The regulation of renal acid secretion: new observations from studies of distal nephron segments

On the pleotropic actions of mineralocorticoids

Classical effects of mineralocorticoids include stimulation of Na(+) reabsorption and K(+) secretion in the kidney and other epithelia including colon and several glands. Moreover, mineralocorticoids enhance the excretion of Mg(2+) and renal tubular H(+) secretion. The renal salt retention following mineralocorticoid excess leads to extracellular volume expansion and hypertension. The increase of blood pressure following mineralocorticoid excess is, however, not only the result of volume expansion but may result from stiff endothelial cell syndrome impairing the release of vasodilating nitric oxide. Beyond that, mineralocorticoids are involved in the regulation of a wide variety of further functions, including cardiac fibrosis, platelet activation, neuronal function and survival, inflammation as well as vascular and tissue fibrosis and calcification. Those functions are briefly discussed in this short introduction to the special issue. Beyond that, further contributions of this special issue amplify on mineralocorticoid-induced sodium appetite and renal salt retention, the role of mineralocorticoids in the regulation of acid-base balance, the involvement of aldosterone and its receptors in major depression, the mineralocorticoid stimulation of inflammation and tissue fibrosis and the effect of aldosterone on osteoinductive signaling and vascular calcification. Clearly, still much is to be learned about the various ramifications of mineralocorticoid-sensitive physiology and pathophysiology.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Control of potassium excretion: a Paleolithic perspective

PURPOSE OF REVIEW

Regulation of potassium (K) excretion was examined in an experimental setting that reflects the dietary conditions for humans in Paleolithic times (high, episodic intake of K with organic anions; low intake of NaCl), because this is when major control mechanisms were likely to have developed.

RECENT FINDINGS

The major control of K secretion in this setting is to regulate the number of luminal K channels in the cortical collecting duct. Following a KCl load, the K concentration in the medullary interstitial compartment rose; the likely source of this medullary K was its absorption by the H/K-ATPase in the inner medullary collecting duct. As a result of the higher medullary K concentration, the absorption of Na and Cl was inhibited in the loop of Henle, and this led to an increased distal delivery of a sufficient quantity of Na to raise K excretion markedly, while avoiding a large natriuresis. In addition, because K in the diet was accompanied by 'future' bicarbonate, a role for bicarbonate in the control of K secretion via 'selecting' whether aldosterone would be a NaCl-conserving or a kaliuretic hormone is discussed.

SUMMARY

This way of examining the control of K excretion provides new insights into clinical disorders with an abnormal plasma K concentration secondary to altered K excretion, and also into the pathophysiology of calcium-containing kidney stones.

Hyperkalemic distal renal tubular acidosis associated with Rett syndrome

Renal function was studied in a 7-year-old girl with Rett syndrome (RS) complicated by persistent hyperchloremic hyperkalemic metabolic acidosis. The acidosis was associated with a urine pH above 5.5, positive urinary anion gap and decreased potassium excretion. Plasma renin activity, aldosterone and cortisol levels were normal. Therapy with sodium bicarbonate failed to lower urine pH below 5.5 or increase potassium excretion. Hydrochlorothiazide administration resulted in a fall in urine pH below 5.5 and an increase in potassium excretion as a result of increased distal sodium delivery and increased sodium reabsorption in the distal nephron. We conclude that a voltage-dependent type of derangement in the distal nephron, rather than aldosterone deficiency, is responsible for the impairment in urinary acidification observed in this patient. Early detection of impaired renal acidification in RS may prevent or slow the progression of growth failure.

Dynamic interactions between integrative physiology and molecular medicine: The key to understand the mechanism of action of aldo sterone in the kidney

