The lanthanide cation, gadolinium (Gd) attenuates post-ischemic myocardial stunning. This study tests the hypothesis that Gd also preconditions the myocardium against infarction following ischemia-reperfusion (IR) and explores potential mechanisms underlying Gd-induced cardioprotection. Regional myocardial infarction was induced in rats by occluding the left anterior descending artery for 30 min and reperfusing for 120 min. Rats (n=6/group) were administered intravenous Gd (1 to 100 micromol/kg) 15 min prior to ischemia. Hearts were excised after reperfusion to determine infarct size (IS) and area at risk (AAR). The ratio IS/AAR (%) was reduced by Gd in a "U"-shaped, dose-dependent manner. The minimum dose that reduced IS/AAR was 5 micromol/kg (52+/-5% vs. 64+/-4%), while the dose that reduced IS/AAR maximally was 20 micromol/kg (44+/-4%). Gd also reduced IS/AAR when given 1 min before reperfusion (47+/-3%) but not when given 10 s after reperfusion (60+/-3%). Cardioprotection was maintained if IR was delayed 24-72 h after Gd administration. Cardioprotection by Gd was abolished by inhibition of JAK-2 with AG-490, of p42/44 MAPK with PD98059 or of K(ATP) channels with glibenclamide. None of these agents given alone altered IS/AAR compared with controls. Inhibition of JAK-2 also blocked Gd-induced delayed cardioprotection. Gd may have broad potential roles in IR, as it conferred immediate cardioprotection when given prior to ischemia or prior to reperfusion and delayed cardioprotection for up to 72 h after administration. The mechanism underlying Gd-induced preconditioning appears to be multi-factorial, involving JAK-2, STAT-3 and p44 MAPK pathways, as well as K(ATP) channels.