Effects of birth weight, sex and neonatal glucocorticoid overexposure on glucose-insulin dynamics in young adult horses.

In several species, adult metabolic phenotype is influenced by the intrauterine environment, often in a sex-linked manner. In horses, there is also a window of susceptibility to programming immediately after birth but whether adult glucose-insulin dynamics are altered by neonatal conditions remains unknown. Thus, this study investigated the effects of birth weight, sex and neonatal glucocorticoid overexposure on glucose-insulin dynamics of young adult horses. For the first 5 days after birth, term foals were treated with saline as a control or ACTH to raise cortisol levels to those of stressed neonates. At 1 and 2 years of age, insulin secretion and sensitivity were measured by exogenous glucose administration and hyperinsulinaemic-euglycaemic clamp, respectively. Glucose-stimulated insulin secretion was less in males than females at both ages, although there were no sex-linked differences in glucose tolerance. Insulin sensitivity was greater in females than males at 1 year but not 2 years of age. Birth weight was inversely related to the area under the glucose curve and positively correlated to insulin sensitivity at 2 years but not 1 year of age. In contrast, neonatal glucocorticoid overexposure induced by adrenocorticotropic hormone (ACTH) treatment had no effect on whole body glucose tolerance, insulin secretion or insulin sensitivity at either age, although this treatment altered insulin receptor abundance in specific skeletal muscles of the 2-year-old horses. These findings show that glucose-insulin dynamics in young adult horses are sexually dimorphic and determined by a combination of genetic and environmental factors acting during early life.

Effects of maternal dexamethasone treatment on pancreatic ß cell function in the pregnant mare and post natal foal.

REASONS FOR PERFORMING STUDY: Synthetic glucocorticoids are used to treat inflammatory conditions in horses. In other pregnant animals, glucocorticoids are given to stimulate fetal maturation with long-term metabolic consequences for the offspring if given preterm. However, their metabolic effects during equine pregnancy remain unknown. OBJECTIVE: Thus, this study investigated the metabolic effects of dexamethasone administration on pregnant pony mares and their foals after birth. STUDY DESIGN: Experimental study. METHODS: A total of 3 doses of dexamethasone (200 µg/kg bwt i.m.) were given to 6 pony mares at 48 h intervals beginning at ≈270 days of pregnancy. Control saline injections were given to 5 mares using the same protocol. After fasting overnight, pancreatic ß cell responses to exogenous glucose were measured in the mares before, during and after treatment. After birth, pancreatic ß cell responses to exogenous glucose and arginine were measured in the foals at 2 and 12 weeks. RESULTS: In mares during treatment, dexamethasone but not saline increased basal insulin concentrations and prolonged the insulin response to exogenous glucose. Basal insulin and glucose concentrations still differed significantly between the 2 groups 72 h post treatment. Dexamethasone treatment significantly reduced placental area but had little effect on foal biometry at birth or subsequently. Foal ß cell function at 2 weeks was unaffected by maternal treatment. However, by 12 weeks, pancreatic ß cell sensitivity to arginine, but not glucose, was less in foals delivered by dexamethasone- than saline-treated mares. CONCLUSIONS: Dexamethasone administration induced changes in maternal insulin-glucose dynamics, indicative of insulin resistance and had subtle longer term effects on post natal ß cell function of the foals. The programming effects of dexamethasone in horses may be mediated partially by altered maternal metabolism and placental growth.

Neonatal glucocorticoid overexposure programs pituitary-adrenal function in ponies.

The present study tested the hypothesis that overexposure to endogenous glucocorticoids in neonatal life alters the reactivity of the hypothalamic-pituitary-adrenal (HPA) axis in ponies at 1 and 2 yr of age. Newborn foals received saline (0.9% NaCl, n = 8, control) or long-acting adrenocorticotropic hormone (ACTH1-24) (Depot Synacthen 0.125 mg intramuscularly twice daily, n = 9) for 5 d after birth to raise cortisol concentrations 5- to 6-fold. At 1 and 2 yr of age, HPA axis function was assessed by bolus administration of short-acting ACTH1-24 (1 µg/kg intravenous) and insulin (0.5 U/kg intravenous) to induce hypoglycemic on separate days. Arterial blood samples were taken at 5 to 30-min intervals before and after drug administration to measure plasma ACTH and/or cortisol concentrations. There were no differences in the basal plasma ACTH or cortisol concentrations or in the cortisol response to exogenous ACTH1-24 with neonatal treatment or age. At 1 and 2 yr of age, the increment in plasma ACTH but not cortisol at 60 min in response to insulin-induced hypoglycemia was greater in ponies treated neonatally with ACTH than saline (P < 0.05). Neonatal cortisol overexposure induced by neonatal ACTH treatment, therefore, alters functioning of the HPA axis in adult ponies.

