Studies on equine prematurity 3: Insulin secretion in the foal during the perinatal period

Energy endocrine physiology, pathophysiology, and nutrition of the foal

Most homeostatic systems in the equine neonate should be functional during the transition from intra- to extrauterine life to ensure survival during this critical period. Endocrine maturation in the equine fetus occurs at different stages, with a majority taking place a few days prior to parturition and continuing after birth. Cortisol and thyroid hormones are good examples of endocrine and tissue interdependency. Cortisol promotes skeletal, respiratory, cardiovascular, thyroid gland, adrenomedullary, and pancreatic differentiation. Thyroid hormones are essential for cardiovascular, respiratory, neurologic, skeletal, adrenal, and pancreatic function. Hormonal imbalances at crucial stages of development or in response to disease can be detrimental to the newborn foal. Other endocrine factors, including growth hormone, glucagon, catecholamines, ghrelin, adipokines (adiponectin, leptin), and incretins, are equally important in energy homeostasis. This review provides information specific to nutrition and endocrine systems involved in energy homeostasis in foals, enhancing our understanding of equine neonatal physiology and pathophysiology and our ability to interpret clinical and laboratory findings, therefore improving therapies and prognosis.

Comparison of insulin sensitivity between healthy neonatal foals and horses using minimal model analysis

The equine neonate is considered to have impaired glucose tolerance due to delayed maturation of the pancreatic endocrine system. Few studies have investigated insulin sensitivity in newborn foals using dynamic testing methods. The objective of this study was to assess insulin sensitivity by comparing the insulin-modified frequently sampled intravenous glucose tolerance test (I-FSIGTT) between neonatal foals and adult horses. This study was performed on healthy neonatal foals (n = 12), 24 to 60 hours of age, and horses (n = 8), 3 to 14 years of age using dextrose (300 mg/kg IV) and insulin (0.02 IU/kg IV). Insulin sensitivity (SI), acute insulin response to glucose (AIRg), glucose effectiveness (Sg), and disposition index (DI) were calculated using minimal model analysis. Proxy measurements were calculated using fasting insulin and glucose concentrations. Nonparametric statistical methods were used for analysis and reported as median and interquartile range (IQR). SI was significantly higher in foals (18.3 L·min^-1· μIU^-1 [13.4–28.4]) compared to horses (0.9 L·min^-1· μIU^-1 [0.5–1.1]); (p < 0.0001). DI was higher in foals (12 × 10³ [8 × 10³−14 × 10³]) compared to horses (4 × 10² [2 × 10²−7 × 10²]); (p < 0.0001). AIRg and Sg were not different between foals and horses. The modified insulin to glucose ratio (MIRG) was lower in foals (1.72 μIU_insulin²/10·L·mg_glucose [1.43–2.68]) compared to horses (3.91 μIU _insulin²/10·L·mg_glucose [2.57–7.89]); (p = 0.009). The homeostasis model assessment of beta cell function (HOMA-BC%) was higher in horses (78.4% [43–116]) compared to foals (23.2% [17.8–42.2]); (p = 0.0096). Our results suggest that healthy neonatal foals are insulin sensitive in the first days of life, which contradicts current literature regarding the equine neonate. Newborn foals may be more insulin sensitive immediately after birth as an evolutionary adaptation to conserve energy during the transition to extrauterine life.

Abundance of the skeletal muscle Glut-4 glucose transport protein in Standardbred foals during development and exercise

