Sex differences in mitochondrial Ca<sup>2+</sup> handling in mouse fast-twitch skeletal muscle in vivo
Daiki Watanabe, Koji Hatakeyama, Ryo Ikegami, Hiroaki Eshima, Kazuyoshi Yagishita, David C. Poole, Yutaka Kano
Abstract
We investigated sex differences in mitochondrial Ca 2+ handling properties in mouse fast-twitch skeletal muscle. Changes in cytoplasmic Ca 2+ concentration ([Ca 2+ ] cyto ) were measured in vivo using tibialis anterior muscles from male and female mice. The muscles were exposed to increasing concentrations of cyclopiazonic acid [CPA; sarcoplasmic reticulum (SR) Ca 2+ -ATPase inhibitor] (from 10 to 30 to 50 μM at 10 min intervals). Thirty minutes after treatment, [Ca 2+ ] cyto was increased by 31.6 ± 2.0% and 13.5 ± 4.5% of initial [Ca 2+ ] cyto in male and female muscles, respectively, and there was a significant difference between sexes. However, muscle preincubation for 5 min with 10 μM carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (an inhibitor of mitochondria Ca 2+ uptake) eradicated this difference between sexes with respect to the CPA-induced [Ca 2+ ] cyto increase. Both intermyofibrillar mitochondrial number and volume, assessed in longitudinal fiber sections, were higher in females compared with males (mitochondria number: 13.1 ± 1.0 in males vs. 19.9 ± 2.3 in females; mitochondrial volume: 0.034 ± 0.004 μm 3 /μm 3 fiber volume in males vs. 0.066 ± 0.008 μm 3 /μm 3 fiber volume in females, both P < 0.05). There were no sex differences in the content of SR Ca 2+ -ATPase, mitochondrial Ca 2+ uniporter, mitofusin (Mfn) 1, or Mfn2. These results suggest that 1) mitochondrial Ca 2+ uptake ability is greater in female than male myocytes, and 2) this superior Ca 2+ uptake ability of female myocytes is due, partly, to the higher intermyofibrillar mitochondrial content but not to the expression of mitochondrial proteins related to mitochondrial Ca 2+ uptake. NEW & NOTEWORTHY This investigation presents evidence that female versus male fast-twitch muscle exhibits a greater mitochondrial calcium ion uptake capability that is partly conferred by the higher intermyofibrillar mitochondrial volume density.