Importance of medullary events in ammonium excretion: studies in acute respiratory and acute metabolic acidosis

The renal medulla can play an important role in acid excretion by modulating both hydrogen ion secretion in the medullary collecting duct and the medullary [Formula: see text]. The purpose of these experiments was to characterize the intrarenal events associated with ammonium excretion in acute acidosis. Cortical events were monitored in two ways: first, the rates of glutamine extraction and ammoniagenesis were assessed by measuring arteriovenous differences and the rate of renal blood flow; second, the biochemical response of the ammoniagenesis pathway was examined by measuring glutamate and 2-oxoglutarate, key renal cortical metabolites in this pathway. There were no significant differences noted in any of these cortical parameters between acute respiratory and metabolic acidosis. Despite a comparable twofold rise in ammonium excretion in both cases, the urine pH, [Formula: see text], and the urine minus blood [Formula: see text] difference (U-B [Formula: see text]) were lower during acute hypercapnia. In these experiments, the urine [Formula: see text] was 34 mmHg (1 mmHg = 133.322 Pa) lower than that of the blood during acute respiratory acidosis while the U-B [Formula: see text] was 5 ± 3 mmHg in acute metabolic acidosis. Thus there were significant differences in medullary events during these two conditions. Although the urine pH is critical in determining ammonium excretion in certain circumstances, these results suggest that regional variations in the medullary [Formula: see text] can modify this relationship.

Download Full-text

Cellular response to acute respiratory acidosis in rat medullary collecting duct

AJP Renal Physiology ◽

10.1152/ajprenal.1983.245.6.f670 ◽

1983 ◽

Vol 245 (6) ◽

pp. F670-F679 ◽

Cited By ~ 8

Author(s):

K. M. Madsen ◽

C. C. Tisher

Keyword(s):

Plasma Membrane ◽

Surface Density ◽

Collecting Duct ◽

Respiratory Acidosis ◽

Hydrogen Ion ◽

Apical Plasma Membrane ◽

Apical Region ◽

Intercalated Cells ◽

Ion Secretion ◽

Medullary Collecting Duct

The collecting duct of the mammalian kidney is involved in urine acidification. Recent studies in the turtle bladder suggest that hydrogen ion secretion in response to elevated CO2 is regulated by insertion of hydrogen pumps into the luminal membrane of the mitochondria-rich cells. Because intercalated cells of the collecting duct are structurally similar to mitochondria-rich cells of the amphibian bladder, we studied the rat outer medullary collecting duct (OMCD) during respiratory acidosis to determine whether changes compatible with hydrogen ion secretion occur in the intercalated cells. Rats were studied during normal acid-base conditions and after 4-5 h of respiratory acidosis. After collection of physiologic data, the kidneys were fixed by in vivo perfusion and processed for electron microscopy. No changes were observed in the principal cells of the OMCD. Morphometric analysis revealed a significant increase in the surface density of the apical plasma membrane and a decrease in the number of tubulovesicular profiles in the apical region of the intercalated cells throughout the OMCD with respiratory acidosis. There were no changes in surface density of the basolateral membrane. These findings suggest that in response to respiratory acidosis there is transport of membrane from the tubulovesicular membrane compartment to the apical plasma membrane of the intercalated cells.

Download Full-text

Effect of respiratory acidosis on acidification by the medullary collecting duct

AJP Renal Physiology ◽

10.1152/ajprenal.1983.244.1.f89 ◽

1983 ◽

Vol 244 (1) ◽

pp. F89-F94 ◽

Cited By ~ 1

Author(s):

H. H. Bengele ◽

M. L. Graber ◽

E. A. Alexander

Keyword(s):