Sex-associated differences in pancreatic ß cell function in healthy preweaning pony foals.

REASONS FOR PERFORMING STUDY: Pancreatic ß cells are responsive to a range of stimuli during early post natal life in healthy pony foals. However, little is known about whether these responses are sex-linked. OBJECTIVES: To determine pancreatic ß cell responses to the insulin secretagogues, glucose, arginine and tolbutamide, in fillies and colts during the first 3 months after birth. STUDY DESIGN: In vivo experiment examining sex differences in pancreatic ß cell function in foals. METHODS: Female (n = 8) and male (n = 5) pony foals were infused i.v. with glucose (0.5 g/kg bwt 40% dextrose), arginine (100 mg/kg bwt) or tolbutamide (10 or 20 mg/kg bwt) over 5 min, at 48 h intervals, to assess pancreatic ß cell function at ages 2 and 12 weeks. Blood samples (4 ml) were taken through a jugular catheter at -30, -15 and 0 min (immediately before) and 5, 15, 30, 45, 60, 90 and 120 min after glucose, arginine and tolbutamide administration for measurements of plasma glucose, α-amino-nitrogen and insulin concentrations. RESULTS: The maximum increment in plasma insulin concentration in response to glucose was significantly higher in female (395 ± 58 ng/l) than male (172 ± 37 ng/l, P<0.05) pony foals 2 weeks after birth and the area under the insulin curve was significantly greater in females at this age. At 12 weeks, the insulin increment in response to glucose was significantly greater in fillies 45 min post infusion. The ß cell responses to arginine and tolbutamide were not sex-linked at either age. CONCLUSIONS: These data show that in ponies, fillies have a greater ß cell response to glucose than colts in early post natal life. Since glucose clearance was unaffected by sex, the results suggest that fillies may be less insulin sensitive than colts shortly after birth. Innate sex differences in the secretion and action of insulin in early post natal life may influence tissue development and growth with potentially more long-term metabolic consequences.

Glucocorticoid overexposure in neonatal life alters pancreatic beta-cell function in newborn foals.

Studies in humans and animals have linked abnormal programming of adult tissue function to excess glucocorticoids during perinatal development. The current study investigated the hypothesis that physiological variations in glucocorticoid concentrations during early neonatal life of the foal alter the secretory responses of the pancreatic ß cells 2 and 12 wk after treatment. Spontaneously delivered foals received either saline or long-acting ACTH for 5 d from 1 d after birth to maintain an endogenous rise in cortisol concentrations. Starting at d 10, pancreatic ß cell function was studied using an intravenous (i.v.) glucose tolerance test, an i.v. arginine challenge, and an i.v. tolbutamide challenge. The maximum increment in plasma insulin achieved in response to exogenous glucose was less in ACTH-treated foals at both 2 and 12 wk of age (P<0.05). By 12 wk of age, developmental changes also occurred in the magnitude and biphasic pattern of glucose-stimulated insulin release. The area under the insulin curve during the early phase of insulin secretion (0 to 30 min) was not different between the 2- and 12-wk-old animals but was significantly greater during the later phase (30 to 120 min) at 12 wk than at 2 wk (P<0.05). Arginine infusion induced a brief 5 to 15 min increase in plasma concentrations of insulin that was not different in saline- and ACTH-treated foals. The ß-cell response to tolbutamide infusion was rapid and monophasic, and there was no difference (P>0.05) in the area under the insulin curve with treatment at 2 or at 12 wk. However, after tolbutamide, plasma insulin concentrations remained increased for a longer period in the ACTH-treated than in the saline-treated foals at 12 wk of age (P<0.05). Hence, this is the first study to show altered pancreatic ß-cell function after ACTH-induced glucocorticoid overexposure during early postnatal life in foals.

Pancreatic endocrine function in newborn pony foals after induced or spontaneous delivery at term.

