Correlation between urine anion gap and urine ammonia-creatinine ratio in healthy cats and cats with kidney disease

BACKGROUND

Ammonium excretion decreases as kidney function decreases in several species, including cats, and may have predictive or prognostic value in patients with chronic kidney disease (CKD). Urine ammonia measurement is not readily available in clinical practice, and urine anion gap (UAG) has been proposed as a surrogate test.

OBJECTIVES

Evaluate the correlation between urine ammonia-to-creatinine ratio (UACR) and UAG in healthy cats and those with CKD and determine if a significant difference exists between UAG of healthy cats and cats with CKD.

ANIMALS

Urine samples collected from healthy client-owned cats (n = 59) and those with stable CKD (n = 17).

METHODS

Urine electrolyte concentrations were measured using a commercial chemistry analyzer and UAG was calculated as ([sodium] + [potassium]) - [chloride]. Urine ammonia and creatinine concentrations had been measured previously using commercially available enzymatic assays and used to calculate UACR. Spearman's rank correlation coefficient between UAG and UACR was calculated for both groups. The UAG values of healthy cats and cats with CKD were assessed using the Mann-Whitney test (P < .05).

RESULTS

The UAG was inversely correlated with UACR in healthy cats (P < .002, r0  = -0.40) but not in cats with CKD (P = .55; r0  = -0.15). A significant difference was found between UAG in healthy cats and those with CKD (P < .001).

CONCLUSIONS AND CLINICAL IMPORTANCE

The UAG calculation cannot be used as a substitute for UACR in cats. The clinical relevance of UAG differences between healthy cats and those with CKD remains unknown.

Development of a reference interval for urinary ammonia-to-creatinine ratio in feline patients

BACKGROUND

Disruption of acid-base homeostasis can lead to many clinical problems. Ammonia excretion by the kidneys is critical to maintaining acid-base homeostasis through bicarbonate production. Measurement of ammonia excretion may help determine if the kidneys are properly functioning in maintaining acid-base balance. Reference intervals are essential tools for clinical decision-making but do not currently exist for urinary ammonia-to-creatinine ratio (UACR) in feline patients.

OBJECTIVE

This study aimed to generate a reference interval (RI) for UACR in healthy adult cats.

METHODS

The study used samples from client-owned adult healthy cats that presented to the University of Florida Primary Care and Dentistry service (n = 92). Physical examination, serum biochemistry, urinalysis, urine ammonia, and creatinine concentrations were measured. Cats were excluded if there were significant abnormalities in their urinalysis or biochemistry panel. The RI for UACR was calculated according to the recommendation of the American Society for Veterinary Clinical Pathology. The UACR was evaluated for correlation with serum bicarbonate, weight, age, and sex.

RESULTS

The RI for UACR was 3.4-20.7 with 90% confidence intervals for the lower and upper limits of (3.0-3.7) and (16.0-23.7), respectively. No significant correlation with age, sex, or weight was found. There was no discernable relationship between serum bicarbonate and UACR.

CONCLUSIONS

Establishing an RI for UACR in healthy adult cats will allow further studies to determine if changes in UACR are observed during specific disease states.

Sex differences in renal transporters: assessment and functional consequences

Mammalian kidneys are specialized to maintain fluid and electrolyte homeostasis. The epithelial transport processes along the renal tubule that match output to input have long been the subject of experimental and theoretical study. However, emerging data have identified a new dimension of investigation: sex. Like most tissues, the structure and function of the kidney is regulated by sex hormones and chromosomes. Available data demonstrate sex differences in the abundance of kidney solute and electrolyte transporters, establishing that renal tubular organization and operation are distinctly different in females and males. Newer studies have provided insights into the physiological consequences of these sex differences. Computational simulations predict that sex differences in transporter abundance are likely driven to optimize reproduction, enabling adaptive responses to the nutritional requirements of serial pregnancies and lactation - normal life-cycle changes that challenge the ability of renal transporters to maintain fluid and electrolyte homeostasis. Later in life, females may also undergo menopause, which is associated with changes in disease risk. Although numerous knowledge gaps remain, ongoing studies will provide further insights into the sex-specific mechanisms of sodium, potassium, acid-base and volume physiology throughout the life cycle, which may lead to therapeutic opportunities.

Use of serum osmolality to identify heart disease stage in dogs and relationship to mathematical chloride correction

BACKGROUND

Heart failure-associated hypochloremia can be depletional from diuretics or dilutional from water retention. Serum osmolality reflects water balance but has not been evaluated in dogs with heart disease.

HYPOTHESIS

To determine if serum osmolality is related to heart disease stage and amount of mathematical correction of serum chloride (Cl- ) concentrations in healthy dogs and dogs with myxomatous mitral valve degeneration (MMVD).

ANIMALS

Seventy-seven dogs (20 healthy, 25 Stage B MMVD, 32 Stage C/D MMVD).

METHODS

Serum Cl- concentrations were mathematically corrected. Osmolality was calculated (calOsm) and directly measured by freezing point depression (dmOsm) and compared by Bland-Altman analysis. Biochemical variables and osmolality were compared among healthy, Stage B, and Stage C/D dogs. Correlations were explored between osmolality and biochemical variables. Median and range are presented. P < .05 was considered significant.

RESULTS

The calOsm was different among groups (P = .003), with Stage B (310 mOsm/kg; 306, 316) and C/D dogs (312 mOsm/kg; 308, 319) having higher calOsm than healthy dogs (305 mOsm/kg; 302, 308). Osmolality methods were moderately correlated (P < .0001, rs  = .46) but with proportional bias and poor agreement. The amount of Cl- correction was negatively correlated with calOsm (P < .0001, rs  = -.78) and dmOsm (P = .004, rs  = -.33). Serum bicarbonate concentration was negatively correlated with Cl- (P < .0001, rs  = -.67).

CONCLUSIONS AND CLINICAL IMPORTANCE

Dogs with Stage B and Stage C/D heart disease had higher calOsm than healthy dogs. Osmolality was inversely related to the amount of Cl- correction, which supports its use in assessing relative body water content. Poor agreement between calOsm and dmOsm prevents methodological interchange.

