Alkalotic Rats Research Articles

NBCn1 is a basolateral \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{Na}^{+}\mathrm{-HCO}_{3}^{-}\) \end{document} cotransporter in rat kidney inner medullary collecting ducts

Primary cultures of rat inner medullary collecting duct (IMCD) cells Na(+) dependently import HCO(3)(-) across the basolateral membrane through an undefined transport protein. We used RT-PCR, immunoblotting, and immunohistochemistry to identify candidate proteins for this basolateral Na(+)-HCO(3)(-) cotransport. The mRNA encoding the electroneutral Na(+)-HCO(3)(-) cotransporter NBCn1 was detected as the only Na(+)-HCO(3)(-) cotransporter in the rat inner medulla (IM) among the five characterized Na(+)-dependent HCO(3)(-) transporters. The mRNA of a yet uncharacterized transporter-like protein, BTR1, was also present in the IM, but its expression in microdissected tubules seemed restricted to the thin limbs of Henle's loop. Immunoblotting confirmed the presence of NBCn1 as an approximately 180-kDa protein of the rat IM. Anti-NBCn1 immunolabeling was confined to the basolateral plasma membrane domain of IMCD cells in the papillary two-thirds of the IM. Consistent with the presence of NBCn1, IMCD cells possessed stilbene-insensitive, Na(+)- and HCO(3)(-)-dependent pH recovery after acidification, as assessed by fluorescence microscopy using a pH-sensitive intracellular dye. In furosemide-induced alkalotic rats, NBCn1 protein abundance was decreased in both the IM and inner stripe of outer medulla (ISOM) as determined by immunoblotting and immunohistochemistry. In contrast, NBCn1 abundance in the IM and ISOM was unchanged in NaHCO(3)-loaded animals, and the NBCn1 abundance increased only in the ISOM after NH(4)Cl loading. In conclusion, NBCn1 is a basolateral Na(+)-HCO(3)(-) cotransporter of IMCD cells and is differentially regulated in IMCD and medullary thick ascending limb.

Dopamine enhances the phosphaturic effect of PTH during acute respiratory alkalosis

The phosphaturic response to parathyroid hormone (PTH) is blunted during acute respiratory alkalosis. The objective of the present study was to determine the effect of dopamine on the blunted phosphaturic response to PTH during acute respiratory alkalosis. The phosphaturic response to PTH was determined in thyroparathyroldectomized (TPTX) normocapnic and respiratory alkalotic rats in the absence and presence of the infusion of exogenous dopamine (25 μg/kg/min) or of 3,4-dihydroxyphenylalanine (l-DOPA, 250 μg/kg/min) to increase endogenous dopamine synthesis. In normocapnic rats, PTH infusion (33 U/kg plus 1 U/kg/min) significantly increased the fractional excretion of phosphate (FEpi), from 1.5% ± 0.5% to 28.4% ± 4.0%, (ΔFEpi 26.9% ± 4.1%, n = 11, P < .05); in respiratory alkalotic rats, the increase was from 0.4% ± 0.1% to 11.4% ± 1.7% (ΔFEpi 11.0% ± 1.8%, n = 13, P < .05). However, the phosphaturic response to PTH was attenuated in respiratory alkalotic rats (ΔFEpi 26.9% ± 4.1% vs 11.0% ± 1.9%, P < .05). In normocapnic rats, in the presence of dopamine or l-DOPA infusions, PTH infusion significantly increased the FEpi, from 6.1% ± 2.3% to 33.4% ± 8.0% (ΔFEpi 27.3% ± 7.0%, n = 5) and from 3.2% ± 0.6% to 32.5% ± 3.3% (ΔFEpi 29.3% ± 3.2%, n = 7), respectively. In respiratory alkalotic rats, in the presence of dopamine infusion, PTH significantly increased the FEpi, from 0.6% ± 0.2% to 19.3% ± 3.3% (ΔFEpi 18.7% ± 3.3%, n = 6); in the presence of l-DOPA infusion it increased from 1.0% ± 0.3% to 20.5% ± 2.8% (ΔFEpi 19.5% ± 2.9%, n = 8, P < .05 as compared with PTH alone). Thus the phosphaturic effect of PTH that was attenuated in respiratory alkalotic rats was enhanced by stimulation of endogenous dopamine synthesis by the infusion of l-DOPA.

