The role of Na-K-activated adenosine triphosphatase in potassium adaptation. Stimulation of enzymatic activity by potassium loading

Metabolic acidosis and hyperkalemia differentially regulate cation HCN3 channel in the rat nephron

The kidney controls body fluids, electrolyte and acid-base balance. Previously, we demonstrated that hyperpolarization-activated and cyclic nucleotide-gated (HCN) cation channels participate in ammonium excretion in the rat kidney. Since acid-base balance is closely linked to potassium metabolism, in the present work we aim to determine the effect of chronic metabolic acidosis (CMA) and hyperkalemia (HK) on protein abundance and localization of HCN3 in the rat kidney. CMA increased HCN3 protein level only in the outer medulla (2.74 ± 0.31) according to immunoblot analysis. However, immunofluorescence assays showed that HCN3 augmented in cortical proximal tubules (1.45 ± 0.11) and medullary thick ascending limb of Henle's loop (4.48 ± 0.45) from the inner stripe of outer medulla. HCN3 was detected in brush border membranes (BBM) and mitochondria of the proximal tubule by immunogold electron and confocal microscopy in control conditions. Acidosis did not alter HCN3 levels in BBM and mitochondria but augmented them in lysosomes. HCN3 was also immuno-detected in mitoautophagosomes. In the distal nephron, HCN3 was expressed in principal and intercalated cells from cortical to medullary collecting ducts. CMA did not change HCN3 abundance in these nephron segments. In contrast, HK doubled HCN3 level in cortical collecting ducts and favored its basolateral localization in principal cells from the inner medullary collecting ducts. These findings further support HCN channels contribution to renal acid-base and potassium balance.

Hyperkalemia: Disorders of Internal and External Potassium Balance

The serum potassium level is normally preserved de spite changes in potassium intake and output (the exter nal potassium balance) and changes in its distribution in the body (the internal potassium balance). External potassium homeostasis depends primarily on renal ex cretion of the daily exogenous potassium burden. Inter nal homeostasis depends on the extrarenal regulation of potassium. Skeletal muscle and liver are the dominant sites of that regulation. The two chief regulators of inter nal balance are insulin and catecholamines, the latter acting through β-adrenergic receptors. Acid-base bal ance and the cellular potassium content are other important regulators of internal balance. The major disorders of external balance are renal failure, hypo reninemic hypoaldosteronism, interstitial nephritis, and a variety of drugs that impair renal potassium excretion. The major disorders of internal balance are diabetes mellitus, acidosis, medications, and release of endoge nous potassium during vigorous exercise, traumatic muscle injury, or tumor lysis chemotherapy. These dis orders frequently result in troublesome elevations of serum potassium in the intensive care setting. Their re view in this article includes a thorough discussion of the evaluation and proper management of the hyperkalemic patient.

Immunolocalization of hyperpolarization-activated cationic HCN1 and HCN3 channels in the rat nephron: regulation of HCN3 by potassium diets

Hyperpolarization-activated cationic and cyclic nucleotide-gated channels (HCN) comprise four homologous subunits (HCN1-HCN4). HCN channels are found in excitable and non-excitable tissues in mammals. We have previously shown that HCN2 may transport ammonium (NH4 (+)), besides sodium (Na(+)), in the rat distal nephron. In the present work, we identified HCN1 and HCN3 in the proximal tubule (PT) and HCN3 in the thick ascending limb of Henle (TALH) of the rat kidney. Immunoblot assays detected HCN1 (130 kDa) and HCN3 (90 KDa) and their truncated proteins C-terminal HCN1 (93 KDa) and N-terminal HCN3 (65 KDa) in enriched plasma membranes from cortex (CX) and outer medulla (OM), as well as in brush-border membrane vesicles. Immunofluorescence assays confirmed apical localization of HCN1 and HCN3 in the PT. HCN3 was also found at the basolateral membrane of TALH. We evaluated chronic changes in mineral dietary on HCN3 protein abundance. Animals were fed with three different diets: sodium-deficient (SD) diet, potassium-deficient (KD) diet, and high-potassium (HK) diet. Up-regulation of HCN3 was observed in OM by KD and in CX and OM by HK; the opposite effect occurred with the N-terminal truncated HCN3 in CX (KD) and OM (HK). SD diet did not produce any change. Since HCN channels activate with membrane hyperpolarization, our results suggest that HCN channels may play a role in the Na(+)-K(+)-ATPase activity, contributing to Na(+), K(+), and acid-base homeostasis in the rat kidney.

Distal convoluted tubule

The distal convoluted tubule is the nephron segment that lies immediately downstream of the macula densa. Although short in length, the distal convoluted tubule plays a critical role in sodium, potassium, and divalent cation homeostasis. Recent genetic and physiologic studies have greatly expanded our understanding of how the distal convoluted tubule regulates these processes at the molecular level. This article provides an update on the distal convoluted tubule, highlighting concepts and pathophysiology relevant to clinical practice.

Hypertension of Kcnmb1-/- is linked to deficient K secretion and aldosteronism

Mice lacking the beta1-subunit (gene, Kcnmb1; protein, BK-beta1) of the large Ca-activated K channel (BK) are hypertensive. This phenotype is thought to result from diminished BK currents in vascular smooth muscle where BK-beta1 is an ancillary subunit. However, the beta1-subunit is also expressed in the renal connecting tubule (CNT), a segment of the aldosterone-sensitive distal nephron, where it associates with BK and facilitates K secretion. Because of the correlation between certain forms of hypertension and renal defects, particularly in the distal nephron, it was determined whether the hypertension of Kcnmb1(-/-) has a renal origin. We found that Kcnmb1(-/-) are hypertensive, volume expanded, and have reduced urinary K and Na clearances. These conditions are exacerbated when the animals are fed a high K diet (5% K; HK). Supplementing HK-fed Kcnmb1(-/-) with eplerenone (mineralocorticoid receptor antagonist) corrected the fluid imbalance and more than 70% of the hypertension. Finally, plasma [aldo] was elevated in Kcnmb1(-/-) under basal conditions (control diet, 0.6% K) and increased significantly more than wild type when fed the HK diet. We conclude that the majority of the hypertension of Kcnmb1(-/-) is due to aldosteronism, resulting from renal potassium retention and hyperkalemia.

