Identification and pharmacological characterization of somatostatin receptors in rat lung

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Abstract
[125I]‐[LTT]SRIF‐28 and [125I]‐SMS 201‐995 were used to identify and characterize somatostatin (SRIF) receptors localized in rat lung tissue. In vitro autoradiography of rat lung tissue sections showed the existence of specific, high affinity binding sites for [125I]‐[LTT]SRIF‐28 without any significant specific binding of the sst2/sst5‐receptor selective ligand [125I]‐SMS 201‐995. In radioligand binding studies, specific binding of [125I]‐[LTT]SRIF‐28 to membranes of rat lung was linearly related to the concentration of membrane protein used with only a small portion of nonspecific binding. With [125I]‐SMS 201‐995 no specific binding could be observed up to a membrane concentration of 0.1 mg of protein/assay tube. [125I]‐[LTT]SRIF‐28 bound rapidly to rat lung membranes with an apparent association rate constant (kapp) of 1.8±0.1 h−1 (n=3). The equilibrium of specific binding was reached after an incubation period of approximately 90 min at room temperature and remained constant for the next 3 h. The association rate constant (k1) was calculated to be 3.7×1010 M−1 h−1. The dissociation reaction followed first order kinetics with a dissociation rate constant (k−1)=0.44±0.07 h−1 corresponding to a half‐time of 95±15 min (n=3). From these kinetic experiments an equilibrium dissociation constant (KD) for the binding of [125I]‐[LTT]SRIF‐28 was calculated to be 11.9 pM. Saturation binding of [125I]‐[LTT]SRIF‐28 revealed an equilibrium dissociation constant (KD) of 50.1 pM (pKD=10.3±0.1; n=3) and a receptor density (Bmax) of 78±3 fmol mg−1 protein. A Hill coefficient not significantly different from 1 indicated saturable binding to a single class of high affinity binding sites. Specific binding of [125I]‐[LTT]SRIF‐28 to rat lung membranes was inhibited by SRIF‐14, SRIF‐28 and different SRIF analogues. SRIF and different synthetic short chain SRIF analogues exhibited the following rank order of potency: SRIF‐28>SRIF‐14>CGP 23996>>RC 160>BIM 23014>SMS 201‐995>BIM 23056>MK 678. The binding affinities for SRIF and the various SRIF analogues determined using rat lung tissue were in close correlation to those obtained with Chinese hamster ovary (CHO) cells stably expressing sst1 (r=0.92) and sst4(r=0.95) receptors, respectively. Reverse transcriptase ‐ polymerase chain reaction (RT‐PCR) showed the predominant expression of mRNA specific for sst4 receptors as well as some weak sst1 mRNA expression. The findings suggest that sst4 receptor expression is the predominant form of the somatostatin receptors identified in rat lung tissue. In this study we demonstrated for the first time the existence of sst4 receptors in mammalian tissue.