Peripheral dopamine synthesis and metabolism in spontaneously hypertensive rats

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.

Overexpression of non-functional LAT1/4F2hc in renal proximal tubular epithelial cells from the spontaneous hypertensive rat

The present study examined the expression of type 1 L-amino acid transporter (LAT1) and its associated glycoprotein 4F2hc in freshly isolated renal proximal tubules and immortalized renal proximal tubular epithelial (PTE) cells from spontaneously hypertensive (SHR) and normotensive (WKY) rats. The study also examined the inward and outward transport of [(14)C]-L-leucine, the preferred substrate of LAT1. The abundance of LAT1 and 4F2hc was greater in SHR than in WKY, both in freshly isolated renal proximal tubules and immortalized renal proximal tubular cells. In the absence of extracellular Na(+) the BCH (2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid)-sensitive [(14)C]-L-leucine uptake in SHR PTE cells was approximately 50% that observed in WKY PTE cells (77+/-4 vs 164+/-7 pmol/mg protein). In the absence of extracellular Na(+) the affinity of the transporter for the substrate in WKY PTE cells was 7.7-fold that in SHR cells, as evidenced by lower K(0.5) values. Gene silencing with a LAT1 siRNA and a 4F2hc siRNA significantly reduced LAT1 and 4F2hc expression, which was accompanied by a marked reduction in Na(+)-independent [(14)C]-L-leucine uptake in both SHR and WKY PTE cells. The spontaneous and L-leucine-stimulated outward transfer of [(14)C]-L-leucine was Na(+)-independent in both SHR and WKY PTE cells. The spontaneous [(14)C]-L-leucine efflux was higher in WKY than in SHR PTE cells and the potency of L-leucine to stimulate [(14)C]-L-leucine efflux in WKY (EC(50) = 9 microM) was greater than in SHR PTE cells (EC(50) = 41 microM). It is concluded that the SHR kidney overexpress LAT1/4F2hc units which display low affinity for L-leucine transport.

High-salt intake and the renal expression of amino acid transporters in spontaneously hypertensive rats

This study evaluated in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) the response to salt loading of the renal dopaminergic system and transcript abundance of Na+-independent (LAT1 and LAT2) and Na+-dependent (ASCT2 and B0AT1) amino acid transporters potentially involved in renal tubular uptake of l-DOPA. Rats were fed normal (NS)- or high (HS; 1% saline as drinking water)-salt intake for 24 h. Transcript abundance of amino acid transporters was age dependent, differently regulated in WKY and SHR and responded differently to salt intake. HS intake similarly increased urinary dopamine in 4-wk-old SHR and WKY. At 12 wk of age, HS intake increased urinary dopamine in SHR, but not in WKY. Changes in urinary dopamine paralleled changes in the uptake of l-DOPA in isolated renal tubules from 4- and 12-wk-old WKY and SHR on NS and HS intake. At 12 wk of age, HS intake was accompanied by decreases in LAT1 and LAT2 transcript abundance in WKY and SHR. ASCT2 and B0AT1 expression was significantly decreased in both 4- and 12-wk-old WKY and in 4-wk-old SHR on HS intake. By contrast, HS intake increased ASCT2 and B0AT1 expression in 12-wk-old SHR. It is concluded that salt-sensitive mechanisms influence LAT1, LAT2, ASCT2, and B0AT1 gene transcription. Differences in urinary dopamine and tubular uptake of l-DOPA between WKY and SHR during HS intake, namely in 12-wk-old animals, may result from increases in the ASCT2 and B0AT1 mRNA levels and less pronounced decreases in LAT2 expression.

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.

