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

Nishi , A., Bertorello , A. M. & Aperia , A. 1992. 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. Acta Ptiysiol Scand 144 , 263–267. Received 26 July 1991, accepted 25 October 1991. ISSN 0001–6772. Department of Paediatrics, Karolinska Institute, Sweden 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 < 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.     

Calcium supplementation and thyroid hormone protect against gentamicin-induced inhibition of proximal tubular Na+,K(+)-ATPase activity and other renal functional changes.

Gentamicin can cause proximal tubule necrosis. We have shown that inhibition of PT Na+,K(+)-ATPase activity is rapidly induced by gentamicin. We have now investigated whether manipulations known to attenuate the negative effects of gentamicin on renal excretory capacity, i.e. high calcium intake and L-thyroxine treatment, will also attenuate gentamicin-induced inhibition of Na+,K(+)-ATPase activity and ameliorated signs of proximal tubule damage. Rats were gentamicin- or vehicle-treated for 7 days. Sub-groups were given 4% calcium (Ca) supplements or L-thyroxine 20 micrograms 100 g-1 body weight daily. Gentamicin significantly reduced the glomerular filtration rate and increased the urinary excretion of the proximal tubule lysosomal enzyme, N-acetyl-beta-D-glucosaminidase. Gentamicin significantly reduced proximal tubule Na+,K(+)-ATPase activity, measured in single permeabilized proximal tubule segments. Sodium excretion was inversely correlated to proximal tubule Na+,K(+)-ATPase activity. Both calcium and L-thyroxine alleviated all gentamicin-induced side-effects on renal function as well as on proximal tubule Na+,K(+)-ATPase activity. Calcium and L-thyroxine had no significant effect on renal function. L-thyroxine, but not calcium, increased proximal tubule Na+,K(+)-ATPase activity in control rats. Renal cortical tissue gentamicin concentration was not influenced by calcium but was significantly lowered by L-thyroxine. Two procedures which, via different mechanisms, afford protection from gentamicin-induced changes in renal function also give protection from gentamicin-induced inhibition of Na+,K(+)-ATPase activity. This suggests that loss of integrity of the Na+,K(+)-ATPase enzyme contributes to gentamicin-induced nephrotoxicity.     

Increase of urocortin-like immunoreactivity in the supraoptic nucleus of Dahl rats given a high salt diet

Urocortin-like immunoreactivity (Ucn-LI) in the supraoptic nucleus (SON) of Dahl rats was examined. Dahl salt-sensitive (S) rats fed with a high salt diet developed hypertension. Numbers of Ucn-LI neurons in the SON in Dahl S on a high salt diet were markedly increased, compared with those in Dahl salt-resistant (R) rats on the same. Sporadic Ucn-LI neurons were found in the SON of both Dahl S and R on a normal diet. Numbers of Ucn-LI neurons in the SON of spontaneously hypertensive rat (SHR) and stroke-prone SHR, genetic models of hypertension, and control rats (Sprague-Dawley and Wistar-Kyoto) were similar. These results suggest that Ucn in the SON is associated with salt loading-induced hypertension rather than spontaneous hypertension.     

Effect of salt intake on jejunal dopamine, Na+,K+-ATPase activity and electrolyte transport

