204. IMPLICATION OF THE CIRCULATORY SYSTEM IN SKELETAL MUSCLE TO MEAT QUALITY R. A. MERKEL MI CH I GAN STATE UN I VERSI TY __--__-__------------------------------------------------------------------ The internal equilibria of living, functioning muscle depends primarily upon the blood circulation. Blood carries oxygen and other nutrients to the muscles and heat, carbon dioxide and other waste products of metabolism are transported away from the tissues where they are produced to the lungs, kidneys and skin for renoval from the body. Thus zn adequate and essentially continual blood supply is necessary for the main- tenance of a stable internal equilibrium. Skeletal muscle has a rather extensive blood supply which is derived from branches of neighboring arteries (Wall, 1960). The aY-teries enter into the rnuscle from the epimysium and traverse along the strands of perimysium, dividing into branches and freely anastomosing with' one another. The finer branches of supply axe oriented transversely to the long axes of the muscle fibers, and from them arise the capillaries which course between the fibers in the endomysial ma%rix. The arteries and veins are contiguous to the point at which the terminal arterioles and venules originate, but subsequently these small vessels arise alternately. This orientation presumably allows the intervening capillaries to run in a relatively direct course from arteriole to venule to expedite rapid removal of metabolites (Clark, 1952) . Capillaries are generally oriented longitudionally between the individual muscle fibers as shown in figure 1.. A fine capillary net- work is formed as the longitudional vessels give rise to frequent trans- verse vessels which surround the intervening muscle fibers. The capil- lay network was first described morphologically in detail almost a century ago (Ranvier, 1874a, 187413; Spalteholz, 1888). Capillary anastomoses are especially well developed at the sites of the motor end plates (Wilkinson, 1929), a site which is especially active metabolically. Additionally, Ranvier (1874a,b) stated that the cross con- necting vessels in the muscle bed frequently exhibit considerable dilations. These enlargements have been suggested to act as reservoirs for oxygen from which muscle fibers can be supplied during sustained activity and during contraction, a time when capillary circulation is impeded. Such reservoirs have not been confirmed in more recent studies (Romanul, 1964; 1965) however. Capillary density in skeletal muscle has been shown to be directly proportionate to the metabolic rate, particularly in laboratory animals (Krogh 1919a, 1919b; Stoel1925-26; Duyff and Bouman, 1927; Paff, 1930; Martin --et al., 1932; Schmidt-Nielsen and Pennycuik, 1961). However varia- tion in capillary density does exist between muscles within the same species. Also the relationship of capillary density to metabolic rate does not apply unequivocally to the larger domestic animals used for meat production. In fact the pig has a higher metabolic rate than either sheep or cattle, but possesses a similar or slightly lower muscle capillary density than either of these two species (Schmidt-Nielsen and Pennycuik, 1961) . ___-_______ This manuscript is published with the approval of the Director of the Michigan Agricultural Experiment Station. 205. Capillary density is higher in red muscles than in white muscles (Ranvier, 187La, 187413; Krogh, 1919; Stoel, 1925-26; Smith and Giovacchini, 1956) and in fact the type I or red fibers within a given muscle have greater capillary density than the type I1 or white fibers (Schmidt-Nielsen and Pennycuikj 1961; Rornanul, 1964, 1965; Weatherspoon, 1968) . This distribution of the czpillaries in a cross section of muscle stained for elkaline phosphatase activity results in a positive reaction for sorne as yet unde- termined coriponent of the endothelium of small arteries, arterioles ana capillaries. The red muscle fibers are surrounded by approximately twice as many capillaries as white fibers and consequently reference to the capillary: fiber ratio appears in the literature as 2:1 or 1:l fibers (Schmidt-Nj-elsen and Pennycuik, 1961) . Thus far I have attemlsted to provide a brief description of the capillary bed of skeletal mscle. The previous description and subsequ-ent discussion of the circulatory system is not a cornplete treatise of the subject, but is intentionally restricted to those parameters necessaxy for definition and to those physiological events and characteristics which probably are associated with the ultimate qualitative properties of post-mortem muscle. The role of the vascular bed in normal, functioning muscle --in situ and the possible implications of the circulatory system to post-mortem muscle properties will now be considered. I