Sprint interval and traditional endurance training increase net intramuscular triglyceride breakdown and expression of perilipin 2 and 5

Abstract

Intramuscular triglyceride (IMTG) utilization is enhanced by endurance training (ET) and is linked to improved insulin sensitivity. This study first investigated the hypothesis that ET-induced increases in net IMTG breakdown and insulin sensitivity are related to increased expression of perilipin 2 (PLIN2) and perilipin 5 (PLIN5). Second, we hypothesized that sprint interval training (SIT) also promotes increases in IMTG utilization and insulin sensitivity. Sixteen sedentary males performed 6 weeks of either SIT (4-6, 30 s Wingate tests per session, 3 days week(-1)) or ET (40-60 min moderate-intensity cycling, 5 days week(-1)). Training increased resting IMTG content (SIT 1.7-fold, ET 2.4-fold; P < 0.05), concomitant with parallel increases in PLIN2 (SIT 2.3-fold, ET 2.8-fold; P < 0.01) and PLIN5 expression (SIT 2.2-fold, ET 3.1-fold; P < 0.01). Pre-training, 60 min cycling at ∼65% pre-training decreased IMTG content in type I fibres (SIT 17 ± 10%, ET 15 ± 12%; P < 0.05). Following training, a significantly greater breakdown of IMTG in type I fibres occurred during exercise (SIT 27 ± 13%, ET 43 ± 6%; P < 0.05), with preferential breakdown of PLIN2- and particularly PLIN5-associated lipid droplets. Training increased the Matsuda insulin sensitivity index (SIT 56 ± 15%, ET 29 ± 12%; main effect P < 0.05). No training × group interactions were observed for any variables. In conclusion, SIT and ET both increase net IMTG breakdown during exercise and increase in PLIN2 and PLIN5 protein expression. The data are consistent with the hypothesis that increases in PLIN2 and PLIN5 are related to the mechanisms that promote increased IMTG utilization during exercise and improve insulin sensitivity following 6 weeks of SIT and ET.

Figures

Figure 4. Analysis of the colocalization of perilipin 2 (PLIN2) and perilipin 5 (PLIN5) with lipid droplets (LDs) pre- and post-60 min steady state cycling, before and after 6 weeks of sprint interval training (SIT) or endurance training (ET)

Representative immunofluorescence images of PLIN5 (A), IMTG (B), and the merged images (C) with the yellow areas describing regions of overlap between PLIN5 and IMTG images. The yellow objects were then extracted (to measure the number of PLIN5-LDs; D), and the number subtracted from the total number of LDs to quantify the number of PLIN5-null-LD. The same procedure was used to obtain values of colocalization between PLIN2 and IMTG. White bar, 5 μm. The number of PLIN2-LD (filled bars) and PLIN2-null-LD (open bars) were quantified before (PreEx) and after exercise (PostEx), prior to (E) and following (F) training in both the SIT and ET groups. The same analysis was repeated for PLIN5 for pre- (G) and post-training (H). *Main effect of exercise bout (P < 0.05 vs. pre-exercise).

Figure 1. Effect of 6 weeks of sprint interval training (SIT) and endurance training (ET) on fibre type-specific intramuscular triglyceride (IMTG) content and net IMTG breakdown during exercise

Representative immunofluorescence images of IMTG (stained red) in combination with WGA to identify the cell border (stained blue) in skeletal muscle, pre- (A) and post- (B) 6 weeks of SIT. Type I fibres are indicated with a ‘I’, all other fibres are assumed type II fibres. White bar, 50 μm. C and D, the ORO signal obtained after the intensity threshold was applied during quantitation (showing the LDs in white), before and after SIT, respectively. IMTG content, quantified from immunofluorescence images of oil red O-stained muscle sections, before (Pre-training) and after (Post-training) 6 weeks of SIT (filled bars) or ET (open bars). IMTG content was measured before (PreEx) and after (PostEx) 60 min steady state cycling, and net IMTG breakdown calculated (Breakdown) in type I (E) and type II fibres (F). Values are presented as means ± SEM (n= 8 per group). *Main effect of training intervention (P < 0.05 vs. pre-training).

Figure 2. Effect of 6 weeks of sprint interval training (SIT) and endurance training (ET) on fibre type-specific perilipin 2 (PLIN2) expression

Representative immunofluorescence images of PLIN2 (stained red) in combination with WGA to identify the cell border (stained blue) in skeletal muscle, pre- (A) and post- (B) 6 weeks of SIT. Type I fibres are indicated with a ‘I’, all other fibres are assumed type II fibres. White bar, 50 μm. PLIN2 expression, quantified from immunofluorescence images of PLIN2, in type I and type II fibres obtained before (C) and after (D) 6 weeks of SIT (filled bars) or ET (open bars). PLIN2 expression quantified from immunofluorescence images correlated with PLIN2 expression determined following immunoblotting of whole muscle homogenates (E). Values are presented as means ± SEM (n= 8 per group). *Main effect of fibre type (P < 0.05 vs. type I fibres). †Main effect of training intervention (P < 0.05 vs. pre-training).

Figure 3. Effect of 6 weeks of sprint interval training (SIT) and endurance training (ET) on fibre type-specific perilipin 5 (PLIN5) expression

Representative immunofluorescence images of PLIN5 (stained green) in combination with WGA to identify the cell border (stained blue) in skeletal muscle, pre- (A) and post- (B) 6 weeks of SIT. Type I fibres are indicated with a ‘I’, all other fibres are assumed type II fibres. White bar, 50 μm. PLIN5 expression, quantified from immunofluorescence images of PLIN5, in type I and type II fibres obtained before (C) and after (D) 6 weeks of SIT (filled bars) or ET (open bars). PLIN5 expression quantified from immunofluorescence images correlated with PLIN5 expression determined following immunoblotting of whole muscle homogenates (E). Values are presented as means ± SEM (n= 8 per group). *Main effect of fibre type (P < 0.05 vs. type I fibres). †Main effect of training intervention (P < 0.05 vs. pre-training).