Effect of free sarcoplasmic Ca2+ concentration on maximum unloaded shortening velocity: measurements on single glycerinated rabbit psoas muscle fibres

Abstract

Isotonic shortening velocity in tension-controlled quick releases was studied at different sarcomere lengths, loads and Ca2+ concentrations using single glycerinated rabbit psoas muscle fibres. During a single quick release shortening velocity decreases with increasing shortening. Length-velocity relations, that is, momentary shortening velocity as a function of the corresponding sarcomere length, of a single quick release yield straight lines when graphed on a logarithmic scale. This indicates an exponential decrease of the isotonic shortening velocity during a quick release. A given shortening affects the subsequent part of the length-velocity relation in a manner which can be simulated by loading the fibre. The decrease in shortening velocity during a quick release could therefore be interpreted as the result of a shortening-induced increase in contractile system load. Thetotal amount of this additional load is proportional to the amount of shortening but is independent of the Ca2+ concentration. It follows that the change in loadper crossbridge due to a given shortening Δl, is inversely proportional to the degree of activation of the contractile system, provided that the number of ‘cycling’ crossbridges during isotonic shortening is dependent on sarcoplasmic Ca2+ concentration as is known for the isometric state. These shortening-induced changes in crossbridge load should be considered in interpreting Ca2+ effects on shortening velocity. One approach may be extrapolation of the length-velocity relation to the starting length of the respective quick release, that is, the point at which all length-velocity relations of different Ca2+ concentrations converge. Thus, when an increase in crossbridge load due to shortening is taken into account, an effect of the free Ca2+ concentration on maximum unloaded shortening velocity cannot be proved.