OBJECTIVE Carnitine and its derivatives, namely propionyl-carnitine (PC), have been shown to protect cardiac metabolism and function in diabetes mellitus and ischaemic heart disease. Since diabetes is associated with abnormalities in mitochondrial metabolism of fuels, we examined the effects of PC on mitochondrial respiration in ischaemic hearts from streptozotocin-diabetic rats. METHODS Diabetes was induced in Sprague-Dawley rats by an intravenous injection of streptozotocin. Following the diagnosis of diabetes, oral PC treatment was initiated for a period of 6 weeks. After treatment, cardiac function was determined from working hearts perfused under aerobic conditions and in a separate group of hearts subjected to ischaemia and reperfusion. Mitochondrial respiration was determined under aerobic conditions and following low-flow ischaemia. RESULTS Rates of state 3 mitochondria respiration with pyruvate were significantly lower in diabetic (n = 4) hearts compared with control (n = 6) hearts (80 +/- 5 vs 112 +/- 5 nanoatoms O2/mg protein/min, respectively), but those with palmitoylcarnitine were similar (101 +/- 11 vs 106 +/- 6 nanoatoms O2/mg protein/min). Diabetic rat heart (n = 8) function, expressed as rate pressure product, was also significantly decreased compared with control (n = 8) hearts (21.5 +/- 1.0 vs 29.5 +/- 0.9 beats x mm Hg x 10(-3)/min, respectively). In PC-treated diabetic (n = 6) hearts, state 3 respiration with pyruvate was increased, and a marked improvement in left ventricular function from 21.5 +/- 1.0 to 26.0 +/- 0.6 beats x mm Hg x 10(-3)/min was observed. During low-flow ischaemia, state 3 respiration with pyruvate remained lower in diabetic (n = 5) hearts compared with control (n = 5) hearts (64 +/- 3 vs 46 +/- 5 nanoatoms O2/mg protein/min, respectively). Following treatment with PC (n = 4), however, respiration with this substrate was significantly increased to 57 +/- 4 nanoatoms O2/mg protein/min. PC was also associated with a significant improvement in cardiac function in reperfused diabetic rat (n = 4) hearts (18.4 +/- 0.2 beats x mm Hg x 10(-3)/min). CONCLUSIONS Our results showed that PC has a beneficial effect on cardiac function and increases ischaemic tolerance of the diabetic rat heart. This beneficial effect of PC can be explained, in part, as an improvement in mitochondrial metabolism of pyruvate during the actual ischaemic period.