The present study addresses the question of the relevance of salt intake on jejunal dopamine, Na+,K+-ATPase activity and electrolyte transport. Low salt, but not high salt, intake for 2 weeks increased dopamine levels in the jejunal mucosa accompanied by a marked decrease in L-3,4-dihydroxyphenylalanine tissue levels. By contrast, in rats fasted for 72 h the effect of refeeding for 24 h with a low salt diet failed to change dopamine tissue levels, although it significantly increased those of L-3,4-dihydroxyphenylalanine. By contrast, high salt intake markedly increased the tissue levels of both dopamine and L-3,4-dihydroxyphenylalanine, without changes in dopamine/L-3,4-dihydroxyphenylalanine tissue ratios. Tissue levels of both L-3,4-dihydroxyphenylalanine and dopamine in control conditions (normal salt intake for 2 weeks) were markedly higher (P < 0.05) than in rats submitted to 72 h fasting plus 24 h refeeding. The effect of fasting for 72 h followed by 24 h refeeding was a marked decrease in jejunal Na+,K+-ATPase activity, particularly evident for rats fed a normal salt and high salt diets during the refeeding period. Basal short circuit current was similar in rats fed a normal salt diet for 2 weeks and 24 h, and the type of diet failed to alter basal short circuit current after refeeding with normal, low and high salt diets. On the other hand, the effect of prolonged low salt intake was a marked decrease in jejunal Na+, K+-ATPase activity and basal short circuit current, whereas high salt intake failed to alter enzyme activity and basal short circuit current. In rats fed for 2 weeks a high salt diet ouabain was found to be more potent in reducing jejunal short circuit current than in rats fed normal and low salt diets. The effect of furosemide was more marked in rats fed for 2 weeks high and low salt diets than in animals receiving a normal salt intake. Dopamine (up to 1 micromol L-1) was found not to alter Na+,K+-ATPase and basal short circuit current in jejunal epithelial sheets, in rats fed with normal, low and high salt diets for 2 weeks and 24 h.     

Role of TRPV1 channels in renal haemodynamics and function in Dahl salt-sensitive hypertensive rats

This study tests the hypothesis that dysfunction of transient receptor potential vanilloid type 1 (TRPV1) channels occurs and contributes to the decrease in the glomerular filtration rate (GFR) and sodium/water excretion in Dahl salt-sensitive hypertensive rats. Recirculating Krebs-Henseleit buffer added with inulin was perfused at a constant flow in the isolated kidneys of Dahl salt-sensitive (DS) or Dahl salt-resistant (DR) rats fed a high-salt (HS) or low-salt (LS) diet for 3 weeks. Perfusion pressures (PP) were pre-adjusted to three levels ( approximately 100, approximately 150 or approximately 190 mmHg) with or without phenylephrine. Capsaicin, a selective TRPV1 agonist, in the presence or absence of capsazepine, a selective TRPV1 antagonist, was perfused. Basal GFR, urine flow rate (UFR) and Na(+) excretion (U(Na)V) were significantly lower in DS-HS than in DR-HS, DS-LS and DR-LS rats. Capsaicin caused pressure-dependent decreases in PP and increases in GFR, UFR and U(Na)V in all groups, with less magnitude of decreases in PP and increases in GFR, UFR and U(Na)V in DS-HS than in DR-HS, DS-LS and DR-LS rats. Capsazepine completely blocked the effect of capsaicin on PP, GFR, UFR and U(Na)V in all groups. Thus, these results show that TRPV1 function is impaired in the kidney of DS rats fed a high-salt diet, which may contribute to the decrease in GFR and renal excretory function in DS rats in the face of salt challenge.     

Morphometric evaluation of the renal arterial system of Dahl salt-sensitive and salt-resistant rats on a high salt diet. I. Interlobar and arcuate arteries

Structural changes in intrarenal arteries of inbred Dahl salt-sensitive and salt-resistant rats with acute hypertension were studied morphometrically. After a week on a normal salt diet (1% NaCl), animals were placed on a high salt diet (8% NaCl) for 4 weeks. Systolic blood pressure (BP) and body weights (BW) were recorded, and six salt-sensitive and six salt-resistant animals were sacrificed weekly for a total of five sampling periods. Corrected cross-sectional (C/S) areas of adventitia, media (MAC), intima, wall (WAC), and lumen (LAC) were measured by planimetry. Although significant increases (p less than 0.01) in both BW and systolic BP were observed over time in both strains, salt-sensitive rats became hypertensive (systolic BP greater than 150 mm Hg) by week 2 on a high salt diet, while salt-resistant rats remained normotensive. In interlobar arteries, significant increases over time were observed for the WAC, MAC, and LAC in salt-resistant rats and in the WAC, adventitia, MAC, and LAC for salt-sensitive rats. Significant increases over time were observed for the WAC, adventitia, MAC, and LAC in arcuate arteries from salt-sensitive rats only. Increased C/S areas observed over time in both strains were observed by week 3 on the high salt diet, after the elevated systolic BP. Analysis of covariance indicated that increased C/S areas observed over time in salt-sensitive rats paralleled elevated systolic BP but did not follow an increase in BW. On the other hand, in salt-resistant rats, increased C/S areas observed over time correlated with BW but not systolic BP. The documented rapid development of vascular changes in salt-sensitive rats in conjunction with the development of acute hypertension demonstrates the potential usefulness of this model for investigating the pathogenesis of hypertensive renal vascular alterations.     

