Measurement of C-peptide concentrations and responses to somatostatin, glucose infusion, and insulin resistance in horses F. TÓTH, N. FRANK*, T. MARTIN-JIMÉNEZ † , S. B. ELLIOTT, R. J. GEOR ‡ and R. C. BOSTON § The Departments of Large Animal Clinical Sciences and † Comparative Medicine, College of Veterinary Medicine, University of Tennessee, Knoxville, Tennessee 37996; ‡ Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan 48824; and § Department of Clinical Studies, University of Pennsylvania, Kennett Square, Pennsylvania 19348, USA. Keywords: horse; insulin; C-peptide; insulin resistance Summary Reasons for performing study: Hyperinsulinaemia is detected in horses with insulin resistance (IR) and has previously been attributed to increased pancreatic insulin secretion. Connecting peptide (C-peptide) can be measured to assess pancreatic function because it is secreted in equimolar amounts with insulin and does not undergo hepatic clearance. Hypothesis: A human double antibody radioimmunoassay (RIA) detects C-peptide in equine serum and concentrations would reflect responses to different stimuli and conditions. Methods: A validation procedure was performed to assess the RIA. Six mature mares were selected and somatostatin administered i.v. as a primed continuous rate infusion, followed by 50 nmol human C-peptide i.v. Insulin and C-peptide concentrations were measured in horses (n = 6) undergoing an insulin-modified frequently sampled i.v. glucose tolerance test, and in horses with insulin resistance (n = 10) or normal insulin sensitivity (n = 20). Results: A human RIA was validated for use with equine sera. Endogenous C-peptide secretion was suppressed by somatostatin and median (range) clearance rate was 0.83 (0.15–1.61) ml/min/kg bwt. Mean  s.d. C-peptide-to-insulin ratio significantly (P = 0.004) decreased during the glucose tolerance test from 3.60  1.95 prior to infusion to 1.03  0.18 during the first 20 min following dextrose administration. Median C-peptide and insulin concentrations were 1.5- and 9.5-fold higher, respectively in horses with IR, compared with healthy horses. Conclusions: Endogenous C-peptide secretion decreases in response to somatostatin and increases after dextrose infusion. Results suggest that relative insulin clearance decreases as pancreatic secretion increases in response to dextrose infusion. Hyperinsulinaemia in insulin resistant horses may be associated with both increased insulin secretion and decreased insulin clearance. Potential relevance: Both C-peptide and insulin concentrations should be measured to assess pancreatic secretion and insulin clearance in horses. Introduction Insulin is a peptide hormone containing A and B chains that is synthesised by the b-cells of the pancreatic islets of Langerhans (Wahren et al. 2000; Wilcox 2005). Upon translation of insulin mRNA, pre-proinsulin is generated and this molecule is comprised of a signal peptide, the B chain, the connecting peptide (C-peptide) and the A chain (Wilcox 2005). Proinsulin is synthesised by removal of the signal peptide from pre-proinsulin in the ribosomes of the rough endoplasmic reticulum of b-cells. Proinsulin is then transported to the Golgi apparatus, where it forms soluble, zinc- containing hexamers. Enzymes acting outside the Golgi convert proinsulin to insulin by cleaving the C-peptide from the molecule during the formation of immature storage vesicles. C-peptide has an important role in insulin synthesis by linking the A and B chains of insulin in a manner that facilitates folding and interchain disulphide bond formation (Wahren et al. 2000). Proteolytic removal of C-peptide from proinsulin allows the carboxy terminal of the B-chain of the insulin molecule to assume a conformation that facilitates interaction with the insulin receptor. Insulin and C-peptide are cosecreted in equimolar amounts when mature granules release their contents into the portal circulation (Wilcox 2005). Compensatory hyperinsulinaemia associated with insulin resistance typically predates the development of human type II DM (Brunton et al. 2006). Similarly, horses suffering from decreased insulin sensitivity triggered by endotoxaemia or obesity exhibit an augmented pancreatic insulin response to exogenous glucose challenge, indicated by higher acute insulin response to glucose (AIRg) values (Hoffman et al. 2003; Toth et al. 2008). In man, overproduction of insulin by pancreatic b-cells eventually induces b-cell exhaustion, impaired insulin secretion and relative insulin deficiency (Brunton et al. 2006). As glucose levels rise, b-cell function further deteriorates, with diminishing sensitivity to glucose, ultimately resulting in the development of type II DM (Wilcox 2005). Insulin-resistant horses and ponies rarely develop pancreatic b-cell insufficiency and usually maintain a state of compensated IR (Johnson et al. 2005; Treiber et al. 2005). Nevertheless, DM has been described in a Spanish Mustang in *Author to whom correspondence should be addressed. [Paper received for publication 23.04.09; Accepted 15.07.09] EQUINE VETERINARY JOURNAL 149 Equine vet. J. (2010) 42 (2) 149-155 doi: 10.2746/042516409X478497 © 2009 EVJ Ltd