SOMATOSTATIN RECEPTOR SUBTYPES SSTR2 AND SSTR5 COUPLE NEGATIVELY TO AN L-TYPE CA2-LINE ATT-20( CURRENT IN THE PITUITARY CELL)

The somatostatin receptor subtypes SSTR2 and SSTR5 mediate distinct en docrine and exocrine functions of somatostatin and may also be involve d in mediating the neuromodulatory actions of somatostatin in the brai n. To investigate whether these receptors couple to voltage-sensitive Ca2+ channels, SSTR2 and SSTR5 selective agonists were tested for thei r effects on AtT-20 cells using whole cell patch clamp techniques. The SSTR2 selective agonist MK 678 inhibited Ca2+ currents in AtT-20 cell s. The effects of MK 678 were reversible and blocked by pertussis toxi n pretreatment, suggesting that SSTR2 couples to the L-type Ca2+ chann els via G proteins. Other SSTR2-selective agonists, including BIM 2302 7 and NC8-12, were able to inhibit the Ca2+ current in these cells. Th e SSTR5 selective agonist BIM 23052 also inhibited the Ca2+ currents i n these cells and this effect was reversible and blocked by pertussis toxin treatment. The ability of SSTR5 to mediate inhibition of the Ca2 + current was greatly attenuated by pretreatment with the SSTR5-select ive agonist BIM 23052, whereas SSTR2-mediated inhibition of the Ca2+ c urrent was not altered by pretreatment with the SSTR2-selective agonis t MK 678. Thus, the SSTR2 and SSTR5 couplings to the Ca2+ current are differentially regulated. The peptide L362,855, which we previously ha ve shown to have high affinity for the cloned SSTR5, had minimal effec ts on Ca2+ currents in AtT-20 cells at concentrations up to 100 nM and did not alter the ability of MK 678 to inhibit Ca2+ currents. However , it completely antagonized the effects of the SSTR5-selective agonist BIM 23052 on the Ca2+ currents. L362,855 is an antagonist/partial ago nist at SSTR5 since it can reduce Ca2+ currents in these cells at conc entrations above 100 nM. L362,855 is also an antagonist/partial agonis t at the cloned rat SSTR5 expressed in CHO cells since it is able to b lock the inhibition of cAMP accumulation induced by somatostatin at co ncentrations below 100 nM but at higher concentrations can inhibit cAM P formation itself. Structural analysis of L362,855 reveals that only a single hydroxyl group at residue seven in the peptide is needed to c onvert the compound from an antagonist/partial agonist to a full agoni st at SSTR5. These studies reveal that two different somatostatin rece ptor subtypes, SSTR2 and SSTR5, can mediate the inhibition of an L-typ e Ca2+ channel in AtT-20 cells by somatostatin. The receptor subtype r esponses can be distinguished by selective agonists and antagonists an d are regulated differently by agonist pretreatment. The inhibition of Ca2+ influx into endocrine cells and neurons may be a major cellular mechanism by which somatostatin modulates hormone and neurotransmitter release. Our results reveal that at least two receptor subtypes can m ediate this cellular response.