Increases in transepithelial vectorial Na+ transport facilitates Na+-dependent L-DOPA transport in renal OK cells

The present study evaluated the hypothesis of whether increases in vectorial Na+ transport translate into facilitation of Na+-dependent L-DOPA uptake in cultured renal epithelial tubular cells. Increases in vectorial Na+ transport were obtained in opossum kidney (OK) cells engineered to overexpress Na+-K+-ATPase after transfection of wild type OK cells with the rodent Na+-K+-ATPase alpha1 subunit. The most impressive differences between wild type and transfected OK cells are that the latter overexpressed Na+-K+-ATPase accompanied by an increased activity of the transporter. Non-linear analysis of the saturation curve for l-DOPA uptake revealed a Vmax value (in nmol mg protein/6 min) of 62 and 80 in wild type and transfected cells, respectively. The uptake of a non-saturating concentration (0.25 microM) of [14C]-L-DOPA in OK-WT cells was not affected by Na+ removal, whereas in OK-alpha1 cells accumulation of [14C]-L-DOPA was clearly dependent on the presence of extracellular Na+. When Na+ was replaced by choline, the inhibitory profile of neutral l-amino acids, but not of basic and acidic amino acids, upon [14C]-L-DOPA uptake in both cell types, was significantly greater than that observed in the presence of extracellular Na+. It is concluded that enhanced ability of OK cells overexpressing Na+-K+-ATPase to translocate Na+ from the apical to the basal cell side correlates positively with their ability to accumulate L-DOPA, which is in agreement with the role of Na+ in taking up the precursor of renal dopamine.

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.

Apical and basolateral 4F2hc and the amino acid exchange of L-DOPA in renal LLC-PK1 cells

The present study aimed to examine the presence and define the role of 4F2hc, a glycoprotein associated with the LAT2 amino acid transporter, in L-DOPA handling by LLC-PK1 cells. For this purpose we have measured the activity of the apical and basolateral inward and outward transport of [14C] L-DOPA in cell monolayers and examined the influence of 4F2hc antisense oligonucleotides on [14C] L-DOPA handling. The basal-to-apical transepithelial flux of [14C] L-DOPA progressively increased with incubation time and was similar to the apical-to-basal transepithelial flux. The spontaneous and the L-DOPA-stimulated apical fractional outflow of [14C] L-DOPA were identical to that through the basal cell side. The L-DOPA-induced fractional outflow of [14C] L-DOPA through the apical or basal cell side was accompanied by marked decreases in intracellular levels of [14C] L-DOPA. In cells treated with an antisense oligonucleotide complementary to 4F2hc mRNA for 72 h, [14C] L-DOPA inward transport and 4F2hc expression were markedly reduced. Treatment with the 4F2hc antisense oligonucleotide markedly decreased the spontaneous fractional outflow of [14C] L-DOPA through the apical or the basal cell side. It is likely that the Na+-independent and pH-sensitive uptake of L-DOPA include the hetero amino acid exchanger LAT2/4F2hc, which facilitates the trans-stimulation of L-DOPA and its outward transfer at both the apical and basal cell sides.

Cloning and gene silencing of LAT2, the L-3,4-dihydroxyphenylalanine (L-DOPA) transporter, in pig renal LLC-PK1 epithelial cells

Organ-specific overexpression of type 2 L-amino acid transporter (LAT2) in the kidney of the spontaneous hypertensive rat (SHR), accompanied by an enhanced ability to take up L-DOPA, may constitute the basis for the enhanced renal production of dopamine in the SHR in an attempt overcome the deficient dopamine-mediated natriuresis. To understand the physiological role of LAT2-mediated L-DOPA handling, we used 21-nucleotide small interfering RNA duplexes (siRNA) to specifically suppress LAT2 expression in LLC-PK1 cells, a cell line that retains several properties of proximal tubular epithelial cells and takes up L-DOPA largely through Na+-independent transporters. After cloning the LLC-PK1 LAT2 gene, one target region of LAT2 mRNA (nt 97-117) was selected by scanning the length of the LAT2 gene for AA-dinucleotide sequences and downstream 19 nucleotides. Levels of LAT2 cDNA, determined by real-time quantitative RT-PCR, were markedly (P<0.05) reduced by LAT2 siRNA but not by the mismatch LAT2 siRNA. The LAT2 siRNA but not the mismatch LAT2 siRNA, reduced by 85% [14C]-L-DOPA accumulation, a time- and concentration-dependent effect. The efflux of intracellular [14C]-L-DOPA was markedly increased (P<0.05) by L-DOPA and L-leucine. The [14C]-L-DOPA outward transport was decreased 90% by LAT2 siRNA, but not by the mismatch LAT2 siRNA. However, treatment with the siRNA LAT2 did not affect the L-DOPA-induced fractional outflow of [14C]-L-DOPA. The Na+-independent and pH-sensitive L-DOPA transporter may include the hetero amino acid exchanger LAT2, whose activation results in trans-stimulation of L-DOPA outward transfer.-Soares-da-Silva, P., Serrão, M. P., João Pinho, M., Bonifácio, M. J. Cloning and gene silencing of LAT2, the L-3,4-dihydroxyphenylalanine (L-DOPA) transporter, in pig renal LLC-PK1 epithelial cells.

