Ca²⁺ loading and adrenergic stimulation reveal male/female differences in susceptibility to ischemia-reperfusion injury

To compare ischemia-reperfusion injury in males versus females under hypercontractile conditions, perfused hearts from 129J mice pretreated with 3 mmol/l Ca²⁺ or 10^-8 mol/l isoproterenol ± 10^-6 mol/l N^ω-nitro-L-arginine methyl ester (L-NAME) were subjected to 20 min of ischemia and 40 min of reperfusion while ³¹P NMR spectra were acquired. Basal contractility increased equivalently in female versus male hearts with isoproterenol- or Ca²⁺ treatment. Injury was equivalent in untreated male versus female hearts but was greater in isoproterenol or Ca²⁺-treated male than female hearts, as indicated by lower postischemic contractile function, ATP, and PCr. Endothelial nitric oxide (NO) synthase (eNOS) expression was higher in female than male hearts, neuronal NOS (nNOS) did not differ, and inducible NOS (iNOS) was undetectable. Ischemic NO production was higher in female than male hearts, and L-NAME increased injury in female isoproterenol-treated hearts. In summary, isoproterenol or high Ca²⁺ pretreatment increased ischemia-reperfusion injury in males more than females. eNOS expression and NO production were higher in female than male hearts, and L-NAME blocked female protection. Females were therefore protected from the detrimental effects of adrenergic stimulation and Ca²⁺ loading via a NOS-mediated mechanism.