Changes of tension and [Ca²⁺]_i during β-adrenoceptor activation of single, intact fibres from mouse skeletal muscle

Abstract

β-Adrenergic agonists increase tension production in fast-twitch skeletal muscle, but the underlying mechanism is unknown. In the present study we have exposed intact, single fibres from a mouse muscle to the β ₂-adrenergic agonist terbutaline. Fibres were stimulated to produce 350-ms tetani at 20–100 Hz while measuring the myoplasmic Ca²⁺ concentration ([Ca²⁺]_i) and tension. The fluorescent indicator Indo-1 was used to measure [Ca²⁺]_i. Application of terbutaline resulted in marked increases of both tetanic [Ca²⁺]_i and tension. Terbutaline had no significant effect on myofibrillar function as judged from normal Ca²⁺ sensitivity and tension production at saturating [Ca²⁺]_i. The rate of [Ca²⁺]_i and tension decline during relaxation was not affected by terbutaline, thus indicating a normal function of the sarcoplasmic reticulum (SR) Ca²⁺ pumps. The effect of terbutaline developed gradually over 5–10 min when fibres were stimulated each minute; the full effect of terbutaline was also obtained after a 10-min rest period in terbutaline. The [Ca²⁺]_i at rest was not affected by terbutaline. In conclusion, β-adrenergic stimulation increases tetanic tension by enhancing SR Ca²⁺ release.