Gentamicin inhibition of Na+,K(+)-ATPase in rat kidney cells.

Na,K(+)-ATPase activity is decreased in homogenized renal tissue from GM-treated rats. This study examines whether the site of the active effect of GM on Na,K(+)-ATPase activity in the kidney can be localized to the proximal convoluted tubules (PCT) where the drug is taken up and where it will produce necrosis. In rats treated with gentamicin (50 micrograms.kg-1.day-1 i.m.) for 7 days, PCT Na,K(+)-ATPase activity was reduced as compared to vehicle-treated rats but returned to control levels 7 days after treatment withdrawal. In another nephron segment, the medullary thick ascending limb of Henle (mTAL), where GM induced lesions are uncommon, Na,K(+)-ATPase activity was the same in GM- and vehicle-treated rats treatment. To study the in vitro effect of GM, dissected PCT and mTAL segments from untreated rats were preincubated for 30 min with GM 10(-3) M, a dose similar to the tissue concentration in chronically treated rats. In tubule segments that were permeabilized to allow the drug to enter the cells, GM 10(-3) M significantly inhibited Na,K(+)-ATPase activity both in PCT and mTAL. In non-permeabilized mTAL segments GM did not inhibit Na,K(+)-ATPase activity. GM inhibition of Na,K(+)-ATPase activity in permeabilized PCT segments persisted after the tubules were rinsed in GM free medium. GM does not inhibit Na,K(+)-ATPase partly purified from the renal cortex. Conclusion. Gentamicin inhibits Na,K(+)-ATPase activity in renal tubule cells when it has access to the cytoplasm. Treatment with GM will therefore cause a selective inhibition of Na,K(+)-ATPase in the proximal tubule cells.     

The morphology and antihypertensive effect of renomedullary interstitial cells derived from Dahl sensitive and resistant rats

The renomedullary interstitial cell (RIC) has been implicated in the antihypertensive action of the kidney. This cell has been isolated in tissue culture and shown to have an antihypertensive action in several models of experimental hypertension. Morphometric studies of RIC in vivo from Dahl rats sensitive and resistant to the hypertensive effects of high-salt diets indicate major differences between the RICs. These cells were therefore isolated from salt-sensitive and salt-resistant strains of rat, grown, and maintained in tissue culture. Major morphologic differences between the two cell lines were noted and persisted for multiple tissue culture passages. The cells from resistant animals were larger and had more lipid granules. These differences were similar to those seen in vivo. In short-term experiments these cells were compared for their antihypertensive effect. The two cell lines were injected subcutaneously into two groups of hypertensive recipient rats, one group of Dahl salt-sensitive rats on a high-salt diet and one group of Wistar rats subjected to the one-kidney, one-clip Goldblatt procedure. In both cases differences were noted between the cell lines. These data support the concept that differences between the Dahl salt-sensitive and salt-resistant rats may be related to variations in their RIC.     

