Gastric and Plasma Volume in the Control of Milk Intake in Suckling Rats

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Abstract
A method is described for measuring the milk intake of infant rats accurately in short (2-h) tests. It depends upon the prevention of urine loss by occluding the external urethra with collodion during the period of the test. With this method, the role of two internal stimuli—gastric volume and plasma volume—in the control of milk intake were investigated. Saline isotonic with plasma, given by stomach tube in loads of 1.0 ml reduced milk intake. Loads of 1.5 and 2.0 ml prevented also the actual attachment of the pups to the nipple. If, however, the saline was allowed to clear completely from the stomach before the pups were allowed to feed, its effect on milk intake was completely lost. This suggests that any expansion of plasma volume that results from the load after absorption cannot be responsible for inhibiting milk intake. Consistently with this, peritoneal dialysis with the hyperoncotic colloid polyethylene glycol, a procedure which reduces plasma volume, did not stimulate milk intake, though it reduced the rate of urine formation. These results show, therefore, that milk intake responds to changes in gastric volume; but not to changes in plasma volume.

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