Gialpha3 protein-coupled dopamine D3 receptor-mediated inhibition of renal NHE3 activity in SHR proximal tubular cells is a PLC-PKC-mediated event

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

The Contribution of Phospholipase C in Vomiting in the Least Shrew (Cryptotis Parva) Model of Emesis

Gq and Gβγ protein-dependent phospholipase C (PLC) activation is extensively involved in G protein-coupled receptor (GPCR)-mediated signaling pathways which are implicated in a wide range of physiological and pathological events. Stimulation of several GPCRs, such as substance P neurokinin 1-, dopamine D2/3-, histamine H1- and mu-opioid receptors, can lead to vomiting. The aim of this study was to investigate the role of PLC in vomiting through assessment of the emetic potential of a PLC activator (m-3M3FBS), and the antiemetic efficacy of a PLC inhibitor (U73122), in the least shrew model of vomiting. We find that a 50 mg/kg (i.p.) dose of m-3M3FBS induces vomiting in ∼90% of tested least shrews, which was accompanied by significant increases in c-Fos expression and ERK1/2 phosphorylation in the shrew brainstem dorsal vagal complex, indicating activation of brainstem emetic nuclei in m-3M3FBS-evoked emesis. The m-3M3FBS-evoked vomiting was reduced by pretreatment with diverse antiemetics including the antagonists/inhibitors of: PLC (U73122), L-type Ca2+ channel (nifedipine), IP3R (2-APB), RyR receptor (dantrolene), ERK1/2 (U0126), PKC (GF109203X), the serotoninergic type 3 receptor (palonosetron), and neurokinin 1 receptor (netupitant). In addition, the PLC inhibitor U73122 displayed broad-spectrum antiemetic effects against diverse emetogens, including the selective agonists of serotonin type 3 (2-Methyl-5-HT)-, neurokinin 1 receptor (GR73632), dopamine D2/3 (quinpirole)-, and muscarinic M1 (McN-A-343) receptors, the L-type Ca2+ channel (FPL64176), and the sarco/endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin. In sum, PLC activation contributes to emesis, whereas PLC inhibition suppresses vomiting evoked by diverse emetogens.

Renal Dopamine Receptors and Oxidative Stress: Role in Hypertension

Significance: The kidney plays an important role in the long-term control of blood pressure. Oxidative stress is one of the fundamental mechanisms responsible for the development of hypertension. Dopamine, via five subtypes of receptors, plays an important role in the control of blood pressure by various mechanisms, including the inhibition of oxidative stress. Recent Advances: Dopamine receptors exert their regulatory function to decrease the oxidative stress in the kidney and ultimately maintain normal sodium balance and blood pressure homeostasis. An aberration of this regulation may be involved in the pathogenesis of hypertension. Critical Issues: Our present article reviews the important role of oxidative stress and intrarenal dopaminergic system in the regulation of blood pressure, summarizes the current knowledge on renal dopamine receptor-mediated antioxidation, including decreasing reactive oxygen species production, inhibiting pro-oxidant enzyme nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and stimulating antioxidative enzymes, and also discusses its underlying mechanisms, including the increased activity of G protein-coupled receptor kinase 4 (GRK4) and abnormal trafficking of renal dopamine receptors in hypertensive status. Future Directions: Identifying the mechanisms of renal dopamine receptors in the regulation of oxidative stress and their contribution to the pathogenesis of hypertension remains an important research focus. Increased understanding of the role of reciprocal regulation between renal dopamine receptors and oxidative stress in the regulation of blood pressure may give us novel insights into the pathogenesis of hypertension and provide a new treatment strategy for hypertension.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Abnormalities of Dopamine D3 Receptor Signaling in the Diseased Brain

Dopamine D₃ receptors (D₃R) modulate neuronal activity in several brain regions including cortex, striatum, cerebellum, and hippocampus. A growing body of evidence suggests that aberrant D₃R signaling contributes to multiple brain diseases, such as Parkinson’s disease, essential tremor, schizophrenia, and addiction. In line with these findings, D₃R has emerged as a potential target in the treatment of neurological disorders. However, the mechanisms underlying neuronal D₃R signaling are poorly understood, either in healthy or diseased brain. Here, I review the molecular mechanisms involved in D₃R signaling via monomeric D₃R and heteromeric receptor complexes (e.g., D₃R-D₁R, D₃R-D₂R, D₃R-A_2aR, and D₃R-D₃nf). I focus on D₃R signaling pathways that, according to recent reports, contribute to pathological brain states. In particular, I describe evidence on both quantitative (e.g., increased number or affinity) and qualitative (e.g., switched signaling) changes in D₃R that has been associated with brain dysfunction. I conclude with a description of basic mechanisms that modulate D₃R signaling such as desensitization, as disruption of these mechanisms may underlie pathological changes in D₃R signaling. Because several lines of evidence support the idea that imbalances in D₃R signaling alter neural function, a better understanding of downstream D₃R pathways is likely to reveal novel therapeutic strategies toward dopamine-related brain disorders.

