Heterogeneity of metabolic activity in the canine parasternal intercostalsduring breathing

In the dog, the inspiratory mechanical advantage of the parasternal interco stals shows a marked spatial heterogeneity, whereas the expiratory mechanic al advantage of the triangularis sterni is relatively uniform. The contribu tion of a particular respiratory muscle to lung volume expansion during bre athing, however, depends both on the mechanical advantage of the muscle and on its neural input. To evaluate the distribution of neural input across t he canine parasternal intercostals and triangularis sterni, we have examine d the distribution of metabolic activity among these muscles in seven spont aneously breathing animals by measuring the uptake of the glucose tracer an alog [F-18]fluorodeoxyglucose (FDG). FDG uptake in any given parasternal in tercostal was greatest in the medial bundles and decreased rapidly toward t he costochondral junctions. In addition, FDG uptake in the medial parastern al bundles increased from the first to the second interspace, plateaued in the second through fifth interspaces, and then decreased progressively towa rd the eighth interspace. In contrast, uptake in the triangularis sterni sh owed no significant rostrocaudal gradient. These results overall strengthen the idea that the spatial distribution of neural input within a particular set of respiratory muscles is closely matched with the spatial distributio n of mechanical advantage.