Background—The cardioprotective effect of preconditioning can be exerted within 1 to 2 hours after initial ischemia, termed classical or early preconditioning, or can reappear 24 hours later as second window or late preconditioning. The objective of this study was to study the interaction between late and early preconditioning and to determine the potential underlying mechanism.

Methods and Results—Adenosine receptor agonists and a K_ATP channel opener were used to achieve early preconditioning, and Monophosphoryl lipid A (MLA) was used to induce late preconditioning. Cultured chick ventricular myocytes were used as a myocyte model of simulated ischemia and preconditioning. Prior treatment of the myocyte with MLA caused a dose-dependent decrease in the ischemia-induced myocyte injury 24 hours later, consistent with a late preconditioning effect. L-NMMA, glibenclamide, or 5-hydroxydecanoic acid administered during the ischemia blocked the MLA effect. Twenty four hours after MLA treatment, a 5-minute exposure to ischemia, adenosine, adenosine A₁ agonist CCPA, or A₃ agonist resulted in less myocyte injury during the subsequent prolonged ischemia, as compared with cells pretreated with the vehicle and subsequently exposed to the same early preconditioning stimuli. In addition to its ability to enhance the early preconditioning effect by A₁ and A₃ agonists, MLA pretreatment also increased the phorbol ester- and pinacidil-mediated early preconditioning effect.

Conclusions—This study defined a novel interaction in which the cardioprotective effect of early preconditioning is additive to that of late preconditioning and raised the possibility that both agents can be used as combined therapy in the treatment of ischemic heart disease.