Active Sodium Reabsorption Research Articles

A concise review on analytical profile of chlorthalidone

Chlorthalidone (CHL) is an anti-hypertensive that reduces active sodium reabsorption and peripheral vascular resistance. Also, it is a diuretic commonly known as a thiazide diuretic. Due to the huge amount of use of CHL in different dosage forms for the management of hypertension, it needs qualitative and quantitative estimation in the pharmaceutical and biological formulation. Therefore, the main objective of this analysis of CHL in the pharmaceutical and biological formulation is in both qualitative and quantitative terms. In this review article, we have summarized UV/Vis Spectroscopy, high-performance liquid chromatography (HPLC), High-performance thin-layer chromatography (HPTLC), Ultra performance liquid chromatography (UPLC), etc. based methods for estimation of chlorthalidone. In addition to that, we have discussed the bioanalytical methods for CHL analysis. In conclusion, this review article will help to research scholars for further method development for drug estimation in pharmaceutical dosage forms and biological fluids.

Magnesium Handling in the Kidney.

Magnesium is a divalent cation that fills essential roles as regulator and cofactor in a variety of biological pathways, and maintenance of magnesium balance is vital to human health. The kidney, in concert with the intestine, has an important role in maintaining magnesium homeostasis. Although micropuncture and microperfusion studies in the mammalian nephron have shone a light on magnesium handling in the various nephron segments, much of what we know about the protein mediators of magnesium handling in the kidney have come from more recent genetic studies. In the proximal tubule and thick ascending limb, magnesium reabsorption is believed to occur primarily through the paracellular shunt pathway, which ultimately depends on the electrochemical gradient setup by active sodium reabsorption. In the distal convoluted tubule, magnesium transport is transcellular, although magnesium reabsorption also appears to be related to active sodium reabsorption in this segment. In addition, evidence suggests that magnesium transport is highly regulated, although a specific hormonal regulator of extracellular magnesium has yet to be identified.

Role of Na+/K+-ATPase in Natriuretic Effect of Prolactin in a Model of Cholestasis of Pregnancy.

Participation of Na+/K+-ATPase in the natriuretic effect of prolactin in a cholestasis of pregnancy model was investigated. The Na+/K+-ATPase activity in rat kidney medulla, where active sodium reabsorption occurs, decreased in the model of cholestasis of pregnancy and other hyperprolactinemia types compared with intact animals. This effect was not connected with the protein level of α1- and β-subunits of Na+/K+-ATPase measured by Western blotting in the kidney medulla. Decrease in Na+/K+-ATPase activity in the kidney cortex was not significant, as well as decrease in the quantity of mRNA and proteins of the α1- and β-subunits of Na+/K+-ATPase. There were no correlations between the Na+/K+-ATPase activity and sodium clearance, although sodium clearance increased significantly in the model of cholestasis of pregnancy and other hyperprolactinemia groups under conditions of stable glomerular filtration rate measured by creatinine clearance. We conclude that the Na+/K+-ATPase is not the only mediator of the natriuretic effect of prolactin in the model of cholestasis of pregnancy.

Danno Renale Acuto da Mezzi Iodati di Contrasto. Fisiopatologia e Suggerimenti di Prevenzione

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ENaC mediates human extravillous trophblast cell line (HTR8/SVneo) invasion by regulating levels of matrix metalloproteinase 2 (MMP2)

