Studies of Insulin, Growth Hormone and Prolactin Binding: Tissue Distribution, Species Variation and Characterization

The specific binding of 125I-labeled insulin, hGH and oPRL was surveyed in crude membrane preparations of various tissues of the monkey, rat, guinea pig, rabbit, sheep, pigeon, and frog. Binding varied greatly with the species examined. The highest binding of 125I-insulin was seen in guinea pig tissues, especially kidney, fetal placenta and liver. Significant binding of 125I-insulin was seen in many but not all known target tissues, and in other tissues as well (viz. placental, adrenal, brain). The binding of 125I-hGH and 125I-oPRL paralleled each other, except in rabbit liver, and was highest in rabbit liver and adrenal, female rat liver, frog kidney, and several sheep tissues. There was no relation between phylogenetic proximity to man and the level of specific binding of 125I-hGH observed. The lowest binding of 125I-hGH was in male rat liver and various tissues of the mpnkey and guinea pig, a high proportion of which showed significant binding of 125I-insulin. 125I-insulin was displaced from the membranes of 4 guinea pig tissues by insulin, proinsulin, and desoctapeptide insulin in approximate parallel with their biological effectiveness. Human GH and oPRL were without influence. Specificity studies of 125I-labeled hGH, oPRL and bGH binding indicated a predominance of prolactin binding sites in pregnant rat liver and rabbit mammary gland, and of both prolactin and growth hormone binding sites in rabbit liver. The low binding of 125I-hGH in monkey and guinea pig liver membranes was not due to increased degradation by these membranes. Mixing of both tissue and membrane preparations revealed no evidence of an inhibitor in guinea pig and male monkey liver but suggested its presence in female monkey liver. (Endocrinology95: 521, 1974)