Dopamine D3 receptor inhibits the ubiquitin-specific peptidase 48 to promote NHE3 degradation

The dopamine D3 receptor (D3R) is crucial in the regulation of blood pressure and sodium balance, in that Drd3 gene ablation in mice results in hypertension and failure to excrete a dietary salt load. The mechanism responsible for the renal sodium retention in these mice is largely unknown. We now offer and describe a novel mechanism by which D3R decreases sodium transport in the long term by inhibiting the deubiquitinylating activity of ubiquitin-specific peptidase 48 (USP48), thereby promoting Na(+)-H(+) exchanger (NHE)-3 degradation. We found that stimulation with the D3R-specific agonist PD128907 (1 μM, 30 min) promoted the interaction and colocalization among D3R, NHE3, and USP48; inhibited USP48 activity (-35±6%, vs. vehicle), resulting in increased ubiquitinylated NHE3 (+140±10%); and decreased NHE3 expression (-50±9%) in human renal proximal tubule cells (hRPTCs). USP48 silencing decreased NHE3's half-life (USP48 siRNA t1/2=6.1 h vs. vehicle t1/2=12.9 h), whereas overexpression of USP48 increased NHE3 half-life (t1/2=21.8 h), indicating that USP48 protects NHE3 from degradation via deubiquitinylation. USP48 accounted for ∼30% of the total deubiquitinylating activity in these cells. Extending our studies in vivo, we found that pharmacologic blockade of D3R via the D3R-specific antagonist GR103691 (1 μg/kg/min, 4 d) in C57Bl/6J mice increased renal NHE3 expression (+310±15%, vs. vehicle), whereas an innovative kidney-restricted Usp48 silencing via siRNA (3 μg/d, 7 d) increased ubiquitinylated NHE3 (+250±30%, vs. controls), decreased total NHE3 (-23±2%), and lowered blood pressure (-24±2 mm Hg), compared with that in control mice that received either the vehicle or nonsilencing siRNA. Our data demonstrate a crucial role for the dynamic interaction between D3R and USP48 in the regulation of NHE3 expression and function.

Role of GRK4 in the regulation of arterial AT1 receptor in hypertension

G-protein-coupled receptor kinase 4 (GRK4) gene variants, via impairment of renal dopamine receptor and enhancement of renin-angiotensin system functions, cause sodium retention and increase blood pressure. Whether GRK4 and the angiotensin type 1 receptor (AT(1)R) interact in the aorta is not known. We report that GRK4 is expressed in vascular smooth muscle cells of the aorta. Heterologous expression of the GRK4γ variant 142V in A10 cells increased AT(1)R protein expression and AT(1)R-mediated increase in intracellular calcium concentration. The increase in AT(1)R expression was related to an increase in AT(1)R mRNA expression via the NF-κB pathway. As compared with control, cells expressing GRK4γ 142V had greater NF-κB activity with more NF-κB bound to the AT(1)R promoter. The increased AT(1)R expression in cells expressing GRK4γ 142V was also associated with decreased AT(1)R degradation, which may be ascribed to lower AT(1)R phosphorylation. There was a direct interaction between GRK4γ and AT(1)R that was decreased by GRK4γ 142V. The regulation of AT(1)R expression by GRK4γ 142V in A10 cells was confirmed in GRK4γ 142V transgenic mice; AT(1)R expression was higher in the aorta of GRK4γ 142V transgenic mice than control GRK4γ wild-type mice. Angiotensin II-mediated vasoconstriction of the aorta was also higher in GRK4γ 142V than in wild-type transgenic mice. This study provides a mechanism by which GRK4, via regulation of arterial AT(1)R expression and function, participates in the pathogenesis of conduit vessel abnormalities in hypertension.

Gastrin induces sodium-hydrogen exchanger 3 phosphorylation and mTOR activation via a phosphoinositide 3-kinase-/protein kinase C-dependent but AKT-independent pathway in renal proximal tubule cells derived from a normotensive male human

Gastrin is natriuretic, but its renal molecular targets and signal transduction pathways are not fully known. In this study, we confirmed the existence of CCKBR (a gastrin receptor) in male human renal proximal tubule cells and discovered that gastrin induced S6 phosphorylation, a downstream component of the phosphatidylinositol 3 kinase (PI3 kinase)-mammalian target of rapamycin pathway. Gastrin also increased the phosphorylation of sodium-hydrogen exchanger 3 (NHE3) at serine 552, caused its internalization, and decreased its expression at the cell surface and NHE activity. The phosphorylation of NHE3 and S6 was dependent on PI3 kinases because it was blocked by 2 different PI3-kinase inhibitors, wortmannin and LY294,002. The phosphorylation of NHE3 and S6 was not affected by the protein kinase A inhibitor H-89 but was blocked by a pan-PKC (chelerythrine) and a conventional PKC (cPKC) inhibitor (Gö6976) (10 μM) and an intracellular calcium chelator, 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester, suggesting the importance of cPKC and intracellular calcium in the gastrin signaling pathway. The cPKC involved was probably PKCα because it was phosphorylated by gastrin. The gastrin-mediated phosphorylation of NHE3, S6, and PKCα was via phospholipase C because it was blocked by a phospholipase C inhibitor, U73122 (10 μM). The phosphorylation (activation) of AKT, which is usually upstream of mammalian target of rapamycin in the classic PI3 kinase-AKT-p70S6K signaling pathway, was not affected, suggesting that the gastrin-induced phosphorylation of NHE3 and S6 is dependent on both PI3 kinase and PKCα but not AKT.