IntroductionPlacenta dysfunction is thought to be the major etiological factor related to preeclampsia. The epithelial sodium channel (ENaC) has been localized in the apical plasma membrane of epithelia, mediating the active reabsorption of sodium in kidney, and be involved in the regulation of blood pressure. In previous studies, we found that the reduced expression of ENaC on placenta in preeclampsia patients. The aim of this study was to determine the role of MMP2 in the ENaC-induced trophoblast cell invasion ability, which is closely related to the occurrence of preeclampsia. MethodsHere we checked whether pregnancy related hormones human chorionic gonadotropin (HCG), prolactin and aldosterone could affect ENaC expression in the first trimester extravillous trophoblast cell line (HTR8/SVneo) by RT-PCR and Western blot. Cell invasion was studied by matrigel invasion assay. Tube formation assay was used to investigate the interaction between trophoblast cells and endothelial cells. The effects of ENaC on MMP2 were further determined by RT-PCR, western blot and gelatin zymography. ResultsWe demonstrated that HCG, prolactin and aldosterone could up-regulate the expression of αENaC in protein levels. Trophoblast cell invasion ability is stimulated when αENaC was up-regulated by aldosterone, and inhibited when ENaC was down-regulated by amiloride and αENaC specific RNAi (SiENA/ENaC). The interaction between HTR8/SVneo cells and HUVEC cells was enhanced when treated with aldosterone and weakened when treated with amiloride and SiRNA/ENaC. Amiloride and SiRNA/ENaC could inhibit MMP2 expression and activity. DisscutionAldosterone induced ENaC activity is important for trophoblast cells invasion. The results also indicate that ENaC could mediate trophoblast cells invasion ability through regulating expression and activity of matrix metalloproteinase-2 (MMP2).

Increased expression of sodium transport proteins and Na,K‐ATPase activity in the outer medulla of kangaroo rat is related to its greater urine concentrating ability compared to Sprague‐Dawley rat (1137.7)

The kangaroo rat (Dipodomys merriami), a desert rodent, is able to concentrate its urine to more than 6,000 mosmol/kg water, nearly twice that of the SD rat. We hypothesize that active sodium reabsorption in the thick ascending limb (TAL) of the kangaroo rat outer medulla (OM) is greater than that of the SD rat, and plays a critical role in generating a steeper corticopapillary osmotic gradient. The OM was dissected and prepared for semi‐quantitative western blotting. We measured a four‐fold greater abundance of Na,K‐ATPase α‐1 subunit protein (NKA‐α1) (n=5, p<0.0001), and nearly two‐fold greater abundance of NKCC2 cotransporter and NHE3 protein expression in OM homogenates of kangaroo rat compared to SD rat (n=3, p<0.01). Three‐day water restriction in SD rat (water reduced to 40% of normal consumption) did not significantly affect NKA‐α1, NKCC2, and NHE3 expression (n=3, p>0.1). Na,K‐ATPase (NKA) activity was measured as the difference in ATP hydrolyzed in the absence and presence of ouabain, as determined by measuring release of inorganic phosphate. OM NKA activity in kangaroo rat was significantly higher than in control SD rat, 7778 ± 973 and 4886 ± 296 nmoles ATP hydrolyzed/mg protein/30 min respectively (n=3). However, the volume of mitochondria per unit inner stripe TAL cell volume is identical in both species. We conclude that the greater urine concentrating ability in kangaroo rat compared to the SD rat is associated with greater expression of active Na transport proteins and higher NKA activity, which increases sodium reabsorption in the TAL of the OM.Grant Funding Source: APS, NSF IOS‐0952885 and NIH DK08338

Stability-indicating HPTLC method for simultaneous estimation of amlodipine besylate, hydrochlorothiazide and olmesartan medoxomil in combined tablet dosage forms

