Hyperglycemia after protein ingestion concurrent with injection of a GLP-1 receptor agonist in rats: a possible role for dietary peptides

Effects of Hydrolyzed Whey versus Other Whey Protein Supplements on the Physiological Response to 8 Weeks of Resistance Exercise in College-Aged Males

OBJECTIVE

The objective of this study was to compare the chronic effects of different whey protein forms on body composition and performance when supplemented with resistance training.

METHODS

Resistance-trained men (N = 56, 21.4 ± 0.4 years, 79.5 ± 1.0 kg) participated in an 8-week resistance training regimen (2 upper-body sessions and 2 lower-body sessions per week) and received one of 4 double-blinded treatments: 30 g/serving carbohydrate placebo (PLA) or 30 g/serving protein from either (a) 80% whey protein concentrate (WPC), (b) high-lactoferrin-containing WPC (WPC-L), or (c) extensively hydrolyzed WPC (WPH). All subjects consumed 2 servings of treatment per day; specifically, once immediately before and after training and between meals on nontraining days. Blood collection, one repetition maximum (1RM) testing for bench press and hack squat, and body composition assessment using dual-energy x-ray absorptiometry (DXA) occurred prior to training and 48 hours following the last training session.

RESULTS

Total body skeletal muscle mass increased in all groups (p < 0.0125). There were similar between-group increases in upper-body (4%-7%, analysis of covariance [ANCOVA] interaction p = 0.73) and lower-body (24%-35%, ANCOVA interaction p = 0.85) 1RM strength following the intervention. Remarkably, WPH reduced fat mass (-6%), which was significantly different from PLA (+4.4%, p < 0.0125). No time or between-group differences were present for serum markers of health, metabolism, or muscle damage, with the exception of blood urea nitrogen being significantly lower for WPH than WPC (p < 0.05) following the intervention.

CONCLUSIONS

WPH may augment fat loss but did not provide any other advantages when used in combination with resistance training. More mechanistic research is needed to examine how WPH affects adipose tissue physiology.

The serine protease, dipeptidyl peptidase IV as a myokine: dietary protein and exercise mimetics as a stimulus for transcription and release

Dipeptidyl-peptidase IV (DPP-IV) is an enzyme with numerous roles within the body, mostly related to regulating energy metabolism. DPP-IV is also a myokine, but the stimulus for its release is poorly understood. We investigated the transcription and release of DPP-IV from skeletal muscle in a three-part study using C2C12 myotube cultures, an acute rat exercise and postexercise feeding model, and human feeding or human exercise models. When myotubes were presented with leucine only, hydrolyzed whey protein, or chemicals that cause exercise-related signaling to occur in cell culture, all caused an increase in the mRNA expression of DPP-IV (1.63 to 18.56 fold change, P < 0.05), but only whey protein caused a significant increase in DPP-IV activity in the cell culture media. When rats were fed whey protein concentrate immediately following stimulated muscle contractions, DPP-IV mRNA in both the exercised and nonexercised gastrocnemius muscles significantly increased 2.5- to 3.7-fold (P < 0.05) 3-6 h following the exercise/feeding bout; of note exercise alone or postexercise leucine-only feeding had no significant effect. In humans, plasma and serum DPP-IV activities were not altered by the ingestion of whey protein up to 1 h post consumption, after a 10 min bout of vigorous running, or during the completion of three repeated lower body resistance exercise bouts. Our cell culture and rodent data suggest that whey protein increases DPP-IV mRNA expression and secretion from muscle cells. However, our human data suggest that DPP-IV is not elevated in the bloodstream following acute whey protein ingestion or exercise.

Short-term administration of the GLP-1 analog liraglutide decreases circulating leptin and increases GIP levels and these changes are associated with alterations in CNS responses to food cues: A randomized, placebo-controlled, crossover study

BACKGROUND

GLP-1 agonists, including liraglutide, have emerged as effective therapies for type 2 diabetes (DM) and obesity. Here, we attempted to delineate how liraglutide, at doses approved for DM, may impact circulating hormones influencing energy homeostasis in diabetics.