D5 dopamine receptor knockout mice and hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. All of the five dopamine receptor genes (D1, D2, D3, D4, and D5) expressed in mammals and some of their regulators are in loci linked to hypertension in humans and in rodents. Under normal conditions, D1-like receptors (D1 and D5) inhibit sodium transport in the kidney and the intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats, and humans with essential hypertension, the D1-like receptor-mediated inhibition of sodium transport is impaired because of an uncoupling of the D1-like receptor from its G protein/effector complex. The uncoupling is genetic, and receptor-, organ-, and nephron segment-specific. In human essential hypertension, the uncoupling of the D1 receptor from its G protein/effector complex is caused by an agonist-independent serine phosphorylation/desensitization by constitutively active variants of the G protein-coupled receptor kinase type 4. The D5 receptor is also important in blood pressure regulation. Disruption of the D5 or the D1 receptor gene in mice increases blood pressure. However, unlike the D1 receptor, the hypertension in D5 receptor null mice is caused by increased activity of the sympathetic nervous system, apparently due to activation of oxytocin, V1 vasopressin, and non-N-methyl D-aspartate receptors in the central nervous system. The cause of the activation of these receptors remains to be determined.

Over-expression of renal LAT1 and LAT2 and enhanced L-DOPA uptake in SHR immortalized renal proximal tubular cells

BACKGROUND

Spontaneously hypertensive rats (SHR) may have an increased renal production of dopamine. LAT2 promotes L-DOPA renal uptake, and this may determine the rate of dopamine synthesis. The present study evaluated L-DOPA inward and outward transfer in immortalized renal proximal tubular epithelial cells of SHR and Wistar-Kyoto rats (WKY).

METHODS

Uptake of [(14)C]-L-DOPA was initiated by the addition of 1 mL Hanks' medium with a given concentration of the substrate. The apical fractional outflow of intracellular [(14)C]-L-DOPA was evaluated in cells loaded with [(14)C]-L-DOPA for 6 minutes, and then the corresponding efflux was monitored over 12 minutes. The presence of LAT1 and LAT2 transcripts and protein in WKY and SHR cells was examined, respectively, by reverse transcription-polymerase chain reaction (RT-PCR) and immunobloting.

RESULTS

LAT2 in WKY cells contributed almost exclusively for [(14)C]-L-DOPA uptake. In SHR cells [(14)C]-L-DOPA uptake was 25% through system B(0), 25% through LAT2 (resulting from inhibition by 1 mmol/L glycine, L-alanine, L-serine, and L-threonine), and the remaining 50% through LAT1. The efflux of [(14)C]-L-DOPA from WKY and SHR cells corresponded to approximately 65% and approximately 25%, respectively, of the amount accumulated in the cells. The LAT1 and LAT2 transcripts were present in both SHR and WKY cells, but the abundance of both LAT1 and LAT2 proteins in SHR cells was greater than in WKY cells.

CONCLUSION

Differences in L-DOPA handling between SHR and WKY cells may result from over-expression of LAT1 and LAT2 transporters in the former. The unique role of Na(+)-dependent transporters (system B(0)) in SHR cells also contributes to the enhanced L-DOPA uptake in these cells.

