title: β2-adrenergic stimulation enhances Ca2+ release and contractile properties of skeletal muscles, and counteracts exercise-induced reductions in Na+-K+-ATPase Vmax in trained men.
abstract: The aim of the present study was to examine the effect of β2-adrenergic stimulation on skeletal muscle contractile properties, sarcoplasmic reticulum (SR) rates of Ca(2+) release and uptake, and Na(+)-K(+)-ATPase activity before and after fatiguing exercise in trained men. The study consisted of two experiments (EXP1, n = 10 males, EXP2, n = 20 males), where β2-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 (V̇O2, 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 45 s 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 Ca(2+) release and uptake at 400 nm [Ca(2+)] 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 (Vmax) at time of fatigue. In conclusion, increased contractile force induced by β2-adrenergic stimulation is associated with enhanced rate of Ca(2+) release in humans. While β2-adrenergic stimulation elicits positive inotropic and lusitropic effects on non-fatigued m. quadriceps, these effects are blunted when muscles fatigue.
authors.affiliation: Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark Department of Respiratory Research, Bispebjerg University Hospital, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Hostrup
authors.affiliation: Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark Department of Respiratory Research, Bispebjerg University Hospital, Denmark.
authors.forename: A
authors.initials: A
authors.lastname: Kalsen
authors.affiliation: Department of Sports Science and Biomechanics, University of Southern Denmark, Denmark Swedish Winter Sports Research Centre, Mid Sweden University, Sweden.
authors.forename: N
authors.initials: N
authors.lastname: Ortenblad
authors.affiliation: Department of Biology, University of Copenhagen, Denmark.
authors.forename: C
authors.initials: C
authors.lastname: Juel
authors.affiliation: Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark.
authors.forename: K
authors.initials: K
authors.lastname: Mørch
authors.affiliation: Norwegian Doping Control Laboratory, Oslo University Hospital, Norway.
authors.forename: S
authors.initials: S
authors.lastname: Rzeppa
authors.affiliation: Department of Respiratory Research, Bispebjerg University Hospital, Denmark.
authors.forename: S
authors.initials: S
authors.lastname: Karlsson
authors.affiliation: Department of Respiratory Research, Bispebjerg University Hospital, Denmark.
authors.forename: V
authors.initials: V
authors.lastname: Backer
authors.affiliation: Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark jbangsbo@nexs.ku.dk.
authors.forename: J
authors.initials: J
authors.lastname: Bangsbo
mesh_terms: D058666:Adrenergic beta-2 Receptor Agonists / Q000494:pharmacology*; D000328:Adult; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects; D015444:Exercise; D006801:Humans; D008297:Male; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D010101:Oxygen Consumption; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D058666:Adrenergic beta-2 Receptor Agonists; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium