Effects on shortening velocity of rabbit skeletal muscle due to variations in the level of thin‐filament activation.

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Abstract
The effect of Ca2+ upon maximum shortening velocity (Vmax) has been investigated in skinned single fibres from rabbit psoas muscles. Vmax was obtained at 15 degrees C by measuring the amounts of time (delta t) required to take up various amounts of slack (delta l) imposed at one end of the fibre. During maximal activation with Ca2+, plots of delta l vs. delta t were well fitted by a single straight line. Calculation of Vmax from the slopes of the fitted lines yielded a Vmax of 4.44 +/‐ 0.15 (S.E. of mean) muscle lengths per second (m.l./s). However, at lower levels of Ca2+ activation, plots of delta l vs. delta t were biphasic, containing an initial phase of steady high‐velocity shortening and a subsequent phase of steady low‐velocity shortening. The transition between these two phases occurred following active shortening equivalent to 60‐80 nm/half‐sarcomere. Vmax during the high‐velocity phase was relatively insensitive to Ca2+ concentration between pCas (i.e. ‐log [Ca2+]) of 4.5 and 6.0; however, Vmax fell to 3.58 +/‐ 0.40 m.l./s at pCa 6.1 and further to 1.02 +/‐ 0.30 m.l./s at pCa 6.2. Vmax during the low‐velocity phase decreased as Ca2+ was lowered within the entire range of pCas studied to a minimum value of 0.35 +/‐ 0.09 m.l./s at pCa 6.2. The degree of thin‐filament activation at a particular pCa was varied by partial extraction of troponin‐C, which resulted in a permanent though reversible inactivation of parts of the thin filaments. Partial extraction of troponin‐C altered the plots of delta l vs. delta t obtained at pCa 4.5 to a biphasic form. In addition, Vmax during the high‐ and low‐velocity phases of shortening was reduced at each pCa greater than 4.5. Vmax values obtained in control fibres at low Ca2+ concentrations and extracted fibres were in good agreement when generated isometric tensions were equivalent. This was the case for both the high‐ and low‐velocity phases of shortening. Fibres were also activated in the absence of Ca2+ by partial removal of total troponin complexes. These fibres developed steady tensions less than 30% of maximum and underwent biphasic shortening, indicating that this phenomenon cannot be the result of shortening‐induced dissociation of Ca2+ from troponin‐C.(ABSTRACT TRUNCATED AT 400 WORDS)