Secretion of HCO3-/OH- in cortical distal tubule of the rat.

Secretion of bicarbonate has been described for distal nephron epithelium and attributed to apical Cl-/HCO3- exchange in beta-intercalated cells. We investigated the presence of this mechanism in cortical distal tubules by perfusing these segments with acid (pH 6) 10 mM phosphate Ringer. The kinetics of luminal alkalinization was studied in stationary microperfusion experiments by double-barreled pH (ion-exchange resin)/1 M KCl reference microelectrodes. Luminal alkalinization may be due to influx (into the lumen) of HCO3- or OH-, or efflux of H+. The magnitude of the Cl-/HCO3- exchange component was measured by perfusing the lumen with solutions with or without chloride, which was substituted by gluconate. This component was not different from zero in control and alkalotic (chronic plus acute) Wistar rats. Homozygous Brattleboro rats (BRB), genetically devoid of antidiuretic hormone, were used since this hormone has been shown to stimulate H+ secretion, which could mask bicarbonate secretion. In these rats, no evidence for Cl-/HCO3- exchange was found in control BRB and in early distal segments of alkalotic animals, but in late distal tubule a significant component of 0.14 +/- 0.033 nmol/cm2.sec was observed, which, however, is small when compared to the reabsorptive flow found in control Wistar rats, of 0.95 +/- 0.10 nmol/cm2.sec. In addition, 5 x 10(-4) M SITS had no effect on distal bicarbonate reabsorption in controls as well as on secretion in alkalotic Wistar and Brattleboro rats, which is compatible with the absence of effect of this drug on the apical Cl-/HCO3- exchange in other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid-base regulation of hepatic glutamine metabolism and ureagenesis

The metabolism of (5-15N)glutamine and (2-15N) glutamine has been studied by isolated hepatocytes obtained from either control, chronically acidotic, or alkalotic rats. The main goal was to elucidate the mechanism(s) by which altered acid-base state affects hepatic ureagenesis from glutamine. Isolated hepatocytes were incubated in Krebs buffer (pH 7.4) supplemented with 0.1 mM ornithine plus either 1 mM (5-15N)glutamine or (2-15N)glutamine. To elucidate the role of glutamine cycling in net ammonia metabolism, a separate series of experiments were performed with 1 mM unlabeled glutamine plus 1 mM (15N)H4Cl. Net glutamine utilization was significantly lower in hepatocytes obtained from chronically acidotic rats compared with control or alkalotic rats. The sum of the rates of 15NH3 and (15N)urea production from (5-15N)glutamine was decreased in acidosis compared with alkalosis. After incubations of 50 min, approximately 75, 65, or 90% of the N in carbamoyl-phosphate was derived from the 5-N of glutamine in control, acidosis, or alkalosis respectively. In experiments with (2-15N)glutamine, the production of singly and doubly labeled (15N)urea as well as (15N)aspartate and (15N)H3 was significantly smaller in acidosis compared with alkalosis. Furthermore, a correlation was observed between production rates of (15N)aspartate and (15N)urea, suggesting that alterations in urea production may depend on aspartate formed from glutamine. However, the production of (15N)alanine was higher in acidosis compared with alkalosis with apparent correlation between the production of (15N)alanine and 2-oxoglutaramate, a product of the glutamine aminotransferase pathway. In addition, the rate of glutamine recycling was significantly higher in acidosis compared with control or alkalosis, indicating that both flux through glutamine aminotransferase and flux through glutamine synthetase were elevated in acidosis compared with alkalosis. These data suggest that decreased formation of aspartate from glutamine may limit ureagenesis in chronic metabolic acidosis. The formation of aspartate may depend on the availability of oxaloacetate rather than diminished flux through transaminase reaction. The enhancement of alanine production and glutamine synthesis may provide an alternate route of N disposal in cases of diminished urea formation.

Effect of metabolic acidosis and alkalosis on NEM-sensitive ATPase in rat nephron segments.

