Fasting decreases free fatty acid turnover in mice overexpressing skeletal muscle lipoprotein lipase

Skeletal muscle lipoprotein lipase (LPL) overexpression in mice results in whole-body insulin resistance and increased intramuscular triglyceride stores, but decreased plasma triglyceride concentration and unchanged plasma free fatty acid (FFA) concentration. The effects of skeletal muscle LPL overexpression and fasting duration on FFA kinetics are unknown. Transgenic mice with muscle-specific LPL overexpression (MCKhLPL) and control mice (Con) were studied at rest during a 50-minute constant infusion of [9,10- 3H]palmitate to determine FFA kinetics after both 4 and 16 hours of fasting. FFA concentration was not different between groups after the 4-hour (Con, 0.80 +/- 0.06 mmol/L; MCKhLPL, 0.83 +/- 0.07 mmol/L) and 16-hour (Con, 0.83 +/- 0.04 mmol/L; MCKhLPL, 0.80 +/- 0.07 mmol/L) fast. FFA turnover (Ra) was not significantly different between MCKhLPL and Con groups after the 4-hour fast (Con Ra = 2.52 +/- 0.36 micromol/min; MCKhLPL Ra = 2.37 +/- 0.27 micromol/min). However, FFA turnover was significantly decreased after the 16-hour fast in MCKhLPL mice vs controls (Con Ra = 2.89 +/- 0.52 micromol/min; MCKhLPL Ra = 1.64 +/- 0.17 micromol/min; P < .05). The significantly lower FFA Ra in MCKhLPL vs control mice was due to a decrease in MCKhLPL FFA turnover from the 4- to 16-hour fast, whereas FFA turnover was unchanged in controls. The changes in FFA appearance after the 16-hour fast in MCKhLPL mice are most likely explained by increased reliance by skeletal muscle on plasma triglyceride as a fuel. These data suggest increased skeletal muscle LPL expression decreases dependence on plasma FFA during prolonged fasting in mice.

Effect of impaired glucose uptake on postexercise glycogen repletion in skeletal muscles of insulin-treated streptozotocin-diabetic fasted rats

During recovery from intense exercise performed while fasting, the replenishment of muscle glycogen stores from glucose requires the activation of glucose transport. This study examines if insulin-treated streptozotocin (STZ) diabetes in rats impairs the rate of muscle glucose utilization and glycogen repletion when no food is ingested during recovery from high-intensity exercise. Rats fasted for 24 hours were injected with high doses of STZ (150 mg/kg) to cause severe diabetes, and their glycemia was normalized for 10 days with twice-daily insulin injections. High-intensity exercise in these rats resulted in a marked increase in plasma glucose, which remained higher than preexercise levels thereafter, whereas in control animals, the rise in glycemia was only of a short duration. During recovery, the rates of 2-deoxy-[(3)H]glucose utilization in muscles rich in fast twitch red fibers (red and mixed gastrocnemius muscles) were much lower in STZ-diabetic than in control rats, but were not affected by diabetes in muscles comprised mainly of fast twitch white fibers (white gastrocnemius muscle). Despite these effects on glucose utilization, STZ diabetes had no inhibitory effect on the rate and extent of glycogen deposition and fractional velocities of glycogen synthase across all muscles. In conclusion, although insulin-treated STZ diabetes in fasted rats inhibits glucose transport rates in fast twitch red muscle fibers post-intense exercise, this has no effect on muscle glycogen repletion either because glucose transport does not control the rate of glycogen synthesis or because of a compensatory increase in the activity of lactate glyconeogenesis in these muscles.

Increased expression of the SNARE accessory protein Munc18c in lipid-mediated insulin resistance

Fatty acids inhibit insulin-mediated glucose metabolism in skeletal muscle, an effect largely attributed to defects in insulin-mediated glucose transport. Insulin-resistant mice transgenic for the overexpression of lipoprotein lipase (LPL) in skeletal muscle were used to examine the molecular mechanism(s) in more detail. Using DNA gene chip array technology, and confirmation by RT-PCR and Western analysis, increases in the yeast Sec1p homolog Munc18c mRNA and protein were found in the gastrocnemius muscle of transgenic mice, but not other tissues. Munc18c has been previously demonstrated to impair insulin-mediated glucose transport in mammalian cells in vitro. Of interest, stably transfected C2C12 cells overexpressing LPL not only demonstrated increases in Munc18c mRNA and protein but also in transcription rates of the Munc18c gene. To confirm the relevance of fatty acid metabolism and insulin resistance to the expression of Munc18c in vivo, a 2-fold increase in Munc18c protein was demonstrated in mice fed a high-fat diet for 4 weeks. Together, these data are the first to implicate in vivo increases in Munc18c as a potential contributing mechanism to fatty acid-induced insulin resistance.