REASONS FOR PERFORMING STUDY: During the switch from parenteral to enteral nutrition at birth, endocrine glands such as the pancreas must assume a glucoregulatory role for the first time if the neonate is to survive the transition to extrauterine life. OBJECTIVES: To determine the adaptations in pancreatic endocrine function during the neonatal period in term pony foals delivered by different methods. METHODS: By measuring insulin and glucagon concentrations, pancreatic alpha and beta cell responses to exogenous glucose (0.5 g/kg bwt) and arginine (100 mg/kg bwt) and to endogenous muzzling for 90 min were determined periodically over the first 10 post natal days in foals born spontaneously (n = 8) or by induction of labour with oxytocin at term (n = 7). RESULTS: Pancreatic alpha and beta cell function changed with post natal age in a manner related to the method of delivery. Induced foals had 2-3 fold greater beta cell responses to exogenous glucose and arginine, despite similar glucose and alpha-amino nitrogen clearances compared with spontaneously delivered foals. Pancreatic beta cell responses to glucose decreased by 50% while those to arginine doubled with increasing age in induced but not spontaneously delivered foals. In contrast, pancreatic alpha cell responses to arginine doubled with increasing age in foals born spontaneously but not by induction. These differences in pancreatic endocrine cell function with delivery method were associated with 2-3 fold higher cortisol levels in the induced foals and with differences in the absolute and age-related changes in basal concentrations of glucose, alpha-amino nitrogen and insulin. CONCLUSIONS: Induced delivery leads to changes in pancreatic beta cell sensitivity to glucose and/or tissue insulin resistance in association with persistent neonatal hypercortisolaemia. POTENTIAL RELEVANCE: The altered post natal development of pancreatic endocrine function with induced delivery may compromise glucoregulation and adaptation to enteral nutrition in neonatal foals with potential consequences long after birth.

Hypothalamic-pituitary-adrenal axis function in pony foals after neonatal ACTH-induced glucocorticoid overexposure.

REASONS FOR PERFORMING THE STUDY: The effects of overexposure to glucocorticoids during early life of the foal on the subsequent HPA programming of the hypothalamic-pituitary-adrenal axis are unknown. OBJECTIVES: To test the hypotheses that excess glucocorticoid exposure in early life subsequently increases both basal plasma concentrations of cortisol and the adrenocortical responsiveness to exogenous adrenocorticotropic hormone (ACTH). METHODS: Foals received either saline (0.9% NaCl, n = 9) or long-acting ACTH (0.125 mg i.m. b.i.d., n = 6) for 5 days from Day 1 to increase endogenous cortisol concentrations. Long-term indwelling catheters were inserted under local anaesthesia into the jugular veins of foals aged 2 and 12 weeks. After recovery, short-acting ACTH1-24 was given as a single i.v. injection (2 microg/kg bwt) and blood samples were taken at 5-30 min intervals before and after ACTH administration to measure plasma cortisol concentrations. RESULTS: Basal plasma cortisol concentrations were higher in ACTH- than in saline-treated foals at age 3 weeks, but not at 13 weeks. There were no significant differences in either the time profile or the area under the cortisol curve in response to ACTH between the 2 groups. CONCLUSIONS: These data suggest that ACTH-induced overexposure to glucocorticoids during early post natal life of the foal does not have a programming effect on HPA axis function at 13 weeks. In foals, the effects of ACTH-induced overexposure to glucocorticoids, if any, may not become apparent until much later in life in a long-lived species such as the horse. POTENTIAL RELEVANCE: These studies suggest that clinical and other stressful conditions that raise plasma cortisol concentrations during early life are unlikely to programme cardiovascular and metabolic function in horses in the short term.

Development of baroreflex and endocrine responses to hypotensive stress in newborn foals and lambs.

