Calcium dependence of the apparent rate of force generation in single frog and rabbit skeletal muscle myofibrils activated by rapid solution changes

Single myo®brils or small bundles of 2±3 myo®brils 50±100 lm long prepared from glycerinated frog tibialis anterior muscle or from rabbit psoas muscle (Colomo et al. (1997) J. Physiol 500.2, 535) were activated using a novel method which allows solution changes within 10 ms. The preparations were mounted horizontally between the lever arms of an isometric force transducer and a length control motor (Colombo et al. (1994) J. Physiol. 475, 347) in a temperature-controlled trough ®lled with relaxing solution (pCa 8, 15° C). Sarcomere length was set just above slack length. Mounted myo®brils were continuously perfused by one of two parallel streams of solution jetted by gravity from a theta-style glass pipette. Each pipette channel was connected to reservoirs ®lled with either relaxing or activating solutions of different calcium concentration (MgATP 3 mM plus CP/CPK regenerating system). The perfusion system was ®rmly attached to a stepping motor for rapid alternation of the streams over the myo®brils. When frog or rabbit skeletal muscle myo®brils were activated, tension rapidly rose to steady values that were strongly dependent on calcium concentration. In both cases, the force/pCa relations obtained were consistent with those reported for larger preparations. With all the myo®brils tested, the time course of force development was approximately exponential and superimposable on the time course of force redevelopment following a release±restretch manoeuvre applied at the contraction plateau (Brenner (1988) Proc. Natl Acad. Sci. 85, 3265). At saturating pCa (4.75), the apparent rate constant of the process leading to force generation was 15± 20 s)1 for the frog myo®brils and half that for the rabbit myo®brils, independently of the experimental method used. These values are in reasonable agreement with those for single skinned ®bres from the same muscles of the frog and the rabbit following their activation by the photolysis of caged calcium. In both preparations, the rate constant of force generation was slowed down by decreasing calcium concentration, but the effect was much larger for rabbit than for frog skeletal myo®brils, where it could be detected only at very low levels of activation. These results do not support the idea that the effect of calcium on the kinetics of force generation is larger in fast than in slow muscles (Metzger & Moss (1990) Science 247, 1088; Campbell (1997) Biophys. J. 72, 254).