Cross-Bridge-Dependent Change of the Ca2+ Sensitivity During Relaxation in Aequorin-Injected Tetanized Ferret Papillary Muscles

Abstract

BACKGROUND The aim of the present study was to indicate the cross-bridge-dependent change in the Ca2+ affinity of troponin-C (TnC) during relaxation in an intact preparation, because the intracellular mechanism of relaxation is not fully understood, although several methods of evaluating global diastolic function have been reported. The aequorin method was used with intact ferret papillary muscles and a tetanic contraction was induced by a repetitive electrical stimulation in the presence of ryanodine. The extra-Ca2+, the transient increase in the intracellular Ca2+ concentration in response to a rapid reduction in muscle length, which reflects the change in the Ca2+ affinity of TnC because of cross-bridge detachment, was measured, and the cross-bridge-dependent change in the Ca2+ affinity of TnC was estimated by observing the change in the slope of the extra-Ca2+ -tension relation. The extra-Ca2+ -tension relation measured during relaxation became steeper than that during contraction in all cases. The extra-Ca2+ -tension relation became steeper in the presence of 20 mmol/L caffeine during contraction in all cases. During relaxation, the downstream-dependent change in the Ca2+ affinity of TnC was enhanced, compared with that during contraction, because of a decrease in the number of attached cross-bridges.