An Update on Kidney Ammonium Transport Along the Nephron

Acid-base homeostasis is critical to the maintenance of normal health. The kidneys have a central role in bicarbonate generation, which occurs through the process of net acid excretion. Renal ammonia excretion is the predominant component of renal net acid excretion under basal conditions and in response to acid-base disturbances. Ammonia produced in the kidney is selectively transported into the urine or the renal vein. The amount of ammonia produced by the kidney that is excreted in the urine varies dramatically in response to physiological stimuli. Recent studies have advanced our understanding of ammonia metabolism's molecular mechanisms and regulation. Ammonia transport has been advanced by recognizing that the specific transport of NH3 and NH₄⁺ by specific membrane proteins is critical to ammonia transport. Other studies show that proximal tubule protein, NBCe1, specifically the A variant, significantly regulates renal ammonia metabolism. This review discusses these critical aspects of the emerging features of ammonia metabolism and transport.

The proximal tubule through an NBCe1-dependent mechanism regulates collecting duct phenotypic and remodeling responses to acidosis

The renal response to acid-base disturbances involves phenotypic and remodeling changes in the collecting duct. This study examines whether the proximal tubule controls these responses. We examined mice with genetic deletion of proteins present only in the proximal tubule, either the A variant or both A and B variants of isoform 1 of the electrogenic Na+-bicarbonate cotransporter (NBCe1). Both knockout (KO) mice have spontaneous metabolic acidosis. We then determined the collecting duct phenotypic responses to this acidosis and the remodeling responses to exogenous acid loading. Despite the spontaneous acidosis in NBCe1-A KO mice, type A intercalated cells in the inner stripe of the outer medullary collecting duct (OMCDis) exhibited decreased height and reduced expression of H+-ATPase, anion exchanger 1, Rhesus B glycoprotein, and Rhesus C glycoprotein. Combined kidney-specific NBCe1-A/B deletion induced similar changes. Ultrastructural imaging showed decreased apical plasma membrane and increased vesicular H+-ATPase in OMCDis type A intercalated cell in NBCe1-A KO mice. Next, we examined the collecting duct remodeling response to acidosis. In wild-type mice, acid loading increased the proportion of type A intercalated cells in the connecting tubule (CNT) and OMCDis, and it decreased the proportion of non-A, non-B intercalated cells in the connecting tubule, and type B intercalated cells in the cortical collecting duct (CCD). These changes were absent in NBCe1-A KO mice. We conclude that the collecting duct phenotypic and remodeling responses depend on proximal tubule-dependent signaling mechanisms blocked by constitutive deletion of proximal tubule NBCe1 proteins.NEW & NOTEWORTHY This study shows that the proximal tubule regulates collecting duct phenotypic and remodeling responses to acidosis.

Case Report: Septic Pericarditis With Achromobacter xyloxidans in an Immunosuppressed Dog

A 5-year-old female spayed French Bulldog presented for anorexia and increased respiratory rate. On presentation, she was dyspneic with stridor and increased bronchovesicular sounds. Point-of-care ultrasound identified pericardial effusion. Thoracic radiographs identified pleural effusion, a wide cranial mediastinum, and multifocal unstructured interstitial pulmonary opacities. Bloodwork revealed a moderate leukocytosis characterized by a mature neutrophilia with a left shift, hypoalbuminemia, mildly increased alkaline phosphatase activity, and moderate hypokalemia. Thoracic CT findings revealed moderate pericardial and bilateral pleural effusion, mediastinal effusion, and moderate cranial mediastinal lymphadenopathy. Diagnostic thoracocentesis and pericardiocentesis revealed septic exudates with bacilli. Two days later, a median sternotomy and pericardiectomy were performed. Aerobic cultures of the effusions grew Achromobacter xylosoxidans ss deitrificans. The patient was treated with Amoxicillin-clavulanate and enrofloxacin for 12 weeks and clinically fully recovered. Achromobacter xylosoxidans has not been reported as a cause of purulent pericarditis and pyothorax in a dog. Uniquely, this patient is suspected of developing this infection secondary to immunosuppression.

Acid-base effects of combined renal deletion of NBCe1-A and NBCe1-B

The molecular mechanisms regulating ammonia metabolism are fundamental to acid-base homeostasis. Deletion of the A splice variant of Na+-bicarbonate cotransporter, electrogenic, isoform 1 (NBCe1-A) partially blocks the effect of acidosis to increase urinary ammonia excretion, and this appears to involve the dysregulated expression of ammoniagenic enzymes in the proximal tubule (PT) in the cortex but not in the outer medulla (OM). A second NBCe1 splice variant, NBCe1-B, is present throughout the PT, including the OM, where NBCe1-A is not present. The purpose of the present study was to determine the effect of combined renal deletion of NBCe1-A and NBCe1-B on systemic and PT ammonia metabolism. We generated NBCe1-A/B deletion using Cre-loxP techniques and used Cre-negative mice as controls. As renal NBCe1-A and NBCe1-B expression is limited to the PT, Cre-positive mice had PT NBCe1-A/B deletion [PT-NBCe1-A/B knockout (KO)]. Although on a basal diet, PT-NBCe1-A/B KO mice had severe metabolic acidosis, yet urinary ammonia excretion was not changed significantly. PT-NBCe1-A/B KO decreased the expression of phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase and increased the expression of glutamine synthetase, an ammonia-recycling enzyme, in PTs in both the cortex and OM. Exogenous acid loading increased ammonia excretion in control mice, but PT-NBCe1-A/B KO prevented any increase. PT-NBCe1-A/B KO significantly blunted acid loading-induced changes in phosphate-dependent glutaminase, phosphoenolpyruvate carboxykinase, and glutamine synthetase expression in PTs in both the cortex and OM. We conclude that NBCe1-B, at least in the presence of NBCe1-A deletion, contributes to PT ammonia metabolism in the OM and thereby to systemic acid-base regulation.NEW & NOTEWORTHY The results of the present study show that combined deletion of both A and B splice variants of electrogenic Na+-bicarbonate cotransporter 1 from the proximal tubule impairs acid-base homeostasis and completely blocks changes in ammonia excretion in response to acidosis, indicating that both proteins are critical to acid-base homeostasis.

Familial nephropathy in Bracchi Italiani: 8 cases (2012-2019)

OBJECTIVE

To characterize the signalment, clinical signs, clinical pathological and histologic findings, and outcome in 8 related Bracchi Italiani with proteinuric kidney disease.