Plasma catecholamines in salt-sensitive hypertensive (Dahl) rats

We examined the effects of low (0.4%) and high (8.0%) salt diets on basal and stress-induced increments in plasma levels of norepinephrine (NE) and epinephrine (EPI) in the Dahl lines of salt-sensitive (DS) and salt-resistant (DR) rats. DS rats develop sustained increases in blood pressure when maintained on a high salt diet while DR rats remain normotensive. For this study, blood samples were obtained via a chronic tail artery catheter from DS and DR rats under resting conditions or following exposure to stress. Plasma samples were later assayed for content of NE and EPI by a radioenzymatic assay. Basal plasma levels of both catecholamines were similar in DS and DR rats, irrespective of dietary salt content and mean arterial blood pressure. The mild stress of handling and transfer of rats to a different cage resulted in greater increments in plasma NE but not EPI in DS rats fed a high salt diet compared to DS rats fed a low salt diet. There was a significant effect of line and an interaction of line and diet with respect to the effects of immobilization stress on plasma catecholamines. DS rats had greater immobilization-induced increments in plasma NE and EPI compared to DR rats when both lines were fed a low salt diet. Maintenance on a diet high in salt resulted in lesser immobilization-induced increments in plasma catecholamines for DS rats and greater immobilization-induced increments in plasma catecholamines for DR rats when compared to their respective controls on a low salt diet. There was a significant effect of diet on plasma levels of both catecholamines when blood samples were obtained by decapitation. DS and DR rats that were fed a high salt diet had lower plasma levels of NE and EPI following decapitation compared to rats of the two lines that were fed a low salt diet. These findings demonstrate that dietary salt and genetic factors are important in regulating the activity and responsiveness of the sympathetic-adrenal medullary system to a variety of stressors. Our findings do not provide evidence for a critical role of the sympathetic nervous system in maintaining the diet-induced increase in the arterial blood pressure of DS rats.     

Regional intestinal adaptations in Na+,K+-ATPase in experimental colitis and the contrasting effects of interferon-gamma

AIMS: This study evaluated Na+,K+-ATPase activity and the abundance of alpha1 subunit Na+,K+-ATPase in experimental colitis and gathered evidence on the effects of interferon-gamma (IFN-gamma) on intestinal Na+,K+-ATPase. METHODS: Colitis was induced by the intrarectal administration of 2,4,6-trinitrobenzene sulphonic acid (TNBS, 30 mg/250 microL). Na+,K+-ATPase activity was determined as the difference between total and ouabain-insensitive ATPase. The abundance of Na+,K+-ATPase was analysed by immunoblotting. RESULTS: Na+,K+-ATPase activity was markedly reduced in the proximal colonic mucosa of TNBS-treated rats, whereas upstream in the terminal ileal mucosa a marked increase in sodium pump activity was observed. At the jejunal level no significant changes in Na+,K+-ATPase activity were observed between TNBS-treated rats and corresponding controls (ethanol-treated rats). No changes were observed in the abundance of alpha1 subunit Na+,K+-ATPase in the proximal colon, terminal ileum and jejunum. The administration of IFN-gamma (50,000 U) 48 h before sacrifice reduced both Na+,K+-ATPase activity and the abundance of alpha1 subunit Na+,K+-ATPase in the proximal colon. Dexamethasone prevented colonic inflammation and decreases in proximal colonic Na+,K+-ATPase activity in TNBS-treated rats, but did not affect the INF-gamma-induced decrease in colonic Na+,K+-ATPase activity. CONCLUSIONS: The increase in ileal Na+,K+-ATPase activity upstream to the lesioned colonic mucosa, where Na+,K+-ATPase activity was markedly reduced, might indicate a compensatory process to counteract the decrease in water and electrolyte absorption at the colonic level. This decrease in colonic Na+,K+-ATPase activity is likely not related to INF-gamma-induced downregulation of Na+,K+-ATPase.     

