Changes in muscle lipoprotein lipase activity during exercise in dogs fed on a mixed fat-rich meal

Reply by Zafeiridis and Mougios

We thank Drs Burns and Stensel for their interest in our work. We agree that the published articles on the delayed effect of resistance exercise (RE) on postprandial lipaemia (PL) provide controversial results. Three studies1–3 employed comparable methodologies in terms of exercise protocol and feeding plan of the subjects, that is, two to four sets of eight to eleven exercises at relatively similar intensities (about 10–12 repetitions maximum (RM)) with 1·5–2·0 min of rest between sets and a standardised meal on the night prior to the fat tolerance test. These studies reported a decrease1,3 or no change2 in the postprandial lipaemic response. A fourth study4 employed a similar RE protocol but focused on maintaining the subjects in a state of energy balance by increasing food intake up to two-fold after RE v. control on the night prior to the fat tolerance test. This study found no significant effect of RE on PL.

Physical inactivity amplifies the sensitivity of skeletal muscle to the lipid-induced downregulation of lipoprotein lipase activity

Physical inactivity is a risk factor for lipoprotein disorders and the metabolic syndrome. Physical inactivity has a powerful effect on suppressing lipoprotein lipase (LPL) activity in skeletal muscle, the rate-limiting enzyme for hydrolysis of triglyceride (TG)-rich lipoproteins. We tested the ability of several compounds to prevent the decrease in LPL. The present study minimized standing and ordinary light nonexercise movements in rats to compare the effects of inactivity and nonexercise activity thermogenesis (NEAT) on LPL activity. The key new insight was that the typically quick decrease in LPL activity of oxidative muscle caused by physical inactivity was prevented by nicotinic acid (NA), whereas inhibitors of TNF-alpha, inducible nitric oxide synthase, and NF-kappaB had no such effect. NA was administered at a dose known to acutely impede the appearance of plasma TG from the liver and free fatty acids from adipose tissue, and it was effective at intentionally lowering plasma lipid concentrations to the same level in active and inactive groups. As measured from heparin-releasable LPL activity, LPL in the microvasculature of the most oxidative muscles was approximately 90% lower in the inactive group compared with controls, and this suppression was completely blocked by NA. In contrast to inactivity, NA did not raise muscle LPL in ambulatory controls, whereas a large exogenous fat delivery did decrease LPL activity. In vitro control studies revealed that NA did not have a direct effect on skeletal muscle LPL activity. In conclusion, physical inactivity amplifies the ability of plasma lipids to suppress muscle LPL activity. The light ambulatory contractions responsible for NEAT are sufficient for mitigating these deleterious effects.

Effects of exercise training and feeding on lipoprotein lipase gene expression in adipose tissue, heart, and skeletal muscle of the rat

Lipoprotein lipase (LPL) is found in adipose tissue and muscle, and is important for the uptake of triglyceride-rich lipoproteins from plasma. This study examined the regulation of LPL in adipose tissue and muscle by exercise training in combination with the fed or fasted state. After training male rats on a treadmill for 6 weeks, LPL activity, mass, and mRNA levels were measured in adipose tissue, heart, soleus, and extensor digitorum longus (EDL) muscles and compared with levels in sedentary rats. Tissue LPL was measured as the heparin-released (HR) and cellular-extracted (EXT) fractions 16 hours following the last bout of exercise, during which time some animals were fasted and others were allowed free access to food. Training led to an increase in HR LPL activity and LPL protein mass in soleus and EDL, but had no effort on adipose tissue and heart LPL. The increase in soleus LPL with exercise was in the HR fraction only, whereas the increase in EDL LPL with training was in both the HR and EXT fractions. All these changes in LPL activity were accompanied by similar changes in LPL immunoreactive mass. However, there were no changes in LPL mRNA levels with training. Feeding induced a large increase in adipose tissue LPL activity and mass in both the HR and EXT fractions: however, there was no change in mRNA levels. In heart, feeding yielded a decrease in HR but no consistent change in EXT activity or mass, and a consistent decrease in mRNA levels. As compared with control rats, trained rats demonstrated different responses to feeding in all tissues, especially in soleus and EDL. Whereas feeding had no effect on LPL in soleus and EDL of control rats, feeding induced a decrease in HR and EXT LPL in the soleus of trained rats. In addition, feeding yielded a significant decrease in EXT LPL of the EDL of trained rats. Thus, these data demonstrate that adipose tissue and heart LPL are highly regulated by feeding and are not responsive to long-term exercise training. On the other hand, skeletal muscle LPL is increased in trained rats, but this increase is blunted considerably by feeding following the last bout of exercise. These changes in LPL activity and mass are mostly unaccompanied by changes in LPL mRNA levels, demonstrating that much physiologic regulation occurs posttranscriptionally.

