BackgroundIt is generally accepted that the adrenergic nervous system provides inotropic support for the failing heart. However, the magnitude of this support has never been studied extensively. The present study was performed to test the hypothesis that the adrenergic nervous system is capable of maintaining indexes of pump and contractile function in the normal range despite significant innate myocardial depression.Methods and ResultsWe used our model of experimental canine mitral regurgitation, which produces left ventricular dysfunction after 3 months of volume overload. We studied indexes of contractile function on and off β-blockade at baseline and again on and off β-blockade 3 months after chronic mitral regurgitation had induced significant contractile dysfunction. At baseline, acute β-blockade caused insignificant reductions in the mass-corrected slope of the endejection stress-volume relation (EESVR), the end-systolic stiffness constant, and the ejection fraction-end-systolic stress and the mean velocity of circumferential fiber shortening (VCF)-end-systolic stress relations. After 3 months of chronic mitral regurgitation, all indexes of contractile function were normal in the unblocked state except for the VCF-stress relation, which was mildly reduced. However, after acute β-blockade after 3 months of chronic mitral regurgitation, the EESVR fell to 303±27 versus 443±24 during acute P-blockade before mitral regurgitation was created (P< .05), and the end-systolic stiffness constant was reduced to 2.54±0.15 versus 3.27±0.11 (P< .05). Only after,β-blockade was the ejection fraction-stress relation significantly reduced for dogs with chronic mitral regurgitation. The VCF-stress relation became markedly more abnormal. The viscosity-velocity relation of myocytes isolated from the ventricles of the dogs with mitral regurgitation confirmed that substantial innate contractile depression was present.ConclusionsAfter 3 months of chronic mitral regurgitation, the adrenergic nervous system was able to maintain most indexes of contractile function in the normal range despite significant depression in innate contractile function. Thus, in the absence of β-blockade, significant innate contractile depression may be obscured by adrenergic support.