Defective renal dopamine D1-like receptor signal transduction in obese hypertensive rats

It is reported that dopamine promotes renal sodium excretion via activation of D1-like dopamine receptors located on the proximal tubules. In spontaneously hypertensive rats the natriuretic and diuretic response to exogenously administered and endogenously produced dopamine is reduced, which results from a diminished dopamine-induced inhibition of the enzyme, Na+,K+-ATPase. The present study was designed to examine dopamine-receptor mediated inhibition of Na+,K+-ATPase and its associated signal transduction pathway in the proximal tubules of Zucker obese and lean control rats. The obese animals were hypertensive, hyperinsulinaemic and hyperglycaemic compared with the lean rats. While dopamine caused inhibition of Na+,K+-ATPase activity in lean rats, this effect was significantly attenuated in the obese animals. There was significant reduction in D1-like receptor numbers in the basolateral membranes of obese rats compared with lean rats with no change in the affinity to the ligand [3H]SCH 23390 between the two groups of rats. Dopamine failed to stimulate G proteins as measured by [35S]GTPgammaS binding in the obese rats. Also, dopamine was unable to cause phospholipase-C activation in obese rats, but it did activate phospholipase-C in lean rats. These results show that reduction in D1-like receptor numbers and a defect in receptor-G protein coupling may account for the inability of dopamine to activate the D1-like receptor-coupled signal transduction pathway and cause inhibition of Na+,K+-ATPase in the obese hypertensive rats.     

Intracellular sodium modulates the state of protein kinase C phosphorylation of rat proximal tubule Na+,K+-ATPase

The natriuretic hormone dopamine and the antinatriuretic hormone noradrenaline, acting on alpha-adrenergic receptors, have been shown to bidirectionally modulate the activity of renal tubular Na+,K+-adenosine triphosphate (ATPase). Here we have examined whether intracellular sodium concentration influences the effects of these bidirectional forces on the state of phosphorylation of Na+,K+-ATPase. Proximal tubules dissected from rat kidney were incubated with dopamine or the alpha-adrenergic agonist, oxymetazoline, and transiently permeabilized in a medium where sodium concentration ranged between 5 and 70 mM. The variations of sodium concentration in the medium had a proportional effect on intracellular sodium. Dopamine and protein kinase C (PKC) phosphorylate the catalytic subunit of rat Na+,K+-ATPase on the Ser23 residue. The level of PKC induced Na+,K+-ATPase phosphorylation was determined using an antibody that only recognizes Na+,K+-ATPase, which is not phosphorylated on its PKC site. Under basal conditions Na+,K+-ATPase was predominantly in its phosphorylated state. When intracellular sodium was increased, Na+,K+-ATPase was predominantly in its dephosphorylated state. Phosphorylation of Na+,K+-ATPase by dopamine was most pronounced when intracellular sodium was high, and dephosphorylation by oxymetazoline was most pronounced when intracellular sodium was low. The oxymetazoline effect was mimicked by the calcium ionophore A23187. An inhibitor of the calcium-dependent protein phosphatase, calcineurin, increased the state of Na+,K+-ATPase phosphorylation. The results imply that phosphorylation of renal Na+,K+-ATPase activity is modulated by the level of intracellular sodium and that this effect involves PKC and calcium signalling pathways. The findings may have implication for the regulation of salt excretion and sodium homeostasis.     

Renal Na+, K+-ATPase in Dahl salt-sensitive rats: K+ dependence,effect of cell environment and protein kinases

Na⁺, K⁺-ATPase in renal epithelial cells plays an important role in the regulation of Na⁺ balance, extracellular volume and blood pressure. The function of renal Na⁺, K⁺-ATPase in Dahl salt-sensitive (DS) rats, an animal model for salt-sensitive hypertension, and Dahl salt-resistant (DR) rats has been studied. In Na⁺, K⁺-ATPase partially purified from renal cortex, affinities and the Hill coefficients for Na⁺ and K⁺ activation were similar in DS and DR rats. Only one component of low ouabain affinity site was found in both strains, indicating the presence of the al isoform. Protein kinase C and cAMP-dependent protein kinase phosphorylated Na⁺, K⁺-ATPase α subunit in DS and DR rats, and the phosphorylation by protein kinase C was associated with an inhibition of enzyme activity. The kinetic parameters for K⁺ activation were also studied in a preparation of basolateral membranes and were found to be similar in DS and DR rats. In a preparation of cortical tubule cells, Na⁺, K⁺-ATPase activity was determined as ouabain-sensitive oxygen consumption (OS Qo₂). Maximal OS Qo₂, measured in Na⁺ loaded cells, was the same in DS and DR rats. The K₀₆ for K⁺ was significantly lower in DS than DR rats (0.163 ±0.042 vs. 0.447 + 0.061 mM, P < 0.05), indicating that factors regulating Na⁺, K⁺-ATPase activity in intact cells are altered in DS rats. Kinetic parameters for Na⁺ activation in cells were the same in both strains. In summary, the function of renal Na⁺, K⁺-ATPase molecule is not altered in DS rats. The intracellular systems that regulate renal Na⁺, K⁺-ATPase activity might be different in DS and DR rats.     

