Adaptability of the hamster diaphragm to exercise and/or emphysema

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Abstract
In vitro contractile properties of the diaphragm were measured in four groups of inbred adult hamsters (greater than 40wk), randomly divided into sedentary control (SC), exercise control (EC), sedentary emphysematous (SE), and exercise emphysematous (EE) groups. Emphysema was induced by a single endotracheal instillation of elastase. Exercise consisted of running 1 h/day, 7 days/wk for 20 wk. Functional residual capacity (FRC), measured by means of a pressure box, was approximately 2.5 times greater in both emphysematous groups compared with control groups. Small diaphragmatic bundles were then isolated and subjected to in vitro analysis of isometric contractile properties. No differences were observed among the four groups in time to peak tension, half-relaxation time, and shape of the force-frequency curve. The diaphragmatic length-tension curve of emphysematous animals was displaced toward the left; maximal tetanic tension was similar in all groups, while optimal length (Lo), defined as the length at which maximal twitch tension occurred, was significantly shorter in both emphysematous groups. The Lo was negatively correlated with the FRC. Exercise tended to increase the in vitro endurance of the diaphragm bundles in control animals only. Diaphragms from both emphysematous groups, however, did show the greatest resistance to fatigue. It is concluded that 1) daily running for 1 h does not influence the diaphragmatic contractile properties in the hamster, but fatigue was reduced; 2) the load of chronic emphysema decreased the hamster's diaphragm fatiguability; and 3) the diaphragms of emphysematous hamsters chronically adapt by decreasing diaphragmatic length in proportion to the degree of hyperinflation and thus probably continue to operate at an Lo.