The objective of this study was to investigate the changes in Glut-4 expression in gluteus muscle, and in the plasma concentrations of hormones and metabolites during the suckling period, after weaning, and after exercise training in foals. Our hypotheses were Glut-4 abundance will decrease following metabolite and hormonal concentration between neonatal period and late development but will increase with exercise training. Eight clinically normal Standardbred foals were used in this study, and they stayed with their mothers from birth until weaning at 6 months. After weaning, the foals were randomly divided in two groups: an exercise group (EX) (n=4; ~282 kg) which trained 3 d/wk for 12 wks and a control group (n=4; ~271 kg) without structured exercise. Venous blood samples were obtained immediately after birth (<30 min), at 24 h, at day 7 and day 14, and at 1, 3, 6, 9 and 12 months after parturition. Results were analysed using one-way ANOVA for repeated measures. Post-hoc differences were identified using the Tukey test and correlations were obtained using Pearson product moment analysis. The abundance of Glut-4 in skeletal muscle was highest immediately after birth and with growth and development (P<0.05). Exercise training did not change Glut-4 abundance (P>0.05). Plasma lactate, insulin and cortisol concentrations decreased during the first three months of lactation while glucose and leptin concentrations increased (P<0.05). There were no effects of exercise training on any of the above plasma parameters (P>0.05). In conclusion, Glut-4 abundance in gluteus muscle was highest at the birth and decreased during development and exercise training for 12 weeks did not change its’ abundance.

Enteroinsular axis response to carbohydrates and fasting in healthy newborn foals

Background

The enteroinsular axis (EIA) comprises intestinal factors (incretins) that stimulate insulin release after PO ingestion of nutrients. Glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are the main incretins. The EIA has not been investigated in healthy neonatal foals but should be important because energy demands are high in healthy foals and dysregulation is frequent in sick foals.

Objectives and Hypothesis

To evaluate the EIA response to carbohydrates or fasting in newborn foals. We hypothesized that incretin secretion would be higher after PO versus IV carbohydrate administration or fasting.

Animals

Thirty‐six healthy Standardbred foals ≤4 days of age.

Methods

Prospective study. Blood was collected before and after a PO glucose test (OGT; 300, 500, 1000 mg/kg), an IV glucose test (IVGT; 300, 500, 1000 mg/kg), a PO lactose test (OLT; 1000 mg/kg), and fasting. Foals were muzzled for 240 minutes. Blood was collected over 210 minutes glucose, insulin, GIP, and GLP‐1 concentrations were measured.

Results

Only PO lactose caused a significant increase in blood glucose concentration (P < .05). All IV glucose doses induced hyperglycemia and hyperinsulinemia. Concentrations of GIP and GLP‐1 decreased until foals nursed (P < .05), at which time rapid increases in glucose, insulin, GIP, and GLP‐1 concentrations occurred (P < .05).

Conclusions and Clinical Importance

Healthy newborn foals have a functional EIA that is more responsive to milk and lactose than glucose. Non‐carbohydrate factors in mare's milk may be important for EIA activity. Constant exposure of intestinal cells to nutrients to maintain EIA activity could be relevant to management of sick foals. Foals can be fasted for 4 hours without experiencing hypoglycemia.

Enhanced or reduced fetal growth induced by embryo transfer into smaller or larger breeds alters post-natal growth and metabolism in pre-weaning horses

In equids, placentation is diffuse and nutrient supply to the fetus is determined by uterine size. This correlates with maternal size and affects intra-uterine development and subsequent post-natal growth, as well as insulin sensitivity in the newborn. Long-term effects remain to be described. In this study, fetal growth was enhanced or restricted through ET using pony (P), saddlebred (S) and draft (D) horses. Control P-P (n = 21) and S-S (n = 28) pregnancies were obtained by AI. Enhanced and restricted pregnancies were obtained by transferring P or S embryos into D mares (P-D, n = 6 and S-D, n = 8) or S embryos into P mares (S-P, n = 6), respectively. Control and experimental foals were raised by their dams and recipient mothers, respectively. Weight gain, growth hormones and glucose homeostasis were investigated in the foals from birth to weaning. Fetal growth was enhanced in P-D and these foals remained consistently heavier, with reduced T3 concentrations until weaning compared to P-P. P-D had lower fasting glucose from days 30 to 200 and higher insulin secretion than P-P after IVGTT on day 3. Euglycemic clamps in the immediate post-weaning period revealed no difference in insulin sensitivity between P-D and P-P. Fetal growth was restricted in S-P and these foals remained consistently lighter until weaning compared to S-D, with elevated T3 concentrations in the newborn compared to S-S. S-P exhibited higher fasting glycemia than S-S and S-D from days 30 to 200. They had higher maximum increment in plasma glucose than S-D after IVGTT on day 3 and clamps on day 200 demonstrated higher insulin sensitivity compared to S-D. Neither the restricted nor the enhanced fetal environment affected IGF-1 concentrations. Thus, enhanced and restricted fetal and post-natal environments had combined effects that persisted until weaning. They induced different adaptive responses in post-natal glucose metabolism: an early insulin-resistance was induced in enhanced P-D, while S-P developed increased insulin sensitivity.

