Biochemistry of the anterior, medial, and posterior genioglossus in the aged rat

Abstract

Age-related tongue weakness may contribute to swallowing deficits in the elderly. One contributing factor may be an alteration in muscle-fiber-type properties with aging. However, it is not clear how muscle fiber types within the aged tongue may vary from those found in young adults, or how fiber types may vary across the anteroposterior axis of the extrinsic tongue muscles. We examined the myosin heavy chain (MHC) composition of anterior, medial, and posterior sections of the genioglossus muscle (GG) in ten old male Fischer 344/Brown Norway rats and compared findings to previously reported data from young adult male rats. Significant differences (p < 0.01) between young adult and old rats were found in the distribution of MHC isoforms along the anteroposterior axis of the muscle. In the anterior, medial, and posterior regions, there was a significantly smaller proportion of type IIb MHC in the old rat GG muscles, while the proportion of type IIx MHC was significantly greater. In the medial region, the proportion of type I MHC was found to be significantly greater in the old rats. Thus, we found a shift to more slowly contracting muscle fibers in the aged rat tongue.

Figure 1

Sagittal schematic of the genioglossus (GG) muscle in the adult rat. The muscle is sectioned to illustrate anterior, medial, and posterior GG regions (courtesy of Dr. Hiromi Nagai).

Figure 2

Identification of myosin heavy chain (MHC) isoforms with Western blotting technique in the genioglossus (GG) and soleus muscle of the rat. The soleus, a muscle composed of predominantly type I MHC, is provided as a control to demonstrate accuracy of identification of type I MHC. In the first lane from the left in the panel, MHC isoforms (IIa and I) from a protein stained gel from the soleus muscle. In the second lane in the panel, MHC isoforms (IIa, IIx, IIb, I) from a protein stained gel from the GG muscle. In the next four lanes, Western blots of the four MHC isoforms in the GG muscle stained with monoclonal antibodies SC-71 for type IIa, 6H1 for type IIx, BF-F3 for type IIb, and BA-D5 for type I. Arrow indicates the small percentage of type I found in the GG muscle. In the final column of the panel, Western blot of MHC isoform type I in the soleus muscle stained with BA-D5.

Figure 3

Representative silver-stained SDS-PAGE gels from one animal. Varying myosin heavy chain isoform distribution in genioglossis (GG) muscle regions is shown. The anterior GG contained a significantly greater proportion of type IIa than the posterior (p<.01), but not the medial. No other significant differences were found.

Figure 4

The percent of type IIa, type IIx, type IIb and type I myosin heavy chain (MHC) isoforms in the genioglossus (GG) muscles from old rats. Type IIx MHC predominated in the anterior, medial, and posterior regions of the GG muscle, followed by IIa, IIb, and type I. The proportions of MHC isoforms within the anterior, medial, and posterior GG were compared using a Wilcoxon matched-pairs signed-ranks test. A significantly greater proportion of type IIa MHC was found in the anterior GG versus the posterior GG (p <.01). Error bars represent standard deviation. GGa= anterior, GGm= medial, GGp=posterior.

Figure 5

Percentage of myosin heavy chain (MHC) in young adult rats (from Volz et al., 2007) and old rats from the present study. Type IIx was found to be significantly greater in old rats, while type IIb was found to be significantly greater in young rats, across all regions (p<.01). Error bars represent standard deviations.