Thermoregulatory responses to intra-abdominal heating of sheep

Spinal cord thermosensitivity: An afferent phenomenon?

We review the evidence for thermoregulatory temperature sensors in the mammalian spinal cord and reach the following conclusions. 1) Spinal cord temperature contributes physiologically to temperature regulation. 2) Parallel anterolateral ascending pathways transmit signals from spinal cooling and spinal warming: they overlap with the respective axon pathways of the dorsal horn neurons that are driven by peripheral cold- and warm-sensitive afferents. 3) We hypothesize that these ‘cold’ and ‘warm’ ascending pathways transmit all extracranial thermosensory information to the brain. 4) Cutaneous cold afferents can be activated not only by cooling the skin but also by cooling sites along their axons: we consider that this is functionally insignificant in vivo. 5) By a presynaptic action on their central terminals, local spinal cooling enhances neurotransmission from incoming ‘cold’ afferent action potentials to second order neurons in the dorsal horn; this effect disappears when the spinal cord is warm. 6) Spinal warm sensitivity is due to warm-sensitive miniature vesicular transmitter release from afferent terminals in the dorsal horn: this effect is powerful enough to excite second order neurons in the ‘warm’ pathway independently of any incoming sensory traffic. 7) Distinct but related presynaptic mechanisms at cold- and warm-sensitive afferent terminals can thus account for the thermoregulatory actions of spinal cord temperature.

Short-term exercise-heat acclimation enhances skin vasodilation but not hyperthermic hyperpnea in humans exercising in a hot environment

We tested the hypothesis that short-term exercise-heat acclimation (EHA) attenuates hyperthermia-induced hyperventilation in humans exercising in a hot environment. Twenty-one male subjects were divided into the two groups: control (C, n = 11) and EHA (n = 10). Subjects in C performed exercise-heat tests [cycle exercise for ~75 min at 58% [Formula: see text] (37°C, 50% relative humidity)] before and after a 6-day interval with no training, while subjects in EHA performed the tests before and after exercise training in a hot environment (37°C). The training entailed four 20-min bouts of exercise at 50% [Formula: see text] separated by 10 min of rest daily for 6 days. In C, comparison of the variables recorded before and after the no-training period revealed no changes. In EHA, the training increased resting plasma volume, while it reduced esophageal temperature (T (es)), heart rate at rest and during exercise, and arterial blood pressure and oxygen uptake ([Formula: see text]) during exercise. The training lowered the T (es) threshold for increasing forearm vascular conductance (FVC), while it increased the slope relating FVC to T (es) and the peak FVC during exercise. It also lowered minute ventilation ([Formula: see text]) during exercise, but this effect disappeared after removing the influence of [Formula: see text] on [Formula: see text]. The training did not change the slope relating ventilatory variables to T (es). We conclude that short-term EHA lowers ventilation largely by reducing metabolism, but it does not affect the sensitivity of hyperthermia-induced hyperventilation during submaximal, moderate-intensity exercise in humans.

Mechanosensitive pelvic nerve afferent fibers innervating the colon of the rat are polymodal in character

This report describes the chemical and thermal sensitivity of mechanosensitive pelvic nerve afferent fibers innervating the colon of the rat. A total of 51 fibers in the S1 dorsal root, identified by electrical stimulation of the pelvic nerve, were studied. An approximately 7 cm length of descending colon was isolated in situ to permit intracolonic perfusion and distension with Krebs solution. Reproducibility of responses to repetitive colorectal distension (CRD, 40 mmHg, 30 s, every 4 min) was documented. All fibers gave monotonic, incrementing responses to graded CRD (5 to 60 mmHg). Increases (n = 6) or decreases (n = 6) in pH of the perfusate failed to produce any change in resting activity or responses to CRD. Infusion of bile salts increased the resting activity of 6/6 fibers in a concentration-dependent manner, but did not affect the magnitude of responses to CRD. After intracolonic instillation of an inflammatory soup (bradykinin 10(-5) M, PGE2 10(-5) M, serotonin 10(-5) M, histamine 10(-5) M and KCl 10(-3) M), 13/22 fibers exhibited sensitization of responses to CRD. Seventy-three percent of 45 fibers tested responded to intracolonic perfusion of heated Krebs solution. The estimated threshold for response was 45 degreesC and response magnitude increased with the temperature. A smaller proportion (30%) of 37 fibers tested responded to intracolonic perfusion of cold Krebs solution. The estimated threshold for response was 28 degreesC. Of 36 fibers tested, 8 were activated by both heat and cold; typically, fibers activated by heat did not respond to cold. In a sample of 26 fibers tested for response to all three modalities of stimulation, 11 responded to mechanical, chemical and thermal stimuli; the remaining 15 responded to mechanical and either chemical or thermal stimulation. Changes in intracolonic pressure in response to chemical and thermal stimuli were also evaluated. Inflammatory soup and bile salts did not change intracolonic pressure; heat and cold produced a modest decrease and increase in muscle tension, respectively. These results document that mechanosensitive pelvic nerve afferent fibers are also chemosensitive and/or thermosensitive, supporting the notion that visceral mechanoreceptors in general are likely polymodal in character.

Heart rate and blood pressure changes during radiofrequency irradiation and environmental heating

1. Whole-body exposure of animals to radiofrequency radiation (RFR) can cause an increase in body temperature. 2. Responses to heating, whether due to RFR or to more conventional means, include changes in heart rate and blood pressure. 3. Although cardiovascular responses to various types of heating are similar, differences in the magnitude of changes may result from different thermal gradients within the body. 4. This review compares the effects of RFR and conventional environmental heating on heart rate and blood pressure.

Localization of intra-abdominal thermoreceptors in the ewe

1. In neutral environmental temperatures, shivering was induced in sheep by intra-ruminal cooling. Shivering was then depressed for a period by intra-abdominal heating. Intra-abdominal heating without ruminal cooling induced panting and a reduction of caval temperatures. Posterior caval temperatures were found to be inappropriate to the responses observed.2. Intra-abdominal heating of sheep in cold and warm environments depressed shivering and augmented panting respectively. Unilateral splanchnotomy abolished these responses on the ipsilateral side.3. The results are interpreted to indicate that the thermoreceptors stimulated lie within the walls of the rumen and intestine, and possibly the mesenteric veins.4. The splanchnic nerves are indicated as the afferent pathway for these receptors, and differential splanchnic innervation of the gut is suggested to explain unilateral abolition of the response to warming.