Validation of ^99mTc-labeled "4+1" fatty acids for myocardial metabolism and flow imaging Part 2: Subcellular distribution

Our group has synthesized technetium labeled fatty acids which are extracted into the myocardium and sequestered due to H-FABP binding. In this paper we further address the detailed subcellular distribution and potential myocardial metabolism of "4+1" ^99mTc-fatty acids (FA).

Experiments were conducted using isolated hearts of wistar rats as well as wildtype and H-FABP^-/--mice. Myocardium samples underwent subcellular fractionation (subsarcolemmal and intermyofibrillar mitochondria, cytosol with microsomes as well as nuclei and crude membranes) and analysis by TLC and HPLC.

The largest fraction of tissue radioactivity was associated with the cytosol (79.69 ± 8.88% of the infused dose =ID). 9.07 ± 0.95% and 3.43 ± 1.38% of the ID were associated with the subsarcolemmal and the intermyofibrillar mitochondrial fractions, respectively. In the rat heart, etomoxir, an inhibitor of the carnitin-palmitoyl transferase I, did not significantly decrease radioactivity associated with the mitochondrial fractions, whereas the myocardial extraction of ¹²³I-IPPA (13.26% vs. 49.49% in control) and the radioactivity associated with the subsarcolemmal and the intermyofibrillar mitochondrial fractions was blunted. The percentage of the ID in the mitochondrial and crude fractions increased with the number of NH-amide groups of the FA derivative. Absence of H-FABP significantly decreased the radioactivity count in the cytosolic fraction (P < 0.001). No metabolic products of a ^99mTc "4+1" FA could be detected in any isolated hearts. Myocardial ^99mTc "4+1" FA extraction reflects binding to H-FABP and membrane structures (including the mitochondrial membrane). However, the compounds do not undergo mitochondrial metabolism, because they do not reach the mitochondrial matrix.