Acetylcholine potentiated the glucose-induced insulin release from microdissected mouse islets of Langerhans but had no effect on basal insulin release. Significant potentiation was obtained with 0.1 micron acetylcholine in the presence of 10 micron eserine and with 1 micron or more acetylcholine in the absence of a choline esterase inhibitor. Carbamylcholine, too, potentiated insulin release. Potentiation was blocked by methylatropine, whereas methylatropine alone had no effect on insulin release. Acetylcholine or carbamylcholine (5-500 micron) had no obvious effect on cyclic GMP or cyclic AMP in the islets. In the presence of 11.1 mM D-glucose, the membrane potential of beta-cells oscillated slowly between a polarized silent state of -50 to -55 mV and a depolarized active state of -33 to -39 mV, at which a fast spike activity occurred. Acetylcholine made the potential stay at the plateau and induced a continuous spike activity pattern. Atropine inhibited the electrical effects of acetylcholine but not those of glucose alone. It is suggested that cholinergic potentiation of insulin release is mediated by changes of transmembrane ionic fluxes, probably without the intervention of cyclic GMP or cyclic AMP.

1. Acetylcholine potentiated glucose‐stimulated insulin release from ob/ob‐mouse islets in salt‐balanced bicarbonate buffer and to a lesser extent in Tris buffer; basal insulin release at 3 m M-D‐glucose was not affected. Potentiation required the presence of Ca2+. 2. In bicarbonate buffer, ACh stimulated the islet uptake of 45Ca2+ at 3 m M‐glucose but not significantly at 11 m M; no effect was seen in Tris buffer. 3. At 11 m M‐glucose, ACh increased the fluorescence from Ca2+‐chlorotetracycline in dispersed islet cells; the effect was inhibited by atropine. 4. At both 3 and 11 m M‐glucose, ACh stimulated the islet uptake of 22Na+ in 60 min. At 11 m M‐glucose, 22Na+ uptake in 5 min was also enhanced significantly, and this effect was inhibited by atropine. 5. At 3 m M‐glucose, ACh probably stimulated the islet uptake of 86Rb+ in 10 min. 6. ACh had no effect on 36Cl− retention at 3 or 11 m M‐glucose, or on the oxidation of D‐[U‐14C]glucose (11 m M). 7. The insulin secretory potentiator, ACh, does not act by accelerating glucose oxidation and does not induce the same ionic effects as the secretory initiator, D‐glucose. Increased Na+ permeability and altered interaction of Ca2+ with the plasma membrane may play roles in the cholinergic depolarization of β‐cells and potentiation of insulin release.

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Isolated pancreatic islets of noninbred ob/ob mice were used to test the hypothesis that adenylate cyclase responds to changes of the transmembrane milieu or electric field in intact beta-cells. In the presence of a phosphodiesterase inhibitor, ouabainstimulated both the release of insulin and the islet content of cAMP. Ouabain had no noticeable effect on the islet content of cGMP. These results support the hypothesis at test. However, because ouabain also had some stimulatory effect on cAMP in islet homogenates, a direct action of ouabain on adenylate cyclase cannot be ruled out.

The sensitivity of the radioimmunoassay for cGMP was considerably increased by previous 2\m='\-O-succinylation of the nucleotide. The basal content of cGMP in \g=b\-cell-richpancreatic islets isolated from ob/ob-mice was similar to that of cAMP, i. e. about 3 \g=m\molesper kg dry weight. Extracellular Ca2+ was a prerequisite for maintaining this amount of cGMP. The islet cGMP differed from cAMP in being only slightly enhanced or not affected at all when the islets were exposed to high concentrations of glucose, the sulphydryl reagents chloromercuribenzene-p-sulphonic acid and iodoacetamide, or the potent phosphodiesterase inhibitor 3-isobutyl-l\x=req-\ methylxanthine. The data obtained suggest that the turnover rate for cGMP is much slower than that for cAMP in the pancreatic \g=b\-cells. The interrelationships between the two cyclic nucleotides do not seem to fit into a simple pattern of antagonism.It is well established that cyclic 3',5'-adenosine monophosphate (cAMP) mo¬ dulates insulin secretion by potentiating the secretory response to glucose and other initiators of insulin release (Montague 8c Howell 1976). In contrast to the extensive studies on cAMP only little is known about the regulation and Presented in part at