Immunohistochemical expression of somatostatin receptor subtypes in various normal human tissues

Somatostatin is a tetradecapeptide, which mediates its action via five receptor subtypes (sst1–5). Because of the inhibitory and antiproliferative effects, long-acting somatostatin analogues were developed. While octreotide binds with high affinity to sst2a only, the new analogue SOM230 demonstrates high affinity to sst1, 3 and 5 in addition. We examined somatostatin receptor protein expression in 24 normal human tissues. Immunostaining was performed with specific polyclonal antibodies for sst1, 2a, 3, 4 and 5. Staining pattern, distribution and subcellular localization of somatostatin receptor subtypes were evaluated. Sst 1 was detected in 25%, sst2a in 46%, sst3 in 33%, sst4 in 17% and sst5 in 42% of the investigated tissues. Pituitary, stomach (both antrum and corpus), and kidney were positively stained for all five ssts. Ileum, colon and cerebellum (Purkinje cells) expressed sst5 only. Pancreatic islet cells exhibited a positive staining for sst1–3. The adrenal cortex expressed sst1–3 and 5, whereas the medulla was positive for sst3 only. Testis, ovary and bone marrow demonstrated a positive staining for sst2a only. The salivary gland expressed sst2a, 3 and 5, and the myocardium sst2a and 5. No expression of ssts was found in thyroid, trachea, lung, liver, spleen, prostate, placenta and skeletal muscles. This study demonstrates that somatostatin receptor subtypes are expressed in a variety of normal human tissues. Somatostatin analogues may offer new diagnostic and therapeutic implications for tumors related to these tissues. However, treatment of defined tumors with somatostatin analogues may also alter other normal tissues.