Epithelial-mesenchymal transdifferentiation of renal tubular epithelial cells induced by urinary proteins requires the activation of PKC-α and βI isozymes

Proteinuria is a common feature for almost all glomerular diseases and reflects the severity of the glomerular lesion. The presence of a large amount of proteins in tubular fluid, however, may also contribute to the development of RIF (renal interstitial fibrosis). Endocytosis of albumin in proximal tubular cells triggers PKC (protein kinase C)-dependent generation of reactive oxygen species and secretion of chemokines. As a family including 12 isozymes, which PKC isozymes participate in RIF is still unclear. EMT (epithelial-mesenchymal transdifferentiation) of RTECs (renal tubular epithelial cells) plays a crucial role in the progress of RIF induced by proteinuria. In the present study, we investigated the role of classical PKC isozymes in the proteinuria-induced EMT of RTECs. Employing immunochemical staining, we found that PKC-α, -βI and -βII were expressed in glomerulus and in RTECs in both normal and diseased renal tissues, while PKC-γ was only expressed in podocytes in the glomerulus. Treatment of HK-2 cells with extracted urinary proteins resulted in EMT, as evidenced by morphological changes, decreased E-cadherin expression, increased α-SMA (α-smooth muscle actin) expression, as well as production of type I collagen and fibronectin. Western blot analysis of PKC isozymes in the cytosolic compared with membrane fraction revealed translocation of PKC-α and -βI, but not PKC-βII, in HK-2 cells undergoing EMT. Pretreatment with selective PKC-α inhibitor G-6976 or PKC-β inhibitor significantly attenuated EMT induced by urinary proteins. In summary, the present study suggested that PKC-α and -βI play critical roles in the EMT of RTECs in response to urinary proteins.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

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.

Inhibitory effect of the D(3) dopamine receptor on insulin receptor expression and function in vascular smooth muscle cells

BACKGROUND

Vascular smooth muscle cell (VSMC) proliferation is regulated by numerous hormones and humoral factors. Our previous study found that stimulation of D(1)-like dopamine receptors inhibited insulin receptor expression and function in VSMCs. We hypothesize that there is also an interaction between D(3) dopamine and insulin receptors, i.e., stimulation of the D(3) receptor inhibits insulin receptor expression and function.

METHODS

Receptor expression was determined by immunoblotting, immunohistochemisty, and reverse transcriptase-PCR; VSMC proliferation was determined by 3-(4,5-dimethylthiazol-2-yl)-diphenyl-tetrazolium bromide (MTT) assay and cell number.

RESULTS

Insulin receptor protein is increased in the aorta of D(3) receptor deficient mice. Stimulation of the D(3) receptor inhibited insulin receptor mRNA and protein expression and insulin-mediated VSMC proliferation, and increased protein kinase A (PKA) activity, insulin receptor phosphorylation, and degradation in immortalized aortic VSMCs (A10 cells). These effects were blocked by a PKA inhibitor, indicating that the D(3) receptor-mediated decrease in insulin receptor expression was related to a decrease in transcription/post-transcription and increased degradation, involving PKA signaling.

CONCLUSIONS

D(3) receptor stimulation may be a target to reduce the adverse effect of insulin in hypertension by inhibition of insulin receptor expression and function in arterial VSMCs.

Upregulation of D2-class signaling in dopamine-denervated striatum is in part mediated by D3 receptors acting on Ca V 2.1 channels via PIP2 depletion

