Arterial ammonemia changes of renal origin induced in the rat by acid and alkaline diets

Protein- and diabetes-induced glomerular hyperfiltration: role of glucagon, vasopressin, and urea

A single protein-rich meal (or an infusion of amino acids) is known to increase the glomerular filtration rate (GFR) for a few hours, a phenomenon known as “hyperfiltration.” It is important to understand the factors that initiate this upregulation because it becomes maladaptive in the long term. Several mediators and paracrine factors have been shown to participate in this upregulation, but they are not directly triggered by protein intake. Here, we explain how a rise in glucagon and in vasopressin secretion, directly induced by protein ingestion, might be the initial factors triggering the hepatic and renal events leading to an increase in the GFR. Their effects include metabolic actions in the liver and stimulation of sodium chloride reabsorption in the thick ascending limb. Glucagon is not only a glucoregulatory hormone. It is also important for the excretion of nitrogen end products by stimulating both urea synthesis in the liver (along with gluconeogenesis from amino acids) and urea excretion by the kidney. Vasopressin allows the concentration of nitrogenous end products (urea, ammonia, etc.) and other protein-associated wastes in a hyperosmotic urine, thus allowing a very significant water economy characteristic of all terrestrial mammals. No hyperfiltration occurs in the absence of one or the other hormone. Experimental results suggest that the combined actions of these two hormones, along with the complex intrarenal handling of urea, lead to alter the composition of the tubular fluid at the macula densa and to reduce the intensity of the signal activating the tubuloglomerular feedback control of GFR, thus allowing GFR to raise. Altogether, glucagon, vasopressin, and urea contribute to set up the best compromise between efficient urea excretion and water economy.

Renal metabolism of amino acids: its role in interorgan amino acid exchange

The kidneys play a role in the synthesis and interorgan exchange of several amino acids. The quantitative importance of renal amino acid metabolism in the body is not, however, clear. We review here the role of the kidney in the interorgan exchange of amino acids, with emphasis on quantitative aspects. We reviewed relevant literature by using a computerized literature search (PubMed) and checking relevant references from the identified articles. Our own data are discussed in the context of the literature. The kidney takes up glutamine and metabolizes it to ammonia. This process is sensitive to pH and serves to maintain acid-base homeostasis and to excrete nitrogen. In this way, the metabolism of renal glutamine and ammonia is complementary to hepatic urea synthesis. Citrulline, derived from intestinal glutamine breakdown, is converted to arginine by the kidney. Renal phenylalanine uptake is followed by stoichiometric tyrosine release, and glycine uptake is accompanied by serine release. Certain administered oligopeptides (eg, glutamine dipeptides) are converted by the kidneys to their constituent components before they can be used in metabolic processes. The kidneys play an important role in the interorgan exchange of amino acids. Quantitatively, for several important amino acids, the kidneys are as important as the gut in intermediary metabolism. The kidneys may be crucial "mediators" of the beneficial effects of specialized, disease-specific feeding solutions such as those enriched in glutamine dipeptides.

Enhanced renal vein ammonia efflux after a protein meal in the pig

BACKGROUND/AIMS

The intake of dietary protein has been associated with increased arterial ammonia levels. However, the origin of this rise in ammonia levels is unknown. This study was designed to examine whether this increase is caused by ammonia formed by the gut escaping hepatic clearance, or ammonia formed by the kidney and subsequently released into the circulation.

METHODS

Splanchnic and renal fluxes of ammonia and amino acids were studied in 10 pigs that were fed in a randomized cross-over design with a protein meal (n = 8), a meal with an equimolar amount of free amino acids (n = 8) or an iso-osmolar NaCl solution (n = 6).

RESULTS

After the protein meal, and less pronounced after the amino acid meal, arterial ammonia levels increased from approximately 25 to 75 micromol/l. Arterial pH changes and splanchnic ammonia release were negligible. The renal vein ammonia efflux increased after the protein meal (0.67+/-0.10 to 1.94+/-0.35 micromol/kg bw/min) and to a lesser degree after the amino acid meal (to 1.20+/-0.39 micromol/kg bw/ min). Renal uptake of alanine, and not glutamine, increased stoichiometrically, paralleling the enhanced renal vein ammonia efflux.

CONCLUSIONS

Arterial ammonia increases after a meal in pigs, coinciding with a negligible splanchnic ammonia release, but increased renal vein ammonia efflux. Thus, post-prandial plasma ammonia levels appear to be mainly related to renal ammoniagenesis. Alanine appears to be the main precursor for this renal ammoniagenesis in the pig.

Ammonia and glutamine metabolism during liver insufficiency: the role of kidney and brain in interorgan nitrogen exchange

BACKGROUND

During liver failure, urea synthesis capacity is impaired. In this situation the most important alternative pathway for ammonia detoxification is the formation of glutamine from ammonia and glutamate. Information is lacking about the quantitative and qualitative role of kidney and brain in ammonia detoxification during liver failure.

METHODS

This review is based on own experiments considered against literature data.

RESULTS AND CONCLUSIONS

Brain detoxifies ammonia during liver failure by ammonia uptake from the blood, glutamine synthesis and subsequent glutamine release into the blood. Although quantitatively unimportant, this may be qualitatively important, because it may influence metabolic and/or neurotransmitter glutamate concentrations. The kidney plays an important role in adaptation to hyperammonaemia by reversing the ratio of ammonia excreted in the urine versus ammonia released into the blood from 0.5 to 2. Thus, the kidney changes into an organ that netto removes ammonia from the body as opposed to the normal situation in which it adds ammonia to the body pools.

