Characterisation of human recombinant somatostatin receptors. 3. Modulation of adenylate cyclase activity

Abstract.

The five human somatostatin receptor subtypes (hsst_1–5) were stably expressed in CCL39 cells (Chinese hamster lung fibroblast cells) to study the inhibition of forskolin-stimulated adenylate cyclase (FSAC) activity induced by somatostatin (somatotropin release inhibiting factor, SRIF), cortistatin (CST) and SRIF peptide analogues. Inhibition of FSAC was observed with all five receptors, although the maximal effects produced by SRIF₁₄ varied from around 40% (sst₁, sst₂, sst₄) to 67% (sst₃, sst₅) reflecting to some extent differences in receptor density (B _max values published in accompanying paper, this journal). SRIF₂₈ was slightly more potent than SRIF₁₄ to inhibit FSAC at all five receptors, although the potency of the natural peptides SRIF₁₄, SRIF₂₈ and CST₁₇ was generally similar with pEC₅₀-values ranging from 7.5 to 8.7 depending on receptor and peptide.

At SRIF₁ receptors (sst₂, sst₃, sst₅) most of the peptide analogues displayed full agonism (with some exceptions e.g. BIM 23056 at sst_1–3 and sst₅ receptors, and L362,855 and cycloantagonist SA at sst₃ receptors), whereas at SRIF₂ receptors these analogues tended to behave as partial agonists. BIM 23056 was an antagonist at sst₃ receptors (antagonist binding constant pK _B=6.33), but not at other receptors.

The AC inhibition profiles of sst_1–5 receptors were compared with the different radioligand binding profiles as well as with [³⁵S]guanosine 5'-O-(3-thiotriphosphate) ([³⁵S]GTPγS) binding profile for sst_2–5 receptors. High correlations were observed between FSAC inhibition, radioligand binding and [³⁵S]GTPγS binding profiles at sst₃, sst₄ and sst₅ receptors; by contrast, correlation coefficients at sst₁ and sst₂ receptors were low, and the binding profiles of [¹²⁵I][Tyr¹⁰]CST₁₄ correlated poorly. In line with these findings, the FSAC inhibition and [³⁵S]GTPγS binding correlated poorly at sst₂ receptors (sst₁ receptors show no significant induction of [³⁵S]GTPγS binding). The apparent lack of, or only weak, relationship between FSAC, radioligand or [³⁵S]GTPγS binding observed for some SRIF receptors suggests that different active states may exist for these receptors, which may favour one transduction cascade over others.