Thermoregulatory responses of the newborn pig during experimentally induced hypothermia and rewarming.

Abstract

Exposure to a temperature of 14 degrees C was used to induce a progressive hypothermia in fourteen conscious newborn piglets. Heat production, body (rectal) and skin (between the shoulders) temperatures and shivering intensity assessed as the electromyographic activity (EMG) of longissimus thoracis muscle were measured until body temperature reached 30 degrees C and during a recovery period of 2 h at an ambient temperature of 24 degrees C (n = 7) or 34 degrees C (n = 7). During body cooling, heat production increased up to 9.67 +/- 1.28 W (kg BW)-1, but started to decrease below a body temperature threshold of 34.4 +/- 0.7 degrees C. EMG activity increased (P < 0.023) curvilinearly during body cooling; the main increase occurred between body temperatures of 38 and 33 degrees C (+142%, P < 0.001), and changes in EMG activity between 33 and 30 degrees C were not significant (+18%, P > 0.1). A marked increase in circulating levels of glucose (+312%, P < 0.001), glucagon (+76%, P < 0.05), adrenaline (+172%, P < 0.05) and noradrenaline (+113%, P < 0.05) occurred during body cooling. Insulin levels were not detectable at 2 h of life and increased during body cooling. During 2 h of rewarming at 24 degrees C, heat production and EMG activity remained elevated, changes in carbohydrate metabolism were not completely reversed and the final body temperature was only 35.6 +/- 0.9 degrees C. Rewarming of the piglets was faster at 34 degrees C. There was a net influx of heat into the animals and heat production and shivering activity decreased when body temperature reached 33.9 +/- 0.5 degrees C; the final body temperature was 37.5 +/- 0.2 degrees C. Circulating levels of lactate, glucagon and catecholamines returned to control levels. These results show that in conscious piglets exposed to a constant cold temperature there is an inverse relationship between EMG activity and body temperature during moderate hypothermia and that the thermoregulatory response and carbohydrate metabolism of the piglet are seriously impaired below a body temperature of 34 degrees C.