The loss of dopaminergic neurons in the substantia nigra compacta followed by striatal dopamine depletion is a hallmark of Parkinson's disease. After dopamine depletion, dopaminergic D(2) receptor (D(2)R)-class supersensitivity develops in striatal neurons. The supersensitivity results in an enhanced modulation of Ca(2+) currents by D(2)R-class receptors. However, the relative contribution of D(2)R, D(3)R, and D(4)R types to the supersensitivity, as well as the mechanisms involved, have not been elucidated. In this study, whole cell voltage-clamp recordings were performed to study Ca(2+) current modulation in acutely dissociated striatal neurons obtained from rodents with unilateral 6-hydroxydopamine lesions in the substantia nigra compacta. Selective antagonists for D(2)R, D(3)R, and D(4)R types were used to identify whether the modulation by one of these receptors experiences a selective change after dopaminergic denervation. It was found that D(3)R-mediated modulation was particularly enhanced. Increased modulation targeted Ca(V)2.1 (P/Q) Ca(2+) channels via the depletion of phosphatidylinositol 4,5-bisphosphate, an intracellular signaling cascade hard to detect in control neurons and hypothesized as being amplified by dopamine depletion. An imbalance in the striatal expression of D(3)R and its splice variant, D(3)nf, accompanied enhanced D(3)R activity. Because Ca(V)2.1 Ca(2+) channels mediate synaptic GABA release from the terminals of striatal neurons, reinforcement of their inhibition by D(3)R may explain in part the profound decrease in synaptic strength in the connections among striatal projection neurons observed in the dopamine-depleted striatum.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

The regulation of proximal tubular salt transport in hypertension: an update

PURPOSE OF REVIEW

Renal proximal tubular sodium reabsorption is regulated by sodium transporters, including the sodium glucose transporter, sodium amino acid transporter, sodium hydrogen exchanger isoform 3 and sodium phosphate cotransporter type 2 located at the luminal/apical membrane, and sodium bicarbonate cotransporter and Na+/K+ATPase located at the basolateral membrane. This review summarizes recent studies on sodium transporters that play a major role in the increase in blood pressure in essential/polygenic hypertension.

RECENT FINDINGS

Sodium transporters and Na+/K+ATPase are segregated in membrane lipid and nonlipid raft microdomains that regulate their activities and trafficking via cytoskeletal proteins. The increase in renal proximal tubule ion transport in polygenic hypertension is primarily due to increased activity of NHE3 and Cl/HCO3 exchanger at the luminal/apical membrane and a primary or secondary increase in Na+/K+ATPase activity.

SUMMARY

The increase in renal proximal tubule ion transport in hypertension is due to increased actions by prohypertensive factors that are unopposed by antihypertensive factors.

The D(3) dopamine receptor inhibits dopamine release in PC-12/hD3 cells by autoreceptor signaling via PP-2B, CK1, and Cdk-5

The function of the D(3) dopamine (DA) receptor remains ambiguous largely because of the lack of selective D(3) receptor ligands. To investigate the function and intracellular signaling of D(3) receptors, we established a PC-12/hD3 clone, which expresses the human D(3) DA receptor in a DA producing cell line. In this model, we find that the D(3) receptor functions as an autoreceptor controlling neurotransmitter secretion. Pre-treatment with 3,6a,11, 14-tetrahydro-9-methoxy-2 methyl-(12H)-isoquino[1,2-b] pyrrolo[3,2-f][1,3] benzoxanzine-1-carboxylic acid, a D(3) receptor preferring agonist, dose-dependently suppressed K+-evoked [3H]DA release in PC-12/hD3 cells but not in the control cell line. This effect was prevented by D(3) receptor preferring antagonists GR103691 and SB277011-A. Furthermore, activation of D(3) receptors significantly inhibits forskolin-induced cAMP accumulation and leads to transient increases in phosphorylation of cyclin-dependent kinase 5 (Cdk5), dopamine and cAMP-regulated phosphoprotein of M(r) 32 000 and Akt. Because we observed differences in Cdk5 phosphorylation as well as Akt phosphorylation after DA stimulation, we probed the ability of Cdk5 and phosphatidylinositol-3 kinase (PI3K) to influence DA release. Cdk5 inhibitors, roscovitine, or olomoucine, but not the PI3K inhibitor wortmannin, blocked the D(3) receptor inhibition of DA release. In a complimentary experiment, over-expression of Cdk5 potentiated D(3) receptor suppression of DA release. Pertussis toxin, 3-[(2,4,6-trimethoxyphenyl)methylidenyl]-indolin-2-one and cyclosporine A also attenuated D(3) receptor-mediated inhibition of DA release indicating that this phenomenon acts through Gi/oalpha and casein kinase 1, and phosphatase protein phosphatase 2B (calcineurin), respectively. In support of previous data that D(3) DA receptors reduce transmitter release from nerve terminals, the current results demonstrate that D(3) DA receptors function as autoreceptors to inhibit DA release and that a signaling pathway involving Cdk5 is essential to this regulation.

