Sarcomere length effects on the Sr 2+- and Ca 2+-activation curves in skinned frog muscle fibres

Abstract

1. 1. A procedure recently described to produce rapid changes in [Ca 2+ and [Sr 2+ within the whole cross-section of skinned muscle preparations (Moisescu, D.G. (1976) Nature 262, 610–613, and Moisescu, D.G. and Thieleczek, R. (1978) J. Physiol. 275, 241–262) has enabled us to obtain whole Ca 2+- or Sr 2+-activation curves at different sacromere lengths with the same preparation. 2. 2. The maximal isometric force response was found to be very similar in Ca 2+- and Sr 2+-buffered solutions for otherwise identical conditions. 3. 3. The change in sarcomere length between approx. 2.2 and 2.6 μm reversibly shifted both the Ca 2+- and the Sr 2+-activation curves by approx. 0.1 log units towards lower concentrations of the activator, without affecting their shape. 4. However, the change in sarcomere length in the range above 2.6 μm did not have an effect upon the relative isometric force response-pCa (and -pSr) relationship. 5. 4. All the Ca 2+- and Sr 2+-activation curves present a similar steepness and indicate that the relative isometric force increases from approx. 10 to 90% if the concentration of the activator is increased 3-fold. 6. 5. The half time for force development in these experiments did not appear to be influenced by the length of the sarcomeres. 7. 6. A potentiometric method for determining the apparent affinity constants of Ca 2+, Mg 2+ and Sr 2+ to EGTA and ATP under various conditions is described.