Intermediary metabolism of the lung.

The lung is a metabolically active organ that is engaged in secretion, clearance and other maintenance functions that require reducing potential, energy and substrates for biosynthesis. These metabolic requirements are met in part through uptake and catabolism of glucose which represents the major fuel utilized by lung tissues. Gluconeogenesis does not occur, and glycogen stores are limited so that the lung depends on the circulation for its glucose requirement. Other substrates can be metabolized by lung and contribute to the metabolic pool although their role has been less thoroughly studied. Glucose is catabolized in the lung by cytoplasmic and mitochondrial pathways that are responsive to regulatory mechanisms as in other tissues. Activity of the pentose cycle pathway of glucose catabolism is relatively high and generates the NADPH required for biosynthesis of lipid, detoxification reactions, and protection against oxidant stress. The ATP content of the lung is maintained by oxidative metabolism at levels comparable to other metabolically active organs. Alterations in lung intermediary metabolism may depress amine clearance, alter lung permeability, and influence the lung response to oxidant stress.