OBJECTIVE: The aims of this study were to compare and contrast the development of the cardiac baroreflex and endocrine responses to acute hypotensive stress in healthy newborn pony foals and lambs during the first two weeks of postnatal life. METHODS: Under general anaesthesia, seven Welsh pony foals and six Welsh Mountain lambs were catheterised with hind limb artery and vein catheters. Following post-surgical recovery, at 1 week and 2 weeks of age, blood pressures of the animals were raised and lowered acutely by intravenous infusion of phenylephrine and sodium nitroprusside, respectively. During hypotension, blood samples were taken for measurement of plasma hormones associated with activation of the stress axis. RESULTS: Basal arterial blood pressure increased significantly (P<0.05) between week 1 and week 2 in the absence of any significant change in basal heart rate in foals and with a significant reduction in basal heart rate in lambs. In foals, the slope of the heart rate-blood pressure relationship decreased in response to acute hypertension, and it increased in response to acute hypotension, from week 1 to week 2 (all P<0.05). In contrast, in lambs, the slope of the heart rate-blood pressure relationship decreased with both acute hypertension and acute hypotension from week 1 to week 2 (all P<0.05). In foals, there were significant increases in plasma concentrations of noradrenaline, neuropeptide Y (NPY), vasopressin, adrenocorticotrophic hormone (ACTH) and cortisol in response to hypotension (P<0.05). In lambs, there were also significant increases in plasma concentrations of ACTH and cortisol during hypotension. Plasma concentrations of noradrenaline, NPY and vasopressin were not measured during hypotension in lambs. In foals, although the magnitude of the ACTH response to hypotension was smaller at week 2 than week 1, the increment in plasma cortisol was similar in the two age groups. In contrast, in lambs, the profile of both the ACTH and cortisol responses was similar at week 1 and week 2. CONCLUSION: These data suggest that the increase in basal arterial blood pressure in the foal and the lamb during the first 2 weeks of postnatal life is accompanied by differential maturational changes in the vagal and sympathetic components of the cardiac baroreflex between the two species. These developmental cardiac baroreflex changes occur together with increased adrenocortical responsiveness to acute hypotensive stress, which appears comparatively more mature in lambs than in foals.

Development of insulin and proinsulin secretion in newborn pony foals.

At birth, the endocrine pancreas must assume a glucoregulatory role if the neonate is to survive the transition from parenteral to enteral nutrition. In species like the horse, neonatal hypoglycaemia is common, which suggests that the glucoregulatory mechanisms are not always fully competent at birth. Hence, this study examined pancreatic beta cell function in newborn foals during nutritional adaptation over the first 10 days post partum. Over a 48 h period at three time intervals after birth (days 1-2, 5-6 and 9-10 post partum), the beta cell responses to suckling and to intravenous administration of glucose, arginine and saline were measured in seven normal pony foals. Basal plasma concentrations of proinsulin, but not insulin or glucose, increased significantly between days 1 and 10. Suckling caused a gradual increase in plasma glucose, which was accompanied by a significant increase in plasma insulin concentrations 15 min after the onset of suckling on days 5 and 9, but not day 1. There was no significant change in plasma proinsulin concentrations in response to suckling at any age. At all ages studied, glucose and arginine administration stimulated an increase in the plasma concentrations of insulin and proinsulin; these beta cell responses did not change significantly with postnatal age. The insulin responses to glucose were significantly greater than those of arginine at each time period. Glucose clearance was significantly slower on day 1 than subsequently. Proinsulin and glucose, but not insulin, concentrations decreased significantly after saline administration at all three ages. At each time period, there was a significant positive relationship between the plasma insulin and proinsulin concentrations, the slope of which was significantly shallower on days 1-2 than subsequently. These results show that equine beta cells are responsive to glucose and arginine and release both insulin and proinsulin during the immediate postnatal period. They also suggest that newborn foals may be insulin resistant on the first day after birth.

Developmental changes in pulmonary and renal angiotensin-converting enzyme concentration in fetal and neonatal horses.

Angiotensin-converting enzyme (ACE) has an active role in the control of blood pressure and body fluid homeostasis both before and after birth. This study investigated the ontogeny of pulmonary and renal ACE concentrations in fetal and neonatal horses. Fetal pulmonary ACE concentration increased from 250 days towards term (c. 335 days). Newborn foals showed significantly higher mean concentrations of pulmonary ACE (4.40 +/- 0.62 nmol min(-1) mg protein(-1)) than both fetuses during late gestation (1.23 +/- 0.51 nmol min(-1) mg protein(-1)) and animals aged 1 day to 2 weeks of postnatal age (0.85 +/- 0.15 nmol min(-1) mg protein(-1)). Renal ACE was detected in fetal horses from 100 days of gestation but showed no developmental trend during the second half of gestation or in early postnatal life. Overall in the fetus, mean concentrations of renal ACE were also approximately 10 times lower than mean pulmonary values. Renal ACE concentration may be related to the functional immaturity of the equine kidneys. The increase in pulmonary ACE concentration seen towards term in the fetal horse may be induced by the prepartum cortisol surge that occurs very close to delivery in this species. Therefore, premature delivery in this species may interrupt the onset of ACE production in the fetal lungs and circumvent the normal maturation of the renin-angiotensin system.