Collecting Duct ◽

Respiratory Acidosis ◽

Hydrogen Ion ◽

Membrane Electrodes ◽

Acid Excretion ◽

Acid Phosphate ◽

Arterial Ph ◽

Glass Membrane ◽

Medullary Collecting Duct ◽

Acute Metabolic Acidosis

The effect of acute respiratory acidosis (ARA) on inner medullary collecting duct (IMCD) acidification was studied and the results were compared with previously obtained data by our laboratory in rats with acute metabolic acidosis (AMA). We employed the microcatheterization technique to directly measure pH and PCO2 with glass-membrane electrodes, and fluid samples were obtained for measurement of bicarbonate, phosphate, and ammonium. Arterial pH was 7.18 +/- 0.01 and PCO2 was 88 +/- 2 mmHg. The IMCD data were analyzed as a function of IMCD length (approximately 6 mm). pH decreased from 5.78 +/- 0.07 to 5.27 +/- 0.03 and PCO2 increased from 55 +/- 4 to 75 +/- 2 mmHg between origin and tip. Bicarbonate delivery decreased from 154 +/- 34 to 25 +/- 3 nmol/min but no change was noted in acid phosphate, ammonium, or net acid addition along the IMCD. However, net acid excretion was not different from that found previously in AMA. We conclude that during ARA acidification is augmented prior to, but not along, the IMCD. In contrast, during AMA we previously found that IMCD plays a major regulatory role in urinary acidification, accounting for about 50% of the excreted hydrogen ion.

Download Full-text

Adaptive changes in renal acidification in response to chronic respiratory acidosis

AJP Renal Physiology ◽

10.1152/ajprenal.1985.248.4.f492 ◽

1985 ◽

Vol 248 (4) ◽

pp. F492-F499 ◽

Cited By ~ 1

Author(s):

R. L. Tannen ◽

B. Hamid

Keyword(s):

Metabolic Acidosis ◽

Proton Transport ◽

Respiratory Acidosis ◽

Hydrogen Ion ◽

Distal Nephron ◽

Urine Ph ◽

Chronic Metabolic Acidosis ◽

Adaptive Changes ◽

Secretory Capacity

To examine whether chronic respiratory acidosis results in adaptive changes in renal acidification, rats were housed for 3 days in an environmental chamber with an ambient CO2 content of 10% and their kidneys were perfused in vitro according to two protocols. To assess hydrogen ion secretory capacity of the distal nephron, perfusions were carried out with a low bicarbonate concentration, in the absence of ammoniagenic substrate, and with saturating quantities of the buffer creatinine. Under these conditions, the titration of creatinine at a pH less than 6.0 (TA pH 6.0) reflects the H+ secretory capacity of a discrete functional segment of the distal nephron. Kidneys from rats with chronic respiratory acidosis exhibited a significantly lower urine pH and higher rate of TA pH 6.0 than controls perfused in this fashion, indicative of an adaptive increase in the distal nephron capacity for proton transport. This adaptation was comparable with that reported previously for rats exposed to chronic metabolic acidosis. Furthermore, evidence of adaptation persisted in the presence of amiloride (10(-5) M), suggesting that it reflects, at least in part, a sodium-independent mechanism of proton transport. Hydrogen ion secretion by the proximal nephron was assessed by performing standard bicarbonate titration curves with kidneys from rats with chronic respiratory acidosis, chronic metabolic acidosis, and controls using a perfusate equilibrated with 95% O2/5% CO2.(ABSTRACT TRUNCATED AT 250 WORDS)

Download Full-text

Acute metabolic acidosis augments collecting duct acidification rate in the rat

AJP Renal Physiology ◽

10.1152/ajprenal.1981.241.6.f669 ◽

1981 ◽

Vol 241 (6) ◽

pp. F669-F676 ◽

Cited By ~ 3

Author(s):

M. L. Graber ◽

H. H. Bengele ◽

E. Mroz ◽

C. Lechene ◽

E. A. Alexander

Keyword(s):

