Rationale: Reduction of myocardial infarct size by remote ischemic preconditioning (RIPC), that is, cycles of ischemia/reperfusion in an organ remote from the heart before sustained myocardial ischemia/reperfusion, was confirmed in all species so far, including humans. Objective:

Connexin 43 (Cx43) is present at the sarcolemma and the inner membrane of cardiomyocyte subsarcolemmal mitochondria (SSM). Lack or inhibition of mitochondrial Cx43 is associated with reduced mitochondrial potassium influx, which might affect mitochondrial respiration. Therefore, we analysed the importance of mitochondrial Cx43 for oxygen consumption. Acute inhibition of Cx43 in rat left ventricular (LV) SSM by 18α glycyrrhetinic acid (GA) or Cx43 mimetic peptides (Cx43-MP) reduced ADP-stimulated complex I respiration and ATP generation. Chronic reduction of Cx43 in conditional knockout mice (Cx43Cre-ER(T)/fl + 4-OHT, 5–10% of Cx43 protein compared with control Cx43fl/fl mitochondria) reduced ADP-stimulated complex I respiration of LV SSM to 47.8 ± 2.4 nmol O2/min.*mg protein (n = 8) from 61.9 ± 7.4 nmol O2/min.*mg protein in Cx43fl/fl mitochondria (n = 10, P < 0.05), while complex II respiration remained unchanged. The LV complex I activities (% of citrate synthase activity) of Cx43Cre-ER(T)/fl+4-OHT mice (16.1 ± 0.9%, n = 9) were lower than in Cx43fl/fl mice (19.8 ± 1.3%, n = 8, P < 0.05); complex II activities were similar between genotypes. Supporting the importance of Cx43 for respiration, in Cx43-overexpressing HL-1 cardiomyocytes complex I respiration was increased, whereas complex II respiration remained unaffected. Taken together, mitochondrial Cx43 is required for optimal complex I activity and respiration and thus mitochondrial ATP-production.

Ischemic conditioning before (ischemic preconditioning, IPC) or after (ischemic postconditioning, POCO) sustained myocardial ischemia/reperfusion (I/R), induced locally or remotely from the heart (remote IPC, RIPC), reduces infarct size. However, none of the identified signaling steps of ischemic conditioning was robust across models and species to be successfully translated to humans. In prior separate studies in pigs, activation of signal transducer and activator of transcription 3 (STAT3) was causal for infarct size reduction by IPC, POCO, and RIPC but it remains unclear whether or not STAT3 is truly a common denominator of cardioprotective signaling. We therefore, now analyzed the phosphorylation of STAT3 and other signaling proteins in left ventricular biopsies from our prior studies on IPC, POCO and RIPC in one approach. We developed a strategy for the quantification of protein phosphorylation in multiple samples from many experiments on different gels/membranes by Western blot. Along with reduced infarct size, the ratio of STAT3 phosphorylation/total STAT3 protein at early reperfusion was significantly increased by IPC (IPC 2.0 ± 0.3 vs. I/R 1.2 ± 0.2 arbitrary units), but only trendwise by POCO and RIPC (1.3 ± 0.2; 1.4 ± 0.2 arbitrary units); storage time for IPC samples was shorter than for POCO and RIPC samples. No other signaling protein phosphorylation was associated with reduced infarct size. We confirmed STAT3 phosphorylation with IPC. For POCO and RIPC we could not reproduce the findings from our earlier more focused studies. At this point, we can not distinguish between lack of robustness of the biological signal and methodological issues of our retrospective approach.

SummaryAlthough remote ischemic pre-conditioning (RIPC) reduced infarct size in animal experiments and proof-of-concept clinical trials, recent phase III trials failed to confirm cardioprotection during cardiac surgery. Here, we characterized the kinetic properties of humoral factors that are released after RIPC, as well as the signal transduction pathways that were responsible for cardioprotection in an ex vivo model of global ischemia reperfusion injury. Venous blood from 20 healthy volunteers was collected at baseline and 5 min, 30 min, 1 h, 6 h, and daily from 1 to 7 days after RIPC (3 × 5/5 min upper-limb ischemia/reperfusion). Plasma-dialysates (cut-off: 12 to 14 kDa; dilution: 1:20) were infused into Langendorff-perfused mouse hearts subjected to 20/120 min global ischemia/reperfusion. Infarct size and phosphorylation of signal transducer and activator of transcription (STAT)3, STAT5, extracellular-regulated kinase 1/2 and protein kinase B were determined. In a subgroup of plasma-dialysates, an inhibitor of STAT3 (Stattic) was used in mouse hearts. Perfusion with baseline-dialysate resulted in an infarct size of 39% of ventricular mass (interquartile range: 36% to 42%). Perfusion with dialysates obtained 5 min to 6 days after RIPC significantly reduced infarct size by ∼50% and increased STAT3 phosphorylation beyond that with baseline-dialysate. Inhibition of STAT3 abrogated these effects. These results suggest that RIPC induces the release of cardioprotective, dialyzable factor(s) within 5 min, and that circulate for up to 6 days. STAT3 is activated in murine myocardium by RIPC-induced human humoral factors and is causally involved in cardioprotection.

Ischemic preconditioning (IPC), i.e., brief episodes of nonlethal myocardial ischemia-reperfusion (I/R) before sustained ischemia with subsequent reperfusion, reduces infarct size in all species tested so far, including humans. In rodents, the cardioprotective signal transduction causally involves an activation of Akt, ERK1/2, and STAT3. However, there are apparent species differences in the signal transduction between rodents and larger mammals such as pigs, where data on IPC's signal transduction are inconsistent for Akt and ERK1/2. The role of STAT3 has not yet been analyzed. Pigs were subjected to 60 min of left anterior descending coronary artery occlusion and 180 min of reperfusion without or with IPC (2 cycles of 3-min occlusion separated by 2 min of reperfusion 15 min before sustained I/R). Infarct size was analyzed by triphenyl tetrazolium chloride staining, and Akt, ERK1/2, and STAT3 phosphorylation was quantified in myocardial biopsies taken at baseline and early reperfusion. AG490 was used to block the STAT3 signaling pathway. IPC reduced infarct size (%area at risk; mean ± SE, I/R, 45 ± 3 vs. IPC, 18 ± 3, < 0.05). Akt and ERK1/2 phosphorylation was increased at early reperfusion without and with IPC. In contrast, STAT3 phosphorylation at early reperfusion was only increased with IPC (%baseline; mean ± SE, I/R, 126 ± 29 vs. IPC, 408 ± 147, < 0.05). AG490 prevented the IPC-related increase of STAT3 phosphorylation at reperfusion (%baseline; mean ± SE, 82 ± 12) and abolished IPC's cardioprotection (%area at risk; mean ± SE, 35 ± 4). In pigs, increased phosphorylation of STAT3 is causally involved, whereas Akt and ERK1/2 seem to play no role in IPC's cardioprotection. In pig hearts in situ, ischemic preconditioning (IPC) causally involves increased phosphorylation of STAT3, whereas Akt and ERK1/2 play no role for cardioprotection. The cardioprotective signal transduction of IPC is similar to that of ischemic postconditioning and remote IPC in pigs.