Amlodipine besylate, or 3-ethyl-5-methyl-(4RS)-2-[(2-amino-ethoxy)methyl]-4-(2-chlorophenyl)-6-methyl-1,4-dihydropyri-dine-3,5-dicarboxylate benzene sulphonate is a cardiovascular(antianginal), dihydropyridine calcium channel blocker widelyused in antihypertensive pharmaceutical formulations [1–2].Hydrochlorothiazide, or 6-chloro-3,4-dihydro-2H-1,2,4-benzo-thiadiazine-7-sulfonamide-1,1-dioxide, is a diuretic of the class of benzothiadiazines widely used in antihypertensive phar-maceutical formulations, alone or in combination with otherdrugs, which decreases active sodium reabsorption and reducesperipheral vascular resistance [3]. Olmesartan, the medoxomilsalt of (5-methyl-2-oxo-1,3-dioxol-4-yl)methyl ester of 4-(1-hydroxy-1-methylethyl)-2-propyl-1-{[2’-(1H-tetrazol-5-yl)[1,1’-biphenyl]-4-yl]methyl}-1H-imidazole-5-carboxylic acid,is the ester prodrug of a new generation of effective and orallyactive angiotensin-II receptor antagonist. It blocks the vasocon-strictor and aldosterone-secreting effects of angiotensin-II, oneof the most important regulators of blood pressure [4]. Thedetermination of OLME from tablet formulation has been car-ried out by high-performance liquid chromatography (HPLC),high-performance thin-layer chromatography (HPTLC) andspectrophotometricaly, alone or in combination [5–7]. Assay ofAMLO and HCTZ in bulk and in dosage form is official in Indi-an Pharmacopoeia and British Pharmacopoeia in differentdosage forms. Several analytical methods have been reportedfor their determination alone or in combination with other drugsin different dosage forms, biological fluids and urine using dif-ferent analytical techniques [8–13]. Forced degradation studiesare used to facilitate the development of analytical methodolo-gy, to gain better understanding of the stability of the activepharmaceutical ingredient (API), drug product, and to provideinformation about degradation pathways and degradation prod-ucts. The objective of this work was to develop a stability-indi-cating HPTLC method for simultaneous analysis of AMLO,HCTZ, and OLME as no reported method is available for theirsimultaneous estimation in combined formulations. Chemicalstructure of the drugs is shown in Figure 1.HPTLC has advantages over other methods because of rapidity,selectivity, economy, and overall versatility in QC aspects ofdrugs. Literature survey revealed that various HPTLC methodshave been reported for determination of AMLO, HCTZ, andOLME individually and in combination with other drugs [3,5–15]. Currently, the HPLC method is the most frequently usedanalytical method for this kind of determinations. However, nosimultaneous estimation of these drugs was reported so far;hence, it was found worthwhile to develop an HPTLC methodfor the simultaneous determination of AMLO, HCTZ, andOLME in the presence of degradation products and relatedimpurities in pharmaceutical formulations.

Erythropoietin is reduced by combination of diuretic therapy and RAAS blockade in proteinuric renal patients with preserved renal function

Renin-angiotensin-aldosterone system (RAAS) blockade improves prognosis in renal patients, but usually requires diuretic co-treatment. RAAS blockade can decrease erythropoietin (EPO) and/or haemoglobin (Hb) levels. Diuretics decrease EPO in rodents, but their effect on EPO and Hb in humans is unknown. Proteinuric renal patients with preserved renal function were treated during 6-week periods with placebo, losartan 100 mg/day (LOS) and LOS plus hydrochlorothiazide 25 mg/day (LOS/HCT), in random order. Hb was inversely related to proteinuria, and EPO levels were inappropriately low in relation to Hb. Hb was lowered by LOS with and without HCT. EPO was decreased by LOS/HCT, but not by LOS. EPO and Hb are reduced by HCT added to LOS in proteinuric renal patients with preserved renal function. We hypothesize that EPO reduction by HCT is caused by a decrease in renal oxygen requirement, which is the main stimulus for EPO production, due to the inhibition of active tubular sodium reabsorption. Further studies should explore the exact mechanism of this phenomenon and its clinical impact.

