Age-dependent changes of relaxation and its load sensitivity in rat cardiac muscle

Abstract

The relaxation phase and its load dependence were studied in papillary muscles isolated from the left ventricle of rats of the following ages: 20 days, 2, 8, 18, and 24 months. The myofibrillar ATPase activity and the force-velocity relation were determined in each age group in order to characterize the kinetic properties of the contractile material. Both shortening velocity and myofibrillar ATPase activity showed a progressive reduction with maturation and aging. This observation suggested an age-dependent decrease in cross bridge formation rate. The relaxation phase was characterized by its duration and the maximum rate of tension decline in isometric conditions, and by the speed of relengthening in isotonic conditions. Relaxation became faster and of shorter duration with maturation from 20 days to 2 months and then became slower and of longer duration with further maturation and aging. The sensitivity of relaxation to changes in length or load was evaluated by measuring how much earlier tension declined in the presence of a given length change. An increase in load sensitivity of relaxation was observed during maturation from 20 days to 8 months. This increase was followed by a reduction during aging from 8 to 24 months. Such a biphasic trend of the age-related changes in load sensitivity of relaxation could result from the interplay between the progressive decrease in cross bridge formation rate and a reduction in activation decay rate. The latter was suggested by the prolongation of the relaxation phase and by the maintenance of developed tension during aging.

Shortening velocity and myosin and myofibrillar ATPase activity related to myosin isoenzyme composition during postnatal development in rat myocardium.

“…The resulting homogenate was then placed in a centrifuge (model J2-21, Beckman Instrs., Fullerton, California) for 15 minutes at 17,3OOg at 4° C. From this preparation, washed and purified myofibrils were obtained according to a procedure derived from that proposed by Solaro et al 19 and described in detail elsewhere. 20 The ATPase activity of the myofibrillar preparation was determined from the rate of release of inorganic phosphate in an incubation medium of the following composition (mM): KQ 50, MgCla 2, NaATP 2, imidazole 20, CaCl 2 0-1.5, EGTA 0-1.6, pH 7, at 27° C. The concentrations of CaCl 2 and EGTA were varied in order to obtain pCa values from 4.3 to 8. The pCa value in the incubation medium was calculated by means of a computer program similar to that described by Fabiato and Fabiato.…”

Sarcomeric Auto-Oscillations in Single Myofibrils From the Heart of Patients With Dilated Cardiomyopathy

“…Indeed, it has been reported that myocardial shortening velocity decreases with aging in association with lowering of actomyosin ATPase. 38 However, no significant correlations were found between age and any parameter tested in either myofibrils or fibers, with or without DCM ( Figure V in the Data Supplement). It can therefore be said that the changes in the Ca-SPOC parameters on DCM are not associated with aging.…”

Intrauterine exposure to chronic hypoxia in the rat leads to progressive diastolic function and increased aortic stiffness from early postnatal developmental stages

“…In humans, for instance, decreasing heart rate may provide more time for early filling resulting in progressively increasing E and E’ velocities (Eidem et al, ). Finally, IVRT (both absolute and relative to the cardiac cycle length) progressively decreases in the developing rat, a finding suggestive of more efficient relaxation with age (Cappelli, Tortelli, Zani, Poggesi, & Reggiani, ), whereas in humans, the absolute IVRT remains unchanged from neonatal to adolescent stages, shortening only relative to the cardiac cycle length (Eidem et al, ).…”