Dopamine D3 receptor mRNA and renal response to D3 receptor activation in spontaneously hypertensive rats

Defective dopamine receptors may be involved in the development of hypertension. Recently, it has been shown that gene expression and function of the renal dopamine D3 receptor is impaired in salt-sensitive Dahl rats, a model of salt-dependent hypertension. Here, the functional response to D3 receptor activation was investigated in spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto rats (WKY). In addition, expression of the D3 receptor gene was studied in both rat strains. In clearance experiments, Ringer solution was infused at baseline in thiopental-anesthetized SHR and WKY (each n = 8), followed by an infusion of R(+)-7-hydroxy-dipropylaminotetralin (DPAT), a specific D3 receptor agonist. DPAT was infused in two consecutive doses of 0.01 and 0.1 microg/min per kg body weight. During the entire experiment mean arterial blood pressure was significantly higher (1.5-fold) in adult SHR when compared to age-matched WKY. In both groups DPAT infusion induced a similar dose-dependent increase in urinary flow rate and sodium excretion by a maximum of 2.3-fold and 3.5-fold, respectively. DPAT also increased the glomerular filtration rate in both SHR and WKY. Reverse transcription-polymerase chain reaction studies of whole kidney samples showed no significant differences between young prehypertensive and adult hypertensive SHR when compared to age-matched normotensive WKY. In summary, pharmacological dopamine D3 receptor activation induces a uniform renal response in SHR and WKY. Together with the similar D3 receptor gene expression in both rat strains, which is independent of age or blood pressure levels, the results do not support the notion that the dopamine D3 receptor system is involved in the pathogenesis of hypertension in the SHR model.

Expression and function of sodium transporters in two opossum kidney cell clonal sublines

The present study describes characteristic features of two clonal subpopulations of opossum kidney (OK) cells (OK(LC) and OK(HC)) that are functionally different but morphologically identical. The most impressive differences between OK(HC) and OK(LC) cells are the overexpression of Na+-K+-ATPase and type 3 Na+/H+ exchanger by the former, accompanied by an increased Na+-K+-ATPase activity (57.6 +/- 5.6 vs. 30.0 +/- 0.1 nmol P(i). mg protein(-1). min(-1)); the increased ability to translocate Na+ from the apical to the basolateral surface; and the increased Na+-dependent pH(i) recovery (0.254 +/- 0.016 vs. 0.094 +/- 0.011 pH units/s). Vmax values (in pH units/s) for Na+-dependent pHi recovery in OK(HC) cells (0.00521 +/- 0.0004) were twice (P < 0.05) those in OK(LC) (0.00202 +/- 0.0001), with similar Km values (in mM) for Na+ (OK(LC), 21.0 +/- 5.5; OK(HC), 14.0 +/- 5.6). In addition, we measured the activities of transporters (organic ions, alpha-methyl-D-glucoside, L-type amino acids, and Na+ and enzymes (adenylyl cyclase, aromatic L-amino acid decarboxylase, and catechol-O-methyltransferase). The cells were also characterized morphologically by optical and scanning electron microscopy and karyotyped. It is suggested that OK(LC) and OK(HC) cells constitute an interesting cell model for the study of renal epithelial physiology and pathophysiology, namely, hypertension.

Elevated renal cortical calmodulin-dependent protein kinase activity and blood pressure

The spontaneously hypertensive rat (SHR) exhibits not only hypertension but also behavioral hyperactivity which are not genetically linked. Two strains of rats, one hypertensive but normoactive (WKHT) and another, hyperactive but normotensive (WKHA), have been generated from SHR. We have reported that in renal proximal tubules, the linkage between D1-like receptors an adenylyl cyclase was impaired in SHR and WKHT but intact in WKHA. The impaired renal D1-like receptor function in the SHR was associated with increased phosphorylation of the D1 receptor, presumably caused by increased phosphorylation by G protein-coupled receptor kinases (GRK) or decreased dephosphorylation by protein phosphatase 2A. Because calmodulin kinase (CaMK) can regulate GRK activity, CaMK activity in renal cortical membranes of WKHA and WKHT were studied. We found that CaMK-dependent phosphorylation was two-fold higher in WKHA than in WKHT. In addition, serine phosphorylation of a 36 KDa and a 24 KDa protein was 5-fold and 3-fold greater in WKHA than in WKHT. We hypothesize that the increased CaMK activity in the renal cortical membrane may serve to inhibit GRK activity in WKHA and prevent the development of hypertension.

