Catecholamine physiology in the sheep

Abstract

A study of catecholamine physiology in the sheep has been carried out with emphasis on the measurement of plasma catecholamine levels and their interactions with lactic acid and cortisol. The plasma dynamics of the catecholamines were first examined under laboratory conditions followed by an assessment of sympathetic responses to a variety of environmental stimuli. Catecholamine levels were monitored in the plasma of sheep during and after 90 minute infusions of 1-dopa, dopamine, noradrenaline and adrenaline. A 2-compartmental pharmacokinetic model of 1-dopa plasma dynamics was constructed and the half-lives of the fast and slow decay curves determined. Estimation of the half-lives of dopamine, noradrenaline and adrenaline was prevented by a period of increased endogenous secretion that occurred on the termination of the infusions. A hypothesis based on the hypotension developed by sheep on the arrest of catecholamine infusions of this nature was developed to explain the adrenergic activity observed. Lactic acid levels rose markedly during the adrenaline infusions with a much smaller response during noradrenaline infusions while a biphasic cortisol response to both the noradrenaline and adrenaline infusions was observed. A set of blood samples was collected from each of 6 cannulated lambs at various stages of a journey from the farm to the abattoir. Catecholamine levels remained consistently elevated throughout the experiment before rising to a very high peak upon electrical stunning prior to slaughter. Plasma lactic acid peaks were associated with mustering and washing while cortisol remained high throughout the experiment until penning at the abattoir when it fell sharply to baseline levels. Subsequently it rose overnight. The physiological basis of these observations is discussed in relation to the transport of stock and the use of these blood substances in assessing the reactions of domestic animals to their environment. The massive catecholamine release in response to electrical stunning was further studied in a series of experiments. Significant differences between lamb mobs slaughtered at an abattoir appeared but a relationship was not found between the catecholamine plasma concentrations and the haemorrhagic lesion known as “blood splash” caused by electrical stunning. Short-term stressors encountered by the lambs at the abattoir had no effect on the blood splash incidence but some evidence suggested a link with the previous behavioural and handling conditions of slaughtered lambs. Both the α-adrenergic and the β-adrenergic blocking drugs phentolamine and propranolol were effective in suppressing blood splash and the physiological basis of this observation is considered. Some preliminary experiments to investigate catecholamine involvement in the neurological disease of ryegrass staggers in sheep are described together with a description of the intense ingestive behaviour and the possible taming effects induced by the central dopaminergic receptor blocker haloperidol. The application of these and other results to future agricultural research programmes is considered in the light of recent developments in the understanding of the multiple roles of these substances in mammalian physiology.