Regulation of hepatic inorganic phosphate and ATP in response to fructose loading: an in vivo 31P-NMR study

Abstract

Fructose loading results in hepatic accumulation of fructose 1-phosphate (Fru1 P). The goals of the present experiments were: first, to distinguish between ATP, intracellular inorganic phosphate (P i), and extracellular P i as sources of phosphate for the phosphorylation of fructose, and second, to examine the influence of ATP and Fru1 P on movement of phosphate into and out of these three pools. To achieve these goals, 31P-NMR was used to monitor the response of hepatic ATP, P i and Fru1 P to two consecutive injections of fructose. The first was administered with ATP at the control level, and the second, 1 h after the first, with ATP at 65% of the control level. Changes in intra- and extracellular P i were distinguished by correlating measurements of total NMR-detectable phosphorus and NMR-detectable P i with measurements of plasma P i. The initial fructose injection resulted in rapid accumulation of Fru1 P, small decreases in plasma and NMR-detectable P i and a dramatic decrease in ATP. Total NMR-detectable phosphorus did not change, suggesting that phosphate did not enter or leave the liver. Therefore, accumulation of Fru1 P was initially balanced by an equivalent decrease in ATP, without large changes in P i. Following the second injection, when ATP was at 65% of control, Fru1 P accumulated at approximately the same rate and to the same level as achieved following the first injection. There was little further change in ATP and a marked decrease in NMR-detectable P i, while plasma P i was higher than after the first injection. Therefore the greater decrease in NMR-detectable P i following the second injection represented a significant decrease in intracellular P i. Return of Fru1 P to control coincided with a dramatic increase in plasma P i, and a decrease in total NMR-detectable phosphate. This suggests that phosphate released from Fru1 P entered the extracellular space. These data suggest the mechanisms by which intracellular P i is regulated. When sufficient ATP is available, ATP hydrolysis supplies phosphate for the synthesis of Fru1 P, and prevents a significant decrease in intracellular P i. When ATP is reduced, accumulation of Fru1 P depletes intracellular P i. Therefore, decreased availability of ATP correlates with increased utilization of intracellular P i. When Fru1 P returns to control, the increase in intracellular P i is limited by release of P i into the plasma.