Gaseous [13N]ammonia produced by the 16O(p,α)13N reaction was swept into buffered solutions to synthesize 13N-labeled L-glutamate, L-alanine, L-glutamine, and L-aspartate using enzyme columns. The myocardial residue fraction of these amino acids was studied in open-chest dogs via a single pass uptake technique, showing a triexponential time-activity curve with a slow clearance phase. Tomographic imaging with ECAT in ischemic dogs revealed that ratios of 13N-labeled amino acid activity to [13N]ammonia flow indices were consistently >1, indicating retained activity in ischemic myocardium. Additionally, a series of 16 benz-fused mesoionic thiazolo[3,2-a]pyrimidine analogues were prepared as cyclic-AMP phosphodiesterase (PDE) inhibitors. These benz-fused analogues were more active than nonfused counterparts against bovine heart PDE. Structural modifications (e.g., R1 cyclopropyl substitution, 8-ethoxy group introduction) enhanced activity, and mesoionic character was critical for potency, as nonmesoionic compounds were less active. Solubility issues precluded reliable IC50 determination for some analogues.