Investigations on isolated islets of langerhans in vitro. 16.Modification of the glucose-dependent inhibition of glucagon secretion

Neprilysin Inhibition Increases Glucagon Levels in Humans and Mice With Potential Effects on Amino Acid Metabolism

Context

Inhibitors of the protease neprilysin (NEP) are used for treating heart failure, but are also linked to improvements in metabolism. NEP may cleave proglucagon-derived peptides, including the glucose and amino acid (AA)-regulating hormone glucagon. Studies investigating NEP inhibition on glucagon metabolism are warranted.

Objective

This work aims to investigate whether NEP inhibition increases glucagon levels.

Methods

Plasma concentrations of glucagon and AAs were measured in eight healthy men during a mixed meal with and without a single dose of the NEP inhibitor/angiotensin II type 1 receptor antagonist, sacubitril/valsartan (194 mg/206 mg). Long-term effects of sacubitril/valsartan (8 weeks) were investigated in individuals with obesity (n = 7). Mass spectrometry was used to investigate NEP-induced glucagon degradation, and the derived glucagon fragments were tested pharmacologically in cells transfected with the glucagon receptor (GCGR). Genetic deletion or pharmacological inhibition of NEP with or without concomitant GCGR antagonism was tested in mice to evaluate effects on AA metabolism.

Results

In healthy men, a single dose of sacubitril/valsartan significantly increased postprandial concentrations of glucagon by 228%, concomitantly lowering concentrations of AAs including glucagonotropic AAs. Eight-week sacubitril/valsartan treatment increased fasting glucagon concentrations in individuals with obesity. NEP cleaved glucagon into 5 inactive fragments (in vitro). Pharmacological NEP inhibition protected both exogenous and endogenous glucagon in mice after an AA challenge, while NEP-deficient mice showed elevated fasting and AA-stimulated plasma concentrations of glucagon and urea compared to controls.

Conclusion

NEP cleaves glucagon, and inhibitors of NEP result in hyperglucagonemia and may increase postprandial AA catabolism without affecting glycemia.

Glucose decouples intracellular Ca2+ activity from glucagon secretion in mouse pancreatic islet alpha-cells

The mechanisms of glucagon secretion and its suppression by glucose are presently unknown. This study investigates the relationship between intracellular calcium levels ([Ca²⁺]_i) and hormone secretion under low and high glucose conditions. We examined the effects of modulating ion channel activities on [Ca²⁺]_i and hormone secretion from ex vivo mouse pancreatic islets. Glucagon-secreting α-cells were unambiguously identified by cell specific expression of fluorescent proteins. We found that activation of L-type voltage-gated calcium channels is critical for α-cell calcium oscillations and glucagon secretion at low glucose levels. Calcium channel activation depends on K_ATP channel activity but not on tetrodotoxin-sensitive Na⁺ channels. The use of glucagon secretagogues reveals a positive correlation between α-cell [Ca²⁺]_i and secretion at low glucose levels. Glucose elevation suppresses glucagon secretion even after treatment with secretagogues. Importantly, this inhibition is not mediated by K_ATP channel activity or reduction in α-cell [Ca²⁺]_i. Our results demonstrate that glucose uncouples the positive relationship between [Ca²⁺]_i and secretory activity. We conclude that glucose suppression of glucagon secretion is not mediated by inactivation of calcium channels, but instead, it requires a calcium-independent inhibitory pathway.

Stimulation by 2-deoxy-D-glucose tetraacetates of hormonal secretion from the perfused rat pancreas

The effects of alpha- and beta-2-deoxy-D-glucose tetraacetate (1.7 and 8.5 mM) on insulin, somatostatin, and glucagon secretion from isolated rat pancreases perfused in the presence of 8.3 mM D-glucose were compared with those of unesterified 2-deoxy-D-glucose tested at the same two concentrations. The unesterified glucose analog caused, in a concentration-related manner, inhibition of glucose-induced insulin and somatostatin release and augmentation of glucagon secretion. The two anomers of 2-deoxy-D-glucose tetraacetate, however, increased the secretion rate of all three hormones; this effect was also related to the concentration of the esters. No obvious anomeric specificity of the secretory response to 2-deoxy-D-glucose tetraacetate was observed. These findings indicate that the insulinotropic action of hexose esters cannot be accounted for solely by the metabolic effect of their glucidic moieties. They suggest that the A, B, and D cells of the endocrine pancreas are each equipped with a receptor system responsible for the direct recognition of monosaccharide esters as secretagogues. They further support the view that a paracrine effect of insulin on glucagon-producing cells does not represent a major component in the regulation of their secretory activity.

