Growth retardation during the suckling period in expanded litters of rats: observations of growth patterns and protein turnover.

Postnatal patterns of growth and protein turnover were studied in suckling rats derived from expanded (EL) as compared to contracted (control) suckling rats (Widdowson and McCance, 1960). Maximum deviation of body weight growth velocity in EL vs. control litters of animals occurred early in the postnatal period (days 3-9). Fractional protein synthesis (Ks) in heart, diaphragm and gastrocnemius at 15 postnatal days were not significantly different in animals from expanded vs. control litters (e.g., Ks in heart 35.5 +/- 3.8% per day in control and 36.1 +/- 9.2% in expanded litters). Fractional growth rates in these 3 tissues were comparable in EL and controls, while fractional rates of protein breakdown (Kd) were somewhat reduced in diaphragm and gastrocnemius from EL animals (e.g., Kd in diaphragm 10.4% per day in control and 6.1% per day in EL). Absolute rates of protein synthesis and degradation were lower in EL animals, with rates of degradation reduced more than rates of synthesis. RNA activity of striated muscle tissues at 15 postnatal days in EL animals was not reduced vs. controls in contrast to results obtained in malnourished weanling and adult rats.