Oxygen transport across vasa recta in the renal medulla

In this model of oxygen transport in the renal medullary microcirculation, we predicted that the net amount of oxygen reabsorbed from vasa recta into the interstitium is on the order of 10(-6) mmol/s, i.e., significantly lower than estimated medullary oxygen requirements based on active sodium reabsorption. Our simulations confirmed a number of experimental findings. Low medullary PO(2) results from the countercurrent arrangement of vessels and an elevated vasa recta permeability to oxygen, as well as high metabolic needs. Diffusional shunting of oxygen between descending vasa recta (DVR) and ascending vasa recta also explains why a 20-mmHg decrease in initial PO(2) at the corticomedullary junction only leads to a small drop in papillary tip PO(2) (<2 mmHg with baseline parameter values). Conversely, small changes in the consumption rate of DVR-supplied oxygen, in blood flow rate, in hematocrit, or in capillary permeability to oxygen, beyond certain values sharply reduce interstitial PO(2). Without erythrocytes, papillary tip PO(2) cannot be maintained above 10 mmHg, even when oxygen consumption is zero.

Everything you wanted to know about the bladder epithelium but were afraid to ask.

The mammalian urinary bladder epithelium (urothelium) performs the important function of storing urine for extended periods, while maintaining the urine composition similar to that delivered by the kidneys. The urothelium possesses four properties to perform this function. First, it offers a minimum epithelial surface area-to-urine volume; this reduces the surface area for passive movement of substances between lumen and blood. Second, the passive permeability of the apical membrane and tight junctions is very low to electrolytes and nonelectrolytes. Third, the urothelium has a hormonally regulated sodium absorptive system; thus passive movement of sodium from blood to urine is countered by active sodium reabsorption. Last, the permeability properties of the apical membrane and tight junctions of the urothelium are not altered by most substances found in the urine or blood. The importance of the barrier function of the urothelium is illustrated by infectious cystitis. The loss of the barrier function results in the movement of urinary constituents into the lamina propria and underlying muscle layers, resulting in suprapubic and lower back pain and frequent, urgent, and painful voiding.

Developmental Change in Na,K-ATPase α1 and β1 Expression in Normal and Hypothyroid Rat Renal Cortex

Renal Na,K-ATPase drives active reabsorption of sodium and cotransported solutes along the nephron. There is a large increase in net Na⁺ reabsorption in the postnatal rat kidney. It has previously been established that the postnatal increase in expression of sodium pump isoforms in the brain, but not the heart, is blunted in the hypothyroid neonate. The aims of this study were to establish whether the developmental increase in renal sodium transport is associated with coordinate increases in the abundance of the sodium pump α₁ catalytic and β₁ glycoprotein subunits and Na,K-ATPase activity, and to determine whether thyroid status influences the postnatal increase in renal Na,K-ATPase expression. Pregnant rats were made hypothyroid with low iodine diet, propylthiouracil and perchlorate. Offspring were hypothyroid assessed by triiodothyronine/thyroxine RIA. Renal cortical membranes were prepared from euthyroid and hypothyroid rats from 6 to 24 days of age. There was no change in Na,K-ATPase activity or expression between 6 and 15 days. Between 15 and 24 days, Na,K-ATPase activity increased 1.35-fold while sodium pump α₁ and β₁ subunit abundance increased coordinately to 1.7- and 2-fold over the previous period, respectively. In hypothyroid neonates, kidney weight was less than in euthyroids, and Na,K-ATPase activity, α₁ and β₁ subunit pool sizes did not significantly increase as a function of age between 6 and 24 days. We conclude that the postnatal increase in sodium pump activity can be accounted for by coordinate increases in the pool sizes of α₁ and β₁ subunits and that, like in brain, this increased Na,K-ATPase expression is dependent on normal thyroid status.

Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance.

