OBJECTIVE - In diabetes, when glucose consumption is restricted, the heart adapts to use fatty acid (FA) exclusively. The majority of FA provided to the heart comes from the breakdown of circulating triglyceride (TG), a process catalyzed by lipoprotein lipase (LPL) located at the vascular lumen. The objective of the current study was to determine the mechanisms behind LPL processing and breakdown after moderate and severe diabetes. RESEARCH DESIGN AND METHODS - To induce acute hyperglycemia, diazoxide, a selective, ATP-sensitive K+ channel opener was used. For chronic diabetes, streptozotocin, a β-cell-specific toxin was administered at doses of 55 or 100 mg/kg to generate moderate and severe diabetes, respectively. Cardiac LPL processing into active dimers and breakdown at the vascular lumen was investigated. RESULTS - After acute hyperglycemia and moderate diabetes, more LPL is processed into an active dimeric form, which involves the endoplasmic reticulum chaperone calnexin. Severe diabetes results in increased conversion of LPL into inactive monomers at the vascular lumen, a process mediated by FA-induced expression of angiopoietin-like protein 4 (Angptl-4). CONCLUSIONS-In acute hyperglycemia and moderate diabetes, exaggerated LPL processing to dimeric, catalytically active enzyme increases coronary LPL, delivering more FA to the heart when glucose utilization is compromised. In severe chronic diabetes, to avoid lipid oversupply, FA-induced expression of Angptl-4 leads to conversion of LPL to inactive monomers at the coronary lumen to impede TG hydrolysis. Results from this study advance our understanding of how diabetes changes coronary LPL, which could contribute to cardiovascular complications seen with this disease.