Dopamine- and cAMP-regulated phosphoprotein (DARPP-32) and dopamine DA1 agonist-sensitive Na+,K+-ATPase in renal tubule cells

The epinephrine-induced PGE2 reduces Na+/K+ ATPase activity in Caco-2 cells via PKC, NF-κB and NO

We showed previously an epinephrine-induced inhibition of the Na⁺/K⁺ ATPase in Caco-2 cells mediated via PGE2. This work is an attempt to further elucidate mediators downstream of PGE2 and involved in the observed inhibitory effect. The activity of the Na⁺/K⁺ ATPase was assayed by measuring the amount of inorganic phosphate liberated in presence and absence of ouabain, a specific inhibitor of the enzyme. Changes in the protein expression of the Na⁺/K⁺ ATPase were investigated by western blot analysis which revealed a significant decrease in the abundance of the ATPase in plasma membranes. Treating the cells with epinephrine or PGE2 in presence of SC19220, a blocker of EP1 receptors abolished completely the effect of the hormone and the prostaglandin while the effect was maintained unaltered in presence of antagonists to all other receptors. Treatment with calphostin C, PTIO, ODQ or KT5823, respective inhibitors of PKC, NO, soluble guanylate cyclase and PKG, abrogated completely the effect of epinephrine and PGE2, suggesting an involvement of these mediators. A significant inhibition of the ATPase was observed when cells were treated with PMA, an activator of PKC or with 8-Br-cGMP, a cell permeable cGMP analogue. PMA did reduce the protein expression of IκB, as shown by western blot analysis, and its effect on the ATPase was not manifested in presence of an inhibitor of NF-κB while that of SNAP, a nitric oxide donor, was not affected. The results infer that NF-κB is downstream PKC and upstream NO. The data support a pathway in which epinephrine induces the production of PGE2 which binds to EP1 receptors and activates PKC and NF-κB leading to NO synthesis. The latter activates soluble guanylate cyclase resulting in cGMP production and activation of PKG which through direct or indirect phosphorylation inhibits the Na⁺/K⁺ ATPase by inducing its internalization.

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.

DARPP-32: from neurotransmission to cancer

Dopamine and cAMP-regulated phosphoprotein Mr 32,000 (DARPP-32), also known as phosphoprotein phosphatase-1 regulatory subunit 1B (PPP1R1B), was initially discovered as a substrate of dopamine-activated protein kinase A (PKA) in the neostriatum in the brain. While phosphorylation at Thr-34 by PKA converts DARPP-32 into a potent inhibitor of protein phosphatase 1 (PP1), phosphorylation at Thr-75 transforms DARPP-32 into an inhibitor of PKA. Through regulation of DARPP-32 phosphorylation and modulation of protein phosphatase and kinase activities, DARPP-32 plays a critical role in mediating the biochemical, electrophysiological, and behavioral effects controlled by dopamine and other neurotransmitters in response to drugs of abuse and psychostimulants. Altered expression of DARPP-32 and its truncated isoform (t-DARPP), specifically in the prefrontal cortex, has been associated with schizophrenia and bipolar disorder. Moreover, cleavage of DARPP-32 by calpain has been implicated in Alzheimer's disease. Amplification of the genomic locus of DARPP-32 at 17q12 has been described in several cancers. DARPP-32 and t-DARPP are frequently overexpressed at the mRNA and protein levels in adenocarcinomas of the breast, prostate, colon, and stomach. Several studies demonstrated the pro-survival, pro-invasion, and pro-angiogenic functions of DARPP-32 in cancer. Overexpression of DARPP-32 and t-DARPP also promotes chemotherapeutic drug resistance and cell proliferation in gastric and breast cancers through regulation of pro-oncogenic signal transduction pathways. The expansion of DARPP-32 research from neurotransmission to cancer underscores the broad scope and implication of this protein in disparate human diseases.

The evolutionary strata of DARPP-32 tail implicates hierarchical functional expansion in higher vertebrates

DARPP-32 (dopamine and adenosine 3', 5'-monophosphate-regulated phosphoprotein of 32 kDa), which belongs to PPP1R1 gene family, is known to act as an important integrator in dopamine-mediated neurotransmission via the inhibition of protein phosphatase-1 (PP1). Besides its neuronal roles, this protein also behaves as a key player in pathological and pharmacological aspects. Use of bioinformatics and phylogenetics approaches to further characterize the molecular features of DARPP-32 can guide future works. Predicted phosphorylation sites on DARPP-32 show conservation across vertebrates. Phylogenetics analysis indicates evolutionary strata of phosphorylation site acquisition at the C-terminus, suggesting functional expansion of DARPP-32, where more diverse signalling cues may involve in regulating DARPP-32 in inhibiting PP1 activity. Moreover, both phylogenetics and synteny analyses suggest de novo origination of PPP1R1 gene family via chromosomal rearrangement and exonization.

Protein phosphatase 1 inhibitor-1 deficiency reduces phosphorylation of renal NaCl cotransporter and causes arterial hypotension

The thiazide-sensitive NaCl cotransporter (NCC) of the renal distal convoluted tubule (DCT) controls ion homeostasis and arterial BP. Loss-of-function mutations of NCC cause renal salt wasting with arterial hypotension (Gitelman syndrome). Conversely, mutations in the NCC-regulating WNK kinases or kelch-like 3 protein cause familial hyperkalemic hypertension. Here, we performed automated sorting of mouse DCTs and microarray analysis for comprehensive identification of novel DCT-enriched gene products, which may potentially regulate DCT and NCC function. This approach identified protein phosphatase 1 inhibitor-1 (I-1) as a DCT-enriched transcript, and immunohistochemistry revealed I-1 expression in mouse and human DCTs and thick ascending limbs. In heterologous expression systems, coexpression of NCC with I-1 increased thiazide-dependent Na(+) uptake, whereas RNAi-mediated knockdown of endogenous I-1 reduced NCC phosphorylation. Likewise, levels of phosphorylated NCC decreased by approximately 50% in I-1 (I-1(-/-)) knockout mice without changes in total NCC expression. The abundance and phosphorylation of other renal sodium-transporting proteins, including NaPi-IIa, NKCC2, and ENaC, did not change, although the abundance of pendrin increased in these mice. The abundance, phosphorylation, and subcellular localization of SPAK were similar in wild-type (WT) and I-1(-/-) mice. Compared with WT mice, I-1(-/-) mice exhibited significantly lower arterial BP but did not display other metabolic features of NCC dysregulation. Thus, I-1 is a DCT-enriched gene product that controls arterial BP, possibly through regulation of NCC activity.