At birth, fetal distal lung epithelial (FDLE) cells switch from active chloride secretion to active sodium (Na+) reabsorption. Sodium ions enter the FDLE and alveolar type II (ATII) cells mainly through apical nonselective cation and Na(+)-selective channels, with conductances of 4-26 pS (picoSiemens) in FDLE and 20-25 pS in ATII cells. All these channels are inhibited by amiloride with a 50% inhibitory concentration of < 1 microM, and some are also inhibited by [N-ethyl-N-isopropyl]-2'-4'-amiloride (50% inhibitory concentration of < 1 microM). Both FDLE and ATII cells contain the alpha-, beta-, and gamma-rENaC (rat epithelial Na+ channels) mRNAs; reconstitution of an ATII cell Na(+)-channel protein into lipid bilayers revealed the presence of 25-pS Na+ single channels, inhibited by amiloride and [N-ethyl-N-isopropyl]-2'-4'-amiloride. A variety of agents, including cAMP, oxygen, glucocorticoids, and in some cases Ca2+, increased the activity and/or rENaC mRNA levels. The phenotypic properties of these channels differ from those observed in other Na(+)-absorbing epithelia. Pharmacological blockade of alveolar Na+ transport in vivo, as well as experiments with newborn alpha-rENaC knock-out mice, demonstrate the importance of active Na+ transport in the reabsorption of fluid from the fetal lung and in reabsorbing alveolar fluid in the injured adult lung. Indeed, in a number of inflammatory diseases, increased production of reactive oxygen-nitrogen intermediates, such as peroxynitrite (ONOO-), may damage ATII and FDLE Na+ channels, decrease Na+ reabsorption in vivo, and thus contribute to the formation of alveolar edema.

Torsemide: A new loop diuretic

The pharmacology, pharmacokinetics, clinical efficacy, adverse effects, and dosage and administration of torsemide are reviewed. Torsemide belongs to the pyridine-sulfonylurea class of loop diuretics. Its primary site of activity is the thick ascending limb of the loop of Henle, where it blocks active reabsorption of sodium and chloride, resulting in diuresis, natriuresis, and other effects. Torsemide has high bioavailability, a relatively long half-life, and a prolonged duration of activity. It is highly protein bound. Clinical trials indicate that torsemide is effective in the treatment of hypertension and of edema and other symptoms in patients with chronic renal failure (CRF), hepatic dysfunction, or congestive heart failure (CHF). Torsemide has infrequent, mild, and transient adverse effects; among the most common are orthostatic hypotension, fatigue, dizziness, and nervousness. The recommended initial oral dosages of torsemide are 10-20 mg/day for CHF, 20 mg/day for CRF, 5 mg/day for hypertension, and 5-10 mg/day (in combination with a potassium-sparing diuretic or aldosterone antagonist) for hepatic cirrhosis. In most patients, the pharmacokinetic advantages of torsemide over other loop diuretics are unlikely to translate into a substantial edge in clinical outcomes, and in practice there may be no cost advantages. Although torsemide does not offer major advantages over other loop diuretics, it may be of benefit in patients who do not respond to or cannot tolerate other agents.

Hypoxia of the renal medulla--its implications for disease.

In land mammals, a major task of the kidney is to reabsorb water to allow survival in a dry environment. Water conservation is enhanced by the renal medulla, which concentrates the urine to a level up to four times the osmolality of plasma. To produce this unique gradient of osmolality, the medulla has a countercurrent system of vessels and tubules that dictates active reabsorption of sodium in a milieu poor in oxygen (Figure 1).1 In this review, we describe how hypoxia of the medulla may relate to susceptibility to acute and chronic renal injury.The Renal Medullary Concentrating Mechanism as . . .

Measurement of solute fluxes in isolated rat lungs

Most previous studies in isolated perfused lungs have utilized measurements of solute flow from alveolar to vascular space to characterize the barrier and transport properties of the alveolar epithelium. In this study, we measured flux of a series of nonionic hydrophilic solutes and sodium across the alveolar epithelium of the isolated rat lung from perfusate to airspace(P→A), as well as from airspace to perfusate(A→P). Apparent permeability-surface area products (PS) were calculated from the rates of isotope appearance downstream in either the airspace or the perfusate. Equivalent pore analysis of data forP→A solute flow demonstrated a small pore population with radius 0.6 nm occupying 85% of the total pore area and a large pore population with radius 3.8 nm occupying 15% of the total area. Similar analysis ofA→P solute flux demonstrated a small pore population of 0.6 nm occupying 86% of the total pore area and a large pore population with radius 2.9 nm occupying 14% of total pore area. The ratio (R) ofPS P → A divided byPS A → P was 0.8 for the nonionic hydrophilic solutes, while R for sodium was 0.5. In the presence of amiloride and ouabain, R for sucrose was unchanged while R for sodium increased to 0.8 due to a fall inPS A → P. The difference between R for sodium and R for the passively transported solutes, and the reduction in this difference in the presence of sodium transport inhibitors, are consistent with active sodium reabsorption by the intact alveolar epithelium. Differences in measured unidirectional passive solute fluxes probably result from unequal effective surface areas for diffusion from vascular space and vice versa in the anatomically complex mammalian lung.

