M-BAND STRUCTURE, M-BRIDGE INTERACTIONS AND CONTRACTION SPEED IN VERTEBRATE CARDIAC MUSCLES

Cardiac muscle M-band structures in several mammals (guinea pig, rabbi t, rat and cow) and also from three teleosts (plaice, carp and roach), have been studied using electron microscopy and image processing. Axi al structure seen in negatively stained isolated myofibrils or negativ ely stained cryo-sections shows the presence of five strong M-bridge l ines (M6, M4, M1, M4' and M6') except in the case of the teleost M-ban ds in which the central M-line (M1) is absent, giving a four-line M-ba nd. The M4 (M4') lines are consistently strong in all muscles, support ing the suggestion that bridges at this position are important for the structural integrity of the A-band myosin filament lattice. Across th e vertebrate kingdom, cardiac M-band ultrastructure appears to correla te roughly with heartbeat frequency, just as in skeletal muscles it co rrelates with contraction speed, reinforcing the suggestion that some M-band components may have a significant physiological role. Apart fro m rat heart, which is relatively fast and has a conventional five-line M-band with M1 and M4 approximately equal, the rabbit, guinea pig and beef heart M-bands form a new 1+4 class; M1 is relatively very much s tronger than M4. Transverse sections of the teleost (roach) cardiac A- band show a simple lattice arrangement of myosin filaments, just as te leost skeletal muscles. Almost all other vertebrate striated muscles, including mammalian heart muscles, have a statistical superlattice str ucture. The high degree of filament lattice order in teleost cardiac m uscles indicates their potential usefulness for ultrastructural studie s. It is shown that, in four-line M-bands in which the central (M1) M- bridges are missing, interactions at M4 (M4') are sufficient to define the different myosin filament orientations in simple lattice and supe rlattice A-bands. However the presence of M1 bridges may improve the a xial order of the A-band.