G protein-coupled receptor kinase 4 gene variants in human essential hypertension

Essential hypertension has a heritability as high as 30-50%, but its genetic cause(s) has not been determined despite intensive investigation. The renal dopaminergic system exerts a pivotal role in maintaining fluid and electrolyte balance and participates in the pathogenesis of genetic hypertension. In genetic hypertension, the ability of dopamine and D(1)-like agonists to increase urinary sodium excretion is impaired. A defective coupling between the D(1) dopamine receptor and the G protein/effector enzyme complex in the proximal tubule of the kidney is the cause of the impaired renal dopaminergic action in genetic rodent and human essential hypertension. We now report that, in human essential hypertension, single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK4gamma, increase G protein-coupled receptor kinase (GRK) activity and cause the serine phosphorylation and uncoupling of the D(1) receptor from its G protein/effector enzyme complex in the renal proximal tubule and in transfected Chinese hamster ovary cells. Moreover, expressing GRK4gammaA142V but not the wild-type gene in transgenic mice produces hypertension and impairs the diuretic and natriuretic but not the hypotensive effects of D(1)-like agonist stimulation. These findings provide a mechanism for the D(1) receptor coupling defect in the kidney and may explain the inability of the kidney to properly excrete sodium in genetic hypertension.

G protein-coupled receptor kinase 4 gene variants in human essential hypertension

Essential hypertension has a heritability as high as 30-50%, but its genetic cause(s) has not been determined despite intensive investigation. The renal dopaminergic system exerts a pivotal role in maintaining fluid and electrolyte balance and participates in the pathogenesis of genetic hypertension. In genetic hypertension, the ability of dopamine and D(1)-like agonists to increase urinary sodium excretion is impaired. A defective coupling between the D(1) dopamine receptor and the G protein/effector enzyme complex in the proximal tubule of the kidney is the cause of the impaired renal dopaminergic action in genetic rodent and human essential hypertension. We now report that, in human essential hypertension, single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK4gamma, increase G protein-coupled receptor kinase (GRK) activity and cause the serine phosphorylation and uncoupling of the D(1) receptor from its G protein/effector enzyme complex in the renal proximal tubule and in transfected Chinese hamster ovary cells. Moreover, expressing GRK4gammaA142V but not the wild-type gene in transgenic mice produces hypertension and impairs the diuretic and natriuretic but not the hypotensive effects of D(1)-like agonist stimulation. These findings provide a mechanism for the D(1) receptor coupling defect in the kidney and may explain the inability of the kidney to properly excrete sodium in genetic hypertension.

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.

Dopamine D2-like receptors in the kidney of spontaneously hypertensive rats: a radioligand binding assay and light microscope autoradiography study

1. Dopamine D2-like receptors were investigated in sections of kidney from male spontaneously hypertensive rats (SHRs) at 6 and 14 weeks of age using radioligand binding assay and autoradiographic techniques with [3H]-spiperone as a ligand. 2. Systolic blood pressure values were slightly higher in 6-week-old SHRs in comparison with age-matched normotensive Wistar-Kyoto (WKY) rats and considerably higher in 14-week-old SHRs in comparison with the other groups investigated. Renal dopamine levels were higher in SHRs aged 6 and 14 weeks in comparison with age-matched WKY rats. Noradrenaline concentrations were similar in 6-week-old SHRs and normotensive WKY rats, and increased slightly in SHRs aged 14 weeks. 3. The density of [3H]-spiperone binding sites was similar in SHRs and WKY rats at 6 weeks of age, and decreased in SHRs at 14 weeks in comparison with age-matched normotensive animals. Light microscope autoradiography revealed the accumulation of silver grains in the tunica adventitia, in the adventitia-media border of intrarenal arteries and within cortical tubules. A few specific silver grains were also developed in the glomerular tuft. No changes in the density and pattern of silver grains were noticeable between SHRs and WKY rats at 6 weeks of age, whereas a reduction in silver grains largely affecting vascular binding sites was observed at 14 weeks of age. 4. Renal denervation considerably decreased the density of [3H]-spiperone binding sites in sections of rat kidney, with an almost complete loss of vascular binding sites. 5. The above findings indicate the occurrence of a decrease of dopamine D2-like receptors in the kidney of SHRs with the progress of hypertension. Dopamine D2-like receptors which are mainly prejunctional are involved in the modulation of sympathetic neurotransmission in the kidney. The loss of these receptors in SHRs may contribute to the pathophysiology of hypertension.

