Identification of distinct signalling pathways for somatostatin receptors SSTR1 and SSTR2 as revealed by microphysiometry

Somatostatin receptors (SSTRs) are known to mediate diverse cellular respon ses. Most target cells express more than one SSTR isoform, making it diffic ult to define the signalling pathway used by individual receptor subtypes, Thus, we have expressed SSTR1 or SSTR2 in rat pituitary F4C1 cells which la ck endogenous SSTRs. Using a silicon-based biosensor system, the Cytosensor microphysiometer, which measures the extracellular acidification rate (ECA R) in real time, we have studied the responses to SS mediated by either SST R1 or SSTR2. In control F4C1 cells, SS had no effect on the basal ECAR. In transfected cells expressing only SSTR1, SS caused a unique decrease in ECA R in a concentration-dependent manner. Receptor-mediated decreases in ECAR have not been reported previously. In F4C1 cells expressing only SSTR2, SS induced a bidirectional ECAR response, a rapid increase followed by a decre ase below basal. Two SS analogues, MK678 and CH275, induced characteristic ECAR responses with the expected receptor selectivities for SSTR1 or SSTR2. Pretreatment of F4C1 cells with pertussis toxin abolished the decreases in ECAR mediated by both SSTR1 and SSTR2, but only partially reduced the incr ease in ECAR mediated by SSTR2. The decrease in ECAR did not depend on a de crease in intracellular cAMP. The ECAR responses to SS were modestly attenu ated by methylisobutylamiloride (MIA), an inhibitor of the ubiquitous Na+-H + exchanger NHE1. Removal of extracellular Na+ greatly inhibited the ECAR r esponses to SS, demonstrating a role for both amiloride sensitive and -inse nsitive Na+-dependent acid transport mechanisms in SS induced extracellular acidification. In conclusion, we have identified and characterized differe nt signalling pathways for SSTR1 and SSTR2 in pituitary cells as measured b y crophysiometry. (C) 1999 Elsevier Science Inc.