Bioenergetic characteristics of the costal and crural diaphragm in mammals

Abstract

These experiments compared oxidative and glycolytic enzyme activity in the costal and crural diaphragm in seven adult mammals (mouse, rat, rabbit, ferret, sheep, pig, cow) ranging in body mass from approximately 0.03 to 422 kg. Segments of the costal and crural diaphragm from the aforementioned species were homogenized to determine the activities of the glycolytic enzyme, lactate dehydrogenase (LDH), and the Krebs cycle enzyme, citrate synthase (CS). The results indicated that metabolic differences between the costal and crural diaphragm do not exist in all mammalian species. Specifically, CS activity differed (P<0.05) between the costal and crural diaphragm (costal approximately 36% greater than crural) in only two species (rat and rabbit). Further, the oxidative capacity of the costal and crural diaphragm was significantly correlated with both breathing frequency and resting metabolic rate (r=0.92–0.57; P<0.05) across the species investigated. In contrast, glycolytic capacity was not significantly correlated (P>0.05) with either breathing frequency or resting metabolic rate.

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.