THE NOVEL INOTROPIC AGENT CGP-48506 ALTERS FORCE PRIMARILY BY CA2-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-met hano-1 ,5-benzodiazocine-2,4-dione (BA 41899) and its enantiomers dire ctly alter the Ca2+ sensitivity of cardiac myofilaments. Methods: Porc ine ventricular trabeculae were permeabilised with Triton X-100, The r elationship between force and pCa (-log[Ca2+]) was determined in the p resence and absence of ATP. Troponin I was extracted, using vanadate, to produce unregulated maximally activated myofilaments. Force and act omyosin ATPase activity were measured simultaneously to determine tens ion 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 mu M BA 41899 had a pronounced Ca2+ se nsitising 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 unaffe cted. These actions were primarily caused by CGP 48506 and were concen tration dependent, At concentrations less than 100 mu M, CGP 48506 als o increased twitch amplitude in intact papillary muscles with no effec t on resting tension, whereas 100 mu M CGP 48506 increased resting for ce due to a slowing of relaxation. 100 mu M CGP 48506 potentiated Ca2-independent rigor tension in skinned trabeculae, indicating a Ca2+ se nsitising mechanism unrelated to Ca2+ binding to troponin C, Tension c ost was unaffected by 100 mu M CGP 48506 over the entire range of acti vating 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 su bmaximal activation and the depression of maximum force which are indu ced by CGP 48506 in skinned trabeculae occur at least partly through C a2+-independent mechanisms.