An N-ethylmaleimide (NEM)-sensitive adenosinetriphosphatase (ATPase) displaying the kinetic and pharmacological properties of an electrogenic proton pump has been described in the different segments of rat nephron, where it mediates part of the active tubular proton secretion. This study was therefore designed to evaluate whether changes in urinary acidification observed during metabolic acidosis or alkalosis were associated with alterations of the activity of tubular NEM-sensitive ATPase, and if so, to localize the nephron segments responsible for these changes. Within 1 wk after the onset of ammonium chloride treatment, rats developed a metabolic acidosis, and NEM-sensitive ATPase activity was markedly increased in the medullary thick ascending limb of Henle's loop and outer medullary collecting tubule, and slightly increased in the cortical collecting tubule. Conversely, treatment with sodium bicarbonate induced a metabolic alkalosis that was accompanied by decreased NEM-sensitive ATPase activity in medullary thick ascending limb and outer medullary collecting tubule. NEM-sensitive ATPase activity was not altered in any other nephron segment tested in alkalotic and acidotic rats, i.e., the proximal tubule and the cortical thick ascending limb of Henle's loop. Changes qualitatively similar were observed as soon as 3 h after the onset of NaHCO3 or NH4Cl-loading. In the medullary collecting tubule, alterations of NEM-sensitive ATPase activity are in part due to hyperaldosteronism observed in both acidotic and alkalotic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Maximal proton secretory rate of rat distal tubules is higher during chronic metabolic alkalosis.

In vivo microperfusion studies show augmented proton secretion in the distal tubule of rats with chronic metabolic alkalosis. The present studies used the same technique to determine whether this augmented proton secretion is due predominantly to an increase in substrate affinity or alternatively to a predominant increase in the maximal proton secretory rate. Surface distal tubules of alkalotic and control rats were microperfused in vivo with solutions containing increasing concentrations of HCO3. Proton secretion was determined as the difference between measured net HCO3 reabsorption and passive HCO3 transport calculated by use of the permeability derived from perfusing with a HCO3-free solution. Proton secretion was expressed as a function of the initial luminal HCO3 concentration and was assumed to follow saturable Michaelis-Menten kinetics. Alkalotic animals had a significantly higher Km (33.9 vs. 21.6 mM, P less than 0.03) and Vmax (223.8 vs. 99.1 pmol.mm-1.min-1, P less than 0.001) compared with control. These data are consistent with the augmented proton secretion in the distal tubule of alkalotic animals as predominantly due to an increased maximal proton secretory rate rather than to increased substrate affinity.

Absence of a regulatory role of angiotensin II in acute chloride-depletion alkalosis in rats

Chloride-depletion alkalosis (CDA) has been characterized by hypereninemia. To determine whether angiotensin II (ANG II) has an important role in its maintenance or correction, anesthetized alkalotic rats, chloride depleted by peritoneal dialysis, were infused with 5% dextrose and saralasin (1 microgram.kg-1.min-1) (SAR) or vehicle (SAR-C), 5% dextrose and pretreatment with enalapril (1-1.5 mg/kg) (ENP) or vehicle (ENP-C), or 80 mM Cl solution with ANG II (20 micrograms/min) (ANG) or vehicle (ANG-C). Rats infused with 5% dextrose showed no differences in the magnitude of the alkalosis, inulin clearance, or urinary total CO2 excretion; both SAR and ENP were associated with decreased blood pressure. In SAR, tCO2 delivery out of late proximal convoluted tubule did not differ from that in SAR-C. Rats infused with 80 mM Cl corrected CDA similarly (delta plasma [Cl] - ANG-C + 6 +/- 1, ANG + 5 +/- 1 mM; P = not significant). These data suggest that, although ANG II can importantly influence vascular tone and early proximal tubule bicarbonate reabsorption, it does not have an important role in the renal maintenance or correction of acute CDA.