Dahl salt-sensitive and salt-resistant rats: examination of learning and memory performance, blood pressure, and the expression of central nicotinic acetylcholine receptors

Substantial human and animal data suggest a correlation between hypertension and memory impairment that may appear prior to overt manifestations of cerebrovascular pathology. It is unclear, however, whether hypertension plays a causal role in these memory deficits, whether hypertension and cognitive impairment are each based in family history and not interdependent, or whether a combination of these factors is important. The purpose of this study was to assess whether deficits in memory performance and nicotinic acetylcholine receptors were present in Dahl salt-sensitive rats (as observed previously in spontaneously hypertensive rats) and whether the presence of hypertension per se (induced with an 8% Na(+) diet) contributed to the deficits. Memory was assessed in a passive avoidance task, an eight-arm radial arm maze and in a water maze task, and nicotinic receptors were measured via quantitative receptor autoradiography utilizing [125I]alpha-bungarotoxin and [3H]epibatidine. Salt-sensitive rats exhibited impaired performance in both spatial learning tasks, but not the passive avoidance task, compared to controls (salt-resistant strain) and they exhibited reductions in nicotinic receptors labeled by [125I]alpha-bungarotoxin but not [3H]epibatidine in some brain regions, including some areas important for memory (e.g. the hippocampus and amygdala). In the radial arm maze, the degree of memory impairment and in binding studies the reduced expression of nicotinic receptors each failed to correlate with the highest blood pressures, and the salt-sensitive animals were impaired relative to controls whether or not the high Na(+) diet was administered. In contrast, higher blood pressures did correlate with inferior task performance in the water maze. These findings may suggest that the genetics of the subjects were critical for performance when appetitive drives were involved, but diet (and perhaps hypertension) were key to performance when memory did not involve appetitive drives or mechanisms. Overall, the data obtained from Dahl rats appear to support the role of family history (selective breeding in rats) as underlying the reductions in central nicotinic acetylcholine receptors, whereas both family history and hypertension may contribute to poor cognitive performance.     

Morphometric evaluation of the renal arterial system of Dahl salt-sensitive and salt-resistant rats on a high salt diet. II. Interlobular arteries and intralobular arterioles

The progression of small vessel renal vascular disease was studied in inbred Dahl salt-sensitive (SS/Jr) and salt-resistant rats with acute hypertension induced by a high salt diet. Corrected cross-sectional areas of wall (WAC) and lumen were measured by planimetry and histologic staining for fibrin, hyalin deposition, and elastic lamellae was performed. In SS/Jr rats on the high salt diet, the hallmarks of malignant hypertension (fibrinoid necrosis, hyperplastic and necrotizing arteritis) appeared by week 2 and were intensified after 4 weeks on the high salt diet. Renal vascular lesions from SS/Jr rats were characterized by: hyperplasia and/or hypertrophy of medial smooth muscle cells; intimal proliferation; fibrin, basophilic mucoid, and hyalin deposition within the the subendothelial space and media; variable adventitial fibrosis; and accumulation of mononuclear inflammatory cells in the adventitia and media. Interlobular arteries from both rat strains exhibited significantly increased cross-sectional areas over time for all measured parameters. Intralobular arterioles from both rat strains exhibited significantly increased cross-sectional areas over time for all measured parameters except lumen from SS/Jr rats. For SS/Jr rats, increased WAC from both arterial divisions correlated positively with systolic blood pressure, but not body weight. In salt-resistant rats, increased WAC from both arterial divisions correlated positively with body weight, but not systolic blood pressure. We concluded that the rapid increase in WAC from SS/Jr rats could not be attributed solely to the normal growth of the rat. With the development of acute hypertension in the SS/Jr rat, these results demonstrate the potential usefulness of this model to investigate the pathogenesis of similar renal vascular alterations which are observed in man.     

