Fatty acid cycling in the fasting rat

Abstract

Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands.

Extending food deprivation reverses the short-term lipolytic response to fasting: role of the triacylglycerol/fatty acid cycle

“…Variation in both primary and secondary cycling is responsible for causing these changes, because fasting does not cause any significant shift in their relative contribution to total reesterification (Fig.5). Under all conditions, rabbits support higher fluxes of secondary than primary cycling (Fig.5) (Reidy and Weber, 2002), as previously reported for humans ) but opposite to the case for rats (Kalderon et al, 2000;McClelland et al, 2001). It is unclear whether body size (or mass-specific metabolic rate) plays a role in setting these interspecific differences in the relative importance of primary vs secondary cycling.…”

“…Calculation of primary and secondary TAG/FA cycling using these equations has been widely used in human studies (Bahr et al, 1990;Elia et al, 1987;Romijn et al, 1993;Vallerand et al, 1999;Wolfe et al, 1987a;Wolfe et al, 1990) and in comparative physiology (Kalderon et al, 2000;McClelland et al, 2001;Reidy and Weber, 2002;Weber et al, 1993). The equations were derived from the fact that glycerol and fatty acids released by lipolysis do not behave similarly.…”

Role of energy charge and AMP-activated protein kinase in adipocytes in the control of body fat stores

“…In this respect, the effects of several typical treatments such as administration of leptin, [57][58][59][60][61] bezafibrate, 62,63 adrenoreceptor agonists, 48,[64][65][66][67][68] dietary n-3 polyunsaturated FA [69][70][71] or fasting 66,[72][73][74] can be compared with a phenotype of the aP2-Ucp1 transgenic mice. 20,21,26 In all of these situations, fat accumulation is reduced and disturbances related to the metabolic syndrome are improved.…”

Lipid mobilization of long-distance migrant birdsin vivo: the high lipolytic rate of ruff sandpipers is not stimulated during shivering

“…mitochondria. Mobilization of fat stores, or lipolysis, has been measured as the rate of appearance of glycerol (R a glycerol) in many species including humans (Beylot et al, 1987), other mammals (Himms-Hagen, 1968;Kalderon et al, 2000;McClelland et al, 2001;Shaw et al, 1975;Weber et al, 1993), rainbow trout (Bernard et al, 1999), and one bird: the king penguin (Bernard et al, 2002a;Bernard et al, 2003). R a glycerol can only be measured by continuous infusion of labelled glycerol, and it is therefore not surprising that avian lipolytic rate is only known for large (>13·kg), easy-going penguins.…”