Xenin-25 delays gastric emptying and reduces postprandial glucose levels in humans with and without type 2 diabetes

Abstract

Xenin-25 (Xen) is a neurotensin-related peptide secreted by a subset of glucose-dependent insulinotropic polypeptide (GIP)-producing enteroendocrine cells. In animals, Xen regulates gastrointestinal function and glucose homeostasis, typically by initiating neural relays. However, little is known about Xen action in humans. This study determines whether exogenously administered Xen modulates gastric emptying and/or insulin secretion rates (ISRs) following meal ingestion. Fasted subjects with normal (NGT) or impaired (IGT) glucose tolerance and Type 2 diabetes mellitus (T2DM; n = 10-14 per group) ingested a liquid mixed meal plus acetaminophen (ACM; to assess gastric emptying) at time zero. On separate occasions, a primed-constant intravenous infusion of vehicle or Xen at 4 (Lo-Xen) or 12 (Hi-Xen) pmol · kg(-1) · min(-1) was administered from zero until 300 min. Some subjects with NGT received 30- and 90-min Hi-Xen infusions. Plasma ACM, glucose, insulin, C-peptide, glucagon, Xen, GIP, and glucagon-like peptide-1 (GLP-1) levels were measured and ISRs calculated. Areas under the curves were compared for treatment effects. Infusion with Hi-Xen, but not Lo-Xen, similarly delayed gastric emptying and reduced postprandial glucose levels in all groups. Infusions for 90 or 300 min, but not 30 min, were equally effective. Hi-Xen reduced plasma GLP-1, but not GIP, levels without altering the insulin secretory response to glucose. Intense staining for Xen receptors was detected on PGP9.5-positive nerve fibers in the longitudinal muscle of the human stomach. Thus Xen reduces gastric emptying in humans with and without T2DM, probably via a neural relay. Moreover, endogenous GLP-1 may not be a major enhancer of insulin secretion in healthy humans under physiological conditions.

Trial registration: ClinicalTrials.gov NCT00949663.

Keywords: GIP; GLP-1; gastric emptying; glucagon; incretin; insulin secretion; xenin.

Fig. 1.

Plasma xenin-25 (Xen) levels in response to Xen infusions. Immunoreactive (IR)-Xen levels were determined in plasma prepared from subjects with normal glucose tolerance (NGT; A), impaired glucose tolerance (IGT; B), and Type 2 diabetes mellitus (T2DM; C) at the indicated time after Boost Plus ingestion with a primed 300-min continuous infusion of albumin (Alb; open circles), 4 pmol·kg⁻¹·min⁻¹ Xen (Lo-Xen; open squares), or 12 pmol·kg⁻¹·min⁻¹ Xen (Hi-Xen; open triangles). Group averages ± SE are shown and n = x, y, z represents the number of Alb, Lo-Xen, and Hi-Xen infusions, respectively, included in the measurements.

Fig. 2.

Infusion of Hi-Xen delays gastric emptying and reduces postprandial glucose levels in humans with NGT, IGT, and T2DM. Plasma levels of acetominophen (ACM; A–C), glucose (D–F), and C-peptide (not shown) were measured at the indicated times after Boost Plus ingestion during infusion with Alb (open blue circles), Lo-Xen (open red squares), and Hi-Xen (open green triangles). Insulin secretion rates (ISRs; G–I) were calculated by deconvolution of plasma C-peptide levels. Values represent the group means ± SE for subjects with NGT (A, D, G), IGT (B, E, H), and T2DM (C, F, I). The number of subjects (n) is shown as described in Fig. 1. Statistical analyses are shown in Fig. 3, and time frames used for calculations of incremental areas under the curve (iAUCs) are shown by the dotted lines in each panel. The lower limit of detection (LLOD) for ACM is shown in A–C. Differences in the number of subjects within each group are due to the fact that several subjects did not receive ACM with meal ingestion and some subjects did not complete all study visits.

Fig. 3.

Infusion of Hi-Xen reduces the iAUCs for gastric emptying and postprandial glucose levels in humans with NGT, IGT, and T2DM. The iAUCs and ratio of the iAUCs with infusions of Alb (open bars), Lo-Xen (hatched bars), and Hi-Xen (solid bars) from Fig. 2 were calculated for each individual for the indicated time frame following Boost Plus ingestion. Values represent group means ± SE for plasma ACM (from 0–240 min; A–C), plasma glucose (from 0–120 min; D–F), ISRs (from 0–120 min; G–I), and the ratios of the glucose and ISR iAUCs (from 0–120 min; J–L). Data for subjects with NGT (A, D, G, J), IGT (B, E, H, K), and T2DM (C, F, I, L) are shown. In K, P = 0.05 is for the 1-way ANOVA comparing the means between all 3 groups.

Fig. 4.

Infusion of Hi-Xen does not affect glucose-stimulated insulin secretion. Group average ISRs from Fig. 2, D–F, were plotted as a function of the group average plasma glucose levels from Fig. 2, G–I, at each time point. Data from 0 to 30 min (A–C) and from 60 to 300 min (D–F) are shown. X and Y error bars are the same as those in Fig. 2 but not shown. The direction of the arrow indicates that plasma glucose levels are increasing from 0 to 30 min whereas they are typically decreasing from 60 to 300 min.

Fig. 5.

Plasma levels of IR-glucose-dependent insulinotropic polypeptide (GIP) during Xen infusions. A–C: IR-GIP levels at the indicated times were measured in subjects with NGT, IGT, and T2DM (A–C, respectively) during infusion of Alb (open blue circles), Lo-Xen (open red squares), or Hi-Xen (open green triangles). D–F: for statistical analyses of data in A–C, IR-GIP iAUCs from 0–120 min were determined for each subject during infusion of Alb, Lo-Xen, or Hi-Xen. Group averages ± SE are shown. The number of subjects (n) is shown as described in Fig. 1. P values by 1-way ANOVA are shown in E and F.

Fig. 6.

Hi-Xen reduces plasma levels of intact glucagon-like peptide-1 (GLP-1). Intact GLP-1 levels and iAUCs were determined as described in Fig. 5. P value by 1-way ANOVA is shown in E.

Fig. 7.

Plasma levels of glucagon during Xen infusions. Changes in plasma glucagon levels and iAUCs were determined as described in Figs. 2 and 3, respectively.

Fig. 8.

Xen effects on diarrhea. The percentage of subjects who experienced diarrhea in response to a particular infusion is shown (A; ****P < 0.0001). The average number of diarrhea episodes ± SE per affected subject is shown (B). Hi-Xen (Hi) infusions for shorter duration were administered only to subjects with NGT. Effects of 30-, 90-, and 300-min Hi-Xen infusions are shown (C). P value by Fisher's exact test for affect of Xen (any time or dose) vs. albumin is <0.0001. Comparison of dose and time for Xen P = 0.12 (Fisher's exact test). Lo, Lo-Xen.

Fig. 9.

A high density of Xen receptors is present in the longitudinal muscle (LM) in the human stomach. A single paraffin-embedded section of human stomach was stained for smooth muscle α-actin (green) plus NTSR1 (red). Nuclei were counterstained blue. Staining was visualized by confocal microscopy with a ×40 objective. A merged image was generated in Photoshop.

Fig. 10.

Receptors for Xen are expressed on nerve fibers in the longitudinal muscle in the human stomach. A single paraffin-embedded section of human stomach was stained for PGP9.5 (red) plus NTSR1 (green). Nuclei were counterstained blue. Staining was visualized by confocal microscopy with a ×120 objective. A merged image was generated in Photoshop.