Expression of the GCG gene and secretion of active glucagon-like peptide-1 varies along the length of intestinal tract in horses

BACKGROUND

Active glucagon-like peptide-1 (aGLP-1) has been implicated in the pathogenesis of equine insulin dysregulation (ID), but its role is unclear. Cleavage of proglucagon (coded by the GCG gene) produces aGLP-1 in enteral L cells.

OBJECTIVES

The aim in vivo was to examine the sequence of the exons of GCG in horses with and without ID, where aGLP-1 was higher in the group with ID. The aims in vitro were to identify and quantify the expression of GCG in the equine intestine (as a marker of L cells) and determine intestinal secretion of aGLP-1.

STUDY DESIGN

Genomic studies were case-control studies. Expression and secretion studies in vitro were cross-sectional.

METHODS

The GCG gene sequence of the exons was determined using a hybridisation capture protocol. Expression and quantification of GCG in samples of stomach duodenum, jejunum, ileum, caecum and ascending and descending colon was achieved with droplet digital PCR. For secretory studies tissue explants were incubated with 12 mM glucose and aGLP-1 secretion was measured with an ELISA.

RESULTS

Although the median [IQR] post-prandial aGLP-1 concentrations were higher (p = 0.03) in animals with ID (10.2 [8.79-15.5]), compared with healthy animals (8.47 [6.12-11.7]), there was 100% pairwise identity of the exons of the GCG sequence for the cohort. The mRNA concentrations of GCG and secretion of aGLP-1 differed (p < 0.001) throughout the intestine.

MAIN LIMITATIONS

Only the exons of the GCG gene were sequenced and breeds were not compared. The horses used for the study in vitro were not assessed for ID and different horses were used for the small, and large, intestinal studies.

CONCLUSIONS

Differences in post-prandial aGLP-1 concentration were not due to a variant in the exons of the GCG gene sequence in this cohort. Both the large and small intestine are sites of GLP-1 secretion.

Associations between feeding and glucagon-like peptide-2 in healthy ponies

BACKGROUND

Gastrointestinal peptides, such as glucagon-like peptide-2 (GLP-2), could play a direct role in the development of equine hyperinsulinaemia.

OBJECTIVES

To describe the secretory pattern of endogenous GLP-2 over 24 h in healthy ponies and determine whether oral administration of a synthetic GLP-2 peptide increases blood glucose or insulin responses to feeding.

STUDY DESIGN

A cohort study followed by a randomised, controlled, cross-over study.

METHODS

In the cohort study, blood samples were collected every 2 h for 24 h in seven healthy ponies and plasma [GLP-2] was measured. In the cross-over study, 75 μg/kg bodyweight of synthetic GLP-2, or carrier only, was orally administered to 10 ponies twice daily for 10 days. The area under the curve (AUC0-3h ) of post-prandial blood glucose and insulin were determined before and after each treatment.

RESULTS

Endogenous [GLP-2] ranged from <0.55 to 1.95 ± 0.29 [CI 0.27] ng/mL with similar peak concentrations in response to meals containing 88-180 g of non-structural carbohydrate, that were ~4-fold higher (P < 0.001) than the overnight nadir. After GLP-2 treatment peak plasma [GLP-2] increased from 1.1 [0.63-1.37] ng/mL to 1.54 [1.1-2.31] ng/mL (28.6%; P = 0.002), and AUC0-3h was larger (P = 0.01) than before treatment. The peptide decreased (7%; P = 0.003) peak blood glucose responses to feeding from 5.33 ± 0.45 mmol/L to 5.0 ± 0.21 mmol/L, but not AUC0-3h (P = 0.07). There was no effect on insulin secretion.

MAIN LIMITATIONS

The study only included healthy ponies and administration of a single dose of GLP-2.

CONCLUSIONS

The diurnal pattern of GLP-2 secretion in ponies was similar to other species with no apparent effect of daylight. Although GLP-2 treatment did not increase post-prandial glucose or insulin responses to eating, studies using alternative dosing strategies for GLP-2 are required.

A starch-rich treat affects enteroinsular responses in ponies

OBJECTIVE

To determine the effect of a starch-rich treat, added to the daily diet of ponies for 10 days, on enteroinsular responses to meal consumption.

ANIMALS

10 mixed-breed adult ponies owned by Queensland University of Technology were used in the study. Six ponies were metabolically healthy, and 4 were insulin dysregulated at the start of the study, according to the results of an in-feed oral glucose test.

PROCEDURES

A bread-based treat was offered twice daily for 10 days, adding 0.36 ± 0.04 g/kg body weight (BW) carbohydrates to the daily diet. Before and after treatment, the intestinal capacity for simple carbohydrate absorption was approximated with a modified D-xylose absorption test. Plasma glucagon-like peptide-2 (GLP-2), blood glucose, and serum insulin responses to eating were also measured before and after treatment.

RESULTS

The absorption of D-xylose (area under the curve [AUC]) increased 1.6-fold (P < .001) after 10 days of eating the treat. In addition, while basal (fasted) GLP-2 concentrations were not affected, GLP-2 AUC increased 1.4-fold in response to eating (P = .005). The treat did not change blood glucose or serum insulin concentrations, before, during, or after eating.

CLINICAL RELEVANCE

A small amount of additional carbohydrate each day in the form of a treat can cause a measurable change in the enteroinsular responses to eating.