Intensity Of Shivering Research Articles

Physiological responses of vagotomized pigeons exposed to ambient temperatures gradually reduced from 34°C to 2°C

In pigeons subjected to bilateral cervical vagotomy and sham-vagotomy, measurements of heart rate, breathing frequency, cloacal temperature (T c), foot temperature (T f), shivering and metabolic heat production (M) were made during gradual ambient cooling from 34 to 2°C on Day 2 as well as on Day 3 post-surgery. Severe tachycardia was observed in both Day 2 and Day 3 vagotomized (VX) pigeons. Lowering of ambient temperature (T a) below 28°C produced increased heart rate in both Day 2 as well as Day 3 sham-vagotomized (SVX) and VX pigeons, except with Day 3 VX pigeons exposed to T a below 12°C when a reversal of the trend became apparent. Breathing frequency in VX pigeons was lower than that in SVX on both days tested, the difference being greater on Day 2 than on Day 3. On both days, the difference in breathing frequency between VX and SVX pigeons was minimal at T a 30°C and became increasingly greater with T a above or below this point. T c dropped significantly in both VX and SVX pigeons in response to ambient cooling on Days 2 and 3. The Day 2 VX pigeons in general had a lower T c than that in SVX and as T a continued to drop below 28°C, T c in VX pigeons dropped more rapidly than in SVX, showing the maximal difference in T c between the two at T a 2°C. A similar trend was also observed with Day 3 VX pigeons when T a dropped below 22°C, but at T a between 32°C and 24°C, these pigeons showed a trend to have a higher T c than that in SVX. T f in both Day 2 and Day 3 SVX as well as VX pigeons dropped significantly during ambient cooling, and with fall in T a below 20°C, T f in VX pigeons dropped considerably below that in SVX, the magnitude of the difference being greater with increased ambient cooling. It is suggested that norepinephrine (NE) and thyroxine (T 4) are involved in the thermoregulatory process in VX pigeons during changes in T a. Shivering response in SVX pigeons was observed as early as at T a 26°C, while in VX the initial response was observed only at 22°C on both days. VX pigeons showed significantly lower shivering intensity than SVX, the difference between the groups being greater with Day 2 than Day 3 birds. As T a dropped, shivering intensity continued to increase in both the groups, but this increase was substantially slower in VX than in SVX, resulting in an increased difference in the shivering intensity between VX and SVX as T a dropped to the lowest (2°C). Metabolic heat production (M) increased significantly in Day 2 and Day 3 SVX as well as VX pigeons at low T a. In Day 2 VX pigeons, M was lower than in SVX at or below T a 24°C, but no significant difference in M was observed between Day 3 VX and SVX pigeons. Because the effects of vagotomy on M, shivering and breathing frequency were less pronounced on Day 3 than on Day 2, it may be inferred that a transient correctional process toward a steady state might have been underway.

Thermoregulatory heat production in cold-reared and warm-reared pigs.

Pigs were reared from 2 wk of age in either 10 or 35 degrees C and fed ad libitum. At 8 wk of age they were tested for the presence of regulatory nonshivering thermogenesis by administration of norepinephrine (NE) and propranolol. In addition, an electromyogram and carotid temperature, as well as a heat flow and skin temperature from one site, were monitored while the pigs were at ambient temperatures of 10, 15, 20, 25, and 30 degrees C. Cold-reared pigs showed a heightened response to NE in cold compared to thermally neutral environments. This was not observed in warm-reared pigs. Propranolol depressed O2 consumption more in cold-reared than in warm-reared pigs. Pigs reared in the cold also showed a higher intensity of shivering, tissue conductance, and skin temperature than warm-reared littermates. The shivering response of cold-reared pigs was more sensitive to changes in skin temperature than in warm-reared pigs.

Thermal and electromyographic correlates of shivering thermogenesis in the pigeon

1. 1. Electromyographic, thermal and metabolic measurements were made on pigeons subjected to stepwise ambient cooling. Subsequent signal analysis of the EMG-recordings from the pectoral muscle showed that the mean rectified value ( U mrv) is the most reliable EMG-based indicator of metabolic heat production (M). 2. 2. The muscle-abdomen temperature difference was linearly related to M and correlated strongly with U mrv. 3. 3. The long-term and short-term fluctuations of shivering intensity ( U mrv) highly synchronous in the two main muscles, m. pectoralis and m. supracoracoideus. 4. 4. The EMG-signal approached a Gaussian process at high shivering intensities. Evaluation of the changes in EMG median frequency suggested a size-dependent recruitment of motor units for shivering. 5. 5. It is pointed out that U mrv and the integrated value of EMG are interconvertible and that U mrv should be preferred, because it has more physiological relevance.

Effect of changes in baroreceptor input on the intensity of shivering in the anaesthetised cat.

1. Shivering in the barbitone-anaesthetised cat exposed to a lowered ambient temperature was reduced by transient haemorrhagic or drug induced hypotension such that there was a positive correlation between mean arterial blood pressure and the intensity of shivering. 2. Shivering which had been reduced by haemorrhage could be restored by the reinfusion of blood, centripetal stimulation of a buffer nerve or by the intravenous administration of methoxamine. 3. The intensity of shivering was increased by the intravenous injection of methoxamine and by stimulation of a buffer nerve, although the latter response was attenuated by intact buffer nerves. 4. There was a marked variation in the importance of the different buffer nerves in maintaining shivering. 5. Shivering was abolished by deafferentation of the baroreceptors and although it could be restored by electrical stimulation of a buffer nerve methoxamine was then without effect. 6. A prolonged period (90-120 min) of hypotension (50-60 mm Hg) impaired the ability to shiver even after the hypovolaemia and hypotension had been corrected.

Brainstem control of shivering in the cat. I. Inhibition.

