Kidney Function for the Birds #expbio

Apr 05 2016 Published by under EB2016

The Integration of Gastrointestinal and Renal Function in Nectar-Feeding Birds

McWhorter TJ.

King of the Feeder

King of the Feeder

Regular Whizbangers know that I love hummingbirds. I spent hours watching these tiny feathered warriors at our feeder last summer. When I came across an abstract about their kidney function, it had to be blogged.

Most birds we encounter have very little fluid in their diets. They primarily ingest seeds and bugs, and they maximally retain water from their food. Nectar-feeding birds have an all-liquid diet. Hummingbirds, sunbirds, and honeyeaters must deal with high water loads during their daylight hours. For example, a hummingbird requires the calories in an amount of nectar 1.6 times the bird’s body weight during ideal environmental conditions to meet minimum metabolic needs. With cool temperatures or other stresses, intake may go as high as 3.3 times its body mass. Imagine the mythical 70 kg male drinking more than 200 liters of fluid each day! My kidney stone patients freak out about 2.5 to 3 liters daily!

Birds have much different anatomy than mammals as well (see diagram below). Food enters the crop where digestion begins, then moves into the proventiculus or stomach. After a pass through the gizzard, it hits the intestine where absorption occurs. The remaining material passes into the cloaca from where it leaves the body. Water absorbed from the intestine can be filtered by the kidney. Urine passes into the cloaca. From there it can be directed into the lower intestine for more processing or pass directly out of the body.

From Beuchat 1990 Physiological Zoology 63:1059

From Beuchat 1990 Physiological Zoology 63:1059

These birds have different renal structures from mammals as well. Two types of nephrons occur in birds, looped or mammalian nephrons which reach into medullary pyramids and produce a countercurrent multiplier system for concentrating urine, and reptilian nephrons without loops. Hummingbird kidneys consist primary of unlooped nephrons, so their kidneys are built for maximal urine dilution.

This paper shows that the two classes of nectar feeding birds have different strategies for dealing with massive water intake. Both groups of birds drop their filtration rate to zero overnight when they do not feed. It then increases again during the day as they drink. Hummingbirds primarily absorb water and filter it out via their kidneys, while Passeriformes (those sunbirds and honeyeaters) reduce gastrointestinal water absorption. Imagine instrumenting tiny 4.5 g birds to see how much fluid remains in the intestine as they eat more. Yup, this group did it! The hummingbirds had stable intestinal water excretion across all levels of intake, while water absorption decreased dramatically with higher intake in the Passeriformes, allowing them to send it straight on through the intestine.

These birds have evolved different strategies to handle the same problem, namely high water intake. There are interesting physiological lessons here. And besides, birds are fun! And bird kidneys? Well, what more could you want in a blog post...?

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Damage Control in the Cortical Collecting Duct #expbio

Apr 04 2016 Published by under EB2016, Uncategorized

Vasopressin-Escape Does Not Involve marked Changes in the Ratio of Intercalated-to-Principal Cells in the Cortical Collecting Duct

Chou C-L, et al.

Vasopressin, also known as anti-diuretic hormone (ADH), promotes absorption of water from the kidney’s cortical collecting duct. Under certain conditions, ADH can be inappropriately secreted, resulting in excess water retention and lowering of the body’s osmolality. Changes in osmolality can be quite dangerous, especially for the brain, so it is not surprising that the collecting duct can “escape” the effect of ADH to limit low plasma sodium and osmolality. This group previously showed that such vasopressin-escape occurs in association with lowered levels of expression for aquaporin 2 (AQP2), a water channel that allows ADH to do its job.

Autocrine and paracrine regulation of collecting duct principal cell ENaC and AQP2. Much commonality exists in regulation of ENaC (left) and AQP2 (right). Flow stimulates ATP, PGE2, and ET-1, which act on their cognate receptors to inhibit Na and water reabsorption. Similarly, bradykinin, adenosine, and NE act on their receptors to inhibit ENaC and AQP2. Flow-stimulated EET uniquely inhibits Na, but not water, transport. Compared with the wide variety of inhibitors, relatively few autocrine or paracrine factors stimulate ENaC and/or AQP2 activity. Renin, ultimately via AngII, as well as PGE2 binding to EP4 receptors, are potentially capable of augmenting principal cell Na and water transport. TZDs (via PPARγ) and kallikrein (via cleavage of an autoinhibitory domain in ENaC) may increase Na reabsorption. See the text for more detailed descriptions of each regulatory factor. ACE, angiotensin-converting enzyme; AGT, angiotensinogen; Ang, angiotensin; AQP, aquaporin; EET, eicosataetranoic acid; EP, PGE receptor; ET, endothelin; NE, norepinephrine; NO, nitric oxide; PPARγ, peroxisome proliferator–activated receptor-γ; TZD, thiazolidinedione.

