Bioenergetic characteristics of the costal and crural diaphragm in mammals
Introduction
The diaphragm is the principal muscle of inspiration and is considered the most important ventilatory muscle in mammals (Sieck, 1988, DeTroyer et al., 1982). The diaphragm has been classically considered one muscle with a single functional entity. However, recent reports have provided new insights into diaphragmatic structure and function. The diaphragm is now thought to function as two muscles that act differently on the rib cage. For example, DeTroyer et al. (1981), DeTroyer et al. (1982)separately stimulated the costal and crural parts of the diaphragm in anesthetized dogs; the results indicated that the costal and crural parts of the diaphragm have a different mechanical action on the rib cage. Stimulation of the costal diaphragm increased the dimensions of the lower rib cage, whereas stimulation of the crural diaphragm had no effect on the lower rib cage dimensions. In contrast to the costal region, the crural diaphragm has no attachments to the rib cage, and thus may only act on the rib cage by means of changes in pleural or abdominal pressure (DeTroyer et al., 1981, DeTroyer and Estenne, 1988).
In addition to functional differences between the costal and crural diaphragm, recent evidence from our laboratory (Powers et al., 1990b) and work by Sugiura et al. (1992)have demonstrated that metabolic differences exist between these two diaphragmatic regions in the rat. Specifically, the oxidative capacity of the costal diaphragm is significantly higher than the crural diaphragm (Powers et al., 1990b, Sugiura et al., 1992). Also, we have shown that similar costal vs. crural metabolic differences exist in the dog (Powers et al., 1994b). However, at present, it is unknown if these same metabolic differences exist between the costal and crural diaphragm in other mammalian species. Therefore, the purpose of this study was to compare oxidative and glycolytic enzyme activities in seven different mammalian species (mouse, rat, rabbit, ferret, sheep, pig and cow) ranging in body mass from approximately 0.03 to 422 kg. Based upon our previous findings in the rat and dog, we tested the hypothesis that costal vs. crural bioenergetic (i.e. oxidative capacity) differences would exist in these non-primate mammals.
Section snippets
Animals
Sections of the costal and crural diaphragm muscle from seven different adult mammalian species were used for biochemical analyses in this investigation. The mean (±SD) body mass of the species investigated were as follows: Holstein dairy cow (n=8), 420.94±18.34 kg; pigs (n=8), 115±4.47 kg; Hampshire and Suffolk sheep (n=8), 56.9±1.70 kg; New Zealand White rabbits (n=4), 3.45±0.451 kg; ferrets (n=8), 1.41±0.188 kg; Sprague–Dawley rats (n=7), 0.298±0.013 kg; DBA/C57 mice (n=10), 0.032±0.004 kg.
Removal of tissue
Enzyme activities in costal and crural diaphragm
Table 1 contains the mean (±SEM) costal and crural diaphragm LDH and CS enzyme activities across species. Costal vs. crural CS activity differed (P<0.05) in only two species (i.e. rat and rabbit). Also, note that CS activity in both the costal and crural diaphragm was significantly higher in the smaller animals (e.g. mouse, rat, ferret) compared to all animals of larger mass (rabbit, sheep, pig, cow).
Regional diaphragmatic differences in LDH activity existed in two species: LDH activity was
Overview of principal findings
To our knowledge, this is the first study to examine the bioenergetic enzyme activities in both the costal and crural diaphragm across a range of mammalian species differing widely in body mass and resting metabolic rates. The principal finding in these experiments was that metabolic differences between the costal and crural diaphragm do not exist in all mammalian species. Further, the data indicate that the oxidative capacity (represented by CS activity) of both the costal and crural diaphragm
Summary and conclusions
These experiments tested the hypothesis that costal vs. crural oxidative capacity differs in many non-primate mammals. The results indicated that the costal and crural diaphragm oxidative capacity did not differ in five of the seven species investigated. Indeed, oxidative capacity differed between the costal diaphragm and crural diaphragm in only the rat and rabbit. Hence, the hypothesis was not supported.
Further, our data indicate that the oxidative capacity of both the costal and crural
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