Dietary protein-induced changes in excretory function: a general animal design feature

Why is chronic kidney disease progressive? Evolutionary adaptations and maladaptations

Despite significant advances in renal physiology, the global prevalence of chronic kidney disease (CKD) continues to increase. The emergence of multicellular organisms gave rise to increasing complexity of life resulting in trade-offs reflecting ancestral adaptations to changing environments. Three evolutionary traits shape CKD over the lifespan: 1) variation in nephron number at birth, 2) progressive nephron loss with aging, and 3) adaptive kidney growth in response to decreased nephron number. Although providing plasticity in adaptation to changing environments, the cell cycle must function within constraints dictated by available energy. Prioritized allocation of energy available through the placenta can restrict fetal nephrogenesis, a risk factor for CKD. Moreover, nephron loss with aging is a consequence of cell senescence, a pathway accelerated by adaptive nephron hypertrophy that maintains metabolic homeostasis at the expense of increased vulnerability to stressors. Driven by reproductive fitness, natural selection operates in early life but diminishes thereafter, leading to an exponential increase in CKD with aging, a product of antagonistic pleiotropy. A deeper understanding of the evolutionary constraints on the cell cycle may lead to manipulation of the balance between progenitor cell renewal and differentiation, regulation of cell senescence, and modulation of the balance between cell proliferation and hypertrophy. Application of an evolutionary perspective may enhance understanding of adaptation and maladaptation by nephrons in the progression of CKD, leading to new therapeutic advances.

Mechanisms of nitrogen excretion in insects

Avoiding the toxic effects of ammonia derived from catabolism of proteins and nucleic acids typically involves synthesis of the less soluble compound uric acid in insects, although some species which are not water stressed excrete ammonia directly. Some dipterans metabolize uric acid further to allantoin or urea. Uric acid plays diverse roles as a nitrogenous waste, nitrogen store, pigment, antioxidant and possibly a signaling molecule. Multiple transporters are implicated in urate transport, including members of the ABC and SLC families. Excretion of ammonia by the Malpighian tubules, hindgut, or anal papillae involves multiple transporters, including Na⁺/K⁺-ATPase, Rhesus glycoproteins, ammonia transporters (AMTs) and possibly a hyperpolarization-activated cyclic nucleotide-gated K⁺ channel (HCN).

Why Is the GFR So High?: Implications for the Treatment of Kidney Failure

The high GFR in vertebrates obligates large energy expenditure. Homer Smith's teleologic argument that this high GFR was needed to excrete water as vertebrates evolved in dilute seas is outdated. The GFR is proportional to the metabolic rate among vertebrate species and higher in warm-blooded mammals and birds than in cold-blooded fish, amphibians, and reptiles. The kidney clearance of some solutes is raised above the GFR by tubular secretion, and we presume secretion evolved to eliminate particularly toxic compounds. In this regard, high GFRs may provide a fluid stream into which toxic solutes can be readily secreted. Alternatively, the high GFR may be required to clear solutes that are too large or too varied to be secreted, especially bioactive small proteins and peptides. These considerations have potentially important implications for the understanding and treatment of kidney failure.

Investigation of the effects of cricket ingestion on plasma uric acid concentration in inland bearded dragons (Pogona vitticeps)

OBJECTIVE

To determine whether plasma uric acid concentration in inland bearded dragons (Pogona vitticeps) was affected by recent ingestion of a meal of crickets.

ANIMALS

12 healthy adult inland bearded dragons.

PROCEDURES

Food was withheld for 48 hours prior to experiments. Animals (6/group) were randomly assigned to receive a meal of crickets (equivalent to 1% of the animal's body weight; 10 g/kg [4.5 g/lb]; treatment group) or have food withheld for an additional 48 hours (control group). Blood samples were collected for plasma uric acid measurement just before (time 0) and 4, 24, and 48 hours after feeding. Effects of feeding and time on the targeted measurement were assessed by repeated-measures ANOVA.

RESULTS

Mean plasma uric acid concentration for the treatment group was significantly increased from the time 0 value (2.5 ± 1.5 mg/dL) 24 hours following meal ingestion (6.5 ± 1.2 mg/dL), but not at the 4-hour time point, and returned to the time 0 value by the 48-hour time point. No significant changes in plasma uric acid concentration were detected for the control group.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested food should be withheld for ≥ 48 hours prior to blood collection if inland bearded dragons are used to establish reference intervals for plasma uric acid concentration or if feasible when obtaining samples from these animals for clinical evaluation. Veterinarians should consider the time from last meal consumption when interpreting plasma uric acid concentration for this species and potentially other terrestrial insectivorous and omnivorous lizards.