ANIMALS

8 client-owned Bracchi Italiani.

PROCEDURES

Health records submitted to the Bracco Italiano Health Foundation and the Bracco Italiano Club of America between 2012 and 2019 were reviewed for dogs with evidence of nephropathy for which histologic diagnoses were obtained. Pedigree, signalment, clinical signs, diagnostic test results (including microscopic examination of kidney tissue samples collected ante- or postmortem), and outcome were acquired. Results were presented as descriptive statistics.

RESULTS

The most common clinical sign in affected dogs was inappetence. All dogs were proteinuric, and 4 dogs were azotemic. Seven dogs developed clinical signs of kidney disease and were euthanized a median of 75 days postdiagnosis. Six dogs had glomerular amyloidosis, and 1 dog each had nephrosclerosis and nonamyloidotic fibrillar glomerulopathy.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that the clinical presentation may vary in affected dogs, and proteinuria in young or middle-aged Bracchi Italiani should raise the concern for hereditary nephropathy. Prognosis is likely poor once clinical signs are noted.

Establishment of an RI for the urine ammonia-to-creatinine ratio in dogs

Background

Ammonia is produced and excreted by the kidney, contributing to systemic acid‐base homeostasis through the production of bicarbonate. Disorders of acid‐base balance can lead to many clinical problems and measuring ammonia excretion helps in determining if the kidneys are responding to acid‐base challenges appropriately. Reference intervals are integral to clinical decision‐making, and there is no current RI for the urine ammonia‐to‐creatinine ratio (UACR) in dogs.

Objective

This study aimed to generate an RI for the UACR in healthy adult dogs.

Methods

The study used adult, client‐owned dogs that were presented to the University of Florida Primary Care and Dentistry service (n = 60). Physical examinations were performed and serum chemistry and urinalysis samples were obtained. Urine ammonia and creatinine concentrations were determined. Dogs were excluded if there were significant abnormalities in either their urinalysis or serum chemistry results. The RI for the UACR was calculated according to the recommendation of the American Society for Veterinary Clinical Pathology. Data were evaluated for correlation with serum bicarbonate, weight, age, and sex.

Results

The RIs for the UACR were 0.16‐23.69 with 90% confidence intervals for the lower and upper limits of (0.13‐1.17) and (20.50‐23.75), respectively. No significant impact of age, sex, or weight was found. There was no discernable relationship between serum bicarbonate and UACR.

Conclusions

Establishing an RI for UACR in healthy adult dogs will allow for further studies to determine if alterations are observed during specific disease states.

Role of the renal androgen receptor in sex differences in ammonia metabolism

There are sex differences in renal ammonia metabolism and structure, many of which are mediated by testosterone. The goal of the present study was to determine the role of renal expression of testosterone's canonical receptor, androgen receptor (AR), in these sexual dimorphisms. We studied mice with kidney-specific AR deletion [KS-AR-knockout (KO)] generated using Cre/loxP techniques; control mice were Cre-negative littermates (wild type). In male but not female mice, KS-AR-KO increased ammonia excretion, which eliminated sex differences. Although renal structural size typically parallel ammonia excretion, KS-AR-KO decreased kidney size, cortical proximal tubule volume density, and cortical proximal tubule cell height in males-neither were altered in females and collecting duct volume density was unaltered in both sexes. Analysis of key protein involved in ammonia handling showed in male mice that KS-AR-KO increased both phosphoenolpyruvate carboxykinase (PEPCK) and Na+-K+-2Cl- cotransporter (NKCC2) expression and decreased Na+/H+ exchanger isoform 3 (NHE3) and electrogenic Na+-bicarbonate cotransporter 1 (NBCe1)-A expression. In female mice, KS-AR-KO did not alter these parameters. These effects occurred even though KS-AR-KO did not alter plasma testosterone, food intake, or serum Na+, K+, or [Formula: see text] significantly in either sex. In conclusion, AR-dependent signaling pathways in male, but not female, kidneys regulate PEPCK and NKCC2 expression and lead to the sexual differences in ammonia excretion. Opposing effects on NHE3 and NBCe1-A expression likely limit the magnitude of ammonia excretion changes. As AR is not present in the thick ascending limb, the effect of KS-AR-KO on NKCC2 expression is indirect. Finally, AR mediates the greater kidney size and proximal tubule volume density in male compared with female mice.NEW & NOTEWORTHY Sexual dimorphisms in ammonia metabolism involve androgen receptor (AR)-dependent signaling pathways in male, but not female, kidneys that lead to altered proximal tubule (PT), phosphoenolpyruvate carboxykinase, and thick ascending limb Na+-K+-2Cl- cotransporter expression. Adaptive responses in Na+/H+ exchanger 3 and electrogenic Na+-bicarbonate cotransporter 1-A expression limit the magnitude of the effect on ammonia excretion. Finally, the greater kidney size and PT volume density in male mice is the result of PT androgen signaling through AR.

Sex differences in renal ammonia metabolism

Sexual dimorphic variations are present in many aspects of biology and involve the structure and/or function of nearly every organ system. Acid-base homeostasis is critical for optimal health, and renal ammonia metabolism has a major role in the maintenance of acid-base homeostasis. Recent studies have shown sex-dependent differences in renal ammonia metabolism with regard to both basal ammonia excretion and the response to an exogenous acid load. These sexual dimorphisms are associated with structural changes in the proximal tubule and the collecting duct and variations in the expression of multiple proteins involved in ammonia metabolism and transport. Studies using orchiectomy-induced testosterone deficiency and physiological testosterone replacement have shown that testosterone underlies much of the sex-dependent differences in the proximal tubule. This parallels the finding that the canonical testosterone target receptor, androgen receptor (AR), is present exclusively in the proximal tubule. Thus testosterone, possibly acting through AR activation, regulates multiple components of renal structure and ammonia metabolism. The lack of detectable AR in the remainder of the nephron and collecting duct suggests that some dimorphisms in renal structure and ammonia transporter expression are mediated through mechanisms other than direct testosterone-dependent AR activation. A better understanding of the mechanism and biological implications of sex's effect on renal structure and ammonia metabolism is critical for optimizing our ability to care for both men and women with acid-base disturbances.

