Use of Tritiated Water for Measurement of 24-Hour Milk Intake in Suckling Lean and Genetically Obese (ob/ob) Mice

Abstract

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.

Obese gene expression: reduction by fasting and stimulation by insulin and glucose in lean mice, and persistent elevation in acquired (diet-induced) and genetic (yellow agouti) obesity.

“…The One of the earliest detectable differences between ob/ob mice (which lack functional ob gene product) and wild-type controls (which produce the ob gene product) is an elevation of plasma insulin in ob/ob mice before the development of obesity. Fourteen days after birth, plasma insulin is elevated in ob/ob mice, compared to wild-type controls (16), whereas at this age neither food intake nor body weight differs between ob/ob and wild-type mice (17). Such results suggest that an early effect of leptin is to inhibit insulin secretion, independent of its effects on feeding.…”

Lysosomal unesterified cholesterol content correlates with liver cell death in murine Niemann-Pick type C disease

“…The present studies suggest that these abnormalities are related to the load of cholesterol reaching the liver during the neonatal period. The suckling mouse, for example, takes in each day ?54 mg cholesterol/kg bw from its mother's milk under circumstances in which the pup is synthesizing daily in excess of 150 mg/kg sterol (60,61). In the NPC mouse, this leads to hepatomegaly, increased hepatic cholesterol concentrations, and cholestasis with very high plasma alkaline phosphatase levels (31).…”

Effects of fasting and refeeding on ob gene expression in white adipose tissue of lean and obese (ob/ob) mice

“…One possibility is that it acts as a satiety factor [3,4], although this is not consistent with the observations that the early development of the obesity in ob/ob mice takes place without an increase in food intake [2,[5][6][7]. Nevertheless, given the importance of ob protein in energy balance, it would be predicted that expression of the ob gene should be highly sensitive to nutritional status.…”