Protection afforded by ischemic preconditioning is not mediated by effects on cell-to-cell electrical coupling during myocardial ischemia-reperfusion

Abstract

The end-effectors of ischemic preconditioning (IPC) are not well known. It has been recently shown that transgenic mice underexpressing the gap junction protein connexin43 (Cx43) cannot be preconditioned. Because gap junctions allow spreading of cell death during ischemia-reperfusion in different tissues, including myocardium, we hypothesized that the protection afforded by IPC is mediated by effects on gap junction-mediated intercellular communication. To test this hypothesis, we analyzed the effect of IPC (5 min ischemia-5 min reperfusion x 2) on the changes in electrical impedance (four electrode probe) and impulse propagation velocity (transmembrane action potential) induced by ischemia (60 min) and reperfusion (60 min) in isolated rat hearts. IPC (n = 8) reduced reperfusion-induced lactate dehydrogenase release by 65.8% with respect to control hearts (n = 9) (P = 0.04) but had no effect on the time of onset of rigor contracture (increase in diastolic tension), electrical uncoupling (sharp changes in tissue resistivity and phase angle in impedance recordings), or block of impulse propagation during ischemia. Normalization of electrical impedance during reperfusion was also unaffected by IPC. The lack of effect of IPC on ischemic rigor contracture and on changes in tissue impedance during ischemia-reperfusion were validated under in vivo conditions in pigs submitted to 48 min of coronary occlusion and 120 min of reperfusion. IPC (n = 12) reduced infarct size (triphenyltetrazolium) by 64.9% (P = 0.01) with respect to controls (n = 17). We conclude that the protection afforded by IPC is not mediated by effects on electrical coupling. This result is consistent with recent findings suggesting that Cx43 could have effects on cell survival independent on changes in cell-to-cell communication.

Loss of ischemic preconditioning's cardioprotection in aged mouse hearts is associated with reduced gap junctional and mitochondrial levels of connexin 43

“…Because IP's cardioprotection is not dependent on the presence of gap junctions (23) and is not mediated by effects of cell-cell electrical coupling (29), it is unlikely that the loss of gap junctional Cx43 during aging is involved in the loss of cardioprotection by IP in the aged mice. Rather, the reduced content of mitochondrial Cx43, which is implicated in the trigger phase of preconditioning (17), may be related to the abolition of IP's cardioprotection in the Ͼ13-mo-old mice.…”

Molecular Basis of Cardioprotection

“…In the sarcolemma, Cx 43 is the constituent protein of gap junctions, and its inhibition prevents the spread of injury and extension of IS, 355 but whether this function is related to IPC is contentious. [356][357][358] However, with IPC, sarcolemmal Cx 43 phosphorylation and thus activity during the sustained ischemia is better preserved, 179 and this IPC-related Cx 43 activation may serve a protective function in attenuating cellular edema. 359 The predominant role of Cx 43 in cardioprotection, however, resides in the inner mitochondrial membrane.…”

Ischemic Preconditioning Protects Against Gap Junctional Uncoupling in Cardiac Myofibroblasts

“…This would result in less dephosphorylated Cx43 in gap junctions that could be acted on by PKC and subsequently become internalized, thereby preserving functional Cx43 in preconditioned hearts since preconditioning resulted in a marked preservation of Cx43 in gap junctions. A recent study in isolated rat hearts and in situ pig hearts found no effects of ischemic preconditioning on the ischemia-induced changes in electrical impedance, suggesting that intercellular electrical coupling is not important for protection (35). A very recent study by Li et al (36) found that PC protected wild-type isolated cardiomyocytes, obviously without forming gap junctions, but not Cx43-deficient myocytes, suggesting a volume regulating role of Cx43.…”