Functional hepatocyte heterogeneity

Abstract

1. In isolated perfused rat liver maximal rates of 2-[1-14C] oxoglutarate uptake were about 0.4 .mu.mol .times. g-1 .cntdot. min-1; half-maximal rates of 2-[14C]oxoglutarate uptake were observed with influent concentrations of about 100 .mu.M. 2-[14C]Oxoglutarate uptake by the liver was not affcted by the direction of perfusion, but was decreased by about 80-90% when Na+ in the perfusion fluid was substituted by choline+, suggesting a Na+-dependence of hepatic 2-oxoglutarate uptake. In the absence of added ammonia, [14C]oxoglutarate uptake by the liver was about twice the net oxoglutarate uptake, indicating a simultaneous release of unlabeled oxoglutarate from perfused rat liver. 2. 14C-Labeled metabolites derived from [1-14C]oxoglutarate and recovered in the effluent perfusate were 14CO2 and 14C-labeled glutamate and glutamine; they accounted for 85-100% of the radiolabel taken up by the liver. 14CO2 was the major product (more than 70%) form [1-14C]oxoglutarate taken up the liver, provided glutamine synthesis was either inhibited by methionine sulfoximine or the endogenous rate of glutamine production was below 40 nmol .cntdot.g-1 .cntdot. min-1. 3. Stimulation of glutamine synthesis by ammonia did not affect [14C]oxoglutarate uptake by the liver, but considerably increased net hepatic oxoglutarate uptake, indicating a decreased release of unlabeled oxoglutarate from the liver. Stepwise stimulation of hepatic glutamine synthesis led to a gradual decrease of 14CO2 production and radiolabel was recovered increasingly as [14C]glutamine in the effluent. At high rates of glutamine formation (i.e. about 0.6 .mu.mol .cntdot. g-1 .cntdot. min-1), about 60% of the [1-14C]oxoglutarate taken up by the liver was recovered in the effluent as [14C]glutamine. 14CO2 and [14C]glutmine production from added [1-14C]oxoglutarate were dependent on the rate of hepatic glutamine synthesis but not on the direction of perfusion. Extrapolation of 14C incorporation into glutamine to maximal rates of hepatic glutamine synthesis yielded an about 100% utilization of the [14C] oxoglutarate taken up by the liver for glutamine synthesis. This was again true for both the antegrade and the retrograde perfusion directions. On the other hand, addition of ammonia did not affect 14CO2 production from labeled oxoglutarate, when glutamine synthetase was inhibited by methionine sulfoximine. 4. The data suggest that vascular oxoglutarate is almost exclusively taken up by the small perivenous hepatocyte population containing glutamine synthetase, i.e. a cell population comprising only 6-7% of all hepatocytes. Thus, the findings demonstrate the existence of a, to date, uniquely zonally distributed oxoglutarate transport system which is probably Na+-dependent in the plasma membrane. Accordingly, our findings extend the concept of functional hepatocyte heterogeneity, as was most impressively shown for hepatic nitrogen metabolism to plasma membrane transport systems. The data suggest that in addition to glutamate, oxoglutarate delivered via the portal vein is also a source for the carbon skeleton required for glutamine synthesis in perivenous hepatocytes, thereby underlining the metabolic specialization of this small cell population.