UNLABELLED: 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. METHODS AND RESULTS: 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. CONCLUSION: 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.
UNLABELLED: 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. METHODS AND RESULTS: 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. CONCLUSION: 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.