McLeay, Lance M.Smith, Barry L.Wilkins, DickVerkerk, Gwyneth A.2026-11-092026-11-091995https://hdl.handle.net/10289/18749The hypothalamo-pituitary-adrenal (HPA) axis acts an integrator which recognises challenges to survival and initiates an appropriate neuroendocrine response with the intent of maintaining homeostasis. The end-point of the response is secretion of glucocorticoids by the adrenal cortex. The aim of these studies was to develop understanding of factors which may modify adrenocortical function in dairy cattle. Cows, bulls and steers were treated with a range of doses (0.0125 to 0.4 mg/500 kg live-weight) of an adrenocorticotropic hormone (ACTH). Peak plasma cortisol concentrations (PCC) following ACTH were independent of dose except at the lowest dose; higher doses had a more prolonged effect. There was a linear relationship between the integrated adrenocortical response and log₁₀ dose of ACTH. In post-pubertal bulls, mean peak PCC were less (68.4 ± 2.0, 72.5 ± 4.0 and 43.8 ± 2.4 ng/ml in cows, steers and bulls, respectively; P<0.001 ). Measurement of PCC in samples at 0, 50 and 120 min in relation to iv administration of 0.05 mg ACTH/500 kg live-weight was defined as an adrenal response test (ART) for assessment of adrenocortical function. Treatment of cows with a range of doses (6.25 to 25 μg/500 kg live-weight) of synthetic bovine corticotropin-releasing hormone (bCRH) demonstrated that peak plasma ACTH concentrations (PAC) and PCC, as well as integrated pituitary but not adrenocortical responses were influenced by dose. There was a linear relationship between integrated pituitary responses and dose of bCRH, and between integrated adrenocortical responses and log₁₀ PAC at 15 min after treatment. Injection of bCRH with measurement of PAC and PCC is suggested as a test of pituitary-adrenal function in cattle. Injection of bacterial lipopolysaccharide was investigated as a test for HPA axis integrity in cattle, but found to be unsuitable because results were highly variable. There was a significant linear decline in results of ART during the third trimester of pregnancy. They also declined in bulls from 6 to 17 months of age and were low in bulls tested at 22 months and 5 years of age. Injection of dexamethasone suppressed results of ART for at least 17, 71 and 66 days in cows treated with sodium phosphate, trimethyl-acetate and 21- isonicotinate formulations, respectively. Suppression lasted 9 and 16 days in bulls and steers, respectively, with dexamethasone sodium phosphate. Results of ART were suppressed until at least 3 weeks post-partum following dexamethasone-induced parturition. Plasma concentrations of progesterone, oestrone sulfate and prolactin followed a similar temporal pattern to those seen in naturally calving cows. Plasma α-lactalbumin concentrations were initially lower in more premature cows but were similar at parturition; plasma immunoglobulin G1 concentrations declined at a greater rate in more premature cows. These studies have demonstrated that aspects of adrenocortical function in cattle are unique. There is marked sexual dimorphism in adrenocortical response to ACTH and responses may be further reduced with increasing maturity. The role of the adrenal cortex for the maintenance of metabolic homeostasis during stress may not be as critical in cattle as has been identified in other species; alternative mechanisms of response to stress may be more important.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis