Beta(2)-Adrenergic Stimulation Enhances Ca2+ Release And Contractile Properties Of Skeletal Muscles, And Counteracts Exercise-Induced Reductions In Na+-K+-Atpase V-Max In Trained Men

The aim of the present study was to examine the effect of (2)-adrenergic stimulation on skeletal muscle contractile properties, sarcoplasmic reticulum (SR) rates of Ca2+ release and uptake, and Na+-K+-ATPase activity before and after fatiguing exercise in trained men. The study consisted of two experiments (EXP1, n=10males, EXP2, n=20males), where (2)-adrenoceptor agonist (terbutaline) or placebo was randomly administered in double-blinded crossover designs. In EXP1, maximal voluntary isometric contraction (MVC) of m. quadriceps was measured, followed by exercise to fatigue at 120% of maximal oxygen uptake ((V)over dot(O2), max ). A muscle biopsy was taken after MVC (non-fatigue) and at time of fatigue. In EXP2, contractile properties of m. quadriceps were measured with electrical stimulations before (non-fatigue) and after two fatiguing 45s sprints. Non-fatigued MVCs were 6 +/- 3 and 6 +/- 2% higher (P<0.05) with terbutaline than placebo in EXP1 and EXP2, respectively. Furthermore, peak twitch force was 11 +/- 7% higher (P<0.01) with terbutaline than placebo at non-fatigue. After sprints, MVC declined (P<0.05) to the same levels with terbutaline as placebo, whereas peak twitch force was lower (P<0.05) and half-relaxation time was prolonged (P<0.05) with terbutaline. Rates of SR Ca2+ release and uptake at 400nm [Ca2+] were 15 +/- 5 and 14 +/- 5% (P<0.05) higher, respectively, with terbutaline than placebo at non-fatigue, but declined (P<0.05) to similar levels at time of fatigue. Na+-K+-ATPase activity was unaffected by terbutaline compared with placebo at non-fatigue, but terbutaline counteracted exercise-induced reductions in maximum rate of activity (V-max) at time of fatigue. In conclusion, increased contractile force induced by (2)-adrenergic stimulation is associated with enhanced rate of Ca2+ release in humans. While (2)-adrenergic stimulation elicits positive inotropic and lusitropic effects on non-fatigued m. quadriceps, these effects are blunted when muscles fatigue.