Hepatic sublobular synthesis of urea: control by oxygen or by pH gradient?

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 O2 consumed upon infusion of NH4Cl 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 (0.15 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 NH4Cl concentration was increased. 3 Sublobular rates of O2 uptake were determined by placing miniature O2 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 O2 uptake were calculated in the presence and absence of NH4Cl and local rates of urea synthesis were calculated from the extra O2 consumed in the presence of NH4Cl 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 NH4CI infusion in each region, measured with micro-light-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 μmol g−1 h−1 and 61 ± 24 μmolg−1 h−1 in periportal and pericentral regions, respectively. When perfusion was in the retrograde direction, however, rates were 76 ±23 μmol g−1 h−1 inn periportal areas and 152 ±19 μmol g−1 h−1 inn pericentral regions. 5 During perfusion in the anterograde direction, urea synthesis, calculated by changes in O2 uptake, was 307 ± 76 μmol g−1 h−1 and 72 ±34 μmol g−1 h−1 inn periportal and pericentral regions, respectively. When perfusion was in the retrograde direction, urea was synthesized at rates of 54 ± 17 μmol g−1 h−1 and 387 ± 99 μmol g−1 h−1 in periportal and pericentral regions, respectively. Thous, maximal rates of urea synthesis were dependent upon the direction of perfusion. In addition, rates of urea synthesis were elevated dramatically in periportal regions when the flow rate per gram liver was increased (e.g., 307 versus 177 μmol g−1 h−1). 6 These results demonstrate that periportal regions of the liver lobule have a greater maximal capacity to synthesize urea when flow is in the physiological anterograde direction. In contrast, the primary site of urea synthesis can be shifted rapidly to pericentral regions when the direction of flow is reversed. Thus, maximal rates of urea synthesis in the perfused liver are observed in regions of the liver lobule with the highest O2 tensions.