Inhibition of insulin release by norepinephrine has been attributed to activation of ATP-sensitive K
+ channels, inactivation of voltage-dependent Ca
2+ channels, and inhibition of adenylyl cyclase. However, direct inhibitory action of norepinephrine at a distal site of stimulus-secretion coupling has also been suggested. To obtain more direct evidence for norepinephrine inhibition of insulin release at a distal site, we performed experiments in intact, non-permeabilized β cells. In rat pancreatic islets, a combination of glucose, phorbol ester and forskolin under stringent Ca
2+-free conditions was used as a trigger of insulin exocytosis at a distal site. Norepinephrine inhibited this Ca
2+-independent insulin release in a concentration-dependent manner, with an IC
50 of 50nM. The inhibition was complete, reversible, and pertussis toxin-sensitive, and not associated with any reduction of cAMP content in the islet cells. In conclusion, norepinephrine strongly, yet reversibly, inhibits insulin release in intact β cells at a late step of exocytosis, through pertussis toxin-sensitive, G protein-mediated mechanism(s).
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