Improved skeletal muscle Ca2+ regulation in vivo following contractions in mice overexpressing PGC-1{alpha}

In skeletal muscle, resting intracellular Ca²⁺ concentration ([Ca²⁺]_i) homeostasis is exquisitely regulated by Ca²⁺ transport across the sarcolemmal, mitochondrial, and sarcoplasmic reticulum (SR) membranes. Of these three systems, the relative importance of the mitochondria in [Ca²⁺]_i regulation remains poorly understood in in vivo skeletal muscle. We tested the hypothesis that the capacity for Ca²⁺ uptake by mitochondria is a primary factor in determining [Ca²⁺]_i regulation in muscle at rest and following contractions. Tibialis anterior muscle of anesthetized peroxisome proliferator-activated receptor- coactivator-1α (PGC-1α)-overexpressing (OE, increased mitochondria model) and wild-type (WT) littermate mice was exteriorized in vivo and loaded with the fluorescent probe fura 2-AM, and Rhod 2-AM Ca²⁺ buffering and mitochondrial [Ca²⁺] were evaluated at rest and during recovery from fatiguing tetanic contractions induced by electrical stimulation (120 s, 100 Hz). In addition, the effects of pharmacological inhibition of SR (thapsigargin) and mitochondrial [carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (FCCP)] function were examined at rest. [Ca²⁺]_i in WT remained elevated for the entire postcontraction recovery period (+6 ± 1% at 450 s), but in PGC-1α OE [Ca²⁺]_i returned to resting baseline within 150 s. Thapsigargin immediately and substantially increased resting [Ca²⁺]_i in WT, whereas in PGC-1α OE this effect was delayed and markedly diminished (WT, +12 ± 3; PGC-1α OE, +1 ± 2% at 600 s after thapsigargin treatment, P < 0.05). FCCP abolished this improvement of [Ca²⁺]_i regulation in PGC-1α OE. Mitochondrial [Ca²⁺] accumulation was observed in PGC-1α OE following contractions and thapsigargin treatment. In the SR, PGC-1α OE downregulated SR Ca²⁺-ATPase 1 (Ca²⁺ uptake) and parvalbumin (Ca²⁺ buffering) protein levels, whereas mitochondrial Ca²⁺ uptake-related proteins (Mfn1, Mfn2, and mitochondrial Ca²⁺ uniporter) were upregulated. These data demonstrate a heretofore unappreciated role for skeletal muscle mitochondria in [Ca²⁺]_i regulation in vivo following fatiguing tetanic contractions and at rest.

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