A molecular characterization of the choroid plexus and stress-induced gene regulation

The role of the choroid plexus (CP) in brain homeostasis is being increasingly recognized and recent studies suggest that the CP has a more important role in physiological and pathological brain functions than currently appreciated. To obtain additional insight on the CP function, we performed a proteomics and transcriptomics characterization employing a combination of high resolution tandem mass spectrometry and gene expression analyses in normal rodent brain. Using multiple protein fractionation approaches, we identified 1400 CP proteins in adult CP. Microarray-based comparison of CP gene expression with the kidney, cortex and hippocampus showed significant overlap between the CP and the kidney. CP gene profiles were validated by in situ hybridization analysis of several target genes including klotho, CLIC 6, OATP 14 and Ezrin. Immunohistochemical analyses were performed for CP and enpendyma detection of several target proteins including cytokeratin, Rab7, klotho, tissue inhibitor of metalloprotease 1 (TIMP1), MMP9 and glial fibrillary acidic protein (GFAP). The molecular functions associated with various proteins of the CP proteome indicate that it is a blood-cerebrospinal fluid (CSF) barrier that exhibits high levels of metabolic activity. We also analyzed the gene expression changes induced by stress, an exacerbating factor for many illnesses, particularly mood disorders. Chronic stress altered the expression of several genes, downregulating 5HT2C, glucocorticoid receptor and the cilia genes IFT88 and smoothened while upregulating 5HT2A, BDNF, TNFα and IL-1b. The data presented here attach additional significance to the emerging importance of CP function in brain health and CNS disease states.

2011 Homer Smith Award: To serve and protect: classic and novel roles for Na+, K+ -adenosine triphosphatase

The ability of cells to maintain sharp ion gradients across their membranes is the foundation for the molecular transport and electrical excitability. Across animal species and cell types, Na(+),K(+)-adenosine triphosphatase (ATPase) is arguably the most powerful contributor to this phenomenon. By producing a steep concentration difference of sodium and potassium between the intracellular and extracellular milieu, Na(+),K(+)-ATPase in the tubules provides the driving force for renal sodium reabsorption. Pump activity is downregulated by natriuretic hormones, such as dopamine, and is upregulated by antinatriuretic hormones, such as angiotensin. In the past decade, studies have revealed a novel and surprising role: that Na(+),K(+)-ATPase is a transducer of signals from extracellular to intracellular compartments. The signaling function of Na(+),K(+)-ATPase is activated by ouabain, a mammalian steroid hormone, at far lower concentrations than those that inhibit pump activity. By promoting growth and inhibiting apoptosis, activation of Na(+),K(+)-ATPase exerts tissue-protective effects. Ouabain-stimulated Na(+),K(+)-ATPase signaling has recently shown clinical promise by protecting the malnourished embryonic kidney from adverse developmental programming. A deeper understanding of the tissue-protective role of Na(+),K(+)-ATPase signaling and the regulation of Na(+),K(+)-ATPase pumping activity is of fundamental importance for the understanding and treatment of kidney diseases and kidney-related hypertension.

DARPP-32, Jack of All Trades… Master of Which?

DARPP-32 (PPP1R1B) was discovered as a substrate of cAMP-dependent protein kinase (PKA) enriched in dopamine-innervated brain areas. It is one of three related, PKA-regulated inhibitors of protein phosphatase-1 (PP1). These inhibitors seem to have appeared in early vertebrate ancestors, possibly Gnathostomes. DARPP-32 has additional important biochemical properties including inhibition of PKA when phosphorylated by Cdk5 and regulation by casein kinases 1 and 2. It is highly enriched in specific neuronal populations, especially striatal medium-size spiny neurons. As PP1 inhibitor DARPP-32 amplifies and/or mediates many actions of PKA at the plasma membrane and in the cytoplasm, with a broad spectrum of potential targets and functions. DARPP-32 also undergoes a continuous and tightly regulated cytonuclear shuttling. This trafficking is controlled by phosphorylation of Ser-97, which is necessary for nuclear export. When phosphorylated on Thr-34 and dephosphorylated on Ser-97, DARPP-32 can inhibit PP1 in the nucleus and modulate signaling pathways involved in the regulation of chromatin response. Recent work with multiple transgenic and knockout mutant mice has allowed the dissection of DARPP-32 function in striato-nigral and striato-pallidal neurons. It is implicated in the action of therapeutic and abused psychoactive drugs, in prefrontal cortex function, and in sexual behavior. However, the contribution of DARPP-32 in human behavior remains poorly understood. Post-mortem studies in humans suggest possible alterations of DARPP-32 levels in schizophrenia and bipolar disorder. Genetic studies have revealed a polymorphism with possible association with psychological and psychopathological traits. In addition, a short isoform of DARPP-32, t-DARPP, plays a role in cancer, indicating additional signaling properties. Thus, DARPP-32 is a non-essential but tightly regulated signaling hub molecule which may improve the general performance of the neuronal circuits in which it is expressed.

Beyond the dopamine receptor: regulation and roles of serine/threonine protein phosphatases

Dopamine plays an important modulatory role in the central nervous system, helping to control critical aspects of motor function and reward learning. Alteration in normal dopaminergic neurotransmission underlies multiple neurological diseases including schizophrenia, Huntington’s disease, and Parkinson’s disease. Modulation of dopamine-regulated signaling pathways is also important in the addictive actions of most drugs of abuse. Our studies over the last 30 years have focused on the molecular actions of dopamine acting on medium spiny neurons, the predominant neurons of the neostriatum. Striatum-enriched phosphoproteins, particularly dopamine and adenosine 3′:5′-monophosphate-regulated phosphoprotein of 32 kDa (DARPP-32), regulator of calmodulin signaling (RCS), and ARPP-16, mediate pleiotropic actions of dopamine. Notably, each of these proteins, either directly or indirectly, regulates the activity of one of the three major subclasses of serine/threonine protein phosphatases, PP1, PP2B, and PP2A, respectively. For example, phosphorylation of DARPP-32 at Thr34 by protein kinase A results in potent inhibition of PP1, leading to potentiation of dopaminergic signaling at multiple steps from the dopamine receptor to the nucleus. The discovery of DARPP-32 and its emergence as a critical molecular integrator of striatal signaling will be discussed, as will more recent studies that highlight novel roles for RCS and ARPP-16 in dopamine-regulated striatal signaling pathways.

Na+/K+-ATPase-mediated signal transduction and Na+/K+-ATPase regulation

A number of studies suggest that Na(+)/K(+)-ATPase in caveolae interacts with neighboring membrane proteins and organizes cytosolic cascades of signaling proteins to send messages to intracellular organelles in different tissues, mostly in cardiac myocytes. Low concentration of ouabain binding to Na(+)/K(+)-ATPase activates Src/epidermal growth factor receptor complex to initiate multiple signal pathways, which include PLC/IP3/CICR, PI3K, reactive oxygen species (ROS), PLC/DG/PKC/Raf/MEK/ERK1/2, and Ras/Raf/MEK/ERK1/2 pathways. In cardiac myocytes, the resulting downstream events include the induction of some early response proto-oncogenes, activation of transcription factors, activator protein-1, and nuclear factor-kappaB, the regulation of a number of cardiac growth-related genes, and the stimulation of protein synthesis and myocyte hypertrophy and apoptosis. Conversely, several factors acting through signal pathways, such as protein kinases, Ca(2+), ROS, etc., can modulate the activity of the Na(+)/K(+)-ATPase.

