Oxygen supply dependence of urea production in the isolated perfused rat liver

To determine whether hepatic urea production is limited at low hepatic O₂ delivery (Do₂) by O₂ itself or by the availability of substrate for urea synthesis, we isolated livers from normal rats and perfused them with Krebs-Henseleit bicarbonate (KHB) buffer, KHB + 5 mM NH₄Cl, or KHB + 5 mM glutamine (Gln) as an NH₃ donor. The pump flow was lowered in stages, and we determined at each flow rate inflow and outflow O₂ content and urea levels in the outflow perfusate. Urea production in Gln-perfused livers remained constant at high Do₂ and declined in direct proportion to Do₂ below a critical oxygen delivery (Do₂crit, the point below which the hepatic O₂ consumption [Vo₂ becomes limited by the hepatic Do₂). The Do₂crit calculated from the urea release-Do₂ relationship (147 ± 32 μl/min/ dry g) was similar to the Do₂crit calculated from the Vo₂-Do₂ relationship (158 ± 26 μl/min/dry g). When Gln concentration and flow rate were maintained constant while decreasing Po₂ in the inflow perfusate (as well as hepatic Do₂), urea production declined below the Do₂crit. Furthermore, when Gln concentration in the perfusate was gradually reduced while keeping hepatic Do₂ constant, urea production decreased proportionally with Gln concentrations in the perfusate. Consequently, urea production is dependent on Gln and O₂ availability and becomes limited at the same Do₂crit deter mined by the Vo₂-Do₂ relationship.