Somatostatin receptor subtypes SSTR2 and SSTR5 couple negatively to an L-type Ca2+ current in the pituitary cell line AtT-20.

The somatostatin receptor subtypes SSTR2 and SSTR5 mediate distinct endocrine and exocrine functions of somatostatin and may also be involved in mediating the neuromodulatory actions of somatostatin in the brain. To investigate whether these receptors couple to voltage-sensitive Ca2+ channels, SSTR2 and SSTR5 selective agonists were tested for their effects on AtT-20 cells using whole cell patch clamp techniques. The SSTR2 selective agonist MK 678 inhibited Ca2+ currents in AtT-20 cells. The effects of MK 678 were reversible and blocked by pertussis toxin pretreatment, suggesting that SSTR2 couples to the L-type Ca2+ channels via G proteins. Other SSTR2-selective agonists, including BIM 23027 and NC8-12, were able to inhibit the Ca2+ currents in these cells. The SSTR5 selective agonist BIM 23052 also inhibited the Ca2+ currents in 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-selective agonist BIM 23052, whereas SSTR2-mediated inhibition of the Ca2+ current was not altered by pretreatment with the SSTR2-selective agonist MK 678. Thus, the SSTR2 and SSTR5 couplings to the Ca2+ current are differentially regulated. The peptide L362,855, which we previously have shown to have high affinity for the cloned SSTR5, had minimal effects 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 agonist at SSTR5 since it can reduce Ca2+ currents in these cells at concentrations above 100 nM. L362,855 is also an antagonist/partial agonist at the cloned rat SSTR5 expressed in CHO cells since it is able to block the inhibition of cAMP accumulation induced by somatostatin at concentrations below 100 nM but at higher concentrations can inhibit cAMP formation itself. Structural analysis of L362,855 reveals that only a single hydroxyl group at residue seven in the peptide is needed to convert the compound from an antagonist/partial agonist to a full agonist at SSTR5. These studies reveal that two different somatostatin receptor subtypes, SSTR2 and SSTR5, can mediate the inhibition of an L-type Ca2+ channel in AtT-20 cells by somatostatin. The receptor subtype responses can be distinguished by selective agonists and antagonists and 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 mediate this cellular response.