Metabolic Acidosis ◽

Collecting Duct ◽

Direct Evaluation ◽

Acid Addition ◽

Arterial Ph ◽

Nephron Segment ◽

Medullary Collecting Duct ◽

Acute Metabolic Acidosis

It is generally assumed that at least part of distal acidification occurs along the collecting duct. Complete and direct evaluation of acidification along this nephron segment is unavailable, however. The purpose of these experiments was to quantify the net acidification rate along the inner medullary collecting duct (IMCD) and to measure the effect of acute HCl acidosis. In 13 control rats (arterial pH, 7.39 +/- 0.01; PCO2, 39 +/- 1 mmHg) and 11 HCl-infused rats (arterial pH 7.18 +/- 0.01; PCO2, 40 +/- 1 mmHg) we obtained four to eight IMCD samples by a modified microcatheterization technique that also permitted measurement of in situ and in vitro pH. Tubular fluid pH decreased along the IMCD in both groups and was more acidic by 0.2–0.4 pH units in the acid-infused rats. Bicarbonate reabsorption was noted in both groups as delivery along the IMCD decreased from 205 +/- 127 to 26 +/- 6 nmol/min in control rats and from 219 +/- 118 to 17 +/- 4 nmol/min in the acidotic group. Ammonia delivery to and addition along the IMCD was significantly greater in the acidotic rats--from 193 +/-- 59 to 462 +/- 53 nmol/min in control and from 887 +/- 126 to 1,396 +/- 90 nmol/min in acidotic rats. No significant change in total or titrated phosphate was seen. Net acid addition along the IMCD was over twice as great in acidotic rats, 450 vs. 970 nmol/min. Our results indicate that significant net acid addition occurs along the IMCD and that during acidosis this acidification rate increases.

Download Full-text

pH and PCO2 profiles of the rat inner medullary collecting duct

AJP Renal Physiology ◽

10.1152/ajprenal.1981.241.6.f659 ◽

1981 ◽

Vol 241 (6) ◽

pp. F659-F668 ◽

Cited By ~ 1

Author(s):

M. L. Graber ◽

H. H. Bengele ◽

J. H. Schwartz ◽

E. A. Alexander

Keyword(s):

Collecting Duct ◽

Hydrogen Ion ◽

Arterial Pco2 ◽

Inner Medullary Collecting Duct ◽

Ion Secretion ◽

Medullary Collecting Duct ◽

Disequilibrium Ph ◽

Ph Profiles

To directly characterize acidification by the collecting duct, we developed pH and PCO2 microelectrodes suitable for microcatheterization of the inner medullary collecting duct (IMCD). In saline-infused control rats apparent in situ pH fell significantly along the IMCD, from 5.95 at 60% length to 5.49 at the papilla tip. Luminal PCO2 averaged 34 +/- 1 mmHg and PD averaged +3 mV. In rats acutely infused with 0.1 N HCl, apparent in situ pH also decreased significantly from 5.56 to 5.28, PD averaged +2 mV, and luminal PCO2 31 +/- 1 mmHg. The luminal PCO2 of HCl-infused rats was significantly less than controls and both levels were significantly below arterial PCO2. Corroborating the in situ pH profiles, equilibrium pH measured on collected IMCD samples also decreased significantly with percent length. In samples measured in situ and at equilibrium, a small but significant acid disequilibrium pH ws seen in both groups. We interpret these results to indicate that the IMCD actively participates in distal acidification. It is proposed that acidification by the IMCD is predominantly mediated by hydrogen ion secretion which simultaneously acidifies luminal fluid and generates a cellular sink for CO2, thereby inducing an acid disequilibrium pH by two mechanisms.

