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J Appl Physiol 92: 941-948, 2002; doi:10.1152/japplphysiol.00576.2001
8750-7587/02 $5.00
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Vol. 92, Issue 3, 941-948, March 2002

Alteration of contractile force and mass in the senescent diaphragm with beta 2-agonist treatment

Wesley N. Smith1, Amie Dirks1, Takao Sugiura1, Susan Muller1, Phillip Scarpace2, and Scott K. Powers1,3

Departments of 1 Exercise and Sport Sciences, 2 Pharmacology, and 3 Physiology, University of Florida, Gainesville, Florida 32611

Aging is associated with a decrease in diaphragmatic maximal tetanic force production (Po) in senescent rats. Treatment with the beta 2-agonist clenbuterol (CB) has been shown to increase skeletal muscle mass and Po in weak locomotor skeletal muscles from dystrophic rodents. It is unknown whether CB can increase diaphragmatic mass and Po in senescent rats. Therefore, we tested the hypothesis that CB treatment will increase specific Po (i.e., force per cross-sectional area) and mass in the diaphragm of old rats. Young (5 mo) and old (23 mo) male Fischer 344 rats were randomly assigned to one of the following groups (n = 10/group): 1) young CB treated; 2) young control; 3) old CB treated; and 4) old control. Animals were injected daily with either CB (2 mg/kg) or saline for 28 days. CB increased (P < 0.05) the mass of the costal diaphragm in both young and old animals. CB treatment increased diaphragmatic-specific Po in old animals (~15%; P < 0.05) but did not alter (P > 0.05) diaphragmatic-specific Po in young animals. Biochemical analysis indicated that the improved maximal specific Po in the diaphragm of CB-treated old animals was not due to increased myofibrillar protein concentration. Analysis of the myosin heavy chain (MHC) content of the costal diaphragm revealed a CB-induced increase (P < 0.05) in type IIb MHC and a decrease in type I, IIa, and IIx MHC in both young and old animals. These data support the hypothesis that CB treatment can restore the age-associated decline in both diaphragmatic-specific Po and muscle mass.

sarcopenia; clenbuterol; myosin heavy chain


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