Lues I, Beier N, Jonas R, Klockow M, Haeusler G
Pharmaceutical Research Department, E. Merck, Darmstadt, Germany.
J Cardiovasc Pharmacol. 1993 Jun;21(6):883-92. doi: 10.1097/00005344-199306000-00006.
The novel cardiotonic EMD 53,998 increases contractile force in vitro through both inhibition of phosphodiesterase III (PDE III) activity and increase in the responsiveness of the contractile proteins to calcium ("calcium sensitization"). Because EMD 53,998 is a racemate, the possibility arose that the two modes of action do not reside equally in the enantiomers. Therefore, the effects of the racemate and its two enantiomers [(+)EMD 57,033 and (-)EMD 57,439] were analyzed in guinea pig and rat cardiac tissue with respect to Ca2+ sensitization (Ca(2+)-induced force development in skinned cardiac myofibers and myofibrillar ATPase activity) and PDE III inhibition (isolated PDE isoenzymes and cyclic AMP level in isolated cardiac myocytes). In addition, the positive inotropic effects were compared in isometrically contracting papillary muscles. Enhancement of force of contraction (Fc) in submaximally activated skinned fibers showed a selectivity for the (+)enantiomer with EC50 = 1.7, 4.8, and > 100 microM for EMD 57,033, EMD 53,998, and EMD 57,439, respectively. Ca2+ concentration for half-maximal activation was decreased by 0.5 log units, and Cmax was increased by 15% at 10 microM EMD 57,033. Similarly, myofibrillar ATPase activity was most potently enhanced by the (+)enantiomer, with EC50 values of 1.8, 2.5, and > 30 microM for EMD 57,033, EMD 53,998, and EMD 57,439, respectively. PDE III activity was inhibited with greater potency by the (-)enantiomer, with IC50 values of 0.05, 0.06, and 1.94 microM for EMD 57,439, EMD 53,998, and EMD 57,033, respectively. The cyclic AMP content of isoprenaline-stimulated rat cardiac myocytes was increased by 50% at 13.6 and 0.71 microM for EMD 57,033 and EMD 57,439, respectively. In intact guinea pig papillary muscle, the positive inotropic effect of the (+)enantiomer was insensitive to isoprenaline pretreatment; in contrast, the (-)enantiomer showed only a weak positive inotropic action which was strongly enhanced in the presence of isoprenaline. We conclude that one of the two different mechanisms underlying the overall positive inotropic activity of EMD 53,998 can be assigned, almost exclusively, to one of the two enantiomers. Thus, the (-)enantiomer EMD 57,439 is a "pure" PDE III inhibitor with almost no Ca2+ sensitizing activity; the (+)enantiomer EMD 57,033 is a potent Ca2+ sensitizer with only a weak PDE III inhibitory activity as compared with the racemate. In contrast to other compounds with mixed activity, EMD 57,033 is unique in possessing both a high absolute potency at the level of the contractile elements and a favorable relation of Ca2+ sensitization to PDE inhibition.
新型强心剂EMD 53,998通过抑制磷酸二酯酶III（PDE III）活性和增强收缩蛋白对钙的反应性（“钙敏化”）在体外增加收缩力。由于EMD 53,998是一种外消旋体，因此存在两种作用模式在对映体中分布不均的可能性。因此，分析了外消旋体及其两种对映体[（+）EMD 57,033和（-）EMD 57,439]在豚鼠和大鼠心脏组织中对Ca2+敏化（在脱膜心肌纤维中Ca(2+)诱导的力产生和肌原纤维ATP酶活性）和PDE III抑制（分离的PDE同工酶和分离心肌细胞中的环磷酸腺苷水平）的影响。此外，在等长收缩的乳头肌中比较了正性肌力作用。在次最大激活的脱膜纤维中，收缩力（Fc）增强对（+）对映体具有选择性，EMD 57,033、EMD 53,998和EMD 57,439的EC50分别为1.7、4.8和>100 microM。在10 microM EMD 57,033时，半最大激活的Ca2+浓度降低0.5个对数单位，Cmax增加15%。同样，肌原纤维ATP酶活性最有效地被（+）对映体增强，EMD 57,033、EMD 53,998和EMD 57,439的EC50值分别为1.8、2.5和>30 microM。（-）对映体对PDE III活性的抑制作用更强，EMD 57,439、EMD 53,998和EMD 57,033的IC50值分别为0.05、0.06和1.94 microM。在13.6和0.71 microM时，EMD 57,033和EMD 57,439分别使异丙肾上腺素刺激的大鼠心肌细胞中环磷酸腺苷含量增加50%。在完整的豚鼠乳头肌中，（+）对映体的正性肌力作用对异丙肾上腺素预处理不敏感；相反，（-）对映体仅表现出微弱的正性肌力作用，在异丙肾上腺素存在下其作用显著增强。我们得出结论，EMD 53,998总体正性肌力活性的两种不同机制之一几乎可以完全归因于两种对映体中的一种。因此，（-）对映体EMD 57,439是一种“纯”PDE III抑制剂，几乎没有Ca2+敏化活性；（+）对映体EMD 57,033是一种有效的Ca2+敏化剂，与外消旋体相比，其PDE III抑制活性较弱。与其他具有混合活性的化合物不同，EMD 57,033的独特之处在于它在收缩元件水平上具有高绝对效力，并且Ca2+敏化与PDE抑制之间具有良好的关系。