Ion concentrations in the rat CCD: differences between cell types and effect of alkalosis

We have previously shown that the isolated perfused cortical collecting duct (CCD) from chloride-depleted alkalotic (CDA) rats continues to secrete HCO3 for up to 3 h. To determine whether the sustained alteration in transport was associated with changes in intracellular ion concentrations, we performed energy-dispersive X-ray microanalysis in microdissected tubule bundles obtained from CDA rats and rats with normal acid-base status (CON). Before analysis, the bundles from both groups were incubated for 1 h in vitro in a modified Ringer solution (pH 7.4, 105 mM Cl). Principal (PC) and intercalated cells (IC) of the CCD from CON animals differed in the nuclear concentration of Na (17.0 vs. 24.7 mmol/l cell water), K (192.5 vs. 177.0 mmol/l cell water), and Cl (17.8 vs. 47.8 mmol/l cell water). Cells of the cortical thick ascending limb of Henle (CTAL) had the lowest Na and Cl values (11.5 and 14.8 mmol/l cell water, respectively). CDA resulted in no systematic Cl changes. In the IC the nuclear Na concentration was significantly increased (32.0 vs. 24.7 mmol/l cell water) and in all cells a small reduction in K concentration was detectable. These findings suggest that 1) the different transport functions of IC, PC, and CTAL are associated with differences in the intracellular ion composition, and 2) the sustained HCO3 secretion seen in CCD from CDA rats cannot be explained as the result of intracellular Cl depletion.

Site of restoration of the effect of PTH by propranolol in respiratory alkalosis

Respiratory alkalosis decreases the phosphaturic response to parathyroid hormone (PTH). beta-adrenoreceptor blockade by propranolol infusion restores the phosphaturic effect of PTH in respiratory alkalotic rats; however, the nephron site of these changes in phosphate reabsorption by propranolol is not known. The present study was performed to localize the nephron segment(s) involved in the restoration of the phosphaturic effect of PTH by propranolol infusion in respiratory alkalotic rats. PTH infusion increased the fractional delivery of phosphate (FDPi) to the late proximal tubule to similar levels in the propranolol and vehicle-infused respiratory alkalotic rats (FDPi 46.6 +/- 4.4% and 48.6 +/- 4.2%, respectively). In contrast, PTH only increased FDPi to the early distal tubules (to 17.1 +/- 0.9%) in the absence of propranolol compared to FDPi 41.9 +/- 2.5% in the presence of propranolol in respiratory alkalotic animals. We conclude that the restoration of the phosphaturic effect of PTH in respiratory alkalotic rats by propranolol infusion is due primarily to decreased reabsorption of phosphate by the straight segment of the proximal tubule.

Role of angiotensin II in glomerulotubular balance

Inhibition of angiotensin II activity reduces reabsorption of both bicarbonate and chloride predominantly in the S1 subsegment of the proximal convoluted tubule (PCT). Because the S2 PCT is intrinsically better able to compensate for the increased delivery of bicarbonate compared with chloride under normal conditions, we reasoned that angiotensin II inhibition might selectively raise the amount of sodium chloride emerging from the PCT. Free-flow micropuncture techniques were used in normal and alkalotic Munich-Wistar rats that were euvolemic or plasma volume depleted. In the normal volume-depleted animals, saralasin caused a small rise in single-nephron glomerular filtration rate (29.5 +/- 0.6 to 31.5 +/- 0.6 nl/min, P less than 0.025) and fall in bicarbonate and chloride reabsorption in the 1st mm (S1) PCT (387 +/- 22 to 348 +/- 23 peq.mm-1.min-1, P less than 0.05, and 341 +/- 27 to 217 +/- 57 peq.mm-1.min-1, P less than 0.05, respectively). Reabsorptive compensation by the S2 PCT maintained the end-PCT delivery of bicarbonate unchanged (76 +/- 4 to 78 +/- 3 peq.mm-1.min-1, NS), but end-PCT chloride delivery increased significantly (2,014 +/- 41 to 2,248 +/- 29 peq.mm-1.min-1, P less than 0.01). Early distal convoluted tubule (DCT) and urinary bicarbonate excretion were unchanged, but DCT chloride delivery increased associated with a chloruresis. When metabolic alkalosis was present, however, S2 compensation for increased bicarbonate delivery was attenuated so that end-PCT, DCT, and urinary bicarbonate as well as chloride delivery rates increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Superficial distal and deep nephrons in correction of metabolic alkalosis