Regulation of erythropoietin production is related to proximal tubular function.

Regulation of renal erythropoietin (EPO) production is based on an intrarenal oxygen sensor. Whereas the sensitivity of this oxygen sensor to variations in renal oxygen supply is well established, the influence of changes in renal oxygen consumption has not yet been elucidated. Diuretic drugs, which inhibit active sodium reabsorption, reduce tubular oxygen consumption. We therefore investigated the effects of acetazolamide, furosemide, hydrochlorothiazide, and amiloride, known to preferentially inhibit sodium reabsorption at different segments of the nephron, on hypoxia-induced EPO formation in mice. Those drugs that are considered to act mainly in the loop of Henle, distal tubule, and collecting duct (furosemide, hydrochlorothiazide, and amiloride) did not impair EPO formation. Acetazolamide on the other hand, which is thought to act predominantly at the proximal tubular site, significantly reduced EPO formation in response to normobaric hypoxia (8 and 14% O2) and functional anemia (0.1% carbon monoxide). This inhibitory effect of acetazolamide was dose dependent and correlated with the natriuresis induced. It appeared not to depend on the metabolic acidosis induced by the drug, since the simultaneous administration of sodium bicarbonate, which restored standard bicarbonate levels to normal, did not diminish the inhibitory effect of acetazolamide on EPO production. In conclusion the data suggest that the regulation of EPO production is likely to be related to proximal tubular function.

Potassium-Sparing Diuretics

Amiloride, triamterene, and the spirolactones are potassium-sparing diuretics which act on the distal parts of the nephron, from the late distal tubule to the collecting duct. In these segments, active sodium reabsorption occurs through the following mechanism: sodium ions enter the cell through specific channels present in the luminal membrane and are extruded out of the cell into the peritubular medium by a sodium-potassium exchange pump, the Na-K-ATPase. Amiloride in micromolar concentrations reduces the sodium transport by blocking the luminal membrane sodium channel. Triamterene has a similar effect, although with a lower affinity; the available studies do not allow to determine if an inhibitory effect of triamterene on the Na-K-ATPase plays an additional role in its diuretic action. The spirolactones are competitive inhibitors of aldosterone, the mineralocorticoid hormone which promotes sodium reabsorption by increasing both the number of active sodium channels in the luminal membrane and the number of active Na-K pumps in the peritubular membrane. By the inhibitory effect on the electrogenic sodium transport, amiloride, triamterene, and the spirolactones decrease the lumen-negative transepithelial potential difference. This reduces the driving force for potassium movement into the tubular lumen and thus decreases potassium excretion.

Ionic environment of cochlear hair cells.

