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

We previously showed that beta-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 (23)Na 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(omega)-nitro-l-arginine methyl ester (1 micromol/l). Furthermore, in ISO-treated hearts, the Na(+)/K(+)-ATPase inhibitor, ouabain (200 micromol/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.