Insulin dysregulation

Abnormalities of insulin metabolism include hyperinsulinaemia and insulin resistance, and these problems are collectively referred to as insulin dysregulation in this review. Insulin dysregulation is a key component of equine metabolic syndrome: a collection of endocrine and metabolic abnormalities associated with the development of laminitis in horses, ponies and donkeys. Insulin dysregulation can also accompany prematurity and systemic illness in foals. Causes of insulin resistance are discussed, including pathological conditions of obesity, systemic inflammation and pituitary pars intermedia dysfunction, as well as the physiological responses to stress and pregnancy. Most of the discussion of insulin dysregulation to date has focused on insulin resistance, but there is increasing interest in hyperinsulinaemia itself and insulin responses to feeding. An oral sugar test or in-feed oral glucose tolerance test can be performed to assess insulin responses to dietary carbohydrates, and these tests are now recommended for use in clinical practice. Incretin hormones are likely to play an important role in postprandial hyperinsulinaemia and are the subject of current research. Insulin resistance exacerbates hyperinsulinaemia, and insulin sensitivity can be measured by performing a combined glucose-insulin test or i.v. insulin tolerance test. In both of these tests, exogenous insulin is administered and the rate of glucose uptake into tissues measured. Diagnosis and management of hyperinsulinaemia is recommended to reduce the risk of laminitis. The term insulin dysregulation is introduced here to refer collectively to excessive insulin responses to sugars, fasting hyperinsulinaemia and insulin resistance, which are all components of equine metabolic syndrome.

Factors affecting clinical assessment of insulin sensitivity in horses

Insulin resistance is thought to be involved in the pathogenesis of many equine conditions such as pars intermedia dysfunction, equine metabolic syndrome, diabetes mellitus, hyperlipaemia, laminitis, endotoxaemia and osteochondrosis dissecans (OCD); whereas polysaccharide storage myopathy in Quarter Horses and equine motor neuron disease (EMD) have been associated with increased insulin sensitivity. However, it is clear that there is not one ideal test, in terms of both practicality and accuracy, for evaluating insulin sensitivity in horses and improved diagnostic techniques are required. This review sets out the background to the subject and identifies current knowledge regarding the measurement of insulin sensitivity by tolerance testing and clamping techniques. Factors affecting insulin sensitivity, such as breed, pregnancy, lactation, obesity and nutritional factors are discussed. In addition, the relationship with training, nutritional supplementation and drug administration are considered.

Maturity of the Neonatal Foal

The immature foal frequently represents a significant management challenge to even the most experienced clinician. The clinical course typically involves complications to a range of body systems,including the musculoskeletal, respiratory, and gastrointestinal systems. Before the commencement of treatment, it is important to provide the owner with an estimation of short-term and long-term survival, expected costs, and possible complications. Formulation of an accurate prognosis can be a difficult task but is aided by knowledge not only of normal maturation but of the factors that affect this process.

Thermoregulation in sick foals aged less than one week

Metabolic rate, rectal temperature Tr and respiratory quotient (RQ) were determined in 16 sick foals, aged 0-182 h. The foals were categorized into three groups: premature, dysmature or those suffering from neonatal maladjustment syndrome. The mean metabolic rate of the premature foals was 71 watts per unit area of body surface (W m(-2)), significantly lower than that of the other two groups. The overall mean metabolic rate for the sick foals was 82 W m(-2), about 25% below that of healthy foals of similar age. Air temperature (Ta) was 9.5-26.3 degrees C, and several foals shivered despite the provision of additional 'warmth' and insulation. Values of Tr were below 38 degrees C until after 4 h postpartum. Thereafter, Tr averaged 38.1 degrees C but fluctuated widely. The RQ (mean value 0.83) was not correlated with Ta. The lower critical temperature for the sick foals was estimated at 24 degrees C, on average. Methods to prevent excessive heat loss from sick foals are suggested.