High-Protein Diet-Induced Glomerular Hyperfiltration Is Dependent on Neuronal Nitric Oxide Synthase β in the Macula Densa via Tubuloglomerular Feedback Response

It is well known that high protein intake increases glomerular filtration rate. Evidence from several studies indicated that NO and tubuloglomerular feedback (TGF) mediate the effect. However, a recent study with a neuronal NO synthase-α knockout model refuted this mechanism and concluded that neither neuronal NO synthase nor TGF response is involved in the protein-induced hyperfiltration. To examine the discrepancy, this study tested a hypothesis that neuronal NO synthase-β in the macula densa mediates the high-protein diet-induced glomerular hyperfiltration via TGF mechanism. We examined the effects of high protein intake on NO generation at the macula densa, TGF response, and glomerular filtration rate in wild-type and macula densa-specific neuronal NO synthase KO mice. In wild-type mice, high-protein diet increased kidney weight, glomerular filtration rate, and renal blood flow, while reduced renal vascular resistance. TGF response in vivo and in vitro was blunted, and NO generation in the macula densa was increased following high-protein diet, associated with upregulations of neuronal NO synthase-β expression and phosphorylation at Ser1417. In contrast, these high-protein diet-induced changes in NO generation at the macula densa, TGF response, renal blood flow, and glomerular filtration rate in wild-type mice were largely attenuated in macula densa-specific neuronal NO synthase KO mice. In conclusion, we demonstrated that high-protein diet-induced glomerular hyperfiltration is dependent on neuronal NO synthase β in the macula densa via TGF response.

Higher protein intake is associated with increased risk for incident end-stage renal disease among blacks with diabetes in the Southern Community Cohort Study

BACKGROUND AND AIMS

Diabetes, a risk factor for end-stage renal disease (ESRD), is associated with impaired protein metabolism. We investigated whether protein intake is associated with ESRD and whether the risk is higher among blacks with diabetes.

METHODS AND RESULTS

We conducted a nested case-control study of ESRD within the Southern Community Cohort Study, a prospective study of low-income blacks and whites in the southeastern US (2002-2009). Through 2012, 1057 incident ESRD cases were identified by linkage with the United States Renal Data System and matched to 3198 controls by age, sex, and race. Dietary intakes were assessed from a validated food frequency questionnaire at baseline. Odds ratios (ORs) and 95% confidence intervals (CIs) were computed from logistic regression models that included matching variables, BMI, education, income, hypertension, total energy intake, and percent energy from saturated and polyunsaturated fatty acids. Mean (±SD) daily energy intake from protein was higher among ESRD cases than controls (15.7 ± 3.3 vs. 15.1 ± 3.1%, P < 0.0001). For a 1% increase in percent energy intake from protein, the adjusted ORs (95% CIs) for ESRD were 1.06 (1.02-1.10) for blacks with diabetes, 1.02 (0.98-1.06) for blacks without diabetes, 0.99 (0.90-1.09) for whites with diabetes and 0.94 (0.84-1.06) for whites without diabetes. Protein intake in g/kg/day was also associated with ESRD (4th vs. 1st quartile OR = 1.76; 95% CI: 1.17-2.65).

CONCLUSION

Our results raise the possibility that among blacks with diabetes, increased dietary protein is associated with increased incidence of ESRD. Studies on how protein intake and metabolism affect ESRD are needed.

Sex-specific effects of protein and carbohydrate intake on reproduction but not lifespan in Drosophila melanogaster

Modest dietary restriction extends lifespan (LS) in a diverse range of taxa and typically has a larger effect in females than males. Traditionally, this has been attributed to a stronger trade-off between LS and reproduction in females than in males that is mediated by the intake of calories. Recent studies, however, suggest that it is the intake of specific nutrients that extends LS and mediates this trade-off. Here, we used the geometric framework (GF) to examine the sex-specific effects of protein (P) and carbohydrate (C) intake on LS and reproduction in Drosophila melanogaster. We found that LS was maximized at a high intake of C and a low intake of P in both sexes, whereas nutrient intake had divergent effects on reproduction. Male offspring production rate and LS were maximized at the same intake of nutrients, whereas female egg production rate was maximized at a high intake of diets with a P:C ratio of 1:2. This resulted in larger differences in nutrient-dependent optima for LS and reproduction in females than in males, as well as an optimal intake of nutrients for lifetime reproduction that differed between the sexes. Under dietary choice, the sexes followed similar feeding trajectories regulated around a P:C ratio of 1:4. Consequently, neither sex reached their nutritional optimum for lifetime reproduction, suggesting intralocus sexual conflict over nutrient optimization. Our study shows clear sex differences in the nutritional requirements of reproduction in D. melanogaster and joins the growing list of studies challenging the role of caloric restriction in extending LS.

