Mechanical stunning during R is sensitive to MDL-28170. Depressed mechanical function is reflected in a hyposensitization of trabecular myofilaments to Ca2+. Western analysis shows that PKM epsilon is present in R hearts.

This study compared the contractile performance of a canine right atrial trabecula with that of a macroscopically indistinguishable trabecula isolated from the right ventricular apex. The heart was removed from nine mongrel puppies weighing 6-8 kg and placed in Krebs-Ringer's bicarbonate solution. The bathing solution contained only 1.25 mmoles of Ca 2+ and was bubbled with a 95% O 2 -5% CO 2 gas mixture. Each atrial trabecula was specially selected from the right atrial appendage. Histologically, these trabeculae showed a remarkable longitudinal orientation of the fibers. At L max (the length of the muscle at which developed tension was maximum) under identical conditions of temperature, rate of stimulation, ionic milieu, pH, and O 2 and CO 2 supply, right atrial trabeculae achieved the same developed and total tensions but in a much shorter time than did ventricular trabeculae. In both muscle groups the maximum developed tension averaged about 2.5 g/mm 2 . Since L o (expressed as a fraction of L max ) was less in atrial muscle than it was in ventricular muscle, we concluded that atrial muscle can be stretched considerably more than can ventricular muscle before optimum length is reached. At any given initial muscle length, the maximum of tension rise for atrial trabeculae amounted to at least twice that for ventricular trabeculae. At any given load up to 1.5 g/mm 2 , the maximum velocity of shortening of an atrial trabecula was about three to four times that of a ventricular trabecula. These results collectively indicate that the contractile performance of the right atrial muscle is in many respects superior to that of the right ventricle, at least under the conditions of these experiments.• The working myocardial cells of the cardiac atria differ morphologically from those of the ventricles (1-14). Atrial myocytes are smaller, branch less often, have fewer if any transverse tubules, and contain specific granules that are not found in ventricular myocytes. There are also significant differences between the pharmacological responses of atrial and ventricular muscles (15,16). Whereas abundant information is available about the mechanical behavior of ventricular myocardium (15-23), much less is known about atrial myocardial performance, and conflicting results have been reported (15,16,24). Differences in the reported results may be due in part to considerable species variability (15, 16) and in part to the fact that published accounts have often failed to emphasize how the results were obtained under significantly different experimental conditions (15, 16). Although dogs are among the most frequently used animals in the experimental cardiovascular laboratory, we can find no report comparing the mechanical properties of canine right atrial and right ventricular trabeculae. This work was supported by U. S. Public Health Service Grant HL 11,310 from the National Heart and Lung Institute and by MIRU Contract 4367-1441.Received March 10, 1975. Accepted for publication September 12, 1975. The present study...

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...

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