Chloride is necessary and sufficient to correct alkalosis induced by dialysis vs. 0.15 M NaHCO3. To determine the contribution of the cortical (SC) distal convolution (DCT) and juxtamedullary (JM) nephrons to correction, we examined Cl and total CO2 (tCO2) transport in alkalotic Sprague-Dawley rats infused with 5% dextrose (group DM) or with 5% dextrose and 80 mM Cl (group CC); in papillary studies in alkalotic Munich-Wistar rats, 6% albumin was added to the infusate. In cortical studies, changes in plasma Cl and tCO2 were 4.9 +/- 0.7 vs. 0.7 +/- 0.9 and -6.0 +/- 0.8 vs. 0.4 +/- 0.9 meq/l and in tCO2 excretion (133 +/- 28 vs. -8 +/- 10 mueq/min) in groups CC and DM, respectively; results in papillary studies were similar. Delivery of tCO2 out of late SC DCT (CC 146 +/- 20 and DM 146 +/- 23 pmol/min) and Henle's loop (CC 145 +/- 18 and DM 202 +/- 56 pmol/min) and reabsorption within DCT (CC 15 +/- 24 and DM 45 +/- 19 pmol/min) did not differ. During correction of chloride-depletion alkalosis, the increment in bicarbonate excretion does not emanate from DCT of SC nephrons or JM nephrons but rather from the collecting duct.

Regulation of Na/H exchange in renal microvillus vesicles in chronic hypercapnia

Several disturbances of acid-base balance, including chronic metabolic and respiratory acidoses and metabolic alkalosis, are associated with enhanced proximal tubule bicarbonate reabsorption. To determine whether augmented brush border Na/H exchange might mediate enhanced proximal tubule bicarbonate reabsorption in these disorders, we measured Na/H exchange activity in cortical brush border membrane vesicles (BBMV) prepared from rats and rabbits adapted to hypercapnia and other chronic acid-base disturbances. BBMV prepared from control animals and animals with chronic acid-base disturbances were similar as judged by marker enzymes, alkaline phosphatase, and ouabain-sensitive phosphatase. Despite profound respiratory acidosis, no increase in Na/H exchange activity could be detected in vesicles prepared from rats adapted to chronic (8 to 10 days) or subacute (24 hr) respiratory acidosis. In addition, vesicles prepared from rabbits exposed to chronic hypercapnia did not show increased Na/H exchange when compared with contemporaneous controls. By contrast, in agreement with previously published results, amiloride-sensitive sodium uptake was increased by 30% in vesicles derived from animals with ammonium chloride-induced acidosis compared with contemporaneous controls. Two models of chronic metabolic alkalosis were also studied; vesicles from alkalotic rats did not show any alteration in Na/H exchange. We conclude that metabolic acidosis, but not respiratory acidosis or metabolic alkalosis, leads to enhanced activity of the luminal Na/H exchanger.

Alkalosis and renal excretion of ammonia by rat kidney.

Upon sulfate administration, UpH falls more in alkalotic rats than in controls. Alkalosis can lead to a reduction in UNH3 V at highly acidic urine. The significance of this process is doubtful at UpH ranging from about 6 to 7. At lower UpH less NH3 would be excreted, thereby less H+ would be trapped in urine and some acid would be conserved.

Acidotic alterations in oxidative metabolism influencing rat renal slice amnoniagenesis

