Adrenaline Dose-Response on Fatigue Resistance of the Gastrocnemius Muscle: An Integrated Functional and Biochemical Analysis in an Amphibian Model

Abstract

Background: Adrenaline modulates skeletal muscle contractile function and metabolism through β₂-adrenergic receptor activation, yet the dose-response relationship governing these effects on fatigue resistance remains uncharacterised. Aim: This study investigated the concentration-dependent effects of adrenaline on gastrocnemius muscle fatigue resistance. Methods: Adult male Rana temporaria (n=40, 100-120g) were randomly assigned to five groups (n=8 each): control (saline), low (0.01 mg/kg), moderate (0.1 mg/kg), high (0.5 mg/kg), and supraphysiological (1.0 mg/kg) adrenaline doses. Using an in situ gastrocnemius-sciatic nerve preparation, peak twitch tension, peak tetanic tension, time to fatigue (T₅₀), and contraction/relaxation rates were measured during continuous high-frequency stimulation. Results: Adrenaline produced significant, dose-dependent enhancements in all contractile parameters up to an optimal dose, followed by diminished efficacy at supraphysiological concentrations. The high dose (0.5 mg/kg) maximally increased twitch tension (26.9%), tetanic tension (26.0%), and T₅₀ (49.3%) compared to control (p<0.001). The supraphysiological dose (1.0 mg/kg) showed significantly smaller improvements (17.4%, 17.7%, and 35.6%, respectively; p<0.01 vs. high dose). Quadratic regression models provided superior fit over linear models (p<0.01), confirming a biphasic (inverted-U) dose-response relationship. Biochemical Findings: In a parallel cohort of frogs (n=30) drawn from the same experimental population and dosed identically, adrenaline produced a dose-dependent fall in the phosphocreatine-to-inorganic phosphate (PCr/Pi) ratio (-15.5% at 0.5 mg/kg; p<0.001), increased phosphorylase activity (+20.3%; p<0.001), elevated cAMP (+41.2%; p<0.001), increased blood lactate (+27%; p<0.001), and increased Na+/K+-ATPase activity (+5.9%; p=0.010). Plasma adrenaline and the PCr/Pi ratio were the strongest predictors of T50 (multiple regression, R2=0.87, p<0.001), and the biochemical changes tracked the same biphasic (high > supra) pattern seen in the contractile measures. Conclusion: Adrenaline enhances gastrocnemius fatigue resistance through a biphasic dose-response mechanism, with optimal effects at 0.5 mg/kg and declining efficacy at supraphysiological concentrations, underpinned by a measurable shift in energy metabolism, glycogenolytic enzyme activity, cAMP signalling, and ionic regulation. This pattern is consistent with a shifting balance between β₂-mediated potentiation and α-mediated inhibition. These findings establish a therapeutic window for adrenergic modulation of muscle performance in this amphibian model; extrapolation to mammalian muscle-wasting disorders or critical illness weakness will require confirmation in mammalian and clinical studies.