The effect of oral sodium acetate administration on plasma acetate concentration and acid-base state in horses

Aim

Sodium acetate (NaAcetate) has received some attention as an alkalinizing agent and possible alternative energy source for the horse, however the effects of oral administration remain largely unknown. The present study used the physicochemical approach to characterize the changes in acid-base status occurring after oral NaAcetate/acetic acid (NAA) administration in horses.

Methods

Jugular venous blood was sampled from 9 exercise-conditioned horses on 2 separate occasions, at rest and for 24 h following a competition exercise test (CET) designed to simulate the speed and endurance test of 3-day event. Immediately after the CETs horses were allowed water ad libitum and either: 1) 8 L of a hypertonic NaAcetate/acetic acid solution via nasogastric tube followed by a typical hay/grain meal (NAA trial); or 2) a hay/grain meal alone (Control trial).

Results

Oral NAA resulted in a profound plasma alkalosis marked by decreased plasma [H⁺] and increased plasma [TCO₂] and [HCO₃^-] compared to Control. The primary contributor to the plasma alkalosis was an increased [SID], as a result of increased plasma [Na⁺] and decreased plasma [Cl^-]. An increased [A_tot], due to increased [PP] and a sustained increase in plasma [acetate], contributed a minor acidifying effect.

Conclusion

It is concluded that oral NaAcetate could be used as both an alkalinizing agent and an alternative energy source in the horse.

Animal and human tissue Na,K-ATPase in normal and insulin-resistant states: regulation, behaviour and interpretative hypothesis on NEFA effects

The sodium(Na)- and potassium(K)-activated adenosine-triphosphatase (Na,K-ATPase) is a membrane enzyme that energizes the Na-pump by hydrolysing adenosine triphosphate and wasting energy as heat, so playing a role in thermogenesis and energy balance. Na,K-ATPase regulation by insulin is controversial; in tissue of hyperglycemic-hyperinsulinemic ob/ob mice, we reported a reduction, whereas in streptozotocin-treated hypoinsulinemic-diabetic Swiss and ob/ob mice we found an increased activity, which is against a genetic defect and suggests a regulation by hyperinsulinemia. In human adipose tissue from obese patients, Na,K-ATPase activity was reduced and negatively correlated with body mass index, oral glucose tolerance test-insulinemic area and blood pressure. We hypothesized that obesity is associated with tissue Na,K-ATPase reduction, apparently linked to hyperinsulinemia, which may repress or inactivate the enzyme, thus opposing thyroid hormones and influencing thermogenesis and obesity development. Insulin action on Na,K-ATPase, in vivo, might be mediated by the high level of non-esterified fatty acids, which are circulating enzyme inhibitors and increase in obesity, diabetes and hypertension. In this paper, we analyse animal and human tissue Na,K-ATPase, its level, and its regulation and behaviour in some hyperinsulinemic and insulin-resistant states; moreover, we discuss the link of the enzyme with non-esterified fatty acids and attempt to interpret and organize in a coherent view the whole body of the exhaustive literature on this complicated topic.

Hyperkalemia, renal failure, and converting-enzyme inhibition: an overrated connection

Hyperkalemia is widely viewed as a common complication of ACE inhibition in azotemic patients. These renal failure patients are the patients who benefit most from ACE inhibition. Because we could not confirm this notion after a retrospective evaluation of 236 azotemic patients, we studied 2 models of renal mass reduction. In the first, we did a 5/6 nephrectomy (Nx) on rats and studied them 2 weeks after surgery (before chronic renal changes had developed). A second group was studied 16 weeks after Nx, once chronic renal failure was established. Rats in both models were treated with quinapril in drinking water. After baseline evaluation, we challenged them either by a high-K(+) diet or by blocking aldosterone receptors. We found that although quinapril blocked the K(+)-induced increase in aldosterone, serum K(+) levels and K(+) balance were maintained before and during high K(+) intake or during simultaneous spironolactone administration. We conclude that in hemodynamically stable rats with reduced renal mass and renal dysfunction, the administration of an ACE inhibitor does not cause severe hyperkalemia.

NaHCO(3) and KHCO(3) ingestion rapidly increases renal electrolyte excretion in humans

This paper describes and quantifies acute responses of the kidneys in correcting plasma volume, acid-base, and ion disturbances resulting from NaHCO(3) and KHCO(3) ingestion. Renal excretion of ions and water was studied in five men after ingestion of 3.57 mmol/kg body mass of sodium bicarbonate (NaHCO(3)) and, in a separate trial, potassium bicarbonate (KHCO(3)). Subjects had a Foley catheter inserted into the bladder and indwelling catheters placed into an antecubital vein and a brachial artery. Blood and urine were sampled in the 30-min period before, the 60-min period during, and the 210-min period after ingestion of the solutions. NaHCO(3) ingestion resulted in a rapid, transient diuresis and natriuresis. Cumulative urine output was 44 +/- 11% of ingested volume, resulting in a 555 +/- 119 ml increase in total body water at the end of the experiment. The cumulative increase (above basal levels) in renal Na(+) excretion accounted for 24 +/- 2% of ingested Na(+). In the KHCO(3) trial, arterial plasma K(+) concentration rapidly increased from 4.25 +/- 0.10 to a peak of 7.17 +/- 0.13 meq/l 140 min after the beginning of ingestion. This increase resulted in a pronounced, transient diuresis, with cumulative urine output at 270 min similar to the volume ingested, natriuresis, and a pronounced kaliuresis that was maintained until the end of the experiment. Cumulative (above basal) renal K(+) excretion at 270 min accounted for 26 +/- 5% of ingested K(+). The kidneys were important in mediating rapid corrections of substantial portions of the fluid and electrolyte disturbances resulting from ingestion of KHCO(3) and NaHCO(3) solutions.

