Pretreatment with Pertussis Toxin Blocks the Protective Effects of Preconditioning: Evidence for a G-protein Mechanism

We tested whether a pertussis toxin-sensitive G-protein mediates the protective effects of preconditioning. Thirty-one small (1 kg) rabbits were used. All rabbits experienced 30 min regional myocardial ischaemia followed by 3 h reperfusion. Infarct size was determined by tetrazolium. The percentage of the risk zone which infarcted in controls was 37.4 ± 4.6%. Preconditioning with a 5 min occlusion followed by 10 min reperfusion prior to the 30 min ischaemic insult protected against infarction (5.2 ± 2.1% infarction). 25 μg/kg pertussis toxin, administered 48 h prior to surgery, blocked G-protein signal transduction as tested by challenge with iv acetylcholine which normally slows the heart. Pertussis toxin treatment had no effect on infarct size in non-preconditioned rabbits (37.6 ± 4.7% infarction) but blocked protection in preconditioned rabbits (27.3 ± 4.3% infarction). To test the involvement of G-proteins further in preconditioning, we tested the ability of the M2 agonist carbachol to substitute for ischaemic preconditioning and protect the heart from infarction. Rabbits hearts were excised and perfused with Krebʹs buffer. Control animals subjected to 30 min of regional ischaemia and 2 h reperfusion yielded 29.4 ± 2.9% infarction. Preconditioning with 5 min global ischaemia and 10 min reperfusion prior to the 30 min regional ischaemia significantly protected the isolated heart from infarction (8.41 ± 3.26% infarction). Carbachol, infused for 5 min at 1 μM concentration, protected the hearts as well as preconditioning, yielding 8.90 ± 2.08% infarction. We concluded that preconditioning involves a pertussis toxin-sensitive G-protein and that the same G-protein couples to muscarinic M2 receptors.