IDENTIFICATION AND CHARACTERIZATION OF HIGH AND LOW AFFINITY BINDING-SITES FOR GROWTH-HORMONE IN RABBIT LIVER

The specific binding of rat (r) and rabbit (rb) [l25I]iodo-GH was studied in liver membrane preparations isolated from male, virgin female, late pregnant, and midlactating rabbits. Scatchard plots of dose-response displacement curves were consistently nonlinear and suggested the existence of at least two classes of binding sites for both GHs. The average binding affinity constants (Ka) for rGH on the low affinity (1.7 × 108M-1) and high affinity sites(s) (1.0 × 1010M-1) were similar to those for the rbGH. Displacement of [125I]iodo-rGH with preparations of GH, PRL, and placental lactogen (PL) from different species demonstrated that the potencies and slopes of the dose-response curves varied considerably, indicating that the displacement activities of many hormones differ at the two sites. Neither the potencies nor the slopes of the dose-response curves correlated with the growth-promoting activities of the various hormones. On the bases of parallelism and potency, rbGH, equine GH, and bovine (b) PRL were the hormones most closely related to the rGH standard. Human (h), ovine (o), and bGH and oPL exhibited steeper slopes of displacement than did rGH and, with the exception of oPL, they had greater potencies. Inhibition of [125I]iodo-rGH binding by oPRL and hPL exhibited lower slope values and lesser potencies than rGH. hPRL, rbPRL, rPRL, and equine PRL effected minimal displacement of [l25]iodo-rGH. The potencies of different hormones relative to the rGH standard varied significantly among different rabbit liver preparations. This variation in apparent potency correlated with the proportion of high affinity sites on the membranes and with the relative affinities of the high and low affinity sites for the different hormones. Neither the proportion of high affinity sites nor the total binding capacity appeared to correlate with the sex or physiological state of the rabbits. © 1979 by The Endocrine Society.