Deficiency in parvalbumin increases fatigue resistance in fast-twitch muscle and upregulates mitochondria

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Abstract
The soluble Ca2+-binding protein parvalbumin (PV) is expressed at high levels in fast-twitch muscles of mice. Deficiency of PV in knockout mice (PV −/−) slows down the speed of twitch relaxation, while maximum force generated during tetanic contraction is unaltered. We observed that PV-deficient fast-twitch muscles were significantly more resistant to fatigue than were the wild type. Thus components involved in Ca2+ homeostasis during the contraction-relaxation cycle were analyzed. No upregulation of another cytosolic Ca2+-binding protein was found. Mitochondria are thought to play a physiological role during muscle relaxation and were thus analyzed. The fractional volume of mitochondria in the fast-twitch muscle extensor digitorum longus (EDL) was almost doubled in PV −/− mice, and this was reflected in an increase of cytochrome coxidase. A faster removal of intracellular Ca2+concentration ([Ca2+]i) 200–700 ms after fast-twitch muscle stimulation observed in PV −/− muscles supports the role for mitochondria in late [Ca2+]iremoval. The present results also show a significant increase of the density of capillaries in EDL muscles of PV −/− mice. Thus alterations in the dynamics of Ca2+ transients detected in fast-twitch muscles of PV −/− mice might be linked to the increase in mitochondria volume and capillary density, which contribute to the greater fatigue resistance of these muscles.