Canine atrial and ventricular muscle mechanics studied as a function of age.

Abstract

SUMMARY Age-dependent differences in mechanical performance and morphometric and electron microscopic characteristics of atrial and ventricular trabeculae are described. At 3 months, atrial and ventricular trabeculae develop the same amount of force. At 9 months, the ventricular muscle develops twice as much force as its atrial counterpart, although shortening is almost identical in both muscles. At any age, velocity of shortening of atrial trabeculae is at least twice that of the ventricular muscles. StereologicaJ data indicate that atrial and ventricular working myocytes maintain fixed volume fractions of myofibrils (70%) and mitochondria (25%) between 3 and 9 months of age. A broad frequency distribution of sarcomere lengths was measured at Lmax in muscles of the younger age groups. More than 80% of sarcomeres of adult atrial and ventricular myocytes clustered around 2.05 to 2.25 /im; only 30% of sarcomeres of younger atrial myocytes and 45% of sarcomeres of younger ventricular myocytes were within that length bracket. About 45% of sarcomeres in younger atrial muscles had lengths in excess of 2.35 pm; less than 3% of sarcomeres were longer than 2.35 fun in adult atrial muscles. Sarcomere lengths cluster more and more around the mean with increasing age, suggesting that with maturation there is a more homogeneous recruitment of sarcomeres. At both ages, there is a marked difference between atrial and ventricular myocytes whether examined in terms of morphological development or functional performance. We conclude that any work correlating myocardial structure and function must account for two things: the site from which the muscle was excised and the age of the donor heart. AGE-RELATED changes in developed tension, 1 passive elastic stiffness, 2 or maximum velocity of shortening' 1 are as well established as age-related differences in the sensitivity to digitalis. 4 It also is well known that the heart is not made of a homogeneous population of working myocytes and that the differences in structure 5 '" and function 7 -a between atria and ventricles are quite marked. Age-related changes in ventricular contractility have been studied in some detail in lambs and sheep 1 and in cats, 9 " 11 but little or no attention has been given to the effects of maturation on atrial contractile performance in any species. 1 -9 In this study we describe age-dependent differences in the mechanical performance of isolated canine atrial and ventricular trabeculae. Presentation of the physiological observations is combined with morphometric and electron microscopic data from atrial and ventricular trabeculae previously studied in the myograph. MethodsThirty-six mongrel dogs of either sex were anesthetized with sodium pentobarbital, 30 mg/kg, iv. Two age groups were studied: one group was (n = 18) 13 ± 4 weeks old (3 months) and the second group (n = 18) 35 ± 5 weeks old (9 months). The entire heart was rapidly removed and placed in a beaker of Krebs-Ringer's bicarbonate solution that was gassed at 37°C with a 95% O 2 -5% CO 2 g...

Role of Thyroid State in Age-Dependent Cardiac Effects of Ouabain in Guinea Pigs

“…It should be clearly stated that thyroid hormone levels, used here to represent the thyroid state, is certainly not the only relevant parameter, and features such as thyroid hormone receptor density Our findings that the sensitivity to oua bain of the myocardium of the newborn guinea pig can be augmented by hypothyroid state are in agreement with previous studies in cat, canine and rabbit atrial and ventricu lar preparations [14,18,19,21,26], As indi cated in several studies, the age-related dif ferences in twitch characteristics can be ac counted for by developmental changes in the proportion between the fast myosin ATPase isoenzyme Vi and the slower isoenzyme V3 [18,25]. However, what has not been shown previously, and is of considerable impor tance, are the marked differences in twitch characteristics between the euthryroid and hypothyroid newborns.…”

Age-Related Changes in Tetanic Contraction of Ventricular Myocardium

“…Muscle relaxation is dependent on removal of calcium from the con tractile apparatus, largely a result of calcium sequestration by the sarcoplas mic reticulum. The age-related differences in contraction time indices dem onstrated in this and other studies [21,26], as well as the different responses of contraction time indices to caffeine, are strong evidence for maturational variation of sarcoplasmic reticulum function.…”

Hydroxyproline and passive stiffness of pressure-induced hypertrophied kitten myocardium.

“…The observaReceived for publication 14 July 1980 and in revised form 29 September 1981. tion of Bishop and Melsen (8) that pulmonary artery banding in the cat produced myocardial necrosis and fibrosis, whereas neither was found in cats with congenital pulmonary valve stenosis, supports the latter possibility. Furthermore, it is clear that collagen concentration can increase, decrease, or remain unchanged during development of hypertrophy depending on the stimulus for increased growth (9) and that an inverse relationship between connective tissue and passive stiffness can be found under certain conditions (10). Therefore, we attempted to develop a model of pressure-induced hypertrophy in which hypertrophy developed more gradually than in the usual animal models and compared passive stiffness and the hydroxyproline response in these hearts.…”