Long-term protection and mechanism of pacing-induced postconditioning in the heart

Abstract

Brief periods of ventricular pacing during the early reperfusion phase (pacing-induced postconditioning, PPC) have been shown to reduce infarct size as measured after 2 h of reperfusion. In this study, we investigated (1) whether PPC leads to maintained reduction in infarct size, (2) whether abnormal mechanical load due to asynchronous activation is the trigger for PPC and (3) the signaling pathways that are involved in PPC. Rabbit hearts were subjected to 30 min of coronary occlusion in vivo, followed by 6 weeks of reperfusion. PPC consisted of ten 30-s intervals of left ventricular (LV) pacing, starting at reperfusion. PPC reduced infarct size (TTC staining) normalized to area at risk, from 49.0 +/- 3.3% in control to 22.9 +/- 5.7% in PPC rabbits. In isolated ejecting rabbit hearts, replacing LV pacing by biventricular pacing abolished the protective effect of PPC, whereas ten 30-s periods of high preload provided a protective effect similar to PPC. The protective effect of PPC was neither affected by the adenosine receptor blocker 8-SPT nor by the angiotensin II receptor blocker candesartan, but was abrogated by the cytoskeletal microtubule-disrupting agent colchicine. Blockers of the mitochondrial K(ATP) channel (5HD), PKC (chelerythrine) and PI3-kinase (wortmannin) all abrogated the protection provided by PPC. In the in situ pig heart, PPC reduced infarct size from 35 +/- 4 to 16 +/- 12%, a protection which was abolished by the stretch-activated channel blocker gadolinium. No infarct size reduction was achieved if PPC application was delayed by 5 min or if only five pacing cycles were used. The present study indicates that (1) PPC permanently reduces myocardial injury, (2) abnormal mechanical loading is a more likely trigger for PPC than electrical stimulation or G-coupled receptor stimulation and (3) PPC may share downstream pathways with other modes of cardioprotection.

Fig. 1

Experimental protocol for studying the role of abnormal mechanical load and electrical stimulation in isolated rabbit hearts. Regional ischemia was induced by coronary ligation for 30 min. The hearts were atrially paced at 240 bpm during the entire protocol, except during the first 10 min of reperfusion (gray blocks), where simultaneous atrial and LV posterior wall (or biventricular in one series) pacing at 240 bpm was performed. Drug infusion was started 5 min before the start of reperfusion and was continued till the end of LV pacing. The ten open blocks in the high preload group indicate the periods with increased preload (from 6 to 12 mmHg) by raising the atrial reservoir

Fig. 2

Experimental protocol for the in vivo studies of pig hearts. The format is equal to that of Fig. 1

Fig. 3

Infarct size, normalized for the area at risk, in rabbit hearts 6 weeks after the ischemia/reperfusion procedure (left) and representative hearts (right). Open bar represents controls and filled bar represents PPC

Fig. 4

Time course of fractional shortening and LV end-diastolic diameter during the 6-week follow-up in rabits (right). Open bar and open symbols represent controls and filled bar and symbols represent PPC. * P < 0.05 as compared to time =0 (pre-ischemia)

Fig. 5

Infarct size, normalized to the area at risk, in isolated rabbit hearts. Infarct size in the control group is compared with that in hearts subjected to ten cycles of 30 s of: left ventricular (LV) pacing, high preload and biventricular (BiV) pacing as well as ten cycles of 30 s of LV pacing plus administration of 8-SPT (adenosine receptor blocker, PPC+8-SPT), colchicine (disrupting microtubules, PPC+colchicine), 5HD (mitochondrial K_ATP channel blocker, PPC+5HD), chelerythine (PKC inhibitor, PPC+Chel) and wortmannin (PI3-kinase inhibitor, PPC+Wort). The drugs were added to the perfusion medium between 25 min of coronary occlusion and 10 min of reperfusion (see Fig. 1). Values are mean ± SD. *P < 0.05, as compared to control. The number of animals is indicated in Table 1

Fig. 6

Algorithm for pacing postconditioning. Infarct size (expressed as percentage of the area at risk) is plotted as a function of the total duration of pacing. Each symbol represents one rabbit heart

Fig. 7

Infarct size in pig hearts in vivo (expressed as percentage of the area at risk), comparing control hearts (n = 5) with those subjected to ten cycles of RV pacing (PPC, n = 6), five cycles of RV pacing (PPC-5, n = 5), ten cycles of RV pacing, but starting 5 min after reperfusion (PPC-D, n = 5) and ten cycles of RV pacing in combination with administration of gadolinium (PPC-Gd, n = 5). Values are mean ± SD. *P < 0.05, as compared to control