Mechanism of force inhibition by 2,3‐butanedione monoxime in rat cardiac muscle: roles of [Ca2+]i and cross‐bridge kinetics.

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Abstract
1. We investigated the mechanism of force inhibition by 2,3-butanedione monoxime (BDM) on rat cardiac trabeculae. [Ca2+]i was measured by iontophoretic injection of fura-2 salt. Isometric force was recorded at an end-systolic sarcomere length of 2.1-2.2 microns. 2. With an external [Ca2+] of 1 mM, peak twitch force was monotonically reduced with increasing [BMD]; at 5 and 20 mM [BDM], force was 35 and 1% of the control force. In contrast, the mean peak [Ca2+]i during transients was only reduced at [BDM] > or = 10 mM. 3. The duration of the twitch was dramatically reduced by BDM in a dose-dependent fashion with no significant change in the time course of the underlying Ca2+ transients. The abbreviation of twitch force duration was much greater than expected for the observed reduction in peak force by this agent. 4. The mechanism of the inhibition of force by BDM was explored by examining the relationship between twitch force and Ca2+ transients at various values of external [Ca2+]. In the presence of BDM, the steepness of the relationship between peak force and peak [Ca2+]i was reduced compared to control conditions. As a result, significant elevation in the [Ca2+]i transient was unable to reverse the reduction in force observed in the presence of BDM. 5. The direct inhibitory effects of BDM on the contractile system were examined using ryanodine tetani in intact trabeculae to measure the steady-state force-[Ca2+]i relationship. In contrast to the effects on twitch force at 5 mM BDM, maximal force was only reduced to 71% of control. Furthermore, the [Ca2+]i required for half-maximal activation (Ca50) was increased while the Hill coefficient was reduced slightly by BDM. 6. BDM dramatically slowed the rate of rise of tetanic force. At maximal activation, the time required to reach 90% maximal force was prolonged by a factor of 3-8 in the presence of 5 mM BDM. This suggests that the observed reduction in twitch force and steady-state force may result from slowed kinetics of cross-bridge attachment, consistent with recent biochemical studies. 7. The contribution of altered cross-bridge kinetics to the effects of BDM was investigated using a co-operative cross-bridge model of the contractile system. Changing the rate constants for cross-bridge attachment in the model to mimic the reported biochemical effects of BDM reproduced the observed effects of BDM.(ABSTRACT TRUNCATED AT 400 WORDS)

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