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(Investigative Ophthalmology and Visual Science. 2004;45:1499-1506.)
© 2004 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.03-0835
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Effect of Somatostatin on Nitric Oxide Production in Human Retinal Pigment Epithelium Cell Cultures

Anna Vasilaki,1,2 Thekla Papadaki,1,3 George Notas,4 George Kolios,4 Niki Mastrodimou,1 Daniel Hoyer,5 Miltiadis Tsilimbaris,3 Elias Kouroumalis,4 Ioannis Pallikaris,3 and Kyriaki Thermos1

1From the Departments of Pharmacology, 4Gastroenterology, and 3Ophthalmology, Faculty of Medicine, University of Crete, Heraclion, Crete, Greece; and 5Nervous System Research, Novartis Pharma AG, Basel, Switzerland.

PURPOSE. To investigate the presence of somatostatin and its receptors (sst1–5 receptors) and their possible involvement in the regulation of nitric oxide (NO) production in human RPE cell cultures.

METHODS. Human RPE cells (D407) were used for all studies performed. Somatostatin levels were detected by radioimmunoassay, and RT-PCR and immunocytochemistry studies were performed to identify the somatostatin receptors (sst1-sst5). Radioligand binding assays were also performed examining the ability of certain somatostatin ligands (sst1, sst2, sst5) to compete for [125I]Tyr11 somatostatin binding. The presence of NO synthase in the cultures was assayed with NADPH-diaphorase cytochemistry, and RT-PCR, and NO levels were assessed by examining the production of its stable metabolites NO2– and NO3– (NOx–).

RESULTS. SRIF was detected in a concentration of 0.56 ± 0.13 picomoles/mg protein. sst1, sst2, and sst5 mRNAs were detected, yet only sst2B and sst5 immunoreactivity was observed in human RPE cell cultures. sst1- and sst5- but not sst2-selective ligands displaced the specific [125I]Tyr11 somatostatin binding to RPE cell membranes. NADPH-diaphorase stain and iNOS mRNA were detected. SRIF and the sst2-selective analogue MK678 increased the levels of NOx– in a concentration-dependent manner. This increase was blocked by the sst2 antagonist CYN-154806 (Ac-4NO2-Phe-c(dCys-Tyr-dTrp-Lys-Thr-Cys)-dTyr-NH2).

CONCLUSIONS. These results demonstrate the presence of somatostatin, and its receptors sst1, sst2B, and sst5 in human RPE cells and suggest an autocrine or paracrine role for somatostatin. Somatostatin’s ability to regulate NO production, by activating sst2 receptors, provides a functional role of somatostatin in the RPE.




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