Disproportionate enhancement of myocardial contractility by the xanthine oxidase inhibitor oxypurinol in failing rat myocardium

Objective: Xanthine oxidase (XO) inhibitors enhance myofilament Ca2+ responsiveness of normal rat myocardium. We examined whether this inotropic action is preserved or magnified in failing rat myocardium and whether the magnitude of this effect correlates with tissue xanthine-oxidoreductase (XOR) activity. Methods: Hearts of 18–20 month-old SHHF (spontaneous hypertensive/heart failure) rats with end-stage heart failure, as well as of normal control rats, were perfused with the XO inhibitor oxypurinol. Afterwards, [Ca2+]i and tension were measured simultaneously in fura-2-loaded intact isolated right ventricular trabeculae. XOR activity was determined fluorometrically in myocardial homogenates. Results: In failing myocardium, 100 μM oxypurinol significantly increased systolic twitch tension (by 87 and 92% at 1.0 and 1.5 mM extracellular [Ca2+], respectively), without altering [Ca2+]i transient amplitude. Oxypurinol did not alter the midpoint or cooperativity of the steady-state tension-[Ca2+]i relationship, but significantly enhanced maximum Ca2+-activated tension by 75% in failing myocardium. Oxypurinol also exerted a positive inotropic effect in failing myocardium, which was, however, of significantly smaller relative magnitude. Failing rat myocardium exhibited higher XOR activity than nonfailing myocardium, and this activity was largely suppressed in oxypurinol-treated preparations. Conclusions: The magnitude of functional improvement with XOR inhibitors depends on the initial level of XOR activity. Specifically, the inotropic actions of oxypurinol are more pronounced in failing rat myocardium, a tissue that exhibits enhanced XOR activity. Our findings rationalize how XO inhibitors boost cardiac contractility and improve mechanoenergetic coupling, and why the effects might be relatively ‘selective’ for heart failure.