Endogenous somatostatin receptors mobilize calcium from inositol 1,4,5-trisphosphate-sensitive stores in NG108-15 cells

Somatostatin receptors are members of the G-protein-coupled receptor superfamily and exert their principal effects by coupling to inhibitory G-proteins. We used fura-2-based digital calcium imaging and assayed for [³H]inositol phosphates (IPs) to study the effects of somatostatin on intracellular calcium signaling in neuroblastomaXglioma NG108-15 cells. Both somatostatin-14 and octreotide induced concentration-dependent increases in intracellular Ca²⁺ concentration ([Ca²⁺]_i). Thirty-four percent of the cells responded to treatment with 100 nM somatostatin-14. Somatostatin-induced responses were not blocked by the removal of extracellular calcium; instead, they were abolished by pretreatment with 100 nM thapsigargin, an agent that depletes and prevents refilling of intracellular Ca²⁺ stores. Pretreatment with the inositol 1,4,5-trisphosphate (IP₃) receptor antagonist xestospongin C (10 μM) for 20 min inhibited markedly the somatostatin-induced response. Somatostatin (100 nM) increased [³H]IPs formation. U73122 (1 μM), an inhibitor of phospholipase C (PLC), completely blocked the somatostatin-induced [Ca²⁺]_i increases and the formation of [³H]IPs. Pretreatment with pertussis toxin (PTX, 200 ng/ml) for 24 h blocked the somatostatin-induced responses. Thus, we conclude that activation of endogenous somatostatin receptors in NG108-15 cells induces the release of calcium from IP₃-sensitive intracellular stores through PTX-sensitive G-protein-coupled PLC.