Our objective is to illustrate how an approach that integrates new insights from molecular biology and traditional physiology can lead to the development of new concepts. This dynamic interaction is illustrated by examining the steps taken to improve our understanding of the renal actions of aldosterone. We began by defining the big picture of what aldosterone does in the kidney. This led to the conclusion that aldosterone must at times become a sodium chloride-retaining hormone, while at other times it must function primarily or exclusively as a kaliuretic hormone. The second step was to define the major molecular actions of this hormone. Acting on the principal cells in the cortical collecting duct (CCD), aldosterone leads to the insertion of active epithelial sodium ion channels (ENaC) in their luminal membranes. This active ENaC, however, does not distinguish between the two major renal actions of aldosterone. Accordingly, we returned to integrative physiology and examined a possible role of renal and non-renal events. We implicated the potential importance of the delivery of bicarbonate ions to the CCD to determine which effect of aldosterone will become manifest. This, however, required that we reconsider some of the traditional views in interpretation of acid-base balance. At the clinical level, this global view can help us understand why, for example, a low dietary intake of potassium salts might predispose a person to an elevated blood pressure. Using a similar approach, it is possible to understand how the risk of the formation of kidney stones can be minimized.Key words: acid-base, hypertension, integrative physiology, kidney stones, potassium, sodium.

Prevalence and characterization of renal tubular acidosis in patients with osteopenia and osteoporosis and in non-porotic controls

BACKGROUND

Chronic metabolic acidosis may increase alkali mobilization from the bone and thus promote the development of osteoporosis. The objective of the current study was to compare urinary acidification in patients with reduced bone mineral content with that in control subjects with normal bone density.

METHODS

Forty-six subjects (41 females, 5 males) with osteopenia or osteoporosis were studied. In none of the subjects were overt metabolic acidosis, derangement of potassium homeostasis, or renal insufficiency present. Distal tubular acidification was studied by means of oral ammonium chloride loading test (0.1 g/kg body weight) and the oral frusemide test (40 mg). In addition the frusemide test was performed in 20 healthy age- and sex-matched controls (17 females, 3 males).

RESULTS

In all control subjects a urinary pH <5. 5 was observed following the ingestion of 40 mg frusemide. In contrast, in patients with reduced bone mineral density incomplete renal tubular acidosis type I (RTA I) was diagnosed in 10 of 46 subjects (22%) by oral ammonium chloride loading test. Disorders possibly related to RTA I were detected in eight of these 10 patients. Thirty-six patients had a normal urinary pH response following oral ammonium chloride loading. Oral frusemide, 40 mg, failed to lower urinary pH <5.5 in sixteen patients (35%), these included 10 subjects with incomplete RTA I, and six subjects with a normal oral ammonium chloride loading test. An abnormal frusemide test was found in 35% of patients with reduced bone mass and in none of the normal controls (chi(2)=7.39; P<0.01). With the ammonium chloride test as the gold standard for diagnosis of distal RTA, the frusemide test showed a sensitivity of 1.0 (95% CI, 0.69-1.0) and a specificity of 0.89 (95% CI, 0.78-0.96) for the diagnosis of distal RTA. Patients with incomplete RTA I were younger than those without incomplete RTA I (42+/-16 vs 54+/-14 years; P=0.025; mean+/-SD). Basal serum bicarbonate concentrations and capillary pH did not differ between the groups.

CONCLUSION

Incomplete RTA I may be prevalent in a significant proportion of patients suffering from osteopenia or osteoporosis. The outcome of the frusemide test suggests either a defect of the H(+)ATPase in the cortical collecting tubule (CCT) or a defective Na(+) reabsorption in the CCT. Prospective studies are needed to further elucidate the impact of incomplete RTA I on the development of reduced bone mineral content.

Animal plasma membrane energization by proton-motive V-ATPases

Proton-translocating, vacuolar-type ATPases, well known energizers of eukaryotic, vacuolar membranes, now emerge as energizers of many plasma membranes. Just as Na(+) gradients, imposed by Na(+)/K(+) ATPases, energize basolateral plasma membranes of epithelia, so voltage gradients, imposed by H(+) V-ATPases, energize apical plasma membranes. The energized membranes acidify or alkalinize compartments, absorb or secrete ions and fluids, and underwrite cellular homeostasis. V-ATPases acidify extracellular spaces of single cells such as phagocytes and osteoclasts and of polarized epithelia, such as vertebrate kidney and epididymis. They alkalinize extracellular spaces of lepidopteran midgut. V-ATPases energize fluid secretion by insect Malpighian tubules and fluid absorption by insect oocytes. They hyperpolarize external plasma membranes for Na(+) uptake by amphibian skin and fish gills. Indeed, it is likely that ion uptake by osmotically active membranes of all fresh water organisms is energized by V-ATPases. Awareness of plasma membrane energization by V-ATPases provides new perspectives for basic science and presents new opportunities for medicine and agriculture.

