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2-agonist administration increases sarcoplasmic reticulum Ca2+-ATPase activity in aged rat skeletal muscle
1 Department of Physiology, The University of Melbourne, Melbourne, Victoria, Australia
* To whom correspondence should be addressed. E-mail: gsl{at}unimelb.edu.au.
Aging is associated with a slowing of skeletal muscle contractile properties, including a
decreased rate of relaxation. In rats, the age-related decrease in the maximal rate of relaxation is
reversed after 4 weeks' administration with the 
2-adrenoceptor agonist (2-agonist), fenoterol.
Given the critical role of the sarcoplasmic reticulum (SR) in regulating intracellular Ca2+
transients and ultimately the time course of muscle contraction and relaxation, we tested the
hypothesis that the mechanisms of action of fenoterol are mediated by alterations in SR proteins.
Sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) kinetic properties were assessed in muscle
homogenates and enriched SR membranes isolated from the red (RG) and white (WG) portions
of the gastrocnemius muscle in adult (16 month) and aged (28 month) F344 rats that had been
administered fenoterol for 4 weeks (1.4 mg/kg/day; i.p. in saline) or vehicle only. Aging was
associated with a 29% decrease in the maximal activity (Vmax) of SERCA in the RG, but not in
the WG muscles. Fenoterol treatment increased the Vmax of SERCA and SERCA1 protein levels
the SERCA2a isoform. Our findings demonstrate that the mechanisms underlying age-related
changes in contractile properties are fiber type dependent, whereas the effects of fenoterol
administration are independent of age and fiber type.
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