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 expenditure? We tested this question in mice by imposing peak energy burdens of lactation and of cold stress simultaneously We measured food intake, body and organ masses, and small intestinal brush-border hydrolase and transporter capacities in virgin female mice and in mothers nursing 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 lactational performance at 23 degrees C, while virgin mice at 5 degrees C are near their limit of food intake in response to cold stress. Nevertheless, the increments in food intake due to these two energy stresses applied simultaneously proved to be additive: food intake in lactating mice at 5 degrees C was even higher than the peak intake in lactating 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 expenditure limited by shared machinery for energy assimilation; assimilation could be pushed even higher by adding another energy stress. Masses of the small intestine, liver, and kidney increased with food intake 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 mice. Safety factors (load/capacity ratios) of three intestinal brush-border hydrolases and transporters for nutrients declined toward 1 with increasing food intake. The capacity of the brush-border enzyme sucrase to produce glucose remained matched to the capacity of the brush-border glucose transporter to absorb the resulting glucose, as both varied with food intake.