Hepatocyte heterogeneity in response to extracellular ATP

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Abstract
The metabolic and hemodynamic effects of extracellular ATP in perfused rat liver were compared during physiologically antegrade (portal to hepatic vein) and retrograde (hepatic to portal vein) perfusion. ATP in concentrations up to 100 μM was completely hydrolyzed during a single liver passage regardless of the perfusion direction. The ATP (20 μM)‐induced increases of glucose output, perfusion pressure and ammonium ion release seen during antegrade perfusions were diminished by 85–95% when the perfusion was in the retrograde direction, whereas the amount of Ca2+ mobilized from the liver was decreased by only 60%. The maximal rate of initial K+ uptake following ATP was dependent on the amount of Ca2+ mobilized regardless of the direction of perfusion. In the presence of UMP (1 mM), an inhibitor of ATP hydrolysis by membrane‐bound nucleotide pyrophosphatase, the effect of the direction of perfusion on the glycogenolytic response to ATP (20 μM) was largely diminished. For a maximal response of glucose output, Ca2+ release and perfusion pressure to extracellular ATP, concentrations of about 20 μM, 50 μM and 100 μM were required during antegrade perfusion, respectively. These maximal responses could also be obtained during retrograde perfusion, but higher ATP concentrations were required (120 μM, 80 μM, above 200 μM, respectively). 14CO2 production from [1‐14C]glutamate which occurs predominantly in the perivenous hepatocytes capable of glutamine synthesis was stimulated by extracellular ATP (20 μM); it was only slightly affected by the direction of perfusion. In antegrade perfusions, ATP (20 μM) increased 14CO2 production from 88 to 162 nmol g−1 min−1, compared to an increase from 91 to 148 nmol g−1 min−1 in retrograde perfusion. The data are interpreted to suggest that (a) extracellular ATP is predominantly hydrolyzed by a small hepatocyte population located at the perivenous outflow of the acinus; (b) glycogenolysis to glucose is predominantly localized in the periportal area; (c) contractile elements (sphincters) exist near the inflow of the sinusoidal bed; (d) a considerable portion of the Ca2+ mobilized by ATP is derived from liver cells that do not contribute to hepatic glucose output.