Kidney Function in the Crab-Eating Frog (Rana Cancrivora)

Effects of the environment on the evolution of the vertebrate urinary tract

Evolution of the vertebrate urinary system occurs in response to numerous selective pressures, which have been incompletely characterized. Developing research into urinary evolution led to the occurrence of clinical applications and insights in paediatric urology, reproductive medicine, urolithiasis and other domains. Each nephron segment and urinary organ has functions that can be contextualized within an evolutionary framework. For example, the structure and function of the glomerulus and proximal tubule are highly conserved, enabling blood cells and proteins to be retained, and facilitating the elimination of oceanic Ca+ and Mg+. Urea emerged as an osmotic mediator during evolution, as cells of large organisms required increased precision in the internal regulation of salinity and solutes. As the first vertebrates moved from water to land, acid-base regulation was shifted from gills to skin and kidneys in amphibians. In reptiles and birds, solute regulation no longer occurred through the skin but through nasal salt glands and post-renally, within the cloaca and the rectum. In placental mammals, nasal salt glands are absent and the rectum and urinary tracts became separate, which limited post-renal urine concentration and led to the necessity of a kidney capable of high urine concentration. Considering the evolutionary and environmental selective pressures that have contributed to renal evolution can help to gain an increased understanding of renal physiology.

Transcriptomes reveal the genetic mechanisms underlying ionic regulatory adaptations to salt in the crab-eating frog

The crab-eating frog, Fejervarya cancrivora, is the only frog that lives near seas. It tolerates increased environmental concentrations of sodium, chloride and potassium partly by raising ion and urea levels in its blood plasma. The molecular mechanism of the adaptation remains rarely documented. Herein, we analyze transcriptomes of the crab-eating frog and its closely related saline-intolerant species, F. limnocharis, to explore the molecular basis of adaptations to such extreme environmental conditions. Analyses reveal the potential genetic mechanism underlying the adaptation to salinity for the crab-eating frog. Genes in categories associated with ion transport appear to have evolved rapidly in F. cancrivora. Both positively selected and differentially expressed genes exhibit enrichment in the GO category regulation of renal sodium excretion. In this category, the positively selected sites of ANPEP and AVPR2 encode CD13 and V2 receptors, respectively; they fall precisely on conserved domains. More differentially expressed rapidly evolved genes occur in the kidney of F. cancrivora than in F. limnocharis. Four genes involved in the regulation of body fluid levels show signs of positive selection and increased expression. Significant up-regulation occurs in several genes of F. cancrivora associated with renin-angiotensin system and aldosterone-regulated sodium reabsorption pathways, which relate to osmotic regulation.

Identification and Expression of a Putative Facilitative Urea Transporter in Three Species of True Frogs (Ranidae): Implications for Terrestrial Adaptation

Urea transporters (UTs) help mediate the transmembrane movement of urea and therefore are likely important in amphibian osmoregulation. Although UTs contribute to urea reabsorption in anuran excretory organs, little is known about the protein’s distribution and functions in other tissues, and their importance in the evolutionary adaptation of amphibians to their environment remains unclear. To address these questions, we obtained a partial sequence of a putative UT and examined relative abundance of this protein in tissues of the wood frog (Rana sylvatica), leopard frog (R. pipiens), and mink frog (R. septentrionalis), closely related species that are adapted to different habitats. Using immunoblotting techniques, we found the protein to be abundant in the osmoregulatory organs but also present in visceral organs, suggesting that UTs play both osmoregulatory and nonosmoregulatory roles in amphibians. UT abundance seems to relate to the species’ habitat preference, as levels of the protein were higher in the terrestrial R. sylvatica, intermediate in the semiaquatic R. pipiens, and quite low in the aquatic R. septentrionalis. These findings suggest that, in amphibians, UTs are involved in various physiological processes, including solute and water dynamics, and that they have played a role in adaptation to the osmotic challenges of terrestrial environments.