BASIC PROCEDURES

Using a randomized, placebo-controlled, double-blind, cross-over trial of 20 patients with type 2 diabetes, we examined the effects of liraglutide as compared to placebo on fasting levels of circulating hormones important to energy homeostasis, including leptin, ghrelin, PYY, and GIP. After 17days (0.6mg for 7days, 1.2mg for 7days and 1.8mg for 3days) of treatment, we also studied changes in fMRI responses to food cues.

MAIN FINDINGS

By design, to avoid any confounding by weight changes, subjects were studied for 17days, i.e. before body weight changed. Participants on liraglutide had significantly increased GLP-1 levels (p<0.001), decreased percent change in leptin levels (p<0.01) and increased GIP levels (p<0.03) in comparison to placebo treated subjects. Whole brain regressions of functional activity in response to food cues reveal that increased GIP levels were associated with deactivation of the attention- and reward-related insula. Decreases in leptin levels were associated with activations in the reward-related midbrain, precuneus, and dorsolateral prefrontal cortex (DLPFC), and sensorimotor-related motor cortex and with deactivations in the attention-related parietal cortex and the cognitive control-related thalamus and pre-SMA.

PRINCIPAL CONCLUSIONS

We demonstrate herein short-term changes to circulating levels of GIP and leptin in response to GLP-1 agonist liraglutide therapy. These findings suggest that liraglutide may alter the circulating levels of hormones important in energy homeostasis that, in turn, influence CNS perception of food cues. This could possibly lead to compensatory changes in energy homeostasis that could over time limit the efficacy of liraglutide to decrease body weight. These novel findings, which, pointing to the potential advantages of combination therapies, may have therapeutic implications, will need to be confirmed by larger and longer-term trials.

Brain activation following peripheral administration of the GLP-1 receptor agonist exendin-4

The aim of our study was to investigate the anorectic and brain stimulatory effects of various doses of exendin-4 (Ex-4) and to investigate the role of the vagus nerve in Ex-4-induced brain activation. A dose-related increase in c- fos mRNA expression was observed following Ex-4 administration (0.155–15.5 μg/kg). Doses of Ex-4 that caused anorexia without aversive effects (0.155, 0.775 μg/kg) induced c- fos expression in the hypothalamic arcuate and paraventricular (PVH; parvocellular) nuclei as well as in the limbic and brainstem structures. Doses of Ex-4 that caused aversion (1.55, 15.5 μg/kg) stimulated the same regions (in a more intense way) and additionally activated the magnocellular hypothalamic structures (supraoptic nucleus and PVH magnocellular). The brain c- fos pattern induced by Ex-4 showed both similarities and differences with that induced by refeeding. Subdiaphragmatic vagotomy significantly blunted the stimulation of c- fos mRNA expression induced by Ex-4 in the nodose ganglion, the medial part of nucleus of the solitary tract, and the parvocellular division of the PVH. Pretreatment with Ex-9-39 (330 μg/kg ip) impaired the neuronal activation evoked by Ex-4 in all brain regions and in the nodose ganglion. Effects of Ex-4 on hypothalamic-pituitary-adrenal axis activity were not altered by vagotomy. Results of this study demonstrate and relate the anorectic and brain stimulatory effects of aversive and nonaversive doses of Ex-4 and indicate that the activation of specific central regions induced by the peripheral administration of Ex-4 is, at least in part, dependent on the integrity of the vagus nerve.

Pharmacokinetic and pharmacodynamic modeling of exendin-4 in type 2 diabetic Goto-Kakizaki rats