Brain-specific factors in combination with mutant huntingtin induce gene-specific transcriptional dysregulation

Mutant huntingtin lowered steady-state levels of DARPP-32 mRNA in the brain but not kidney of R6 transgenic HD mice by repressing transcription from one of two promoters. The activity of DARPP-32 promoter deletion constructs were lower in the presence of mutant huntingtin in immortalized striatal cell lines but no difference in transcription factor binding to the promoter was detected. The activity of CMV, TK and HPRT promoters was also affected by mutant huntingtin in these cell lines. Transient transfection experiments demonstrated that short-term expression of mutant huntingtin exerted a cell- and promoter-specific transcriptional repression. In in vitro experiments, transcription of the CMV promoter was reduced in the presence of striatal proteins and mutant huntingtin. It is likely that select combinations of trans-acting factors, co-activators and components of the Pol II holoenzyme acting in concert provide the basis for both the gene- and tissue-specific effects of mutant huntingtin.

Angiotensin II AT2 receptors inhibit proximal tubular Na+-K+-ATPase activity via a NO/cGMP-dependent pathway

Angiotensin II AT2 receptors act as a functional antagonist for the AT1 receptors in various tissues. We previously reported that activation of the renal AT2 receptors promotes natriuresis and diuresis; however, the mechanism is not known. The present study was designed to investigate whether activation of AT2 receptors affects the activity of Na+-K+-ATPase (NKA), an active tubular sodium transporter, in the proximal tubules isolated from Sprague-Dawley rats. The AT2 receptor agonist CGP-42112 (10(-10)-10(-7) M) produced a dose-dependent inhibition of NKA activity (9-38%); the inhibition was attenuated by the presence of the AT2 receptor antagonist PD-123319 (1 microM), suggesting the involvement of the AT2 receptors. The AT1 receptor antagonist losartan (1 microM) did not affect the CGP-42112 (100 nM)-induced inhibition of NKA activity. The presence of guanylyl cyclase inhibitor ODQ (10 microM) and the nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 100 microM) abolished the CGP-42112 (100 nM)-induced NKA inhibition. ANG II (100 nM), in the presence of losartan, significantly inhibited NKA activity; the inhibition was attenuated by PD-123319. CGP-42112 also, in a dose-dependent manner, stimulated NO production (approximately 0-230%) and cGMP accumulation (approximately 25-100%). The CGP-42112 (100 nM)-induced NO and cGMP increases were abolished by the AT2 receptor antagonist PD-123319, ODQ, and L-NAME. The data suggest that the activation of the AT2 receptor via stimulation of the NO/cGMP pathway causes inhibition of NKA activity in the proximal tubules. This phenomenon provides a plausible mechanism responsible for the AT2 receptor-mediated natriuresis-diuresis in rodents.

Dopamine regulation of amiloride-sensitive sodium channels in lung cells

Dopamine increases lung fluid clearance. This is partly due to activation of basolateral Na-K-ATPase. However, activation of Na-K-ATPase by itself is unlikely to produce large changes in transepithelial transport. Therefore, we examined apical and basolateral dopamine's effect on apical, highly selective sodium channels [epithelial sodium channels (ENaC)] in monolayers of an alveolar type 2 cell line (L2). Dopamine increased channel open probability (P(o)) without changing the unitary current. The D(1) receptor blocker SCH-23390 blocked the dopamine effect, but the D(2) receptor blocker sulpiride did not. The dopamine-mediated increase in ENaC activity was not a secondary effect of dopamine stimulation of Na-K-ATPase, since ouabain applied to the basolateral surface to block the activity of Na-K-ATPase did not alter dopamine-mediated ENaC activity. Protein kinase A (PKA) was not responsible for dopamine's effect since a PKA inhibitor, H89, did not reduce dopamine's effect. However, cpt-2-O-Me-cAMP, which selectively binds and activates EPAC (exchange protein activated by cAMP) but not PKA, increased ENaC P(o). An Src inhibitor, PP2, and the phosphatidylinositol-3-kinase inhibitor, LY-294002, blocked dopamine's effect on ENaC. In addition, an MEK blocker, U0126, an inhibitor of phospholipase A(2), and a protein phosphatase inhibitor also blocked the effect of dopamine on ENaC P(o). Finally, since the cAMP-EPAC-Rap1 pathway also activates DARPP32 (32-kDa dopamine response protein phosphatase), we confirmed that dopamine phosphorylates DARPP32, and okadaic acid, which blocks phosphatases (DARPP32), also blocks dopamine's effect. In summary, dopamine increases ENaC activity by a cAMP-mediated alternative signaling pathway involving EPAC and Rap1, signaling molecules usually associated with growth-factor-activated receptors.

Protein phosphatase-1 inhibitor-3 is co-localized to the nucleoli and centrosomes with PP1γ1 and PP1α, respectively

In this study, we show that protein phosphatase-1 (PP1) inhibitor-3 (Inh3) is localized to the nucleoli and centrosomes in interphase HEK 293 cells. Inh3 exhibited a specific co-localization to the nucleoli with PP1gamma1, and to the centrosomes with PP1alpha. These findings indicate that Inh3 may act as a modulator of PP1 functions in the processes of cytokinesis, as well as of nucleolar events. The specificity of the interaction of Inh3 with the PP1 isoforms was also demonstrated in vitro, where Inh3 co-immunoprecipitated with PP1alpha and PP1gamma1, but not with PP1beta. The nuclear localization signal of Inh3 was identified as a N-terminal basic cluster (33RKRK36), while nucleolar localization was shown to be dependent on a C-terminal basic cluster (94HRKGRRR100). The importance of the individual basic residues was quantitatively assessed by site-directed mutagenesis and a novel use of laser scanning cytometry.

Peptide inhibitors of protein kinases-discovery, characterisation and use

Protein kinases are now the second largest group of drug targets, and most protein kinase inhibitors in clinical development are directed towards the ATP-binding site. However, these inhibitors must compete with high intracellular ATP concentrations and they must discriminate between the ATP-binding sites of all protein kinases as well the other proteins that also utilise ATP. It would therefore be beneficial to target sites on protein kinases other than the ATP-binding site. This review describes the discovery, characterisation and use of peptide inhibitors of protein kinases. In many cases, the development of these peptides has resulted from an understanding of the specific protein-binding partners for a particular protein kinase. In addition, novel peptide sequences have been discovered in library screening approaches and have provided new leads in the discovery and/or design of peptide inhibitors of protein kinases. These approaches are therefore providing exciting new opportunities in the development of ATP non-competitive inhibitors of protein kinases.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

DARPP-32 and inhibitor-1 are expressed in pancreatic β-cells

Insulin secretion from pancreatic beta-cells has to be tightly regulated to ensure accurate glucose homeostasis. The capacity of beta-cells to respond to extracellular stimulation is determined by several signaling pathways. One important feature of these pathways is phosphorylation and subsequent dephosphorylation of a wide range of cellular substrates. Protein phosphatase 1 (PP1) is a major eukaryotic serine/threonine protein phosphatase that controls a multitude of physiological processes. We have investigated the expression and cellular distribution of two endogenous inhibitors of PP1 activity in beta-cells. RT-PCR, Western blotting, and immunohistochemistry showed that DARPP-32 and inhibitor-1 are present in insulin-secreting endocrine beta-cells. Subcellular fractionation of mouse islets revealed that both PP1 inhibitors predominantly localized to cytosol-enriched fractions. Inhibitor-1 was also present in fractions containing plasma membrane-associated proteins. These data indicate a potential role for DARPP-32 and inhibitor-1 in the regulation of PP1 activity in pancreatic beta-cell stimulus-secretion coupling.

Quantitative assessment of transcriptome differences between brain territories

Transcriptome analysis of mammalian brain structures is a potentially powerful approach in addressing the diversity of cerebral functions. Here, we used a microassay for serial analysis of gene expression (SAGE) to generate quantitative mRNA expression profiles of normal adult mouse striatum, nucleus accumbens, and somatosensory cortex. Comparison of these profiles revealed 135 transcripts heterogeneously distributed in the brain. Among them, a majority (78), although matching a registered sequence, are novel regional markers. To improve the anatomical resolution of our analysis, we performed in situ hybridization and observed unique expression patterns in discrete brain regions for a number of candidates. We assessed the distribution of the new markers in peripheral tissues using quantitative RT-PCR, Northern hybridization, and published SAGE data. In most cases, expression was higher in the brain than in peripheral tissues. Because the markers were selected according to their expression level, without reference to prior knowledge, our studies provide an unbiased, comprehensive molecular signature for various mammalian brain structures that can be used to investigate their plasticity under a variety of circumstances.

Increased blood pressure and loss of anp-induced natriuresis in mice lacking DARPP-32 gene

Atrial natriuretic peptide (ANP) is an important regulator of sodium metabolism and indirectly of blood pressure. Evidence has accumulated that ANP regulates sodium metabolism through a cascade of steps involving an increase in the level of cGMP, activation of cGMP-dependent protein kinase (PKG), and inhibition of renal tubular Na+, K+-ATPase activity. One of the major substrates for PKG is DARPP-32. In the present study we observed that ANP does not induce natriuresis in mice that lack DARPP-32. In contrast, there was a 4-fold increase in urinary sodium excretion following ANP administration to wild type mice. ANP as well as Zaprinast, a selective inhibitor of cGMP phosophodiesterase, inhibited renal Na+, K+-ATPase activity in wild type mice but had no such effect in mice lacking DARPP-32. Mean arterial blood pressure, measured in conscious animals, was significantly increased in DARPP-32 deficient mice as compared to wild type mice. The results confirm that DARPP-32 acts as a third messenger in the ANP signaling pathway in renal tissue and suggest an important role of DARPP-32 in the maintenance of normal blood pressure.

Functional characterization of basolateral and luminal dopamine receptors in rabbit CCD

Previous studies reported the existence of both D1- and D2-like receptors in the cortical collecting duct (CCD). However, especially with regard to natriuresis, it remains controversial. In the present study, rabbit CCD was perfused to characterize the receptor subtypes responsible for the tubular actions. Basolateral dopamine (DA) induced a dose-dependent depolarization of transepithelial voltage. Basolateral domperidone, a D2-like receptor antagonist, abolished depolarization, whereas SKF-81297, a D1-like receptor agonist, showed no significant change. In addition, bromocriptine, a D2-like receptor agonist, also caused depolarization, whereas SKF-81297, a D1-like receptor agonist, did not depolarize significantly. Moreover, RBI-257, a D4-specific antagonist, reversed the basolateral DA-induced depolarization. In contrast to the basolateral side, luminal DA caused depolarization via a D1-like receptor; however the change was less than that for basolateral DA. For further evaluation, 22Na+ flux (J(Na)) was measured to confirm the effect of DA on Na+ transport. Basolateral DA also caused a suppression of J(Na), and this reaction was abolished by domperidone. These results suggested that the basolateral D2-like receptor is mainly responsible for the natriuretic action of DA in rabbit CCD.

Regulation of sodium/potassium ATPase activity: impact on salt balance and vascular contractility

Na+,K+-ATPase distributes ions between the intracellular and extracellular space and is responsible for total-body sodium homeostasis. The activity of this ion pump is regulated by catecholamines and peptide hormones; by the ligand of Na+,K+-ATPase, ouabain; and by direct interaction with cytoskeleton proteins. This review summarizes recent advances in the field of short-term regulation of Na+,K+-ATPase and the implications of these advances for the regulation of blood pressure. Renal Na+,K+-ATPase activity is bidirectionally regulated by natriuretic and antinatriuretic hormones, and a shift in the balance between these forces may lead to salt retention and hypertension. Dopamine plays a key role in this interactive regulation. By inhibiting vascular Na+,K+-ATPase activity, an excess of circulating ouabain may increase calcium concentration in vascular cells and lead to increased vascular contractility. Finally, mutations in cytoskeleton proteins may stimulate renal Na+,K+-ATPase activity by way of protein/protein interaction and lead to salt retention and hypertension. Abnormalities in the systems regulating Na+,K+-ATPase should be explored further in the search for the multiple causes of essential hypertension.

Detection and quantification of protein phosphatase inhibitor-1 gene expression in total rat liver and isolated hepatocytes

The mRNA expression of protein phosphatase inhibitor-1 (inhibitor-1) in rat liver was demonstrated using highly sensitive semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). Quantification by real-time RT-PCR (LightCycler technology) yielded the same copy number of inhibitor-1 mRNA in total rat liver and isolated hepatocytes (12 copies per cell). This novel finding shows that rat liver expresses indeed inhibitor-1 mRNA, albeit in low amounts. The low copy number explains why the mRNA had not been detected by Northern blotting so far. For comparison, about 425 copies/cell were detected in brain and 2500 copies/cell in skeletal muscle from rat. The full-length coding sequence of rat liver inhibitor-1 was cloned and sequenced, 100% homology with the muscle cDNA was obtained, indicating the expression of the same gene in liver and muscle. In vitro transcription and translation yielded a protein (Mr approximately 30 kDa) which could be detected with a specific antibody by immunoblotting. This indicates an intact open reading frame of inhibitor-1 in rat liver. Immunoblotting of liver extract yielded a very weak band which comigrated with the inhibitor-1 proteins from muscle and brain. It is concluded that mRNA expression of inhibitor-1 may have implications for the regulation of protein phosphatase-1 (PP1) in rat liver.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Characterization of acute inhibition of Na/H exchanger NHE-3 by dopamine in opossum kidney cells

BACKGROUND

Dopamine (DA) is a principal natriuretic hormone that defends extracellular fluid volume from a Na load. Natriuresis is effected partly through inhibiting the proximal tubule Na/H exchanger NHE-3. Changes in NHE-3 phosphorylation is one mechanism by which NHE-3 activity is regulated.