Cutaneous nitrogen excretion in the African clawed frog Xenopus laevis: effects of high environmental ammonia (HEA)

Ammonia is a highly toxic molecule and often introduced in considerable amounts into aquatic environments due to anthropogenic activities. Many aquatic and semi-aquatic amphibians utilize, in addition to their kidneys, the skin for osmoregulation and nitrogen excretion. In the present study the effects of prolonged (7-21 days) exposure to high environmental ammonia (HEA, 1 mmol l(-1) NH4Cl) on cutaneous nitrogen excretion and gene expression of key-transporters involved in nitrogen excretion and acid-base regulation were investigated in the fully aquatic African clawed frog, Xenopus laevis. The study revealed that X. laevis excretes predominately ammonia of which approximately 50% is excreted via the skin. Both the ventral and dorsal skin were capable to generate a net ammonia efflux, which was significantly activated by 10 mmol l(-1) of the phosphodiesterase blocker theophylline. The obtained data further suggest that the ammonia efflux was promoted by an acidification of the unstirred boundary layer, likely generated by an apical localized V-ATPase, with NH3 being transported via cutaneous expressed ammonia transporters, Rhbg and Rhcg. Prolonged HEA exposure did significantly reduce the net-flux rates over the ventral skin with Vmax changing from 256 nmol cm(-2) h(-1) in control frogs to 196 nmol cm(-2) h(-1) in HEA exposed animals. Further, prolonged HEA exposure caused a decrease in mRNA expression levels of the ammonia transporter Rhbg, Na(+)/K(+)-ATPase (α-subunit) and V-ATPase (subunit H) in the ventral and dorsal skin and the kidney. In contrast, Rhcg expression levels were unaffected by HEA in skin tissues.

Seasonal variation and response to osmotic challenge in urea transporter expression in the dehydration- and freeze-tolerant wood frog, Rana sylvatica

Urea accumulation is a universal response to osmotic challenge in anuran amphibians, and facilitative urea transporters (UTs) seem to play an important role in this process by acting in the osmoregulatory organs to mediate urea retention. Although UTs have been implicated in urea reabsorption in anurans, little is known about the physiological regulation of UT protein abundance. We examined seasonal variation in and effects of osmotic challenge on UT protein and mRNA levels in kidney and urinary bladder of the wood frog (Rana sylvatica), a terrestrial species that tolerates both dehydration and tissue freezing. Using immunoblotting techniques to measure relative UT abundance, we found that UT numbers varied seasonally, with a low abundance prevailing in the fall and winter, and higher levels occurring in the spring. Experimental dehydration of frogs increased UT protein abundance in the urinary bladder, whereas experimental urea loading decreased the abundance of UTs in kidney and bladder. Experimental freezing, whether or not followed by thawing, had no effect on UT numbers. UT mRNA levels, assessed using quantitative real-time polymerase chain reaction, did not change seasonally nor in response to any of our experimental treatments. These findings suggest that regulation of UTs depends on the nature and severity of the osmotic stress and apparently occurs posttranscriptionally in response to multiple physiological factors. Additionally, UTs seem to be regulated to meet the physiological need to accumulate urea, with UT numbers increasing to facilitate urea reabsorption and decreasing to prevent retention of excess urea.

Salinity tolerance of eggs of Buergeria japonica (Amphibia, Anura) inhabiting coastal areas

Buergeria japonica is one of a few frogs that breed in coastal areas. To understand why this species can breed in coastal areas, I tested the salinity tolerance of eggs of B. japonica collected from a coastal area of Okinawa Island, Japan. All eggs hatched within four days after oviposition. At 0%. salinity (control), over 94% of eggs hatched normally, and even at 1 per thousand salinity over 85% of eggs hatched. Survival rate of eggs was low at 2, 3, and 4 per thousand, and no eggs hatched at 5 per thousand salinity. These results indicate that low salinity, close to pure water, is necessary for successful egg development, even for populations of B. japonica that breed in coastal areas. Future studies are necessary to examine whether females of B. japonica breeding in coastal areas select appropriate oviposition sites where the environmental salinity level is sufficiently low for eggs.

