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. 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 beta-methyl[1-14C]heptadecanoic acid to glucose uptake determined by 2-deoxy-D-[1-3H]glucose. Free fatty acid uptake was decreased sharply at the ischemic periphery where glucose uptake was preserved. This double-tracer autoradiography with 3H and 14C which has high sensitivity, a high spatial resolution of 50 microns and superior linearity with a wide dynamic range of 10(4) to 10(5) allows accurate quantification of the tissue radioactivity of the two radiopharmaceuticals.