Cartilage Metabolism during Growth Retardation following Irradiation of the Head of the Neonatal Rat

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Abstract
The heads of 2-day-old male and female rats were irradiated with a single dose of 600 rads X-irradiation, a dose which is known to stunt body weight, tibial length and tail length, in order to ascertain its effect on synthesis by cartilage of sulfated proteoglycans, DNA, chondroprotein and collagen as determined by utilization of [35S]sulfate, [Me-3H]thymidine, [1-14C]leucine, and [3,4-3H]proline, respectively. Data have been collected at 20-21, 23, 41-45, and 70-71 days of age. In comparison to controls, growth in body weight, tibial length and tail length was significantly retarded in irradiated rats of both sexes. Although slow catch-up growth was observed with respect to tail length in both sexes and tibial length in females, a significant deficit in body weight in irradiated rats in both sexes remained at 70-71 days. Cartilage metabolism as evidenced by incorporation of the labeled substances showed no significant disturbance just prior to weaning (20-21 days) or after completion of the principal growth surge (70-71 days). Reduced sulfate and thymidine incorporation attributable to a brief period of undernutrition associated with weaning occurred in head-irradiated rats immediately following weaning (23 days). Increased isotope incorporation occurred at 41-45 days of age in cartilage of irradiated rats incubated with labeled sulfate, leucine, and proline; it did not increase with labeled thymidine. Neonatal head irradiation apparently slows the rate of growth through the age of most rapid postnatal growth in normal rats. The pattern of cartilage metabolism during this time can be the result either of stimulation by a factor other than somatomedin, or selective inhibition of cartilage thymidine incorporation acting in combination with somatomedin.