The β-adrenergic receptor stimulation improves endurance in fast twitch muscles and these effects are sensitive to extracellular Ca2+ influx. Present study is aimed to determine the effects of adrenaline, with different concentrations of extracellular Ca2+[Formula: see text], on the contractility and endurance of slow twitch muscles during high frequency stimulations (HFS). Isolated soleus of rabbit was electrically stimulated (strength; 50 Hz, duration; 0.5 ms) in the presence (Test) of adrenaline (1 × 10-7 mM) or without adrenaline (CTL). Fatigue was induced with HFS (80 Hz) for the duration of 20 s. Contractions were recorded through isometric transducer connected with Powerlab. Kreb's buffer was used with three compositions: standard with 2.5 mM Ca2+ (Ca-S), Ca2+ free buffer (Ca-F) and buffer with raised Ca2+ i.e., 10 mM (Ca-R). Muscles endurance was assessed by measuring the decline in tetanic tension in the terms of percentage (%Pmax) and rate of decline in tetanic tension (dP/dt). During 20 s, %Pmax showed reduction of only 10% in Ca-S. This decline was enhanced in Ca-F (50%) and reduced in Ca-R (6%). Effect of adrenaline was observed only in Ca-F where %Pmax was about 20% greater in Test than CTL. These effects were not observed in both Ca-S and Ca-R during 20 s. However, when duration of stimulation was increased to 120 or 150 s in Ca-S and Ca-R respectively, decline in %Pmax was less in Test as compared to CTL. Thus, [Formula: see text] plays protective role against fatigue during continuous HFS in slow twitch muscles. In addition, adrenaline improves the muscles endurance during fatiguing contraction but these effects are not mediated through [Formula: see text] influx.
Keywords: Adrenaline; Calcium transients; High frequency stimulation; Slow twitch muscles.