METHODS

We used opossum kidney (OK) cells to characterize the differential and synergistic effects of DA receptor subtype-1 (DA1) and -2 (DA2) agonists and the effect of blockade of protein kinase A (PKA) or protein kinase C (PKC) on NHE-3 activity and phosphorylation.

RESULTS

DA and DA1 agonists inhibited NHE-3 activity, and DA1 antagonist blocked the effect of either DA or DA1 agonist. DA2 agonist alone had no effect, but DA2 antagonist reduced the DA effect on NHE-3 activity. DA1 and DA2 agonists together were more potent than DA1 alone. PKA inhibition eliminated the effect of DA1 agonist and partially blocked the effect of DA on NHE-3 activity. PKC inhibition did not block the DA effect. DA1 agonist and PKA activation phosphorylated NHE-3 on identical sites. Despite lack of effect on NHE-3 activity, DA2 agonists increased NHE-3 phosphorylation. DA-induced NHE-3 phosphorylation was distinct from DA1 and PKA but closely resembled DA2.

CONCLUSION

We postulate the following: (1) DA modifies NHE-3 phosphorylation by activating PKA through DA1 and by other kinases/phosphatases via DA2. (2) DA1 is sufficient to inhibit NHE-3, while DA2 is insufficient but plays a synergistic role by altering NHE-3 phosphorylation.

Phosphorylation of protein phosphatase-1 inhibitors, inhibitor-1 and DARPP-32, in renal medulla

Inhibitor-1 and DARPP-32 (dopamine and cAMP-regulated phosphoprotein, Mr 32 kDa) are each phosphorylated by cAMP-dependent protein kinase, resulting in their conversion to potent inhibitors of protein phosphatase-1. Protein phosphatase-1 is involved in the regulation of Na(+) reabsorption from renal tubule by modulating the activity of Na(+),K(+)-ATPase. In this study, we have investigated the regulation of inhibitor-1 and DARPP-32 phosphorylation in slices of renal medulla. Activation of cAMP-dependent protein kinase by forskolin and 8-bromo-cAMP increased the level of phosphorylated inhibitor-1. Okadaic acid (1 microM), used to inhibit protein phosphatase-2A, increased the level of phosphorylated inhibitor-1, but cyclosporin A had no effect. DARPP-32, like inhibitor-1, was phosphorylated by cAMP-dependent protein kinase and dephosphorylated only by protein phosphatase-2A. These data demonstrate that the phosphorylation of inhibitor-1 and DARPP-32 is regulated by the balance of phosphorylation by cAMP-dependent protein kinase and dephosphorylation by protein phosphatase-2A in renal medulla. Furthermore, the phosphorylation step is regulated by pharmacological stimuli such as activation of beta(1)-adrenoceptors and dopamine D1 receptors.

Intrarenal dopamine: a key signal in the interactive regulation of sodium metabolism

The kidney regulates sodium metabolism with extraordinary precision and sensitivity. This is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between anti-natriuretic and natriuretic factors. Dopamine, produced in renal proximal tubule cells, plays a central role in this interactive network. Natriuretic hormones that are released from extrarenal sources, such as atrial natriuretic peptide, mediate some of their effects via renal dopamine receptors. On the level of the tubules, dopamine acts by opposing the effects of anti-natriuretic factors, such as angiotensin II and alpha-adrenergic receptors. Sodium retention leads to an increase in renal dopamine tonus, and the natriuretic effects of dopamine are more prominent under this condition. Inhibition or down-regulation of dopamine receptors significantly attenuates the natriuretic response to salt loading. Renal dopamine is modulated by the supply of filtered L-DOPA and the metabolism of dopamine via catechol-O-methyldopamine. The importance of dopamine as a natriuretic hormone is reflected by its capacity to inhibit the majority of renal tubule sodium transporters. Notably, the activity of Na+, K+ ATPase is inhibited in most tubule segments by dopamine. Recent studies have elucidated many of the signaling pathways for renal dopamine receptors. Novel principles for homologous and heterologous sensitization of dopamine receptors have been detected that may explain some of the interaction between dopamine and other first messengers that modulate renal tubule sodium transport. A broad understanding of the renal dopamine system has become increasingly important, since there is now strong evidence from both clinical and experimental studies that dysregulation of the renal dopamine system plays a role in many forms of multigenetic 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.

Renal dopaminergic mechanisms and hypertension: a chronology of advances

Dopamine (DA) has been shown to influence kidney function through endogenous synthesis and subsequent interaction with locally expressed dopamine receptor subtypes (D1, D5 as D1-like and D2, D3, and D4 as D2-like). DA, and DA-receptor specific agonists and antagonists can alter renal water and electrolyte excretion along with renin release when infused systemically or intrarenally. Such effects are brought about by a combination of renal hemodynamic and direct tubular effects evoked along the full length of the nephron. The cellular mechanisms that direct these dopamine-mediated renal electrolyte fluxes have recently been clarified and include alterations in adenylyl cyclase, phospholipase C, and phospholipase A1 activity. The dopaminergic system also interacts directly with the renal kallikrein-kinin, prostaglandin and other neurohumoral systems. Aberrant renal dopamine production and/or dopamine receptor function have been reported in salt-dependent and low-renin forms of human primary hypertension as well as in genetic models of animal hypertension, including the SHR and Dahl SS rat. DA D1 or D3 receptor knockout mice have been shown to develop hypertension.

Mechanisms by which intrarenal dopamine and ANP interact to regulate sodium metabolism

Maintenance of a normal blood pressure requires a precise and fine-tuned regulation of salt metabolism. This is accomplished by a bidirectional regulation of renal tubular sodium transporters by natriuretic and antinatriuretic hormones. Dopamine, produced in the renal proximal tubular cells, plays an important role in this interactive system. Dopamine inhibits the activity of Na+,K+ ATPase as well as of many important sodium influx pathways in the nephron. These effects of dopamine are particularly pronounced in situation of sodium loading. There is an abundance of evidence suggesting that the natriuretic effects of ANP are to a large extent mediated via renal dopamine 1 like receptors. The renal tubular dopamine 1 like receptors are, under basal conditions, mainly located intracellularly. ANP and its second messenger, cGMP, cause a rapid translocation of the dopamine 1 like receptors to the plasma membrane. This phenomenon may explain how ANP and dopamine act in concert to regulate sodium metabolism. Regulation of sodium metabolism and blood pressure is critically dependent on a normal function of the renal dopamine system. Hence, abnormalities in the interaction between dopamine and ANP may predispose to hypertension.

Effects of serine/threonine protein phosphatases on ion channels in excitable membranes

This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.

