Intracellular Ca 2+ , Intercellular Electrical Coupling, and Mechanical Activity in Ischemic Rabbit Papillary Muscle

Abstract

During myocardial ischemia, electrical uncoupling and contracture herald irreversible damage. In the present study, we tested the hypothesis that an increase of intracellular Ca 2+ is an important factor initiating these events. Therefore, we simultaneously determined tissue resistance, mechanical activity, pH o , and intracellular Ca 2+ (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 Ca 2+ was increased under nonischemic conditions by perfusing with 0.1 mmol/L ionomycin and 0.1 μmol/L gramicidin. Ca 2+ transients and contractions rapidly disappeared after the induction of ischemia. In the control group, diastolic Ca 2+ began 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 Ca 2+ 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 Ca 2+ rise, whereas the start of contracture was not different from the rise in Ca 2+ . Perfusion with ionomycin and gramicidin permitted estimation of a threshold [Ca 2+ ] for electrical uncoupling of 685±85 nmol/L. In conclusion, the rise in intracellular Ca 2+ is the main trigger for cellular uncoupling during ischemia. Contracture is closely associated with the increase of intracellular Ca 2+ during ischemia.