FUNCTIONAL TISSUE AND DEVELOPMENTAL SPECIFICITIES OF MYOFIBRILS AND MITOCHONDRIA IN CARDIAC-MUSCLE

To understand the factors underlying the functional differences betwee n atrial and ventricular tissues, intrinsic properties of myofibrils a nd mitochondria of atrial skinned fibers were compared with those of f ibers from adult or immature (1 and 2 weeks old) ventricular muscle. I sometric mechanical parameters were determined at various calcium conc entrations in fibers treated with Triton X-100 to solubilize all cellu lar membranes. Maximal active tension and stiffness measured at pCa 4. 5, as well as calcium sensitivity, were not different in adult atria a nd ventricles. Both force and stiffness increased in adult ventricles, while calcium sensitivity diminished in adult ventricles, compared wi th immature muscles. Myofibrillar contractile kinetics, assessed by th e rate constant of tension fall following quick stretches, were simila r in adult atria (79.7 +/- 6.9 s(-1)) and ventricles (72.4 +/- 6.8 s(- 1)) and higher in adult atria and ventricles than in immature ventricl es (24.1 +/- 2.3 s(-1) in 1-week-old rats and 49.3 +/- 4.2 s(-1) in 2- week-old rats). Sensitivity of rigor tension development to MgATP in t he presence and in the absence of phosphocreatine was not markedly dif ferent in the different tissues. Mitochondrial function was assessed i n saponin-skinned fibers. Tissue oxidative capacities, expressed as nm ol O-2 . min(-1). mg(-1) fiber dry weight, were lower in immature vent ricles and atria than in adult ventricles. Creatine failed to stimulat e respiration in ventricles of young rats and in adult atria, whereas a 74 +/- 10% increase in respiration was observed in adult ventricles. Since mitochondrial creatine kinase was present in adult atria, this suggests an absence of coupling between oxidative phosphorylation and mitochondrial creatine kinase in this tissue. Thus, adult atrial tissu e differs from neonatal ventricular tissue but it exhibits contractile properties similar to adult ventricular properties and differs from a dult ventricle mainly in metabolic properties.