QUANTITATION OF MYOGLOBIN SATURATION IN THE PERFUSED HEART USING MYOGLOBIN AS AN OPTICAL INNER FILTER

Quantitation of metabolic parameters using the technique of cardiac surface fluorescence is complicated by motion and changes in tissue absorption. Because ratio fluorescence methodology can be applied to eliminate motion-induced errors, in the current study, we used a ratio fluorescence technique to evaluate myoglobin saturation in the perfused rat heart, since myoglobin is the major oxygen-dependent light absorbing species in this tissue. Changes in myoglobin saturation can affect surface fluorescence measurements as a result of the inner filter effect. Optical scans of heart extracts indicated the major absorption peak is due to myoglobin and its peak wavelength shifts from 415 to 430 nm upon deoxygenation. To monitor this shift in hearts, the isolated perfused heart was loaded covalently with the fluorescent dye 7-diethylaminocoumarin-3-carboxylic acid by brief perfusion with the succinimidyl ester. This dye has an excitation maximum in the region of maximal absorption by myoglobin and allows for monitoring myoglobin oxygenation using the inner filter effect. The dye localized to endothelial cells and increased the surface fluorescence in this wavelength region similar to 50-fold above background levels without affecting cardiac function. An equation was derived to estimate the fraction of myoglobin in the oxygenated state from changes in the fluorescence 415/430 excitation ratio. From this fraction, the average Po, in the environment of myoglobin was estimated under several perfusion conditions. We report that retrograde perfusion in the Langendorff mode at either 60 or 120 mmHg pressure resulted in full oxygenation of myoglobin with use of cell-free perfusate equilibrated with 95% O-2. Decreased cardiac work with butanedione monoximine or complete arrest with mepivacaine decreased the perfusate oxygen concentration required to maintain myoglobin in the oxygenated state. The data are consistent with 1) the oxygenation of myoglobin being determined by a balance between oxygen delivery to and utilization by the heart, and 2) the existence of steep oxygen gradients from the vascular space to myoglobin in the heart.