Effects of terbutaline on force and intracellular calcium in slow-twitch skeletal muscle fibres of the rat.
1. The effect of the alpha2-adrenoceptor agonist, terbutaline, was investigated on simultaneously measured force and intracellular free calcium ([Ca2+]i) in intact rat soleus muscle fibres, and on contractile protein function and Ca2+ content of the sarcoplasmic reticulum (SR) in skinned fibres. 2. Terbutaline (10 microM) had no significant effect on either resting force or [Ca2+]i. Exposure to terbutaline increased both the integral of the indo-1 ratio transient and peak twitch force by 37%. 3. At sub-maximal (10 Hz) stimulation frequencies, terbutaline accelerated force relaxation but had highly variable effects on tetanic force amplitude. The corresponding indo-1 ratio transients were significantly larger, and faster to decay than the controls. 4. Terbutaline increased tetanic force at near maximal stimulation frequencies (50 Hz) by increasing tetanic [Ca2+]i. Force relaxation was accelerated at this frequency with no significant change in the indo-1 ratio transient decay rate. 5. All of terbutaline's effects on force and indo-1 ratio transients in intact fibres were completely blocked and reversed by ICI 118551 (1 microM). 6. Mechanically skinned fibres isolated from intact muscles pre-treated with terbutaline showed no significant changes in SR Ca2+ content, myofilament [Ca2+]i-sensitivity or maximum force generating capacity. 7. The results suggest that terbutaline primarily modulates force by altering the amplitude and decay rate of the [Ca2+]i transient via phosphorylation of both the ryanodine receptor (RR) and the SR pump regulatory protein, phospholamban (PLB). The high variability of responses of slow-twitch muscles to beta2-agonists probably reflects individual differences in basal phosphorylation levels of PLB relative to that of RR.
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School of Physiology and Pharmacology, University of New South Wales, Sydney, Australia.
Posterino
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Fryer
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D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004347:Drug Interactions; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology*; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology*; D011412:Propanolamines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D013726:Terbutaline / Q000037:antagonists & inhibitors / Q000494:pharmacology*
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D011412:Propanolamines; C026777:ICI 118551; D013726:Terbutaline; D002118:Calcium