Effects of resistance training associated with whey protein supplementation on liver and kidney biomarkers in rats

The aim of this study was to investigate the impact of whey protein (WP) supplementation and resistance training (RT) on liver and kidney biomarkers. The sedentary + WP group showed higher levels of plasma liver and kidney dysfunction markers compared with the other groups. In addition, WP supplementation associated with RT resulted in physiologic cardiac hypertrophy. WP supplementation without RT affected liver and kidney function.

Ethylene glycol induces calcium oxalate crystal deposition in Malpighian tubules: a Drosophila model for nephrolithiasis/urolithiasis

Several animal species are used to study calcium oxalate urolithiasis; however, an ideal model has yet to be identified. We used Drosophila as a model organism and fed the flies lithogenic agents such as ethylene glycol, hydroxyl-L-proline, and sodium oxalate. At different times, the Malpighian tubules, the kidney equivalent of insects, were dissected and a polarized light microscope used to highlight the birefringent crystals. Scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed that the crystal composition was predominately calcium oxalate. Furthermore, administration of potassium citrate successfully reduced the quantity of and modulated the integrity of the ethylene glycol-induced crystals. Thus, the Drosophila model of bio-mineralization produces crystals in the urinary system through many lithogenic agents, permits observation of crystal formation, and is amenable to genetic manipulation. This model may mimic the etiology and clinical manifestations of calcium oxalate stone formation and aid in identification of the genetic basis of this disease.

Identification of a liver-specific cAMP response element in the human argininosuccinate synthetase gene

Argininosuccinate synthetase (ASS), a key enzyme in the urea cycle, participates in many metabolic processes including arginine biosynthesis and the citrulline-nitric oxide (NO) cycle. Factors like diets, hormones and pro-inflammatory stimuli are known to regulate ASS gene expression primarily at the transcription level. However, little is known about the cis-elements for transcriptional regulation of the ASS gene. In this study, we employed DNase I hypersensitive sites mapping to identify potential regulatory sites of the gene and revealed a site located at 10 kb upstream of the transcription start site which is responsible for liver-specific cAMP induction. Furthermore, a cAMP response element (CRE) highly conserved among mammals was identified and was experimentally verified. Our results show that liver-specific enhancement of ASS gene expression is mediated in part by the cAMP signaling pathway through a distal CRE site.

Discovery of an alternate metabolic pathway for urea synthesis in adult Aedes aegypti mosquitoes

We demonstrate the presence of an alternate metabolic pathway for urea synthesis in Aedes aegypti mosquitoes that converts uric acid to urea via an amphibian-like uricolytic pathway. For these studies, female mosquitoes were fed a sucrose solution containing (15)NH4Cl, [5-(15)N]-glutamine, [(15)N]-proline, allantoin, or allantoic acid. At 24 h after feeding, the feces were collected and analyzed in a mass spectrometer. Specific enzyme inhibitors confirmed that mosquitoes incorporate (15)N from (15)NH4Cl into [5-(15)N]-glutamine and use the (15)N of the amide group of glutamine to produce labeled uric acid. More importantly, we found that [(15)N2]-uric acid can be metabolized to [(15)N]-urea and be excreted as nitrogenous waste through an uricolytic pathway. Ae. aegypti express all three genes in this pathway, namely, urate oxidase, allantoinase, and allantoicase. The functional relevance of these genes in mosquitoes was shown by feeding allantoin or allantoic acid, which significantly increased unlabeled urea levels in the feces. Moreover, knockdown of urate oxidase expression by RNA interference demonstrated that this pathway is active in females fed blood or (15)NH4Cl based on a significant increase in uric acid levels in whole-body extracts and a reduction in [(15)N]-urea excretion, respectively. These unexpected findings could lead to the development of metabolism-based strategies for mosquito control.