Complete adaptation to chronic potassium loading after adrenalectomy: possible humoral mechanisms

This study was designed to evaluate the mechanisms of adaptation to chronic potassium loading after bilateral adrenalectomy. Studies were performed in Sprague-Dawley rats subjected to 3 days of normal diet and 9 days of high KCl diet followed by adrenalectomy or sham operation on the thirteenth day and 9 additional days of potassium loading (groups 1 and 2, respectively). Animals that underwent adrenalectomy and intact animals, both receiving a normal diet, served as the control groups (groups 3 and 4, respectively). Plasma potassium, urinary potassium and sodium excretion rates, plasma aldosterone and insulin, and Na+-K+ ATPase activity in renal cortical and medullary homogenates were measured. Within 5 days of adrenalectomy the urinary potassium excretion rate in potassium-loaded rats that underwent adrenalectomy (group 1) reached the level observed in potassium-loaded intact rats (group 2), but a significant elevation in plasma potassium levels among rats in group 1 was noticed. In both of the potassium-loaded groups plasma insulin levels and renal cortical and medullary Na+-K+ ATPase activity were significantly higher compared with those in respective control groups receiving a normal diet. Acute clearance experiments carried out in adrenalectomized rats infusing the sera of the potassium-adapted rats that underwent adrenalectomy (obtained at the end of the chronic experiment) showed an uprise in urinary potassium excretion. This result was not observed after the infusion of control sera. These findings suggest that full renal adaptation to chronic potassium loading can be achieved in the absence of aldosterone through mechanisms that might be related to elevated plasma insulin levels (extrarenal); also, a humoral factor associated with the renal adaptation cannot be ruled out.

K+ supplementation increases muscle [Na+-K+-ATPase] and improves extrarenal K+ homeostasis in rats

Effects of K+ supplementation (approximately 200 mmol KCl/100 g chow) on plasma K+, K+ content, and Na+-K+-adeonsinetriphosphatase (ATPase) concentration ([Na+-K+-ATPase]) in skeletal muscles as well as on extrarenal K+ clearance were evaluated in rats. After 2 days of K+ supplementation, hyperkalemia prevailed (K+-supplemented vs. weight-matched control animals) [5.1 +/- 0.2 (SE) vs. 3.2 +/- 0.1 mmol/l, P < 0.05, n = 5-6], and after 4 days a significant increase in K+ content was observed in gastrocnemius muscle (104 +/- 2 vs. 97 +/- 1 micromol/g wet wt, P < 0.05, n = 5-6). After 7 days of K+ supplementation, a significant increase in [3H] ouabain binding site concentration (344 +/- 5 vs. 239 +/- 8 pmol/g wet wt, P < 0.05, n = 4) was observed in gastrocnemius muscle. After 2 wk, increases in plasma K+, K+ content, and [3H]ouabain binding site concentration in gastrocnemius muscle amounted to 40, 8, and 68% (P < 0.05) above values observed in weight-matched control animals, respectively. The latter change was confirmed by K+-dependent p-nitrophenyl phosphatase activity measurements. Fasting for 1 day reduced plasma K+ and K+ content in gastrocnemius muscle in rats that had been K+ supplemented for 2 wk by 3.1 +/- 0.3 mmol/l (P < 0.05, n = 5) and 15 +/- 2 micromol/g wet wt (P < 0.05, n = 5), respectively. After induction of anesthesia, arterial plasma K+ was measured during intravenous KCl infusion (0.75 mmol KCl x 100 g body wt(-1) x h(-1)). The K+-supplemented fasted group demonstrated a 42% (P < 0.05) lower plasma K+ rise, associated with a significantly higher increase in K+ content in gastrocnemius muscle of 7 micromol/g wet wt (P < 0.05, n = 5) compared with their control animals. In conclusion, K+ supplementation increases plasma K+, K+ content, and [Na+-K+-ATPase] in skeletal muscles and improves extrarenal K+ clearance capacity.

Extrarenal potassium tolerance in chronic renal failure: implications for the treatment of acute hyperkalemia

The role of extrarenal potassium homeostasis is well recognized as a major mechanism for the acute defense against the development of hyperkalemia. The purpose of this report is to examine whether or not the various mechanisms of extrarenal potassium regulation are intact in patients with end-stage renal disease (ESRD). The available data suggest that with the development of ESRD and the uremic syndrome there is impaired extrarenal potassium metabolism that is related to a defect in the Na,K-adenosine triphosphatase (ATPase). The responsiveness of uremic patients to the various effector systems that regulate extrarenal potassium handling is discussed. Insulin is well positioned to play an important role in the regulation of plasma potassium concentration in patients with impaired renal function. The role of basal insulin may be even more important than previously appreciated, since somatostatin infusion causes a much greater increase in the fasting plasma potassium in rats with renal failure than in controls. Furthermore, stimulation of endogenous insulin by oral glucose results in a greater intracellular translocation of potassium in uremic rats than in controls. Under at least two common physiologic circumstances, feeding and vigorous exercise, endogenous catecholamines might also act to defend against acute increments in extracellular potassium concentration. However, it is important to appreciate that the response to beta 2-adrenoreceptor-mediated internal potassium disposal is heterogeneous as judged by the variable responses to epinephrine infusion. Based on the evidence presented in this report, a regimen for the treatment of life-threatening hyperkalemia is outlined. Interpretation of the available data demonstrate that bicarbonate should not be relied on as the sole initial treatment for severe hyperkalemia, since the magnitude of the effect of bicarbonate on potassium is variable and may be delayed. The initial treatment for life-threatening hyperkalemia should always include insulin plus glucose, as the hypokalemic response to insulin is both prompt and predictable. Combined treatment with beta 2-agonists and insulin is also effective and may help prevent insulin-induced hypoglycemia.