Dietary acid increases blood and renal cortical acid content in rats

We examined whether dietary acid that increases net acid excretion (NAE) without measurably decreasing plasma pH or total CO2 (tCO2) causes net acid retention. Control rats drinking distilled H2O were compared with those drinking 40 mM (NH4)2SO4, 40 mM Na2SO4, or drinking Na2SO4 and given aldosterone (Na2SO4 + Aldo) to increase NAE without dietary acid. Systemic plasma tCO2 increased in Na2SO4 + Aldo animals, but systemic and stellate vessel plasma tCO2 and pH were not different from control among remaining groups. NAE increased in (NH4)2SO4 and Na2SO4 + Aldo but not in Na2SO4 animals. Blood base excess (BBE) decreased compared with its respective baseline in (NH4)2SO4 (-0.44 +/- 0.06 vs. 0.66 +/- 0.04 mumol/ml; P < 0.01, paired t-test), increased in Na2SO4 + Aldo (0.79 +/- 0.05 vs. 0.61 +/- 0.03 mumol/ml; P < 0.04, paired t-test), but was unchanged in Na2SO4 animals. Renal cortical H+ content assessed by microdialysis of the renal cortex in situ increased in (NH4)2SO4, decreased in Na2SO4 + Aldo, but was unchanged in Na2SO4 animals. The data show that dietary acid sufficient to increase NAE without decreasing plasma tCO2 or pH nevertheless decreases BBE and increases renal cortical acid content, consistent with net acid retention.

Control of excretion of potassium: lessons from studies during prolonged total fasting in human subjects

A deficit of K+ of close to 300 mmol develops in the first 2 wk of fasting, but little further excretion of K+ occurs, despite high levels of aldosterone and the delivery of ketoacid anions that are not reabsorbed in the distal nephron. Our purpose was to evaluate how aldosterone could have primarily NaCl-retaining, rather than kaliuretic, properties in this setting. To evaluate the role of distal delivery of Na+, four fasted subjects received an acute infusion of NaCl to induce a natriuresis. To assess the role of distal delivery of HCO3-, five fasted subjects were given an infusion containing NaHCO3. The natriuresis induced by an infusion of NaCl caused only a small rise in the rate of excretion of K+ (0.8 +/- 0.1 to 1.9 +/- 0.3 mmol/h); in contrast, when HCO3- replaced Cl- in the infusate, K+ excretion rose to 8.3 +/- 2.2 mmol/h, despite little excretion of HCO3- (urine, pH 5.8) and similar rates of excretion of Na+. The transtubular K+ concentration gradient was 19 +/- 3 with HCO3- and 6 +/- 2 with NaCl. We conclude that the infusion of NaHCO3 led to an increase in K+ excretion, likely reflecting an increased rate of distal K+ secretion. With a low distal delivery of HCO3-, aldosterone acts as a NaCl-retaining, rather than a kaliuretic, hormone.

Endogenous endothelins mediate increased distal tubule acidification induced by dietary acid in rats

We examined if endogenous endothelins mediate the decreased HCO3 secretion and increased H+ secretion in in vivo-perfused distal tubules of rats fed dietary acid as (NH4)2SO4. Animals given (NH4)2SO4 drinking solution had higher endothelin-1 addition to renal interstitial fluid than those given distilled H2O (480+/-51 vs. 293+/-32 fmol g kidney wt(-1) min(-1), respectively, P < 0.03). (NH4)2SO4-ingesting animals infused with bosentan (10 mg/kg) to inhibit A- and B-type endothelin receptors had higher HCO3 secretion than baseline (NH4)2SO4 animals (-4.7+/-0.4 vs. -2.4+/-0.3 pmol mm(-1) min(-1), P < 0.01), but (NH4)2SO4 animals given a specific inhibitor of A-type endothelin receptors (BQ-123) did not (-2.0+/-0.2 pmol mm(-1) min(-1), P = NS vs. baseline). H+ secretion was lower in bosentan-infused compared with baseline (NH4)2SO4 animals (27.7+/-2.5 vs. 43.9+/-4.0 pmol mm(-1) min(-1), P < 0.03), but that for BQ-123-infused (NH4)2SO4 animals was not (42.9+/-4.2 pmol mm(-1) min(-1), P = NS vs. baseline). Bosentan had no effect on distal tubule HCO3 or H+ secretion in control animals. The data show that dietary acid increases endothelin-1 addition to renal interstitial fluid and that inhibition of B- but not A-type endothelin receptors blunts the decreased HCO3 secretion and increased H+ secretion in the distal tubule of animals given dietary acid. The data are consistent with endogenous endothelins as mediators of increased distal tubule acidification induced by dietary acid.