Sodium-dependent regulation of sodium, potassium-adenosine-tri-phosphatase (Na+, K(+)-ATPase) activity in medullary thick ascending limb of Henle segments. Effect of cyclic-adenosine-monophosphate guanosine-nucleotide-binding-protein activity and arginine vasopressin.

This study examine the regulation Na+, K(+)-ATPase activity in the medullary thick ascending limb of Henle Na+, K(+)-ATPase activity was determined in medullary thick ascending limb of Henle (mtal) segments dissected from rat kidneys. The sodium concentration in the medium (Nam) was 20 or 70 mM. Since the segments were permeabilized, intracellular Na+ (Nai) was assumed to be the same as Nam. Dibuturyl cyclic adenosine monophosphate (dbcAMP) and forskolin inhibited Na+, K(+)-ATPase activity independently of Nam. Arginine vasopressin (AVP) receptors coupled to adenylate cyclase have been identified in the medullary thick ascending limb of Henle. At Nam = 20 mMAVP caused a dose-dependent inhibition of Na+, K(+)-ATPase activity with a maximal effect (49%) at 10(-8) M. This inhibition was abolished in the presence of the adenylate cyclase inhibitor 2,5-dideoxyadenosine (2, 5-DDA). AVP had no effect on Na+, K(+)-ATPase activity in the mTAL at Nam = 70 mM. The guanosine-diphosphate analogue GDP beta S inhibited Na+, K(+)-ATPase activity at Nam = 70 mM but not at Nam = 20 mM. We conclude that increased cyclic adenosine monophosphate (cAMP) levels inhibit Na+, K(+)-ATPase activity in mTAL. AVP can, depending on Nai, produce this effect by adenylate cyclase activation. The guanonine nucleotide binding protein G-protein might be the site of Na(+)-dependence.     

Ouabain-insensitive, vanadate-sensitive K(+)-ATPase of rat distal colon is partly similar to gastric H+,K(+)-ATPase.

A membrane fraction from rat distal colon contained both ouabain-sensitive and -insensitive K(+)-ATPase activities, which were measured under Na(+)-free conditions. About 38% of the ouabain-insensitive K(+)-ATPase activity was inhibited by vanadate. It was determined whether the ouabain-insensitive, vanadate-sensitive K(+)-ATPase in the colon is similar or identical to gastric H+,K(+)-ATPase. This colonic K(+)-ATPase activity was inhibited completely by monoclonal antibody HK4001, which inhibits the hog gastric H+,K(+)-ATPase activity but not Na+,K(+)-ATPase or Ca(2+)-ATPase. The colonic ATPase activity was inhibited partly by SCH 28080, which is a specific reversible inhibitor of gastric H+,K(+)-ATPase. The colonic ATPase activity was stimulated by low concentrations of K+ (its half-maximal stimulating concentration was 1 mM) and inhibited by high concentrations of K+ (its half-maximal inhibiting concentration was 10 mM), indicating that high and low K+ affinity sites are present in the colonic enzyme as in gastric H+,K(+)-ATPase and that this enzyme is not fully operative under normal physiological conditions. Two other monoclonal antibodies, which inhibit the gastric H+,K(+)-ATPase activity, did not inhibit the colonic K(+)-ATPase activity. The present results suggest that the colonic ouabain-insensitive K(+)-ATPase is partly similar but not identical to the gastric H+,K(+)-ATPase.     