Testosterone modulates renal ammonia metabolism

There are substantial sex differences in renal structure and ammonia metabolism that correlate with differences in expression of proteins involved in ammonia generation and transport. This study determined the role of testis-derived testosterone in these differences. We studied 4-mo-old male C57BL/6 mice 4 and 8 wk after either bilateral orchiectomy (ORCH) or sham-operated control surgery and determined the effect of testosterone replacement to reverse the effects of ORCH. Finally, we determined the cellular expression of androgen receptor (AR), testosterone's canonical target receptor. ORCH decreased kidney and proximal tubule size, and testosterone replacement reversed this effect. ORCH increased ammonia excretion in a testosterone-dependent fashion; this occurred despite similar food intake, which is the primary component of endogenous acid production. ORCH increased expression of both phosphoenolpyruvate, a major ammonia-generating protein, and Na⁺-K⁺-2Cl^- cotransporter, which mediates thick ascending limb ammonia reabsorption; these changes were reversed with testosterone replacement. Orchiectomy also decreased expression of Na⁺/H⁺ exchanger isoform 3, which mediates proximal tubule ammonia secretion, in a testosterone-dependent pattern. Finally, ARs are expressed throughout the proximal tubule in both the male and female kidney. Testosterone, possibly acting through ARs, has dramatic effects on kidney and proximal tubule size and decreases ammonia excretion through its effects on several key proteins involved in ammonia metabolism.

NBCe1-A is required for the renal ammonia and K+ response to hypokalemia

Hypokalemia increases ammonia excretion and decreases K⁺ excretion. The present study examined the role of the proximal tubule protein NBCe1-A in these responses. We studied mice with Na⁺-bicarbonate cotransporter electrogenic, isoform 1, splice variant A (NBCe1-A) deletion [knockout (KO) mice] and their wild-type (WT) littermates were provided either K⁺ control or K⁺-free diet. We also used tissue sections to determine the effect of extracellular ammonia on NaCl cotransporter (NCC) phosphorylation. The K⁺-free diet significantly increased proximal tubule NBCe1-A and ammonia excretion in WT mice, and NBCe1-A deletion blunted the ammonia excretion response. NBCe1-A deletion inhibited the ammoniagenic/ammonia recycling enzyme response in the cortical proximal tubule (PT), where NBCe1-A is present in WT mice. In the outer medulla, where NBCe1-A is not present, the PT ammonia metabolism response was accentuated by NBCe1-A deletion. KO mice developed more severe hypokalemia and had greater urinary K⁺ excretion during the K⁺-free diet than did WT mice. This was associated with blunting of the hypokalemia-induced change in NCC phosphorylation. NBCe1-A KO mice have systemic metabolic acidosis, but experimentally induced metabolic acidosis did not alter NCC phosphorylation. Although KO mice have impaired ammonia metabolism, experiments in tissue sections showed that lack of ammonia does impair NCC phosphorylation. Finally, urinary aldosterone was greater in KO mice than in WT mice, but neither expression of epithelial Na⁺ channel α-, β-, and γ-subunits nor of H⁺-K⁺-ATPase α₁- or α₂-subunits correlated with changes in urinary K⁺. We conclude that NBCe1-A is critical for the effect of diet-induced hypokalemia to increase cortical proximal tubule ammonia generation and for the expected decrease in urinary K⁺ excretion.

Differences in acidosis-stimulated renal ammonia metabolism in the male and female kidney

Renal ammonia excretion is a critical component of acid-base homeostasis, and changes in ammonia excretion are the predominant component of increased net acid excretion in response to metabolic acidosis. We recently reported substantial sex-dependent differences in basal ammonia metabolism that correlate with sex-dependent differences in renal structure and expression of key proteins involved in ammonia metabolism. The purpose of the present study was to investigate the effect of sex on the renal ammonia response to an exogenous acid load. We studied 4-mo-old C57BL/6 mice. Ammonia excretion, which was less in male mice under basal conditions, increased in response to acid loading to a greater extent in male mice, such that maximal ammonia excretion did not differ between the sexes. Fundamental structural sex differences in the nonacid-loaded kidney persisted after acid loading, with less cortical proximal tubule volume density in the female kidney than in the male kidney, whereas collecting duct volume density was greater in the female kidney. To further investigate sex-dependent differences in the response to acid loading, we examined the expression of proteins involved in ammonia metabolism. The change in expression of phosphoenolpyruvate carboxykinase and Rh family B glycoprotein with acid loading was greater in male mice than in female mice, whereas Na⁺-K⁺-2Cl^- cotransporter and inner stripe of the outer medulla intercalated cell Rh family C glycoprotein expression were significantly greater in female mice than in male mice. There was no significant sex difference in glutamine synthetase, Na⁺/H⁺ exchanger isoform 3, or electrogenic Na⁺-bicarbonate cotransporter 1 variant A protein expression in response to acid loading. We conclude that substantial sex-dependent differences in the renal ammonia response to acid loading enable a similar maximum ammonia excretion response.

Regulation of renal NaDC1 expression and citrate excretion by NBCe1-A

Citrate is critical for acid-base homeostasis and to prevent calcium nephrolithiasis. Both metabolic acidosis and hypokalemia decrease citrate excretion and increase expression of Na+-dicarboxylate cotransporter 1 (NaDC1; SLC13A2), the primary protein involved in citrate reabsorption. However, the mechanisms transducing extracellular signals and mediating these responses are incompletely understood. The purpose of the present study was to determine the role of the Na+-coupled electrogenic bicarbonate cotransporter (NBCe1) A variant (NBCe1-A) in citrate metabolism under basal conditions and in response to acid loading and hypokalemia. NBCe1-A deletion increased citrate excretion and decreased NaDC1 expression in the proximal convoluted tubules (PCT) and proximal straight tubules (PST) in the medullary ray (PST-MR) but not in the PST in the outer medulla (PST-OM). Acid loading wild-type (WT) mice decreased citrate excretion. NaDC1 expression increased only in the PCT and PST-MR and not in the PST-MR. In NBCe1-A knockout (KO) mice, the acid loading change in citrate excretion was unaffected, changes in PCT NaDC1 expression were blocked, and there was an adaptive increase in PST-MR. Hypokalemia in WT mice decreased citrate excretion; NaDC1 expression increased only in the PCT and PST-MR. NBCe1-A KO blocked both the citrate and NaDC1 changes. We conclude that 1) adaptive changes in NaDC1 expression in response to metabolic acidosis and hypokalemia occur specifically in the PCT and PST-MR, i.e., in cortical proximal tubule segments; 2) NBCe1-A is necessary for normal basal, metabolic acidosis and hypokalemia-stimulated citrate metabolism and does so by regulating NaDC1 expression in cortical proximal tubule segments; and 3) adaptive increases in PST-OM NaDC1 expression occur in NBCe1-A KO mice in response to acid loading that do not occur in WT mice.

