Control of evaporative heat loss during changes in plasma osmolality in the cat

Abstract

SUMMARY1. The effects of intravenous infusion of hypertonic saline and distilled water into normally hydrated and dehydrated cats have been examined at both high and neutral ambient temperatures.2. In hydrated cats measurements of body temperature (Tb) and evaporative heat loss (e.h.l.) show that infusion of 30 % saline (1I5 ml./kg) at an ambient temperature of 38 0C, lowers e.h.l. by an average of 0-21 W/kg (P < 0-001) and elevates Tb by 0 43 'C (P < 0-01). 3. At 25 'C alterations in these two parameters were in the same direction, though not statistically different from pre-infusion levels (P > 0 05).4. Infusion of distilled water (15 ml./kg) into dehydrated animals produced significant increases in e.h.l. (+ 0 35 W/kg, P < 0-001) and reductions in Tb (-0 45 OC, P < 0-001) at 38 OC. No significant effects were observed at 25 OC.5. Infusion of water into normally hydrated animals at 380C also significantly increased e.h.l. (+0 13 W/kg, P < 0-05) and insignificantly lowered Tb (-0 03°C, P> 005).6. Local heating of the preoptic hypothalamic area in four animals indicated that hypertonic saline infusion into normally hydrated animals caused a reduction in the slope and displacement to the right of the relationship between hypothalamic temperature and e.h.l.7. Conversely, water infusion into dehydrated animals increased the slope and shifted this relationship to the left.8. These experiments provide evidence for an osmotic interaction in body temperature regulation which acts to alter the responsiveness of the hypothalamus to increasing temperature. This osmotic component may be an important factor in the alterations in thermoregulation seen in dehydrated animals.

Integration of thermal and osmotic regulation of water homeostasis: the role of TRPV channels

“…The subsequent elucidation of a hormonal mechanism underlying cold diuresis (6), and the discovery of VP as an antidiuretic agent linked the phenomenon of cold diuresis to inhibition of VP secretion. The more recent observations that dehydration reduces evaporative cooling responses resulting in hyperthermia (7,57) further supports the importance of mechanisms integrating thermal and fluid homeostasis.…”

Selective brain cooling in goats: effects of exercise and dehydration.

“…In the only study of the activity of these venous structures in intact animals, Johnsen et al (1987) Dehydrated mammals can save water by reducing thermoregulatory evaporation and allowing the body temperature to rise during heat stress (Schmidt-Nielsen, Schmidt-Nielsen, Jarnum & Houpt, 1957). One mechanism for this readjustment of thermoregulation has been thought to be an osmotically induced reduction in the sensitivity of the hypothalamus to increased temperature (Baker & Doris, 1982).…”

Central Mechanisms for Thermoregulation in a Hot Environment

“…The PO/anterior hypothalamus contains abundant warmsensitive neurons 12) , which are involved in various autonomic and behavioral thermoregulatory processes 17,[73][74][75][76][77] . Baker and Doris 6,7) first reported that osmotic stimulation attenuated evaporative heat loss at the level of the hypothalamus. Nakashima et al 78) showed that, in an in vitro slice of rat brain, the warm-sensitive neurons in the medial PO lower the firing rate in a hyperosmotic medium.…”

“…The two factors attenuate both evaporative and non-evaporative heat loss mechanisms, e.g. saliva spreading and tail blood flow in rats, and panting and skin blood flow in dogs [7][8][9][10][11] . These responses would be important in preserving body fluid and preventing excessive blood distribution to the periphery; however, animals may lose thermal homeostasis.…”

“…However, during exercise, we cannot redistribute blood from the body core to the skin at such level, because blood flow to the muscle must be also maintained. Especially, during dehydration, both dry and evaporative heat loss are suppressed even though body temperature elevate [6][7][8][9][10][11] . Thus, it is thought that these suppressions of thermoregulation during dehydration are induced by non-thermoregulatory factors, which are involved in cardiovascular and/or body fluid regulations.…”