We believe that two findings are interconnected and help to comprehend a major mechanism behind the regulation of renal ammonia production during acidosis. First, slices from acidotic compared to control and alkalotic rats produce more ammonia from glutamine. Second, inhibition of renal oxidative metabolism at various points by metabolic inhibitors augments slice ammoniagenesis. Based on this, our purpose was to determine whether enhanced renal ammoniagenesis during acidosis could occur through the same mechanism as the metabolic inhibitors. However, metabolic inhibitors (malonate; arsenite; 2,4-dinitrophenol) usually decrease while acidosis increases slice gluconeogenesis. There is one known exception. Fluorocitrate, which blocks citrate metabolism, simulates the acidotic condition by enhancing both ammonia and glucose production. Accordingly, a block of oxidative metabolism if located prior to citrate oxidation in the tricarboxylic acid cycle could theoretically augment ammoniagenesis during acidosis. Lactate, is a major renal fuel whose oxidative metabolism would be blocked by fluorocitrate. There, we concentrated on the effects of acidosis on lactate as well as glutamine metabolism. Lactate decarboxylation decreases in the face of increased glucose production during acidosis, and lactate inhibition of glutamine decarboxylation decreases in slices from acidotic rats. Also, we found lesser oxygen consumption in the presence of lactate by kidney slices from acidotic rats compared to control and alkalotic rats. We postulate that relatively less incorporation of lactate into the TCA cycle, causing decreased citrate formation and citrate oxidation during acidosis, contributes, at least in part, to acidotic adaptation of ammoniagenesis.

Adaptive changes of H+ and/or bicarbonate transport in chronic metabolic acidosis: its quantitative evaluation by using isolated perfused kidney.

We have recently improved the technique of the isolated perfused rat kidney by addition of amino acids to the perfusion solution. Utilizing this improved preparation, we examined to what extent chronic acid-base status can alter H+ and/or bicarbonate transport in the whole kidney and the effect of acetazolamide (10(-4) M) on this process. Kidneys from rats that had been drinking tap water (control), 1.5% NH4Cl (acidotic), or 1.5% NaHCO3 (alkalotic) were perfused with a solution of identical pH and PCO2. During the control period, the fractional bicarbonate reabsorption was higher in acidotic rats than in control rats by 1.0% and urine pH was lower than those in control. On the other hand, the two parameters were not different between control and alkalotic rats. Acetazolamide increased urine pH and decreased the fractional bicarbonate reabsorption in all, but these parameters in acidotic and alkalotic rats were not different compared with those in control. We conclude that adaptive changes of H+ and/or bicarbonate transport in chronic metabolic acidosis corresponds to about 1.0% as fractional bicarbonate reabsorption by the isolated perfused kidney and is largely carbonic anhydrase dependent.

Distal tubule bicarbonate accumulation in vivo. Effect of flow and transtubular bicarbonate gradients.

We have performed microperfusion studies on distal tubules of normal and alkalotic rats in an attempt to demonstrate in vivo bicarbonate secretion. All perfusion solutions were free of phosphate and other nonbicarbonate buffers. In both normal and alkalotic rats, distal perfusions elicited significant tCO2 entry only at high flow (24 nl/min). Even when perfusate tCO2 concentration closely matched plasma tCO2 concentration (30 mM tCO2), significant tCO2 entry again occurred at high flow. This was associated with a rise of the perfusate tCO2 concentration, which indicated net entry of tCO2 against a concentration gradient. In this "symmetrical" perfusion situation, acetazolamide blockade prevented tCO2 entry. Accordingly: distal tubule tCO2 entry is demonstrable in both alkalotic and normal rats at high flow rates; increasing perfusate tCO2 concentration can suppress tCO2 entry; and entry can occur in the absence of a gradient and this effect can be blocked by acetazolamide.

Acute hypokalemic effect of gossypol.

Clinical trials had demonstrated that chronic ingestion of gossypol induced infertility in males. Hypokalemia of various severities were reported in volunteers taking gossypol. The purpose of this study is to investigate the effects of acute gossypol infusion into alkalotic rats. Plasma potassium concentration decreased from 4.08 to 2.87 mM after gossypol infusion with minimal change in urinary potassium excretion. The hypokalemic effect of gossypol was also observed in nephretomized rats. Some of the factors that are known to affect potassium distribution between the extracellular and intracellular fluid compartment have been investigated in these rats. Plasma glucose concentration was not significantly altered. Gossypol induced hypokalemia was not blocked in rats treated with propranolol. It is therefore believed that the acute hypokalemic effect of gossypol is a direct shifting of potassium into cells.

Nephron site of resistance to phosphaturic effect of PTH during respiratory alkalosis.