Arginine vasotocin promotes urea permeability through urea transporter expressed in the toad urinary bladder cells

We previously isolated a cDNA of a urea transporter (Bufo UT) from the kidney of the marine toad, Bufo marinus, and demonstrated that the Bufo UT was specifically localized on the epithelial membrane of the early distal tubules in the kidney and urinary bladder. In the present study, the function of Bufo UT was investigated using a Xenopus oocytes expression system. Further, we examined the effects of arginine vasotocin (AVT) on urea transport in isolated cells from the toad urinary bladder. When expressed in Xenopus oocytes Bufo UT induced more than a 10-fold increase in [(14)C]urea uptake compared with water-injected control oocytes. Phloretin, a urea transport inhibitor, fully blocked the increase of urea uptake. In epithelial cells isolated from the toad urinary bladder, addition of AVT to the medium increased the urea uptake in a concentration-dependent manner (10(-12)-10(-8)M). To examine the relationship between the Bufo UT protein expression and an increase of urea transportability, we analyzed the time course of the Bufo UT expression levels and urea uptake in the cells treated with 10(-8)M AVT. Treatment of 10(-8)M AVT increased the urea uptake in the cells after 24 and 48h incubation, but not after 12h. According to the immunoblot analysis, UT protein expression was coincident with the results of urea uptake in the AVT-treated cells. These results suggest that Bufo UT isolated from the kidney, functions as an AVT-mediated urea transporter in the urinary bladder of the toad.

Excretion and conservation of glycerol, and expression of aquaporins and glyceroporins, during cold acclimation in Cope's gray tree frog Hyla chrysoscelis

Cope's gray tree frog Hyla chrysoscelis accumulates glycerol during cold acclimation. We hypothesized that, during this process, gray tree frogs adjust renal filtration and/or reabsorption rates to retain accumulated glycerol. During cold acclimation, plasma concentrations of glycerol rose >200-fold, to 51 mmol/l. Although fractional water reabsorption decreased, glomerular filtration rate (GFR) and, consequently, urine flow were <5% of warm levels, and fractional glycerol reabsorption increased. In contrast, dehydrated frogs increased fractional water reabsorption, decreased GFR, and did not accumulate glycerol. We hypothesized that expression of proteins from the aquaporin (AQP)/glyceroporin (GLP) family was associated with changing patterns of water and glycerol movement. We cloned the cDNA for three such proteins, quantified mRNA expression in nine tissues using real-time quantitative PCR, and functionally characterized them using a Xenopus oocyte expression system. HC-1, an AQP1-like water channel conferring low glycerol permeability, is expressed ubiquitously in warm- and cold-acclimated tissues. HC-2, a water channel most similar to AQP2, is primarily expressed in organs of osmoregulation. HC-3, which is most similar to AQP3, is functionally characterized as a GLP, with low permeability to water but high permeability to glycerol. Aspects of expression levels and functional characteristics varied between cold and warm conditions for each of the three AQPs, suggesting a complex pattern of involvement in osmoregulation related to thermal acclimation.

Effect of osmotic stress on expression of a putative facilitative urea transporter in the kidney and urinary bladder of the marine toad, Bufo marinus

Anuran amphibians accumulate a large amount of urea in their extracellular fluids to avoid a severe dehydration under dry and hyper-saline environments. To clarify the mechanisms of urea retention, we examined structure and distribution of the urea transporter (UT) in the kidney of the marine toad (Bufo marinus), and its expression in the kidney and urinary bladder following exposure to dry and hyper-saline conditions by means of cDNA cloning, semi-quantitative RT-PCR, immunoblot analysis and immunohistochemistry. The Bufo UT cDNA cloned from the kidney encodes a 390-amino-acid residue protein, which is 80% identical to Rana esculenta UT with the functional characteristics of a urea transporter. The Bufo UT mRNA was abundantly expressed in the kidney and urinary bladder, but not in the skin. In immunoblot analysis using a specific antibody raised against the Bufo UT, a 52 kDa protein similar to the glycosylated forms of mammalian UT-A2 ( approximately 55 kDa) was detected in extracts from plasma membrane fractions of the kidney and urinary bladder. When toads were acclimated to dry and hyper-saline environments for 7 days, UT mRNA expression was upregulated in the kidney and urinary bladder and there was an elevated plasma urea concentration and osmolality. Immunohistochemistry showed that the UT was specifically localized on the apical membrane of the early distal tubule, known to be the diluting segment, in the kidney and the epithelial cells of urinary bladder. Immunoreactive cells were not detected along the late distal tubule, the connecting tubule or the collecting duct in the kidney. The present findings suggest that the Bufo UT probably contributes to urea transport in the kidney and urinary bladder in response to hyperosmotic stresses such as body fluid hypertonicity and dehydration.