Expression of the B splice variant of NBCe1 (SLC4A4) in the mouse kidney

Sodium-coupled bicarbonate transporters are critical for renal electrolyte transport. The electrogenic, sodium-coupled bicarbonate cotransporter, isoform 1 (NBCe1), encoded by the SLC4A4 geneencoded by the SLC4A4 gene has five multiple splice variants; the A splice variant, NBCe1-A, is the primary basolateral bicarbonate transporter in the proximal convoluted tubule. This study's purpose was to determine if there is expression of additional NBCe1 splice variants in the mouse kidney, their cellular distribution, and their regulation by metabolic acidosis. In wild-type mice, an antibody reactive only to NBCe1-A showed basolateral immunolabel only in cortical proximal tubule (PT) segments, whereas an antibody reactive to all NBCe1 splice variants (pan-NBCe1) showed basolateral immunolabel in PT segments in both the cortex and outer medulla. In mice with NBCe1-A deletion, the pan-NBCe1 antibody showed basolateral PT immunolabel in both the renal cortex and outer stripe of the outer medulla, and immunoblot analysis showed expression of a ~121-kDa protein. RT-PCR of mRNA from NBCe1-A knockout mice directed at splice variant-specific regions showed expression of only NBCe1-B mRNA. In wild-type kidney, RT-PCR confirmed expression of mRNA for the NBCe1-B splice variant and absence of mRNA for the C, D, and E splice variants. Finally, exogenous acid loading increased expression in the proximal straight tubule in the outer stripe of the outer medulla. These studies demonstrate that the NBCe1-B splice variant is present in the PT, and its expression increases in response to exogenous acid loading, suggesting it participates in the PT contribution to acid-base homeostasis.

Differences in renal ammonia metabolism in male and female kidney

Renal ammonia metabolism has a major role in the maintenance of acid-base homeostasis. Sex differences are well recognized as an important biological variable in many aspects of renal function, including fluid and electrolyte metabolism. However, sex differences in renal ammonia metabolism have not been previously reported. Therefore, the purpose of the current study was to investigate sex differences in renal ammonia metabolism. We studied 4-mo-old wild-type C57BL/6 mice fed a normal diet. Despite similar levels of food intake, and, thus, protein intake, which is the primary determinant of endogenous acid production, female mice excreted greater amounts of ammonia, but not titratable acids, than did male mice. This difference in ammonia metabolism was associated with fundamental structural differences between the female and male kidney. In the female mouse kidney, proximal tubules account for a lower percentage of the renal cortical parenchyma compared with the male kidney, whereas collecting ducts account for a greater percentage of the renal parenchyma than in male kidneys. To further investigate the mechanism(s) behind the greater ammonia excretion in female mice, we examined differences in the expression of proteins involved in renal ammonia metabolism and transport. Greater basal ammonia excretion in females was associated with greater expression of PEPCK, glutamine synthetase, NKCC2, Rhbg, and Rhcg than was observed in male mice. We conclude that there are sex differences in basal ammonia metabolism that involve both renal structural differences and differences in expression of proteins involved in ammonia metabolism.

NBCe1-A Regulates Proximal Tubule Ammonia Metabolism under Basal Conditions and in Response to Metabolic Acidosis

Renal ammonia metabolism is the primary mechanism through which the kidneys maintain acid-base homeostasis, but the molecular mechanisms regulating renal ammonia generation are unclear. In these studies, we evaluated the role of the proximal tubule basolateral plasma membrane electrogenic sodium bicarbonate cotransporter 1 variant A (NBCe1-A) in this process. Deletion of the NBCe1-A gene caused severe spontaneous metabolic acidosis in mice. Despite this metabolic acidosis, which normally causes a dramatic increase in ammonia excretion, absolute urinary ammonia concentration was unaltered. Additionally, NBCe1-A deletion almost completely blocked the ability to increase ammonia excretion after exogenous acid loading. Under basal conditions and during acid loading, urine pH was more acidic in mice with NBCe1-A deletion than in wild-type controls, indicating that the abnormal ammonia excretion was not caused by a primary failure of urine acidification. Instead, NBCe1-A deletion altered the expression levels of multiple enzymes involved in proximal tubule ammonia generation, including phosphate-dependent glutaminase, phosphoenolpyruvate carboxykinase, and glutamine synthetase, under basal conditions and after exogenous acid loading. Deletion of NBCe1-A did not impair expression of key proteins involved in collecting duct ammonia secretion. These studies demonstrate that the integral membrane protein NBCe1-A has a critical role in basal and acidosis-stimulated ammonia metabolism through the regulation of proximal tubule ammonia-metabolizing enzymes.

Mechanism of Hyperkalemia-Induced Metabolic Acidosis

Background Hyperkalemia in association with metabolic acidosis that are out of proportion to changes in glomerular filtration rate defines type 4 renal tubular acidosis (RTA), the most common RTA observed, but the molecular mechanisms underlying the associated metabolic acidosis are incompletely understood. We sought to determine whether hyperkalemia directly causes metabolic acidosis and, if so, the mechanisms through which this occurs.Methods We studied a genetic model of hyperkalemia that results from early distal convoluted tubule (DCT)-specific overexpression of constitutively active Ste20/SPS1-related proline-alanine-rich kinase (DCT-CA-SPAK).Results DCT-CA-SPAK mice developed hyperkalemia in association with metabolic acidosis and suppressed ammonia excretion; however, titratable acid excretion and urine pH were unchanged compared with those in wild-type mice. Abnormal ammonia excretion in DCT-CA-SPAK mice associated with decreased proximal tubule expression of the ammonia-generating enzymes phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase and overexpression of the ammonia-recycling enzyme glutamine synthetase. These mice also had decreased expression of the ammonia transporter family member Rhcg and decreased apical polarization of H⁺-ATPase in the inner stripe of the outer medullary collecting duct. Correcting the hyperkalemia by treatment with hydrochlorothiazide corrected the metabolic acidosis, increased ammonia excretion, and normalized ammoniagenic enzyme and Rhcg expression in DCT-CA-SPAK mice. In wild-type mice, induction of hyperkalemia by administration of the epithelial sodium channel blocker benzamil caused hyperkalemia and suppressed ammonia excretion.Conclusions Hyperkalemia decreases proximal tubule ammonia generation and collecting duct ammonia transport, leading to impaired ammonia excretion that causes metabolic acidosis.

