Aim: 

To investigate the effect of a short-term exposure to somatostatin (SS), its receptors (SSTR) selective agonists as well as muscarinic receptors agonists upon acetylcholine-induced release of ³H-MPP⁺ from bovine adrenal medullary cells.

Results: 

Acetylcholine (ACh, 100, 500 mM) was found to increase the release of ³H-MPP⁺ by these cells (to 175 and 171% of basal release, respectively). ACh-elicited ³H-MPP⁺ release was significantly reduced by hexamethonium (100 mM) and atropine (100 mM), selective nicotinic and muscarinic antagonists, respectively. Previous exposure to any of two muscarinic agonists, oxotremorine or pilocarpine, led to a significant reduction of ³HMPP⁺ release in response to 100 mM ACh, to about a maximum of 51% and 78% of control, respectively. Somatostatin (SS, 0.01-0.1 mM), previously applied to the preparation, depressed ACh-elicited ³H-MPP⁺ release by 25-27%, but only when a 500 mM ACh concentration was used. The inhibition exerted by SS upon ACh-evoked ³H-MPP⁺ release appeared to be mediated by its SSTR:

(1) SSTR2, 3 and 4 subtype agonists mimicked the effects seen with SS, and (2) the SSTR non-selective antagonists, cyclo-SS, counteracted the SS inhibitory effect. When SS was tested in the presence of any of the muscarinic agonists, oxotremorine or pilocarpine, its inhibitory effect on 500 mM ACh-induced ³H-MPP⁺ release was no longer detectable. Conclusion:

The similar effect of short-term exposure to SS and muscarinic agonists over ACh-induced release of ³H-MPP⁺, as well as the loss of effect of SS by the presence of the muscarinic agonists, suggest that these compounds may share signaling pathways.