Separation of fluorescence signals from Ca 2+ and NADH during cardioplegic arrest and cardiac ischemia

Abstract

Determinations of intracellular [Ca 2+] i during ischemia using fluorescent indicators are hampered by overlapping cellular autofluorescence (AF), which largely depends on NADH. If Ca 2+ is to be determined under different kinds of ischemia, signal separation merits special attention. We used triple wavelength excitation fluorescence to separate autofluorescence from [Ca 2+]-dependent fura-2 fluorescence. Excitation at 360 nm served as third, Ca 2+-insensitive wavelength. Using an appropriate evaluation procedure, we separated Ca 2+-dependent signals from autofluorescence which is semiquantitatively associated with NADH, an indicator of the cellular redox state. We compared changes of [Ca 2+] i in isolated hearts during ischemia following cardioplegic arrest with those after transient stop of nutritive perfusion. We observed [Ca 2+] transients in spontaneously beating hearts, persisting during ischemic episodes, and an increase of mean [Ca 2+] i. In contrast, cardioplegic arrest stopped periodical [Ca 2+] i transients and heart beats simultaneously. [Ca 2+] i remained at diastolic values, tended to decrease during the first minutes of cardioplegic arrest and then increased slowly. Autofluorescence increased under both conditions. During ischemia, this increase was faster than in cardioplegia experiments. It started after the last heart beat despite persisting perfusion. Our measurements demonstrate that rhythmical heart beat is essential for sufficient perfusion. Reduced [Ca 2+] i under cardioplegic arrest may influence metabolism.