Diet-induced obesity alters skeletal muscle fiber types of male but not female mice

Abstract

Skeletal muscles are highly plastic tissues capable dramatic remodeling in response to use, disuse, disease, and other factors. Growing evidence suggests that adipose tissues exert significant effects on the basic fiber-type composition of skeletal muscles. In the current study, we investigated the long-term effects of a high-fat diet and subsequent obesity on the muscle fiber types in C57 BLK/6J mice. Litters of mice were randomly assigned to either a high-fat diet or a control group at the time of weaning, and were maintained on this diet for approximately 1 year. Single fibers were harvested from the soleus and plantaris muscles, and fiber types were determined using SDS-PAGE. The high-fat diet mice were significantly heavier than the control mice (39.17 ± 2.7 g vs. 56.87 ± 3.4 g; P < 0.0003), but muscle masses were not different. In male mice, the high-fat diet was associated with a significantly lower proportion of slow, type I fibers in the soleus muscle (40.4 ± 3.5% vs. 29.33 ± 2.6%; P < 0.0165). Moreover, the proportion of type I fibers in the soleus of male mice was inversely proportional to the relative fatness of the male mice (P < 0.003; r (2) = 0.65), but no association was observed in female mice. In male mice, the decline in type I fibers was correlated with an increase in type I/IIA hybrid fibers, suggesting that the type I fibers were transformed primarily into these hybrids. The reported trends indicate that type I fibers are most susceptible to the effects of obesity, and that these fiber-type changes can be sex specific.

Keywords: Fiber types; obesity; skeletal muscle.

Figure 1.

Anatomical location and fiber types present in mouse soleus and plantaris muscles. (A) Low magnification image and map (B) of the entire triceps surae complex: gastrocnemius (medial: MG and lateral: LG), plantaris (P), soleus (S). The great majority of fibers (>95%) of the fibers in the gastrocnemius outside the hatched line are type IIB in the mouse. (C) Higher magnification image of the soleus and plantaris to illustrate the basic fiber types present in these muscles. The soleus primarily comprised type I and IIA fibers, whereas the plantaris contains a mixture of fast fiber types. Staining intensity of different fiber types in the mouse is indicated in the key below, and is consistent with previous reports (Hamalainen and Pette ; Glaser et al. 2010). A and B are reproduced from Glaser et al. .

Figure 2.

High‐fat diet induced an increase in mouse body mass but not muscle mass. (A) Mice fed a high‐fat diet (Fat) had a significantly higher body mass than controls (Control) (56.87 ± 3.4 g vs. 39.17 ± 2.7 g; P < 0.0003). (B) Relative fatness (body mass:triceps surae mass) was also significantly greater in the mice maintained on high fat. (C) Mass of the muscles comprising the triceps surae complex was not different between the high‐fat diet and controls.

Figure 3.

SDS‐PAGE used to identify single fiber types. Single fibers were dissected from whole muscles and loaded on SDS‐PAGE gels. The adult MHC isoforms migrated in the pattern IIX < IIA < IIB < I (migration direction from top → bottom). Individual fiber types from a soleus sample are identified above by their migration patterns.

Figure 4.

Skeletal muscle fiber types identified in soleus and plantaris muscles of high‐fat diet and control mice. (A) Soleus muscles have the highest proportions of types I, IIA, and I/IIA hybrids. Minor amounts of other fiber types are also present. When data from male and female mice are combined, no differences between high‐fat diet and control mice are evident. (B) Plantaris muscles primarily contain types IIB, IIX/IIB hybrids, and IIX fibers. No differences in fiber type between the high‐fat diet and control mice were detected within the plantaris.

Figure 5.

Fat male mice have reduced numbers of type I fibers. (A) Male mice maintained on a high‐fat diet had significantly fewer type I fibers than control male mice (*t‐test, P < 0.0165). Letters indicate the results of the Tukey post‐ANOVA test, where means with the same letter is not significantly different. (B) A significant correlation was observed between relative “fatness” and the proportion of type I fibers in male mice (r² = 0.65; P < 0.003).

Figure 6.

Differences in proportions of type I fibers in male and female soleus muscles. (A) Proportions of type IIA fibers are significantly greater in male soleus (*P < 0.015), whereas the proportion of type I fibers is significantly greater in the female soleus (^§P < 0.010). No differences were detected between the proportions of I/IIA hybrids (P > 0.70). Histochemical staining confirms that female male mice (B) typically have a higher proportion of type I fibers (dark) in the soleus compared with males (C).

Figure 7.

Decline in type I fibers is coincident with an increase in type I/IIA hybrids in male soleus. (A) The relative proportions of type I and I/IIA hybrids combined are not different between the high‐fat (Fat) and control diet mice (Control) (t‐test, P > 0.31). In both groups, >90% of the fibers are I, I/IIA hybrid, or IIA. (B) The relative proportions of type I and I/IIA hybrid fibers are inversely correlated with one another (r² = 0.059, P < 0.006).

Figure 8.

Relative proportions of types I and IIA MHC isoforms within I/IIA hybrids from the male soleus. (A) The proportions of isoforms conformed to a pattern similar to a normal distribution, meaning that the majority of hybrids contained roughly equal proportions of the two MHC isoforms (the curve is a normal distribution fit to the data). (B) Examples of SDS‐PAGE identification of I/IIA hybrids arranged as a continuum from hybrids with large proportions of type IIA MHC (left), to those containing mostly type I (right).