Urea synthesis from ammonia in periportal and pericentral regions of the liver lobule

Abstract

1. Rates of urea synthesis were determined in periportal and pericentral regions of the liver lobule in perfused liver from fed, phenobarbital-treated rats by measuring the extra 0 2 consumed upon infusion of NH4C1 with miniature O2 electrodes and from decreases in NADPH fluorescence detected with micro-light-guides.2. Urea synthesis by the perfused rat liver supplemented with lactate (5 mM), ornithine ( 2 mM) and methionine sulfoximine (0.1 5 mM), an inhibitor of glutamine synthetase, was stimulated by stepwise infusion of NH4Cl at doses ranging from 0.24 mM to 3.0 mM. A good correlation (r = 0.92) between decreases in NADPH fluorescence and urea production was observed when the NH4CI concentration was increased.3. Sublobular rates of O 2 uptake were determined by placing miniature oxygen electrodes on periportal or pericentral regions of the lobule on the liver surface, stopping the flow and measuring decreases in oxygen tension. From such measurements local rates of 0, uptake were calculated in the presence and absence of NHLCl and local rates of urea synthesis were calculated from the extra O 2 consumed in the presence of NH4C1 and the stoichiometry between O2 uptake and urea formation. Rates of urea synthesis were also estimated from the fractional decrease in NADPH fluorescence, caused by NH4C1 infusion in each region, measured with microlight-guides and the rate of urea synthesis by the whole organ.4. When perfusion was in the anterograde direction, maximal rates of urea synthesis, calculated from changes in fluorescence, were 177 31 pmol g-' h-' and 61 f 2 4 pmol g-' h-' in periportal and pericentral regions, respectively. When perfusion was in the retrograde direction, however, rates were 76 f 23 pmol g

Endoplasmic reticulum stress eIF2α–ATF4 pathway-mediated cyclooxygenase-2 induction regulates cadmium-induced autophagy in kidney

“…32 However, since urea is synthesized by liver, BUN will be lower than the normal level when the liver is severely damaged. 33 A low BUN/CRE ratio indicates reduction of BUN reabsorption caused by kidney damage. 31 In our study, Cd raised the serum CRE and BUN in the 0.2 and 1 mg/kg groups, indicating kidney filtration failure, but not in the 5 mg/kg group, which might have resulted from severe hepatomegaly and liver damage.…”

Nitrogen metabolism in liver: structural and functional organization and physiological relevance

“…Different techniques (for recent reviews see [5,9,10]) have been employed to study metabolic zonation in hepatic nitrogen metabolism: histochemistry [12], immunohistochemistry [13][14][15][16][17], in situ hybridization [18,19], retrograde/antegrade liver perfusion [20,21], microdissection [22], autoradiography (B. Stoll, H. P. Buscher & D. Haiussinger, unpublished work), radiolabelincorporation studies [23,24], studies on zonal cell damage [25,26], use of micro-lightguides and mini-oxygen electrodes [27], attempts to separate periportal from perivenous hepatocytes [28][29][30] and the dual-digitonin-pulse perfusion [31]. These studies revealed a remarkable development of functional hepatocyte heterogeneity with respect to ammonia and glutamine metabolism.…”

Oxygen tension does not affect urea synthesis in perifused rat hepatocytes

“…On the basis of experiments with livers perfused with Krebs-Henseleit bicarbonate buffer in a non-recirculating system, Kari et al [6] proposed that urea synthesis is controlled by the oxygen tension and not by the concentration of ornithinecycle enzymes. In their experiments, flux through the ornithine cycle varied with the oxygen concentration, even though the concentration of oxygen (0.2-0.5 mM, which is non-physiologically high) was much higher than the value of 20 pM required for half-maximal oxygen uptake.…”

“…Results obtained with suspensions of hepatocytes isolated from the portal and central areas have confirmed that the capacity of portal hepatocytes to synthesize urea is much higher than that of pericentral hepatocytes [5]. An opposing view is held by Kari et al [6]. According to these authors, oxygen tension is a major factor controlling urea synthesis in the intact liver.…”

“…When the flow was in the physiological antegrade direction, periportal hepatocytes synthesized urea at higher rates, whereas urea was predominantly synthesized in pericentral hepatocytes when the flow was reversed. Measurements of oxygen disappearance after stopping the flow in periportal and pericentral regions, at the liver surface, during antegrade and retrograde perfusions, led Kari et al [6] to propose that oxygen tension rather than the local distribution of enzymes of the ornithine cycle determined the rate of urea synthesis in the portal and central areas of the liver lobule, even though the oxygen concentration was much higher than that required for half-maxima1 oxygen uptake.…”