Regions containing heat gain tonic inhibitory areas in mesencephalon and upper pons were studied using topical microinjection of a local anesthetic. This inhibition was compared in the mesencephalic decerebrated and intact animals. In mesencephalic animals that failed to shiver, bilateral injections of procaine into the ventromedial pontine tegmental areas released some heat gain responses and in intact anesthetized cats injections in the same regions increased intensity of shivering. These effects on inhibitory areas are reversible and can be repeated in the same preparation several times. Permanent effects were obtained by electrolytic lesions or by the local microinjections of 1% osmic acid. In summary, it is shown, by means of microinjection of local anesthetic, that a tonic inhibitory area located in the ventromedial pontine reticular formation completely inhibits shivering in the mesencephalic decerebrate preparation and partially inhibits shivering in the intact anesthetized preparation.

Temperature-induced changes in metabolism and body weight of cattle (Bos taurus).

The metabolic and body weight changes in two non-pregnant beef cows were studied during prolonged exposure to warm (20 +/- 3 degrees C, relative humidity 50-70%) and cold (-10 +/- 2 or -25 +/- 4 degrees C) temperatures. Other factors including daily food intake were held constant throughout each 8-week exposure. During cold exposures, metabolic rate, blood hematocrit, and plasma concentrations of glucose and free fatty acid were elevated and respiratory frequencies and skin temperatures decreased. Resting metabolic rates measured at 20 degrees C, i.e., without the direct influence of cold, were 83.4-95.3 litres 02 per hour when the cows were cold acclimated, at either -10 or -25 degrees C, and 30-40% greater than when the cows were warm acclimated. The resting metabolic response and the concomitant reduction in intensity of shivering is indicative of metabolic acclimation to cold in these animals of greater than 500 kg body weight. As well as the expected changes in body weight with changes in energy metabolism there were losses in weight (13-24 kg) during the first 3 days of each cold exposure. Weight gains occurred when the cold stress was abruptly removed. These short term weight changes were associated with changes in water intake and apparent shifts in body fluid content.

Electromyographic aspects of postural, respiratory and thermoregulatory mechanisms in sleeping cats

The electrograms of neck, limb and respiratory muscles were studied in unrestrained cats sleeping at different environmental temperatures. 1. 1. Thermal neutrality. Slow wave sleep: postural activity is present in neck and intercostal records; diaphragmatic and intercostal electrograms show respiratory activity; limb muscles are inactive. Fast wave sleep: neck and intercostal postural activity disappears; intercostal respiratory activity is weak; respiratory activity of the diaphragm persists. 2. 2. Low environmental temperature. Slow wave sleep: postural activity is present in neck and intercostal electrograms; shivering activity is clearly observed in all records except that of the diaphragm; shivering intensity increases from waking to slow wave sleep; interaction occurs between respiratory and shivering rhythms. Fast wave sleep: postural and shivering activities disappear. 3. 3. High environmental temperature. Slow wave sleep: neck and intercostal postural activity is weak; diaphragmatic and intercostal electrograms show panting activity; frequency of panting increases from waking to slow wave sleep. Fast wave sleep: postural and panting activities disappear. Summing up, fast wave sleep is associated with changes in the regulation of postural, respiratory and thermoregulatory muscle activities.

Effect of injury on shivering thermogenesis in the rat.

1. Shivering in response to changes in ambient or core temperature has been measured in control rats and rats injured by bilateral hind-limb ischaemia, both with and without fur.2. Removal of the fur increased the threshold ambient temperature for the onset of shivering.3. This threshold was lower than in uninjured controls during the latter half of a 4 hr period of bilateral hind-limb ischaemia in rats with or without fur.4. The threshold was also lower than in uninjured controls after the period of bilateral hind-limb ischaemia but was not significantly different from that during the period of ischaemia.5. The alterations in threshold were not accompanied by any significant changes in the slope of the regression line relating intensity of shivering and ambient temperature.6. When the core temperature fell after limb ischaemia to levels which provoked shivering in uninjured controls, no shivering was seen in the injured rats.7. These results are interpreted as further evidence for the lowering of the ;set-point' for thermogenesis after injury in the rat and are discussed in relation to the changes in body temperature after injury in that species.

Metabolic changes in some acclimation phenomena

1. Acclimation to constant cold also causes in mice a decrease in the intensity of muscle shivering in cold and an increase in the metabolic rate of the animal and in the succinic dehydrogenase complex activity in the liver and skeletal muscles.

Effects of anti- and pseudo-Parkinson drugs on shivering

Effects of atropine, an anti-Parkinson agent, and reserpine, a pseudo-Parkinson agent, on the intensity of shivering in cats were determined. This intensity was measured by the ratio of shivering oxygen consumption rate (V ∗O 2) to resting V ∗O 2. In six cats the mean ratio was 3.0 before administration of 5 mg/kg atropine sulfate, i.p. After administration the ratio was 2.6, an insignificant difference. Thus atropine, at a dose level in excess of that necessary to suppress Parkinson or experimentally-induced alternating tremor, had no effect on the intensity of shivering. Since the reticulospinal tracts appear to be involved in both shivering and Parkinson tremor, it is suggested that the suppression of the latter by atropine is not by inhibition of multisynaptic extrapyramidal pathways in the midbrain or more caudal regions. In ten cats the mean ratio of shivering to resting V ∗O 2 was 3.2 before the administration of 0.5 mg/kg reserpine, i.v. After administration shivering was suppressed, the mean ratio being 1.7, a significant difference ( P < 0.001). In all cats reserpine evoked an alternating tremor with Parkinson characteristics together with excessive parasympathomimetic activity. It is suggested that reserpine suppressed shivering by activation of an anterior hypothalamic region known to suppress shivering when stimulated thermally or electrically.