Effects of saline acclimation on plasma electrolytes, urea excretion, and hepatic urea biosynthesis in a freshwater stingray, Potamotrygon sp. garman, 1877

We tested the hypothesis that urea transport in rainbow trout (Oncorhynchus mykiss) embryos is dependent, in part, on a bidirectional urea-transport protein. Acute exposure to phloretin and urea analogs [acetamide, thiourea, 1,(4-nitrophenyl)-2-thiourea] reversibly inhibited urea excretion from the embryos to the external water. Unidirectional urea influx was inhibited by acetamide and thiourea, with IC(50) values of 0.04 and 0.05 mmol l(−1), respectively. Influx of urea from the external water to the embryo tended to saturate at elevated external urea concentrations (V(max)=10.50 nmol g(−1) h(−1); K(m)=2 mmol l(−1)). At very high urea concentrations (20 mmol l(−1)), however, a second, non-saturable component was apparent. These results indicate that urea excretion in trout embryos is dependent, in part, on a phloretin-sensitive facilitated urea transporter similar to that reported in mammalian inner medullary collecting ducts and elasmobranch kidney.

Download Full-text

In Reply: Pathogenesis of Hyposthenuria in Sickle Cell Anemia

PEDIATRICS ◽

10.1542/peds.26.6.1051a ◽

1960 ◽

Vol 26 (6) ◽

pp. 1051-1052

Author(s):

Hans G. Keitel

Keyword(s):

Sickle Cell ◽

Sickle Cell Anemia ◽

Normal Level ◽

Urinary Concentration ◽

Urea Excretion ◽

Low Level ◽

Concentration Of Urine

In reply to the question raised by Dr. Morgan, it is now well established that urea plays an important part in the concentration of urine. Starting at a very low level of urea excretion, the urinary concentration maxima (as obtained during prolonged fluid deprivation, or following sustained vasopressin stimulation) is less than normal. When the rate of urea excretion is increased to the normal level, the maximal urinary concentration ability increases, but when the rate of urea excreted increases markedly, the urinary concentration maxima once again decreases.

Download Full-text

The Mode of Excretion of Ammonia and Urea in Xenopus Laevis

Journal of Experimental Biology ◽

10.1242/jeb.38.4.695 ◽

1961 ◽

Vol 38 (4) ◽

pp. 695-705

Author(s):

J. B. BALINSKY ◽

E. BALDWIN

Keyword(s):

Xenopus Laevis ◽

Blood Concentration ◽

Concentration Ratio ◽

Ammonia Excretion ◽

Ammonia Concentration ◽

Mean Value ◽

Urea Excretion ◽

Wide Range ◽

Glomerular Filtrate ◽

The Mean

1. Eighty-two single determinations of ammonia and urea excretion by Xenopus laevis indicated that the percentage of ammonia varied from 40 to 80%, with a mean value of 62%. 2. Measurements of excretion on successive days after feeding showed that a large amount of ammonia was produced soon after feeding, but that ammonia excretion declined rapidly. Urea excretion, not so high initially, remained more or less constant until the third or fourth day, often exceeding ammonia excretion at that time. Thereafter, it also declined and the excretion of both substances reached a constant starvation level by the fifteenth day. 3. Both ammonia and urea excretion were equally affected by temperature. The Q10's were near 2 in the range 20-30° C., but greater in the range 10-20° C. 4. At least 86% of ammonia, and 81% of urea were excreted through the cloaca. 5. The mean 24 hr. urine output of Xenopus at 20% C. was 23.6 ml. per 100 g. body weight. 6. Although the blood ammonia concentration did not appear to be zero, the urine/blood concentration ratio of ammonia was greater than 100. The urine/blood concentration ratio of urea was not significantly different from unity, and constant over a very wide range of concentrations. 7. The above result is interpreted to indicate passive glomerular filtration of urea, and little or no tubular reabsorption of water. 8. It is suggested that ammonia is formed in the kidney, and actively secreted into the glomerular filtrate.

Download Full-text

Ureotelism is inducible in the neotropical freshwater Hoplias malabaricus (Teleostei, Erythrinidae)

Brazilian Journal of Biology ◽

10.1590/s1519-69842004000200012 ◽

2004 ◽

Vol 64 (2) ◽

pp. 265-271 ◽

Cited By ~ 3

Author(s):

G. Moraes ◽

V. L. P. Polez

Keyword(s):

Specific Activity ◽

Excretion Rate ◽

Nitrogen Excretion ◽

Urea Synthesis ◽

Plasma Ammonia ◽

Alkaline Water ◽

Hoplias Malabaricus ◽

Urea Excretion ◽

Ammonia Transport ◽

In The Wild

Increased environmental pH decreases ammonia transport through the gills, impairing nitrogenous waste. The consequent toxicity is usually drastic to most fishes. A few species are able to synthesize urea as a way to detoxify plasma ammonia. We studied three teleosts of the family Erythrinidae living in distinct environments, and assumed the biochemical behaviors would be different in spite of their being closely related species. Adult fish collected in the wild were submitted to alkaline water and the urea excretion rate was determined. The specific activity of urea cycle enzymes was determined in liver samples of fish from neutral waters. The studied species Hoplias lacerdae, Hoplerithrynus unitaeniatus, and Hoplias malabaricus are ureogenic. Urea synthesis is not a metabolic way to detoxify ammonia in H. lacerdae and Hoplerithrynus unitaeniatus exposed to an alkaline environment. The plasma ammonia profile of both species showed two distinct biochemical responses. Urea excretion of H. malabaricus was high in alkaline water, and the transition to ureotelism is proposed. The nitrogen excretion rate of H. malabaricus was among the highest values reported and the high urea excretion leads us to include this species as ureotelic in alkaline water.

Download Full-text