Lactate elimination and O 2 consumption of the liver in narcotized dogs

Studies in Rats of Combined Muscle and Liver Perfusion and of Muscle Extract Indicate That Contractions Release a Muscle Hormone Directly Enhancing Hepatic Glycogenolysis

Background: Established neuroendocrine signals do not sufficiently account for the exercise-induced increase in glucose production. Using an innovative, yet classical cross-circulation procedure, we studied whether contracting muscle produces a factor that directly stimulates hepatic glycogenolysis. Methods: Isolated rat hindquarters were perfused in series with isolated livers. Results: Stimulation of the sciatic nerve of one or both legs resulted in an increase in force, which rapidly waned. During one-legged contractions, hepatic glucose production increased initially (from −0.9 ± 0.5 (mean ± SE) to 3.3 ± 0.7 µmol/min, p < 0.05). The peak did not differ significantly from that seen after 20 nM of epinephrine (5.1 ± 1.2 µmol/min, p > 0.05). In response to two-legged contractions, the increase in hepatic glucose production (to 5.4 ± 1.3 µmol/min) was higher (p < 0.05) and lasted longer than that seen during one-legged contractions. During contractions, peak hepatic glucose output exceeded concomitant hepatic lactate uptake (p < 0.05), and glucose output decreased to basal levels, while lactate uptake rose to a plateau. Furthermore, in separate experiments an increase in lactate supply to isolated perfused livers increased lactate uptake, but not glucose output. In intact rats, intra-arterial injection of extract made from mixed leg muscle elicited a prolonged increase (p < 0.05) in plasma glucose concentration (from 5.2 ± 0.1 mM to 8.3 ± 1.5 mM). In perfused livers, muscle extract increased glucose output dose dependently. Fractionation by chromatography of the extract showed that the active substance had a MW below 2000. Conclusion: This study provides evidence that contracting skeletal muscle may produce a hormone with a MW below 2000, which enhances hepatic glycogenolysis according to energy needs. Further chemical characterization is warranted.

Quantitative aspects of L(+)-lactate metabolism in human beings

The production rate of endogenous L(+)-lactate in the resting human being can be calculated from results of experiments during which isotopic or unlabelled lactate is infused. The merits of the different experimental techniques are discussed, although all published methods have given similar results, namely a resting production rate of about 1.3 mol . (70 kg body wt)-1 . 24 h-1. This result is similar to that found in sheep, but lower than that in dogs and rats. L(+)-lactate production in human subjects is unchanged after fasting for 36-168 h, a finding which contrasts with some animal experiments. It is not known whether this represents a true species difference or a difference in diet. Net splanchnic uptake of L(+)-lactate in the resting human being is about 0.4 mol 24 h-1 although the normal liver has the capacity to metabolize more than this. The kidney is an important site of L(+)-lactate utilization in rats, but there is no information about its role in normal human beings. There are conflicting reports concerning the uptake of L(+)-lactate by resting skeletal muscle, but there is no evidence that resting muscle can metabolize lactate.

Energy metabolism in critically ill patients: lactate is a major oxidizable substrate

The potential role of an energy defect in acute diseases is still in the centre of the pathophysiological understanding of such states and therefore of our attempts to limit or to reverse the possible deleterious consequences of such defect. In fact several recent experimental works have shown that instead of being a negative consequence, the lactate production and the related metabolic acidosis due to the stimulation of anaerobic ATP-production pathway is rather a protective adapted response.

Effect of phenformin on hepatic balances of gluconeogenic substrates in man

The effect of a five day pretreatment with phenformin (3 X 50 mg daily) on hepatic metabolism was studied in six healthy volunteers. Arterial and hepatic venous concentrations of substrates and hepatic blood flow were estimated during a basal period and during a low-dose lactate infusion (0,03 mmol . kg-1 . min-1). The results have been compared with those obtained from untreated normal subjects in a previous study (16). During the baseline period arterial concentration of alanine and the hepatic venous concentration ratios of alanine: pyruvate and beta-hydroxybutyrate: acetoacetate were significantly increased with phenformin treatment, while the balances of carbon dioxide and glucose and the fractional extraction of alanine were decreased compared to the values obtained in untreated subjects. During lactate infusion mean arterial lactate concentration was significantly increased and hepatic lactate extraction was decreased compared to untreated persons under the same conditions. In the phenformin-treated group lactate infusion resulted in hepatic output of pyruvate and the hepatic glucose balance remained unchanged compared to baseline. Since the rate of hepatic blood flow was not increased during lactate infusion a significantly smaller glucose output and lactate uptake was obtained with phenformin. These findings support the present view that the hypoglycaemic effect of biguanides is due, at least in part, to inhibition of hepatic gluconeogenesis.

