Calcium transient alternans in blood-perfused ischemic hearts: observations with fluorescent indicator Fura Red

Ischemia produces striking electrophysiological abnormalities in blood-perfused hearts that may be caused, in part, by effects of ischemia on intracellular calcium. To test this hypothesis, intracellular Ca2+ concentration ([Ca2+](i)) transients were recorded from the epicardial surface of blood-and saline-perfused rabbit hearts using the long-wavelength indicator Fura Red. Calcium transients were much larger than the movement artifact, representing up to 29% of the total signal. Switching the perfusate from saline to blood did not affect the time course of the transients or the apparent level of [Ca2+](i). Compartmentation of Fura Red fluorescence was estimated by exposure to Mn2+. The results were cytosol 60 +/- 3%, organelles 12 +/- 2%, and autofluorescence plus partly deesterified Fura Red 29 +/- 4%. [Ca2+](i) transients were calibrated in situ by perfusion of the extracellular space with high-Ca2+ and Ca2+-free EGTA solutions. Peak systolic [Ca2+]( )was 663 +/- 74 nM, and end-diastolic [Ca2+](i) was 279 +/- 59 nm. Ischemia was produced by interruption of aortic perfusion for 2.5 min during pacing (150 beats/min). Ischemia produced broadening of the [Ca2+](i) transient, along with beat-to-beat alternations in the peak systolic and end-diastolic level of [Ca2+](i) (calcium transient alternans). [Ca2+](i) transient alternans occurred in 82% of blood-perfused hearts vs. 43% of saline-perfused hearts. The discrepancy between large and small transients (mean alternans ratio) was larger in the blood-perfused hearts (0.23 +/- 0.04 vs. 0.07 +/- 0.03, P = 0.005). These observations are important because of the apparent relationship of [Ca2+](i) transient alternans to electrical alternans and arrhythmias during ischemia.