Characteristics of ATPase activity and the subunit composition of myosin in the conduction system of bovine heart

Age-dependent changes in contraction and regional myocardial myosin heavy chain isoform expression in rats

The goals of this study were to measure the relative levels of the alpha- and beta-isoforms of myosin heavy chain (MHC-alpha and MHC-beta, respectively) in multiple, specific regions of the adult rat heart and to determine whether age-dependent changes in isoform levels in different regions are uniform. Relative amounts of MHC-alpha and MHC-beta were determined in right and left atria and left ventricular (LV) Purkinje fibers (PF), papillary muscles, trabeculae, and endo-, mid-, and epicardial regions at 2, 5, 10, 16, and 21 mo. PFs contained substantial amounts of myosin and were striated and capable of generating force and shortening on activation. Levels of MHC-beta increased in all LV compartments with age, especially between 2 and 5 mo. There was more MHC-beta in PFs than other LV sites. There were regional differences in the level of MHC-beta throughout the LV at all ages, and the rates of change within regions differed. Ca(2+)-activated tension in PFs and trabeculae was compared at 2 and 22 mo. PF tension was less than trabecula tension, and this difference may be explained by differences in MHC content. V(max) and tension-generating ability in PFs decreased with age. Maximal tension generated by trabeculae did not change during aging. A large proportion of the increase in the level of MHC-beta that is normally associated with aging occurs at a relatively early age in rat LV. PFs, with their small diameters and short diffusion distance, should be considered for skinned multicellular myocardial studies.

Isolation and characterization of myosin heavy chain isozymes of the bovine conduction system

To determine the characteristics of cardiac myosin in the conduction system, a pure Purkinje fiber preparation, consisting of atrioventricular nodes and the ventricular conduction system, was obtained from bovine hearts. Two types of myosin heavy chain isozymes, alpha-type and beta-type, were fractionated by affinity chromotography using monoclonal antibodies CMA19 and HMC50, which are specific for the alpha-type heavy chain and beta-type heavy chain, respectively. Competitive enzyme-linked immunosorbent assay demonstrated that the content of beta-type in the atrioventricular node (30-40%) was higher than that in atrial ordinary myocardium (10-20%) and that of the alpha-type was 30-40% in the ventricular conduction system, which was much higher than that in the ventricular ordinary myocardium (less than 10%). By one- and two-dimensional electrophoresis of the peptides produced by partial and complete digestion, the peptide compositions of alpha-type and beta-type in the conduction system were shown to be very similar to those of alpha-type and beta-type in ordinary myocardium, respectively. The CA2+-activated ATPase activity of myosin of the atrioventricular nodes was lower than that of ordinary atrial myosin (0.46 +/- 0.03 versus 0.58 +/- 0.02 mumol Pi/mg/min, mean +/- SEM, p less than 0.05) and in contrast, that of ventricular specialized myocardium was higher than that of myosin in the ventricular ordinary working myocardium (0.32 +/- 0.03 versus 0.22 +/- 0.01 mumol Pi/mg/min, p less than 0.05). This was in good agreement with the relative proportion of myosin isozymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of alpha- and beta-myosin heavy chains in the ventricular fibers of the postnatal developing rat

Four monoclonal antibodies, two raised against alpha-myosin heavy chain (MHC) and two against beta-MHC, have been used to investigate in situ the fiber distribution of alpha- and beta-MHC in rat cardiac ventricles during postnatal development. Eighteen ventricles from 2-day-old to 1-year-old rats were analyzed. Three fiber populations were determined according to their immunofluorescent labeling: one with only alpha-MHC, one only beta-MHC, and one with mixed alpha- and beta-MHC. Large variations in the proportions of these three fiber populations according to age indicate that: (1) alpha-MHC are expressed in all fibers until the second month; they then disappear in a small endocardial fiber population and in a few apparently conductive fibers around the vessels. (2) beta-MHC are also first expressed in all fibers and then disappear gradually from epicardium to endocardium between the second and fourth weeks, except in the conductive fibers; they reappear during the second month sequentially from endocardium to epicardium; and they are then expressed in almost all fibers, except in a small epicardial fiber population, proportionally larger in the right ventricle than in the left. Immunological characterization of MHC isolated from a 22-day-old-rat ventricle, using anti-beta immunoaffinity chromatography, suggests that MHC of conductive fibers are probably at least partially in an alpha beta heterodimeric form.

Distribution of light chains and ATPase activity of myosin in the atrioventricular conducting tissue of bovine heart

The relations of the light chains of myosins of the atria, ventricles, and atrioventricular conducting tissue (specialized myocardial tissue) and the distribution of the light chains of myosin in different regions of the atrioventricular conducting tissue in bovine heart were examined. Two-dimensional gel electrophoresis showed that the atrial and ventricular myosins each had two light chains (LC1 and LC2). Ventricular LC1 differed from atrial LC1, but ventricular LC2 corresponded to atrial LC2. The specialized myocardial tissue myosin had three light chains (named here SL1, SL2, and SL3). SL1 comigrated with ventricular LC1, SL2 with atrial LC1, and SL3 with ventricular LC2 and atrial LC2. The compositions of the three light chains of myosins in various regions of the atrioventricular conducting tissue were determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The percentage proportion of SL1 decreased in the order--atrioventricular node (AVN), right and left bundle branches (RLBB), His bundle (HIS), false tendon (FT) myosin; while the percentage proportion of SL2 decreased in the order--FT and HIS, RLBB, AVN myosin. The percentages of SL3 in these four regions were similar. The Ca2+-activated ATPase activity of myosin was highest in the AVN and lowest in the FT. The activities in the HIS and RLBB were intermediate between those in the AVN and FT. Thus, the composition of the light chains and the Ca2+-activated ATPase activity were different in various regions of the atrioventricular conducting tissue.

Distribution of cardiac myosin isozymes in human conduction system. Immunohistochemical study using monoclonal antibodies

To determine the presence and distribution of cardiac myosin isozymes in the human conduction system, we performed an immunohistochemical study using monoclonal antibodies CMA19 and HMC14, which are specific for myosin heavy chains of human atrial type (alpha-type) and ventricular type (beta-type), respectively. Serial frozen sections of human hearts were obtained from autopsy samples and examined by indirect immunofluorescence. Alpha-type was found in all myofibers of sinus node and atrio-ventricular node, and in 55.2 +/- 10.2% (mean +/- SD, n = 5) of the myofibers of ventricular conduction tissue, which consists of the bundle of His, bundle branches, and the Purkinje network. In contrast, beta-type was found in all myofibers of the atrio-ventricular node and ventricular conduction tissue, whereas almost all myofibers of the sinus node were unlabeled by HMC14. Although the number of ventricular myofibers labeled by CMA19 was small, the labeled myofibers were more numerous in the subepicardial region than in the subendocardial region. These findings show that the gene coding for alpha-type is expressed predominantly in specialized myocardium compared with the adjacent ordinary working myocardium.