Autocrine and paracrine regulation of collecting duct principal cell ENaC and AQP2. Much commonality exists in regulation of ENaC (left) and AQP2 (right). Flow stimulates ATP, PGE2, and ET-1, which act on their cognate receptors to inhibit Na and water reabsorption. Similarly, bradykinin, adenosine, and NE act on their receptors to inhibit ENaC and AQP2. Flow-stimulated EET uniquely inhibits Na, but not water, transport. Compared with the wide variety of inhibitors, relatively few autocrine or paracrine factors stimulate ENaC and/or AQP2 activity. Renin, ultimately via AngII, as well as PGE2 binding to EP4 receptors, are potentially capable of augmenting principal cell Na and water transport. TZDs (via PPARγ) and kallikrein (via cleavage of an autoinhibitory domain in ENaC) may increase Na reabsorption. See the text for more detailed descriptions of each regulatory factor. ACE, angiotensin-converting enzyme; AGT, angiotensinogen; Ang, angiotensin; AQP, aquaporin; EET, eicosataetranoic acid; EP, PGE receptor; ET, endothelin; NE, norepinephrine; NO, nitric oxide; PPARγ, peroxisome proliferator–activated receptor-γ; TZD, thiazolidinedione. Click image to access full review article.

Their current question centers on how AQP2 gets down regulated. It could be an intracellular mechanism or remodeling of the collecting duct, with a change in the ratio of principal and intercalated cells in that structure. Principal cells regulate sodium, potassium, and water reabsorption in the collecting duct, while intercalated cells influence acid-base balance. Decreasing the number of principal cells could decrease the effect of ADH. A full review of principal function can be found here; the image above comes from this paper.

After micro dissecting cortical collecting duct segments from animals in the early phases of vasopressin escape, the investigators probed them with a marker for all cells; an antibody to H+-ATPase, a marker of alpha intercalated cells; and an antibody to pendrin, found in in beta intercalated cells. They could then calculate the number of principal cells and intercalated cells to see if the principal cells decreased to explain the diminished AQP2 expression.

The cellular ratios did not differ between normal and vasopressin-escape animals.

So what intracellular process could be involved? Further exploration suggests a shift in cell cycle from G0 (resting) to mitosis. How this reduces AQP2 expression is not yet clear.

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Exercise and the Damaged Kidney #expbio

Apr 04 2016 Published by under EB2016

Endothelial Dysfunction Predicts Systolic Blood Pressure Slope During Whole Body Maximal Exercise in Patients with Chronic Kidney Disease

Downey RM, et al

Exercise is good for almost anything that ails someone. Patients with chronic kidney disease (CKD) often find it difficult to exercise, even before their disease advances to requiring intervention. Little is known about the vascular response of CKD patients to exercise.

BpKidneysThis study looked at the vascular effects of high intensity exercise in patients with stage 3 CKD compared to age-matched control subjects. CKD3 patients are often asymptomatic, diagnosed only through an abnormal lab test. CKD3 is the level of kidney dysfunction that gets flagged as abnormal by most labs. CKD3 has a wide range of function, from 20 mL/min/1.73m^2 to  59 mL/min/1.73m^2. Patients above 40 mL/min/1.73m^2 are much less likely to have secondary complications of CKD than those in the lower half of the range.

They then subjected these participants to maximal treadmill exercise while monitoring blood pressure, heart rate, and peak oxygen uptake. Brachial artery flow-mediated dilation (FMD) was measured to assess endothelial dysfunction. This study occurred just before and 1 hour after the treadmill test.

CKD patients had similar maximal blood pressure to controls; however, the rate of rise to maximal blood pressure was much greater in CKD patients. Maximum heart rate was lower in CKD patients, but the rate of rise was higher than the age-matched controls. Finally, peak oxygen uptake was significantly lower in CKD patients than in control subjects. FMD did not change pre- and post-exercise in either group, but they were significantly lower in the CKD patients. Basal levels of FMD predicted the slope of rise of blood pressure with exercise in CKD patients. Those with lower FMD had accelerated blood pressure and heart rate response during exercise.

