Abstract
One of the most important cytosolic Ca2+ buffers present in mouse fast-twitch myofibers, but not in human myofibers, is parvalbumin (PV). Previous work using conventional PV gene (PV) knockout (PV-KO) mice suggests that lifelong PV ablation increases fatigue resistance, possibly due to compensations in mitochondrial volume. In this work, PV ablation was induced only in adult mice (PV-KO), and contractile and cytosolic Ca2+ responses during fatigue were studied in isolated muscle and intact single myofibers. Results were compared with control littermates (PV-Ctr). We hypothesized that the reduced myofiber cytosolic Ca2+ buffering developed only in adult PV-KO mice leads to a larger cytosolic free Ca2+ concentration ([Ca2+]c) during repetitive contractions, increasing myofiber fatigue resistance. Extensor digitorum longus (EDL) muscles from PV-KO mice had higher force in unfused stimulations (∼50%, P < 0.05) and slowed relaxation (∼46% higher relaxation time, P < 0.05) versus PV-Ctr, but muscle fatigue resistance or fatigue-induced changes in relaxation were not different between genotypes (P > 0.05). In intact single myofibers from flexor digitorum brevis (FDB) muscles, basal and tetanic [Ca2+]c during fatiguing contractions were higher in PV-KO (P < 0.05), accompanied by a greater slowing in estimated sarcoplasmic reticulum (SR) Ca2+-pumping versus PV-Ctr myofibers (∼84% reduction, P < 0.05), but myofiber fatigue resistance was not different between genotypes (P > 0.05). Our results demonstrate that although the estimated SR Ca2+ uptake was accelerated in PV-KO, the total energy demand by the major energy consumers in myofibers, the cross-bridges, and SR Ca2+ ATPase were not altered enough to affect the energy supply for contractions, and therefore fatigue resistance remained unaffected.NEW & NOTEWORTHY Parvalbumin (PV) is a cytosolic Ca2+ buffer that is present in mouse myofibers but not in human muscle. We show that inducible knockout of PV leads to increases in myofiber cytosolic free Ca2+ concentrations and slowing of Ca2+ pumping during fatigue versus control mice. However, PV ablation does not interfere with fatigue-induced slowing in relaxation or fatigue resistance. These data support the use of mouse muscle as a suitable model to investigate human muscle fatigue.