The scala media of the adult cochlea in mammals comprises a morphologically closed compartment sealed with tight junctions of the intermediate to tight types. The unique ionic composition of endolymph is maintained by the stria vascularis through active reabsorption of sodium and active secretion of potassium against ionic gradients. The subtectorial space is only a partially closed compartment which communicates with the endolymph via holes in the tectorial membrane at its outer insertion to the organ of Corti. Hardesty's membrane divides the subtectorial space into two compartments: one facing the surfaces of inner hair cells and one facing the surfaces of outer hair cells. In the study of comparative anatomy, hair cells, e.g. in the lizard, basilar papilla are of two types: those covered with a tectorial membrane and those being free-standing lacking the tectorial membrane. The ionic environment of the hair cell surface seems to be the same, independent of whether covered with a tectorial membrane or not. The tectorial membrane itself is semipermeable to ions in the endolymphatic space. Only the surface structures of the hair cell with the sensory hairs facing the subtectorial space are exposed to the high concentration of potassium, whereas the remaining parts of the hair cell are surrounded by a fluid having a more normal extracellular type of ionic composition (cortilymph/perilymph). During embryonic development the ionic composition of endolymph develops in parallel with the morphologic maturation of the stria vascularis. A completely mature composition of endolymph is reached before any electrophysiological potentials in the cochlea can be elicited. The sensory hair surface of hair cells has reached a mature morphology prior to the maturation of endolymph. In several species the tectorial membrane is morphologically only partially mature when the increase of the potassium concentration of endolymph starts. Drugs primarily affecting the stria vascularis causing a transient change of the ionic composition of endolymph result in a transient dysfunction of inner ear potentials. If the ionic changes persist for longer time, morphological changes can occur in both the stria vascularis and the hair cells of the organ of Corti. Whether such changes are primarily caused by the ototoxic drug itself or by changes in the ionic composition of endolymph has to be explored further.

Heterogeneity in the distal nephron of the salamander (Ambystoma tigrinum): a correlated structure function study of isolated tubule segments.

Studies on isolated perfused tubules of the tiger salamander (Ambystoma tigrinum) have shown that the distal nephron is heterogeneous with respect to function (Stoner, 1977). In this study, the initial portion of the distal tubule (diluting segment) exhibited a voltage, positive in the lumen, and a net absorption of chloride. Since the chloride was transported against an electro-chemical gradient, its transport was active. More distad, the junctional segment exhibited a lumen-negative voltage and sodium, rather than chloride, was transported actively. More recently Delaney and Stoner (1981) have demonstrated in vitro that the collecting duct of this species also has a lumen-negative voltage which is probably associated with active sodium reabsorption. The primary objective of the present paper was to correlate the morphology of the diluting and junctional segments of the Ambystoma distal tubules with the physiologic data from the same isolated perfused tubules. The results indicate that the morphological heterogeneity previously demonstrated in distal tubules of Necturus exists with respect to both structure and function in Ambystoma. The cell types found in the amphibian distal nephron appear to be homologous to those seen in the mammalian nephron.

Studies on the regulation of hydrogen ion secretion in the collecting duct in vivo: Evaluation of factors that influence the urine minus blood PCO2 difference

The purpose of these studies was to clarify the basis of the relationship between the urine bicarbonate concentration and the urine minus blood PCO2 difference in alkaline urine (U-B PCO2) and hence shed light on factors that influence hydrogen ion secretion in the collecting duct in vivo. The U-B PCO2 was used to monitor this latter parameter. In dogs with a normal extracellular fluid (ECF) volume, the U-B PCO2 was not primarily influenced by the urine bicarbonate concentration but rather it was related to the rate of sodium excretion. The U-B PCO2 could be abolished by amiloride when the urine bicarbonate concentration was less than 60 mm. At higher urine bicarbonate concentrations, there was a linear correlation between the U-B PCO2 and the urine bicarbonate concentration in normovolemic dogs given amiloride, but the absolute values were lower than they were in normovolemic animals not treated with amiloride. In the dogs with an expanded ECF volume, the U-B PCO2 was lower than it was in the normovolemic animals, and the U-B PCO2 was nor directly related to the urine bicarbonate concentration and not influenced by the rate of sodium excretion. Amiloride had little influence on the U-B PCO2 under these conditions. These results are interpreted to suggest that the magnitude of collecting duct hydrogen ion secretion is determined primarily by the electrical gradient generated by sodium reabsorption in normovolemic dogs and by the intracellular and lumenal hydrogen ion concentrations when the ECF volume is expanded or when active sodium reabsorption is inhibited by amiloride.