The crab-eating frog,Rana cancrivora, up-regulates hepatic carbamoyl phosphate synthetase I activity and tissue osmolyte levels in response to increased salinity

The crab-eating frog Rana cancrivora is one of only a handful of amphibians worldwide that tolerate saline waters. They typically inhabit brackish water of mangrove forests of Southeast Asia, but live happily in freshwater and can be acclimated to 75% seawater (25 ppt) or higher. We report here that after transfer of juvenile R. cancrivora from freshwater (1 ppt) to brackish water (10 -->20 or 20 -->25 ppt; 4-8 d) there was a significant increase in the specific activity of the key hepatic ornithine urea cycle enzyme (OUC), carbamoyl phosphate synthetase I (CPSase I). At 20 ppt, plasma, liver and muscle urea levels increased by 22-, 21-, and 11-fold, respectively. As well, muscle total amino acid levels were significantly elevated by 6-fold, with the largest changes occurring in glycine and beta-alanine levels. In liver, taurine levels were 5-fold higher in frogs acclimated to 20 ppt. There were no significant changes in urea or ammonia excretion rates to the environment. As well, the rate of urea influx (J(in) (urea)) and efflux (J(out) (urea)) across the ventral pelvic skin did not differ between frogs acclimated to 1 versus 20 ppt. Taken together, these findings suggest that acclimation to saline water involves the up-regulation of hepatic urea synthesis, which in turn contributes to the dramatic rise in tissue urea levels. The lack of change in urea excretion rates, despite the large increase in tissue-to-water gradients further indicates that mechanisms must be in place to prevent excessive loss of urea in saline waters, but these mechanisms do not include cutaneous urea uptake. Also, amino acid accumulation may contribute to an overall rise in the osmolarity of the muscle tissue, but relative to urea, the contribution is small.

Renal response of euryhaline toad (Bufo viridis) to acute immersion in tap water, NaCl, or urea solutions

Green toads (Bufo viridis) were acclimated to either tap water, 230 mOsmol NaCl kg-1 H2O (saline), 500 mOsmol NaCl kg-1 H2O (high saline), or 500 mmol L-1 urea. Renal functions for each acclimation group were studied on conscious animals that had one ureter chronically catheterized. Reciprocal immersion of tap-water- and saline-acclimated toads in the opposite solution did not stress the animals osmotically, and plasma osmolality increased or decreased by no more than 15%. However, urine osmolality and ionic composition changed immediately and profoundly on exposure to the other solution. Exposure of tap-water-acclimated toads to saline decreased urine flow by 30%, whereas the reciprocal immersion led to an increase of 30%. Immersion of tap-water-acclimated toads in high saline led to immediate cessation of urine flow, whereas immersion of 500 NaCl- or urea-acclimated toads in tap water led to a large increase in urine flow, with an overshoot that lasted 10 h (as a result of either salt or urea diuresis). Urine flow then stabilized at a level 5-6 times higher than the value attained at high-salt environment. On immersion of 500 urea-acclimated toads in 500 NaCl, urine flow doubled, accompanied by a change in ion composition, without change in the osmolality. In all experimental conditions, plasma potassium concentration was maintained within a narrow range. The results show that the toad's kidneys contributed efficiently both to osmo- and ionoregulation in a wide range of ambient solutions.

Urea and amphibian water economy

Accumulation of urea in the body fluids enables some amphibians to tolerate high ambient salinities (Bufo viridis, Xenopus laevis, Rana cancrivora, Ambystoma tigrinum, Batrachoseps spp.) or to estivate in soil with low water potentials (Scaphiopus spp.). These species are assumed not only to accumulate urea produced in the normal metabolism, but to synthesize urea in response to water shortage. Re-examination of the data did not support the view of an osmoregulatory urea synthesis. Increased urea synthesis on exposure to high salinities in X. laevis, R. cancrivora and Batrachoseps spp. seemed to reflect reactions to an adverse environment. It is suggested that in amphibians, solute concentration in the plasma and rate of excretion of urea are coordinated so that at a certain plasma concentration, urea is excreted at the same rate at which it is produced. The higher the level of urea in the body fluids at balance between production and excretion, the higher the tolerance of the species of low external water potentials. The mechanisms that integrate the relationship between plasma solute concentration and handling of urea by the kidneys are not known.

