Pyruvate Sparing by Fatty Acid and Glutamate in Sparrow Skeletal Muscle Mitochondria

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The ability of avian muscle mitochondria (MITO) to oxidize fatty acids is crucial as only lipid can provide the high energy density fuel required for flight. When exercising at ∼70-80% VO2max, a flying pigeon has an RQ of ∼.73, whereas a running rat has an RQ of ∼.96, indicating the bird is oxidizing primarily fat (FAT) for fuel while the rat is almost exclusively oxidizing carbohydrate (CHO). PURPOSE: To assess the control sensitivity of CHO and FAT oxidation in MITO isolated from avian skeletal muscle and examine fuel selection at the level of the mitochondrial matrix when multiple substrates are provided. METHODS: MITO were isolated from English sparrow pectoralis and rat mixed hindlimb muscles and the rate of oxygen consumption (Jo) at varying values of ΔGATP were assessed with either pyruvate (P) + malate (M) (CHO) or palmitoyl-l-carnitine (PC) + M (FAT) as substrates. ΔGATP was established by using the creatine kinase energetic clamp. The slope of ΔGATP:Jo was determined with each fuel. Next, sparrow MITO were incubated at ΔGATP = -13.0, -13.5, and -14.0 kcal/mol with P+M, P+ glutamate (G)+M, P+PC+M, G+PC+M, or P+G+PC+M as substrates. The steady state Jo and total O2 consumed were measured polarographically. Samples were taken just after MITO addition and ∼10.0 min later and fluorometrically assayed to determine P utilization. RESULTS: Sparrow MITO showed dramatic increases in Jo as ΔGATP decreased from -14.0 to -13.0 kcal/mol with both P+M and PC+M as substrates, whereas rat MITO only showed this relationship with P+M. Specifically, the slope of the ΔGATP:Jo relationship, a measure of the elasticity of oxidative flux to ΔGATP, is 7 fold greater with CHO than with FAT in rat MITO, whereas in sparrow MITO, CHO and FAT oxidation provide identical control sensitivity. This indicates sparrow MITO are able to fuel high ATP turnover rates with FAT or CHO as fuel, while in rat MITO only CHO oxidation can fuel high turnover rates, which is consistent with the mammalian requirement to oxidize CHO during exercise at and above moderate intensities. Substrate competition assays indicate, at the level of the mitochondrial matrix, either PC or G addition to P+M results in more than a 50% decrease in the rate of P utilization over the three values of ΔGATP examined, with more pronounced suppression as flux decreased and energy state increased. Adding G+PC to P+M oxidation decreased P utilization further. CONCLUSIONS: Avian MITO, at the level of the matrix, are able to maintain a high ATP turnover rate with FAT as fuel and preferentially oxidize PC (FAT) and/or G over P (CHO), possibly due to PDH inhibition by matrix redox potential and/or inhibition due to acetyl-CoA. Supported by NSF (IBN-0116997).