31 ALTERED HYPOTHALAMIC-PITUITARY-ADRENAL AXIS RESPONSE IN WEANED CLONED CALVES

Abstract

Somatic cell nuclear transfer (NT) is associated with an increased incidence of abnormal placental and fetal development. Postnatal survival of NT offspring to weaning and even into adult life is lower when compared to contemporary offspring produced by artificial insemination (AI). Neonatal NT calves are hypoglycemic and lethargic initially but show increased frequency of feeding later on. These and other symptoms suggest that NT animals have impaired homeostatic mechanisms. Despite this, NTs that survive beyond weaning appear normal, but underlying health problems can become apparent when NTs are subjected to physiological stressors. The health and physiology of predominantly (75%) Jersey NT bull calves (n = 9), derived from a fetal myogenic cell line, and control Jersey AI bull calves (n = 5) were investigated. At 6 months of age, animals were subjected to a series of physiological intravenous hormonal challenges to test the responses of individual organs. Pancreatic insulin secretion response was assessed via a glucose tolerance test (GTT) and direct adrenal resistance via ACTH administration. The response of the hypothalamic-pituitary-adrenal (HPA) axis was assessed via a glucagon challenge. Except for glucagon, the doses given were adjusted for body weight at the time of challenge and all animals were fasted overnight. Plasma glucose, insulin, glucagon, and cortisol concentrations were measured using commercial assays. All animals were euthanized at approximately 7 months of age and a full postmortem (PM) was undertaken. Calf and organ weights were recorded. Liver glycogen content was also determined. Parameters and challenge data were analyzed by one-way or as repeated measures ANOVA. NT and AI calves appeared healthy at the time of the physiological challenges, and at PM, no major gross or histological organ abnormalities were recorded. There were no differences in the relative mean liver and adrenal weights between NT and AI. Basal plasma glucose concentrations were similar between NT and AI controls but the post-fasting decrease in glucose concentration was greater (P < 0.05) in controls than in NTs. There was no difference in response between the groups to the GTT. The NTs showed a slower response to ACTH than did AI controls. Glucose and insulin secretion were significantly higher (P < 0.05) whereas initial cortisol release was significantly lower (P < 0.05) after glucagon administration. Plasma glucagon levels and liver glycogen content did not differ between NT and controls. The current study indicates that surviving NT calves appear healthy, but when physiologically challenged, demonstrate reduced adrenal sensitivity and altered HPA axis response. These response deviations are indicative of underlying physiological differences and could explain the increased susceptibility of NT animals to physiological stressors. Ultimately, the cloning procedures may affect the long-term health of cloned offspring.