Distal urinary acidification from Homer Smith to the present

Since Smith's time, the essential role of collecting duct intercalated cells in controlling net acid excretion has been recognized. Rather than employing an H(+)-exchange mechanism, intercalated cells have V-ATPase on the plasma membrane and in plasmalemma-associated tubulovesicles, which functions in the bicarbonate reabsorption, regeneration, and bicarbonate secretion required for acid-base homeostasis. Several distinct mechanisms participate in regulating V-ATPase-driven H+ secretion in different cell types: (1) Renal epithelial cells have the capacity to express different structural forms of V-ATPase that have intrinsic differences in their enzymatic properties. 2) The kidney produces cytosolic regulatory proteins, capable of interacting directly with the V-ATPase, that may modify its activity. V-ATPases in different cell types may differ in the degree to which their activity is affected by regulatory factors, as a result of variations in V-ATPase structure. (3) In the alpha intercalated cell, the number of active V-ATPases on the luminal membrane is controlled in vivo by membrane vesicle-mediated traffic that may require unidentified mediators. In the beta intercalated cell, the number of active V-ATPases on the basolateral membrane may be controlled by regulated assembly and disassembly, responding directly to extracellular pH.

Renal bicarbonate reabsorption in the rat. IV. Bicarbonate transport mechanisms in the early and late distal tubule

Bicarbonate transport was studied in vivo by separate microperfusion experiments of early and late distal tubules. Total CO2 was measured by microcalorimetry and fluid absorption by 3H-inulin. Significant bicarbonate absorption was observed in all experimental conditions. Bicarbonate transport was load-dependent upon increasing the luminal bicarbonate concentration from 15 to 50 mM in both early and late distal tubule segments and remained constant at higher concentrations at a maximum rate of 100-110 pmol/min per mm. At low lumen bicarbonate concentrations (15 mM), higher rates of bicarbonate absorption were observed in early (32.9 +/- 4.57 pmol/min per mm) as compared to late distal tubules (10.7 +/- 3.1 pmol/min per mm). Amiloride and ethyl-isopropylamiloride both inhibited early but not late distal tubule bicarbonate absorption whereas acetazolamide blocked bicarbonate transport in both tubule segments. Fluid absorption was significantly reduced in both tubule segments by amiloride but only in early distal tubules by ethyl-isopropylamiloride. Substitution of lumen chloride by gluconate increased bicarbonate absorption in late but not in early distal tubules. Bafilomycin A1, an inhibitor of H-ATPase, inhibited late and also early distal tubule bicarbonate absorption, the latter at higher concentration. After 8 d on a low K diet, bicarbonate absorption increased significantly in both early and late distal tubules. Schering compound 28080, a potent H-K ATPase inhibitor, completely blocked this increment of bicarbonate absorption in late but not in early distal tubule. The data suggest bicarbonate absorption via Na(+)-H+ exchange and H-ATPase in early, but only by amiloride-insensitive H+ secretion (H-ATPase) in late distal tubules. The study also provides evidence for activation of K(+)-H+ exchange in late distal tubules of K depleted rats. Indirect evidence implies a component of chloride-dependent bicarbonate secretion in late distal tubules and suggests that net bicarbonate transport at this site results from bidirectional bicarbonate movement.

