Characterization of high affinity receptors for insulin-like growth factors I and II on rat anterior pituitary cells

Primary cultures of rat anterior pituitary cells were assessed for the presence of specific receptors for insulin and for the somatomedin peptides, insulin-like growth factors I and II (IGF-I and IGF-II). Specific binding per 100,000 pituitary cells averaged 9.45 ± 1.69% (mean ± sd) for [¹²⁵I]IGF-II, 0.83 ± 0.06% for [¹²⁵I]IGF-I, and only 0.11% for [¹²⁵I]insulin. IGF-II was twice as potent as IGF-I in displacing [¹²⁵I]IGF-II, while insulin was totally nonreactive. IGF-I was 5-fold more potent than IGF-II at displacing [¹²⁵I]IGF-I and 1000-fold more potent than insulin. Scatchard analysis of [¹²⁵I]IGF-II binding revealed a curvilinear plot, which could be resolved into a high affinity receptor with a K_a of 7.0 × 10⁸ m^-1 and 120,000 receptor sites/cell, and a low affinity receptor with a K_a of 1.1 × 10⁸ m^-1 and 720,000 receptor sites/cell. The existence of abundant high affinity somatomedin receptors (especially for IGF-II) on rat anterior pituitary cells is consistent with a potential role for these peptides in the regulation of GH secretion.