Renal medullary Na-K-ATPase and hypoxic injury in perfused rat kidneys

We wished to see if chronic alterations in Na-K-ATPase activity in the medullary thick ascending limb would modify the susceptibility of its cells to the hypoxic injury produced by perfusion of the isolated kidney. Rats were fed a diet high (64%) or low (8%) in protein for three weeks. Renal medullary Na-K-ATPase was 75 +/- 12 U/mg protein/hr (mean +/- SE) in the high protein group and 44 +/- 3 in rats given low protein. After 90 minutes of perfusion, the kidneys of rats fed a high protein diet showed almost all mTAL cells near the inner medulla with severe damage (93 +/- 4.8%), whereas the same zone in perfused kidneys of rats on a low protein diet showed only 47 +/- 7.7% injury. In a similar fashion, damage to mTAL cells seen in perfused kidneys was greatly augmented by compensatory renal hypertrophy produced by removal of the contralateral kidney two weeks earlier, and by a diet high in potassium given for two weeks, procedures which also increased the activity of medullary Na-K-ATPase. The results suggest that the level of transport work of medullary cells mediated by Na-K-ATPase is a determinant of the vulnerability of mTAL cells to hypoxic injury.

Potassium tolerance

The maintenance of potassium homeostasis with an increased potassium intake or decreased renal function is dependent in part on the renal adaptation observed in 'potassium tolerance'. However other factors, including control of ingestion, and increased distal delivery of fluid, also play a role.

The role of aldosterone in potassium tolerance: studies in anephric humans

Since the role of aldosterone in mediating extrarenal potassium transport remains uncertain, the effect of mineralocorticoid on potassium metabolism was assessed in anephric patients. Seven anephric patients underwent three identical 72-hour periods between hemodialyses during which treatment with either 10 mg/day deoxycorticosterone acetate (DOCA) intramuscularly or 300 mg/day spironolactone orally was compared to a baseline control period. The serum potassium rise, plasma aldosterone, salivary and stool electrolytes were measured in response to potassium loading over 48 hours with a metabolic diet containing 38 mEq/day followed by an "acute" oral potassium load of 0.5 mEq/kg. Acute potassium loading with DOCA resulted in a lower increment in serum potassium than with spironolactone (P less than 0.01). The volume of distribution of the acute potassium load at three hours was 55% of body weight with DOCA, which was significantly greater (P less than 0.05) than with either spironolactone (35%) or control (34%). However, with the dietary load of potassium, the increments in serum potassium measured at 24 and 48 hours (13 hours post-prandial) were similar in all three periods. The volume of distribution of the dietary potassium was not altered by DOCA or spironolactone but had risen to an average of 172% at 24 hours and 243% at 48 hours in the three periods. Plasma aldosterone levels were low, positively correlated to the serum potassium and similar in all three periods without evidence of feedback inhibition by DOCA or stimulated by spironolactone.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of potassium adaptation on the distribution of potassium, sodium and chloride across the apical membrane of renal tubular cells

To assess the effect of K adaptation on the electrolyte concentrations of renal tubular cells and on the concentration gradients across the luminal membrane, electron microprobe analysis was employed on freeze-dried cryosections of the renal cortex and on freeze-dried samples of tubular fluid in control and high-K rats. The measurements were performed in individual cells of the proximal and superficial distal tubule and on samples of tubular fluid obtained by free flow micropuncture from proximal and early and late distal collection sites. The ingestion of a potassium-rich diet for at least 10 days together with an acute potassium load of 0.4 mmol/kg/h led to a small increase in potassium concentration of about 7 mmol/kg wet weight (w.w.) in all cell types analysed. In distal convoluted tubule, connecting tubule and principal cells sodium concentration was markedly decreased by 4, 4, and 6 mmol/kg w.w., respectively, while no significant changes in sodium concentration were found in proximal tubule and intercalated cells. No consistent changes in cell chloride could be observed under K adaptation. Analysis of the tubular fluid samples showed that the K concentration gradient across the apical cell membrane of all distal tubular cell types investigated was diminished in the high-K rats. The concentration gradient for sodium entry, however, was clearly enhanced in the distal convoluted tubule, connecting tubule and principal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of potassium bicarbonate on distal nephron Na-K-ATPase in adrenalectomized rabbits

Na-K-ATPase activity in the connecting tubule (CNT) and cortical collecting duct (CCD) has been shown to be influenced by KCl both in the presence and in the absence of aldosterone. To investigate if the aldosterone-independent effect of K+ on Na-K-ATPase can be produced by other K+ salts, we studied the effects of dietary KHCO3 on Na-K-ATPase and ouabain-insensitive Mg-ATPase activities in four nephron segments of adrenalectomized (ADX) rabbits. The segments examined were: the distal convoluted tubule (DCT), CNT, CCD and medullary collecting duct (MCD). All diets were similar in composition except their KHCO3 contents which were 100, 300, 500 and 700 meq/kg in groups 1 to 4 respectively. Increasing KHCO3 in the diet increased K+ excretion (7 X) and urine pH (6.6 to 8.3). Na-K-ATPase activity in the CCD increased greater than 200% as dietary KHCO3 was increased to 700 meq/kg. There was a linear relation between Na-K-ATPase activity in this segment and steady state plasma K+ as well as K+ excretion in the urine. However, Na-K-ATPase activity in the CCD was lower in KHCO3-fed ADX rabbits than the KCl-fed animals studied previously under similar conditions. There were no significant differences in Na-K-ATPase activities in DCT, CNT and MCD among the four groups given different KHCO3-diets. It is concluded that dietary intake of KHCO3 can also influence Na-K-ATPase activity in the CCD independent of aldosterone.

Role of aldosterone in the mechanism of renal potassium adaptation

Chronic potassium loading results in an adaptive change in renal tubular epithelium which increases the capacity for potassium excretion. The present study was performed to evaluate the role of aldosterone in renal potassium adaptation, since hyperaldosteronism stimulates potassium secretion, and potassium loading increases the production of aldosterone. Experiments were performed in animals with intact adrenal glands, and in adrenalectomized animals (Adx) replaced with basal physiologic amounts of corticosterone, which were not replaced with aldosterone, or were chronically infused with aldosterone to achieve either basal plasma levels or elevated levels. Chronic potassium loading in adrenal intact animals was associated with a statistically significant higher rate of urinary potassium excretion (3.57 +/- 0.30 microEq/min/100 g BW) compared to the control rate (2.54 +/- 0.25 microEq/min/100 g BW, p less than 0.05), during acute infusion of KCl. In potassium loaded Adx animals, with selective replacement of adrenal hormones, the maximum rate of potassium excretion was blunted in the absence of aldosterone, compared to potassium loaded animals with intact adrenal glands. In contrast, when Adx animals were infused chronically with aldosterone, to achieve basal or elevated plasma levels, the maximum rate of potassium excretion was not blunted, although at basal aldosterone levels increased potassium excretion was due, at least in part, to hyperkalemia. These results indicate that the induction of renal potassium adaptation after a week or more of chronic potassium loading is dependent on the action of hyperaldosteronism on renal tubular epithelium.

