Phenotype‐Specific Permeability Transition Pore Regulation in Rat Skeletal Muscle

Abstract

The aim of this study was to determine whether the susceptibility of mitochondria to opening of the permeability transition pore (PTP) varies according to muscle phenotype represented by the soleus (SOL: 95% type I fibers) and superficial white gastrocnemius (WG: 92% type IIb fibers). Sensitivity to Ca2+‐induced PTP opening was determined with a novel approach using permeabilized myofibers devoid of their contractile apparatus (phantom fibers). Ca2+ uptake in this preparation is specific to mitochondria and calcium retention capacity (CRC) is a good indicator of PTP sensitivity. Mitochondrial volume density was ~ 2 fold greater in SOL vs. WG, as judged by respiratory capacity per mg fiber weight and activity of the marker enzyme citrate synthase (CS). CRC values were 2.2 fold lower in phantom fibers from SOL compared to WG when expressed per unit of CS (CRC: 36.6±9.9 vs. 81.0±26.2 pmol Ca2+/mIU CS; p<0,001; n=4), indicating a pronounced vulnerability to PTP opening in type I vs. IIb fibers. A similar phenomenon was also observed in isolated mitochondria (CRC: <83 and 121 ± 60 nmol Ca2+/mg prot. in SOL and WG). This phenomenon was partly due to a 2‐fold increase in endogenous matrix Ca2+ levels in mitochondria from SOL compared to WG, but not to enhanced ROS production. Expression of the PTP regulatory protein cyclophilin‐D was similar in mitochondria from both muscles. However, expression of ANT, a protein reported to be involved in pore formation and which contributes to Ca2+‐sensitivity of the PTP, was significantly 2.3 fold higher in SOL compared to WG. The present study provides evidence that regulation of the PTP varies according to the phenotype of muscle fibers, which may impact on muscle cell death under conditions of stress.Funded by NSERC.