Short-term regulation of the Cl-/HCO3(-) exchanger in immortalized SHR proximal tubular epithelial cells

The present study evaluated the activity of Cl(-)/HCO(3)(-) exchanger and the abundance of Slc26a6 in immortalized renal proximal tubular epithelial (PTE) cells from the Wistar-Kyoto rat (WKY) and spontaneously hypertensive rat (SHR) and identified the signaling pathways that regulate the activity of the transporter. The affinity for HCO(3)(-) was identical in WKY and SHR PTE cells, but V(max) values (in pH units/min) in SHR PTE cells (0.4016) were significantly higher than in WKY PTE cells (0.2304). The expression of Slc26a6 in SHR PTE cells was sevenfold that in WKY PTE cells. Dibutyryl-cAMP (db-cAMP) or forskolin, which increased endogenous cAMP, phorbol-12,13-dibutyrate (PDBu) and anisomycin, significantly (P<0.05) increased the Cl(-)/HCO(3)(-) exchanger activity in WKY and SHR PTE cells to a similar extent. The stimulatory effects of db-cAMP and forskolin were prevented by the PKA inhibitor H89, but not by chelerythrine. The stimulatory effects of PDBu were prevented by both chelerythrine and SB 203580, but not by H89 or the MEK inhibitor PD 98059. The stimulatory effect of anisomycin was prevented by SB 203580, but not by chelerythrine. Increases in phospho-p38 MAPK by anisomycin were identical in WKY and SHR PTE cells, this being sensitive to SB 203580 but not to chelerythrine. It is concluded that SHR PTE cells, which overexpress the Slc26a6 protein, are endowed with an enhanced activity of the Cl(-)/HCO(3)(-) exchanger. The Cl(-)/HCO(3)(-) exchanger is an effector protein for PKA, PKC and p38 MAPK in both WKY and SHR PTE cells.

Oxidative stress and alpha1-adrenoceptor-mediated stimulation of the Cl-/HCO3- exchanger in immortalized SHR proximal tubular epithelial cells

BACKGROUND AND PURPOSE

This study evaluated the signalling coupled to the alpha1-adrenoceptor-induced stimulation of the Cl-/HCO3- exchanger in hypertension.

EXPERIMENTAL APPROACH

The Na+ -independent HCO3- transport system activity was assayed as the initial rate of pHi recovery after an alkaline load (CO2/HCO3 removal) in immortalized renal proximal tubular epithelial cells from spontaneously hypertensive rat (SHR) and their normotensive control (Wistar Kyoto rat; WKY).

KEY RESULTS

Noradrenaline increased Cl-/HCO3- exchanger activity with EC50 values of 0.6 and 5.3 microM in SHR and WKY cells, respectively. These effects were abolished by prazosin, but not by yohimbine. Phenylephrine increased Cl-/HCO3- exchanger activity in SHR and WKY cells (EC50 of 2.6 and 4.9 microM, respectively). Phenylephrine-mediated increase in Cl-/HCO3- exchanger activity in WKY and SHR cells was inhibited by protein kinase C (PKC), MAPK/ERK kinase (MEK) and p38 mitogen-activated protein kinase (p38 MAPK) inhibitors. The expression of alpha1A- and alpha1B-adrenoceptors was identical in WKY and SHR cells. SHR cells generated more H2O2 than WKY cells. In SHR cells, the NADPH oxidase inhibitor apocynin reduced their increased ability to generate H2O2 and abolished their hypersensitivity to phenylephrine, but failed to affect basal Cl-/HCO3- exchanger activity. H2O2-dependent stimulation of Cl-/HCO3- exchange activity was significantly higher in SHR than in WKY cells.

CONCLUSIONS AND IMPLICATIONS

Differences between WKY and SHR cells on their sensitivity to alpha1-adrenoceptor stimulation did not correlate with the abundance of alpha1A- and alpha1B-adrenoceptors and may be related to the increased generation of H2O2, which may amplify the response downstream of alpha1-adrenoceptor activation.

Oxidative Stress Causes Renal Dopamine D1 Receptor Dysfunction and Salt-Sensitive Hypertension in Sprague-Dawley Rats

Renal dopamine plays an important role in maintaining sodium homeostasis and blood pressure (BP) during increased sodium intake. The present study was carried out to determine whether renal dopamine D1 receptor (D1R) dysfunction contributes to increase in salt sensitivity during oxidative stress. Male Sprague-Dawley rats, divided into various groups, received tap water (vehicle); 1% NaCl (high salt [HS]); l -buthionine sulfoximine (BSO), an oxidant; and HS plus BSO with or without Tempol, an antioxidant, for 12 days. Compared with vehicle, HS intake increased urinary dopamine production and decreased basal renal Na/K-ATPase activity but did not affect BP. BSO-treated rats exhibited oxidative stress and a mild increase in BP. In these rats, D1R expression and G protein coupling were reduced, and SKF38393, a D1R agonist, failed to inhibit Na/K-ATPase activity and promote sodium excretion. Concomitant administration of BSO and HS caused oxidative stress, D1R dysfunction, and a marked increase in BP. Although renal dopamine production was increased, it failed to reduce the basal Na/K-ATPase activity in these animals. Treatment of BSO plus HS rats with Tempol decreased oxidative stress and restored endogenous, as well as exogenous, D1R agonist-mediated Na/K-ATPase inhibition and normalized BP. In conclusion, during HS intake, the increased dopamine production via Na/K-ATPase inhibition prevents an increase in BP. During oxidative stress, D1R function is defective, and there is mild hypertension. However, in the presence of oxidative stress, HS intake causes marked elevation in BP, which results from a defective renal D1R function leading to the failure of dopamine to inhibit Na/K-ATPase and promote sodium excretion.

