Calcium Sensitivity after Active Shortening in Rabbit Psoas Fibres

Abstract

Introduction It has been shown that isometric force after active shortening of muscles is lower than the purely isometric force performed at the corresponding lengths. This property is termed force depression. Force depression has been observed in isolated muscle preparations and muscles in situ and in vivo (Rassier et al., J Applied Physiol 2004). The origin of force depression is still unknown. It has been suggested that active shortening may affect calcium sensitivity by changing calcium-troponin binding equilibrium leading to decreased force after active shortening (Ekelund and Edman, Acta Physiol Scand 1982). Our aim in the present study was to test whether calcium sensitivity was decreased after active shortening. Methods Experiments were performed using skinned fibres (n=7) isolated from rabbit psoas muscle. Calcium sensitivity was characterized by establishing the force-pCa curves for isometric reference contractions performed at an average sarcomere length of 2.4 µm and active shortening contractions from an average sarcomere length of 3.0 µm to an average sarcomere length of 2.4 µm. pCa50 and the coefficient of cooperativity (nH) were compared between reference and active shortening contractions. Results and Discussion pCa50 for the reference contractions was 5.99±0.02. No change in pCa50 was observed in the force depressed state (6.00±0.01). Furthermore, the coefficient of cooperativity did not change after active shortening (3.9±0.3 versus 4.3±0.3 for the reference and force depressed states respectively). These results suggest that calcium sensitivity does not decrease after active shortening and that the binding interactions between troponin and calcium are not altered in the force depressed state.