Quantitative double-tracer autoradiography with tritium and carbon-14 using imaging plates: Application to myocardial metabolic studies in rats

A system for ³H- and ¹⁴C-labeled macroautoradiography was developed that is able to quantify the tissue radioactivity of two tracers using imaging plates. Methods: Discrimination between electrons emitted from ³H and ¹⁴C is possible on the basis of their different energy distributions. The general use imaging plate with a protective layer detects ¹⁴C radioactivity, but it does not detect ³H radioactivity which has a lower energy distribution than ¹⁴C. Recently, a ³H-sensitive imaging plate without a protective layer was developed. The ³H distribution image is obtained by subtracting the UR image from the TR image. For quantification of the tissue radioactivity of ³H and ¹⁴C, we obtained tissue equivalent values (Bq/mg) of commercially available ³H- and ¹⁴C-labeled graded standards using different dilutions of labeled heart paste and liquid scintillation counting. Using the ³H- and ¹⁴C-labeled graded standards, we confirmed the validity of the quantification of the ³H-autoradiographic intensity using this subtraction method. We applied this method to a rat model of acute myocardial ischemia to compare regional myocardial free fatty acid uptake determined by β-methyl[1-¹⁴C]heptadecanoic acid to glucose uptake determined by 2-deoxy-D-[1-³H]glucose. Results: Free fatty acid uptake was decreased sharply at the ischemic periphery where glucose uptake was preserved. Conclusion: This double-tracer autoradiography with ³H and ¹⁴C which has high sensitivity, a high spatial resolution of 50 μm and superior linearity with a wide dynamic range of 10⁴ to 10⁵ allows accurate quantification of the tissue radioactivity of the two radiopharmaceuticals.