Adaptive changes in insulin and glucagon secretion during cold acclimation in the rat

Arginine-stimulated insulin and glucagon outputs from isolated perfused pancreata of warm-acclimated and 2-, 4-, and 6-wk cold-acclimated rats (4 degrees C) were determined to assess whether observed changes in these parameters were a result of cold exposure per se or a part of the adaptive process of cold acclimation. Progressive and sequential changes were seen in both insulin and glucagon outputs. At 2 wk cold acclimation, glucagon rose and insulin output tended to fall, at 4 wk, glucagon output remained elevated and insulin output was further reduced, and at 6 wk, glucagon output had returned to control levels, whereas insulin output was substantially further reduced. These changes resulted in reduction of the insulin-to-glucagon molar ratio of the total arginine-induced output from 7.27 +/- 1.76 (SE) in the warm acclimate to 2.31 +/- 0.79 (SE) at 2 wk, 1.42 +/- 0.29 (SE) at 4 wk, and 1.26 +/- 0.21 (SE) at 6 wk cold acclimation. The data do not provide in vitro support for the hypothesis that changes in pancreatic hormone secretion in vivo are a consequence of cold exposure and not cold acclimation.

Consequences of ventromedial hypothalamic lesions upon insulin and glucagon secretion by subsequently isolated perfused pancreases in the rat

The existence of a relationship between the ventromedial hypothalamic area (VMH) and the activity of the endocrine pancreas has been shown previously. This relationship has been further tested and extended in the present study, using isolated perfused pancreases from rats previously lesioned (4-7 d) in the VMH. It was found that in isolated pancreases obtained from rats fed ad lib. for 4 d after VMH lesions (i.e., that were hyperphagic), the typical biphasic pattern of insulin secretion was observed following glucose stimulation (20 mM) and that the total insulin output was much greater than that of controls. The increased insulin output was not a result of hyperphagia because similar results were obtained using pancreases obtained from VMH-lesioned rats in which a food restriction matching exactly that of control rats was started either immediately of 3 d after the lesions. Pancreases from such food-restricted VMH-lesioned rats oversecreted insulin, when compared with controls fed the same amount, from 7 mM of glucose concentration in perfusion medium onwards. After the addition of arginine (10 mM), the total output of glucagon by pancreases from food-restricted VMH-lesioned rats was twice that of controls. Qualitatively, the arginine-induced glucagon secretion by pancreases from food-restricted VMH-lesioned rats retained its biphasic pattern. Similarly, epinephrine (0.1 muM) elicited a greater glucagon release by pancreases from food-restricted VMH-lesioned rats when compared with controls. These data further support the concept of a link (as yet undefined) between the hypothalamus and the endocrine pancreas, as lesions of the VMH area resulted in abnormal secretion not only of insulin, but of glucagon as well.

Different effects of hypothermia on insulin and glucagon secretion from the isolated perfused rat pancreas

Two series of experiments with the isolated perfused rat pancreas were performed in parallel. The conditions differed only with respect to temperature, which was 37.5 degrees C in one series and 28 degrees C in the other. The lowering of the temperature decreased insulin secretion induced by glucose as well as the insulin response to tolbutamide and acetylcholine. Unlike insulin, glucagon secretion was not significantly modified by hypothermia. Our results suggest that the mechanisms involved in glucagon and insulin secretion are different.

Interactions of alpha-ketoisocaproate, glucose and arginine in the secretion of glucagon and insulin from the perfused rat pancreas

The effects of alpha-ketoisocaproate (KIC, 10 mmol/l) on glucagon and insulin release were studied in the in vitro perfused rat pancreas. The experiments were performed at low glucose concentration (3.3 mmol/l) in the absence or presence of arginine (10 mmol/l). In all the experiments KIC induced a marked and not rapidly reversible inhibition of glucagon release. This inhibition was more pronounced in the absence (76 percent) than presence of arginine (61 percent). These inhibitory patterns closely duplicated those which were seen in parallel experiments which included a rise in the concentration of glucose (from 3.3 to 11.1 mmol/l). KIC was also a potent stimulator of insulin release. The results are compatible with the view that the intracellular metabolism of KIC and glucose plays an essential role in the regulation of glucagon release by exogenous substrates.