Download Full-text

Endothelin receptor mRNA expression in renal medulla identified by in situ RT-PCR

AJP Renal Physiology ◽

10.1152/ajprenal.1995.269.3.f449 ◽

1995 ◽

Vol 269 (3) ◽

pp. F449-F457 ◽

Cited By ~ 19

Author(s):

L. H. Chow ◽

S. Subramanian ◽

G. J. Nuovo ◽

F. Miller ◽

E. P. Nord

Keyword(s):

Mrna Expression ◽

Collecting Duct ◽

Renal Medulla ◽

Receptor Subtypes ◽

Rt Pcr ◽

Receptor Mrna ◽

Pcr Products ◽

Medullary Collecting Duct ◽

Labeled Probe

Three subtypes of endothelin (ET) receptors have been identified by cDNA cloning, namely ET-RA, ET-RB, and ET-RC. In the current study the precise cellular distribution of the ET receptor subtypes in the renal medulla was explored by detecting the corresponding polymerase chain reaction (PCR)-amplified cDNAs by in situ reverse transcription (RT)-PCR. The PCR-amplified cDNAs were detected either by direct incorporation using digoxigenin-dUTP (dig-dUTP) as a nucleotide substrate in the PCR reaction or by in situ hybridization with the dig-dUTP-labeled probe. ET-RB mRNA was detected exclusively in the epithelial cells of the inner and outer medullary collecting duct. In contrast, ET-RA message was observed primarily in interstitial cells and pericytes of the vasae rectae in the outer and inner medulla. Southern blot analysis of PCR-amplified cDNAs reverse transcribed from extracted RNA of rat renal medulla confirmed the specificity of the RT-PCR products. ET-RC mRNA was not detected. We conclude that ET-RB is the major ET receptor found in rat renal medulla and is expressed exclusively on inner medullary collecting duct cells. The pattern of ET receptor mRNA expression described suggests different physiological actions for ET on the diverse cellular structures of the renal medulla.

Download Full-text

Prompt Rise in Urinary Ammonium Excretion Suffices To Mitigate Metabolic Acidosis in an Experimental Animal Model of Severe Normovolemic Hemodilution

Physiological Research ◽

10.33549/physiolres.933491 ◽

2017 ◽

pp. 615-620 ◽

Cited By ~ 1

Author(s):

J. K. TELOH ◽

I. N. WAACK ◽

H. DE GROOT

Keyword(s):

Metabolic Acidosis ◽

Absolute Amount ◽

Ammonium Excretion ◽

Acid Excretion ◽

Urinary Ph ◽

Normovolemic Hemodilution ◽

Proton Excretion ◽

Acute Metabolic Acidosis ◽

Experimental Group ◽

Hasselbalch Equation

Recently, we have established a model of severe stepwise normovolemic hemodilution to a hematocrit of 10 % in rats employing three different colloidal volume replacement solutions (Voluven, Volulyte and Gelafundin) that are routinely used in clinical practice at present. We did not see severe dilutional acidosis as to be expected, but a decline in urinary pH. We here looked on further mechanisms of renal acid excretion during normovolemic hemodilution. Bicarbonate, which had been removed during normovolemic hemodilution, was calculated with the help of the Henderson-Hasselbalch equation. The urinary amount of ammonium as well as phosphate was determined in residual probes. The absolute amount of free protons in urine was obtained from the pH of the respective samples. The amount of protons generated during normovolemic hemodilution was approximately 0.6 mmol. During experimental time (5.5 h), distinct urinary ammonium excretion occurred (Voluven 0.52 mmol, Volulyte 0.39 mmol and Gelafundin 0.77 mmol). Proton excretion via the phosphate buffer constituted 0.04 mmol in every experimental group. Excretion of free protons was in the range of 10-6 mmol. The present data prove that the prompt rise in urinary ammonium excretion is also valid for acute metabolic acidosis originating from severe normovolemic hemodilution.