Investigation of an N-Terminal Prohormone of Brain Natriuretic Peptide Point-of-Care ELISA in Clinically Normal Cats and Cats With Cardiac Disease

Background

N‐terminal prohormone of brain natriuretic peptide (NT‐proBNP) concentrations may be increased in cats with various cardiac disorders. The point‐of‐care (POC) ELISA assay uses the same biologic reagents as the quantitative NT‐proBNP ELISA. Previous studies have evaluated the sensitivity and specificity of the POC ELISA in cats with cardiac disease.

Objectives

To prospectively evaluate the diagnostic utility of the POC ELISA in a select population of cats.

Animals

Thirty‐eight client‐owned cats presented to the University of Florida Cardiology Service for cardiac evaluation. Fifteen apparently healthy cats recruited as part of another study.

Methods

Physical examination and echocardiography were performed in all cats. The POC ELISA was assessed visually as either positive or negative by a reader blinded to the echocardiographic findings, and results were analyzed relative to quantitative assay results.

Results

Twenty‐six cats were diagnosed with underlying cardiac disease, and 27 cats were considered free of cardiac disease. Cats with cardiac disease included: 21 with hypertrophic cardiomyopathy, 2 with unclassified cardiomyopathy, 2 with restrictive cardiomyopathy, and 1 with 3rd degree atrioventricular (AV) block. The POC ELISA differentiated cats with cardiac disease with a sensitivity of 65.4% and specificity of 100%.

Conclusions and Clinical Importance

The POC NT‐proBNP ELISA performed moderately well in a selected population of cats. A negative test result cannot exclude the presence of underlying cardiac disease, and a positive test result indicates that cardiac disease likely is present, but further diagnostic investigation would be indicated for a definitive diagnosis.

Biologic variability of N-terminal pro-brain natriuretic peptide in adult healthy cats

Objectives The biologic variability of N-terminal pro-brain natriuretic peptide (NT-proBNP) and its impact on diagnostic utility is unknown in healthy cats and those with cardiac disease. The purpose of this study was to determine the biologic variation of NT-proBNP within-day and week-to-week in healthy adult cats. Methods Adult cats were prospectively evaluated by complete blood count (CBC), biochemistry, total thyroxine, echocardiography, electrocardiography and blood pressure, to exclude underlying systemic or cardiac disease. Adult healthy cats were enrolled and blood samples were obtained at 11 time points over a 6 week period (0, 2 h, 4 h, 6 h, 8 h, 10 h and at weeks 2, 3, 4, 5 and 6). The intra-individual (coefficient of variation [CV_I]) biologic variation along with index of individuality and reference change values (RCVs) were calculated. Univariate models were analyzed and included comparison of the six different time points for both daily and weekly samples. This was followed by a Tukey's post-hoc adjustment, with a P value of <0.05 being significant. Results The median daily and weekly CV_I for the population were 13.1% (range 0-28.7%) and 21.2% (range 3.9-68.1%), respectively. The index of individuality was 0.99 and 1 for daily and weekly samples, respectively. The median daily and weekly RCVs for the population were 39.8% (range 17.0-80.5%) and 60.5% (range 20.1-187.8%), respectively. Conclusions and relevance This study demonstrates high individual variability for NT-proBNP concentrations in a population of adult healthy cats. Further research is warranted to evaluate NT-proBNP variability, particularly how serial measurements of NT-proBNP may be used in the diagnosis and management of cats with cardiac disease.

Use of endoscopic-assisted argon plasma coagulation for the treatment of colonic vascular ectasia (angiodysplasia) in an adult dog

CASE DESCRIPTION

A 10-year-old neutered male mixed-breed dog was evaluated for a 5-year history of intermittent hematochezia and chronic anemia that were unresponsive to medical treatment.

CLINICAL FINDINGS

Colonoscopy revealed multifocal areas of coalescing tortuous mucosal blood vessels throughout the colon and rectum. Colonic vascular ectasia (angiodysplasia) was diagnosed on the basis of the endoscopic appearance of the lesions.

TREATMENT AND OUTCOME

The dog failed to respond to traditional medical treatments for colonic vascular ectasia and required multiple plasma and blood transfusions. The dog received 4 endoscopic-assisted argon plasma coagulation treatments, which resulted in long-term resolution of gastrointestinal hemorrhage. Colonic perforation occurred during the third argon plasma coagulation treatment. The perforation was surgically repaired. The dog remained free from clinical signs of colonic vascular ectasia for > 1 year after the third argon plasma coagulation treatment and was euthanized because of clinical deterioration associated with progressive heart disease.

CLINICAL RELEVANCE

Endoscopic-assisted argon plasma coagulation treatment is a novel treatment for dogs with colonic vascular ectasia and provided long-term resolution of clinical signs for the dog of this report. In human patients, complications associated with endoscopic-assisted argon plasma coagulation treatment include colonic perforation, which also occurred in the dog of this report.

