METABOLIC CEILINGS UNDER A COMBINATION OF PEAK ENERGY DEMANDS

Is energy expenditure limited by shared metabolic machinery for energy assimilation or by bottlenecks specific to each mode of energy expend iture? We tested this question in mice by imposing peak energy burdens of lactation and of cold stress simultaneously We measured food intak e, body and organ masses, and small intestinal brush-border hydrolase and transporter capacities in virgin female mice and in mothers nursin g approximately 5, 8, or 14 pups, at either 5 degrees C or 23 degrees C. We had already observed that mothers of 14 pups are at a limit of l actational performance at 23 degrees C, while virgin mice at 5 degrees C are near their limit of food intake in response to cold stress. Nev ertheless, the increments in food intake due to these two energy stres ses applied simultaneously proved to be additive: food intake in lacta ting mice at 5 degrees C was even higher than the peak intake in lacta ting mice at 23 degrees C or in virgins at 5 degrees C. Thus, neither during peak lactation nor during peak cold stress alone was energy exp enditure limited by shared machinery for energy assimilation; assimila tion could be pushed even higher by adding another energy stress. Mass es of the small intestine, liver, and kidney increased with food intak e even more than expected from increases in body mass. These increased organ masses are adaptive and permit energy-stressed mice to process ingested nutrients at rates exceeding the capacities of unstressed mic e. Safety factors (load/capacity ratios) of three intestinal brush-bor der hydrolases and transporters for nutrients declined toward 1 with i ncreasing food intake. The capacity of the brush-border enzyme sucrase to produce glucose remained matched to the capacity of the brush-bord er glucose transporter to absorb the resulting glucose, as both varied with food intake.