Abstract 1560: Role of Gap Junctions in Pharmacological Preconditioning
Introduction: It is known that phosphorylation of Cx43 by PKC induces hemichannels to close; inhibition of PKC opens them. Prolonged ischemia dephosphorylates (opens) hemichannels; ischemic preconditioning (IPC) preserves the phosphorylation state, perhaps through activation of PKC, and induces cardioprotection. However heptanol, a gap junction uncoupler, blocks IPC mediated protection.
Hypothesis: It remains unknown whether ischemic protection is better afforded by cellular isolation (uncoupling) or by maintenance of cell-to-cell coupling to distribute injurious factors such as calcium overload, or sodium that induces edema.
Methods: Accordingly, we tested pharmacologically induced preconditioning without antecedent ischemia (no IPC) in 36 isolated, Langendorff perfused, rabbit hearts exposed to 5 min infusions of 1mM Heptanol (Hept), 40 μM 18-beta-Glycyrrhetinic acid (18-β-GA), or 500nM Staurosporine (Stau) followed by 1 hr of LAD ischemia and 2 hrs of reperfusion.
Results: Neither coronary flow nor LV peak developed pressure data discriminated the groups. Infarct size (IS) assessed by tetrazolium staining related to area at risk supported our hypotheses (Data: Mean ± SEM, statistics by ANOVA). Pharmacological preconditioning by gap junction uncoupling was protective (IS: Control 48.5±4%; Hept 19.5±1%; 18-β-GA 16.3±4%, P<0.001 vs. Control). Interestingly, PKC inhibition by Staurosporine was also protective (IS: Stau 26.9±2%, P<0.001 vs. Control), but this finding was abolished when Stau infusion was followed by either Heptanol (IS: Stau + Hept 45.9± 5%, P<0.01 vs. Stau) or 18-β-GA (IS: Stau + 18-β-GA 51.6±4%, P<0.001 vs. Stau).
Conclusion: These data suggest that opening hemichannels prior to prolonged ischemia without antecedent IPC is protective (Stau alone), as is gap junction uncoupling (Hept, 18-β-GA). However, the balance of these effects was disturbed when the agents were combined, suggesting that myocytes must dynamically equilibrate cell-to-cell communication by gap junctions and intracellular signaling by PKC in the absence of IPC. Our findings also imply the existence of an alternative protective pathway, perhaps related to mitochondrial Cx43 or to the phosphorylation state of Cx43, which remains to be elucidated.