Pertussis toxin induces parallel loss of neuropeptide Y Y1 receptor dimers and Gi alpha subunit function in CHO cells

Treatment with pertussis toxin in addition to a stable inhibition of G(i)alpha subunits of G-proteins also strongly reduced human neuropeptide Y Y(1) receptors expressed in Chinese hamster ovary (CHO) cells. This was reflected in abolition of the inhibition by Y(1) agonists of forskolin-stimulated adenylyl cyclase in intact cells, and of Y(1) agonist stimulation of GTPgammaS binding to particulates from disrupted cells. The loss of both receptor and G(i)alpha subunit function was attenuated by ammonium chloride, an inhibitor of acid proteinases, pointing to a chaperoning co-protection of active pertussis toxin-sensitive Galpha subunits and Y(1) receptors. The surface complement of the Y(1) receptor was changed a little in conditions of approximately 85% decrease of the Y(1) population, but the rate of the Y(1) receptor-linked internalization of agonist peptides was reduced about 70%. The preserved receptor fraction consisted of monomers significantly coupled to G(q)alpha subunits. The persistent pertussis toxin-insensitive internalization of agonists with the Y(1) receptor may reflect a rescue or alternative switching that could be important for cell functioning in neuropeptide Y-rich environments. The results are compatible with a loss, due to G(i)alpha subunit inactivation by the toxin, of a large Y(1) receptor reserve constituted of oligomers associating with heterotrimeric G-proteins.

Organ specific underexpression renal of Na+-dependent B0AT1 in the SHR correlates positively with overexpression of NHE3 and salt intake

The present study examines the renal and intestinal expression of Na(+)-dependent amino acid transporter B(0)AT1 during the development of hypertension in the spontaneous hypertensive rats (SHR) and its normotensive control (Wistar-Kyoto rat; WKY), and evaluates whether the expression of renal B(0)AT1 correlates with changes in the expression of Na(+) transporters, type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-K(+)-ATPase, known to occur in the SHR. The effect of high salt (HS) intake on the expression of renal and intestinal B(0)AT1 transcript abundance was also evaluated. For this purpose, the cloning of rat homolog of B(0)AT1 was performed. Rat B(0)AT1 shows high sequence homology to the mouse ortholog. Renal B(0)AT1 transcript abundance was lower in SHR than WKY at both 4 and 12 weeks of age. No significant differences between strains were observed in terms of intestinal expression of B(0)AT1. The decreased B(0)AT1 expression in SHR kidney was accompanied with an increase in NHE3 expression, suggesting an impaired Na(+) uptake. HS intake decreased renal B(0)AT1 mRNA in SHR and WKY at 4 weeks of age. In 12-week-old SHR, HS intake increased renal B(0)AT1 transcript abundance. Intestinal B(0)AT1 transcript was significantly increased by HS intake, though the effect was considerably more pronounced in the SHR. It is concluded, that underexpression of B(0)AT1 in the SHR kidney is organ specific, precedes the onset of hypertension and correlates negatively with the renal tubular transport of Na(+). The regulation of B(0)AT1 gene transcription appears to be under the influence of Na(+) delivery, being organ specific.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Underexpression of the Na+-dependent neutral amino acid transporter ASCT2 in the spontaneously hypertensive rat kidney

This study examined the inward transport of l-[14C]alanine, an ASCT2 preferential substrate, in monolayers of immortalized renal proximal tubular epithelial (PTE) cells from Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. The expression of ASCT2 in WKY and SHR PTE cells and kidney cortices from WKY and SHR was also evaluated. l-[14C]alanine uptake was highly dependent on extracellular Na+. Replacement of NaCl by LiCl or choline chloride abolished transport activity in SHR and WKY PTE cells. In the presence of the system L inhibitor BCH, Na+-dependent l-alanine uptake in WKY and SHR PTE cells was inhibited by alanine, serine, and cysteine, which is consistent with amino acid transport through ASCT2. The saturable component of Na+-dependent l-alanine transport under Vmax conditions in SHR PTE cells was one-half of that in WKY PTE cells, with similar Km values. Differences in magnitude of Na+-dependent l-alanine uptake through ASCT2 between WKY and SHR PTE cells correlated positively with differences in ASCT2 protein expression, this being more abundant in WKY PTE cells. Abundance of ASCT2 transcript and protein in kidney cortices of SHR rats was also lower than that in normotensive WKY rats. In conclusion, immortalized SHR and WKY PTE cells take up l-alanine mainly through a high-affinity Na+-dependent amino acid transporter, with functional features of ASCT2 transport. The activity and expression of the ASCT2 transporter were considerably lower in the SHR cells.

