Zusammenfassung

Our aim is to describe the acute effects of catecholamines/β‐adrenergic agonists on contraction of non‐fatigued skeletal muscle in animals and humans, and explain the mechanisms involved. Adrenaline/β‐agonists (0.1–30 μm) generally augment peak force across animal species (positive inotropic effect) and abbreviate relaxation of slow‐twitch muscles (positive lusitropic effect). A peak force reduction also occurs in slow‐twitch muscles in some conditions. β2‐Adrenoceptor stimulation activates distinct cyclic AMP‐dependent protein kinases to phosphorylate multiple target proteins. β‐Agonists modulate sarcolemmal processes (increased resting membrane potential and action potential amplitude) via enhanced Na+–K+ pump and Na+–K+–2Cl− cotransporter function, but this does not increase force. Myofibrillar Ca2+ sensitivity and maximum Ca2+‐activated force are unchanged. All force potentiation involves amplified myoplasmic Ca2+ transients consequent to increased Ca2+ release from sarcoplasmic reticulum (SR). This unequivocally requires phosphorylation of SR Ca2+ release channels/ryanodine receptors (RyR1) which sensitize the Ca2+‐induced Ca2+ release mechanism. Enhanced trans‐sarcolemmal Ca2+ influx through phosphorylated voltage‐activated Ca2+ channels contributes to force potentiation in diaphragm and amphibian muscle, but not mammalian limb muscle. Phosphorylation of phospholamban increases SR Ca2+ pump activity in slow‐twitch fibres but does not augment force; this process accelerates relaxation and may depress force. Greater Ca2+ loading of SR may assist force potentiation in fast‐twitch muscle. Some human studies show no significant force potentiation which appears to be related to the β‐agonist concentration used. Indeed high‐dose β‐agonists (∼0.1 μm) enhance SR Ca2+‐release rates, maximum voluntary contraction strength and peak Wingate power in trained humans. The combined findings can explain how adrenaline/β‐agonists influence muscle performance during exercise/stress in humans.