Intracellular Ca2+, intercellular electrical coupling, and mechanical activity in ischemic rabbit papillary muscle - Effects of preconditioning and metabolic blockade

During myocardial ischemia, electrical uncoupling and contracture herald irreversible damage. In the present study, we tested the hypothesis that an increase of intracellular Ca2+ is an important factor initiating these events. Therefore, we simultaneously determined tissue resistance, mechanical activity, pH(o), and intracellular Ca2+ (with the fluorescent indicator indo 1, Molecular Probes, Inc) in arterially perfused rabbit papillary muscles. Sustained ischemia was induced in three experimental groups: (1) control, (2) preparations preconditioned with two 5-minute periods of ischemia followed by reperfusion, and (3) preparations pretreated with 1 mmol/L iodoacetate to block anaerobic metabolism and minimize acidification during ischemia. In a fourth experimental group, intracellular Ca2+ was increased under nonischemic conditions by perfusing with 0.1 mmol/L ionomycin and 0.1 mu mol/L gramicidin. Ca2+ transients and contractions rapidly disappeared after the induction of ischemia. In the control group, diastolic Ca2+ be an to rise after 12.6+/-1.3 minutes of ischemia; uncoupling, after 14.5+/-1.2 minutes of ischemia; and contracture, after 12.6+/-1.5 minutes of ischemia (mean+/-SEM). Preconditioning significantly postponed Ca2+ rise, uncoupling, and contracture (21.5+/-4.0, 24.0+/-4.1, and 23.0+/-5.3 minutes of ischemia, respectively). Pretreatment with iodoacetate significantly advanced these events (1.9+/-0.7, 3.6+/-0.9, and 1.9+/-0.2 minutes of ischemia, respectively). In all groups, the onset of uncoupling always followed the start of Ca2+ rise, whereas the start of contracture was not different from the rise in Ca2+. Perfusion with ionomycin and gramicidin permitted estimation of a threshold [Ca2+] for electrical uncoupling of 685+/-85 nmol/L. In conclusion, the rise in intracellular Ca2+ is the main trigger for cellular uncoupling during ischemia. Contracture is closely associated with the increase of intracellular Ca2+ during ischemia.