Short-term effect on intestinal epithelial Na+/H+ exchanger by Giα1,2-coupled 5-HT1A and Gq/11-coupled 5-HT2 receptors

The present study evaluated the effect of 5-hydroxytryptamine (5-HT) on intestinal Na(+)/H(+) exchanger (NHE) activity and the cellular signaling pathways involved in T84 cells. T84 cells express endogenous NHE1 and NHE2 proteins, detected by immunoblotting, but not NHE3. The rank order for inhibition of NHE activity in acid-loaded T84 cells was 5-(N-ethyl-N-isopropyl)-amiloride (EIPA; IC(50)=519 [465, 579] nM)>cariporide (IC(50)=630 [484, 819] nM)>amiloride (IC(50)=19 [16, 24] microM); the NHE3 inhibitor S3226 was found to be devoid of effect. This different inhibitory sensitivity indicates that both NHE1 and NHE2 isoforms may play an active role in Na(+)-dependent intracellular pH (pH(i)) recovery in T84 cells. Short-term exposure (0.5 h) of T84 cells to 5-HT increased NHE activity in a concentration-dependent manner. The stimulation induced by 5-HT (30 microM) was partially inhibited by both WAY 100135 (300 nM) and ketanserin (300 nM), antagonists of 5-HT(1A) and 5-HT(2) receptors, respectively. NHE activity was significantly increased by 8-OH-DPAT and alpha-methyl-5-HT, agonists of, respectively, 5-HT(1A) and 5-HT(2) receptors. An incubation of T84 cells with anti-G(s) and anti-G(beta) antibodies complexed with lipofectin did not prevent the 5-HT-induced stimulation of NHE activity. Overnight treatment with anti-G(ialpha1,2) and anti-G(q/11) antibodies complexed with lipofectin blocked the stimulatory effect induced by 8-OH-DPAT and alpha-methyl-5-HT, respectively. It is concluded that in T84 cells 5-HT enhances intestinal NHE activity through stimulation of G(ialpha1,2)-coupled 5-HT(1A) and G(q/11)-coupled 5-HT(2) receptors.

The dopaminergic system in hypertension

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport, vascular smooth muscle contractility and production of reactive oxygen species and by interacting with the renin–angiotensin and sympathetic nervous systems. Dopamine receptors are classified into D1-like (D1 and D5) and D2-like (D2, D3 and D4) subtypes based on their structure and pharmacology. Each of the dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central and/or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. This review summarizes the physiology of the different dopamine receptors in the regulation of blood pressure, and the relationship between dopamine receptor subtypes and hypertension.

Distinct contributions of microtubule subtypes to cell membrane shape and stability

Microtubules (MTs) are linked to cell mechanobiology. "Stable" and "dynamically unstable" microtubule (MT) subtypes are differentially sensitive to growth and distribution in serum starved (SS) versus full serum (FS) conditions. Atomic Force and Immunofluorescence microscopies were used to study the nanomechanical properties of the cell membrane in response to serum conditions and nocodazole. Nanomechanical properties of the cell membrane remain unchanged under SS/FS conditions even though there are drastic MT changes. The cell membrane is shown to depend on unstable MTs and the intermediate filament (IF) networks to maintain local stiffness. Measurements of local membrane nanomechanics in response to nocodazole display characteristic serum dependent decays. The responses suggest that the cell exists in a mechanical transition state. Stiffness is shown to depend on the interplay between dynamically unstable MTs, stable MTs and IFs which all act to impart a distinct cellular type of transient "metastability".

Mechanisms of disease: the role of GRK4 in the etiology of essential hypertension and salt sensitivity

Hypertension and salt sensitivity of blood pressure are two conditions the etiologies of which are still elusive because of the complex influences of genes, environment, and behavior. Recent understanding of the molecular mechanisms that govern sodium homeostasis is shedding new light on how genes, their protein products, and interacting metabolic pathways contribute to disease. Sodium transport is increased in the proximal tubule and thick ascending limb of Henle of the kidney in human essential hypertension. This Review focuses on the counter-regulation between the dopaminergic and renin-angiotensin systems in the renal proximal tubule, which is the site of about 70% of total renal sodium reabsorption. The inhibitory effect of dopamine is most evident under conditions of moderate sodium excess, whereas the stimulatory effect of angiotensin II is most evident under conditions of sodium deficit. Dopamine and angiotensin II exert their actions via G protein-coupled receptors, which are in turn regulated by G protein-coupled receptor kinases (GRKs). Polymorphisms that lead to aberrant action of GRKs cause a number of conditions, including hypertension and salt sensitivity. Polymorphisms in one particular member of this family-GRK4-have been shown to cause hyperphosphorylation, desensitization and internalization of a member of the dopamine receptor family, the dopamine 1 receptor, while increasing the expression of a key receptor of the renin-angiotensin system, the angiotensin II type 1 receptor. Novel diagnostic and therapeutic approaches for identifying at-risk subjects, followed by selective treatment of hypertension and salt sensitivity, might center on restoring normal receptor function through blocking the effects of GRK4 polymorphisms.

