Pharmacokinetics of radioiodinated growth hormones in the turtle Chrysemysdorbigni

Growth hormone binding proteins (GHBP) have been identified in the blood of many species. The aim of the present work is to study the physiological ro le of the GHBP in the turtle serum which we recently described. Binding stu dies were carried out using in vivo pharmacokinetic and chromatographic tec hniques as well as in vitro methods. When I-125-GH was injected in physiolo gical concentration into Chrysemys dorbigni turtles, the first step of phar macokinetics was the binding of a significant fraction of the labeled GH by the GHBPs present in serum. The decay curve followed a three compartments model and gave the equation: Ae(-alpha t) + Be-beta t + Ce-gamma t. The fas t compartment with t1/2 of 14.4 min or 25.2 min, for hGH and bGH represents 30.3% and 18.9% of total radioactivity, respectively, at hypothetical time zero (not experimental). Chromatographic studies reveal that this rapid co mpartment represents free GH. The second and third compartments represent c omplex forms between GH and GHBPs present in the turtle serum, and represen t 70% and 80% of total radioactivity for hGH and bGH, respectively. In vitr o chromatographic studies showed direct evidence of the presence of GHBPs i n the turtle serum. The presence of these GHBPs changed the pharmacokinetic s of labeled GH in plasma and the subsequent liver uptake of GH. The labele d hGH or bGH binds to turtle serum in similar proportion, but maximal liver uptake of these hormones are completely different (L/B ratio of 9.2 +/- 0. 6 (n = 5) for I-125-hGH and 4.8 +/- 0.3 (n = 7) for I-125-bGH). The reasons for these differences could be that human GH binds to lactogenic and somat otropic receptors and bovine GH binds only to somatotropic receptors.