Myosin Types and Fiber Types II. Atrial Myocardium in Cardiac Muscle

Antibodies were produced against myosins isolated from the left atrial myocardium (anti-bAm) and the left ventricular myocardium (anti-bVm) of the bovine heart. Cross-reactive antibodies were removed by cross-absorption. Absorbed anti-bAm and anti-bVm were specific for the myosin heavy chains when tested by enzyme immunoassay combined with SDS gel electrophoresis. Indirect immunofluorescence was used to determine the reactivity of atrial muscle fibers to the two antibodies. Three populations of atrial muscle fibers were distinguished in the bovine heart: (a) fibers reactive with anti-bArn and unreactive with anti-bVm, like most fibers in the left atrium; (b) fibers reactive with both antibodies, especially numerous in the right atrium; (c) fibers reactive with anti-bVm and unreactive with anti-bAm, present only in the interatrial septum and in specific regions of the right atrium, such as the cristaterminalis. These findings can be accounted for by postulating the existence of two distinct types of atrial myosin heavy chains, one of which is antigenically related to ventricular myosin. The tendency for fibers labeled by anti-bVm to occur frequently in bundles and their preferential distribution in the crista terminalis, namely along one of the main conduction pathways between the sinus node and the atrioventricular node, and in the interatrial septum, where different internodal tracts are known to converge, suggests that these fibers may be specialized for faster conduction. Cardiac muscle is a heterogeneous tissue composed of distinct muscle cell populations. Ultrastructural studies have revealed the existence of significant differences between ventricular, atrial, and conduction fibers (see reference 36 for a review). However, a precise characterization of the cellular composition of the heart has been hampered by the lack of weU-defmed molecular markers for the different types of cardiac muscle cells. With the recent discovery that multiple myosin isozymes are present in cardiac muscle (14) and are differentially distributed among cardiac muscle cells (30), a new powerful tool for distinguishing cardiac muscle ceils has become available. In previous immunofluorescence studies, we have analyzed the distribution of different isomyosins in the mammalian ventricular myocardium (9, 29). This study has now been extended to the atrial myocardium, where different muscle cell types have been identified by specific antimyosin antibodies and immunofluorescence procedures. Previous studies have shown that atrial myosin differs in structure and enzymatic activity from ventricular myosin. Koreeky and Michael (17) and Long et al. (19) showed that atrial myosin has a higher Ca2+-activated ATPase activity than ventricular myosin and contains electrophoretically and immunologically distinct light chains. Differences in enzymatic activity, including actin-activated ATPase activity, and light chain pattern between atrial and ventricular myosin were subsequently observed in different mammalian species (37, 43). The structure of atrial myosin heavy chains was also found to differ from that of ventricular myosin heavy chains by polypeptide mapping after cyanogen bromide or proteolytic cleavage (6, 8, 42). Immunohistochemical studies with specific antimyosin antibodies have shown that the difference in atrial and ventricular myosin is a general feature of the vertebrate heart, being found also in birds (7, 30) as well as in amphibians and reptiles (our unpublished observations). These studies also showed that atrial myosin is antigenically related to fast skeletal myosin in birds and mammals (31). The different myosin composition of atrial and ventricular myosin appears to be of physiological significance. Atrial muscle contracts more rapidly than ventricular muscle (17, 40). In cardiac muscle, as in skeletal muscle, there seems to be a close correlation between Ca 2+and actinactivated ATPase activity and speed of muscle shortening (32, 37, 43). Multiple forms of ventricular myosin have been identified by electrophoresis of native myosin under nondenaturing conTHE JOURNAL OF CELL BIOLOGY. VOLUME 95 DECEMBER 1982 838-845 838 © The Rockefeller University Press 0021-9525/82/12/838/08 $1.00 on O cber 0, 2017 jcb.rress.org D ow nladed fom FIGURE 1 Ouchterlony double immunodif fusion assay. The central well contained anti-bVm. The outer wells contained atrial myosin (1), ventricular myosin (2and 6), crude extract of ventricular myosin (3 and 5) and crude extract of atrial myosin (4). dit ions (14) and by immunoaf fmi ty ch roma tog raphy (28). Ventr icular isomyosins show a he terogeneous dis t r ibut ion in different regions of the vent r icu lar myoca rd ium (30) an d thei r relative concent ra t ion can vary dur ing deve lopment an d in a variety of condi t ions (14, 17). Studies on the heterogenei ty of atrial myosin are comparat ively scanty. In different m a m m a l ian species, atr ial myosin can be separa ted into two b a n d s by pyrophospha te gel electrophoresis with relative mobil i t ies different f rom those of ventr icular myosins (4, 14). However , these two componen t s have not yet been character ized biochemical ly and it is not k n o w n whe the r they differ in the heavy or l ight chains. Myosin l ight chains of vent r icu lar type, in addi t ion to atr ialtype light chains, have been identif ied in h u m a n atrial muscle undergoing pressure over load hype r t rophy (5, 26). In a previous immunof luorescence study o f the ch icken hear t we described a n u m b e r of atrial muscle fibers, some o f which h ad features of Purkinje fibers, showing an t imyos in immunoreac tivity different f rom tha t o f normal atr ial fibers (30). In this s tudy we have used two an t imyos in ant ibodies wi th dist inct specificities to investigate the cellular d is t r ibut ion o f atr ial myos in in the bov ine heart . The funct ional significance of these f indings will be discussed with par t icular reference to the p rob lem of specialized conduc t ion tracts. M A T E R I A L S A N D M E T H O D S