Kidney extracts from spontaneously hypertensive rats (SHR) have greater dopamine 1A receptor RNA-binding activity than extracts from normotensive Wistar-Kyoto (WKY) rats

Rat kidney extracts contain a 52 kDa protein that binds to the 3' untranslated region of the dopamine 1A (D1A) receptor mRNA at a 243 base-long cis element starting at the stop codon and ending approximately 220 bases upstream of an AUUUA-rich region. The D1A receptor RNA-binding protein (D1A-BP) is redox-sensitive; free sulfhydryl groups on the protein are required for binding. Kidney extracts from SHR have significantly more D1A-BP activity than extracts from WKY rats. When kidney extracts were tested for binding to an 80-base RNA containing four AUUUA repeats, there was also greater binding activity in extracts from SHR. These increases are at least partly specific because there was no difference in catalase RNA-binding protein activity between the two rat strains. These data suggest D1A-BP and AUUUA-binding protein may play a role in posttranscriptional regulation of the D1A receptor in the hypertensive rat.

Transgenic mice to study the role of dopamine receptors in cardiovascular function

Dopamine, an intrarenal regulator of sodium transport, is important in the pathogenesis of hypertension. The transduction of D1-like receptors in renal proximal tubules is defective in animal models of genetic hypertension. The defect is associated with an impaired regulation of proximal tubular sodium transport and cosegregates with hypertension in rats. Moreover, mice lacking one or both D1A receptor alleles develop hypertension. Extrasynaptic D3 receptors in renal tubules and juxtaglomerular cells may also regulate renal sodium transport and renin secretion while presynaptic D3 receptors may act as autoreceptors to inhibit neural norepinephrine release. Mice lacking one or both D3 alleles have elevated systolic blood pressure and developed diastolic hypertension. Although basal urine flow, sodium excretion, and glomerular filtration rate are similar, mice homozygous to the D3 receptor have an impaired ability to excrete an acute saline load compared to heterozygous and wild type mice. These studies suggest that abnormalities in dopamine receptor genes or their regulation may lead to the development of hypertension via different pathogenetic mechanisms.

Dopamine receptor blockade and synthesis inhibition during exaggerated natriuresis in spontaneously hypertensive rats

The influence of dopamine receptor blockade and synthesis inhibition on natriuresis induced by isotonic saline volume expansion was investigated in anaesthetized spontaneously hypertensive rats and normotensive Wistar-Kyoto rats. The aim of the study was to elucidate the mechanisms underlying the phenomenon of exaggerated natriuresis during volume expansion that has been observed in spontaneously hypertensive rats. Volume expansion, at 5% of body weight, resulted in a larger and faster natriuretic response in spontaneously hypertensive rats than in Wistar-Kyoto rats. Sixty minutes after commencement of volume expansion the natriuretic response (accumulated sodium excretion) in Wistar-Kyoto rats (n = 8) was only 24% of that in spontaneously hypertensive rats (n = 17). When spontaneously hypertensive rats were pretreated with the dopamine receptor blockers haloperidol (n = 14, 1 mg kg-1), SCH23390 (n = 8, 30 micrograms h-1 kg-1) or the dopamine synthesis inhibitor benserazide (n = 8, 50 mg kg-1; n = 5, 100 mg kg-1), the natriuretic response to volume expansion was only 16, 35, 59 and 42%, respectively, of that in untreated SHR. The corresponding proportion in the haloperidol-treated (n = 8) compared with untreated Wistar-Kyoto rats was 22%. In conclusion, isotonic volume loading results in more pronounced natriuresis in spontaneously hypertensive than in Wistar-Kyoto rats. Dopamine receptor blockade and synthesis inhibition attenuate the expansion of exaggerated natriuresis in spontaneously hypertensive rats and reduces the volume expansion natriuresis in Wistar-Kyoto rats, indicating that the dopamine system plays an important role.

