The Pancreatic β-Cell Recognition of Insulin Secretagogues: Does Cyclic AMP Mediate the Effect of Glucose?

Abstract

Insulin release and the content of cAMP were studied in microdissected pancreatic islets of noninbred ob/ob (obese) mice. In the absence of 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, 20 mM glucose had no effect on cAMP save a very small initial rise detectable by a freeze-stop perifusion technique only. However, combined with this methylxanthine, 20 mM glucose produced significant increases of cAMP both in perifused islets and in islets conventionally incubated in closed vials. Glucose shared this capacity to raise the cAMP level with D-glyceraldehyde and 1,3-dihydroxyacetone. Isobutylmethylxanthine (0.05-1.0 mM) or 5 ug/ml of cholera toxin, an activator of adenylate cyclase, also increased the islet cAMP level; the effects of the methylxanthine, whether or not combined with cholera toxin, were potentiated by glucose. Isobutylmethylxanthine (0.05-1.0 mM) or 5 4g/mIl of cholera toxin potentiated insulin release in response to 20 mM glucose. However, only 0.5-1.0 mM isobutylmethylxanthine stimulated insulin release in the presence of 3 mM glucose, whereas 0.05-0.1 mM isobutylmethylxanthine or 5 jig/ml of cholera toxin had no effect on secretion at the low glucose concentration. These discrepancies between cAMP-promoting and insulin-releasing activities suggest that glucose does not initiate insulin release by activating the f-cell adenylate cyclase. By being metabolized in the B-cells, glucose may both create a release-initiating signal not identical with cAMP and enhance cAMP formation, leading to potentiation of the effect of the initiator signal.Several compounds can raise the concentration of cAMP and stimulate protein kinase in pancreatic islets (1). These compounds include glucagon, which activates islet adenylate cyclase (2-4), and methylxanthines, which inhibit cyclic nucleotide phosphodiesterase activity in the islets (5-7). Glucagon and methylxanthines also potentiate the insulin-releasing action of glucose (8, 9), effects that may be due to the cAMP-mediated activation of protein kinase (1).In contrast to the above potentiators of insulin release, glucose in most studies was found not to activate the adenylate cyclase (2-4), not to inhibit the phosphodiesterase (5-7), not to increase the cAMP concentration (9, 10), and not to stimulate the protein kinase (1) in pancreatic islets. Although in a dissenting study Charles et al. (11) reported that glucose raised the cAMP level in rat islets, this effect was smaller than that of theophylline; theophylline, however, was less effective than glucose in stimulating insulin release. (14,15). The effects of D-glyceraldehyde and 1,3-dihydroxyacetone were also studied to test the hypothesis that glucose-induced changes of cAMP are due to interaction of the unmetabolized glucose molecule with a specific receptor. Previous studies suggested that D-glyceraldehyde and 1,3-dihydroxyacetone feed into the glycolytic pathway of the f,-cells and so mimic the capacity of glucose to initiate insulin release (16). MATERIALS AND METHODSAdult noninbred ...

Factors governing glucose induced elevation of cyclic 3?5? AMP levels in pancreatic islets

“…The present experiments demonstrated that glucose stimulation of islets produced marked elevation of cAMP levels, a result also reported by Charles et al [8], by Selawry et al [10] by Grill and Cerasi [9,12] as well as by Capito and Hedeskov [14] but not confirmed by Cooper, Ashcroft and Randle [16] and Hellman et al [17]. Reasons for these discrepancies are not readily apparent, but subtle differences in the procedures for isolating pancreatic islets with crude collagenase used by the several laboratories, species differences, or the nutritional state of the animal [14] seem to be the most likely causes.…”

“…It remains an open question whether insulin release resulting from glucose, the major physiological B-cell stimulus, involves elevation of cAMP levels as an obligatory step. In several studies it was found that cAMP was elevated in isolated pancreatic islets exposed to high glucose [8--14]; however in an equally large number of papers it was reported that glucose alone did not alter the cAMP levels of islet tissue [15,16] or had only a small transient effect [17[.-Some have found that high glucose elevated cAMP when a phosphodiesterase inhibitor was present [8,9,17] others, however, did not confirm this phenomenon [15,16]. This is a report of some of the results we have obtained in an effort to help clarify the issue.…”

Oscillatory control of insulin secretion

“…cAMP is formed by adenylyl cyclases in response to G s -coupled receptor agonists, such as glucagon, the incretin hormones glucagon-like peptide-1 (GLP-1) and gastric inhibitory popypeptide (GIP), as well as the neuropeptides pitutitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal popypeptide (VIP). Also glucose-stimulated islets show a modest elevation of cAMP (Charles et al, 1973;Grill and Cerasi, 1973;Hellman et al, 1974;Sharp, 1979), an effect that has been attributed to the elevation of [Ca 2+ ] i (Charles et al, 1975;Valverde et al, 1979). From the observation that purified β-cells, lacking influence from the glucagon-producing α-cells, show reduced cAMP content and impaired secretory capacity, it was suggested that cAMP has a permissive role in glucose-induced insulin release and that the effect of the sugar on cAMP content represents amplification of glucagon-induced cAMP production (Schuit and Pipeleers, 1985).…”

Inhibition of insulin and glucagon release from the perfused rat pancreas by cyproheptadine (Periactinol�, Nuran�)

“…These results indicate that a specific block of glucose utilization is not responsible for the observed inhibition. Although cyclic AMP probably does not mediate glucose induced insulin release [22], it is widely accepted that the nucleotide potentiates the effects of glucose and other stimulators [23]. Thus the potentiating effect of theophylline on glucose-induced insulin release may be mediated by an increase of the intracellular cyclic AMP [24], which in turn was supposed to translocate calcium from organelle-bound pools to the cytoplasm [25].…”