This study was performed to evaluate the nephron site(s) responsible for the blunted phosphaturic effect of parathyroid hormone during respiratory alkalosis. In normocapnic thyroparathyroidectomized rats, parathyroid hormone administration markedly increased the fractional excretion of phosphate (FEp) from 2.1 +/- 0.5 to 36.6 +/- 5.0%. However, in the respiratory alkalotic rats, parathyroid hormone administration did not significantly increase the FEp (1.4 +/- 0.9 to 5.9 +/- 2.2%). This blunted phosphaturic response to parathyroid hormone was not due to a blunted inhibition of phosphate reabsorption by the superficial proximal tubule, since parathyroid hormone administration significantly increased the fractional delivery of phosphate (FDp) at the superficial late proximal tubule in both normal (25.3 +/- 3.0 to 36.2 +/- 3.8%, delta 10.9 +/- 3.2%) and respiratory alkalotic rats (12.2 +/- 3.1 to 30.3 +/- 4.9%, delta 18.0 +/- 4.7%). Parathyroid hormone administration significantly increased the FDp at the superficial early distal tubule from 9.3 +/- 3.9 to 38.7 +/- 7.4% (delta 29.4 +/- 5.1%) in normal rats and from 4.5 +/- 1.7 to 12.9 +/- 3.4% (delta 8.5 +/- 3.2%) in the respiratory alkalotic rats. We conclude that the blunted phosphaturic response to parathyroid hormone in respiratory alkalotic rats is not due to a blunted inhibition of phosphate reabsorption by the proximal convoluted tubule but is primarily due to enhanced reabsorption by the pars recta segment of the proximal tubule.

Atrial natriuretic factor ameliorates chronic metabolic alkalosis by increasing glomerular filtration.

The kidney maintains the elevated plasma concentration of bicarbonate that occurs in chronic metabolic alkalosis. A reduction in the glomerular filtration rate (GFR) can maintain the filtered bicarbonate load at a normal level so that a normal rate of bicarbonate reabsorption suffices to prevent urinary excretion of this anion. It is also possible that bicarbonate reabsorption might increase so as to maintain the alkalosis if GFR were not reduced. To examine this latter possibility, atrial natriuretic factor was used in alkalotic rats to restore a more normal GFR and to increase the amount of bicarbonate filtered by the glomerulus. Proximal bicarbonate reabsorption remained relatively static. Higher than normal amounts of bicarbonate were then delivered out of the proximal tubule, bicarbonate appeared in the urine, and the plasma concentration of bicarbonate fell. A reduction in GFR is thus necessary for the maintenance of chronic metabolic alkalosis. Normalizing GFR induces bicarbonaturia and initiates repair of the alkalosis.

Juxtamedullary nephrons during acute metabolic alkalosis in the rat.

The renal handling of bicarbonate during acute metabolic alkalosis was examined in Munich-Wistar rats using micropuncture techniques. Group I received an acute bicarbonate load, and fractional delivery of total CO2 (tCO2) (FDtCO2) to the superficial late distal tubule (LD) was significantly lower than to the base of the papillary collecting duct (B) (18.4 +/- 1.7 vs. 22.9 +/- 1.5%; P less than 0.01), indicating net addition of bicarbonate between LD and B. When acutely bicarbonate-loaded rats had their deep nephrons destroyed with bromoethylamine hydrobromide (BEA) (group II), net addition of tCO2 between LD and B was abolished and net reabsorption uncovered (FDtCO2 LD: 28.0 +/- 3.6 vs. B: 17.5 +/- 2.5%; P less than 0.01). The infusion of amiloride (2.5 mg/kg body wt) to alkalotic rats treated with BEA (group III) completely inhibited distal bicarbonate reabsorption but did not reestablish addition (FDtCO2 LD: 27.6 +/- 1.6 vs. B: 26.1 +/- 3.7%; P = NS). The values obtained for sham-operated animals (group IV) were the same for group I. The patterns that were observed between LD and B were reproduced for the four groups of animals when FDtCO2 LD was compared with the fractional excretion of bicarbonate in the urine of the intact contralateral kidney. These studies suggest that juxtamedullary nephrons contribute a higher load of bicarbonate than superficial nephrons to the final urine during acute metabolic alkalosis in the rat.