The novel inotropic agent CGP 48506 alters force primarily by Ca(2+)-independent mechanisms in porcine skinned trabeculae.

OBJECTIVES: The aim was to determine whether, and by what mechanism(s), a novel inotropic agent 5-methyl-6-phenyl-1,3,5,6-tetrahydro-3, 6-methano-1,5-benzodiazocine-2,4-dione (BA 41899) and its enantiomers directly alter the Ca2+ sensitivity of cardiac myofilaments. METHODS: Porcine ventricular trabeculae were permeabilised with Triton X-100. The relationship between force and pCa (-log[Ca2+]) was determined in the presence and absence of ATP. Troponin I was extracted, using vanadate, to produce unregulated maximally activated myofilaments. Force and actomyosin ATPase activity were measured simultaneously to determine tension cost (ATPase activity/tension). The effects of the (+) enantiomer (CGP 48506) on the twitch of intact muscle were demonstrated using rat papillary muscle. RESULTS: 100 microM BA 41899 had a pronounced Ca2+ sensitising effect on force production by porcine skinned cardiac fibres, increasing the pCa required for 50% maximal activation by 0.64 units, while suppressing maximum force by 18.3%. Resting tension was unaffected. These actions were primarily caused by CGP 48506 and were concentration dependent. At concentrations less than 100 microM, CGP 48506 also increased twitch amplitude in intact papillary muscles with no effect on resting tension, whereas 100 microM CGP 48506 increased resting force due to a slowing of relaxation. 100 microM CGP 48506 potentiated Ca(2+)-independent rigor tension in skinned trabeculae, indicating a Ca2+ sensitising mechanism unrelated to Ca2+ binding to troponin C. Tension cost was unaffected by 100 microM CGP 48506 over the entire range of activating Ca2+ concentrations. Suppression of maximum force by CGP 48506 was independent of both Ca2+ concentration and the regulatory troponin complex. CONCLUSIONS: Both the increase in Ca2+ sensitivity during submaximal activation and the depression of maximum force which are induced by CGP 48506 in skinned trabeculae occur at least partly through Ca(2+)-independent mechanisms.

The effects of EMD 57033 on rigor tension in porcine skinned cardiac trabecula.

5-[1-(3,4-Dimethoxybenzoyl)-1,2,3,4-tetrahydro-6-quinolyl]-6-methy l-3,6- dihydro-2H-1,3,4-thiadiazin-2-one (EMD 57033) is a cardiotonic agent which acts directly on the myofilaments, increasing their sensitivity to Ca2+. We studied the effects of EMD 57033 on porcine cardiac skinned fibres to investigate the roles of Ca2+ and crossbridges in its Ca2+ sensitisation. EMD 57033 potentiated resting and maximum tensions and caused a leftward shift of the force-Ca2+ relationship. Under rigor conditions, in which fibres developed Ca(2+)-independent force, EMD 57033 was still able to potentiate tension, provided the compound was added prior to rigor development. These results show that EMD 57033 increases force by a Ca(2+)-independent mechanism in the intact myofilament lattice and that the Ca2+ sensitiser acts during the transition of crossbridge state rather than on crossbridges that are fixed in the strongly attached state corresponding to rigor.