Renal function at steady state in a toad (Bufo viridis) acclimated in hyperosmotic NaCl and urea solutions

Kidney function of the euryhaline toad Bufo viridis was studied in animals acclimated to tap water and solutions of NaCl (230 and 500 mosmol.kg-1 H2O) and urea (500 mmol.l-1) in steady-state conditions. An ureter was catheterized for continuous urine collection and blood was sampled from an iliac artery. A single injection of 3H-inulin served for estimation of glomerular filtration rate: this was in the range of 15-27 ml.kg-1.h-1 and did not differ significantly in any of the acclimation conditions. Urine flow, on the other hand, varied considerably and was highest in tap water (18.2 +/- 3.2 ml.kg-1.h-1; urine/plasma inulin ratio = 0.88), lower in 230 mosmol.kg-1 H2O NaCl solution (13.5 +/- 3.9 ml.kg-1.h-1; u/p inulin ratio = 1.73) and lowest in 500 mosmol.kg-1 H2O NaCl or urea acclimation solutions (5-7 ml.kg-1.h-1; u/p inulin = 3.7-4.2). Clearance of free water was high in the tap water group, lower in 230 mosmol.kg-1 H2O NaCl solution, and much lower in the hyperosmotic acclimation conditions. Clearances of both Na+ and Cl- were similar under our experimental conditions, but changed independently in accordance to the composition of the acclimation solution. Potassium clearance was similar in all acclimation conditions, and a constant plasma K+ concentration was maintained. Urea clearance was high in tap water and 500 mmol.l-1 urea acclimation groups and low in the NaCl acclimations.(ABSTRACT TRUNCATED AT 250 WORDS)

The crab-eating frog Rana cancrivora: serum mineral concentrations and histology of the ultimobranchial gland and the parathyroid gland

In the crab-eating frog Rana cancrivora, taken from a full-strength seawater pond, the in situ levels of serum Na, Cl, K, Ca, Mg, Pi, urea, and osmolarity were examined. The levels were higher than those usually reported for freshwater anurans. However, values for the monovalent salts, urea, and osmolarity were lower than those reported by Gordon et al. (1961, J. Exp. Biol. 38, 659-678) following immersion of this species in 80% seawater for 7 days. The histological features of the ultimobranchial gland and parathyroid gland were coincident with those known in usual freshwater frogs, in spite of the peculiarity of the habitat of this species. The ultimobranchial gland was composed of a single follicle or multiple follicles. In most parenchymal cells, immunoreactive calcitonin was detected using the PAP method with anti-salmon calcitonin antiserum. In the parathyroid gland, cells in the central part of the gland were smaller than those in the peripheral part.

Structure of the kidney in the crab-eating frog, Rana cancrivora

The structure of the nephron in the ranid frog, Rana cancrivora, was studied by light and electron microscopy. This frog is the only amphibian species to live in mangrove swamps of very high salinity. The nephron consists of the following parts: renal corpuscle, ciliated neck segment, proximal tubule, ciliated intermediate segment, distal tubule, connecting tubule, and collecting duct. The distal tubule is located in the ventromedial region of the kidney, and the other tubules are situated in the dorsolateral region. Renal corpuscles are found between the two regions. Some renal corpuscles have a wide Bowman's space because of the small glomerulus within them. The proximal tubules are composed of columnar cells with a dense luminal brush border of long microvilli and numerous apical vesicles and vacuoles. The initial part of the distal tubule consists of heavily interdigitated cells, characterized by a very regular palisade arrangement of mitochondria. In the terminal part of the distal tubule, shorter mitochondria of the infolding cells are situated irregularly around the nucleus. The connecting tubule consists of principal cells and canaliculus cells. The collecting duct consists of columnar or cuboidal cells; cytoplasmic organelles are relatively sparse. The canaliculus cells are intercalated between principal cells from the terminal distal tubule to the proximal part of the collecting duct. Our findings indicate that the kidney of R. cancrivora is structurally similar to kidneys of other amphibians. These findings are discussed with regard to probable correlations between ultrastructure and function in R. cancrivora.

