High sodium intake increases the urinary excretion of L-3,4-dihydroxyphenylalanine but fails to alter the urinary excretion of dopamine and amine metabolites in Wistar 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.

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.

High- and low-affinity transport ofl-leucine andl-DOPA by the hetero amino acid exchangers LAT1 and LAT2 in LLC-PK1renal cells

The present study examined the functional characteristics of the inward and outward l-[14C]DOPA and l-[14C]leucine transporters in LLC-PK1cells. Uptake was initiated by the addition of Hanks' medium with a given concentration of l-[14C]DOPA or l-[14C]leucine. Saturation experiments were performed in cells incubated for 6 min with 0.25 μM concentration of the substrates in the absence and the presence of increasing concentrations of the nonlabeled substrates. Fractional outflow of intracellular l-[14C]DOPA or l-[14C]leucine was evaluated in cells loaded with 2.5 μM l-[14C]DOPA or 1 μM l-[14C]leucine for 6 min and then the corresponding efflux was monitored over 24 min. The high-affinity ( Km= 5.1 μM) uptake of l-[14C]leucine and the low-affinity ( Km= 120.0 μM) uptake of l-[14C]DOPA were largely promoted through a Na+-independent transporter. The uptake of the substrates was insensitive to N-(methylamino)-isobutyric acid but competitively inhibited by 2-aminobicyclo( 2 , 2 , 1 )-heptane-2-carboxylic acid (BCH). l- And d-neutral amino acids, but not acidic and basic amino acids, markedly inhibited l-[14C]DOPA and l-[14C]leucine accumulation. The uptake of l-[14C]leucine was a pH-insensitive process, whereas that of l-[14C]DOPA was sensitive to pH. The efflux of l-[14C]DOPA and l-[14C]leucine was markedly increased ( P < 0.05) by l-cysteine, l-leucine, BCH, and l-DOPA but not by l-arginine. RT-PCR detected LAT1 and LAT2 transcripts in LLC-PK1cells. It is concluded that LLC-PK1cells express both LAT1 and LAT2 transcripts and transport l-[14C]leucine through the Na+-independent pH-insensitive and high-affinity LAT1 transporter, whereas l-[14C]DOPA is mainly transported through the Na+-independent pH-insensitive and low-affinity LAT2 transporter and a minor component through a Na+-dependent transporter.

Concerted action of dopamine on renal and intestinal Na(+)-K(+)-ATPase in the rat remnant kidney

The present study evaluated renal and intestinal adaptations in sodium handling in uninephrectomized (Unx) rats and the role of dopamine. Two weeks after uninephrectomy, the remnant kidney in Unx rats weighed 33 +/- 2% more than the corresponding kidney in sham-operated (Sham) animals. This was accompanied by increases in urinary levels of dopamine and major metabolites [3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid] and increases in maximal velocity values (169 vs. 115 nmol. mg protein(-1). 15 min(-1)) for renal aromatic L-amino acid decarboxylase, the enzyme responsible for the synthesis of renal dopamine. High salt (HS) intake increased (P < 0.05) the urinary excretion of dopamine and DOPAC in Unx and Sham rats. However, the urinary levels of L-3,4-dihydroxyphenylalanine, dopamine, and DOPAC in Sham rats during HS intake were lower than in Unx rats. Blockade of dopamine D(1) receptors (Sch-23390, 2 x 30 microg/kg) reduced the urinary excretion of sodium in Unx (31% decrease) more pronouncedly than in Sham (19% decrease) rats. However, inhibition of renal Na(+)-K(+)-ATPase activity by dopamine was of similar magnitude in Unx and Sham rats. In parallel, it was observed that uninephrectomy resulted in a significant reduction in jejunal sodium absorption and Na(+)-K(+)-ATPase activity in jejunal epithelial cells. In jejunal epithelial cells from Sham rats, dopamine (1 microM) failed to inhibit Na(+)-K(+)-ATPase activity, whereas in Unx rats it produced a significant reduction. It is concluded that uninephrectomy results in increased renal dopaminergic activity and dopamine-sensitive enhanced natriuresis. Furthermore, it is suggested that decreased jejunal absorption of sodium may take place in response to partial renal ablation, as an example of renal-intestinal cross talk.

Aging, high salt intake, and renal dopaminergic activity in Fischer 344 rats

The present study examined renal dopaminergic activity and its response to high salt (HS) intake in adult (6-month-old) and old (24-month-old) Fischer 344 rats. Daily urinary excretion of L-3, 4-dihydroxyphenylalanine (L-DOPA), dopamine, and its metabolites 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid was similar in adult and old rats; by contrast, daily urinary excretion of norepinephrine in old rats was almost twice that in adult animals. HS intake (1% NaCl) over a period of 24 hours resulted in a 2-fold increase in the urinary excretion of dopamine, DOPAC, and norepinephrine in adult animals but not in old animals. Norepinephrine and L-DOPA plasma levels did not change during HS intake and were similar in both groups of rats. The natriuretic response to an HS intake in old rats (from 4.7+/-0.4 to 10.7+/-2.0 nmol. kg(-1). d(-1); Delta=6.0+/-0.9 nmol. kg(-1). d(-1)) was less than in adult rats (from 5.2+/-0.4 to 13.5+/-2.5 nmol. kg(-1). d(-1); Delta=8.3+/-0.8 nmol. kg(-1). d(-1)). A diuretic response to HS intake was observed in adult rats (from 20.9+/-2.3 to 37.6+/-2.8 mL. kg(-1). d(-1)) but not in old rats (from 37.7+/-5.7 to 42.3+/-6. 0 mL. kg(-1). d(-1)). Dopamine levels and dopamine/L-DOPA ratios in the renal cortex of old rats were greater than in adult rats. HS intake increased both dopamine levels and dopamine/L-DOPA ratios in the renal cortex of adult rats but not in old rats. Aromatic L-amino acid decarboxylase activity was higher in old rats than in adult rats; HS intake increased L-amino acid decarboxylase activity (nmol. mg protein(-1). l5 min(-1)) in adult rats (from 67+/-1 to 93+/-1) but not in old rats (from 86+/-2 to 87+/-2). Dopamine inhibited Na(+),K(+)-ATPase activity in proximal tubules obtained from adult rats, but it failed to exert such an inhibitory effect in old rats. It is concluded that renal dopaminergic tonus in old rats is higher than in adult rats but fails to respond to HS intake as observed in adult rats. This may be due in part to the inability of dopamine to inhibit Na(+),K(+)-ATPase activity in old rats.

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.