Expression of somatostatin receptors 3, 4, and 5 in mouse kidney proximal tubules

Background. Systemic infusion of somatostatin (SRIF) induces many physiological changes in human and rodent kidneys, including alterations in glomerular filtration, solute transport, and water clearance. Although somatostatin can bind to five different G-protein coupled receptors (SSTRs), only SSTR1 and SSTR2A proteins have been described convincingly in rat and/or human kidneys. Both are expressed primarily in collecting ducts, despite clear evidence that somatostatin also can bind to proximal tubules. Our aim was to characterize the expression of somatostatin receptors three to five in adult mouse kidneys. Methods. Reverse transcriptase-polymerase chain reaction (RT-PCR) was performed followed by Southern blotting on mouse kidney RNA for SSTR3, SSTR4, and SSTR5. Immunohistochemistry and dual-labeling immunofluorescence also were performed to localize the receptors in the kidney. Results. Messenger RNA was detected for somatostatin receptors 3 to 5 in the mouse kidney by RT-PCR, with confirmation by Southern blotting. By immunohistochemistry and dual-labeling immunofluorescence, the proteins for all three receptors were abundantly expressed, but exclusively localized to the proximal tubules. SSTR3 was present in intracellular granules, while SSTR4 and SSTR5 were expressed on the lumenal membranes of the tubules. Conclusions. Expression of SSTR3, SSTR4, and SSTR5 in mouse proximal tubules complements the expression of SSTR1 and SSTR2 in collecting ducts as seen in other species. Taken together, the kidney is one of few organs expressing all five somatostatin receptors outside of the nervous system and pancreas.