Acute effects of exercise on triacylglycerol metabolism in fasted, untrained rats treated with nadolol

The current study investigated the early response of some determinants of triacylglycerol (TG) metabolism to acute exercise in rats that were treated or not with the nonselective beta-adrenergic blocker, nadolol (25 mg.kg b.wt.-1.day-1 for 30 days). Measurements of hepatic TG secretion rate (HTGSR), postheparin plasma hepatic TG lipase (HTGL) activity, and that of lipoprotein lipase (LPL) in postheparin plasma, heart, vastus lateralis muscle (VLM), white (WAT) and brown (BAT) adipose tissues were carried out in untrained rats at rest or immediately after a 1 h run on treadmill (22 m.min-1, 0 degrees grade). All animals were in the fasted state. Exercise reduced serum TG levels (46% below resting levels) and doubled those of nonesterified fatty acids. HTGSR was enhanced (+28%) by exercise while HTGL activity was not modified. The decrease in postheparin plasma LPL activity (-24%) caused by exercise was consistent with a reduction in enzyme activity in WAT (-32%), BAT (-45%) and heart (-25%). One h of treadmill running did not influence LPL activity in VLM. beta-Adrenergic blockade did not affect any of the variables of lipid metabolism, except for a slight decrease in specific activity of LPL in the heart (-14%). This study demonstrates that absolute variations in HTGSR and in LPL activity cannot account for the acute fall in serum TG levels caused by moderate exercise, in fasted, untrained rats. In addition, these results show that the beta-adrenergic pathway is either not involved in, or not necessary for, the latter effects of exercise on TG metabolism.

Metabolic and hormonal responses to prolonged physical exercise in dogs after a single fat-enriched meal

Metabolic and hormonal responses to prolonged treadmill exercise in dogs fed a fat-enriched meal 4 h prior to the exercise were compared to those measured 4 h after a mixed meal or in the postabsorptive state. Ingestion of the fat-enriched meal caused significant elevations in the resting values of plasma triglyceride (TG), free fatty acid (FFA), and glycerol concentrations. A reduction of the plasma TG concentration (from 1.6 +/- 0.2 to 1.1 +/- 0.10 mmol X l-1, P less than 0.005) occurred only in dogs exercising after the fat-enriched meal. No significant changes in this variable were noted in dogs fed a mixed meal, whilst in the postabsorptive state exercise caused an increase in the plasma TG level (from 0.42 +/- 0.03 to 0.99 +/- 0.11 mmol X l-1, P less than 0.01). The exercise-induced elevations in plasma FFA and glycerol concentrations were the highest in the dogs given the fat-enriched meal. Plasma glycerol during exercise correlated with the initial values of circulating TG (r = 0.73). The plasma FFA-glycerol ratio, at the end of exercise was lowest in the dogs taking the fat-enriched meal (1.39 +/- 0.19), suggesting an increased utilization of FFA in comparison with that in the postabsorptive state (3.27 +/- 0.37) or after a mixed meal (2.88 +/- 0.55). Basal serum insulin (IRI) concentrations were similarly enhanced in dogs fed fat-enriched and mixed meals, and they were reduced to control values within 60 min of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)