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(0), 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 mumol/L gramicidin. Ca2+ transients and contractions rapidly disappeared after the induction of ischemia. In the control group, diastolic Ca2+ 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 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.