Selective effect of mineralocorticoid replacement therapy on renal acid excretion in congenital adrenal hyperplasia

The renal acid excretion of eight children with salt-losing congenital adrenal hyperplasia, was studied in three different situations: before treatment (period I), under glucocorticoid therapy (period II) and when both glucocorticoid and mineralocorticoid were given as replacement treatment (period III). Although administration of glucocorticoid therapy alone allowed the correction of acidemia, normalization of urinary net acid excretion was achieved only after mineralocorticoid was added to the treatment.

Increased sodium transport by cortical collecting tubules from remnant kidneys

To determine whether intrinsic changes in cortical collecting tubule (CCT) transport contribute to the maintenance of sodium and acid-base balance after loss of renal mass, we studied transport functions in isolated perfused CCT from rabbit remnant kidneys. The rabbits were sacrificed three weeks after surgical reduction of renal mass (by 3/4 to 7/8) at which time they were mildly azotemic but had no systemic electrolyte or acid-base disturbances. When perfused by standard methods in vitro, CCT from remnant kidneys exhibited sodium transport rates (lumen-to-bath 22Na-flux) approximately twice as high as those in CCT from control animals (111 +/- 19 vs. 54 +/- 7 pmol/min mm, P less than 0.02). A similar difference was present in the ouabain-sensitive sodium fluxes (81 +/- 16 vs. 39 +/- 8 pmol/min mm, P less than 0.05). In contrast, there were no significant differences in net bicarbonate transport. Significant hypertrophy of the remnant kidney CCT was reflected by 30 to 45% increases in tubule diameters. To examine the possible role of differences in food intake, we studied a separate group of weight-matched, pair-fed sham-operated and remnant kidney rabbits. Similar differences in total and ouabain-sensitive 22Na-flux, and in tubule size persisted in the pair-fed animals. A dissociation between active sodium transport and tubule hypertrophy was documented in the outer medullary collecting tubule: despite the lack of active sodium transport, hypertrophy was present. Our studies show that loss of renal mass results in a selective augmentation of certain transport processes in the CCT, implying selective or specific signals and mechanisms.

Secretion of bicarbonate by rat distal tubules in vivo. Modulation by overnight fasting

We have performed microperfusion studies on distal tubule bicarbonate reabsorption (JtCO2) of fed and fasted rats to extend our previous observations of in vivo bicarbonate secretion and to resolve certain discrepancies between free-flow and microperfusion data. When rats are fasted overnight, as in previous free-flow studies, distal tubule microperfusion with a 28-mM tCO2 solution results in significant JtCO2 (53 +/- 6 pmol.min-1.mm-1) at normal flow and increases briskly (91 +/- 16 pmol.min-1.mm-1) with bicarbonate load. This response is not influenced by the addition of other normal tubular fluid constituents. However, when normally fed rats are used, as in our previous microperfusion studies, distal tubule JtCO2 is not different from zero when a 28-mM tCO2 solution is perfused at normal flow rates but becomes negative (-54 +/- 13 pmol.min-1.mm-1) at high flow rates, which indicates the existence of bicarbonate secretion against a concentration gradient. Alkali loading of fasted rats also elicits bicarbonate secretion at high flow. These results demonstrate for the first time that normal feeding or alkali loading can induce bicarbonate secretion in a mammalian nephron segment in vivo, and resolves previous discrepancies between free-flow and microperfusion data.

Rat urinary kallikrein localization in kidney: effects of fixation

The influence of fixation on the immunocytochemical localization of tissue kallikrein in the kidney has been evaluated using both monoclonal and polyclonal antibodies. These studies have provided several results relevant to kallikrein localization in kidney: (1) the intensity and distribution of immunostaining with both polyclonal and monoclonal anti-kallikrein antibodies is fixation-dependent; (2) the most intense and consistent localizations of kallikrein are in the connecting tubule and the cortical collecting duct of the nephron; (3) kallikrein-like immunoreactivity is seen in proximal tubules with polyclonal but not with non-cross-reactive monoclonal antibodies; and (4) fixatives which disrupt membranes reveal a kallikrein-like antigen in straight tubules of the outer medulla. However, immunostaining with monoclonal antibodies indicates that much of the observed immunostaining at this site probably represents cross-reactivity with another member of the kallikrein family of enzymes.