Somatostatin (SRIF) modulates distinct signaling pathways in rat pituitary tumor cells; negative coupling of SRIF receptor subtypes 1 and 2 to arachidonic acid release

The somatotropin release-inhibiting factor somatostatin-14 (SRIF) is known to activate distinct receptor subtypes (sst1-5). In rat pituitary tumor cells (GC cells), sst2 but not sst1 receptors mediate the SRIF-induced inhibition of intracellular concentration of Ca2+ ([Ca2+]i) and are negatively coupled to cAMP-dependent pathways. In the present study, transduction mechanisms coupling distinct SRIF receptors to their specific functional role were investigated with the use of both SRIF agonists with well-known affinity at individual SRIF receptors and the sst2 receptor antagonist L-Tyr(8) isomer of Cyanamid 154806 (CYN-154806). Our results demonstrate that sst1 and sst2 receptors are coupled to distinct signaling pathways in GC cells. In particular, sst2 receptors are negatively coupled to the cAMP-dependent pathway and this pathway is partially responsible for the sst2 receptor-mediated inhibition of [Ca2+]i. In addition, sst1 and sst2 receptors are both coupled to a decrease of arachidonic acid (AA) release with an efficacy similar to that of SRIF, suggesting that SRIF reduces AA release through either a partial activation of both receptors or the activation of one at a time. This finding is important given the well-accepted role for phospholipase A2 (PLA2) as a positive signaling component in transduction pathways of SRIF receptors. sst1 and sst2 receptor negative coupling to PLA2/AA pathways does not seem to be implicated in the SRIF-induced inhibition of [Ca2+]i. The possible role for the SRIF-mediated inhibition of AA release in GC cell function remains to be elucidated.