The pharmacokinetics (PK) and pharmacodynamics (PD) of exendin-4 were studied in type 2 diabetic Goto-Kakizaki rats after single doses at 0.5, 1, 5, or 10 μg/kg by intravenous administration and 5 μg/kg by subcutaneous administration. Plasma exendin-4, glucose, and insulin concentrations were determined. A target-mediated drug disposition model was used to characterize the PK of exendin-4. Glucose turnover was described by an indirect response model, with insulin stimulating glucose disposition. Insulin turnover was characterized by an indirect response model with a precursor compartment. After intravenous doses, exendin-4 rapidly disappeared from the circulation, whereas it exhibited rapid absorption (T(max) = 15-20 min) and incomplete bioavailability (F = 0.51) after the subcutaneous dose. Exendin-4 increased insulin release at 2 to 5 min with capacity S(max) = 6.91 and sensitivity SC₅₀ = 1.29 nM, followed by a rebound at 10 to 15 min and a slow return to the baseline. Glucose initially declined because of enhanced insulin secretion, and then gradually increased because of the activation of the neural system by exendin-4. The hyperglycemic action was modeled with increased hepatic glucose production with a linear factor S(RC) = 0.112 1/nM. The mechanistic PK/PD model satisfactorily described the disposition and effects of exendin-4 on glucose and insulin homeostasis in type 2 diabetic rats.

Exendin-4 increases blood glucose levels acutely in rats by activation of the sympathetic nervous system

Exendin-4 (Ex-4), an agonist of the glucagon-like peptide-1 receptor (GLP-1R), shares many of the actions of GLP-1 on pancreatic islets, the central nervous system (CNS), and the gastrointestinal tract that mediates glucose homeostasis and food intake. Because Ex-4 has a much longer plasma half-life than GLP-1, it is an effective drug for reducing blood glucose levels in patients with type 2 diabetes mellitus (T2DM). Here, we report that acute administration of Ex-4, in relatively high doses, into either the peripheral circulation or the CNS, paradoxically increased blood glucose levels in rats. This effect was independent of the insulinotropic and hypothalamic-pituitary-adrenal activating actions of Ex-4 and could be blocked by a GLP-1R antagonist. Comparable doses of GLP-1 did not induce hyperglycemia, even when protected from rapid metabolism by a dipeptidyl peptidase IV inhibitor. Acute hyperglycemia induced by Ex-4 was blocked by hexamethonium, guanethidine, and adrenal medullectomy, indicating that this effect was mediated by sympathetic nervous system (SNS) activation. The potency of Ex-4 to elevate blood glucose waned with chronic administration such that after 6 days the familiar actions of Ex-4 to improve glucose tolerance were evident. These findings indicate that, in rats, high doses of Ex-4 activate a SNS response that can overcome the expected benefits of this peptide on glucose metabolism and actually raise blood glucose. These results have important implications for the design and interpretation of studies using Ex-4 in rats. Moreover, since there are many similarities in the response of the GLP-1R system across mammalian species, it is important to consider whether there is acute activation of the SNS by Ex-4 in humans.

Lesions of area postrema and subfornical organ alter exendin-4-induced brain activation without preventing the hypophagic effect of the GLP-1 receptor agonist

The mechanism and route whereby glucagon-like peptide 1 (GLP-1) receptor agonists, such as GLP-1 and exendin-4 (Ex-4), access the central nervous system (CNS) to exert their metabolic effects have yet to be clarified. The primary objective of the present study was to investigate the potential role of two circumventricular organs (CVOs), the area postrema (AP) and the subfornical organ (SFO), in mediating the metabolic and CNS-stimulating effects of Ex-4. We demonstrated that electrolytic ablation of the AP, SFO, or AP + SFO does not acutely prevent the anorectic effects of Ex-4. AP + SFO lesion chronically decreased food intake and body weight and also modulated the effect of Ex-4 on the neuronal activation of brain structures involved in the hypothalamic-pituitary-adrenal axis and glucose metabolism. The results of the study also showed that CVO lesions blunted Ex-4-induced expression of c-fos mRNA (a widely used neuronal activity marker) in 1) limbic structures (bed nucleus of the stria terminalis and central amygdala), 2) hypothalamus (paraventricular hypothalamic nucleus, supraoptic nucleus, and arcuate nucleus), and 3) hindbrain (lateral and lateral-external parabrachial nucleus, medial nucleus of the solitary tract, and ventrolateral medulla). In conclusion, although the present results do not support a role for the CVOs in the anorectic effect induced by a single injection of Ex-4, they suggest that the CVOs play important roles in mediating the actions of Ex-4 in the activation of CNS structures involved in homeostatic control.