Restitution of defective glucose-stimulated insulin secretion in diabetic GK rat by acetylcholine uncovers paradoxical stimulatory effect of β-cell muscarinic receptor activation on cAMP production

Because acetylcholine (ACh) is a recognized potentiator of glucose-stimulated insulin release in the normal beta-cell, we have studied ACh's effect on islets of the Goto-Kakizaki (GK) rat, a spontaneous model of type 2 diabetes. We first verified that ACh was able to restore the insulin secretory glucose competence of the GK P-cell. Then, we demonstrated that in GK islets 1) ACh elicited a first-phase insulin release at low glucose, whereas it had no effect in Wistar; 2) total phospholipase C activity, ACh-induced inositol phosphate production, and intracellular free calcium concentration ([Ca2+](i))) elevation were normal; 3) ACh triggered insulin release, even in the presence of thapsigargin, which induced a reduction of the ACh-induced [Ca2+](i) response (suggesting that ACh produces amplification signals that augment the efficacy of elevated [Ca2+](i) on GK exocytosis); 4) inhibition of protein kinase C did not affect [Ca2+](i) nor the insulin release responses to ACh; and 5) inhibition of cAMP-dependent protein kinases (PKAs), adenylyl cyclases, or cAMP generation, while not affecting the [Ca2+](i) response, significantly lowered the insulinotropic response to ACh (at low and high glucose). In conclusion, ACh acts mainly through activation of the cAMP/PKA pathway to potently enhance Ca2+-stimulated insulin release in the GK beta-cell and, in doing so, normalizes its defective glucose responsiveness.