Effect of cAMP on the activity and the phosphorylation of Na+,K(+)-ATPase in rat thick ascending limb of Henle

BACKGROUND

In rat kidney medullary thick ascending limb of Henle's loop (MTAL), activation of protein kinase A (PKA) was previously reported to inhibit Na+,K(+)-ATPase activity. This is paradoxical with the known stimulatory effect of cAMP on sodium reabsorption. Because this inhibition was mediated by phospholipase A2 (PLA2) activation, a pathway stimulated by hypoxia, we evaluated the influence of oxygen supply on cAMP action on Na+,K(+)-ATPase in MTAL.

METHODS

Ouabain-sensitive 86Rb uptake and Na+,K(+)-ATPase activity were measured in isolated MTALs. Cellular ATP content and the phosphorylation level of Na+,K(+)-ATPase were determined in suspensions of outer medullary tubules. Experiments were carried out under nonoxygenated or oxygenated conditions in the absence or presence of PKA activators.

RESULTS

cAMP analogues or forskolin associated with 3-isobutyl-1-methylxanthine (IBMX) inhibited ouabain-sensitive 86Rb uptake in nonoxygenated MTALs. In contrast, when oxygen supply was increased, cAMP stimulated ouabain-sensitive 86Rb uptake and Na+,K(+)-ATPase activity. Improved oxygen supply was associated with increased intracellular ATP content. The phosphorylation level of the Na+,K(+)-ATPase alpha subunit was increased by cAMP analogues or forskolin associated with IBMX in oxygenated as well as in nonoxygenated tubules. Under nonoxygenated conditions, the inhibition of Na+,K(+)-ATPase was dissociated from its cAMP-dependent phosphorylation, whereas under oxygenated conditions, the stimulatory effect of cAMP analogues on ouabain-sensitive 86Rb uptake was linearly related and cosaturated with the level of phosphorylation of the Na+,K(+)-ATPase alpha subunit.

CONCLUSION

In oxygenated MTALs, PKA-mediated stimulation of Na+,K(+)-ATPase likely participates in the cAMP-dependent stimulation of sodium reabsorption. Under nonoxygenated conditions, this stimulatory pathway is likely overridden by the PLA2-mediated inhibitory pathway, a possible adaptation to protect the cells against hypoxic damage.

Nitric oxide modulates Na+, K+-ATPase activity through cyclic GMP pathway in proximal rat trachea

The present work demonstrated that nitric oxide (NO) modulates Na+, K+-ATPase activity in the proximal rat trachea. Sodium nitroprusside induced concentration-dependent (10-100 microM) stimulation in proximal trachea Na+, K+-ATPase activity. The effect was specific for Na+, K+-ATPase since Mg-ATPase activity was unaffected. This NO-donor changed neither Na+, K+-ATPase nor Mg-ATPase activity in the distal segment. The modulatory action on Na+, K+-ATPase induced by sodium nitroprusside was linked to an increase in nitrates/nitrites and cyclic GMP levels in proximal segments. Modulation of proximal Na+, K+-ATPase activity by sodium nitroprusside was mimicked by S-nitroso-N-acetylpenicillamine (100 microM) and 8-bromo-cyclic GMP (100 microM). Both sodium nitroprusside and 8-bromo-cyclic GMP effects on Na+, K+-ATPase activity of proximal segments of trachea were blocked by 2 microM of KT 5823 (a cyclic GMP-dependent protein kinase inhibitor), but not by 0.5 microM of KT 5720 (a cyclic AMP-dependent protein kinase inhibitor). Both kinase inhibitors decreased proximal Na+, K+-ATPase activity, but did not change Mg-ATPase activity. Okadaic acid (1 microM), a phosphatase-1 inhibitor, increased proximal Na+, K+-ATPase but not Mg-ATPase activity. The effect of okadaic acid was non-additive with that of 8-bromo-cGMP on Na+, K+-ATPase activity. Our results suggest that NO modulates proximal rat trachea Na+, K+-ATPase activity through cyclic GMP and cyclic GMP-dependent protein kinase.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Short-term regulation of endothelial Na(+)-K(+)-pump activity by cGMP: a 133Cs magnetic resonance study

The effect of nitric oxide radicals (NO) on the activity of porcine aortic endothelial Na(+)-K(+)-ATPase is reported. Measurements were made using an in vitro cell system and 133Cs magnetic resonance (NMR). It is shown that NO, through stimulation of guanylate cyclase, results in a reduction of pump activity. Similar observations were made using 8-Br-cGMP. Measurement of the cytosolic volume indicated no changes in volume during incubation with 8-Br-cGMP. Our measurements indicate a continuous regulation of endothelial Na(+)-K(+)-ATPase activity by endogenous NO. This regulation could be removed by L-NAME, resulting in a small increase in pump activity.

Amyloid beta-peptides inhibit Na+/K+-ATPase: tissue slices versus primary cultures

Abeta1-40 (20 microM) has been reported to selectively inhibit Na+/K+-ATPase activity in rat primary hippocampal cultures after 2-6 days of exposure. We expanded these studies to include Abeta's effects on Na+/K+-ATPase activity in rat primary cortical cultures and hippocampal slices, and we correlated these effects with estimates of cell survival in rat brain primary cultures. Using optimized assay conditions, a 5-day exposure to 50 microM Abeta 25-35, 20 microM Abeta 1-40, and 20 microM Abeta 1-42 decreased Na+/K+-ATPase activity in rat primary cortical cultures 66%, 60%, and 22%, respectively. Abeta 25-35 (50 microM) at 24 h was the only condition that caused inhibition of Na+/K+-ATPase activity in the absence of cell death, defined as an extracellular shift in the localization of the cytoplasmic enzyme lactate dehydrogenase (LDH). We also found that hippocampal slices were sensitive to Abeta, exhibiting a 40-60% reduction in membrane Na+/K+-ATPase activity when exposed to 1-30 nM of Abeta 1-40 for 60 min. This inhibition was not readily reversible, as it withstood homogenization and repeated dilution and centrifugation. Additionally, this inhibition occurred only after amyloid incubation with intact hippocampal slices, not with disrupted membranes. The inhibition of Na+/K+-ATPase in brain slices by physiological, low nM concentrations of Abeta 1-40 is consistent with effects on neurotransmitter release and intrasynaptosomal calcium responses.

Effect of dopamine on NaCl transport in the medullary thick ascending limb of the rat

The aim of the present study was to determine whether dopamine affects NaCl reabsorption in the medullary thick ascending limb of the loop of Henle. Basolateral dopamine (10(-6) M) significantly inhibited Cl- reabsorption in the in vitro microperfused rat medullary thick ascending limb by 21 +/- 2% (P < 0.01). The response to 10(-6) M dopamine was completely blocked by pretreatment with the dopamine D1 receptor antagonist R(+)-SCH-23390 (5 x 10(-5) M), and was mimicked by the dopamine D1 receptor agonist A-77636 (10(-6) M; delta - 36 +/- 2%; P < 0.05). In contrast, basolateral administration of the dopamine D2 receptor agonist (+)-bromocriptine (10(-6) M) transiently increased Cl- reabsorption by 49 +/- 18% (P < 0.05). Neither the D1 nor the D2 agonist affected Cl- reabsorption when selectively administered to the luminal membrane. These data suggest that the predominant effect of dopamine on the medullary thick ascending limb of the rat is to inhibit the reabsorption of NaCl, a response which is mediated by dopamine D1 receptors.