Renal ammonia and glutamine metabolism during liver insufficiency-induced hyperammonemia in the rat

Renal glutamine uptake and subsequent urinary ammonia excretion could be an important alternative pathway of ammonia disposal from the body during liver failure (diminished urea synthesis), but this pathway has received little attention. Therefore, we investigated renal glutamine and ammonia metabolism in midly hyperammonemic, portacaval shunted rats and severely hyperammonemic rats with acute liver ischemia compared to their respective controls, to investigate whether renal ammonia disposal from the body is enhanced during hyperammonemia and to explore the limits of the pathway. Renal fluxes, urinary excretion, and renal tissue concentrations of amino acids and ammonia were measured 24 h after portacaval shunting, and 2, 4, and 6 h after liver ischemia induction and in the appropriate controls. Arterial ammonia increased to 247 +/- 22 microM after portacaval shunting compared to controls (51 +/- 8 microM) (P < 0.001) and increased to 934 +/- 54 microM during liver ischemia (P < 0.001). Arterial glutamine increased to 697 +/- 93 microM after portacaval shunting compared to controls (513 +/- 40 microM) (P < 0.01) and further increased to 3781 +/- 248 microM during liver ischemia (P < 0.001). In contrast to controls, in portacaval shunted rats the kidney net disposed ammonia from the body by diminishing renal venous ammonia release (from 267 +/- 33 to -49 +/- 59 nmol/100 g body wt per min) and enhancing urinary ammonia excretion from 113 +/- 24 to 305 +/- 52 nmol/100 g body wt per min (both P < 0.01). Renal glutamine uptake diminished in portacaval shunted rats compared to controls (-107 +/- 33 vs. -322 +/- 41 nmol/100 g body wt per min) (P < 0.01). However, during liver ischemia, net renal ammonia disposal from the body did not further increase (294 +/- 88 vs. 144 +/- 101 nmol/100 g body wt per min during portacaval shunting versus liver ischemia). Renal glutamine uptake was comparable in both hyperammonemic models. These results indicate that the rat kidney plays an important role in ammonia disposal during mild hyperammonemia. However, during severe liver insufficiency induced-hyperammonemia, ammonia disposal capacity appears to be exceeded.

Intestinal glutamine and ammonia metabolism during chronic hyperammonaemia induced by liver insufficiency

During liver insufficiency, besides portasystemic shunting, high arterial glutamine concentrations could enhance intestinal glutamine consumption and ammonia generation, thereby aggravating hyperammonaemia. To investigate this hypothesis, portal drained viscera (intestines) fluxes and jejunal tissue concentrations of ammonia and glutamine were measured in portacaval shunted rats with a ligated bile duct, portacaval shunted, and sham operated rats, seven and 14 days after surgery, and in normal unoperated controls. Effects of differences in food intake were minimised by pair feeding portacaval shunted and sham operated with portacaval shunted rats with biliary obstruction. At both time points, arterial ammonia was increased in the groups with liver insufficiency. Also, arterial glutamine concentration was raised in all operated groups compared with normal unoperated controls. At both time points, ammonia production by portal drained viscera was reduced in portacaval shunted rats with biliary obstruction, portacaval shunted, and sham operated rats compared with normal unoperated controls, and no major differences were found between these operated groups. At day 7 in all operated groups glutamine uptake by portal drained viscera was lower than in normal unoperated controls, but no major differences were found at day 14. These experiments show that ammonia generation by portal drained viscera remains unchanged in rats with chronic liver insufficiency despite alterations in arterial glutamine concentrations and intestinal glutamine uptake. The hyperammonaemia seems to be mainly determined by the portasystemic shunting.

The role of the kidney in the maintenance of water balance

This chapter shows how the mammalian kidney is able to regulate the excretion of water independently from that of solutes. For this function, which derives from several evolutionary steps among vertebrates, it takes advantage of the diluting ability of the thick ascending limb to produce osmotic energy which is then used to concentrate solutes in the urine. This concentration is permitted by a highly sophisticated architecture of nephrons and vessels in the renal medulla, combined with special permeability characteristics of the different nephron segments and specific hormonal regulation. Two different types of loops of Henle and several well-insulated vascular compartments contribute to this process. The major nitrogenous waste product, urea, is concentrated by an indirect process involving a transfer of osmotic energy from the outer to the inner medulla. As known for several decades, concentrating function is primarily regulated by the effect of antidiuretic hormone (ADH) on water permeability of the collecting duct. However, as discovered more recently, it is also largely dependent upon the effect of the same hormone on urea permeability in the terminal collecting duct. In addition, recent investigations have revealed a much more complex hormonal regulation of the concentrating process than previously thought. ADH itself acts on many other structures in the kidney, and many other hormones and mediators, the secretion of which is not thought to be influenced by the water status, do affect urine concentration either directly or by their interaction with ADH. Rodents display a wide spectrum of morphological and functional renal adaptations improving water conservation. Their study has brought a better understanding of the significant steps and anatomical structures that contribute to the concentrating process. Finally, it is also apparent that the capacity to concentrate urine is influenced in individual animals of a given species by the availability of water, by specific feeding patterns, and by the protein content of the diet.

Intestinal ammonium production in the rat: the role of the colon, small intestine, and circulating glutamine

The intestinal ammonium production and the intestinal uptake of circulating glutamine were investigated in anesthetized intact rats and rats with resected small intestine or colon by simultaneous measurements performed on portal and arterial blood. It was shown that ammonium release into the portal blood by the small intestine is of equal magnitude to that released by the colon, and that circulating glutamine participates in ammonium production by the small intestine. Increased levels of circulating glutamine induced by its i.v. infusion to intact rats were not accompanied by an increase in intestinal ammonium production.