Mode of action of α‐methylnoradrenaline on temperature and oxygen consumption in young chickens

Summary1 Temperature, oxygen consumption, electromyographic activity, plasma non‐esterified fatty acids and blood sugar were estimated in conscious unrestrained young chickens under conditions of thermoneutrality (31° C) and below thermoneutrality (16° C). In some chickens carotid arterial pressure was also recorded. 2 At thermoneutrality, α‐methylnoradrenaline lowered temperature and oxygen consumption in intact or chronically vagotomized chicks. α‐Methylnoradrenaline was ineffective on temperature in chicks with transection of the brain‐stem posterior to the hypothalamus but anterior to the respiratory centre. Hypothermia due to α‐methylnoradrenaline was associated with a significant reduction of plasma non‐esterified fatty acids but blood sugar was not significantly altered. Lowering of temperature by α‐methylnoradrenaline occurred despite vasoconstriction which would hinder heat loss. 3 Temperature and oxygen consumption were reduced by α‐methylnoradrenaline in chronically thyroidectomized chicks to the same extent as in intact chicks but recovery did not occur unless the chicks were taken from the metabolism chamber and warmed artificially. In contrast, chronically thyroidectomized chicks given replacement thyroxine were relatively resistant to α‐methylnoradrenaline. 4 Oxygen consumption of tissue slices from different parts of the chick's brain, including the diencephalon, was not altered by α‐methylnoradrenaline over an extensive dose range. The effects of α‐methylnoradrenaline on temperature and oxygen consumption in intact chickens were unlikely, therefore, to be due to depressed metabolism of neurones. 5 In an environment below thermoneutrality (16° C) temperature was considerably reduced and carotid arterial pressure fell 40–50 mmHg. In contrast, electromyographic activity, oxygen consumption and plasma non‐esterified fatty acids were markedly raised whereas blood sugar was insignificantly elevated. 6 In experiments at 16° C, α‐methylnoradrenaline markedly reduced oxygen consumption although values were still higher than those at thermoneutrality. Temperature fell further, but whereas the reductions in oxygen consumption and temperature were long‐lasting, electromyographic activity (shivering) was only transiently diminished. Plasma non‐esterified fatty acids were reduced after α‐methylnoradrenaline but not significantly so; blood sugar was not significantly altered. The time‐course for recovery of oxygen consumption following α‐methylnoradrenaline paralleled recovery from its blood pressure effects but the effect on oxygen consumption was not a consequence of the blood pressure changes. The effects of α‐methylnoradrenaline on temperature, oxygen consumption and electromyographic activity were similar to those of another central depressant, pentobarbitone.