Regional blood flow antagonism induced by central thermal stimulation in the conscious dog

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

Renal function changes during preoptic-anterior hypothalamic heating in the rabbit

Thermoregulatory reactions evoked by selective preoptic-anterior hypothalamic (PO/AH) heating in conscious rabbits were associated with significant changes in renal function. Urine flow rate decreased from a control value of 0.92 +/- (S.E.) 0.08 to 0.47 +/- 0.07 ml/min after 10-20 min of heating, urine osmolality increased from 273 +/- 34 to 417 +/- 46 Osm/kg H2O, and free water clearance per 100 ml GFR decreased from 1.11 +/- 0.46 to -0.50 +/- 0.23 ml/min. These changes were followed by a gradual recovery despite continued heating. Clearances of exogenous creatinine and p-aminohippurate fell transiently during the first 10 min of heating and then returned to normal. Plasma antidiuretic activity (ADA) measured by rat bioassay increased regularly and markedly during PO/AH heating but was poorly correlated with changes in urine concentration. Moreover, a similar increase in plasma ADA observed with selective heating of a different brain area (supraoptic nucleus) never produced urine concentration or other renal changes. This suggests that a large and variable fraction of ADA appearing in rabbit blood in response to thermal stimuli was not identical with antidiuretic hormone. Therefore, the causal relationship of ADH release and antidiuresis associated with thermoregulatory reactions could not be clearly demonstrated. The physiological role of renal water conservation would be to compensate for extrarenal water loss related to thermal sweating or panting.

Arterial baroreceptor function in differential cardiovascular adjustments induced by central thermal stimulation

Dogs were anesthetized with sodium pentobarbital, relaxed with succinyl choline and were kept under artificial ventilation. Both carotid bifurcations were denervated and the Vagus nerves were cut in the neck. Regional blood flow in the skin and the intestine, cardiac output, heart rate and arterial pressure were determined before, during and after spinal cord heating and cooling. Further experiments were performed in which, in addition, sympathetic effects on the heart were excluded by exstirpation of the caudal cervical and stellate ganglia or by beta-receptor blockade. The cardiovascular responses were compared with those obtained in a preceding investigation from dogs with intact baroreceptors and vagus nerves. As in intact dogs, appropiate thermoregulatory adjustments of skin blood flow were induced by thermal stimulation of the spinal cord after baroreceptor denervation and vagotomy. However, blood pressure homeostasis was lost. The pattern of cardiovascular ajustments during heating consisted in cutaneous vasodilatation intestinal vasoconstriction and, due to sympathetic activation an increase of heart rate and cardiac output. This pattern was qualitatively identical with that intact animals. During spinal cord cooling the cardiovascular response pattern consisted in cutaneous vasoconstriction, intestinal vasoconstriction and, depending on cooling intensity, a reduced or unchanged sympathetic influence on the heart. This pattern differed considerably from what in intact animals but basic features were still present as indicated by opposite changes of cardiac and vascular sympathetic tone during cooling. It is concluded that the baroreceptor signals play no primary role in the generation of differential vasomotor responses under the present experimental conditions. This confirms assumptions made on the basis of observations in animals with intact baroreceptor input. However, baroreceptor signals contribute significantly to blood pressure homeostasis which is normally maintained during spinal thermal stimulation.