Comparison of Plasma Uric Acid Levels in Five Varieties of the Domestic Turkey, Meleagris gallopavo

Plasma uric acid (PUA) is a consensus physiological biomarker for many phenotypes in vertebrates because it is a reliable indicator for processes such as oxidative stress and tubular function. In birds, it is considered a major antioxidant and is also the primary endproduct of nitrogen metabolism. Despite this importance, knowledge of baseline levels of PUA in physiologically normal birds, including the turkey, Meleagris gallopavo, is limited. Here, we compared PUA levels in a total of 106 apparently normal male and female birds at 8 and 32 wk of age from 5 strains of the domestic turkey, including Bourbon Red, Narragansett, Blue Slate, Royal Palm, and Spanish Black. Though differences in PUA were not significant at 8 and 32 wk of age, BW, variety, and sex effects were highly significant. When adjusted for BW, female birds had, on average, a higher PUA per kilogram of BW than male birds. When adjusted for both sex and BW, Royal Palm birds had the lowest average PUA, and Blue Slate had the highest PUA. Results of these investigations represent the first comparative analysis of PUA in physiologically normal turkey varieties. They suggest that differences in basal plasma levels of uric acid in physiologically normal turkeys are influenced by sex, weight, and genetic background but may be independent of age.

The influence of ambient temperature and the energy and protein content of food on nitrogenous excretion in the Egyptian fruit bat (Rousettus aegyptiacus)

The diets of frugivorous and nectarivorous vertebrates contain much water and generally have high energy but low protein contents. Therefore, we tested the prediction that to save energy under conditions of high energy demands and high water intake, frugivorous Egyptian fruit bats (Rousettus aegyptiacus) will increase both the absolute quantity and the proportion of ammonia in their urine. We also examined whether such changes occur when protein intake is low and water intake is high. We did three feeding trials. In trials 1 and 2, bats were fed one of four liquid diets containing constant soy protein concentrations but varying in sucrose concentration and were kept at ambient temperatures (T(a)) of 30 degrees Celsius and 12 degrees Celsius, respectively. In trial 3, bats were kept at Ta=12 degrees Celsius and fed one of four liquid diets with equal sucrose concentrations but varying protein concentrations. In trial 1, food intake at a sucrose concentration of 256 mmol/kg H(2)O was initially high but decreased to a constant rate with further increases in sucrose concentration, while in trial 2, food intake decreased exponentially with increasing sucrose concentration. As predicted, at 12 degrees Celsius with varying sucrose concentration, both the absolute quantity and the fraction of ammonia in the bats' urine increased significantly with food intake (P<0.02), while the absolute quantity of urea and the fraction of urea nitrogen excreted decreased significantly with food intake (P<0.03). Varying sucrose concentration had no significant effect on nitrogen excretion at Ta=30 degrees Celsius. Varying protein concentration had no significant effect on nitrogen excretion at Ta=12 degrees Celsius. We suggest that Egyptian fruit bats can increase ammonia excretion in response to increased energetic demands, and we calculate that they can save energy equal to approximately 2% of their daily metabolic rate by doing so.

Low ambient temperature increases food intake and dropping production, leading to incorrect estimates of hormone metabolite concentrations in European stonechats

Non-invasive methods to measure steroid hormone metabolites in bird droppings or mammalian feces have become very popular. However, the accuracy of these measurements may be affected by many factors. Here, we use the stonechat (Saxicola torquata) as a passerine bird model to test whether differences in ambient temperature affect food intake and dropping production and whether these changes lead to measurement artefacts in hormone metabolite concentrations. In addition, we tested for diurnal patterns in hormone metabolites. We held European stonechats in climate chambers and subjected them to two different long-term ambient temperature regimes, +5 degrees C and +22 degrees C. As expected, food intake and dropping production was higher at +5 degrees C than at +22 degrees C. Plasma concentrations of corticosterone and testosterone did not differ between different ambient temperature regimes. However, corticosterone and testosterone metabolite concentrations (in ng/g) were significantly lower at +5 degrees C than at +22 degrees C. When we measured the rate of hormone metabolite excretion (in picogram per hour) instead of the concentration, there was no difference between treatment groups. Thus, the measurement of hormone metabolite concentrations can be flawed because, depending on the treatment, similar amounts of hormone metabolites can be excreted into very different amounts of droppings. In conclusion, hormone metabolite concentration measurements are sensitive to changes in ambient temperature and probably any other factor that alters metabolic rates. Any study involving systematic changes in metabolism--i.e., during molt, migration, hibernation, egg production, or seasonal comparisons--needs to take these caveats into account.