On gluconeogenesis of human liver. Accelerated hepatic glucose formation induced by increased precursor supply

In 8 subjects in whom portal vein catheters had been inserted 5-6 days previously during cholecystectomy, arterial and portal concentrations of glucose, lactate, pyruvate, glycerol, alanine, free fatty acids, beta-hydroxybutyrate and acetoacetate revealed no significant differences. This provided the basis for the calculation of hepatic balances from arterio-hepatic venous substrate-differences in 17 healthy volunteers. In eight of them metabolic balances were determined during elevated hepatic lactate supply. Kinetics of the substrates throughout the whole test period in 9 controls showed no gross interference from the catheterization or infusion procedure. The elevated hepatic lactate concentration caused a doubling of hepatic glucose output, which could almost entirely be accounted for by a fivefold increase of hepatic lactate uptake. This acceleration of hepatic gluconeogenesis was accompanied by a significant increment of hepatic free fatty acid uptake, whereas hepatic ketone body production did not change. These data seem to support the view that hepatic energy requirements caused by an accelerated gluconeogenesis might be covered from enhanced free fatty acid oxidation.

Oxygen supply and uptake in the liver and the intestine

The oxygen supply to the liver was found to be dependent on the total blood flow only and not on the ratio of arterial to portal contribution. The mean value of O2-uptake in the liver, related to a blood flow of 110 ml/min - 100 g liver, amounted to 6.08 +/- 0.2 ml O2/min - 100 g liver (mean +/- S.E.M.). O2-uptake of the intestine was found to be 1.95 +/- 0.13 ml O2/min - 100 g tissue, related to a normal blood flow of 50 ml/min - 100 g tissue. With low oxygen supply O2 extraction in the liver reaches values of 97%, whereas the intestinal extraction does not surpass 75%. A rise in oxygen supply surmounting normal values does not increase the O2-consumption. Contrary to the intestinal circulation the liver showed no postocclusive vasodilatation. The oxygen debt was payed back by a greater extraction. The portal oxygen supply to the liver can markedly increase due to intestinal metabolic hyperemia. High O2-extraction capacity, rather than vasodilatation, is the main mechanism for matching hepatic oxygen supply with requirements. The hepatic venous blood may leave the liver with an extremely low O2-content.

Effects of abdominal surgery under general anaesthesia and of postoperative analgesic therapy on splanchnic exchange of some blood borne energy metabolites

Sixteen patients with gallbladder disease, but otherwise healthy, were studied in connection with cholecystectomy. For postoperative analgesia, seven patients were given a posterior splanchnic blockade, 30 ml of 0.5% plain lidocaine, and nine patients were given fentanyl intramuscularly in a dose of 3.5 mug/kg b.w. Immediately after the operation and before analgesia, the arterial blood concentrations of glucose, lactate, glycerol, 3-hydroxybutyrate, and free fatty acids increased significantly. The splanchnic exchange of these metabolites was also significantly increased, except for glucose, the splanchnic release of which was significantly reduced. Induction of analgesia, both methods, was followed by a rapid significant decrease of the arterial blood concentration of glycerol, which tended to rise later when the analgesia disappeared. The arterial blood concentration of glucose and lactate showed a tendency to decrease slowly following both kinds of analgesia, while the arterial blood concentration of 3-hydroxybutyrate remained stable. The splanchnic release of glucose was significantly reduced after administration of fentanyl, while it increased significantly following splanchnic blockade, the splanchnic glucose release being correlated to arterial blood pH. The splanchnic release of 3-hydroxybutyrate was significantly reduced during maximal analgesia in both groups. The splanchnic uptakes of glycerol and free fatty acids were correlated to their splanchnic inflows.