Effects of adrenaline on contractility and endurance of isolated mammalian soleus with different calcium concentrations.

Abstract

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; 50Hz, duration; 0.5ms) in the presence (Test) of adrenaline (1 × 10-7mM) or without adrenaline (CTL). Fatigue was induced with HFS (80Hz) for the duration of 20s. Contractions were recorded through isometric transducer connected with Powerlab. Kreb's buffer was used with three compositions: standard with 2.5mM Ca2+ (Ca-S), Ca2+ free buffer (Ca-F) and buffer with raised Ca2+ i.e., 10mM (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 20s, %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 20s. However, when duration of stimulation was increased to 120 or 150s 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.