CKD encompasses a relatively wide range of estimated glomerular filtration rate. Some nephrologists have suggest splitting it into CKD3a (40-59 mL/min/1.73m^2) and CKD3b because of the different risk profiles for these groups. Did dividing the study patients this way predict lower FMD for CKD3b? Apparently not.

So what follows this translational study? The next step will be exercise training for CKD3 patients to see if their endothelial dysfunction can be improved, resulting in better exercise tolerance. After all, exercise is good for almost everything.

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Why Should Kidneys Smell Fungus? #expbio

Apr 03 2016 Published by under EB2016

Renal Olfactory Receptor 90 (Olfr90) Responds to Fungal Metabolites

Halperin Kuhns RL, Pluznick JL

Receptors that help detect smells often have no known ligand, the molecule that turns them on, so to speak. That makes them orphan receptors. As these receptors are found well outside of the nose, we have to figure out what they react with to try and understand their function.

KidneySmellsFirst we need a bit of vocabulary to study the family tree. Olfactory receptors have families, even though they may not have parents. Structurally similar receptors often have overlap in their ligands, as may less similar receptors found in the same tissues. When ligands for an orphan receptor are identified, the receptor is “de-orphanized” (why don’t we say adopted and complete the damn metaphor, please?).

Olfr90 is an orphan olfactory receptor, found in the macula densa of the kidney. After expressing this molecule in a cell line with a reporter so cells would glow when the receptor got turned on, these investigators exposed these cells to a number of potential ligands. These included mixtures of odorant chemicals (after all, these are olfactory receptors), common ligands for “siblings” of the receptor, physiologically relevant ligands of “sibling” receptors, and physiologically relevant odorants. This strategy produced 9 ligands with little in common structurally; however, 4 of the 9 represented fungal metabolic products. When tested against other fungal-derived metabolites as well as conditioned media from various fungi, an additional 7 ligands occurred. Thus, 11 of 16 ligands for Olfr90 are of fungal origin.

So why does the kidney need to react to fungus? After all, this is an internal organ that should not regularly be exposed to yeasty-beasties, even though those wily single-cells run all over out skin and guts. Kidney and urinary tract infections with fungus do occur, but generally in immunosuppressed patients or those with instrumented urinary tracts. Making receptors has metabolic costs for cells, so should have a benefit beyond a relatively rare infection risk.

As noted last at the Cannon Lecture, microbes do not always have to enter the body to wreck havoc. Under certain circumstances the gut and other mucosa can become “leaky” and allow microbial metabolites into the circulation. These sensors may be waiting for the metabolites as a signal of these processes.

So what response does the kidney make to these ligands? Experiments are still in progress, but given the Olfr90 localization to the macula densa, changes in glomerular filtration rate could occur.

Study of olfactory receptors opens a world well beyond the nose. They do not mean the kidney smells, in any sense of the word!

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Pediatric Screening Urinalysis in the US

Sep 16 2014 Published by under [Medicine&Pharma]

A Brief History of Recommendations

Back in the 1980s when I trained, the American Academy of Pediatrics (AAP) recommended a screening urinalysis at four age points during childhood: infancy, early childhood, late childhood, and adolescence. Getting urine out of a child can be incredibly time consuming. Stick-on bags can be used in children not yet toilet trained, although results are often contaminated by skin flora. Bags can also leak, making the process a frustrating waiting game.

In 2000 the AAP published new guidelines with screening UA recommended only at 2 ages: 5 years old, the typical age of school entry, and in sexually active adolescents.

Hmm...UA doesn't seem to be a procedure...

Hmm...UA doesn't seem to be a procedure...

Today's well child preventive care guidelines are known as Bright Futures. The components of care are enlarged in the figure at the right; recommended lab studies are listed under Procedures, and no urinalysis can be found in this table or elsewhere in the document.

At present, it would appear that otherwise healthy, asymptomatic children do not need screening UAs.

What About Sports?

After exploring a number of professional sites, including the AAP, I found no recommendations for UAs prior to athletic participation. Blood pressure screening is included, with the recommendation that children with unexplained or uncontrolled hypertension should not participate in power lifting or body building. A urinalysis should be included in the work-up of hypertension in children, but that goes beyond the scope of the sports physical.

So the Answer is...?

Poll

None of the above wins!

None of the above. Currently, no UA is recommended at any age or before any activity for healthy, asymptomatic children.

So what are primary care providers actually doing? And why is this an issue? More fun to come, WhizBangers!

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When to Pee in the Cup

Sep 11 2014 Published by under Kidney Function

Screening urinalysis (UA), usually performed by dipstick in a physician's office, ultimately results in a lot of referrals for nephrologists. I am reviewing this topic, and I will have a series of posts about UAs over the coming weeks. First, I want to start with a poll about what is really recommended for healthy, asymptomatic children:

What are the current recommendations for screening urinalysis by the American Academy of Pediatrics?Next week I will reveal the answers from the crowd, as well as what the real answer is.

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Forty-Six

Sep 06 2014 Published by under Kidney Function, Life of a Physician

It's been a dry week.

Zer0. Zip. Nada.

Today, the urine output box shows 46 mL overnight.

Less than an ounce, but an important sign of the return of kidney function.

Keep it up, kid.

Urine is golden.

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Is the Answer at the RIVUR? #NephJC

Jul 16 2014 Published by under Evidence Based Medicine, Journal Club

This study will be discussed as part of the online, twitter-based Nephrology Journal Club on July 22,2014. More information about the workings of #NephJC can be found here.

The Problem of Reflux

Vesicoureteroreflux (VUR) occurs in approximately 10% of children overall, but about one-third of those with a febrile or otherwise symptomatic urinary tract infection (UTI). VUR is associated with an increased risk of renal "scars." Since it was first described in the 1960's, treatment of this backflow of urine from the bladder to the ureter has been recommended for all affected children. Surgery can create a competent valve at the vesicoureteral junction during voiding, but an early randomized trial showed that prophylactic antibiotics to prevent infection were just as effective as surgery in the scarring outcome.

Despite the recommendations for treatment for 50 years, permanent kidney failure attributed to VUR has not declined in the end-stage database of any country. Improved prenatal diagnosis of infant renal anomalies have allowed us to diagnose VUR in the first weeks of life, prior to any UTIs. Some children without UTIs still get renal scarring, leading some to suspect that "scars" may actually be areas of hypoplasia or other abnormal development due to an abnormal ureteric bud.

The original study showed equivalent results from surgery and antibiotic prophylaxis, but it included no untreated control group to assess the strategy of intermittent treatment of  UTIs when they occurred. The Randomized Intervention for Children with Vesicoureteral Reflux (RIVUR) trial set out to determine if long-term prophylaxis prevented recurrence of UTIs, occurrence of "scars," or contributed to antimicrobial resistance.

The Study

The study was a randomized, double-blind, placebo-controlled trial of prophylaxis with trimethoprim-sulfamethoxazole (TMPS). Children were screened and enrolled after 1 or 2 febrile or otherwise symptomatic UTIs, including positive culture. Bagged urine samples were not allowed. Children in the study ranged in age from 2 months to 6 years and had grades I to IV VUR (severe grade V patients were excluded). Exclusion criteria included other urinary abnormalties, chronic kidney disease, inability to take TMPS, and other selected medical issues.

Studies included dimercaptosuccinic acid (DMSA) scans at baseline and 1 and 2 years later. These scans (the gold standard for kidney scars) were read and scored centrally by two pediatric nuclear medicine radiologists.

Treatment failure was defined as:

  • 2 febrile UTIs
  • 1 febrile and 3 symptomatic UTIs
  • 4 symptomatic UTIs
  • New or worsening "scars" at 1 year

The Results

Baseline characteristics of the children enrolled can be seen here. No significant differences on any parameter existed between the treatment and control groups. Time to first febrile or symptomatic UTI after trial enrollment is shown below:

Figure2

As shown in the paper’s figure 2 above, the two groups separated significantly within the first 6 months of treatment, with TMPS prophylaxis clearly preventing UTIs. By the end of 2 years, approximately one quarter of the placebo group had experienced an infection, while only half that many in the prophylaxis group had fallen ill.

A number of potential modifying factors were assessed for impact on the results, shown in the figure below:

Figure 3

As shown prophylaxis was more valuable for children who presented with febrile, as opposed to symptomatic but afebrile, UTI. Bowel and bladder dysfunction, determined via a standardized survey, also favored the use of TMPS.