NEGATIVE REGULATORY EFFECTS OF PHOSPHATIDYLINOSITOL3-KINASE PATHWAY ON PHAGOCYTOSIS AND MACROPINOCYTOSIS IN BOVINE MONOCYTES

Recent studies have shown that monocytes and macrophages not only present antigens to effector T cells and stimulate and shape T cell-mediated immune responses, but they also prime naïve T cells, thus initiating adaptive immune responses. Phosphatidylinositol 3-kinase functions at an early phase of toll-like receptor signaling pathways, modulates the magnitude of the primary immune responses, and is involved in the reorganization of the actin cytoskeleton during macropinocytic and phagocytic antigen uptakes, important early steps in triggering adaptive immune responses. We assessed by flow cytometry the endocytic capacities of bovine monocytes by using endocytic tracers and Salmonella transformed with a green fluorescence plasmid GFP to evaluate macropinocytosis, mannose receptor-mediated endocytosis, and phagocytosis in bovine professional antigen presenting cells, respectively. Our data reveal that wortmannin, an inhibitor of phosphatidylinositol 3-kinase signaling pathway, significantly increased macropinocytosis and phagocytosis but did not affect the mannose receptor-mediated antigen uptake in bovine monocytes. Protein expression data support these findings by showing decreased levels of phosphoinositide 3-kinase in the presence of wortmannin during macropinocytosis. We expanded further the key role of phosphatidylinositol 3-kinase as an endogenous suppressor of primary immune responses, suggesting a novel mechanism of phosphatidylinositol 3-kinase antigen uptake modulation that may provide a unique therapeutic target for controlling excessive inflammation.

aubergineencodes aDrosophilapolar granule component required for pole cell formation and related to eIF2C

In Drosophila oocytes, activation of Oskar translation from a transcript localized to the posterior pole is an essential step in the organization of the pole plasm, specialized cytoplasm that contains germline and abdominal body patterning determinants. Oskar is a component of polar granules, large particles associated with the pole plasm and the germline precursor pole cells of the embryo. aubergine mutants fail to translate oskar mRNA efficiently and are thus defective in posterior body patterning and pole cell formation. We have found that Aubergine protein is related to eukaryotic translation initiation factor 2C and suggest how it may activate translation. In addition, we found that Aubergine was recruited to the posterior pole in a vas-dependent manner and is itself a polar granule component. Consistent with its presence in these structures, Aubergine is required for pole cell formation independently of its initial role in oskar translation. Unlike two other known polar granule components, Vasa and Oskar, Aubergine remains cytoplasmic after pole cell formation, suggesting that the roles of these proteins diverge during embryogenesis.

The basic helix-loop-helix, leucine zipper transcription factor, USF (upstream stimulatory factor), is a key regulator of SF-1 (steroidogenic factor-1) gene expression in pituitary gonadotrope and steroidogenic cells

Tissue-specific expression of the mammalian FTZ-F1 gene is essential for adrenal and gonadal development and sexual differentiation. The FTZ-F1 gene encodes an orphan nuclear receptor, termed SF-1 (steroidogenic factor-1) or Ad4BP, which is a primary transcriptional regulator of several hormone and steroidogenic enzyme genes that are critical for normal physiological function of the hypothalamic-pituitary-gonadal axis in reproduction. The objective of the current study is to understand the molecular mechanisms underlying transcriptional regulation of SF-1 gene expression in the pituitary. We have studied a series of deletion and point mutations in the SF-1 promoter region for transcriptional activity in alphaT3-1 and L/betaT2 (pituitary gonadotrope), CV-1, JEG-3, and Y1 (adrenocortical) cell lines. Our results indicate that maximal expression of the SF-1 promoter in all cell types requires an E box element at -82/-77. This E box sequence (CACGTG) is identical to the binding element for USF (upstream stimulatory factor), a member of the helix-loop-helix family of transcription factors. Studies of the SF-1 gene E box element using gel mobility shift and antibody supershift assays indicate that USF may be a key transcriptional regulator of SF-1 gene expression.

A novel A/T-rich element mediates ANF gene expression during cardiac myocyte hypertrophy

The induction of the atrial natriuretic factor (ANF) gene during alpha 1-adrenergic stimulation of neonatal rat ventricular myocytes has served as a model for gene expression during cardiac muscle cell hypertrophy. This study describes and identifies a single regulatory element that mediates expression of the ANF gene. Deletional mutations were generated in a 639-bp fragment of the ANF promoter that confers alpha 1-adrenergic inducibility to a luciferase reporter gene in transient transfection assays in ventricular myocytes. The results of gel mobility shift and diethylpyrocarbonate (DEPC) interference studies with nuclear cardiac cell extracts identified the nucleotide contract points for a novel A/T-rich element (ANF-AT) at positions -582/-575 that partially mediates alpha 1-adrenergic inducibility. Mutations in the ANF-AT element reduced alpha-adrenergic inducibility of an ANF-TK-luciferase fusion gene in cardiac cells by 35% but had no effect on expression in other muscle and non-muscle cells tested. Gel mobility supershift assays with antibodies directed against the MEF-2 protein, the homeobox protein MHox, or the zinc finger protein HF-1b, document that these factors are not major components of the endogenous ANF-AT binding activity in cardiac muscle cells. The current study provides evidence for a role for a novel A/T-rich element in the regulation of ANF gene expression in cardiac ventricular myocytes.

Myosin light chain-2 luciferase transgenic mice reveal distinct regulatory programs for cardiac and skeletal muscle-specific expression of a single contractile protein gene

To examine the relationship between the cardiac and skeletal muscle gene programs, the current study employs the regulatory (phosphorylatable) myosin light chain (MLC-2) as a model system. Northern blotting, primer extension, and RNase protection studies documented the high level expression of the cardiac MLC-2 mRNA in both mouse cardiac and slow skeletal muscle (soleus). Transgenic mouse lines harboring a 2100- or a 250-base pair rat cardiac MLC-2 promoter/luciferase fusion gene were generated, demonstrating high levels of luciferase activity in cardiac muscle, and only background luminescence in slow skeletal muscle and non-muscle tissues. As assessed by in situ hybridization, immunofluorescence, and luminescence assays of luciferase reporter activity in various regions of the heart, both the endogenous MLC-2 gene and the MLC-2 luciferase fusion gene were expressed exclusively in the ventricular compartment, with expression in the atrium at background levels. Point mutations within the conserved regulatory sites HF-1a and HF-1b significantly cripple ventricular muscle specificity, while mutation of the single E-box site was without effect, suggesting that ventricular muscle-specific expression occurs through an E-box-independent pathway. This study provides direct evidence that the cis regulatory sequences in the cardiac/slow twitch MLC-2 gene which confer cardiac and skeletal muscle-specific expression can be clearly segregated, suggesting that distinct regulatory programs may have evolved to control the tissue-specific expression of this single contractile protein gene in cardiac and skeletal muscle.

