Caffeine- and Ca2(+)-induced mechanical oscillations in isolated skeletal muscle fibres of the frog.

Isometric force and subthreshold sarcomeric oscillations were studied in isolated muscle fibres of the frog. In intact muscle fibres, caffeine in a concentration of 2 mM caused a subthreshold oscillatory activation of single sarcomeres, called 'sarcomeric oscillations'. They occurred independently of membrane potential and were blocked by agents which directly interfere with Ca2+ release from the sarcoplasmic reticulum (SR). In skinned muscle fibres, sarcomeric oscillations were also induced by the Ca2+ ion itself (pCa = 6.1). When the free EGTA concentration of the bathing solutions was reduced, fibres responded with long lasting oscillations of force. Both types of oscillations were blocked when the membranes of the SR were solubilized by detergent. The results reveal that caffeine- and Ca2+-induced oscillations in skeletal muscle fibres are triggered by a cyclic release of Ca2+ ions from the SR. It is suggested that they interfere with the process of Ca2+-induced Ca2+ release.