Schlenker, Evelyn H.

The purpose of the present study was to evaluate the effects of chronic mass loading produced by obesity on the structural and functional characteristics of the diaphragm in lean and obese Zucker rats. The trapezius muscle served as an internal control. The studies were carried out on 17 lean (303 +/- 24 g) and 16 obese (698 +/- 79 g) Zucker rats. We observed that the diaphragms from obese animals were restructured such that the overall contribution of type I and IIa fibers was significantly increased. As a consequence of this remodeling, overall diaphragm thickness was selectively greater in obese animals. In small isolated diaphragm bundles studied in vitro, we also detected a reduction in specific force in obese animals that was not detected in the trapezius muscle. In vitro fatigue resistance, assessed by repeated stimulation, was similar in muscles of lean and obese animals. Diaphragm fiber oxidative capacity (succinate dehydrogenase activity) was also comparable in lean and obese animals. We conclude that in obesity the diaphragm undergoes modest remodeling that may be beneficial in enhancing force generation.

Abstract The obese Zucker rat, an autosomally genetic model of obesity, represents a good model of relatively early onset human obesity. Although factors associated with the control of metabolism and thermoregulation have been studied extensively in these animals, pulmonary mechanics and ventilation have not been documented and form the basis of this investigation. Studies were carried out in 16 obese and 18 lean female littermates (698 +/- 79 versus 304 +/- 24 g, p < 0.001). Pulmonary function, including lung volumes and respiratory system compliance, was evaluated in supine anesthetized animals. With the exception of residual volume, all other lung volumes, including function residual capacity, total lung capacity, expiratory reserve volume, and inspiratory capacity, were significantly reduced (p < 0.05 or better) in the obese phenotype compared with volumes in the lean littermates. Pressure-volume relationships of the intact respiratory system and the excised lung were also determined. Although lung compliance was similar between the phenotypes, respiratory system compliance was significantly lower (0.85 +/- 0.06 versus 0.67 +/- 0.09 ml/cm H2O, p < 0.01) in the obese rats. Oxygen consumption and ventilatory parameters (including respiratory rate, tidal volume, minute ventilation, inspiratory time, and expiratory time) were similar between phenotypes breathing room air, and the minute ventilation in response to hypoxia was similar in both groups. In marked contrast, obese animals exhibited a blunted ventilatory response to hypercapnia (221 +/- 38 versus 135 +/- 44 ml/min, p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)