THE EFFECT OF ALPHA(2)-ADRENOCEPTOR STIMULATION WITH GUANFACIN ON THERMOGENESIS IN FASTED AND FED SHEEP

Eight sheep with an average body weight of 99 kg were divided into two groups (fasted and fed) of four and kept in individual pens. The shee p were used to examine the effects of alpha, adrenoceptor stimulation on heat production, heart rate, rectal temperature, skin temperature a nd plasma parameters. On the day of the experiment, food but not water was withheld from the fasted group and the fed group received 1.0 kg of pelleted alfalfa diet. The treatments consisting of vehicle (saline ) and three levels (low = 0.8 mg; medium = 1.6 mg; high = 2.4 mg per s heep) of guanfacin were administered intravenously in a split-plot exp erimental design. The heat production was determined by open circuit r espiration calorimetry, the heart rate was recorded by an electrocardi ograph and the rectal and skin temperatures were measured with a Fishe r Digital Thermometer. When compared to saline, guanfacin reduced (P < 0.05) the heat production of fasted sheep (1.21 vs. 0.99, 1.09, 0.98 W kg(-1)) for the low, medium and high doses, respectively. For the fe d sheep the medium and high doses reduced total post-feeding heat prod uction (1.64 vs. 1.43, 1.55 W kg(-1)). The difference in heat producti on between the fed and fasted animals was not altered by guanfacin. Th e skin temperature was reduced by all doses of guanfacin in the fasted sheep but only by the high dose in the fed sheep. The heart rate was not affected by guanfacin. The high dose increased the rectal temperat ure of fed sheep. In both groups the high dose resulted in a decrease (P < 0.05) in blood haemoglobin content. Guanfacin induced a two- to t hreefold increase in plasma glucose and a transient response in insuli n concentration. We conclude that short-term alpha(2)-adrenoceptor sti mulation with guanfacin suppresses the heat production of both fasted and fed sheep but has no effect on the heat increment of feeding.