Pflugers Arch - Eur J Physiol (2002) 445:431–436 DOI 10.1007/s00424-002-0964-0 SKELETAL MUSCLE Susanna Iossa · Maria Pia Mollica · Lillà Lionetti · Raffaella Crescenzo · Monica Botta · Sonia Samec · Giovanni Solinas · Davide Mainieri · Abdul G. Dulloo · Giovanna Liverini Skeletal muscle mitochondrial efficiency and uncoupling protein 3 in overeating rats with increased thermogenesis Received: 24 July 2002 / Accepted: 26 September 2002 / Published online: 6 November 2002  Springer-Verlag 2002 Abstract To establish whether changes in skeletal mus- cle mitochondrial efficiency contribute to increased energy expenditure and decreased metabolic efficiency of overeating rats with increased thermogenesis, we measured basal proton leak, fatty acid-induced uncou- pling and uncoupling protein 3 (UCP3) content in subsarcolemmal and intermyofibrillar skeletal muscle mitochondria. Intermyofibrillar, but not subsarcolemmal, mitochondria from rats with increased thermogenesis exhibited a lower proton leak compared with controls. In both mitochondrial populations from rats with increased thermogenesis, fatty acid-induced uncoupling was in- creased significantly and a small recoupling effect of GDP was detected. In addition, intermyofibrillar and subsarcolemmal mitochondria from rats with increased thermogenesis showed higher UCP3 contents than con- trols. These results point out that metabolic efficiency in subsarcolemmal and intermyofibrillar mitochondria from rats with increased thermogenesis is differently regulated. In fact, in intermyofibrillar mitochondria both basal proton leak and fatty acid-induced uncoupling are altered, while in subsarcolemmal mitochondria only fatty acid- induced uncoupling increases. Both mitochondrial popu- lations in skeletal muscle cells from rats with increased thermogenesis display an increased fatty acid-induced uncoupling and UCP3 content, which could contribute to avoiding obesity. Keywords Uncoupling protein 3 · Mitochondrial efficiency · Uncoupling · Obesity resistance Introduction Previous work from our laboratory has shown that post- weaning rats fed a palatable high-fat diet for 15 days increase their metabolizable energy and lipid intake by about 40 and 200%, respectively [12, 13, 20], but resist the development of obesity by markedly increasing energy expenditure [12, 13, 20]. Such a change in metabolic efficiency at the whole body level, most likely via the activation of diet-induced thermogenesis, is the outcome of changes in metabolic efficiency occurring at the level of organs and tissues, which largely contribute to whole body energy expenditure. Of the candidate sites for dissipating surplus energy as heat, skeletal muscle is particularly relevant since its metabolism accounts for about 30% of resting energy expenditure in the rat [25]. At the cellular level, the major determinants of the efficiency of energy transduction are the mitochondria, where energy coming from the oxidation of metabolic fuels is converted to ATP. It is now accepted that the efficiency of the mitochondrial machinery depends on the presence of a proton leak pathway in the inner mitochon- drial membrane [2, 31], which reportedly accounts for about 20% of resting metabolic rate in rats [24]. In addition, it is well known that free fatty acids (FFA) are responsible for so-called “mild uncoupling” in mitochon- dria [10, 16, 26, 27, 29, 30, 33], due to non-ionic diffusion coupled with the transport of the fatty acid anion out of the mitochondria through various mitochondrial carriers, such as the adenine nucleotide translocase (ANT) and the glutamate-aspartate carrier and possibly uncoupling pro- teins (UCP) 2 and 3. The combination of basal leak and fatty acid-induced uncoupling could thus determine the efficiency of the mitochondrial system. Based on the above considerations, the objective of the present study was to assess whether changes in mito- chondrial efficiency in the skeletal muscle could con- tribute to the elevated energy expenditure and decreased metabolic efficiency of rats fed high-fat diet. To address this issue, we measured the basal proton leak and determined the recoupling effects of carboxyatractyloside S. Iossa ( ) ) · M.P. Mollica · L. Lionetti · R. Crescenzo · M. Botta · G. Liverini Department of General and Environmental Physiology, University of Naples “Federico II”, Via Mezzocannone 8, I-80134, Naples, Italy e-mail: susiossa@unina.it Tel.: +39-81-2535086 Fax: +39-81-2535090 R. Crescenzo · S. Samec · G. Solinas · D. Mainieri · A.G. Dulloo Institute of Physiology, University of Fribourg, Fribourg, Switzerland