Metabolic Clearance Rate of Growth Hormone during Experimental Growth Arrest and Subsequent Recovery in Rats*

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Abstract
The MCR of rat GH (rGH) was determined by a single injection method with [l25I]rGH in young, postweaning, Long-Evans male rats through experimental growth retardation and 28 days of growth recovery. The three experimental models used were 1) fast for 48 h, 2) sc injections of cortisone acetate in a dose of 5 mg/rat-day for 4.days, and 3) feeding 6-propylthiouracil (PTU), added to the stock diet in a concentration of 0.1% for 20 days. The MCR decreased during fasting and hypothyroidism. During cortisone treatment, however, the MCR remained normal. During recovery, the MCR returned to normal by 7 days after the fast and to near normal by 21 days after the end of PTU feeding. In the cortisone-treated animals, the MCR dropped below the control value by recovery day 7 and remained lower than in controls throughout the remaining period of observation. The persistently reduced MCR during recovery in the cortisone model, which does not have catch-up growth acceleration,: may result from reduced tissue mass and a reduced number of. GH receptors in the treated animals. PTU-fed animals displayed a return to normal MCR. Although their mean body weight remained lower than that of controls by recovery day 28, the PTUfed rats had catch-up growth acceleration throughout the 28-day recovery period. The present findings indicate that although MCR is affected by these experimental treatments, the alterations do not cause the previously observed elevation of GH during the recovery period. The latter is, therefore, probably the result of increased secretion of GH.