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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



interrupted. Exact knowledge of the details of chemo- 

 mechanical coupling remains elusive. The discovery 

 of myosin by Weber (252) and of actin by Szent- 

 Gyorgyi in 1942 (232) paved the way for the study 

 of contractility in vitro. Currently, there are no less 

 than six hypotheses regarding the contractile mecha- 

 nism (i 1 1, 166, 229, 234, 235, 254, 255). Five of them 

 deal with the interactions of two major contractile 

 proteins, actin and myosin with ATP, and the sixth 

 (8) relates this system to a third protein of the myo- 

 fibril, tropomyosin. Although these three proteins 

 (myosin, actin, and tropomyosin) appear to be the 

 major elements in the contractile system and ATP 

 the energy source, great controversy centers around 

 the primary and secondary events in the contractile 

 cycle at the molecular level. Although most of these 

 hypotheses have been developed from data obtained 

 in studies of skeletal muscle, the majority appear to 

 be applicable to cardiac muscle as well. The properties 

 of each of the contractile proteins will be discussed 

 briefly and then the mechanisms proposed to account 

 for contraction presented. 



Myosin. Myosin from rabbit skeletal muscle was 

 first isolated by Weber & Portzehl (253, 254) and 

 von Muralt & Edsall (249). Myosin was discovered 

 to have ATPase activity by Engelhardt & Ljubimova 

 in 1939 (67). Its properties have since been studied 

 in a number of laboratories. Estimates of its molecular 

 weight have ranged from 1,500,000 (217) to 389,000 

 (162). The higher molecular weights obtained in 

 earlier studies appear to be due to mild denaturation 

 and aggregation of the native protein (104). Data 

 from several recent studies (104, 130, 163, 246) give 

 a molecular weight of about 440,000 and dimensions 



inferred from measurements of viscosity and diffusion 



00 

 of 1,200 A by 25 A. Skeletal myosin is thus a long 



narrow molecule with an axial ratio of 48. When this 

 protein is digested with trypsin for very short periods, 

 two discrete subunits are obtained which have been 

 called H-meromyosin (heavy) and L-meromyosin 

 (light) (77, 157, 236). Originally the molecular weight 

 of H-meromyosin was estimated to be 240,000 and 

 L-meromyosin 100,000 suggesting that i H-mero- 

 myosin and 2 L-meromyosins were deri\ed from 

 each molecule of skeletal myosin. The ATPase and 

 actin-combining activities resided in the H-mero- 

 myosin. More recently Lowey & Holtzer (142) have 

 presented sound evidence to show that the molecular 

 weight of H-meromyosin is 340,000 so that it now 

 appears that the parent molecule is composed of one 

 of each of the tryptic digestion fragments. Kielley 

 & Harrington (121) have also shown that guanidine 



HCl will depolymerize rabbit skeletal myosin into 

 molecules of about 206,000 in molecular weight and 

 A. G. Szent-Gyorgyi & M. Borbiro (236) have shown 

 that high concentrations of urea may fragment skeletal 

 myosin into small protomyosins of 4,600 in molecular 

 weight which are uniform in size but variable in 

 amino acid composition. These data suggest that 

 myosin is a relatively unstable protein held together 

 by many accessory as well as co-valent iionds. Studies 

 in Olson's laljoratory (66, 226) have shown that 

 myosin isolated from dog heart is of lower molecular 

 weight than that from rabbit skeletal muscle. Esti- 

 mates of molecular weight from sedimentation and 

 diffusion constants and light scattering measurements 

 are in good agreement with a value of 226,000 and 

 a size of 690 by 28 A with an axial ratio of 24. The 

 higher value for the molecular weight for dog heart 

 myosin obtained by Gergely et al. (78) is apparently 

 due either to contamination by actomyosin or mild 

 denaturation (141). Cardiac myosin thus appears to 

 be approximately half as long and half as large as 

 skeletal myosin and does not yield the same mero- 

 myosins upon tryptic digestion. It nevertheless has 

 an amino acid composition very similar to that of 

 rabbit .skeletal myosin and may be related to skeletal 

 myosin as monomer is to dimer. In intact striated 

 muscle, myosin appears to be localized in the primary 

 filaments which compose the A band. 



Actin. Actin was discovered by Banga & Szent- 

 Gyorgyi in 1942 (10). Actin is extracted with more 

 difficulty than myosin from muscle and in prolonged 

 extractions with hypertonic salt solutions at a slightly 

 alkaline pH the much more viscous actomyosin com- 

 plex is obtained which can be dissociated in vitro 

 by ATP to yield actin and myosin. Actin can also 

 be obtained relatively free from myosin by extraction 

 ot an acetone powder of muscle with water. In such 

 freshly prepared extracts, actin exists as a globular 

 protein (G-actin) with a molecular weight of 70,000 

 and molecular dimensions of 290 X 24 A. In the 

 presence of ATP, Mg++ ions, and low concentrations 

 of KCl (o. I m) actin G dimerizes to form an aggre- 

 gate of mol wt 140,000 which is 590 X 24 A. The 

 G-actin-dimer then further aggregates to form actin 

 F, a long fibrous protein with a molecular weight of 

 the order of 1,500,000 which appears as the thin 

 secondary filaments of the sarcomere visible in the 

 electron microscope as noted previously. Actin has 

 been obtained from cardiac muscle and appears to 

 be similar but not identical to skeletal muscle actin 

 (52, 2 18, 21 9). Cardiac actin G appears to polymerize 

 to actin F with more diHicult\- than that from skeletal 



