Contractile dysfunction and altered metabolic profile of the aging rat thyroarytenoid muscle.

The larynx and its muscles are important for ventilation, coughing, sneezing, swallowing, Valsalva's maneuver, and phonation. Because of their functional demands, the intrinsic laryngeal muscles have a unique phenotype: very small and fast fibers with high mitochondrial content. How aging affects their function is largely unknown. In this study, we tested the hypothesis that an intrinsic laryngeal muscle (thyroarytenoid muscle, a vocal fold adductor) would become weaker, slower, and fatigable with age. Muscles from Fischer 344 x Brown Norway F1 hybrid rats (6, 18, and 30 mo of age) were used for in vitro contractile function and histology. Thyroarytenoid muscles generated significantly lower twitch and tetanic forces at 30 mo vs. 6 and 18 mo. Maximal shortening velocity decreased by 20% at 30 mo (vs. 6 mo), and velocity of unloaded shortening was slower at 18 and 30 mo by 19 and 27% vs. 6 mo. There was no histochemical evidence of altered myosin ATPase activity at 18 or 30 mo of age. Fatigue resistance was significantly decreased at 18 and 30 mo. We also found abundant mitochondrial clusters and ragged red fibers in the muscles of 30-mo-old rats, and there was an age-related increase in glycogen-positive fibers. We conclude that rat thyroarytenoid muscles become weaker, slower, and more fatigable with age. These functional changes are not due to alterations in myosin ATPase activity, but a switch in the expression of myosin isoforms remains a possibility. Finally, the alterations in mitochondrial and glycogen content indicate a shift in the metabolic characteristics of these muscles with age.