Salt-induced epithelial-to-mesenchymal transition in Dahl salt-sensitive rats is dependent on elevated blood pressure

Dietary salt intake has been linked to hypertension and cardiovascular disease. Accumulating evidence has indicated that salt-sensitive individuals on high salt intake are more likely to develop renal fibrosis. Epithelial-to-mesenchymal transition (EMT) participates in the development and progression of renal fibrosis in humans and animals. The objective of this study was to investigate the impact of a high-salt diet on EMT in Dahl salt-sensitive (SS) rats. Twenty-four male SS and consomic SS-13^BN rats were randomized to a normal diet or a high-salt diet. After 4 weeks, systolic blood pressure (SBP) and albuminuria were analyzed, and renal fibrosis was histopathologically evaluated. Tubular EMT was evaluated using immunohistochemistry and real-time PCR with E-cadherin and alpha smooth muscle actin (α-SMA). After 4 weeks, SBP and albuminuria were significantly increased in the SS high-salt group compared with the normal diet group. Dietary salt intake induced renal fibrosis and tubular EMT as identified by reduced expression of E-cadherin and enhanced expression of α-SMA in SS rats. Both blood pressure and renal interstitial fibrosis were negatively correlated with E-cadherin but positively correlated with α-SMA. Salt intake induced tubular EMT and renal injury in SS rats, and this relationship might depend on the increase in blood pressure.     

beta-Adrenoceptor agonist sensitizes the dopamine-1 receptor in renal tubular cells

The renal effects of dopamine are mainly mediated via the dopamine-1 receptor (D1 receptor). This receptor is recruited from intracellular compartments to the plasma membrane by dopamine and atrial natriuretic peptide (ANP), via adenylyl cyclase activation. We have studied whether isoproterenol, a beta-adrenoceptor (beta-AR) agonist that may interact with dopamine in the regulation of rat renal Na+, K+-adenosine triphosphatase (ATPase) activity, can recruit D1 receptors to the plasma membrane. The spatial regulation of D1 receptors was examined using confocal microscopy techniques in LLCPK cells and the functional interaction between dopamine and isoproterenol was examined by studying their effects on Na+, K+-ATPase activity in microdissected single proximal tubular segments from rat. Isoproterenol was found to translocate the D1 receptors from the interior of the cell towards the plasma membrane. The recruitment of dopamine 1 receptors was found to be cyclic adenosine phosphate (cAMP) dependent, while protein kinase C (PKC) activation was not involved. The functional studies on Na+, K+-ATPase activity showed that the effect of isoproterenol was abolished by a D1-like receptor antagonist (SCH 23390), and mediated via protein kinase A (PKA) and PKC dependent pathways. The results provide an explanation for the interaction between G protein-coupled receptors. The effects of isoproterenol on Na+, K+-ATPase activity can be explained by a heterologous recruitment of D1 receptors to the plasma membrane.     

Response of jejunal Na+, K+-ATPase to 5-hydroxytryptamine in young and adult rats: effect of fasting and refeeding

The present study is aimed to evaluate the effects of 5-hydroxytryptamine (5-HT) upon jejunal Na+,K+-ATPase in young (20-day-old) and adult (60-day-old) rats, and determine the effect of food intake on the response of the sodium pump to the amine. Basal Na+,K+-ATPase activity in jejunal epithelial cells from young rats was twice that in adult animals and responded to 5-HT with stimulation. In adult rats, fasting reduced by 25% basal jejunal Na+, K+-ATPase activity, whereas in young rats, no such change was observed. The sensitivity of jejunal Na+,K+-ATPase to 5-HT in young fasted rats was similar to that observed in fed animals. The effect of refeeding in young rats was a 2-fold increase in jejunal Na+, K+-ATPase activity, this being accompanied by insensitivity to 5-HT. In adult rats, refeeding was accompanied by an increase in jejunal Na+,K+-ATPase activity. It is concluded that the stimulatory effect of 5-HT upon jejunal Na+,K+-ATPase activity is a phenomenon dependent on both age and type of diet. In young rats, it is the food intake that plays an important role in development of insensitivity of Na+,K+-ATPase to stimulation by 5-HT, while in adult animals fasting or fasting followed by refeeding does not play a major role in regulating its sensitivity to the amine.