When shorn sheep were exposed to ambient temperatures of 30C and the residual fleece was wetted, heart rate and cardiac output increased substantially above levels measured during a control period at 250C. With continued cold exposure and wetting the sheep become hypothermic and heart rate and cardiac output decreased. In most sheep at rectal temperature <32°C cardiac output had fallen below control values while heart rate remained above control. In a minority of hypothermic sheep cardiac output remained above control rates.Our interest in physiological events that occur during cold-stress in sheep originated from deaths of sheep from this cause after shearing in Australia. Hutchinson [1968] reviewed losses suffered by newly-shorn sheep that are exposed to cold, wet conditions and he concluded that approximately one million sheep died annually from cold exposure.Few data on cardiac output during cold exposure and hypothermia have been collected in conscious animals. The majority of results are confounded by anaesthesia of the subjects [Thauer, 1965] and the rates at which anaesthetized subjects were cooled [Brendel et al., 1958]. Data of other indices of cardiac physiology in cold-stressed sheep are also fragmentary. Hutchinson et al. [1960] reported elevated heart rates in sheep exposed to a cold, windy environment that reduced their rectal temperature (Tre) to approximately 37*4°C. In many other studies on newly-shorn sheep, cold exposure has been transient and measurements of body temperature and heart rate have not been concurrent [Slee, 1966; Slee and Sykes, 1967;Sykes and Slee, 1969].We have, therefore, studied cardiovascular parameters in newly-shorn sheep held at a control temperature of 250C and repeated the measurements when the sheep were exposed to a cold (3°C), wet environment. Endocrinological aspects of these experiments are reported elsewhere [Panaretto and Vickery, 1970].
METHODSThe sheep and their pre-experimental treatment. Six adult Merino ewes and one wether (no. 4) each weighing about 30 kg were used; they were accustomed to the presence of the observers. Each animal was given 400 g daily of a mixture of equal parts of lucerne chaff and oats which was sufficient to maintain body weight during the 4-8 weeks prior to the experiments. The same quantity of feed was given during cold exposure.The sheep were shorn and a femoral arterial catheter (clear vinyl, i.d. 1*5 mm, 101