Erythrocyte Na(+)-K+ ATPase activity in the genesis of reducing renal mass in hypertensive rats

To study the relationship between natriuretic factor and experimental low renin hypertension, the erythrocyte Na⁺-K⁺ ATPase activities in the genesis of reducing renal mass in hypertensive rats were measured. The results were as follows : 1. The fractional excretion of Na in hypertensive, reduced renal mass rats (saline-drinking) was higher than that of normotensive, reduced renal mass rats (water-drinking) (p<.001). The plasma renin activities in the hypertensive group were markedly lower than those of the normotensive group (p<.001). 2. The total ATPase activities of rat erythrocyte membrane in the hypertensive group were lower than in those of the normal and the normotensive groups (p<.001, p<.02). 3. The Mg⁺⁺-ATPase activities of rat erythrocyte membrane in the normotensive group tended to decrease significantly (p<.001), but the differences between the normotensive and hypertensive group were not significant. 4. The Na⁺-K⁺ ATPase activities of rat erythrocyte membrane in the hypertensive group were lower than those of the normal and the normotensive group (p<.001). However, the difference between the normal and the normotensive groups was not significant. 5. When erythrocyte membrane taken from normal rats was incubated with supernates of boiled plasma from normotensive rats, the total ATPase activities of erythrocyte membrane were not different from those of hypertensive rats. 6. When erythrocyte membrane taken from normal rats was incubated with supernates of boiled plasma from normotensive rats, the Mg⁺⁺ ATPase activities of erythrocyte membrane were lower than those of hypertensive rats (p<.01). 7. When erythrocyte membrane taken from normal rats was incubated with supernates of boiled plasma from hypertensive rats, the Na⁺-K⁺ ATPase activities of erythrocyte membrane were lower than those of normotensive rats (p<.01). Based on the above findings, it is suggested that the pathogenesis of low renin, reduced renal mass hypertension is primarily mediated by a natriuretic factor (or an ouabain-like factor, inhibitor of Na⁺-K⁺ ATPase) produced by extracellular fluid volume expansion.

Renal adaptation to potassium in the adrenalectomized rabbit. Role of distal tubular sodium-potassium adenosine triphosphatase

Potassium secretion and sodium-potassium adenosine triphosphatase (Na-K-ATPase) activity in the distal nephron segments are known to be influenced by the dietary intake of K+. This has been attributed to a change in the plasma aldosterone level, which also influences K+ secretion and Na-K-ATPase activity in the distal nephron. To investigate whether or not dietary K+ can modulate Na-K-ATPase activity in the distal nephron independently of aldosterone, we determined Na-K-ATPase activity in four distinct nephron segments of adrenalectomized (adx) rabbits given four specific diets for 1 wk before experimentation. Na-K-ATPase activity was determined by a fluorometric microassay in which ATP hydrolysis is coupled to NADH oxidation. The nephron segments examined were the distal convoluted tubule (DCT), the connecting tubule (CNT), the cortical collecting duct (CCD), and the outer medullary collecting duct (MCD). All diets were similar in composition except for their K+ contents, which were 100, 300, 500, and 700 meq/kg in groups 1-4, respectively. In these adx animals, Na-K-ATPase activity increased greater than 200% in the CCD as the dietary intake of K+ increased. There was a linear relationship between K+ excretion and the enzyme activity in this segment. There was a 50% increase in Na-K-ATPase activity in the CNT as the dietary intake of K+ increased in adx animals. However, there were no significant differences in Na-K-ATPase activities in the DCT and MCD among the four treatment groups. It is concluded that dietary K+ intake can influence Na-K-ATPase activity in the CCD and CNT independently of plasma aldosterone levels.

Sodium-dependent modulation of the renal Na-K-ATPase: influence of mineralocorticoids on the cortical collecting duct

Mineralocorticoids play a major role in the regulation of sodium transport in a variety of tissues, including the cortical collecting duct (CCD) of the mammalian nephron. To assess, in part, the underlying mechanism(s) of this control, the present studies were designed to evaluate, first, the influence of mineralocorticoids on the Na-K-ATPase activity in the rabbit CCD and, secondly, a possible role of sodium entry into the cell at the luminal border on the regulation of the Na-K-ATPase. In the first series of studies, rabbits were maintained on a low sodium diet which raised serum aldosterone levels from 16 to 70 ng/dl after 3-4 days, with further elevations being expressed with treatment for two weeks or more. In CCDs isolated from these animals, the Na-K-ATPase increased from 13 to 40 pmol ADP min-1 mm-1 after 3-4 days on the low sodium regimen, but then declined, returning to control values after approximately 2 weeks. This decline in activity was preceded by a decrease in the Na+ concentration of the urine to low levels and hence, likely coincided with a decreased delivery of sodium to, and sodium entry into the cells of, the CCD. If dietary manipulations were used to maintain a high delivery of sodium to the CCD in the animal, elevation of plasma mineralocorticoid levels by treatment with deoxycorticosterone acetate (DOCA) caused a similar elevation in the Na-K-ATPase activity after 3-4 days, which did not decline with continued treatment for up to 2 weeks. Furthermore, it was observed that mineralocorticoids only exerted their effect on the Na-K-ATPase after a latent period of 1 day, well after sodium excretion had fallen, indicating that sodium entry into the CCD cells was already stimulated. If animals were simultaneously treated with DOCA and the sodium channel blocker amiloride for 3-4 days, the effects on the Na-K-ATPase were markedly reduced, whereas amiloride treatment alone had no effect on the enzyme activity. Since others have shown that mineralocorticoids induce synthesis of the Na-K-ATPase subunits in toad bladder cells in an amiloride-insensitive manner, sodium must be exerting its effect on a process after translation. It is concluded that the initial effect of mineralocorticoids in the CCD is on sodium entry with a delayed induction of the Na-K-ATPase, which is regulated by Na-dependent modulation of a posttranslational process.