Regulation of endothelial Na(+)-K(+)-ATPase activity by cAMP

Using an in vitro cell system and Cs+ NMR techniques we were able to show that porcine aortic endothelial cells (PAEC) reduce their Na(+)-K(+)-ATPase activity upon an increase in intracellular cAMP. Reduction in the pump rate was due to phosphorylation of the alpha-subunit of the ATPase as shown by immunoprecipitation. Apart from a pump inhibiton using 8-Br-cAMP and IBMX, we were also able to show that changes in the Na(+)-K(+)-ATPase activity could be mediated by the adenosine-A2 and prostaglandin receptor agonists 5'-N-Ethylcarboxamidoadenosine and Iloprost, respectively. Parallel to a decrease in pump activity we also observed a decrease in intracellular Cs+, indicating opening of K+ channels.

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.

The renal dopamine system

Intrarenally formed dopamine induced natriuresis by inhibiting the activity of renal tubular Na/KATPase. This effect is mediated via a complex signal network, which includes inhibition of PP1 via the adenylyl cyclase-PKA-DARPP32 pathway and activation of PKC via the PLA2-arachidonic acid-20HETE pathway. The renal dopamine availability is a major determinant of the natriuretic effect of dopamine and is to a large extent modulated by the activity of COMT. The possibility that regulation of dopamine storage and release influences renal dopamine effects should be considered.

Differential human renal tubular responses to dopamine type 1 receptor stimulation are determined by blood pressure status

We performed the present studies to determine whether a proximal renal tubular dopamine D1-like receptor defect exists in human essential hypertension. Twenty-four subjects were studied (13 normotensive and 11 hypertensive) in a randomized, double-blind, vehicle-controlled study using fenoldopam, a selective D1-like receptor agonist. Subjects were studied in sodium metabolic balance at 300 mEq/d, after which the salt sensitivity of their blood pressure was determined. Fenoldopam at peak doses of 0.1 to 0.2 microgram/kg per minute decreased mean arterial pressure in hypertensive subjects but did not change mean pressure in normotensive subjects. Fenoldopam increased renal plasma flow to a greater extent in hypertensive than normotensive subjects. Fenoldopam increased both urinary and fractional sodium excretions in the hypertensive and normotensive groups. In normotensive but not hypertensive subjects, fenoldopam increased the fractional excretion of lithium and distal sodium delivery. In contrast, both distal fractional sodium reabsorption and sodium-potassium exchange fell significantly in hypertensive subjects. We conclude that human essential hypertension is associated with a reduction in the proximal tubular response to D1-like receptor stimulation compared with normotensive subjects. Hypertensive subjects appear to have a compensatory upregulation of renal vascular and distal tubular D1-like receptor function that offsets the proximal tubular defect, resulting in an enhanced natriuretic response to D1-like receptor stimulation.

Altered arachidonic acid metabolism contributes to the failure of dopamine to inhibit Na+,K(+)-ATPase in kidney of spontaneously hypertensive rats

Dopamine decreases tubular sodium reabsorption in part by inhibition of Na+,K(+)-ATPase activity in renal proximal tubules. The signaling mechanism involved in dopamine-mediated inhibition of Na+,K(+)-ATPase is known to be defective in spontaneously hypertensive animals. The present study was designed to evaluate the role of phospholipase A2 (PLA2) and its metabolic pathway in dopamine-induced inhibition of Na+,K(+)-ATPase in renal proximal tubules from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Renal proximal tubular suspensions were prepared and Na+,K(+)-ATPase activity was measured as ouabain-sensitive adenosine triphosphate hydrolysis. Dopamine inhibited Na+,K(+)-ATPase activity in a concentration (1 nM-10 microM)-dependent manner in WKY rats while it failed to inhibit the enzyme activity in SHR. Dopamine (10 microM)-induced inhibition of Na+,K(+)-ATPase activity in WKY rats was significantly blocked by mepacrine (10 microM), a PLA2 inhibitor, suggesting the involvement of PLA2 in dopamine-mediated inhibition of Na+,K(+)-ATPase. Arachidonic acid (a product released by PLA2 action) inhibited Na+,K(+)-ATPase in a concentration-dependent (1-100 microM) manner in WKY rats while the inhibition in SHR was significantly attenuated (IC50: 7.5 and 80 microM in WKY rats and SHR, respectively). Furthermore, lower concentrations of arachidonic acid stimulated (30% at 1 microM) Na+,K(+)-ATPase activity in SHR. This suggests a defect in the metabolism of arachidonic acid in SHR. Proadifen (10 microM), an inhibitor of cytochrome P-450 monoxygenase (an arachidonic acid metabolizing enzyme) significantly blocked the inhibition produced by arachidonic acid in WKY rats and abolished the difference in arachidonic acid inhibition of Na+,K(+)-ATPase between WKY rats and SHR. These data suggest that PLA2 is involved in dopamine-induced inhibition of Na+,K(+)-ATPase and altered arachidonic acid metabolism may contribute to reduced dopaminergic inhibition of Na+,K(+)-ATPase activity in spontaneously hypertensive rats.

Dopamine D1A receptor regulation of phospholipase C isoform

In LTK- cells stably transfected with rat D1A receptor cDNA, fenoldopam, a D1 agonist, increased phosphatidylinositol 4, 5-bisphosphate hydrolysis in a time-dependent manner. In the cytosol, phospholipase C (PLC) activity increased (50 +/- 7%) in 30 s, returned to basal level at 4 h, and decreased below basal values by 24 h; in the membrane, PLC activity also increased (36 +/- 13%) in 30 s, returned to basal level at 10 min, and decreased below basal value at 4 and 24 h. Fenoldopam also increased PLC-gamma protein in a time-dependent manner. The latter was blocked by the D1 antagonist SKF83742 and by a D1A antisense oligodeoxynucleotide, indicating involvement of the D1A receptor. The fenoldopam-induced increase in PLC-gamma and activity was mediated by protein kinase A (PKA) since it was blocked by the PKA antagonist Rp-8-CTP-adenosine cyclic 3':5'-monophosphorothioate (Rp-8-CTP-cAMP-S) and mimicked by direct stimulation of adenylyl cyclase with forskolin or by a PKA agonist, Sp-cAMP-S. Protein kinase C (PKC) was also involved, since the fenoldopam-induced increase in PLC-gamma protein was blocked by two different PKC inhibitors, calphostin C and chelerythrine; calphostin C also blocked the fenoldopam-induced increase in PLC activity. In addition, forskolin and a PKA agonist, Sp-8-CTP-cAMP-S, increased PKC activity, and direct stimulation of PKC with phorbol 12-myristate 13-acetate increased PLC-gamma protein and activity, effects that were blocked by calphostin C. We suggest that the D1A-mediated stimulation of PLC occurs as a result of PKA activation. PKA then stimulates PLC-gamma in cytosol and membrane via activation of PKC.

