Evidence that a decrease in sarcomere length reduces the Ca⁺⁺ affinity of troponin in the intact cardiac myofibril

Abstract

Biochemical studies on troponin (TN), such as those described by Dr. Kay, have been invaluable in defining the number and affinity of the Ca²⁺-binding sites in the molecule. Such studies have usually been performed using isolated TN in solution. However, there is indirect evidence that the Ca²⁺-affinity of TN in the intact myofibril may be different from that in the isolated protein and may be altered by processes and constraints involved in the production of force (for review, see [1]). We have been particularly interested in whether the Ca²⁺-affinity of cardiac TN is reduced at shorter sarcomere lengths (SL). If this is so, it could explain such phenomena as (i) the SL-dependence of myofibrillar Ca²⁺-sensitivity [2, 3, 4] and Chapter 6 by ter Keurs et al. in this volume and (ii) the small ‘bump’ on the decay of the Ca²⁺ transient that is seen if an aequorin-injected papillary muscle is rapidly shortened (Allen and Kurihara [5]); this bump was attributed to Ca²⁺ release from troponin because of a reduced Ca²⁺-affinity at the shorter SL, though it could equally well have been due to Ca²⁺ release from the sarcoplasmic reticulum or from mitochondria.