Dynorphin-[1-13], at concentrations of 5.8 × 10−12 to 5.8 × 10−9 M, stimulated insulin secretion from isolated islets of Langerhans of the rat, in medium containing 6 mM glucose. Higher concentrations of dynorphin had no significant effect on secretion. Dynorphin (5.8 × 10−9 M) was unable to initiate insulin release from islets in the presence of 2 mM glucose, or to increase insulin secretion further in the presence of 20 mM glucose or 6 and 12 mM glyceraldehyde. Dynor-phin-induced insulin secretion from islets was blocked by verapamil (5 μM) or by chlorpropamide (72 μM), but not by a mu opiate receptor antagonist, naloxone (0.11 μM), or by ICI 154129, a specific antagonist for the delta receptor (0.25 μM).
Dynorphin had no effect on islet somatostatin secretion, under conditions in which insulin secretion was greatly stimulated. Glucose (20 mM) and glyceraldehyde (6 and 12 mM) significantly increased both insulin and somatostatin secretion.
Dynorphin (5.8×10−9 M) increased 45Ca2+ uptake into islets, and also increased intracellular islet c-AMP levels. These changes persisted when higher concentrations of dynorphin were used.
These results suggest that (1) dynorphin is a very potent stimulus for insulin secretion; (2) dynorphin does not affect somatostatin secretion in static incubations of islets, in the same way as does glucose and glyceraldehyde; (3) dynorphin's effects may involve increased calcium ion movement and can be blocked by verapamil; (4) dynorphin can also increase islet c-AMP, and could thereby modulate the responsiveness of other secretagogues; (5) the actions of dynorphin on insulin secretion are not mediated by delta or mu opiate receptors in islets.