Drinking-induced thermoregulatory panting in rehydrated sheep: influences of oropharyngeal/esophageal signals, core temperature, and thirst satiety

Abstract

Dehydrated mammals conserve body water by reducing thermoregulatory evaporative cooling responses e.g., panting and sweating. Increased core temperature (Tc) may result. Following rehydration and correction of fluid deficits, panting and sweating commence. We investigated the role of oropharyngeal/esophageal, postabsorptive and thermal signals in the panting response, and reduced Tc that occurs when unshorn sheep drink water following water deprivation for 2 days (ambient temperature 20 degrees C). Ingestion of water (at body temperature) resulted in increased respiratory rate (panting) and reduced Tc within 4 min that persisted for at least 90 min. Initially, a similar panting response and reduced Tc occurred following rehydration by drinking isotonic saline solution, but panting was not sustained after 20 min, and Tc began to rise again. Rehydration by intraruminal administration of water, without any drinking, resulted in delayed panting and fall in Tc. Intraruminal infusion of saline was ineffective. Rehydration by drinking cool water (20 degrees C) resulted in a rapid fall in Tc without increased panting. Shorn sheep had lower basal Tc that did not increase during 2 days of water deprivation, and they did not pant on rehydration by drinking water. Our results indicate that signals from the oropharyngeal and/or esophageal region associated with the act of drinking play a crucial role in the initial 20-30 min of the panting response to rehydration. Postabsorptive factors most likely reduced plasma tonicity and cause continued panting and further reduction in Tc. Tc also influences rehydration-induced panting. It occurs only if sheep incur a heat load during bodily dehydration.

Interaction between thermoregulation and osmoregulation in domestic animals

“…marsupials, 36 °C; cats and dogs, 37-38 °C; and sheep and goats, 39 °C Baker and Dawson, 1985;Jessen et al, 1998;Randall et al, 2002;McKinley et al, 2009). This constancy of core temperature comes about because a number of homeostatic responses are engaged when core temperature changes outside the "thermoneutral range" for that species.…”

“…dogs, cats, sheep, and goats, show that when these animals are subjected to a warm environment, they engage in thermoregulatory panting and/or sweating that results in elevated plasma Na + concentration and osmolality if the evaporated water is not replaced Baker and Dawson, 1985;Baker, 1989;McKinley et al, 2009). Such thermoregulatory loss of body fluids does not continue at the same rate if animals become dehydrated and many investigators have shown that dehydration, brought on usually by water deprivation, results in an attenuation of sweating and/or panting Baker and Dawson, 1985;Baker, 1989;McKinley et al, 2009). For example, exercising cats reduce evaporative heat loss if they are deprived of water compared with those permitted to drink water .…”

“…Evaporative cooling by panting is necessary to dissipate excess body heat in such animals, whereas shorn sheep can radiate heat from their exposed skin. When unshorn sheep are dehydrated, panting is suppressed and core temperature increases (McKinley et al, 2009). Because the osmolality of body fluids increases with dehydration, several investigators suggested that hypertonicity of plasma is the stimulus to suppress panting and sweating in dehydrated animals Baker and Dawson, 1985).…”

“…When dehydrated dogs and sheep in hot environments were rehydrated by drinking warm water, respiratory rate increased within 1-3 min, well before any change in plasma osmolality could be detected (Baker and Turlejska, 1989;McKinley et al, 2009). Heat-exposed, dehydrated goats increased sweating as well as panting within 3 min when they drank water (Baker, 1989).…”

“…This led to the suggestion that oropharyngeal factors or filling of the stomach associated with drinking fluid provided neural signals to influence thermoregulatory sweating and panting (Baker, 1989). Further studies in sheep showed that administration of isotonic saline directly into the rumen had no effect on thermoregulatory panting, showing that gastric distention was not a factor (McKinley et al, 2009). Administration of warm water into the rumen of dehydrated sheep caused a slow increase in panting and fall in core temperature after 30-40 min and was probably due to the slow fall in plasma osmolality that occurred.…”