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Use of tritiated water for measurement of 24-hour milk intake in suckling lean and genetically obese (ob/ob) mice.

A non-invasive method has been developed for measuring milk intake of suckling mice under physiological conditions. This method was used to determine whether genetically obese (ob/ob) mice are hyperphagic at 10 and 15 days of age. Lactating dams were injected with tritiated water (3H2O), which equilibrated in body water within 30 minutes. A constant specific activity of 3H2O was maintained over a 24-hour period by provision of 3H2O in drinking water. Tritium accumulation in body water of pups was proportional to their milk intake. After 24 hours, pups were removed from the dam, weighed, and blood samples (less than 10 microliters) obtained for assay of plasma 3H content by liquid scintillation counting. Body water content was computed from body weight. The composition of mouse milk taken from dams on days 10 and 15 of lactation was analyzed both volumetrically and gravimetrically. Water content was 68 to 69%; lipid content was 20% on day 10, 17% on day 15. At 10 days, mean milk intake was 0.96 ml, independent of litter size. At 15 days, intake per pup tended to decrease, from 1.4 to 0.8 ml, with increasing litter size. Using this method we have established that under physiological conditions ob/ob mice, which were identified as such at 4 to 5 weeks of age, do not have increased milk consumption at either 10 or 15 days of age.