INTRACELLULAR CA2-ACTIVITY IN ISCHEMIC RABBIT PAPILLARY-MUSCLE - EFFECTS OF PRECONDITIONING AND METABOLIC BLOCKADE(, INTERCELLULAR ELECTRICAL COUPLING, AND MECHANICAL)

During myocardial ischemia, electrical uncoupling and contracture hera ld irreversible damage. In the present study, we tested the hypothesis that an increase of intracellular Ca2+ is an important factor initiat ing these events. Therefore, we simultaneously determined tissue resis tance, mechanical activity, pH(o), and intracellular Ca2+ (with the fl uorescent indicator indo 1, Molecular Probes, Inc) in arterially perfu sed 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) pre parations pretreated with 1 mmol/L iodoacetate to block anaerobic meta bolism and minimize acidification during ischemia. In a fourth experim ental group, intracellular Ca2+ was increased under nonischemic condit ions by perfusing with 0.1 mmol/L ionomycin and 0.1 mu mol/L gramicidi n. Ca2+ transients and contractions rapidly disappeared after the indu ction 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 min utes of ischemia; and contracture, after 12.6+/-1.5 minutes of ischemi a (mean+/-SEM). Preconditioning significantly postponed Ca2+ rise, unc oupling, and contracture (21.5+/-4.0, 24.0+/-4.1, and 23.0+/-5.3 minut es of ischemia, respectively). Pretreatment with iodoacetate significa ntly advanced these events (1.9+/-0.7, 3.6+/-0.9, and 1.9+/-0.2 minute s of ischemia, respectively). In all groups, the onset of uncoupling a lways followed the start of Ca2+ rise, whereas the start of contractur e was not different from the rise in Ca2+. Perfusion with ionomycin an d gramicidin permitted estimation of a threshold [Ca2+] for electrical uncoupling of 685+/-85 nmol/L. In conclusion, the rise in intracellul ar Ca2+ is the main trigger for cellular uncoupling during ischemia. C ontracture is closely associated with the increase of intracellular Ca 2+ during ischemia.