Stereoselective Actions of Thiadiazinones on Canine Cardiac Myocytes and Myofilaments

• Published in: Circulation research Vol. 73; no. 6; pp. 981 - 990
• Main Authors: Solaro, R John, Gambassi, Giovanni, Warshaw, David M, Keller, Michael R, Spurgeon, Harold A, Beier, Norbert, Lakatta, Edward G
• Format: Journal Article
• Language: English
• Published: Hagerstown, MD American Heart Association, Inc 01-12-1993; Lippincott
• Subjects: Actin Cytoskeleton - drug effects; Actin Cytoskeleton - metabolism; Adenosine Triphosphatases - metabolism; Animals; Biological and medical sciences; Calcium - metabolism; Cardiotonic agents; Cardiotonic Agents - pharmacology; Cardiovascular system; Dogs; Heart - drug effects; Medical sciences; Muscle Proteins - metabolism; Myocardium - cytology; Myocardium - metabolism; Pharmacology. Drug treatments; Quinolines - pharmacology; Stereoisomerism; Thiadiazines - pharmacology; Heart; Fissipedia; Carnivora; Cardiotonic agent; Calcium; Actins; Contractile protein; In vitro; Myocyte; Stereoselectivity; Vertebrata; Mammalia; Animal; Myosin; Mechanism of action; Dog
• ISSN: 0009-7330; 1524-4571
• DOI: 10.1161/01.res.73.6.981

Thiadiazinones are cardiotonic agents that have potent, direct, and stereoselective actions on the myofilament response to Ca in intact myocardium. Their mechanism of action is unknown. We studied the effects of racemic thiadiazinone, EMD 53998 (5-[1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydro-6-quinolyl]-6-methyl-3,6-dihydro-2H-1,3,4-thiadiazin-2-one), and its enantiomers on Ca signaling in myocytes, myofilaments, and myofilament proteins. Intact canine ventricular myocytes responded to the positive enantiomer, EMD 57033, with an increase in the extent of shortening during twitch contractions without increasing the peak amplitude of the Ca transient. The negative enantiomer, EMD 57439, also increased the extent of shortening, but in this case there was a concentration-dependent increase in the peak amplitude of the Ca transient. This is predicted from in vitro data showing that this enantiomer is a relatively potent inhibitor of phosphodiesterase activity. There was no effect of EMD 57439 on the relation between pCa and actomyosin Mg-ATPase activity of canine heart myofibrils. In contrast, EMD 57033 shifted the pCa-Mg-ATPase activity relation to the left. There was no effect of either enantiomer on Ca binding to myofilament troponin C. Moreover EMD 57033, but not EMD 57439, stimulated actomyosin ATPase activity of myofilament preparations in which either troponin or troponin-tropomyosin had been extracted. EMD 57033 had no effect on Mg-ATPase activity of pure ventricular myosin. EMD 57033 also stimulated the velocity of actin filament sliding on myosin heads adhered to nitrocellulose-coated glass coverslips. We propose that the action of EMD 57033 is at the actin-myosin interface on a “ʼreceptor” that may be on actin or the crossbridge. Drug binding to this domain appears to reverse the inhibition of actin-myosin interactions by troponin-tropomyosin and also to promote transition of crossbridges from weak to strong force-generating states.