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

Abstract

Because acetylcholine (ACh) is a recognized potentiator of glucose-stimulated insulin release in the normal β-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 β-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 β-cell and, in doing so, normalizes its defective glucose responsiveness.