Rapid activation of Na+/H+ exchange by EPEC is PKC mediated

Enteropathogenic Escherichia coli (EPEC) increases sodium/hydrogen exchanger 2 (NHE2)-mediated sodium uptake by intestinal epithelial cells in a type III secretion-dependent manner. However, the mechanism(s) underlying these changes are not known. This study examines the role of a number of known secreted effector molecules and bacterial adhesins as well as the signaling pathways involved in this process. Deletion of the bacterial adhesins Tir and intimin had no effect on the increase in sodium/hydrogen exchanger (NHE) activity promoted by EPEC infection; however, there was a significant decrease upon deletion of the bundle-forming pili. Bacterial supernatant also failed to alter NHE activity, suggesting that direct interaction with bacteria is necessary. Analysis of the signal transduction cascades responsible for the increased NHE2 activity during EPEC infection showed that PLC increased Ca2+, as well as PKCalpha and PKCepsilon were involved in increasing NHE activity. The activation of PKCepsilon by EPEC has not been previously described nor has its role in regulating NHE2 activity. Because EPEC markedly increases NHE2 activity, this pathogen provides an exceptional opportunity to improve our understanding of this less-characterized NHE isoform.

D3 dopamine receptor directly interacts with D1 dopamine receptor in immortalized renal proximal tubule cells

D3 receptors act synergistically with D1 receptors to inhibit sodium transport in renal proximal tubules; however, the mechanism by which this occurs is not known. Because dopamine receptor subtypes can regulate and interact with each other, we studied the interaction of D3 and D1 receptors in rat renal proximal tubule (RPT) cells. The D3 agonist PD128907 increased the immunoreactive expression of D1 receptors in a concentration- and time-dependent manner; these effects were blocked by the D3 antagonist U99194A. PD128907 also transiently (15 minutes) increased the amount of cell surface membrane D1 receptors. Laser confocal immunofluorescence microscopy showed that D3 receptor and D1 receptor colocalized in RPT cells more distinctly in Wistar-Kyoto rats than in spontaneously hypertensive rats (SHRs). In addition, D3 and D1 receptors could be coimmunoprecipitated, and this interaction was increased after D3 receptor agonist stimulation for 24 hours in Wistar-Kyoto rats but not in SHRs. We propose that the synergistic effects of D3 and D1 receptors may be caused by a D3 receptor-mediated increase in total, as well as cell surface membrane D1 receptor expression, and direct D3 and D1 receptor interaction, both of which are impaired in SHRs.

Aberrant ETB receptor regulation of AT1 receptors in immortalized renal proximal tubule cells of spontaneously hypertensive rats

BACKGROUND

The renin-angiotensin and endothelin systems interact to regulate blood pressure, in part, by affecting sodium transport in the kidney. Because angiotensin II type 1 (AT(1)) receptor activation increases ETB receptor expression in renal proximal tubule cells from Wistar-Kyoto (WKY) rat, we hypothesize that ETB receptor activation may also regulate AT(1) receptor expression. Furthermore, ETB receptor regulation of the AT(1) receptor may be different in the WKY and spontaneously hypertensive rat (SHR).

METHOD

AT(1) and ETB receptors were studied in immortalized renal proximal tubule cells from WKY and SHRs, using immunoblotting, confocal microscopic colocalization, and immunoprecipitation.

RESULTS

In WKY renal proximal tubule cells, an ETB receptor agonist, BQ3020, decreased AT(1) receptor protein in a time- and concentration-dependent manner [median effective concentration (EC(50)) = 3.2 x 10(-10) mol/L, t(1/2)= 15 hours]. The inhibitory effect of BQ3020 (10(-8) mol/L/24 hours) on AT(1) receptor protein was blocked by an ETB receptor antagonist (BQ788). However, BQ3020 (10(-8) mol/L/24 hours) increased ETB receptor protein in WKY renal proximal tubule cells. In contrast, in SHR renal proximal tubule cells, BQ3020 (10(-8) mol/L/24 hours) no longer affected AT(1) or ETB receptor protein. AT(1)/ETB receptors colocalized and coimmunoprecipitated in WKY and SHRs. BQ3020 (10(-8) mol/L/15 minutes) treatment had no effect on AT(1)/ETB coimmunoprecipitation in WKY but decreased it in SHRs. BQ3020 (10(-8) mol/L/15 minutes) treatment increased AT(1) receptor phosphorylation in WKY, but decreased it in SHRs.

CONCLUSION

ETB receptors regulate AT(1) receptors by direct physical receptor interaction and receptor expression. An impaired ETB receptor regulation of the AT(1) receptor may participate in the pathogenesis of high blood pressure in the SHR.