Effects of arginine vasotocin on water balance of three leiopelmatid frogs

The effects of arginine vasotocin (AVT) on water movements in three frogs of the anatomically primitive genus Leiopelma were compared in vivo and in vitro. Treatment with AVT in vivo (7 x 10(-13) mol/g body weight) led to significant water retention in the terrestrial species L. archeyi and L. hamiltoni, but not in the semiaquatic species L. hochstetteri. All three species showed increasing water retention over some part of the dose range 7 x 10(-15)-7 x 10(-11) mol/g. Isolated pelvic skin from L. archeyi and L. hamiltoni increased in osmotic water flow (OWF) in response to AVT, whereas that from L. hochstetteri did not. Dorsal skin from all three species, as well as pectoral skin from L. archeyi and L. hochstetteri, showed no hydroosmotic response to AVT. Isolated bladders of all species are small and showed small increases in OWF in response to AVT. Glomerular filtration rate in vivo was reduced significantly by AVT in L. archeyi and L. hamiltoni, but not in L. hochstetteri. These results show that water balance responses to AVT among the three species are correlated with habitat, and indicate that the absence of a cutaneous hydroosmotic response in L. hochstetteri is not a characteristic, primitive feature of the entire genus.

Volume regulation in salt-acclimated toad (Bufo viridis): the role of urea and the urinary bladder

Body water (weight) was studied in the euryhaline toad Bufo viridis during high salt (500 mOsm NaCl) acclimation. Plasma osmolality was greatly increased upon salt acclimation mainly by urea, and was always hyperosmotic to the ambient solution. Water content was regulated quite efficiently in slowly acclimated undisturbed toads. Repeatedly catheterized toads behaved like osmometers when transferred to hyperosmotic solutions. Total urea loss was greatly reduced in salt acclimated toads, suggesting urine was not voided under these conditions. It is concluded that urea accumulation, inhibition of the urine voiding response and the urine in the bladder are the principal factors involved in volume regulation under conditions of salt acclimation.

Ornithine-urea cycle activity in xenopus laevis: adaptation in saline

The concentration of urea in the blood and the rate of urea excretion were markedly elevated in Xenopus maintained in hypertonic saline for 2 to 3 weeks. These changes were accompanied by a twofold increase in the activity of the ornithine-urea cycle as measured in liver slices. The activity of carbamoyl phosphate synthetase rose threefold in frogs adapted to saline. These results suggest that changes in activities of urea cycle enzymes may be important in the adaptation of aquatic organisms to environments of varying salinities.

Urea and its formation in coelacanth liver

Urea occurs in liver of the coelacanth Latimeria chalumnae to the extent of about 1.7 percent by weight. It was determined quantitatively by reaction with 1-phenyl-1,2-propanedione-2-oxime (Archibald reagent) and by measurement of ammonia released upon treatment with urease. Arginase and ornithine carbamoyltransferase, enzymes instrumental in the formation of urea in typical ureotelic vertebrates, occur in homogenates of coelacanth liver. Formed in part by the ornithine-urea cycle, urea may have an osmoregulatory function in the coelacanth as it has in elasmobranchs.

Serum osmolality in the coelacanth, Latimeria chalumnae: urea retention and ion regulation

Samples of blood (hemolyzed) were obtained from the renal vein, the hepatic portal vein, and the heart of a freshly thawed specimen of Latimeria chalumnae. The coelacanth uses high concentrations of urea to maintain its serum osmolality at approximately that of sea water. The mean value for the total osmolality was 1181 milliosmoles per liter. The mean values (milliequivalents per liter) were: for sodium, 181; for potassium, 51.3; for calcium, 6.9; for magnesium, 28.7; for chloride, 199; and for bicarbonate, 4.7. The mean urea concentration was 355 millimoles per liter, and the mean nonprotein nitrogen was 1343 milligrams percent. Heart blood showed significantly lower values for osmolality (921 milliosmoles per liter) and nonprotein nitrogen (1030 mg percent) and was probably less severely contaminated with products of protein breakdown. Fluid from the anterior chamber of the eye showed values of 952 milliosmole/liter; the urea value for this fluid was 303 mmole/liter, and the magnesium was 7.3 meq/liter. The magnesium value for the aqueous humor was used to correct the abnormally high concentrations in the hemolyzed serum. The high level of serum potassium also was attributed to hemolysis.

Uricolytic enzymes in liver of the Dipnoan Protopterus aethiopicus

The enzymes uricase, allantoinase, and allantoicase have been measured in liver preparations of the African lungfish Protopterus aethiopicus. The levels for these enzymes in lungfish liver suggest that the amount of urea formed in vivo in Protopterus via a uricolytic pathway may be greater than that derived via the Ornithine-urea cycle. The operation of a "purine cycle" in lungfish liver is proposed.