Cardiac vagal tone, plasma cortisol and DHEA response to an ACTH challenge in lame and non-lame dairy cows

Abstract We studied the adrenocortical and vagal tone response to a single ACTH challenge in lame (L, n=9) vs. non-lame (NL, n=9) cows. Cows were paired according to parity, days in milk and milk yield. Plasma cortisol and dehydroepiandrosterone (DHEA) concentrations and cardiac vagal tone response (high-frequency (HF) component of heart rate variability) were compared following intravenous ACTH administration. Baseline, min/max, amplitude of the response and area-under the response curve (AUC) were compared. No difference could be detected between groups in the cortisol response. DHEA was irresponsive to ACTH treatment and concentrations did not differ between lame and non-lame cows. Vagal tone decreased in response to the ACTH treatment. HF was lower in the lame group at all sampling times. Lameness was associated with delayed return to baseline. We concluded that the adrenal response capacity is not influenced by lameness which supports the concept of lameness being a chronic intermittent rather than a chronically persistent stressor. DHEA concentrations were not proven to be useful indicators of hypothalamus-pituitary axis dysfunction in cattle. A decreased vagal contribution to heart rate variability – possibly coupled with increased sympathetic modulation – was observed in lame cows which suggests that lameness affects the mechanisms underlying the action of ACTH on cardiovascular activity.