Effect of triiodothyronine (T3) excess on fatty acid metabolism in the soleus muscle from endurance-trained rats.

We studied whether short-term administration of triiodothyronine (T3) for the last 3 days of endurance training would influence the rate of uptake of palmitic acid (PA) as well as metabolism in rat soleus muscle, in vitro. Training per se did not affect the rate of PA uptake by the soleus; however, an excess of T3 increased the rate of this process at 1.5 mmol/L PA, as well as the rate that at which PA was incorporated into intramuscular triacylglycerols (TG). The rate of TG synthesis in trained euthyroid rats was reduced after exercise (1.5 mmol/L PA). The rate of PA oxidation in all of the trained rats immediately after exercise was enhanced by comparison with the sedentary values. Hyperthyroidism additionally increased the rate of this process at 1.5 mmol/L PA. After a recovery period, the rate of PA oxidation returned to the control values in both the euthyroid and the hyperthyroid groups. Examination of the high-energy phosphate levels indicated that elevated PA oxidation after exercise-training in euthyroid rats was associated with stable ATP levels and increased ADP and AMP levels, thus reducing energy cellular potential (ECP). In the hyperthyroid rats, levels of ADP and AMP were increased in the sedentary as well as the exercise-trained rats. ECP levels were high as a result of high levels of ATP and decreased levels of ADP and AMP in hyperthyroid rats after the recovery period. In conclusion, short-term hyperthyroidism accelerates PA utilization in well-trained soleus muscle.

Fasting increases palmitic acid incorporation into rat hind-limb intramuscular acylglycerols while short-term cold exposure has no effect.

The aim of the study was to investigate the palmitic acid incorporation into intramuscular acylglycerols in perfused hind-limb skeletal muscles of different fibre types in rats either fasted for 48 h or exposed to cold (6 °C) for 12 h. Hind-limb preparations of fasted and cold exposed rats were perfused with buffers containing tritium labelled and cold palmitic acid. Palmitic acid incorporation into intracellular lipid pools in the soleus, plantaris, red and white gastrocnemius and red and white quadriceps was measured. It was found that fasting increased approximately 2-fold palmitic acid incorporation in all muscles examined regardless of the fibre type composition of the muscle. On the other hand, exposure to cold had no effect on the palmitic acid incorporation into intramuscular acylglycerols regardless the muscle fibre type. The increased incorporation of palmitic acid into acylglycerols in fasted animals is in line with data showing that 48 h fasting stimulates the expression of plasma membrane proteins putatively facilitating fatty acid uptake. It appears that although 12 h cold exposure increases the use of fatty acids as energy substrates it does not alter the incorporation of palmitic acid into intramuscular acylglycerols in the perfused rat hind-limb.

Palmitic acid incorporation into intramuscular acylglycerols depends on both total and unbound to albumin palmitic acid concentration.

Palmitic acid incorporation into the intramuscular acylglycerols in rat skeletal muscles of different fiber types was investigated at various total and unbound to albumin concentrations by means of the hind-limb perfusion technique. It was found that at simultaneously increasing total and unbound to albumin palmitic acid concentrations in the perfusion medium the incorporation of palmitic acid into acylglycerols increased. However, when the concentration of palmitic acid not bound to albumin was kept constant and the total palmitic acid concentration was increased, the incorporation also increased although markedly less than under former conditions. The increase was most apparent in the muscles composed of slow-twitch oxidative and fast-twitch oxidative-glycolytic fibers where fatty acid uptake is the greatest. These findings suggest that fatty acid incorporation into intramuscular acylglycerols depends not only on the unbound to albumin fatty acid concentration but also, to some extent, on the total fatty acid concentration.

Palmitic acid uptake by the rat soleus muscle in vitro.

Abstract: The rate of fatty acid uptake, oxidation, and deposition in skeletal muscles in relation to total and unbound to albumin fatty acids concentration in the medium were investigated in the incubated rat soleus muscle. An immunohistochemical technique was applied to demonstrate whether the albumin-bound fatty acid complex from the medium penetrates well within all areas of the muscle strips. It was found that the percentage of incorporation of palmitic acid into intramuscular lipids was fairly constant, independently of the fatty acid concentration in the medium, and amounted to 63-72% for triacylglycerols, 7-12% for diacylglycerols-monoacylglycerols, and 19-26% for phospholipids. Both palmitic acid incorporation into the muscle triacylglycerol stores and its oxidation to CO2 closely correlated with an increase in both total and unbound to albumin fatty acid concentrations in the incubation medium. Under conditions of increased total but constant unbound to albumin palmitic acid concentrations, the incorporation of palmitic acid into triacylglycerols and its oxidation to CO2 were also increased, but to a lower extent. This supports the hypothesis that the cellular fatty acid metabolism depends not only on the availability of fatty acids unbound to albumin, but also on the availability of fatty acids complexed to albumin.

[Mechanisms for tissue transport of fatty acids]. / Mechanizmy tkankowego wychwytu kwasów tluszczowych.

Fatty acids (FA) play an important role in energy metabolism. They are not only used as substrates for energy production but are also involved in many other metabolic pathways. However, the exact mechanisms by which FA are taken up by the cells are not yet fully understood. Fatty acids circulating in blood cross the cell membranes partly via a mechanism facilitated by plasma membrane proteins and partly via a passive diffusion mechanism, the former mechanism being of major importance.