RESUMO
BACKGROUND: Activation of G-protein coupled-somatostatin receptors induces the release of calcium from inositol 1, 4, 5-trisphosphate-sensitive intracelluar stores. G-protein-coupled receptor signaling decreases with prolonged exposure to an agonist. SEBJECTS and METHODS: Fura-2-based digital Ca2+ imaging was used to study the effects of prolonged exposure to an agonist on the somatostatin-induced intracellular Ca2+ concentration([Ca2+]i) increases in NG108-15 cells, which were differentiated with CO2-independent medium and 10micrometer forskolin. RESULTS: Exposure to somatostatin(1micrometer) for 30 min completely desensitized the NG108-15 cells to a second somatostatin-induced response. The cells recovered gradually over 20 min following washout of the somatostatin. The desensitization was not due to depletion of the intracellular Ca2+ stores, and pretreatment for 30 min with bradykinin(100nM), which activates phospholipase C, or DADLE(D-Ala2-D-Leu5 enkephalin, 1microM), which activates phospholipase C, failed to cross-desensitize the somatostatin-evoked [Ca2+]i increases. Treatment with 8-cpt-cAMP(0.1mM) for 30min did not influence the somatostatin-induced[Ca2+]i increases. Phorbol 12, 13-dibutyrate(PdBu, 1microM) blocked the response completely. Down-regulation of PKC due to 24 h exposure of PdBu (1microM) inhibited the somatostatin-induced desensitization. CONCLUSION: Prolonged exposure of somatostatin to NG108-15 cells desensitized the somatostatin-induced release of Ca2+ from the intracelluar store, with protein kinase C also involved in the desensitization.