Immunoreactive insulin-like growth factor I and insulin in blood plasma and milk of mares and in blood plasma of foals

Concentration of (total) globulin was relatively stable in blood plasma of mares, but rapidly decreased in colostrum to very low levels within 2 days after parturition. In foals, after intake of the first colostrum, globulin increased within 1 day in blood plasma, but remained at lower concentrations than those measured in mare plasma. Concentrations of immunoreactive insulin (iI) were high during the first 2 months of lactation in blood plasma of mares and then decreased, were high in first colostrum and then decreased drastically, and remained at low concentrations up to weaning in blood of foals. In mares, concentration of immunoreactive insulin-like growth factor I (iIGF-I) in plasma increased during late pregnancy, peaked 2 days after parturition, and then gradually declined until weaning. iIGF-I was highest in first colostrum and then dramatically decreased within the first 2 days of lactation. In foals, iIGF-I gradually increased over the first 2 months of life. IGF-I in the horse appears to be bound to proteins of similar molecular weight as in cattle. The study demonstrates different patterns of changes in plasma iIGF-I and iI concentrations in mares and their foals, whereas iI, iIGF-I and globulin changes in colostrum and milk occurred in parallel. Furthermore, plasma iI and iIGF-I behaved differently, while colostrum iI and iIGF-I behaved similarly, in mares compared with dairy cows.

Maturation of insulin and glucose responses to normal feeding in foals

Postprandial insulin and glucose concentrations were measured in 3 Arabian and 3 Thoroughbred foals at 1 day, 1 week, 1 month and 3 months of age. Prefeeding serum insulin concentrations were similar in foals at 1 day (25.9 +/- 5.1 pmol/L), 1 week (32.4 +/- 5.8 pmol/L), and 1 month (38.2 +/- 7.9 pmol/L), but had increased significantly to 131.0 +/- 20.2 pmol/L at 3 months of age (P < 0.05). There was significantly increased serum insulin secretion after a feed in foals at 3 months of age (P < 0.05) when compared with that at younger ages. Prefeeding serum glucose concentrations ranged from 6.0 +/- 0.7 mmol/L at 1 day, to 5.9 +/- 0.9 mmol/L at 1 week, 4.9 +/- 1.7 mmol/L at 1 month, and 4.4 +/- 1.5 mmol/L at 3 months of age. There were lower postprandial glucose concentrations with advancing age. It appeared that there was a period of maturation in pancreatic beta-cell function after birth in foals, which reached adult levels by 3 months of age.

Clinical view of disturbances in equine foetal maturation

This review summarises comparative aspects of equine pregnancy and birth. The allantochorion covers the entire endometrial surface of the mare's uterus and the placenta is microcotyledonary and epitheliochorial in structure. The foetus has, therefore, to pass through the allantochorion at birth. The umbilical cord has amniotic and allantoic portions and remains intact after delivery, enabling an arterial venous circulation to be maintained for several minutes. Maternal IgG does not cross the placental barrier and passive transfer post-natally is essential for immune status. Gestation in Thoroughbreds is 340 days with a wide range (320-360 days). Birth may be induced by oxytocin but dose rate is related inversely to gestational age. Normal foals rise, suck from the mare and gallop within 4 h after birth. Categorisation of newborn foal diseases into infective (Group 1) and non-infective (Groups 2, 3 and 4) conditions is presented. The neonatal maladjustment syndrome (NMS) affects full-term individuals, causing cerebral oedema, haemorrhage and/or ischaemic necrosis. NMS is related to the birth process and myocardial 'injury'. Prematurity and dysmaturity have origins in pre-natal disturbances of foetal maturation and physiology. Prematurity is a term ascribed to foals delivered at less than 320 days gestation whereas dysmaturity, for clinical convenience, describes foals born in the full-term period showing premature-like signs. Dysmature foals are generally associated with placental pathology. However, the distinction between the two groups is tenuos. Recent evidence suggests that 1) placental pathology is often present in premature foals; 2) differences exist in the degree of adrenocortical function. These affect clinical signs, prognosis and course of the condition. Placental pathology results in precocious adrenocortical maturation but the effect on maturation of other organ systems requires further study. Premature/dysmature foals fall into two groups; those with a favourable clinical outcome and those which make progress during the first 24 h post partum but deteriorate with development of neurological, metabolic and respiratory deficits (second day syndrome). Two models have been established to study premature/ dysmature foals. The first, developed in the early 1980s, was based on induction of mares from 280 days gestation to full term, using oxytocin. A premature, intermediary (twilight) and full-term status of adrenocortical function were identified; these categories correlating with changes in mammary secretion electrolytes. The same groupings have been related to other organ systems and metabolic functions of maturation. The second model establishes placental pathology from 220 to 260 days of gestation. A small area of placenta is separated from its uterine attachment, using a videoendoscope introduced through the cervix. Precocious adrenocortical function has been induced although further work is required to confirm the model as a means of investigating the pathogenesis of dysmaturity. Foetal injections of ACTH, CRH or betamethasone cause precocious increases in maternal plasma pregnane concentrations.

