Contrasting dopaminergic patterns in two forms of genetic hypertension

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.

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.

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.

Salt intake and sensitivity of intestinal and renal Na+-K+ atpase to inhibition by dopamine in spontaneous hypertensive and Wistar-Kyoto rats

The present study evaluated the activity of jejunal Na+-K+-ATPase and its sensitivity to inhibition by dopamine in spontaneous hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats during low (LS), normal (NS) and high (HS) salt intake. Basal jejunal Na+-K+-ATPase activity in SHR on LS intake was higher than in WKY rats. Jejunal Na+-K+-ATPase activity in WKY rats, but not in SHR, on LS intake was significantly reduced (20% decrease) by dopamine (1 microM) and SKF 38393 (10 nM), but not quinerolane (10 nM), this being antagonized the D1 receptor antagonist (SKF 83566). Changing from LS to NS or HS intake in WKY rats increased basal jejunal Na+-K+-ATPase activity and attenuated the inhibitory effect of dopamine. In SHR, changing from LS to NS or HS intake increased basal jejunal Na+-K+-ATPase activity. Basal renal Na+-K+-ATPase activity in SHR on LS intake was similar to that in WKY rats and was insensitive to inhibition by dopamine. Changing from LS to NS or HS intake in WKY rats increased basal renal Na+-K+-ATPase activity without affecting the inhibitory effect of dopamine. In SHR, changing from LS to NS or HS intake failed to alter basal renal Na+-K+-ATPase activity. It is concluded that inhibition of jejunal Na+-K+ ATPase activity by D1 dopamine receptor activation is dependent on salt intake in WKY rats, and SHR animals fail to respond to dopamine, irrespective of their salt intake.

High salt diet down-regulates proximal tubule Na+, K(+)-ATPase activity in Dahl salt-resistant but not in Dahl salt-sensitive rats: evidence of defective dopamine regulation

We examined the regulation of Na+,K(+)-ATPase activity in proximal tubule segments during a high salt diet in prehypertensive Dahl salt-sensitive and salt-resistant rats. Rats were placed on normal salt or high salt diets (0.9% saline as drinking water). During the normal salt diet, Na+,K(+)-ATPase activity was not different between Dahl salt-sensitive and salt-resistant rats. After 2 days and 10 days on a high salt diet, Na+,K(+)-ATPase activity in Dahl salt-resistant rats significantly decreased when compared to Dahl salt-resistant rats on a normal salt diet (P less than 0.01). The decreased Na+,K(+)-ATPase activity in Dahl salt-resistant rats during a high salt diet was reversed by treatment with an inhibitor of aromatic L-amino acid decarboxylase (dopamine synthesizing enzyme), benserazide. In contrast, Na+,K(+)-ATPase activity did not decrease during the high salt diet and benserazide had no effect on Na+,K(+)-ATPase activity in Dahl salt-sensitive rats. These results indicate that Dahl salt-sensitive rats do not have the capacity to down-regulate the proximal tubule Na+,K(+)-ATPase activity during a high salt diet. Indirect evidence suggests that the regulation of Na+,K(+)-ATPase activity by locally produced dopamine is absent in Dahl salt-sensitive rats.

Selective role of dopamine in the natriuresis produced by iso-osmotic saline infusion

Endogenous dopamine (DA) selectively contributes to the natriuresis (UNaV) produced by infusion and dietary consumption of sodium chloride. The present study in anesthetized rats determined whether DA has a role in the natriuresis produced by small (2.0 +/- 0.11% increase in body weight) and large (17.9 +/- 0.58% increase in body weight) increments in extracellular fluid volume with iso-osmotic saline. Small volume expansion increased urine flow (V) by 59 +/- 15%, UNaV by 155 +/- 31%, and dopamine excretion by 25 +/- 9%. DA1-receptor blockade with SCH 23390 (SCH, 1.0 microgram/kg/min), attenuated the natriuresis; an increase in UNaV of only 69 +/- 15%. Large volume expansion increased V by 1,026 +/- 215% and UNaV by 2,735 +/- 899%, without affecting dopamine excretion. Increments in V and UNaV were unaffected by increasing doses of SCH (1.0 microgram/kg/min; 10 micrograms/kg/min; and 50 micrograms/kg bolus, followed by 10 micrograms/kg/min). Adequacy of DA1-receptor blockade was demonstrated by the fact that SCH (1.0 microgram/kg/min) attenuated the natriuresis produced by the DA1-receptor agonist, fenoldopam (0.1 micrograms/kg/min ia). We conclude that endogenous DA contributes to the natriuresis produced by small, but not large, increases in extracellular fluid volume with iso-osmotic saline.

Defective dopamine-1 receptor adenylate cyclase coupling in the proximal convoluted tubule from the spontaneously hypertensive rat

The natriuretic effect of DA-1 agonists is less in the spontaneously hypertensive rat (SHR) than its normotensive control, the Wistar-Kyoto rat (WKY). To determine a mechanism of the decreased effect of DA-1 agonists on sodium transport, DA-1 receptors in renal proximal convoluted tubule (PCT) were studied by radioligand binding and by adenylate cyclase (AC) determinations. Specific binding of 125I-SCH 23982 (defined by 10 microM SCH 23390, a DA-1 antagonist) was concentration dependent, saturable, and stereoselective. The dissociation constant, maximum receptor density, and DA-1 antagonist inhibition constant were similar in SHR and WKY. The apparent molecular weight of the DA-1 receptor determined by the photoaffinity D1 probe 125I-MAB was also similar in WKY and SHR. However, DA-1 agonists competed more effectively for specific 125I-SCH 23982 binding sites in WKY than in SHR. Basal as well as forskolin, parathyroid hormone, GTP and Gpp(NH)p-stimulated-AC activities were similar. In contrast DA-1 agonists (fenoldopam, SKF 38393, SND 911C12) stimulated AC activity to a lesser extent in SHR. GTP and Gpp(NH)p enhanced the ability of DA-1 agonists to stimulate AC activity in WKY but not in SHR. These data suggest a defect in the DA-1 receptor-second messenger coupling mechanism in the PCT of the SHR.

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.