Cellular mechanisms for bi-directional regulation of tubular sodium reabsorption

The molecular mechanisms underlying the regulation of sodium excretion are incompletely known. Here we propose a general model for a bi-directional control of tubular sodium transporters by natriuretic and antinatriuretic factors. The model is based on experimental data from studies on the regulation of the activity of Na+,K+-ATPase, the enzyme that provides the electrochemical gradient necessary for tubular reabsorption of electrolytes and solutes in all tubular segments. Regulation is carried out to a large extent by autocrine and paracrine factors. Of particular interest are the two catecholamines, dopamine and norepinephrine. Dopamine is produced in proximal tubular cells and inhibits Na+,K+-ATPase activity in several tubule segments. Renal dopamine availability is regulated by the degrading enzyme, catechol-O-methyl transferase. Renal sympathetic nerve endings contain norepinephrine and neuropeptide Y (NPY). Activation of alpha-adrenergic receptors increase and activation of beta-adrenergic receptors decrease Na+,K+-ATPase activity. alpha-Adrenergic stimulation increases the Na+ affinity of the enzyme and thereby the driving force for transcellular Na+ transport. NPY acts as a master hormone by synergizing the alpha- and antagonizing the beta-adrenergic effects. Dopamine and norepinephrine control Na+,K+-ATPase activity by exerting opposing forces on a common intracellular signaling system of second messengers, protein kinases and protein phosphatases, ultimately determining the phosphorylation state of Na+,K+-ATPase and thereby its activity. Important crossroads in this network are localized and functionally defined. Phosphorylation sites for protein kinase A and C have been identified and their functional significance has been verified.

Mechanisms of dopamine effects on Na-K-ATPase activity in Madin-Darby canine kidney (MDCK) epithelial cells

Dopamine decreases tubular sodium reabsorption, attributed in part to Na-K-ATPase inhibition in the proximal convoluted tubule (PCT). Because the final regulation of sodium excretion occurs in the collecting duct, where specific dopamine DA1 binding sites have been demonstrated, we examined the effects of dopamine, as well as of DA1 and DA2 receptor agonists on Na-K-ATPase activity and on the number of units in Madin-Darby canine kidney (MDCK) cells, which retain differentiated properties of the renal cortical collecting tubule epithelium. Dopamine (10(-5) M) inhibited pump activity (by 50%) and reduced the number of units. This effect was reproduced by the DA1 agonist SKF 38393, which inhibited pump activity in a dose- and time-dependent manner (maximum, 10(-5) M). The DA2 agonist quinpirole hydrochloride was without effect, either alone or in combination with SKF 38393. Inhibition of pump activity by dopamine was totally abolished by H7 (100 microM), an inhibitor of protein kinase (PK), but partially by 2',5'-dideoxy-adenosine (DDA) and H4, respective inhibitors of cAMP production and PKA, which suggests that the dopamine effect on Na-K-ATPase activity may be linked to activation of both PKC and PKA. In these cells, amiloride addition during preincubation did not alter the effect of dopamine on Na-K-ATPase activity; in contrast, furosemide increased further the inhibitory effect of dopamine on the enzyme activity. Monensin addition (10(-3) M) reversed the inhibitory effect of dopamine after a 30-min preincubation.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of calcineurin A isoenzyme mRNAs in rat thymus and kidney

The distribution of the mRNAs encoding the different isoforms of the catalytic subunit (A subunit) of calcineurin has been investigated in rat thymus and kidney using in situ hybridization histochemistry with specific antisense oligonucleotide probes. In the thymus, the mRNAs of the A beta isoforms were the predominant transcripts and showed very intense hybridization signals in the cortical areas. The A alpha mRNAs were expressed at low levels. A beta 2 mRNA was expressed at higher levels than A beta 3 mRNA, but no difference could be detected between the expression levels of A alpha 1 and A alpha 2. In the kidney, highest calcineurin A mRNA hybridization signals were found in the medulla. Signal intensities of A alpha mRNAs were comparable to those of A beta mRNAs. A alpha 1 mRNA level was extremely weak, and A beta 2 mRNA expression was slightly higher than A beta 3 mRNA expression. A tissue-specific distribution pattern of the alternatively spliced isoforms of calcineurin A, as suggested by these preliminary data from thymus and kidney, may be critical in understanding the toxic side-effects associated with the use of the immunosuppressive, calcineurin-inhibiting compounds cyclosporin A and FK506.

Nitric oxide, cGMP, and hormone regulation of active sodium transport

The inter- and intracellular regulator nitric oxide (NO) has been suggested to play a role in the modulation of cellular excitability, but the mechanism(s) by which this occurs remain unclear. Using the kidney as a model system, we report here evidence that NO, produced in response to various hormones and cytokines, can effect long-term alterations in the activity of the membrane sodium pump. This regulation of Na, K-ATPase, which occurs in a system of NO-containing renal tubules, involves cGMP and cGMP-dependent protein kinase. Na, K-ATPase can also be regulated by alterations of cGMP initiated through NO-independent factors, such as atriopeptin, and in nonrenal tissues, such as cerebellum. Regulation of the membrane sodium pump by NO and cGMP, therefore, represents a mechanism for hormonal modulation of ion gradients, not only in kidney but also in other organs, including brain, where NO and cGMP play a prominent role in cellular function.

Dopaminergic defect in hypertension

Reverse genetics and the candidate gene approach have been utilized to identify the genetic defect(s) in hypertension. We have proposed the dopamine receptor gene as one candidate in the pathogenesis of hypertension. Because some forms of hypertension are sodium dependent or aggravated by sodium loading and because dopamine is important in aiding the organism to eliminate "excess" sodium, an abnormality in the renal dopaminergic system may be responsible for the sodium retention in hypertension. Both human and animal models of hypertension are associated with renal dopamine production and/or post first messenger defects. The Dahl salt-sensitive rat, which has a decreased ability to generate renal dopamine, and the spontaneously hypertensive rat (SHR), which has no such limitation, have a defective coupling of a D1 receptor to a G protein/adenylyl cyclase complex. This coupling defect is: (1) genetic, since it precedes the onset of hypertension and co-segregates with the hypertensive phenotype, (2) receptor specific, since it is not shared by other humoral agents, and (3) organ and nephron segment selective, since it occurs in proximal tubules but not in cortical collecting ducts or the brain striatum. A consequence of the defective dopamine receptor/adenylyl cyclase coupling in the SHR is a decreased ability of D1 agonists to inhibit Na+/H+ exchange activity. A resistance to the natriuretic effect of dopamine and D1 agonists in the SHR is due mainly to decreased cyclic AMP production, although with maturation a post cyclic AMP defect is acquired. Radioligand binding studies suggest a "loss" of the high-affinity D1 binding site in the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)