Disposition of a glucose load in fed rats and rats adapted to a high-carbohydrate diet

Role of the rat liver in the disposal of a glucose gavage

An oral gavage of either 3, 1, or 0.1 mmoles of glucose was given to rats under standard feeding conditions or food deprived for 24 hr. The blood flow of the portal and suprahepatic veins as well as the hepatic balances for glucose, lactate, alanine and pyruvate were estimated. In fed rats, after the administration of an oral 3 mmoles load, the liver actually released 310 mumoles of glucose and 90 of lactate, amounts that could be accounted for by the uptake of alanine (148 mumoles) and small loss of glycogen (275 mumoles of glycosyl residues). In starved rats, however, the liver took a very high proportion (c. 71%) of the glucose absorbed, both as glucose (780 mumoles), lactate and pyruvate (892 mumoles) or alanine (134 mumoles). The synthesis of glycogen was considerably limited, accounting for only 205 mumoles, and leaving practically one mmol of glucose equivalent energy available for liver function and the synthesis of other compounds. Practically all glycogen was synthesized directly from glucose, since the synthesis from 3 C carriers was less than a 5%. Smaller gavages (1 or 0.1 mmoles) resulted in a much lower liver uptake activity. The strikingly different activity of the liver with respect to the available glucose and 3 C fragments could not be explained alone by the circulating levels of these compounds, suggesting a very deep influence of the intestine in hepatic function. The liver plays a very passive role in fed animals, with a very small involvement in the disposal of a glucose load, whereas it takes on an important role when the overall availability of energy is diminished.

Rat splanchnic net oxygen consumption, energy implications

1. The blood flow, PO2, pH and PCO2 have been estimated in portal and suprahepatic veins as well as in hepatic artery of fed and overnight starved rats given an oral glucose load. From these data the net intestinal, hepatic and splanchnic balances for oxygen and bicarbonate were calculated. The oxygen consumption of the intact animal has also been measured under comparable conditions. 2. The direct utilization of oxygen balances as energy equivalents when establishing the contribution of energy metabolism of liver and intestine to the overall energy expenses of the rat, has been found to be incorrect, since it incorporates the intrinsic error of interorgan proton transfer through bicarbonate. Liver and intestine produced high net bicarbonate balances in all situations tested, implying the elimination (by means of oxidative pathways, i.e. consuming additional oxygen) of high amounts of H+ generated with bicarbonate. The equivalence in energy output of the oxygen balances was then corrected for bicarbonate production to 11-54% lower values. 3. Intestine and liver consume a high proportion of available oxygen, about one-half in basal (fed or starved) conditions and about one-third after gavage, the intestine consumption being about 15% in all situations tested and the liver decreasing its oxygen consumption with gavage.

Simultaneous synthesis and degradation of rat liver glycogen. An in vivo nuclear magnetic resonance spectroscopic study

Using 13C nuclear magnetic resonance spectroscopic methods we examined in vivo the synthesis of liver glycogen during the infusion of D-[1-13C]glucose and the turnover of labeled glycogen during subsequent infusion of D-[1-13C]glucose. In fasted rats the processes of glycogen synthesis and degradation were observed to occur simultaneously with the rate of synthesis much greater than degradation leading to net glycogen synthesis. In fed rats, incorporation of infused D-[1-13C]glucose occurred briskly; however, over 2 h there was no net glycogen accumulated. Degradation of labeled glycogen was greater in the fed versus the fasted rats (P less than 0.001), and the lack of net glycogen synthesis in fed rats was due to degradation and synthesis occurring at similar rates throughout the infusion period. There was no indication that suppression of phosphorylase a or subsequent activation of glycogen synthase was involved in modulation of the flux of tracer into liver glycogen. We conclude that in both fed and fasted rats, glycogen synthase and phosphorylase are active simultaneously and the levels of liver glycogen reached during refeeding are determined by the balance between ongoing synthetic and degradative processes.