Sodium potassium ATPase activity in human rectal mucosa with and without renal insufficiency

Studies in rats have shown that fecal potassium excretion and colonic mucosa Na-K-ATPase activity are elevated during dietary potassium loading and in chronic renal insufficiency. We studied Na-K-ATPase activity in human rectal mucosa in normal subjects as well as in patients with chronic renal insufficiency (creatinine clearance 2 to 72 mL/min). In normals, Na-K-ATPase activity was 4.34 +/- 0.83 mumol P/mg protein. After 2 weeks on a potassium intake of 300 mmol/d the mean activity did not differ significantly from the control value (2.49 +/- 1.30). In none of the patients with renal failure was Na-K-ATPase activity beyond the range found in the normal subjects, irrespective of serum potassium; the mean activity was 3.50 +/- 0.85. Like others, however, we found a two-fold increase in Na-K-ATPase activity in potassium loaded rats. Possible explanations for these differences are discussed.

Mechanism of intestinal adaptation in rats with acute renal failure

Acute renal failure (ARF) was experimentally induced in rats and the specific activity of mucosal Na-K-ATPase activity in segments of the small intestine and colon was measured. Bilateral nephrectomy (BN) resulted in a significant evaluation of the enzyme activity in all segments examined. With an additional procedure of adrenalectomy (BN + Ax), the enzyme activity failed to show any increase in ARF rats produced by BN. However, a supplementation of a maintenance dose of dexamethasone to adrenalectomized ARF rats (BN + Ax + DM 10) resulted in a significant resumption of the activity in all intestinal segments, although its increase was insignificant in the duodenum. Addition of a high dose of DOCA (BN + Ax + DOCA 500) was effective in increasing the enzyme activity only in the colon but not in the small intestine. With a high dose of DM or a maintenance dose of DM plus a high dose of DOCA (BN + Ax + DM 30 or BN + Ax + DM 10 + DOCA 500), there was an increase in the enzyme activity of all intestinal segments. In ARF rats induced by bilateral lower ureteral ligation (BLUL), the enzyme activity did not show any increase at all. Addition of a high dose of DOCA to this animal model (BLUL + DOCA 500) brought about the increase of the enzyme activity in the intestinal segments but for the jejunum.(ABSTRACT TRUNCATED AT 250 WORDS)

The intestinal profile of Na-K-ATPase in three rat models of acute renal failure

The specific activity of mucosal Na-K-ATPase in segments of the small intestine and colon was examined after bilateral nephrectomies (BN), bilateral upper ureteral ligations (BUUL) and bilateral lower ureteral ligations (BLUL). Animals were studied 22-26 h after these respective operations. Bilateral nephrectomies and bilateral upper ureteral ligations resulted in an increase of the specific activity of the enzyme throughout the mucosa of the intestinal tract. Bilateral lower ureteral ligations induced no significant change in the specific activity of Na-K-ATPase in the intestinal mucosa. There were no differences in the degree of renal failure. Marked aldosteronemia was observed in the BN and the BUUL rats but not in the BLUL rats. These data suggest that the increase in the intestinal Na-K-ATPase activity in the BN and the BUUL rats may be related to the elevation of serum aldosterone as a regulator of the body potassium.

[Primary hypoaldosteronism and secondary pseudo-hypoaldosteronism]

We observed a 23-year-old man with pronounced hyperkalemia (max. 6.8 mmol/l) and hyponatremia (min. 112 mmol/l), which had been existent for 3 years without complaint except a transitory psychorganic syndrome due to hyponatremia. Physical examination showed no abnormality except hypotension (blood pressure 100/70 mmHg). Renal function tests were normal. Fractional clearance of sodium was significantly increased (0.8%), whereas that of potassium was decreased (2.4%). Plasma renin activity was tripled and rose after furosemide. Plasma aldosterone was lowered and showed no rise after furosemide. Suppression of plasma renin and aldosterone by saline infusion was normal. Pressor dose of angiotensin II was increased (17,9 ng AT II/kg/min). Urinary excretion of aldosterone and its conjugates was below normal, and aldosterone precursors were within normal range. The findings were interpreted as selective primary hypoaldosteronism caused by corticosterone methyl oxidase defect type II. However, neither fludrocortisone (0.5 mg/day) nor sodium chloride (200 mmol/day) led to a normalization of sodium and potassium in plasma. Additional pseudohypoaldosteronism was thus assumed. Aldosterone infusion (3 mg in 1 h) decreased renal excretion of sodium; potassium excretion failed, however, to increase in contrast to its pattern in normal man. These findings resemble additional pseudohypo-aldosteronism of type II. After 8 weeks' application of additional 80 mmol sodium (as sodium bicarbonate) plasma sodium and potassium showed normal values under combined treatment with fludrocortisone (0.1 mg/day) and sodium bicarbonate (80 mmol/day). It is to be assumed that the patient suffers from a reduced aldosterone biosynthesis in the presence of an additional transitory secondary pseudohypoaldosteronism.

Impaired renal and extrarenal potassium adaptation in old rats

Young (3 to 4 months) and old (21 to 22 months) rats were fed either a regular or high potassium (K) diet. After acute potassium chloride infusion, the fraction of infused K excreted (K efficiency) was similar in rats on a normal diet (57 +/- 3%, young, vs. 61 +/- 2%, old). With high K feeding there was a significant increase in the young, 69 +/- 4%, but not in the old rats, 62 +/- 2%. Na-K ATPase activity was markedly reduced in the renal medulla of old rats on a regular or high K diet. In addition, the response to acute K loading was compared in acutely nephrectomized rats. In the young rats on a regular diet plasma K increased from 3.72 +/- 0.09 to 5.28 +/- 0.16 mEq/liter while with K ingestion the increase was significantly less, 3.62 +/- 0.07 to 4.75 +/- 0.12 mEq/liter. In the old rats plasma K increased similarly on a regular or high K diet, 3.68 +/- 0.10 to 5.68 +/- 0.33 mEq/liter and 3.76 +/- 0.06 to 5.97 +/- 0.30 mEq/liter, respectively. Thus, old rats have impaired renal and extrarenal adaptation, but they have a normal response to an acute K challenge. A reduction in Na-K ATPase may account for the defect in renal adaptation in the aged rats.

