Replenishment and Depletion of Citric Acid Cycle Intermediates in Skeletal Muscle.

Abstract

The effects of various substrates on the concentrations of free amino acids, citric acid cycle intermediates and acylcarnitines were studied in perfused hindquarter of rat in presence of glucose and insulin in order to assess regulatory mechanisms of the level of citric acid cycle intermediates in skeletal muscle.1. Acetate and acetoacetate effected a significant increase in the level of citrate cycle intermediates and accumulation of acetylcarnitine. These changes were accompanied by a reduction in the level of alanine. The concentration of AMP was significantly elevated.2. Muscle mitochondria fixed 14C02 in the presence of pyruvate. The products were identified as malate or citrate when whole and disintegrated mitochondria were used respectively. The fixation was greatly stimulated by acetylcarnitine.3 . Acetylcarnitine inhibited the production of pyruvate from malate by muscle mitochondria. 4. Perfusion with 2-oxoisocaproate and 2-oxoisovalerate promoted increases in the level of citric cycle intermediates, a drop in both alanine and glutamate, and accumulation of branched-chain acylcarnitines. 2-Oxoisocaproate also caused a reduction of alanine released from the muscle.5. Perfusion with leucine and valine did not change the concentration of citric acid cycle intermediates, but elevated glutamate and still more the concentration of alanine.6. It is concluded that citric cycle intermediate level in the perfused resting muscle is modified by a) conditions which change the concentration of acetyl-CoA and thereby modify the rate of b) conditions which change the concentration of pyruvate cause changes in alanine and cycle c) conditions which change the concentrations of 2-oxoacids which are converted to cycle interpyruvate carboxylation and decarboxylation of malate via malic enzyme intermediates in the same direction via transamination reactions mediates via oxidation.It has been repeatedly shown that starvation, diabetes or perfusion of rat hearts with long-chain fatty acids, acetate or ketone bodies of rat hearts effect an elevation in the level of total citrate cycle intermediates [l -71. The disposition of glucose is suppressed under these conditions [8-lo], this inEnzymes.

Ethanol-induced activation of adenine nucleotide turnover. Evidence for a role of acetate.

“…This activity has been attributed to the vasodilator properties of acetate, which may be related to its ability to increase tissue adenosine (27,29), a well-known vasodilator compound (40)(41)(42). Infusion of acetate elevates myocardial adenosine levels and augments cardiac output and coronary blood flow (27) by first increasing tissue AMP concentrations (29,43) and decreasing the ratio of ATP to ADP. By a similar mechanism, acetate may contribute to the hemodynamic effects of ethanol.…”

Maintenance of the pectoralis muscle during hibernation in the big brown bat,Eptesicus fuscus

“…The maintenance of constant levels of intermediates involves continuous flux into the cycle (Spydevold et al 1976;Lee and Davis 1979). Input of intermediates may come from the carboxylation of pyruvate and proprionate (Lee and Davis 1979) or the deamination of amino acids (Lehninger 1975).…”

“…However, mammals cannot live on fat alone. The oxidation of fats in the citric acid cycle depends on a constant input of 3-and 4-carbon intermediates (Spydevold et al 1976;Lee and Davis 1979), which can only be derived from glucose, amino acids, or glycerol (Lehninger 1975). In addition, the central nervous system requires glucose even during prolonged fasting (Owen et al 1967).…”

Dietary acetic acid reduces serum cholesterol and triacylglycerols in rats fed a cholesterol-rich diet

“…Absorbed acetic acid is taken up and then metabolised via acetyl-CoA in the tricarboxylic acid cycle in the liver and peripheral tissues (Ballard, 1972;Spydevold et al 1976;Crabtree et al 1990). Thus, acetate has been utilised as a substrate of lipogenesis in many cell studies.…”