Cross-bridge-dependent change of Ca²⁺ affinity of troponin C during relaxation in ferret myocardium

Abstract

We investigated the properties of the cross-bridge-dependent change in the Ca²⁺ affinity of troponin C (TnC) (downstream mechanism) during relaxation because it has not been understood due to the difficulty of mechanical perturbation during relaxation. The aequorin method was used to intact ferret papillary muscles and a tetanic contraction was induced by a repetitive electrical stimulation in the presence of ryanodine. The cross-bridge-dependent change in the Ca²⁺ affinity of TnC was estimated by measuring the extra-Ca²⁺, the transient increase in the intracellular Ca²⁺ concentration in response to a rapid reduction in muscle length, and was estimated by the change in the slope of the extra-Ca²⁺-tension relation. The extra-Ca²⁺-tension relation measured during relaxation became steep compared to that during contraction. 20 mM Caffeine, by which the maximal activated tension was significantly reduced, made the extra-Ca²⁺-tension relation steeper during contraction. 3 mM 2,3-butanedione monoxime, an inhibitor of the cross-bridges, made the extra-Ca²⁺-tension relation steeper during contraction. These results indicate that the downstream-dependent change in the Ca²⁺ affinity of troponin-C was reduced during relaxation compared to that during contraction due to the decreased number of attached cross-bridge. [Jpn J Physiol 55 Suppl:S87 (2005)]