think it would generally be agreed that data on the relationship of the circulatory system to mdscle quality are essentially non-existent . Thus my remarks will only suggest possible implica- tions of the circulatory system parameters to ultimate post-mortem quality characteristics of muscle. Furthermore, since we, in OUT laboratory, have been studying some aspects of the circulatory system of the pig in an attempt to relate them to muscle quality, I shall subsequently confine most of my discussion to this species. It is generally conceded that the supply of oxygen to muscle is the most important function of circulation, but whether the delivery of oxygen or the removal of metabolites is more important is not known (Schmidt- Nielsen and Pennycuik, 1961). Oxygen, reversibly bound to hemoglobin, is transported via the capillaies and by the process of diffusion p' dsses through the capillary wall, the extracellular fluids and finally through the sarcolemma into the sarcoplasm. The removal of metabolites likewise occurs by diffusion but in the opposite direction or from the muscle cells into the capillary lumen. An adequate supply of oxygen to skeletal muscle is thus dependent upon a multitude of system; the most obvious of which are: the mbient oxygen pressure, the effectiveness of respiratory exchange within the pulmonary capillaries, the blood flow rate through the muscles, the number and size of the capillaries per c-Jbic unit, the concentration and oxygen binding capacity of the hemoglobin and the oxygen consumption by the muscle cells (Thorling and Ersler, 1968). This complex of systems is integrated by a number of humoral and nervous mechanisms which enable the body to maintain "homeostasisff by counteraction of disequilibria in one system by changes in function of others . Oxygen is a prerequisite for oxidative metabolic activity within the muscle cell, i.e., type I fibers, and fibers so disposed have a high 206. myoglobin concentration, a profuse capillary supply or both. The capillary density associated with an individual muscle fiber is not only correlated with the Oxidative activity, but has further meaning in terms of the energy metabolism of the cell (Roxanul, 1965). Skeletd musclp fibers with low oxidative metabolic activity derive their energy from anaerobic glycolysis utilizing as their substrate the intracellular glycogen store (Dawson and Romanul, 1964). Such fibers produce a maximal amount o€ work of short dura- tions and their activity is independent of oxygen supply. These fibers are more or less self-sufficient, depending upon blood supply primarily for the removal of the lactic acid produced and we provided with few capillaries. By contrast, cardiac and skeletal muscle with high oxidative metabolism derive their energy from the aerobic breakdown of a variety of substrates, especially fatty acids. These fibers act for prolonged periods of time and thus cannot depend upon stored substrate, but must obtain it from the circu- lating blood. In addition, the substrate has to be completely metabolized and oxygen is an absolute essential for these reactions. Therefore such fibers depend upon circulating blood for both oxygen and substrate and consequently are surrounded by numerous capillaries. Electron microscopy further reveals that the capillaries on the surface of the fiber are adjacent to the subsarcolemal mitochondrial accumulations -- the sites of the oxidative metabolic activity (Romanul, 1965). From these statements it is readily apparent that the ratio of fiber types and factors that affect oxygen supply to the muscles, particularly the Mediate preslaughter period and during exsanguination, may have profound implications upon post-mortem muscle quality. Let us now consider the possible implications. Wachtel (1963) compared hemoglobin level, oxygen utilization and cardiac output of the domestic pig with the European wild pig. The wild pigs had considerably higher hemoglobin levels and blood of the wild pigs was capable of carrying 25-40$ more oxygen than that of the domestic pig of the same body weight. He also reported that oxygen utilization in wild pigs is lower (34%) than the domestic pig (44%). Mammals ordinarily meet increased oxygen demand with an increase in cardiac output. When this is insufficient to met bodily requirements, oxygen utilization increases. Wachtel (1963) states that under resting conditions cardiac output in domestic pigs is already relatively high. This might account for his observation that under conditions of increased oxygen demand, oxygen utilization is greatly increased but cardiac output is only moderately increased in domestic pigs. In wild pigs under similar conditions, cardiac output increases most markedly, while change in oxygen utilization is relatively