204 



HANDBOOK OF PHVSIOLOGV 



CIRCULATION I 



delivery of oxygen from the cell membrane to the 

 mitochondria where terminal respiration actually 

 occurs. 



Myoglobin is found in largest quantities in muscles 

 requiring relatively slow repetitive contractions of 

 considerable force, maintained for long periods. Such 

 muscular activity is characteristic of cardiac muscle 

 and the breast muscles in larger flying birds, such 

 as the pigeon and duck, and the leg muscles in running 

 animals such as the horse and dog. The presence of 

 myoglobin furthermore correlates with the presence 

 of large numbers of mitochondria (sarcosomes) and 

 a more aerobic metabolism. Myoglobin is not found 

 in the leg muscles of jumping amphibia, such as the 

 frog, and in the breast muscles of nonflying birds such 

 as the chicken. It is also absent from the flight muscle 

 of insects which contain large numbers of sarcosomes. 

 These relationships suggest that myoglobin is present 

 in those tissues where sustained activity and aerobic 

 metabolism coincide, and rates of anaerobic glycol- 

 ysis are insufficient to sustain the work requirement. 



Cardiac muscle does not always contain the highest 

 myoglobin concentration. The myoglobin content 

 of heart muscle from young dogs is slightly higher 

 than that of their skeletal muscle, but in adult dogs 

 the myoglobin concentration of psoas muscle is twice 

 as high as in cardiac muscle. Likewise, in horses the 

 skeletal myoglobin concentration is higher than that 

 of the heart (131). Values for myoglobin in a number 

 of muscles are shown in the Appendix table. The 

 content of cardiac muscle ranges from 0.40 to 1.50 

 per cent fresh weight. In the leg muscles of horse, 

 dog, and man, the myoglobin concentration may be 

 as high as '^.o to 3.0 per cent (24). 



Lipids of till' Afv 



rdiiim 



The heart is relatively rich in complex lipids. It 

 contains a large variety of phosphatides and related 

 compounds, moderate amounts of cholesterol, and 

 very little, if any, triglyceride under normal condi- 

 tions. These lipids are distributed largely in the 

 organelles of the cell. The mitochondria and micro- 

 somes (endoplasmic reticulum) have the richest con- 

 tent of lipid, 94 per cent of which is phospholipid and 

 6 per cent cholesterol (222). Since cardiac muscle 

 is so rich in sarcosomes, the lipids of the heart re- 

 flect primarily the lipids of the cardiac sarcosome. 

 Marinetti and co-workers (125, 151) have studied 

 the distribution of lipids in pig heart and find that 

 mitochondrial lipid represents approximately 60 per 

 cent of the lipid isolated, the microsomes represent- 



ing about 30 per cent, and the supernatant or the 

 cytoplasm representing about 10 per cent. The myo- 

 fibril (191) contains very little, if any, lipid. The 

 remaining lipids are associated with granules, which 

 can be isolated from the sarcoplasm (189) and the 

 membrane. Ninety-one per cent of the sarcosomal 

 lipid is phosphatide — distributed among inositol 

 phosphatide, sphingomyelin, lecithin, plasmalogen, 

 phosphatidal serine, phosphatidal ethanolamine, and 

 polyglycerol phosphatide (cardiolipin) (228). 



Cardiac microsomes contain more sphingomyelin 

 and phosphatidal ethanolamine and less inositol 

 phosphatide than do the sarcosomes. Marinetti and 

 co-workers (150) have shown further that pig heart 

 lecithin fraction contains 60 per cent of the diester- 

 lecithin and 40 per cent of plasmalogen (monoester 

 monoacetal lecithin). The fatty aldehydes appear 

 to be mainly saturated, whereas the fatty acids as- 

 sociated with phospholipids of the heart are highly 

 unsaturated. 



The lipids of cardiac muscle are integral parts of 

 tissue lipoproteins which provide a suitable matrix 

 for the enzymatic activities of sarcosomes, micro- 

 somes, and nucleus of the cardiac muscle cell (149, 

 175). Stotz and collaborators (228) have shown that 

 various purified enzymes of the sarcosome (cyto- 

 chrome b-c, cytochrome a-3) have phosphatide, 

 cholesterol and other lipids firmly attached to the 

 enzyme micelle. Green & Jarnefelt (88) have further 

 shown that the electron transport system of the heart 

 sarcosome can be fractionated to yield a variety of 

 lipoproteins with enzymatic activity. 



PATHWAYS OF CARDIAC METABOLISM 



Metabolic processes in heart muscle may be di- 

 vided into three general phases: a) energy liberation, 

 b) energy conservation, and c) energy utilization. 

 These phases are presented very schematically in 

 figure 8 and will be discu.ssed in detail. The phase 

 of energy liberation includes those reactions l)y whicii 

 the carbon-carbon and carbon-hydrogen bond energy 

 of oxidizable substrate is liberated as free energy. 

 The processes of glycolysis, fatty acid oxidation, and 

 the common terminal oxidative reactions of the 

 Krebs tricarboxylic acid cycle occur in phase i . The 

 net result of this biochemistry is the near quantitative 

 conversion of the l)ond energy of substrate into the 

 free energy of hydrogen electrons which are then 

 available for transport to oxygen along a chain of 

 electron transport enzymes located on the cristae 



