Ageing Causes Ultrastructural Modification to Calcium Release Units and Mitochondria in Cardiomyocytes

Ageing is associated with an increase in the incidence of heart failure, even if the exist-ence of a real age‐related cardiomyopathy remains controversial. Effective contraction and relaxa-tion of cardiomyocytes depend on efficient production of ATP (handled by mitochondria) and on proper Ca2+ supply to myofibrils during excitation–contraction (EC) coupling (handled by Ca2+ release units, CRUs). Here, we analyzed mitochondria and CRUs in hearts of adult (4 months old) and aged (≥24 months old) mice. Analysis by confocal and electron microscopy (CM and EM, re-spectively) revealed an age‐related loss of proper organization and disposition of both mitochon-dria and EC coupling units: (a) mitochondria are improperly disposed and often damaged (per-centage of severely damaged mitochondria: adults 3.5 ± 1.1%; aged 16.5 ± 3.5%); (b) CRUs that are often misoriented (longitudinal) and/or misplaced from the correct position at the Z line. Immu-nolabeling with antibodies that mark either the SR or T‐tubules indicates that in aged cardiomyo-cytes the sarcotubular system displays an extensive disarray. This disarray could be in part caused by the decreased expression of Cav‐3 and JP‐2 detected by western blot (WB), two proteins involved in formation of T‐tubules and in docking SR to T‐tubules in dyads. By WB analysis, we also detected increased levels of 3‐NT in whole hearts homogenates of aged mice, a product of nitration of protein tyrosine residues, recognized as marker of oxidative stress. Finally, a detailed EM analysis of CRUs (formed by association of SR with T‐tubules) points to ultrastructural modifications, i.e., a decrease in their frequency (adult: 5.1 ± 0.5; aged: 3.9 ± 0.4 n./50 μm2) and size (adult: 362 ± 40 nm; aged: 254 ± 60 nm). The changes in morphology and disposition of mitochondria and CRUs highlighted by our results may underlie an inefficient supply of Ca2+ ions and ATP to the contrac-tile elements, and possibly contribute to cardiac dysfunction in ageing.