Two-phase seed dispersal: linking the effects of frugivorous birds and seed-caching rodents

Frugivorous birds disperse the seeds of many fruit-bearing plants, but the fate of seeds after defecation or regurgitation is often unknown. Some rodents gather and scatter hoard seeds, and some of these may be overlooked, germinate, and establish plants. We show that these two disparate modes of seed dispersal are linked in some plants. Rodents removed large (>25 mg) seeds from simulated bird feces (pseudofeces) at rates of 8-50%/day and scatter hoarded them in soil. Ants (Formica sibylla) also harvested some seeds and carried them to their nests. Rodents carried seeds 2.5+/-3.2 m to cache sites (maximum 12 m) and buried seeds at 8+/-7 mm depth. Enclosure studies suggest that yellow pine chipmunks (Tamias amoenus) and deer mice (Peromyscus maniculatus) made the caches. In spring, some seeds germinated from rodent caches and established seedlings, but no seedlings established directly from pseudofeces. This form of two-phase seed dispersal is important because each phase offers different benefits to plants. Frugivory by birds permits relatively long-range dispersal and potential colonization of new sites, whereas rodent caching moves seeds from exposed, low-quality sites (bird feces on the ground surface) to a soil environment that may help maintain seed viability and promote successful seedling establishment.

Dietary protein intake and renal function

Recent trends in weight loss diets have led to a substantial increase in protein intake by individuals. As a result, the safety of habitually consuming dietary protein in excess of recommended intakes has been questioned. In particular, there is concern that high protein intake may promote renal damage by chronically increasing glomerular pressure and hyperfiltration. There is, however, a serious question as to whether there is significant evidence to support this relationship in healthy individuals. In fact, some studies suggest that hyperfiltration, the purported mechanism for renal damage, is a normal adaptative mechanism that occurs in response to several physiological conditions. This paper reviews the available evidence that increased dietary protein intake is a health concern in terms of the potential to initiate or promote renal disease. While protein restriction may be appropriate for treatment of existing kidney disease, we find no significant evidence for a detrimental effect of high protein intakes on kidney function in healthy persons after centuries of a high protein Western diet.

Age-dependent changes in plasma biochemistry of yellow-legged gulls (Larus cachinnans)

The study of avian plasma chemistry is providing useful reference values for the management of endangered and game species, supporting veterinarians in their diagnostics, and also bringing to light relevant physiological adaptations during periods of food-shortage. Age is an important source of variability for plasma chemistry. Here I report plasma chemistry of yellow-legged gulls Larus cachinnans from different ages, between post-independence and adulthood, a 5-year interval. Increase in plasma cholesterol concentration and decreases in uric acid, inorganic phosphorus and alkaline phosphatase values were seen. Body mass corrected by body size (i.e. body condition) increased with age, plasma cholesterol being positively correlated in females, but not in males. Moreover, cholesterol was also positively correlated to gonad size in both sexes. Long-term developmental changes in this species, such as gonad development and the acquisition of an optimal body mass for reproduction, could explain these findings. Finally, inorganic phosphorus and alkaline phosphatase, both traditionally related to osteogenesis, were not associated to deferred skull ossification, as originally was suggested in other species.

Expression of urea transporter (UT-A) mRNA in papilla and pelvic epithelium of kidney in normal and low protein fed sheep

The identification and cloning of the urea transporter (UT) in papilla and upper pelvic epithelium of sheep kidney and the effect of a 5-week-lasting low protein diet on UT mRNAs expression in these structures are reported. Using degenerate primers we cloned by RT-PCR a 770-base pairs UT-A cDNA fragment. The deduced amino acid sequence shared 92% and 93% identity with UT-A2 protein from rabbit and rat, and from human, respectively. Quantification of UT-A mRNAs expression after LP diet was performed by quantitative RT-PCR using UT-A mutant cRNA. Compared to normal protein fed sheep, low protein diet was associated with a significant reduction of UT-A mRNA levels in pelvic epithelium (852+/-172 vs. 2024+/-260 molecules, P<0.01) and a tendency to its increase in papilla (7959+/-1741 vs. 5447+/-1040 molecules, NS). Functional studies confirmed that kidneys of low protein fed sheep increased their ability to reduce urea losses. The reduction of UT-A expression in the pelvic epithelium lining the outer medulla could be relevant for the renal conservation of urea in protein restricted sheep.