Effects of intravenous xylazine hydrochloride on blood glucose, plasma insulin and rectal temperature in neonatal foals

The effects of intravenous xylazine hydrochloride on blood glucose, plasma insulin and rectal temperature were investigated in six foals at 10 and 28 days of age. These variables were also measured in three foals at 19 days of age when saline alone was injected. Rectal temperature fell significantly after 30 mins in both groups of xylazine treated foals and was still depressed after 120 mins. Hypothermia did not occur in the saline control group. There was no significant change in blood glucose or plasma insulin concentrations during the 120 mins following either xylazine or saline administration and no significant differences between the three groups of foals. When foals were allowed to suckle after being away from their dams for 2 h, there was a significant (P less than 0.01) rise in plasma insulin levels in all the groups. Blood glucose showed a concomitant rise but this was only significant in the saline group. Unlike adults, intravenous xylazine (1.1 mg/kg) does not produce hypoinsulinaemia and hyperglycaemia in foals. This study suggests that the inhibition of insulin release from pancreatic beta-cells by xylazine, which in adults is alpha 2-adrenoceptor mediated, is immature or absent in foals under one month of age.

Studies on equine prematurity 1: Methodology

This paper describes the general management of mares and foals during the perinatal period and the methodology used in a collaborative research project on equine prematurity. Sixteen mares with dated pregnancies delivered 45 foals over three breeding seasons (1981 to 1983). In the majority, parturition was induced with oxytocin and/or fluprostenol; the remainder were allowed to foal spontaneously. Pre-colostral milk analysis provided a means of assessing the pre-foaling status of the mare. All were observed and monitored before, during and after parturition and the sampling protocol for both mare and newborn foal is discussed in detail. The foals were assessed for their degree of maturity at birth using behavioural, haematological, acid-base status and other criteria; they were then assigned to groups for further study and tests.

Studies on equine prematurity 6: Guidelines for assessment of foal maturity

This paper describes criteria used to assess maturity of the newborn foal and their clinical application to field cases of prematurity and dysmaturity. Premature and mature foals may be clearly distinguished by their behavioural and physical characteristics. Measurement of haematological parameters (mean cell volume, total white cell and differential counts), pancreatic beta cell activity (plasma glucose and insulin levels), adrenocortical-medullary function (plasma cortisol, adrenocorticotrophic hormone and catecholamines) and the renin-angiotensin system (plasma renin substrate concentrations) were found useful in evaluating the status of the newborn foal. Confirmation of the initial diagnosis can be made by response to various challenge tests eg, glucose tolerance test, short acting synthetic adrenocorticotrophic hormone (ACTH1-24) and frusemide. In the present investigation a small number of individuals appeared to be intermediate in maturity to the other two groups, indicating that a third state of maturity may be identified. The clinical implications of this work suggest that cortisol replacement therapy and administration of long acting synthetic ACTH1-24 may be of benefit.