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

Abstract

The relation between functional properties of the contractile apparatus, such as shortening velocity and ATPase activity, and myosin isoenzyme composition was studied in ventricular myocardium of adult (60-90-day-old) rats and of newborn (3-day-old) and young (10- and 20-day-old) rats. In adult animals, variations of isomyosin pattern were produced by reducing food intake and by changing the thyroid state. Hyperthyroidism was induced with triiodothyronine daily injection for 15 days; hypothyroidism was induced with iodine-free diet and KClO4 in drinking water for 50-60 days. The following parameters were studied: 1) calcium-magnesium-activated and magnesium-activated ATPase activity of washed and purified myofibrils, 2) calcium-activated ATPase activity of purified myosin, 3) isomyosin composition and relative content of alpha-myosin heavy chains (alpha-MHCs), and 4) force-velocity curve of left and right ventricle papillary muscles. To take into account the difference in excitation-contraction coupling between newborn and adult myocardium, the determination of the force-velocity curve was repeated in Krebs' solution with normal [CaCl2] (2.5 mM) and in Krebs' solution with high [CaCl2] (10 mM). During postnatal growth, the relative content of alpha-MHC increased and reached a maximum at about 20 days. Pronounced increases of myofibrillar and myosin ATPase activity and in shortening velocity occurred during the same period. In adult hyperthyroid rats, alpha-MHC content as well as enzymatic activity and shortening velocity were higher than in control adult animals. Hypothyroidism and food deprivation caused a decrease of alpha-MHC content and a reduction of both enzymatic activities and shortening velocity. The study of the relations between alpha-MHC relative content and functional parameters showed that 1) in ventricular myocardium of adult rats a linear relation existed between alpha-MHC content and myosin and myofibrillar ATPase activity and shortening velocity, and 2) in newborn and young rat ventricular myocardium, both enzymatic activities and shortening velocity were lower than would have been expected on the basis of the linear relation described above. This latter observation could be accounted for by a variation in specific activity of myosin during postnatal development or by the presence of peculiar isomyosins that cannot be detected with usual electrophoretic techniques.

Cyclical Changes in High-Energy Phosphates During the Cardiac Cycle by Pacing-Gated 31P Nuclear Magnetic Resonance

“…Hoh et al 16 and other investigators 22,23 have reported that the ATPase activity of myosin V3 is lower than that of myosin V1, and that the rate of cross-bridge cycling measured by myocardial shortening velocity is correlated with increases and/or decreases in myosin ATPase activity. Therefore, oscillations of energy metabolism may be less obvious in hearts with V3 dominance in myosin isozyme distribution.…”

“…Cappelli et al have reported that the myosin ATPase activity, shortening velocity, and relative composition of V1 myosin are much higher in hyperthyroid rats than in normal rats, although the myosin isozyme distributions in both groups show myosin V1 dominance. 22 The larger magnitude of fluctuations in energy metabolites in the hyperthyroid rat hearts may be attributable to the increased myosin ATPase of these preparations.…”

Lead reduces tension development and the myosin ATPase activity of the rat right ventricular myocardium

“…However, the fact that lead binds to sulfhydryl (SH) groups suggests that the metal could affect contractile proteins (27,28). The activity of myosin, for example, is reduced by Hg, which also binds to SH groups (21).…”

“…To determine if lead acetate is capable of affecting myosin ATPase activity, the effects of this metal were assayed as described previously (20,21). Briefly, myosin was prepared from minced and homogenized right ventricles (N = 7), extracted briefly with KCl phosphate buffer (0.3 M KCl, 0.2 M phosphate buffer, pH 6.5) (22).…”

Eucalyptol, an essential oil, reduces contractile activity in rat cardiac muscle

“…Myosin ATPase activity was assayed (23,24) by measuring Pi liberation from 1 mM ATP in the presence of 50 mM HEPES, pH 7, 0.6 M KCl, 5 mM CaCl 2 or 10 mM EGTA in a final volume of 200 µl. In the presence of this high ionic strength with no addition of Mg 2+ to the incubation medium, only myosin activity is measured and there is no significant activity of Mg 2+ -ATPase, which is an actin-contaminated myosin.…”