Hepatocyte heterogeneity in response to extracellular ATP

The liver's response to extracellular ATP remains a subject of uncertainty, particularly regarding spatial differences in ATP hydrolysis and metabolic outcomes. Prior research has shown that ATP influences glucose output and calcium mobilization in liver cells. However, the specific roles of hepatocyte subpopulations and perfusion direction in these responses remain unclear. It was already known that ATP can modulate hemodynamic parameters like perfusion pressure. Yet, no prior work had resolved how ATP effects vary between periportal and perivenous regions. This gap motivated investigations into whether ATP's metabolic and hemodynamic effects differ based on the direction of liver perfusion. The uncertainty around ATP hydrolysis patterns and calcium mobilization sources also drove the need for further study. No established model yet explains how ATP concentrations influence glycogenolysis in different liver zones. These unresolved questions highlight the need for a more detailed analysis of ATP's effects across liver regions.

This study aimed to compare the effects of extracellular ATP on liver metabolism and hemodynamics during antegrade and retrograde perfusion. The specific problem addressed was the lack of clarity about how ATP concentrations influence glucose output, calcium mobilization, and perfusion pressure in different liver regions. The motivation stemmed from the need to understand spatial heterogeneity in ATP responses. The researchers sought to determine if ATP effects are localized to specific hepatocyte populations. They also aimed to assess how ATP hydrolysis and calcium mobilization vary with perfusion direction. Another objective was to evaluate the role of nucleotide pyrophosphatase in ATP hydrolysis. The study also aimed to clarify whether perivenous hepatocytes contribute differently to ATP responses. The ultimate goal was to interpret these findings in terms of hepatocyte subpopulation functions.

The researchers used perfused rat liver preparations to compare antegrade and retrograde perfusion effects. They measured ATP hydrolysis rates and metabolic outputs like glucose and ammonium ion release. Perfusion pressure and calcium mobilization were monitored using standard physiological techniques. ATP concentrations up to 100 microM were tested in both perfusion directions. UMP was introduced as an inhibitor of nucleotide pyrophosphatase to assess its impact on ATP hydrolysis. The study also evaluated 14CO2 production from [1-14C]glutamate to track metabolic activity in perivenous hepatocytes. Glycogenolysis rates were compared between antegrade and retrograde perfusions. The researchers analyzed how ATP concentrations influenced maximal responses in glucose output and perfusion pressure.

Extracellular ATP was completely hydrolyzed during a single liver passage regardless of perfusion direction. ATP-induced glucose output increased by 85-95% during antegrade perfusion but was significantly reduced in retrograde conditions. Perfusion pressure and ammonium ion release also dropped by similar margins in retrograde perfusion. Calcium mobilization was less affected, with only a 60% decrease in retrograde perfusion. UMP diminished the perfusion direction effect on glycogenolysis by inhibiting ATP hydrolysis. Maximal glucose output required 20 microM ATP during antegrade perfusion but 120 microM during retrograde perfusion. Perfusion pressure reached maximum at 100 microM ATP in antegrade but required over 200 microM in retrograde. 14CO2 production from [1-14C]glutamate increased similarly in both perfusion directions, suggesting perivenous hepatocytes were minimally affected.

The data suggest that ATP is primarily hydrolyzed by perivenous hepatocytes near the acinus outflow. Glycogenolysis to glucose appears localized to the periportal area based on ATP concentration differences. Contractile elements near sinusoidal inflow may influence perfusion pressure responses. Calcium mobilization sources differ between hepatocyte populations, with some not contributing to glucose output. ATP effects on glucose output and perfusion pressure require higher concentrations in retrograde perfusion. UMP reduced the perfusion direction effect on glycogenolysis by inhibiting ATP hydrolysis. The findings imply spatially distinct roles for hepatocyte subpopulations in ATP metabolism. These results support the hypothesis that ATP responses are heterogeneous across liver zones.