Organogenesis of lung and kidney in Thoroughbreds and ponies.

Equine lung and kidney organogenesis has not previously been examined with the use of unbiased stereological techniques. The present study examined healthy (control) pony and Thoroughbred lungs and kidneys to establish baseline data of organ development from before birth until maturity at age 3-18 years. Whole left lungs and kidneys were collected from 45 equine postmortem examinations (34 Thoroughbred, 11 pony). Stereological techniques were used to estimate whole kidney, cortex and medulla volume, total glomerular number and volume-weighted mean glomerular volume, lung volume, total terminal bronchiolar duct ending number and total gas exchange surface area. Lungs were demonstrated to be more developed at birth in ponies compared with Thoroughbreds. Thoroughbreds showed continued lung development after birth, a unique micromorphogenic postnatal development. Kidneys were developed equally in ponies and Thoroughbreds. This study has provided data on the baseline development of the equiune lung and kidney which can be used in further studies to examine whether the development of these organs is affected by specific illnesses.

Establishment and characterization of equine autonomic ganglion cell lines to enable direct testing of candidate toxins involved in equine dysautonomia (grass sickness).

To enable direct testing of a range of potential toxins or pathogens that might be involved in grass sickness, equine thoracic sympathetic chain ganglion cell lines were established from primary cell cultures by retroviral-mediated transduction of the temperature-sensitive mutant of the establishment oncogene encoding SV40 large T antigen. Morphological and behavioral features, temperature dependence, and immunocytochemical characteristics of the cell lines were investigated. The majority of cells were noradrenergic neurons in which dopamine-beta-hydroxylase, the enzyme that catalyzes norepinephrine synthesis, and neuropeptide Y coexisted. Cells treated with plasma from grass sickness cases that had previously been shown to induce autonomic nervous system damage when injected into normal horses showed significantly decreased mitochondrial function after 1 day. After 3 days exposure most cells showed severe degeneration in contrast to those treated with normal plasma. Liver and lung cell lines were also susceptible to plasma, suggesting that the toxin is not specifically neurotoxic.

Investigation of an outbreak of tapeworm-associated colic in a training yard.

A novel serological assay which measures IgG(T) specific for a 12/13 kDa protein of the equine tapeworm Anoplocephala perfoliata was used as part of a colic outbreak investigation. A training/rehabilitation yard for Thoroughbreds and Arabs was found to have an increasing incidence of colic over a 5 year period, culminating in a peak incidence of 1.15 episodes/horse year at risk. Four animals suffered from ileal impaction colic which necessitated surgical management. A case-control study design suggested a strong association between tapeworm infection and colic, with evidence of a dose-response relationship. Intervention, in the form of anticestode anthelminthics, coincided with a decrease in the incidence of colic and a fall in anti-12/13 kDa IgG(T) antibody levels of 8 horses monitored post-treatment. This study demonstrates that anthelminthic regimens, using exclusively ivermectin, may lead to an increase in tapeworm infection intensity which may in turn lead to an increased incidence of colic. Furthermore, it provides support to the hypothesis that the risk of ileal impaction colic and spasmodic colic increases with tapeworm infection intensity. The practical application of the anti-12/13 kDa IgG(T) ELISA is demonstrated by this study.

Glomerular filtration rate, effective renal plasma flow, blood pressure and pulse rate in the equine neonate during the first 10 days post partum.

