13N-labeled L-amino acids for in vivo assessment of local myocardial metabolism.

The hot cell synthesis of sterile, pyrogen-free 13N-labeled L-amino acids was accomplished by employing the appropriate immobilized enzymes on a CNBr-activated Sepharose support and using remote, semiautomated systems. The syntheses were completed 6-12 min after cyclotron production of [13N]ammonia. Myocardial time-activity curves after intracoronary injection of 13N-labeled L-amino acids in dogs were triexponential in both normal and ischemic myocardium. Higher retention of 13N activity was observed in ischemic segments. Positron computed tomography imaging also showed increased uptake of 13N-labeled L-glutamate and L-alanine in ischemic segments compared with normal myocardium when blood flow corrections were made. Myocardial transaminases are primarily responsible for the observed retention fractions. It suggests the participation of the carbon skeletons of these amino acids in the Krebs cycle.