According to the current paradigm, fatty acid (FA) utilization is increased in the diabetic heart. Since plasma levels of competing substrates such as ketone bodies are increased during diabetes, the effect of those substrates on cardiac FA handling was explored. Cardiomyocytes were isolated from control and streptozotocin-treated diabetic rats and incubated with normal (80 microM) and elevated (160 microM) palmitate concentrations in the absence or presence of ketone bodies, including acetoacetate (AcAc). Comparing control cardiomyocytes under normal conditions (80 microM, no AcAc) with diabetic cardiomyocytes (160 microM, 3 mM AcAc) showed that palmitate uptake was increased from 35.2 +/- 4.8 to 60.2 +/- 14.0 nmol x 3 min(-1) x g wet weight(-1) respectively. Under these conditions, palmitate oxidation rates were comparable (58.9 +/- 23.6 versus 53.2 +/- 18.5 nmol x 30 min(-1) x g wet weight(-1)). However, in the absence of AcAc, palmitate oxidation was significantly enhanced in diabetic cardiomyocytes, indicating that ketone bodies are able to suppress cardiac FA oxidation in diabetes. The concomitantly increased FA uptake in diabetic cells, mainly due to the elevated extracellular FA levels, may be responsible for the accumulation of FA and triacylglycerol, as observed in the diabetic heart in situ.