of sodium hydrogen exchanger type 3 was absent in HK-2 (n=6) and uncoupled RPTC cell lines (n=6) but was observed in the 2 normally coupled human RPTC cell lines (-25.41+/-4.7% and -27.36+/-2.70%; P<0.001; n=6), which express wild-type GRK4. Despite the fact that HK-2 cells retain many functional characteristics of RPTCs, they are not normal from the perspective of dopaminergic function.

Intracellular sodium sensing: SIK1 network, hormone action and high blood pressure

Sodium is the main determinant of body fluid distribution. Sodium accumulation causes water retention and, often, high blood pressure. At the cellular level, the concentration and active transport of sodium is handled by the enzyme Na(+),K(+)-ATPase, whose appearance enabled evolving primitive cells to cope with osmotic stress and contributed to the complexity of mammalian organisms. Na(+),K(+)-ATPase is a platform at the hub of many cellular signaling pathways related to sensing intracellular sodium and dealing with its detrimental excess. One of these pathways relies on an intracellular sodium-sensor network with the salt-inducible kinase 1 (SIK1) at its core. When intracellular sodium levels rise, and after the activation of calcium-related signals, this network activates the Na(+),K(+)-ATPase and expel the excess of sodium from the cytosol. The SIK1 network also mediates sodium-independent signals that modulate the activity of the Na(+),K(+)-ATPase, like dopamine and angiotensin, which are relevant per se in the development of high blood pressure. Animal models of high blood pressure, with identified mutations in components of multiple pathways, also have alterations in the SIK1 network. The introduction of some of these mutants into normal cells causes changes in SIK1 activity as well. Some cellular processes related to the metabolic syndrome, such as insulin effects on the kidney and other tissues, also appear to involve the SIK1. Therefore, it is likely that this protein, by modulating active sodium transport and numerous hormonal responses, represents a "crossroad" in the development and adaptation to high blood pressure and associated diseases.

Dopamine-mediated inhibition of renal Na+/K+-ATPase in HK-2 cells is reduced by ouabain

1. Abnormal renal sodium handling is considered a major contributing factor in hypertension associated with chronic ouabain treatment. However, the molecular mechanisms involved in abnormal renal sodium handling have not been elucidated. Therefore, we investigated whether chronic ouabain treatment perturbs dopamine D(1) receptor function. 2. The expression and phosphorylation levels of the D(1) receptor in cells of the human proximal tubule cell line (HK-2) were determined using western blot analysis and reverse transcription polymerase chain reaction. The activity of the renal sodium/potassium pump (Na(+)/K(+)-ATPase) was measured using a colourimetric assay, and cyclic adenosine monophosphate accumulation was determined by performing a radioimmunoassay. 3. We showed that chronic ouabain treatment decreased the protein and mRNA expression levels of the D(1) receptor and increased the basal phosphorylation of the D(1) receptor in HK-2 cells. We also showed that in the presence of ouabain, HK-2 cells did not reveal the cyclic adenosine monophosphate accumulation and Na(+)/K(+)-ATPase inhibition induced by the D(1) receptor agonist fenoldopam. 4. We hypothesize that the ouabain-induced decrease in renal D(1) receptor function is responsible for the increase in renal sodium reabsorption, which eventually leads to ouabain-induced hypertension.