Lewis, Michael I.

The aim of the study was to evaluate the effect of severe short‐term nutritional deprivation (ND) on the fractional synthesis rates (FSR) specific proteins and protein pools in the diaphragm muscle (DiaM). Male rats were divided into two groups: 20% of usual food intake for 4 days (ND) with water provided at libitum and free‐eating control (Ctl). [U‐13‐C] leucine was continuously infused through the jugular vein for 3–3.5 h after a priming dose of the isotope. Blood samples were collected from the carotid artery at baseline and every 30 min at precursor pool plateau. DiaM was excised at the end of the isotope infusion. Mixed muscle protein (MMP), myosin heavy chain (MHC), sarcoplasmic (Sarco) and mitochondrial (Mito) proteins were extracted from DiaM and the incorporation of the label into each protein(s) was determined using mass spectrometry. The FSR of MMP, MHC, Sarco and Mito in the rat DiaM were calculated using the precursor‐product relationship. ND resulted in the reduction of the FSRs of MMP (71%), MHC (69%), Sarco (44%) and Mito (55%) in DiaM. In summary, short‐term ND is associated with a global decrease in the synthetic rate of DiaM proteins, suggesting a compromise in the ability of the DiaM to synthesize functionally important proteins in the ND state. The underlying mechanisms for this depressed state of protein synthesis in DiaM related to ND remain to be elucidated.

The aim of this study was to evaluate the potential mechanisms underlying the improved contractility of the diaphragm (Dia) in adult intact male hamsters after nandrolone (Nan) administration, given subcutaneously over 4 wk via a controlled-release capsule (initial dose: 4.5 mg ⋅ kg−1 ⋅ day−1; with weight gain, final dose: 2.7 mg ⋅ kg−1 ⋅ day−1). Control (Ctl) animals received blank capsules. Isometric contractile properties of the Dia were determined in vitro after 4 wk. The maximum velocity of unloaded shortening ( V o) was determined in vitro by means of the slack test. Dia fibers were classified histochemically on the basis of myofibrillar ATPase staining and fiber cross-sectional area (CSA), and the relative interstitial space was quantitated. Ca2+-activated myosin ATPase activity was determined by quantitative histochemistry in individual diaphragm fibers. Myosin heavy chain (MHC) isoforms were identified electrophoretically, and their proportions were determined by using scanning densitometry. Peak twitch and tetanic forces, as well as V o, were significantly greater in Nan animals compared with Ctl. The proportion of type IIa Dia fibers was significantly increased in Nan animals. Nan increased the CSA of all fiber types (26–47%), whereas the relative interstitial space decreased. The relative contribution of fiber types to total costal Dia area was preserved between the groups. Proportions of MHC isoforms were similar between the groups. There was a tendency for increased expression of MHC2B with Nan. Ca2+-activated myosin ATPase activity was increased 35–39% in all fiber types in Nan animals. We conclude that, after Nan administration, the increase in Dia specific force results from the relatively greater Dia CSA occupied by hypertrophied muscle fibers, whereas the increased ATPase activity promotes a higher rate of cross-bridge turnover and thus increased V o. We speculate that Nan in supraphysiological doses have the potential to offset or ameliorate conditions associated with enhanced proteolysis and disordered protein turnover.

Lewis, Michael I., Thomas J. Lorusso, Wen-Zhi Zhan, and Gary C. Sieck. Interactive effects of denervation and malnutrition on diaphragm structure and function. J. Appl. Physiol. 81(5): 2165–2172, 1996.—The purpose of this study was to examine the interactive effects of unilateral denervation (DN) and prolonged malnutrition (MN) on the structure and function of the diaphragm muscle (Dia). Four groups of rats were studied: control (Con), MN, DN, and DN-MN. MN began 2 wk after DN and lasted 4 wk. In both the DN and DN-MN groups, the relative loss in Dia weight exceeded the relative change in body weight. Compared with the Con group, Dia specific force was reduced by ∼40% in both the DN and DN-MN groups but was unaffected in the MN group. Dia fatigue resistance improved in all experimental groups but to a greater extent in the DN and DN-MN groups. In both the DN and DN-MN groups, ∼50% of Dia fibers were classified as type IIc, whereas fiber type proportions did not change in the MN group. In the DN group, only type IIb/x fibers atrophied, whereas all fiber types atrophied in the MN and DN-MN groups. We conclude that in the DN-MN group the reduction in specific force combined with the reduction in total cross-sectional area of the muscle significantly curtails Dia force-generating capacity.