Segregation of atrial-specific and inducible expression of an atrial natriuretic factor transgene in an in vivo murine model of cardiac hypertrophy

To study the mechanisms that activate expression of the atrial natriuretic factor (ANF) gene during pressure-induced hypertrophy, we have developed and characterized an in vivo murine model of myocardial cell hypertrophy. We employed microsurgical techniques to produce a stable 35- to 45-mmHg pressure gradient across the thoracic aorta of the mouse that is associated with rapid and transient expression of an immediate-early gene program (c-fos/c-jun/junB/Egr-1/nur-77), an increase in heart weight/body weight ratio, and up-regulation of the endogenous ANF gene. These responses that are identical to those in cultured cell and other in vivo models of hypertrophy. To determine whether tissue-specific and inducible expression of the ANF gene can be segregated, we used a transgenic mouse line in which 500 base pairs of the human ANF promoter region directs atrial-specific expression of the simian virus 40 large tumor antigen (T antigen), with no detectable expression in the ventricles. Thoracic aortic banding of these mice led to a 20-fold increase in the endogenous ANF mRNA in the ventricle but no detectable expression of the T-antigen marker gene. This result provides evidence that atrial-specific and inducible expression of the ANF gene can be segregated, suggesting that a distinct set of regulatory cis sequences may mediate the up-regulation of the ANF gene during in vivo pressure overload hypertrophy. This murine model demonstrates the utility of microsurgical techniques to study in vivo cardiac physiology in transgenic mice and should allow the application of genetic approaches to identify the mechanisms that activate ventricular expression of the ANF gene during in vivo hypertrophy.

Co-regulation of the atrial natriuretic factor and cardiac myosin light chain-2 genes during alpha-adrenergic stimulation of neonatal rat ventricular cells. Identification of cis sequences within an embryonic and a constitutive contractile protein gene which mediate inducible expression

To study the mechanisms which mediate the transcriptional activation of cardiac genes during alpha adrenergic stimulation, the present study examined the regulated expression of three cardiac genes, a ventricular embryonic gene (atrial natriuretic factor, ANF), a constitutively expressed contractile protein gene (cardiac MLC-2), and a cardiac sodium channel gene. alpha 1-Adrenergic stimulation activates the expression and release of ANF from neonatal ventricular cells. As assessed by RNase protection analyses, treatment with alpha-adrenergic agonists increases the steady-state levels of ANF mRNA by greater than 15-fold. However, a rat cardiac sodium channel gene mRNA is not induced, indicating that alpha-adrenergic stimulation does not lead to an increase in the expression of all cardiac genes. Studies employing a series of rat ANF luciferase and rat MLC-2 luciferase fusion genes identify 315- and 92-base pair cis regulatory sequences within an embryonic gene (ANF) and a constitutively expressed contractile protein gene (MLC-2), respectively, which mediate alpha-adrenergic-inducible gene expression. Transfection of various ANF luciferase reporters into neonatal rat ventricular cells demonstrated that upstream sequences which mediate tissue-specific expression (-3003 to -638) can be segregated from those responsible for inducibility. The lack of inducibility of a cardiac Na+ channel gene, and the segregation of ANF gene sequences which mediate cardiac specific from those which mediate inducible expression, provides further insight into the relationship between muscle-specific and inducible expression during cardiac myocyte hypertrophy. Based on these results, a testable model is proposed for the induction of embryonic cardiac genes and constitutively expressed contractile protein genes and the noninducibility of a subset of cardiac genes during alpha-adrenergic stimulation of neonatal rat ventricular cells.

Endothelin induction of inositol phospholipid hydrolysis, sarcomere assembly, and cardiac gene expression in ventricular myocytes. A paracrine mechanism for myocardial cell hypertrophy

The present study examined the effects of endothelin-1 on phosphoinositide hydrolysis, diacylglycerol formation, and the induction of myocardial cell hypertrophy utilizing a well characterized cultured neonatal rat myocardial cell model. In this system, a hypertrophic response can be assessed by increases in myocardial cell size, an increase in the assembly of an individual contractile protein (myosin light chain-2) into organized contractile units, accumulation of contractile proteins, the activation of a program of immediate early gene expression, and the induction of genes encoding contractile and embryonic proteins (Iwaki, K., Sukhatme, V., Shubeita, H.E., Chien, K.R., (1990) J. Biol. Chem. 265, 13809-13817). Utilizing these criteria, the present study documents that stimulation with endothelin-1 can produce myocardial cell hypertrophy, induce the expression and release of ANF in ventricular cells, and can activate the transcription of cardiac-specific genes. In addition, endothelin-1 stimulates phosphoinositide hydrolysis and the accumulation of diacylglycerol. It is proposed that endothelin-1 stimulation may represent an important paracrine mechanism for the in vivo regulation of cardiac growth and hypertrophy.

Characterization of prostaglandin production in amnion-derived WISH cells

This study was undertaken to characterize prostaglandin production and its regulation in the human amnion-derived WISH cell line. Epidermal growth factor, tumor growth factor-alpha, tumor growth factor-beta, human interleukin-1, tumor necrosis factor, phorbol 12-myristate 13-acetate, phorbol 12,13-dibutyrate, 4 alpha-phorbol 12,13 didecanoate, and dexamethasone were tested for their ability to modulate prostaglandin production in WISH cells. Quantitatively, the major prostaglandin produced in WISH cells was prostaglandin E2. Treatment with epidermal growth factor, tumor growth factor-alpha, tumor necrosis factor, interleukin-1, phorbol 12,13-dibutyrate, and phorbol 12-myristate 13-acetate resulted in a concentration-dependent stimulation of WISH cell prostaglandin E2 production; tumor growth factor-beta and the inactive phorbol ester analog 4 alpha-phorbol 12,13 didecanoate had no effect. Dexamethasone treatment resulted in concentration-dependent inhibition of prostaglandin E2 production by WISH cells. WISH cells responded in a qualitatively similar manner to that previously observed in primary cultures of human amnion with the exception of the response to dexamethasone. On the basis of the findings of this investigation, we suggest that WISH cells may be a useful model for studying some but not all aspects of the regulation of arachidonic acid release and prostaglandin E2 formation in amnion. WISH cells may also be used to evaluate the mechanisms that link regulation of immune function and arachidonic acid metabolism.