Male/female differences in intracellular Na⁺regulation during ischemia/reperfusion in mouse heart

We previously showed that β-adrenergic stimulation revealed male/female differences in susceptibility to ischemia/reperfusion (I/R) injury. To explore whether altered [Na⁺]_i regulation is involved in the mechanism of this sex difference, we measured [Na⁺] _i by ²³Na NMR spectroscopy in isolated perfused mouse hearts. [Na⁺]_i increased to 195 ± 3% (mean ± S.E.) of the pre-ischemic level at 20 min of ischemia in male hearts, whereas [Na⁺]_i accumulation was slightly less in female hearts (176 ± 2%, P < 0.05). There was no significant difference in the recovery of contractile function after reperfusion (male: 30.6 ± 3.8%; female: 35.0 ± 1.9%; P > 0.05). If hearts were treated with isoproterenol (ISO, 10 nmol/l), males exhibited significantly poorer recovery of post-ischemic contractile function than females (male: 13.0 ± 1.9%; female: 28.1 ± 1.2%; P < 0.05), and a significantly higher [Na ⁺]_i accumulation during ischemia (male: 218 ± 8%; female: 171 ± 2%; P < 0.05). This ISO-induced male/female difference in [Na⁺]_i accumulation or contractile function was blocked by the nitric oxide synthase inhibitor, N_ω-nitro-L-arginine methyl ester (1 μmol/l). Furthermore, in ISO-treated hearts, the Na ⁺/K⁺-ATPase inhibitor, ouabain (200 μmol/l) did not abolish the male/female difference in [Na⁺]_i accumulation during I/R or functional protection. Thus the data show that the sex difference in the [Na⁺]_i regulation is mediated through a NO-dependent mechanism, and the difference in susceptibility to I/R injury appears to result from a difference in Na⁺ influx.