Urea and urine concentrating ability in mice lacking AQP1 and AQP3

Aquaporin-1 (AQP1) and aquaporin-3 (AQP3) water channels expressed in the kidney play a critical role in the urine concentrating mechanism. Mice with AQP1 or AQP3 deletion have a urinary concentrating defect. To better characterize this defect, we studied the influence of an acute urea load (300 μmol ip) in conscious AQP1-null, AQP3-null, and wild-type mice. Urine was collected and assayed every 2 h, from 2 h before (baseline) to 8 h after the urea load. Mice of all genotypes excreted the urea load in ∼4 h with the same time course. Interestingly, despite their low baseline, the AQP3-null mice raised their urine osmolality and urea concentration progressively after the urea load to values almost equal to those in wild-type mice at 8 h. In contrast, urine non-urea solute concentration did not change. Urine volume fell in the last 4 h to about one-fourth of basal values. AQP1-null mice increased their urine flow rate much more than AQP3-null mice and showed no change in urine osmolality and urea concentration. The urea load strongly upregulated urea transporter UT-A3 expression in all three genotypes. These observations show that the lack of AQP3 does not interfere with the ability of the kidney to concentrate urea but impairs its ability to concentrate other solutes. This solute-selective response could result from the capacity of AQP3 to transport not only water but also urea. The results suggest a novel role for AQP3 in non-urea solute concentration in the urine.

Urea and urine concentrating ability: new insights from studies in mice

Urea is the most abundant solute in the urine in humans (on a Western-type diet) and laboratory rodents. It is far more concentrated in the urine than in plasma and extracellular fluids. This concentration depends on the accumulation of urea in the renal medulla, permitted by an intrarenal recycling of urea among collecting ducts, vasa recta and thin descending limbs, all equipped with specialized, facilitated urea transporters (UTs) (UT-A1 and 3, UT-B, and UT-A2, respectively). UT-B null mice have been recently generated by targeted gene deletion. This review describes 1) the renal handling of urea by the mammalian kidney; 2) the consequences of UT-B deletion on urinary concentrating ability; and 3) species differences among mice, rats, and humans related to their very different body size and metabolic rate, leading to considerably larger needs to excrete and to concentrate urea in smaller species (urea excretion per unit body weight in mice is 5 times that in rats and 23 times that in humans). UT-B null mice have a normal glomerular filtration rate but moderately reduced urea clearance. They exhibit a 30% reduction in urine concentrating ability with a more severe defect in the capacity to concentrate urea (50%) than other solutes, despite a twofold enhanced expression of UT-A2. The urea content of the medulla is reduced by half, whereas that of chloride is almost normal. When given an acute urea load, UT-B null mice are unable to raise their urinary osmolality, urine urea concentration (Uurea), and the concentration of non-urea solutes, as do wild-type mice. When fed diets with progressively increasing protein content (10, 20, and 40%), they cannot prevent a much larger increase in plasma urea than wild-type mice because they cannot raise Uurea. In both wild-type and UT-B null mice, urea clearance was higher than creatinine clearance, suggesting the possibility that urea could be secreted in the mouse kidney, thus allowing more efficient excretion of the disproportionately high urea load. On the whole, studies in UT-B null mice suggest that recycling of urea by countercurrent exchange in medullary vessels plays a more crucial role in the overall capacity to concentrate urine than its recycling in the loops of Henle.

Influence of Adjuvants on Peanut (Arachis hypogaea L.) Response to Prohexadione Calcium

Research has demonstrated that prohexadione calcium (calcium salt of 3,5-dioxo-4-propionylcyclohexanecarboxylic acid) retards vegetative growth of peanut (Arachis hypogaea L.) and in some cases increases pod yield, the percentage of extra large kernels, market value ($/kg), and gross value ($/ha). Spray adjuvants such as crop oil concentrate and nitrogen solution most likely will be recommended for application with prohexadione calcium. However, efficacy of prohexadione calcium applied with adjuvants has not been conclusively determined. Twelve experiments were conducted in North Carolina and Virginia during 1997 and 1998 to determine peanut response to prohexadione calcium applied with crop oil concentrate, urea ammonium nitrate, or a mixture of these adjuvants. Applying prohexadione calcium with urea ammonium nitrate, either alone or with crop oil concentrate, increased row visibility and shorter main stems compared with nontreated peanut or prohexadione calcium applied with crop oil concentrate. Prohexadione calcium increased pod yield, the percentage of extra large kernels, and gross value of peanut in seven of 12 experiments regardless of adjuvant when compared with nontreated peanut. Pod yield, the percentage of extra large kernels, and gross value of peanut were not affected in the other experiments. Prohexadione calcium did not affect the percentage of total sound mature kernels, the percentage of other kernels, or market value in any of the experiments regardless of adjuvant.

CHANGES IN AMMONIA CONCENTRATION, BACTERIAL COUNTS, pH AND VOLATILE FATTY ACID CONCENTRATION IN RUMEN OF COWS FED ALFALFA HAY OR CONCENTRATE: UREA-CORN SILAGE

Four rumen-fistulated cows were fed alfalfa hay (15.4% CP) ad libitum for 4 wk prior to the experiment and from day 1 to day 8 of the experiment. From days 9 to 15 (period 2) and from days 16 to 39 (period 3) they were fed ad libitum concentrate: urea-corn silage (15.9% CP). Rumen fluid samples were collected from 0600 to 1600 h during the three periods and changes in the pH, concentrations of ammonia and volatile fatty acids (VFA) and bacterial counts were determined. Ammonia, pH and VFA concentrations showed oscillatory behavior even on an ad libitum feeding schedule. Rates of production and utilization of ammonia were significantly higher with the concentrate: urea-corn silage diet than with the hay diet. VFA concentration and pH were inversely related. Periods of high VFA production coincided with periods of rapid ammonia utilization. The evidence indicated that there was a metabolic adaptation in the rumen for better utilization of ammonia. Concentrations of amino acids and peptides were nearly 1 mM in the rumen fluid throughout the day and increased to 3 mM immediately after feeding. Most of the increases were due to alanine, leucine and aspartic acid. It is concluded that the practice in nutritional studies of measuring and reporting rumen ammonia, VFA and pH only once at one time point in an experiment or even the mean of several determinations does not adequately represent the complex metabolic changes in the rumen. Key words: rumen, ammonia, fermentation, pH, adaptation, amino acids