Renal “scars” showed no difference throughout the study. Rectal swabs showed no significant difference in the rate of resistance of E. coli to TMPS between the prophylaxis and control groups.

Remaining Questions

Clearly antibiotic prophylaxis reduces the risk of recurrence of UTIs in children with VUR. However, about 75% of children receiving placebo had not suffered a recurrence after 2 years of study. UTIs cause discomfort, school absence, and lost work for parents; even after this trial we have no evidence of long-term damage prevention through the use of TMPS. Antibiotic resistance does not seem to be a big problem in this patient population.

So the question remains: what should we do about VUR?

In my mind, the question is still open. Many families today have qualms about long-term exposure to these medications. Other families dread missing a UTI and would far prefer to take the antibiotic. The tolerance of the family for illness vs the small risks of prophylaxis often prove to be a big factor driving therapy.

That leaves us each a lot of flexibility in our approach to VUR. My personal preference is to watch most cases without prophylaxis initially. Those who have further UTIs in the first few months after diagnosis are encouraged to start prophylaxis and consider surgical treatment. Those without significant recurrences receive follow-up on a regular basis. All of this requires ongoing discussion with the parents and input regarding their tolerance for urinary symptoms.

The pediatric nephrology community hoped that RIVUR would answer our managment questions about VUR. It would appear that we still have more we need to know.

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Fair Warning

Jun 11 2014 Published by under Blog Maintenance

Tomorrow I am off into the friendly skies again, headed for that city on the bay for the 74th Scientific Sessions of the American Diabetes Association. That means my posts for the next week or so will involve diabetes and its complications, especially kidney disease.

Other random shiny objects sometimes catch my attention when I travel, so get ready for those as well.

 

 

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Messy, Complex? Math To Make It Simple: #xBio

Apr 30 2014 Published by under EB2014

A number of renal physiology presentations dealt with modeling of kidney functions. They featured Departments of Mathematics, something which I find a bit intimidating:

As usual, XKCD speaks truth

As usual, XKCD speaks truth

Structural organization of the renal medulla has a significant impact on oxygen distribution

Brendan Fry, Anita Layton

Duke University, Durham, NC

 

The first thing that caught my attention with this abstract was its medullary focus. Nephrologists and physiologists often give the medulla little love. It's not the part of the kidney that we biopsy (at least not what we want to get), and while it does a bunch of stuff for water and volume control, most folks only pay attention to their little piece of it.

AJP Renal 287:767, 2004

AJP Renal 287:767, 2004

So what does it take to model the structure of a kidney? Let's look at an example of a medullary reconstruction in the figure on the right. In this study by Pannabecker and Dantzler, segment-specific markers were used to reconstruct the medullary architecture on sections through the kidney. Cross-sectional photos were "assembled" into these tubular models going from corticomedullary junction (a) through the papilla (e). Red structures are descending thin limbs of the loop of Henle, while blue tubes represent collecting ducts.

And these are only two of the tubular structures that course through the medulla and give it a striped appearance.

So these sorts of studies give us a picture of the anatomic relationships within the medulla. Now we need to start adding what we know about functional relationships. For that, we will see the next figure from Lemley and Kriz.

Kidney Int 31:538, 1987

Kidney Int 31:538, 1987

In this cartoon (not as pithy as the XKCD one, huh?), some known transport properties of various segments gets thrown into the mix. This is, of course, one of the simplest diagrams from this paper.

So people have been doing this stuff for many years (that last paper is 26, the same age as my daughter). As time has passed, our understanding of both the structure and function of these areas of the medulla has improved, allowing a mathematical model to be created. The original model by Anita Layton is illustrated in the next figure. I will let you pull the paper if you want the key to the abbreviations.

AJP Renal 300:356, 2011

AJP Renal 300:356, 2011

In this version, certain assumptions were made about the regions in which these segments lie.

The new model adds to this 2011 version, with further refinements about the spatial relationships and how oxygen would traverse the interstitial goo at various levels. They hope to work further functions into their models in the near future, things like the nitric oxide system and acid-base handling.

I have not done this paper justice; I am merely an MD, at best a humble biologist. I can appreciate how an elegant model can direct new hypotheses and future experiments. I'm afraid, though, that when I model a kidney it will be something like my final figure...

Click for original source

Click for original source

 

 

 

 

 

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