Quantitative histochemical assessment of regional differences in hepatic glucose uptake and release

Summary

As a further step in the investigation of the heterogeneity of liver cells in general and regionality of glucose metabolism in particular, requirements for isolation of appropriate tissue samples were defined and procedures for measurement of the biochemical parameters responsible for glucose uptake and release developed and tested. By using enzymatic cycling for chemical amplification, in conjunction with the oil-well technique, sufficient analytical sensitivity was provided to assay samples averaging 20 ng dry weight. Microchemical data on the distribution of glucokinase and glucose-6-phosphatase and of their substrates, glucose and glucose-6-P, were used to, first calculate in vivo rates of these catalytic steps by means of the Michaelis-Menten equation, and then, to determine the direction and rate of net glucose flux, as well as, the rate of substrate cycling between glucose and glucose-6-P.

Calculations from the results indicated a reciprocal distribution of in vivo glucokinase and glucose-6-phosphatase velocities, as well as, sex-specific differences. The distribution of in vivo activities results in a spatial separation of these antagonistic steps. Separation is incomplete, but nevertheless appears to lead to regionally different rates in futile substrate cycling. Glucose gradients permit differentiation between net glucose uptake and release and were, therefore, used as a test of the validity of the calculations of in vivo activities. The observed discrepancies between glucose gradients and calculated in vivo enxyme activities illustrate the power of this approach: it provides a way to compare changes in glucose along the sinusoid with what would be predicted from the levels of enzymes which liberate and tie up glucose and of their respective substrates.