Diminished phospholipase C activation by dopamine in spontaneously hypertensive rats

It is reported that a defect in dopamine-1 (DA-1) receptor adenylate cyclase coupling in the proximal convoluted tubule in the spontaneously hypertensive rat may contribute to the diminished natriuretic response to DA-1 receptor agonists. Since the tubular DA-1 receptor is also coupled to phospholipase C, and both of these cellular signaling processes are involved in DA-1 receptor-mediated diuresis and natriuresis, it is important to know whether a similar defect is also present in DA-1 receptor-coupled phospholipase C pathway. The present study was therefore designed to determine the functional status of DA-1 receptor-phospholipase C coupling system of adult spontaneously hypertensive rats using a renal cortical slice preparation. In addition, the renal response to exogenously administered dopamine (1 microgram/kg/min i.v.) was also determined. We found that basal phospholipase C activity was significantly higher in hypertensive rats than in age-matched Wistar-Kyoto rats (7.36 +/- 0.32% versus 5.61 +/- 0.27%, p less than 0.05). However, compared with the normotensive controls, dopamine-induced increases in phospholipase C activity were significantly attenuated in the preparations of hypertensive rats in a concentration-dependent manner (13 +/- 6% versus 38 +/- 6% for 1 mM dopamine, p less than 0.05; 49 +/- 6% versus 71 +/- 9% for 3 mM dopamine, p less than 0.05; 50 +/- 16% versus 106 +/- 22%, p less than 0.05 for 10 mM dopamine).(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary sodium intake and urinary dopamine and sodium excretion during the course of blood pressure development in Dahl salt-sensitive and salt-resistant rats

Recent studies have suggested that dopamine (DA) formed within the kidney may play an important role in promoting sodium excretion, and that renal production and excretion of DA is determined by dietary sodium intake. Inasmuch as increased sodium consumption produces hypertension in Dahl salt-sensitive (DS) rats but not in Dahl salt-resistant (DR) rats, the present study was designed to examine the relationship between sodium consumption and urinary excretion of DA in these rats. DS and DR rats were placed on either high sodium chloride (8%) or low sodium chloride (0.4%) diets at 4 weeks of age and their systolic blood pressure (SBP), urine volume, urinary sodium and catecholamine excretion were measured once every week for the next 4 weeks. High sodium chloride diet increased SBP in DS rats at 6 weeks of age and SBP continued to rise until they were 8 weeks old. The SBP of DR rats did not reach hypertensive levels when they were given high sodium chloride diet. The SBP of DS rats on low sodium chloride diet was significantly higher than DR rats on the same diet. The urinary DA excretion increased with age in all four groups of rats and was similar when they were 8 weeks old. However, both DS and DR rats on high sodium chloride diet excreted greater amounts of sodium and had increased urine volume compared to the DS and DR rats on low sodium chloride diet. There were no significant differences in urinary NE or E excretion in these four groups of rats. Kidney levels of DA and NE were significantly lower in DS compared to DR rats on high sodium chloride diet. These results show that although there are no differences in urinary DA excretion between rats on low and high sodium intake, both DS and DR rats on high sodium chloride diet are able to exhibit a natriuretic response. The DS rats eliminate sodium at the expense of an elevated SBP whereas DR rats stay normotensive. Therefore, it appears that alterations in mechanisms controlling renal vascular resistance rather than sodium excretion are responsible for the development of hypertension in DS rats.

Abnormal adrenal catecholamine synthesis in salt-sensitive Dahl rats

The possible role of catecholamines in the abnormal renal response to salt loading, a genetic defect resulting in hypertension in the salt-sensitive strain of Dahl rats, was investigated by measuring the adrenal synthesis of norepinephrine, epinephrine, and dopamine as well as their content in several tissues and the urinary excretion of these catecholamines as well as some of their metabolites at the height of salt-induced hypertension. We found that salt-sensitive Dahl rats, compared with salt-resistant Dahl rats, have a higher adrenal synthesis of [3H]norepinephrine following a pulse injection of [3H]tyrosine, a higher adrenal norepinephrine and epinephrine content but a lower kidney and heart ventricle content of dopamine and norepinephrine, and a decreased excretion of urinary dopamine, dihydroxyphenylacetic acid, 3-methoxytyramine, and homovanillic acid. These data suggest that the primary abnormality in salt-sensitive Dahl rats may be their inability to turn off, during high salt intake, their increased adrenal norepinephrine synthesis from dopamine. The abnormal catecholamine response of salt-sensitive Dahl rats to high salt intake indirectly suggests increased noradrenergic activity and decreased dopaminergic activity in the kidney, which may be important mechanisms in the sodium retention and hypertension of these rats.