Changes in the extracellular and intracellular fluid phases of muscle during starvation and dehydration in adult rats

Evaluation of a Mathematical Model of Rat Body Weight Regulation in Application to Caloric Restriction and Drug Treatment Studies

The purpose of this work is to develop a mathematical model of energy balance and body weight regulation that can predict species-specific response to common pre-clinical interventions. To this end, we evaluate the ability of a previously published mathematical model of mouse metabolism to describe changes in body weight and body composition in rats in response to two short-term interventions. First, we adapt the model to describe body weight and composition changes in Sprague-Dawley rats by fitting to data previously collected from a 26-day caloric restriction study. The calibrated model is subsequently used to describe changes in rat body weight and composition in a 23-day cannabinoid receptor 1 antagonist (CB1Ra) study. While the model describes body weight data well, it fails to replicate body composition changes with CB1Ra treatment. Evaluation of a key model assumption about deposition of fat and fat-free masses shows a limitation of the model in short-term studies due to the constraint placed on the relative change in body composition components. We demonstrate that the model can be modified to overcome this limitation, and propose additional measurements to further test the proposed model predictions. These findings illustrate how mathematical models can be used to support drug discovery and development by identifying key knowledge gaps and aiding in the design of additional experiments to further our understanding of disease-relevant and species-specific physiology.

Fluid compartmentation in skeletal muscle and carcass of Mus musculus acclimated to water scarcity

1. Total water (TW), and extracellular water (ECW) (as sodium and chloride space) were determined in skeletal muscle and carcass of Mus musculus acclimated to long-term water shortage. 2. The presence of fat in control mice and those in early stages of acclimation resulted in an apparent increase in TW and ECW as acclimation proceeded. 3. In contrast, fluid volumes per fat-free weight were either unchanged from controls or reduced. 4. Sodium space exceeded chloride space. 5. Muscle and carcass had essentially the same pattern of fluid shifts. 6. We conclude that ECW maintenance is a preeminent component of the acclimation process in this species.

The effect of hunger on water intake in rats

Although reciprocal inhibition between eating and drinking has been postulated, the commonly observed reduction of water intake by hungry rats may not be due to any direct inhibitory mechanism. In one experiment rats deprived of food for 24 hr. and then injected with 2 ml. of NaCl drank the same as rats that had received food ad lib. In the second experiment a stomach load of 10 ml. of water 3 hr. before the salt injection was designed to abolish any water deficit that might have occurred during food deprivation had there been inhibition of drinking by hunger. Here again rats deprived of food did not drink less than undeprived animals. In fact the hungry rats drank slightly but significantly more. This phenomenon may be related to the observation confirmed here that during food deprivation rats excrete about twice as much rather dilute urine as during ad lib food intake. This seems to indicate that though in general food deprived rats drink less than normal they actually drink more than they need. Schedule induced polydipsia also occurs during food deprivation and this too makes it unlikely that water-intake is actually inhibited during hunger.