A comparison of the cardiotonic effects of AR-L115 and AR-L57: evidence for distinct inotropic mechanisms.

AR-L57 and AR-L115 have been of interest as inotropic agents for management of heart failure. Although their physiological effects are well documented, their mechanism(s) of action are unclear. Both AR-L57 and AR-L115 increased contractile force of cat papillary muscles in concentration-dependent manners; these effects were independent of either alpha- or beta-adrenoceptor stimulation. To determine if these effects occurred via a cAMP-dependent mechanism, cardiotonic actions were studied in the presence of carbachol. Muscarinic stimulation of papillary muscles attenuated contractile responses to AR-L115 thus implying a cAMP-mediated response. By contrast, carbachol did not alter the dose-response profile to AR-L57. In addition, AR-L115 potentiated the inotropic actions of isoproterenol whereas AR-L57 was ineffective. Both AR-L57 and ouabain increased diastolic resting tension in papillary muscles--a phenomenon associated with a state of Ca2+ overload; AR-L115 was without effect. In anesthetized dogs, i.v. AR-L57 and AR-L115 increased contractility and heart rate while reducing mean arterial blood pressure. Both agents had similar rates of onset (10-15 s) and durations of action (40-60 min). Although in vitro studies clearly indicate that AR-L57 and AR-L115 enhance inotropic state by distinct mechanisms, their in vivo cardiovascular profiles are comparable.