Decrease in force potentiation and appearance of alpha-adrenergic mediated contracture in aging rat skeletal muscle.

The effect of increasing age on contractile performance and catecholamine receptor activity was investigated in a distal, predominantly fast twitch oxidative glycolytic (FOG) muscle from the plantar surface of the rat hindfoot. The ability of the flexor digitorum brevis (FDB), isolated from anesthetized rats and maintained in vitro, to undergo post-tetanic potentiation and a staircase response declined with age. Potentiation following repetitive stimulation was reduced by 50% in 2 year old rats and eliminated in 3 year old animals. The rate of muscle fatigue during intermittent tetanic stimulation also increased in aging muscles. FDB, regardless of age, did not develop a positive inotropic response to 10(-6) M epinephrine applied in vitro, but 3 year old FDB generated a prolonged contracture. Contracture tension was approximately 25% of twitch tension and was maintained for 2-10 min in the continued presence of catecholamine. Contractures were eliminated by pretreatment with alpha-adrenergic antagonists or by removing Ca2+ from the bathing medium. In addition to decreased contractile capacity, aging muscles acquire a population of alpha-adrenergic receptors which may underlie some of the metabolic and structural changes associated with increasing age.