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Cardiology Division, Stanford University School of Medicine, Stanford, California 94305
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+]i 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.
ischemia; metabolic inhibition; muscle contraction
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