Effect of low potassium-diet on Na-K-ATPase in rat nephron segments

Na-K-ATPase activity was determined in 10 segments of the rat nephron using a fluorometric microassay method [4]. The enzyme activity showed three peaks (greater than 200 pmol ADP min-1 mm-1) along the nephron of normal rats. These peaks were in the S1 portion of the proximal tubule, the medullary thick ascending limb from the inner stripe and the distal convoluted tubule. Feeding the rats a low potassium diet for 8 weeks produced a significant decrease in Na-K-ATPase activity in the cortical collecting duct, but no significant change in this enzyme in any other segment. The low potassium diet did not produce a significant change in Mg-ATPase in any nephron segments. We conclude that Na-K-ATPase activity along the rat nephron shows a pattern that is qualitatively similar to that seen in the rabbit nephron [4]. However, quantitatively the Na-K-ATPase activity in the rat nephron is greater than in the corresponding segments of the rabbit nephron. The results are consistent with the greater rate of glomerular filtration and Na+ reabsorption per rat nephron. Furthermore, our results suggest that the decrease in potassium excretion during potassium deficiency is modulated, at least in part, by the level of Na-K-ATPase activity in the cortical collecting duct.

Distal tubular segments of the rabbit kidney after adaptation to altered Na- and K-intake. II. Changes in Na-K-ATPase activity

Na-K-ATPase activity was measured in the convoluted part of the distal tubule (DCT), the connecting tubule (CNT) and the cortical collecting duct (CCD). The segments were microdissected from freeze-dried kidney tissue of rabbits adapted to various salt diets and exposed to large differences in endogenous and exogeneous mineralocorticoids. The Na-K-ATPase activity in the DCT is not influenced by mineralocorticoids. They do influence the activity in the CNT and in the CCD. In the CNT the highest activity was found with a low Na-, high K-diet. At the beginning of the CNT the enzyme activity is higher than in the end portion. While canrenoate-K treatment has no effect on Na-K-ATPase activity in the initial portions of the CNT, this drug decreases the Na-K-ATPase activity significantly in the end portion of the CNT. DOCA treatment has a significant effect on the enzyme activity in the CNT only in the end-portion of the segment, but provokes the highest Na-K-ATPase activity in the CCD. The changes in Na-K-ATPase are found to be associated with corresponding changes in the baso-lateral cell-membrane area in the segments affected.

Disorders of distal nephron function

In this review, the distal nephron is considered to be that portion of the renal tubule commencing with the thick ascending limb of the loop of Henle and ending with the papillary collecting duct. The collecting duct, including its subdivisions in the cortex and medulla, originates from a different embryologic anlage than more proximal nephron segments, which may explain its morphologic and functional dissimilarities from the thick ascending limb and the distal convoluted tubule. This review summarizes selected aspects of the physiology of the distal nephron, with particular emphasis on the physiology of distal nephron transport of sodium, potassium, chloride and hydrogen ion. The pathophysiologic features of the following disorders of distal nephron function are reviewed: (1) pseudohypoaldosteronism, a heterogenous group of disorders in which the signs and symptoms are suggestive of aldosterone deficiency, but in which aldosterone levels are supernormal and administration of exogenous mineralocorticoid is not ameliorative; (2) pseudohyperaldosteronism (Liddle syndrome), a familial disorder in which the clinical manifestations closely resemble those resulting from an aldosterone-producing adenoma of the adrenal gland (primary aldosteronism), but in which the measured rate of aldosterone secretion and excretion is greatly subnormal; (3) Bartter syndrome and related syndromes of renal potassium wasting; (4) type 1 renal tubular acidosis (classic, distal); (5) type 4 renal tubular acidosis (hyperkalemic). Reference citations are generally to articles reporting recent advances in these areas and to review articles that contain comprehensive bibliographies.

Renal Na+-K+-ATPase in Okamoto and Dahl hypertensive rats

Plasma renin activity (PRA, ng AI/ml/hr), plasma aldosterone (PA, ng%) and renal Na+-K+-ATPase (micron m PO 4/mg protein/hr) were measured in tow groups of eight spontaneously hypertensive rats (SHR), two groups of eight Dahl salt hypertensive rats (SS), and their controls (16 normal Wistar and 16 salt-resistant rats). Measurements were made in one group after 2 weeks on a normal (0.48% sodium) and in the other group after 2 weeks on a low (0.01% sodium) sodium diet. After a normal sodium diet, PRA and PA were lower in both groups of hypertensive rats than in control normotensive animals. Renal NA+-K+-ATPase was lower in SS than in controls: in SHR it was not different from control. On a sodium-free diet, SHR exhibited a rise in renal Na+-K+-ATPase but PRA and PA remained low. In contrast, under similar conditions PRA, PA, and renal Na+-K-ATPase increased in SS rats, although to a lesser extent than in SR. These results suggest that under basal conditions and after low salt diet, renal Na+-K+-ATPase activity in SHR behaves as it does in normal rats. However, the changes are independent of PA in SHR. The reduction in PRA and PA in SS suggest volume expansion hypertension. IN SHR, volume expansion is not present, and renal Na+-K+-ATPase is not altered. Enzyme activity is lower in SS than in SHR and control. This suggests that some factor that results from volume expansion may be responsible for inhibition of renal Na+-K+-ATPase.