The aim of this study was to determine glomerular filtration rate (GFR), effective renal plasma flow (RPF) and filtration fraction (FF) values in Pony foals during the first 10 days post partum in order to assess any age-related changes and the reliability of the single injection inulin/p-aminohippurate (PAH) method. Arterial blood pressure and pulse rate were determined in Pony and Thoroughbred (TB) foals during the first 10 days post partum to establish age-related changes, differences between breeds and possible interrelationships with GFR and RPF. Inulin and PAH were injected intravenously (i.v.) in healthy Pony foals (n = 6) on Days 1, 2, 4, 7 and 10 post partum. Blood samples were collected via the contralateral jugular catheter over a 4 h period. Plasma clearances of inulin and PAH showed that GFR and RPF, respectively, remained relatively constant throughout the post natal period, and were similar to values reported for mature horses. The GFR measurements obtained using the inulin method gave clearance values that were significantly higher (P < 0.01) than those of creatinine clearance performed in a separate group of Pony foals (n = 7). Blood pressure and pulse rates were measured using an electronic sphygmomanometer in TB (n = 6) and Pony foals (n = 7) on Days 1, 2, 3, 4, 7 and 10 postpartum. There were no significant differences in these measurements between the breeds, and both cardiovascular parameters increased during the first 3 days post partum. Values remained unchanged for the rest of the study period. Although measurements of GFR, RPF, blood pressure and pulse rates were not determined in the same animals, blood pressure and pulse rates did not appear to correlate with either GFR or RPF in the Pony foals during the post natal period.

Effects of various feeding regimens on the energy balance of equine neonates.

OBJECTIVE: To determine the effect of diet on energy intake, loss, and metabolism in foals 2 to 7 days old. ANIMALS: 14 pony foals. PROCEDURE: Group-A foals suckled their dams, group-B foals were fed milk replacer, and group-C foals were fed by total parenteral nutrition (TPN). Energy balance studies were performed over 8-hour periods on postpartum days 2, 4, and 7. RESULTS: Mean gross energy (GE) intake of group-A foals increased between days 2 and 7. Approximately 3% of GE was excreted in urine and feces, and energy expenditure remained constant. These foals were in positive energy balance, and mean body weight increased. From day 4 onward, group-B foals consumed more energy than did group-A foals because the milk replacer had a higher energy content than did mares' milk. Mean energy loss in group-B foals was 14% of GE on day 2, but this value decreased subsequently. Energy expenditure in group-B foals was less than that in group-A foals, and energy balance was positive. Group-C foals had the lowest energy intake and expenditure; energy balance was negative on postpartum day 2. These foals also had gastrointestinal tract problems. CONCLUSIONS: Mares' milk is highly digestible and is correlated with positive energy balance in neonatal foals. Milk replacer initially is less digestible than mares' milk. In this study, TPN was associated with negative energy balance. CLINICAL RELEVANCE: The data indicate the advantages of enteral feeding with mares' milk and highlight the clinical and technical difficulties associated with TPN.

Localisation of 15-hydroxy prostaglandin dehydrogenase (PGDH) and steroidogenic enzymes in the equine placenta.

15-hydroxy prostaglandin dehydrogenase (PGDH) is the critical enzyme that determines metabolism of primary prostaglandins. Its expression is determined in part by steroid hormones, particularly progesterone, formed from delta(5) steroids through 3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity. To assess whether the regulation of PGDH might occur in a paracrine, autocrine or intracrine fashion, we used immunohistochemistry (IHC) to determine the localisation of key steroidogenic enzymes in the equine placenta and compared these patterns to the distribution of immunoreactive (IR-) PGDH. Placental tissue was obtained from pony or Thoroughbred mares at about Days 150, 250-280 and >300 of pregnancy (term 320 to 360 days; n=5-8 each group). IR-PGDH, 3beta-HSD, cholesterol side chain cleavage enzyme (P450(scc)) and 17-hydroxylase/lyase (P450(C17)) were localised using specific antibodies and the avidin-biotin peroxidase technique and visualised using diaminobenzidine as substrate. IR-P450(scc) was present in trophoblast cells, but not in maternal tissues of the microcotyledons. In contrast, at Days 150 and 280, IR-PGDH was present in maternal epithelial and interstitial cells in the microcotyledons, but was not detected in trophoblast epithelium, chorioallantois or endometrial glands. After Day 300, IR-PGDH was present in the maternal epithelium and interstitial cells of the placenta and it was also present in trophoblast cells in some specimens.

Immunohistochemical localisation of steroidogenic enzymes and phenylethanolamine-N-methyl-transferase (PNMT) in the adrenal gland of the fetal and newborn foal.

