Half-life of plasma growth hormone in young and old conscious female rats

The kinetics of disappearance of plasma GH was studied in young (3-4 months) and old (24-27 months) Sprague-Dawley female rats. Conscious, free moving animals carrying indwelling atrial and carotid cannulas received a single injection of 125I-rGH via the carotid cannula. Sequential blood samples were removed at intervals during the following hour, and total (TR) and immunoprecipitable radioactivity (IPR) were determined in the corresponding plasmas. Both TR and IPR displayed biexponential kinetics in vivo which did not differ significantly, for each variable, between young and old animals. The volumes of distribution of GH were also similar in both age-groups. The IPR/TR ratio, an estimate of GH inactivation within the plasma space, showed a decreasing sigmoid-shaped kinetics in vivo with a time of semi-inactivation (ti1/2) of 23.8 +/- 1.2 and 29.0 +/- 1.0 min (mean +/- SE) for young and old rats, respectively (P less than 0.02). The estrous status did not significantly affect ti1/2 values in vivo. The in vitro t1/2 was estimated by incubating plasma from the young and old animals at 37 degrees C with 125I-rGH for several hours. The IPR/TR ratio displayed a linear kinetics in vitro with t1/2 values of 23.7 +/- 1.7 and 25.8 +/- 1.9 h (NS) for young and old animals, respectively. The above results show that GH catabolism decreases slightly with age in the female rat, although it is unlikely that this change has a significant effect on plasma levels of GH. The data also suggest that GH is physiologically inactivated in the extravascular space.