title: Stimulation of Ca++ binding and ATPase activity of dog cardiac myofibrils by AR-L 115BS, a novel cardiotonic agent.
abstract: AR-L 115BS, a benzimidazole derivative, is a positive inotropic agent that has been shown to increase active tension development and unloaded shortening velocity of chemically skinned heart muscle preparations at submaximal activating levels of free Ca++. We measured the effect of AR-L on relations between free Ca++, bound Ca++ and ATPase activity of dog cardiac myofibrils. At pCa 6, 100-300 micrometers AR-L increased myofibrillar ATPase activity maximally by about 30%. The concentration of AR-L giving half-maximal activation of myofibrillar ATPase activity was about 10 micrometers, and is similar to plasma concentrations associated with elevated contractility in intact animals. There was no effect of AR-L on myofibrillar ATPase activity at pCa 5 or 8, and the relation between pCa and percent activation of myofibrillar ATPase activity was shifted to the left by 0.4-0.5 pCa units in the presence of 100 micrometers AR-L. Calcium binding by cardiac myofibrils was increased by AR-L in the presence and absence of MgATP by 0.2-0.3 nmol/mg myofibrillar protein over a broad range of free Ca++ concentrations, a result suggesting that AR-L increases the affinity of myofibrillar troponin C for Ca++. The shift in the pCa giving half maximal and myofibrillar ATPase activity induced by raising the free Mg++ from 1.0 to 10 mm was unaffected by AR-L. These results indicate that the positive actions of AR-L 115BS on cardiac contractility may involve direct activation of myofibrils by virtue of an increased affinity of thin filament receptors for Ca++.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002316:Cardiotonic Agents / Q000494:pharmacology; D004285:Dogs; D006352:Heart Ventricles / Q000187:drug effects; D007093:Imidazoles / Q000494:pharmacology*; D008274:Magnesium / Q000378:metabolism; D009210:Myofibrils / Q000378:metabolism*; D014336:Troponin / Q000378:metabolism; D019209:Troponin C
chemical_list: D002316:Cardiotonic Agents; D007093:Imidazoles; D014336:Troponin; D019209:Troponin C; D000251:Adenosine Triphosphatases; C029758:sulmazole; D008274:Magnesium; D002118:Calcium