The two subtype 1 somatostatin receptors of rainbow trout, Tsst1A and Tsst1B, possess both distinct and overlapping ligand binding and agonist-induced regulation features.

In the present study, two isoforms of somatostatin receptor subtype one, previously obtained from the brain of rainbow trout, Tsst1A and Tsst1B, were stably transfected in the Chinese hamster ovary cell line (CHO-K1) and their binding properties were characterized. High affinity binding of somatostatin by expressed receptors was saturable and ligand selective. Both Tsst1A and Tsst1B preferentially bound peptides derived from preprosomatostatin I (PPSS I; e.g., SS-14-I) over those derived from PPSS II (containing Tyr7, Gly10-SS-14-I at their C-terminus; e.g., SS-25-II). The rank order of ligand affinities for Tsst1A was SS-28-I>SS-14-I>SS-26-I?SS-28-II>SS-14-II>SS-25-II. The rank order for Tsst1B was SS-14-I>SS-28-I>SS-26-1?SS-28-II>SS-25-II>SS-14-II. Agonist-induced regulation of Tsst1A and Tsst1B was also investigated. After 30 min of SS-14-I exposure, both Tsst1A and Tsst1B underwent rapid internalization; ca. 60% of membrane Tsst1A was internalized and only about 40% of membrane Tsst1B was internalized. Prolonged agonist exposure (up to 48 h) induced up-regulation of membrane-expressed Tsst1A, but had no effect on Tsst1B. These results indicate that Tsst1s display both distinct and overlapping ligand binding and agonist-induced regulation features. Such features may form the basis of ligand-selection and have important consequences on target organ responsiveness.