Somatostatin Receptors on Rat Anterior Pituitary Membranes

Abstract
[125I]Iodo-Tyr1-somatostatin (SRIF) binds with high affinity to one class of sites in the rat anterior pituitary with a KD of 0.91 ± 0.22 nM and a receptor concentration of 104.4 ± 1.9 fmol/mg protein. This binding is saturable with respect to tissue concentration and is time-, temperature-, pH-, and calcium- dependent. It is also reversible as a function of time. The rates of association and dissociation were calculated to be 5.98 × 107 M-1 min-1 and 0.578 min-1, respectively. Binding of [125<I]iodo-Tyr1-SRIF is not inhibited by morphine, β-endorphin, [D-Ala2]Met-enkephalin, LHRH, TRH, histidylproline diketopiperazine, neurotensin, substance P, bombesin, or vasoactive intestinal peptide. In contrast, SRIF, [Tyr1]SRIF, and [D-Trp8,D-Cys14]SRIF displace [125I]iodo-Tyr1-SRIF binding with Ki values 0.10 ± 0.05, 0.46 ± 0.18, 0.05 ± 0.01 nM, respectively. The constants of inhibition of a series of alanine monosubstituted analogs of SRIF are correlated (r = 0.89) with their biological potency on GH secretion. Furthermore, postnatal development patterns of [125.I]iodo-Tyr1-SRIF binding sites follow the ability of SRIF to inhibit GH release. Thus, [125I]iodo- Tyr1-SRIF binding to adenohypophyseal membranes seems to reflect interaction with SRIF receptors on adenohypophyseal cells. Since biological effects of the peptide have been reported on GH, thyrotropin-stimulating hormone, and PRL secretion, further studies are required to determine the cell types upon which this binding occurs.