Download Full-text

Increased expression of cyclooxygenase-1 and -2 in the diabetic rat renal medulla

AJP Renal Physiology ◽

10.1152/ajprenal.00434.2002 ◽

2003 ◽

Vol 285 (6) ◽

pp. F1068-F1077 ◽

Cited By ~ 13

Author(s):

Rania Nasrallah ◽

Anne Landry ◽

Sonia Singh ◽

Monika Sklepowicz ◽

Richard L. Hébert

Keyword(s):

Collecting Duct ◽

Renal Medulla ◽

Immunohistochemical Analysis ◽

Diabetic Rats ◽

Diabetic Rat ◽

Fourfold Increase ◽

Protein Levels ◽

Cyclooxygenase 1 ◽

Streptozotocin Diabetic Rats ◽

Medullary Collecting Duct

Alterations in renal prostaglandins (PGs) may contribute to some of the renal manifestations in diabetes leading to nephropathy. PG production is dependent on the activity of cyclooxygenases (COX-1 AND -2) and PG synthases. Our present study investigated levels of these enzymes in streptozotocin-diabetic rats at 2, 4, 6, and 8 wk of diabetes. Immunohistochemical analysis revealed an increase in COX signal in the inner and outer medulla of diabetic rats. This was confirmed by Western blotting, showing up to a fourfold increase in both COX isoforms at 4–6 wk of diabetes. Also, Western blot analysis revealed a sixfold increase in PGE2 synthase expression in the outer medullary region of 6-wk diabetic rats but no difference in the inner medulla. In cultured rat inner medullary collecting duct (IMCD), levels of COX were increased two- to threefold in cells exposed for 4 days to 37.5 mM glucose compared with control of 17.5 mM. While no change in PGE2 synthase levels was noted, PGE2 synthesis was increased. Furthermore, levels of EP1 and EP4 mRNA were increased, as well as a twofold increase in EP4 protein levels. Future studies will determine which COX isoform is contributing to the majority of PGE2 produced in the diabetic IMCD and the significance of these findings to disturbances in IMCD function and to the progression of diabetic nephropathy.

Download Full-text

Carbonic anhydrase II and IV mRNA in rabbit nephron segments: stimulation during metabolic acidosis

AJP Renal Physiology ◽

10.1152/ajprenal.1998.274.2.f259 ◽

1998 ◽

Vol 274 (2) ◽

pp. F259-F267 ◽

Cited By ~ 8

Author(s):

Shuichi Tsuruoka ◽

Ann M. Kittelberger ◽

George J. Schwartz

Keyword(s):

Metabolic Acidosis ◽

Carbonic Anhydrase ◽

Mrna Expression ◽

Collecting Duct ◽

Proximal Tubules ◽

Rabbit Kidney ◽

Rt Pcr ◽

Chronic Metabolic Acidosis ◽

Nephron Segments ◽

Medullary Collecting Duct

Carbonic anhydrase (CA) facilitates renal bicarbonate reabsorption and acid excretion. Cytosolic CA II catalyzes the buffering of intracellular hydroxyl ions by CO2, whereas membrane-bound CA IV catalyzes the dehydration of carbonic acid generated from the secretion of protons. Although CA II and IV are expressed in rabbit kidney, it is not entirely clear which segments express which isoforms. It was the purpose of this study to characterize the expression of CA II and CA IV mRNAs by specific segments of the nephron using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and to determine the effect of chronic metabolic acidosis on CA expression by those segments. Individual nephron segments (usually 1–2 mm) were isolated by microdissection and subjected to RT-PCR. Amplification was performed simultaneously for CA IV, CA II, and malate dehydrogenase (MDH), a housekeeping gene. The intensities of the PCR products were quantitated by densitometry. CA IV mRNA was expressed by S1 and S2 proximal tubules and by outer medullary collecting duct from inner stripe (OMCDi) and outer stripe and initial inner medullary collecting duct (IMCDi). CA II mRNA was expressed by S1, S2, and S3 proximal tubules, thin descending limb, connecting segment (CNT), and all collecting duct segments. Acid loading induced CA IV mRNA expression in S1 and S2 proximal tubules and in OMCDi and IMCDi. CA II mRNA was induced by acidosis in all three proximal segments and nearly all distal segments beginning with CNT. No upregulation of MDH mRNA expression occurred. These adaptive increases in CA II and IV mRNAs are potentially important in the kidney’s adaptation to chronic metabolic acidosis.

Download Full-text