Contractile deactivation and uncoupling of crossbridges. Effects of 2,3-butanedione monoxime on mammalian myocardium.

We investigated the effects of 1 and 3 mM 2,3-butanedione monoxime (BDM, diacetyl monoxime) on excitation and contraction of cardiac muscle in several types of preparations at various levels of organization. We selected a concentration of BDM that was not expected to affect sarcolemmal calcium flux and action potential duration in cardiac tissue. Two indicators were used to record intracellular calcium. Aequorin, a bioluminescent calcium indicator, was used in studies with ferret papillary muscle preparations, and fura-2, a fluorescent calcium indicator, was used in studies with guinea pig cardiac myocytes. In both cases, addition of BDM resulted in a reduction of peak intracellular calcium released from the sarcoplasmic reticulum and a reduction of peak twitch force. The duration of the action potential of isolated myocytes was slightly abbreviated in the presence of BDM. In studies on the calcium current in the myocytes, addition of BDM was associated with reduced calcium current at any potential. Peak calcium current was reduced by 7.9 +/- 1% in the presence of BDM. In tetanized ferret papillary muscles, BDM reduced maximal calcium-activated force by 30 +/- 5% and increased the calcium ion concentration required for half-maximal force by 0.1 +/- 0.01 microM. The Hill coefficient was reduced from 5.00 +/- 0.11 to 3.40 +/- 0.20. Maximal shortening velocity of ferret papillary muscles was increased in the presence of BDM from 1.55 +/- 0.24 to 2.04 +/- 0.33 mm/sec. Ca2+ binding to troponin C in skinned fiber preparations from guinea pig, bovine, and canine hearts was unaffected by addition of up to 10 mM BDM. Our results indicate that BDM affects both calcium availability and responsiveness of the myofilaments to Ca2+. Uncoupling of contractile activation from excitation may also result from altered crossbridge kinetics.