CHANGES OF TENSION AND [CA2+](I) DURING BETA-ADRENOCEPTOR ACTIVATION OF SINGLE, INTACT FIBERS FROM MOUSE SKELETAL-MUSCLE

Beta-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 beta(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, beta-adrenergic stimulation increases tetanic tension by enhancing SR Ca2+ release.