The economy of isometric force development, myosin isoenzyme pattern and myofibrillar ATPase activity in normal and hypothyroid rat myocardium.

Abstract

Hypothyroidism was induced in Wistar-Kyoto rats by adding propylthiouracil to the drinking water (0.8 mg/ml). Initial heat, total activity-related heat, and resting heat rate were measured in left ventricular papillary muscle preparations of propylthiouracil-treated and control rats contracting isometrically at 12 beats/min (21 degrees C), using Hill type, planar vacuum-deposited bismuth and antimony thermopiles. In the propylthiouracil preparations, relative to control, time-to-peak tension increased from 288 +/- 27 (mean +/- SD) to 411 +/- 25 msec (P less than 0.001), dp/dtmax decreased from 38.3 +/- 9.5 to 20.4 +/- 3.5 g X mm-2/sec (P less than 0.001), and peak developed tension decreased from 6.11 +/- 1.75 to 4.64 +/- 0.89 g X mm-2 (P less than 0.05). In the propylthiouracil preparations, initial heat was significantly (P less than 0.001) reduced by 27 or 43% when normalized to peak twitch tension or tension-time integral, respectively. In experiments where the papillary muscles were tetanized, the slope of the linear function of total activity-related heat versus tension-time integral was decreased by 43% (P less than 0.001) in the propylthiouracil preparations, indicating an improved economy of isometric tension maintenance. The predominant myosin isoenzyme of the left ventricular wall, as well as the papillary muscle myocardium, was the V3 variety in the propylthiouracil animals, in contrast to V1 in the controls. Myofibrillar actomyosin calcium-magnesium-stimulated adenosine triphosphatase activity was significantly (P less than 0.02) decreased from 55 +/- 18 (control) to 31 +/- 8 nmol inorganic phosphate ion/mg X min (propylthiouracil).(ABSTRACT TRUNCATED AT 250 WORDS)

Forced Expression of α-Myosin Heavy Chain in the Rabbit Ventricle Results in Cardioprotection Under Cardiomyopathic Conditions

“…Compared with V3, V1 has a higher ATPase activity 22 and a faster maximum velocity of shortening but a lower tension-time integral. 23,24 These mechanical properties are inherent to the myosin molecule because V1 myosin, in the in vitro motility assay, generates faster actin filament velocities but lower average force per myosin molecule compared with V3. 22,25 Singlemolecule studies in the laser trap suggest that differences in the kinetics of the cross-bridge cycle for V1 and V3 myosins are the major determinant of the mechanical performance of cardiac myosin.…”

Characterization of Actin and Myosin in the Developing Stomach

“…A change in MHC isoform distribution has been seen in the rodent ventricle with hypothyroidism and in skeletal muscle after hypertroph y of chicken slow muscle from repeated use (19,27).…”

“…In the rabbit heart, hypothyroidism increases the relative amount of slow MHC isoform and is associated with a slower, more economical contraction, whereas hyperthyroidism is associated with an increase in the faster isoform and a more rapid contraction that is less economical in terms of energy expenditure (27). It is also possible that these differences in MHC isoform distribution that affect ATPase activity could affect the ultimate ability of smooth muscle to generate tension (30).…”

Myosin heavy chain gene expression in human heart failure.

“…The ␣ -MyHC has a higher ATPase activity than ␤ -MyHC, and the contractile velocity of the heart is correlated with the relative amount of each MyHC. While hearts expressing more ␣ -MyHC have a more rapid contractile velocity, hearts with more ␤ -MyHC allow for greater economy in force generation (2,3). The MyHC composition of the ventricular myocardium of small mammals has been reported to be Ͼ 90% ␣ -MyHC (4-6), whereas that of humans has been reported to be Ͼ 95% ␤ -MyHC (7-13).…”