Morphologic alterations in the rat medullary collecting duct following potassium depletion

Freeze-fracture and thin-section electron microscopy and morphometry were used to characterize further the response of the rat medullary collecting duct to potassium depletion. In freeze-fracture replicas, principal cells and intercalated cells were identified based on the assumption that intercalated cells possess a high density of rod-shaped intramembrane particles in their luminal membranes. Potassium depletion caused an increase in the relative number of cells with a high density of rod-shaped particles from the control level of 22% to 31% after 2 weeks and to 36% after 4 weeks. The frequency of intercalated cells identified by thin-section criteria was, however, about 35% in controls and unchanged by potassium depletion. This suggests that intercalated cells can have two types of membrane morphology. In potassium depletion, all intercalated cells display a high density of rod-shaped particles in their luminal membranes. In addition, the luminal membrane area of intercalated cells increased more than threefold, and the density of their rod-shaped particles increased by 21%. These observations suggest that the intercalated cell and its rod-shaped particle may be involved with the potassium reabsorption that occurs in this nephron segment with potassium depletion.

Functional profile of the isolated uremic nephron: potassium adaptation in the rabbit cortical collecting tubule

As a renal function declines in patients and experimental animals with chronic renal disease, potassium homeostasis is maintained by a progressive increase in potassium secretion by the surviving nephrons, a phenomenon known as potassium adaptation. To determine the nephron site and the underlying mechanisms responsible for this phenomenon, studies were performed on normal and 75% nephrectomized rabbits maintained on normal or high-potassium diets. Cortical collecting tubules (CCT) were dissected from the normal and remnant kidneys and perfused in vitro in an artificial solution. In normal CCT mean (+/- SE) net K secretion, JK, (peq/cm per s) was 1.26 +/- 0.43 (normal diet) and 3.27 +/- 0.66 (high-K diet). In uremic CCT, JK was 3.55 +/- 0.60 (normal diet) and 6.83 +/- 0.58 (high-K diet). By reducing the dietary intake of potassium in proportion to the reduction of renal mass in these uremic animals, the adaptation in K secretion was prevented (JK: 1.22 +/- 0.40). Transepithelial potential difference was similar in CCT from normal and uremic animals on a normal diet despite the fact that JK was significantly greater in the latter group. However, in both normal and uremic CCT, the increase in JK caused by potassium loading was associated with an increase in luminal negativity. Uremic CCT underwent significant compensatory hypertrophy regardless of the dietary intake or potassium secretory rates. Plasma aldosterone levels were elevated only in the uremic-high potassium rabbits suggesting that a mineralocorticoid effect on the CCT may be exaggerated when potassium loading is superimposed upon decreased excretory capacity. The activity of Na-K ATPase was comparable in normal and uremic CCT from rabbits on either normal or high-K diets indicating that potassium adaptation may occur independently of changes in the activity of this enzyme. Intracellular potassium content measured chemically and by 42K exchange, was not significantly altered in either normal or uremic CCT when dietary potassium intake was increased, despite the fact the JK was increased under these circumstances. These data indicate that the CCT is an important site of potassium adaptation in the surviving nephrons of animals with reduced renal mass. This adaptation is an intrinsic property of the CCT and is expressed in the absence of a uremic milieu. Potassium adaptation by the uremic CCT is not fixed according to the degree of compensatory hypertrophy but varies according to the excretory requirements of the animal. Transepithelial potential difference and circulating aldosterone levels contribute to the adaptation but neither factor can entirely account for the phenomenon. Potassium adaptation by the CCT occurs in the absence of changes in Na-K ATPase activity and intracellular potassium content.

Sodium transport inhibitor from bovine hypothalamus

A low molecular weight, basic, nonpeptidic factor that possesses sodium transport inhibitory properties has been prepared from bovine hypothalamus by acid/acetone extraction and gel filtration. Concentration and partial purification was achieved by ion-exchange chromatography. This substance inhibits active Na+ transport across anuran membranes, inhibits ouabain binding to frog urinary bladder, and directly inhibits renal Na+,K+-ATPase. This substance thus possesses the putative characteristics of a natriuretic factor of hypothalamic origin.

Mechanism of renal potassium conservation in the rat

The mechanisms responsible for renal potassium (K) conservation during dietary potassium deficiency are poorly understood. This study was undertaken to investigate the time course of potassium conservation as well as the roles of distal sodium (Na) delivery, the distal delivery or sodium plus a nonpermeable anion, mineralocorticoid hormone, renal tissue potassium content, and Na-K-ATPase activity in renal potassium conservation. After 72 hours of a low-potassium diet, basal potassium excretion was negligible. After 24 hours, and even more so after 72 hours of potassium restriction, the kaliuretic response to increasing distal delivery of sodium or sodium plus a nonpermeable anion was impaired. After 24 hours of a low-potassium diet, plasma aldosterone levels fell from 180 +/- 25 to 32 +/- 9 pg/ml (P less than 0.001). Mineralocorticoid hormone given in the first 24 hours of a low-potassium diet resulted in a greater potassium loss (1564 +/- 125 muEq) than it did in controls on the same diet not receiving mineralocorticoid hormone (1032 +/- 83 muEq, P less than 0.005). In contrast, after 72 hours of diet, large doses of mineralocorticoid hormone failed to cause a kaliuresis in either anesthetized or conscious rats. After both 24 and 72 hours, outer medullary Na-K-ATPase was increased. At 72 hours, cortical, medullary, and papillary tissue potassium concentrations were significantly depressed. Acute administration of potassium repleted tissue potassium levels and restored basal and saline-stimulated potassium excretion to normal. Although potassium excretion was markedly depressed after 24 hours of the low-potassium diet, 42K microinjection studies of the distal nephron did not suggest any increase in potassium reabsorption. Following 72 hours of diet, potassium reabsorption increased significantly from 26 +/- 2% to 41 +/- 2% (P less than 0.001). We conclude that renal potassium conservation is at first primarily related to a decrease in potassium secretion, which is most likely mediated by falling levels of mineralocorticoid hormone. After 72 hours of the potassium-deficient diet, however, potassium conservation becomes independent of mineralocorticoid hormone, distal delivery of sodium, and Na-K-ATPase. The decreased tissue potassium content appears to be the primary mediator of both the increase in potassium reabsorption by the distal nephron and of renal potassium conservation at this time.