An impairment of excitation-contraction (EC) coupling has been proposed to contribute to the age-related decline of muscle performance that accompanies ageing (EC un-coupling theory). EC coupling in muscle fibers occurs at calcium release units, or triads, which are specifically placed at sarcomere's I-A band transition. In recent publications we have shown that: a) in human muscle, the frequency of triads decreases significantly with age; and b) in mice, triads are tethered to mitochondria placed at the I band. Here we have studied the frequency, sarcomeric-localization, ultra-structure, and coupling of triads/mitochondria in EDL from male WT mice using transmission electron microscopy.

Our results indicates that the number of triads/100mm of longitudinal section in ageing mice (n=4, 25–35 months of age) decreases compared to the adult mice (n=5, 3–12 months of age): 93±9 vs 79±8. Also the i) relative volume and ii) number of mitochondria-profiles/100mm² decrease with age: 8.4±5.1 vs 7.3±4.7 and 54±7 vs. 43±6 respectively. In addition, we have assessed the positioning of mitochondria in respect to myofibrils and triads: a) the number of mitochondria at the A band (misplaced) slightly increases with age (9.3 vs. 2.5%). These changes cause a significant decrease in the number of CRUs-mitochondria couples: 39±5 vs 26±5. Our results shows how the age-related partial disarrangement and spatial re-organization of the EC coupling and mitochondrial apparatuses causes a large decrease in the number of mitochondria functionally coupled to Ca²⁺ release sites. These structural changes may contribute to the decrease of specific force and endurance of skeletal muscle associated to ageing.