Nature of [Ca2+]i transients during ventricular fibrillation and quinidine treatment in perfused rat hearts.

We studied intracellular Ca2+ concentration ([Ca2+]i) and the electrocardiographic signals during pacing-induced ventricular fibrillation (VF) and quinidine treatment (0.4 mg/min) using surface fluorometry in indo 1-acetoxymethyl ester (AM)-loaded perfused rat hearts. [Ca2+]i was evaluated as the indo 1 fluorescence ratio (F400/F510) and expressed as a percentage of the control amplitude of F400/F510 transients. F400/F510 increased to approximately 250% during 2- (n = 14) or 20-min (n = 9) VF. Quinidine significantly decreased F400/F510 by 60% after 2-min VF; however, this effect was blunted after 20-min VF. After 2-min VF, F400/F510 and left ventricular pressure recovered almost to the control level. However, recovery of F400/F510 and ventricular function was poor after 20-min VF. The relationship between [Ca2+]i and the electrocardiogram (ECG) during VF was evaluated by power spectrum analysis of F400/F510 and ECG signals. During VF (25 +/- 3 Hz) with high irregularity, there were no clear [Ca2+]i transients (n = 110). When the cardiac rhythm (22 +/- 3 Hz) was regular, including ventricular tachycardia, there were recognizable [Ca2+]i signals with dominant frequencies that were the same (n = 2), one-half (n = 12), or one-third (n = 1) of the ECG frequencies. The highest frequency of the [Ca2+]i transients was 19 Hz. During quinidine treatment, the VF rate decreased significantly, and clear [Ca2+]i transients were noted in all records responding to every one or two ECG signals. The conclusions were the following: 1) [Ca2+]i responds to electrical signals rapidly (up to 19 Hz) during VF. This fast [Ca2+]i response is a probable cause of high [Ca2+]i during VF. 2) Quinidine decreased [Ca2+]i after 2-min VF possibly in part by slowing the VF and [Ca2+]i transients rates. 3) 20-min VF caused [Ca2+]i overload and poor functional recovery after defibrillation.