An increase in fetal adrenal cortisol output signals the onset of parturition in many animal species but, in the fetal horse, plasma concentrations of cortisol remain low for much of late pregnancy, with a rise occurring only very close to the time of birth (term 320-360 days). Immunohistochemistry was used to determine the localisation and changes in distribution of key steroidogenic enzymes for cortisol production; P450scc, P450C17 and 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) in adrenal tissue from fetal and newborn horses and these findings were correlated with the appearance of immunoreactive (IR)-phenylethanolamine-N-methyl-transferase (PNMT), a cortisol-dependent enzyme. Five micron sections of adrenal tissue from fetuses at Day 100-156 (n = 5), Day 244-295 (n = 8), greater than Day 300 (n = 4) and from newborn foals (n = 6), were stained using specific antibodies and the avidin-biotin-peroxidase technique. All 3 steroidogenic enzymes were present by Day 150, but in less than 20% of the cortical cells. By late gestation the steroidogenic enzymes were present in approximately 30% of the cells, but the distribution varied. P450SCC and P450C17 predominated in cortical cells proximal to the medulla; 3 beta HSD was present throughout the cortex, but more in the zona fasciculata. In foals after birth, IR-3 beta HSD and IR-P450SCC had increased substantially throughout the adrenal cortex, and IR-P450C17 was present in most cells of the presumptive zonae fasciculata and reticularis. IR-PMNT was localised to nuclei of scattered medullary cells at the medullary-cortical interface by Day 150.(ABSTRACT TRUNCATED AT 250 WORDS)

A retrospective study of increased plasma progestagen concentrations in compromised neonatal foals.

Plasma progestagen concentrations were measured daily by radioimmunoassay (RIA) in 35 sick foals for the duration of their illness. The foals were divided into three groups on the basis of time to stand after birth. Foals were given intensive care treatment according to the severity of their illness. Plasma and urine concentrations of pregnenolone (P5) and pregnenediol (P5 beta beta) were measured by gas chromatography--mass spectrometry; plasma cortisol concentrations were measured by RIA and the foals' renal and respiratory status were assessed by creatinine clearance ratios and arterial oxygen concentrations respectively. Five patterns of plasma progestagen concentrations were identified; in general, values increased when the foal's clinical condition deteriorated and decreased as the foal improved. Median progestagen concentrations decreased over the first three days post partum in Group 1 foals but remained elevated in foals from Groups 2 and 3. Similar changes were observed in plasma P5 and P5 beta beta concentrations. Plasma cortisol concentrations were highest in foals from Groups 2 and 3 (P < 0.01) compared with foals from Group 1. Regardless of foal group, mean cortisol concentrations were highest (P < 0.001) in those foals treated with adrenocorticotrophic hormone compared with those treated with dexamethasone or with neither drug. There was no relationship (r2 = 0.21) between plasma cortisol and progestagen concentrations. Results from renal clearance, steroid conjugation and respiratory status suggest that these factors did not play a significant role in elevating progestagen concentrations in sick foals. It is hypothesized that there may be a relationship between adrenal stimulation and an enzyme block resulting in overproduction of P5 and P5 beta beta in the sick neonatal foal.

Increase in plasma progestagen concentrations in the mare after foetal injection with CRH, ACTH or betamethasone in late gestation.

Maternal plasma progestagen concentrations increase about 20 days before parturition. The major contributors to the increase are reduced metabolites (ie 5 alpha-pregnanes). Precocious increases (ie less than 310 days of gestation) in these metabolites may occur in abnormal pregnancies. The effects of CRH, ACTH or betamethasone administered to the foetus at gestational ages ranging from about 250 to 320 days were examined. Sixteen healthy pony mares were used for foetal injection employing aseptic techniques. Water or normal saline were used as controls. Maternal plasma progestagen concentrations were measured using a commercial radioimmunoassay (RIA) progesterone kit and results were confirmed using gas chromatography-mass spectrometry (GC-MS). Results demonstrated clearly that an increase in maternal plasma progestagen concentrations occurred after injection of ACTH, CRH or betamethasone to the foetus, irrespective of gestational age. A comparable increase was not observed in the control animals. Of the 16 mares in which the foetus was injected, 13 produced viable foals at gestational ages ranging from 307 to 339 days whereas 3 mares delivered non-viable foals at 284 to 306 days gestation. The results support the hypothesis that the pre-parturient rise in progestagens occurring in the mare is the result of foetal adrenocortical activity.