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This study investigated the positive inotropic andvasorelaxant activity ofDHQ-11, aconjugate of flavonoid dihydroquercetin with isoquinoline alkaloid 1-aryl-6,7-dimethoxy-1,2.3,4-tetrahydroisoquinoline.A study was performed using anterior papillary muscle removed from the left ventricle and thoracic aorta dissected from rats. DHQ-11 produceda concentration-dependent positive inotropic effect which was more potent than their parent compounds alone. The positive inotropic effect of conjugate DHQ-11was significantly attenuated by the β-adrenoreceptor inhibitor propranolol and L-type Ca2+ channel blocker nifedipine. Also,conjugate DHQ-11 markedly potentiated first post-rest responses indicating that it can modulate Ca2+ loading/release processes in the sarcoplasmic reticulum.These results suggest that positive inotropic effect produced by conjugate DHQ-11may be mediated through activation oftheβ-AR/AC/cAMP/PKA pathway that leads to increased Ca2+ influx and rises in Ca2+ loading/release in the SR, resulting in increased [Ca2+]i and enhanced contraction force. DHQ-11 significantly relaxed both high KCl- and phenylephrine-induced contractions of rat aortic rings whichwere significantly inhibited by lowering extracellular Ca2+ concentration and in the presence of verapamil.DHQ-11 significantly inhibited phenylephrine-induced contractions in a Ca2+-free medium, in the presence of verapamil. The vasorelaxant effect of the DHQ-11 was significantly reduced by the removal of endothelium and in the presenceof L-NAME and methylene blue as well as glibenclamide and TEA.These results suggest that the vasorelaxation produced by conjugate DHQ-11may be mediatedbyan endothelium-independent mechanism involving activation of KATP and BKCa channels and inhibition of L-type VDCCs and Ca2+ release from the sarcoplasmic reticulum and endothelium-dependent mechanism through activation of the NO/sGC/cGMP/PKG signaling pathway resulting in a decrease of intracellular Ca2+levels. These observations reveal that the conjugate DHQ-11 due to its high positive inotropic and vasorelaxant activity could be a promising compound for the design and development of new drugs for the treatment of heart failure.

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