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Investigating high-amplitude oscillations in rat tail skin blood flow during core heating and cooling
ARTICLE IN PRESS
Journal of Thermal Biology 29 (2004) 779–783 www.elsevier.com/locate/jtherbio
Investigating high-amplitude oscillations in rat tail skin blood flow during core heating and cooling Cassandra D. Haley, Christopher J. Gordon, Nigel A.S. Taylor, Arthur B. Jenkins Department of Biomedical Science, University of Wollongong, Wollongong, NSW 2500, Australia
Abstract In a separate paper, we describe high-amplitude oscillations in human skin blood flow (Q˙sk). Using an open-loop model in rats, we independently modulated and clamped hypothalamic and skin temperatures. Central heating reliably induced these high-amplitude oscillations in tail Q˙sk, which occurred at 0.4170.03 Hz spanning 758.1725.7 ms, and were comprised of high-amplitude peaks (496.8787.6 AU) arising from a stable baseline (114.1727.6 AU). Central cooling significantly reduced Q˙sk, but not the amplitude, the frequency, width or baseline of the oscillations. These observations indicate that such high-amplitude oscillations are not primarily mediated via central thermal state. Instead, we believe these oscillations to be turned on by an elevated skin temperature. r 2004 Elsevier Ltd. All rights reserved. Keywords: Body temperature; Laser-Doppler flowmetry; Skin temperature; Rats; Tail skin blood flow; Vasomotion
1. Introduction High-amplitude oscillations in human, non-acral skin blood flow (Q˙sk) during external heating have been reported and described in a companion paper (Haley et al., 2004). These oscillations appeared to be of cardiovascular origin (0.4 Hz), they were of uniform amplitude and wavelength, and could be reliably induced during whole-body heating (Haley et al., 2004), indicating a probable association with temperature regulation. We hypothesised these oscillations could be driven by threshold-dependent changes in skin temperature, and were not related to core temperature changes. To test this hypothesis, we required indepenCorresponding author. Tel.: +61-242-214081; fax: +61242-214096. E-mail address: [email protected] (C.D. Haley).
dent control over both core and skin temperatures, and for this we have turned to an animal model. In another companion paper (Gordon et al., 2004), we have described an open-loop model for investigating mammalian thermosensitivity. This model, developed for use in rats, permitted the modification of deep-body and hypothalamic temperature while independently clamping skin temperature. Since the rat tail is a primary avenue for heat loss, and since its vascular control is analogous to that for human acral skin, this combination was ideally suited to our needs (O’Leary et al., 1985). To test the null hypothesis, that high-amplitude oscillations were not caused by core cooling, we heated (40 1C) then cooled the deep tissues, including the hypothalamus (1.1570.11 1C h1 100 g1), while skin temperature was clamped (36–37 1C). Skin blood flow was measured, using laser-Doppler flowmetry, for evidence of high-amplitude oscillations.
0306-4565/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jtherbio.2004.08.055
ARTICLE IN PRESS 780
C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
2. Methods Eight male Wistar rats (494.8 716.0 g) were studied; housed at 21.6 71.1 1C on a 12-h light–dark cycle, and fed rat chow and water ad libitum. Following surgical procedures (see below), rats were heated internally, via a water-perfused abdominal thermode (50 1C), to a hypothalamic temperature (Thy) of approximately 39.5 1C, to induce the high-amplitude oscillations. Mean skin temperature ðT sk Þ was then clamped at approximately 37 1C via the water-perfused suit. This temperature was chosen since our human studies indicate that a skin temperature threshold of about 36 1C, was required to induce these oscillations. Core temperature was then rapidly reduced (core cooling), via perfusion of the abdominal thermode with water at 20 1C (40 min), with T sk remaining clamped at 37 1C. Mean arterial pressure was also collected from a pressure tranducer in the femoral artery (n=4, Gordon et al., 2004). The Animal Ethics Committee of the University of Wollongong approved all experimental procedures. 2.1. Surgical procedures As described in our companion paper (Gordon et al., 2004), 1 week prior to experimentation rats underwent implantation of an indwelling stainless steel guide cannula into the hypothalamus, to allow subsequent continuous measurement of hypothalamic temperature (Thy). On experimental days, following cannulation of the trachea, to enable mechanical ventilation (60 breaths min1), a 12 cm2 silicone elastomer, water-perfused thermode was placed into the abdominal cavity immediately above the aorta and inferior vena cava, to control central temperature (Gordon et al., 2004). Rats were carefully transferred to a water-perfused suit, to enable the clamping of skin temperatures throughout the trials. The water-perfused suit consisted of two sheets of silicone elastomer, with silicone tubing imbedded at 1 cm intervals, and it completely covered the dorsal and ventral surfaces of the rat, excluding the tail. The tail was placed on an inclined ramp (351) to facilitate venous drainage and was covered by specially designed perspex tubing, to minimise the effects of ambient temperature, but also to allow the collection of tail Q˙sk, through a small window. 2.2. Measurements Hypothalamic, rectal (Tre: 6 cm), skin (T sk : mean of abdomen and back), and tail temperatures (Tt: ventral surface next to the site of laser-Doppler measurement) temperatures were measured using calibrated thermocouples (custom made 21-gauge copper and constantan encased in teflon; Physitemp Instruments Inc. Clifton, NJ, USA). The temperature of the abdominal thermode
was also recorded from a thermocouple secured (during surgery) to the surface in contact with the greater blood vessels of the abdomen (Physitemp Instruments Inc. Clifton, NJ, USA). Temperature data were collected at 0.2 Hz. Tail Q˙sk was estimated at the ventral surface of the tail (1.5 cm from base of tail), using laser-Doppler flowmetry (TSI Laserflo BPM2, Vasamedics Inc., St. Paul, MN, USA), and collected at 20 Hz. Mean arterial pressure was collected from a pressure transducer connected to an amplifier (Onspot, Wollongong, Australia), and collected on a computer at 200 Hz. Mean arterial pressure was calculated using the formula: ((systolicdiastolic)/3)+diastolic. 2.3. Data analysis Data were analysed in 5-min blocks, commencing from the last 5 min of baseline, and prior to core cooling (0–5 min; pre-heating), and for five consecutive blocks during core cooling (5–30 min). Skin blood flow data were analysed first to provide a simple mean flow across the 5-minute blocks. When high-frequency oscillations were detected, data were fractionated to isolate baseline and peak data; then averaged separately. Finally, the peaks were analysed to determine oscillation frequency, the width of each peak and the peak amplitude (peak height minus baseline flow). Data are reported as means with standard errors. Statistical comparisons were made between the last 5 min of core heating and each of the core cooling blocks using a paired t-test. Alpha was set at 0.05.
3. Results The open-loop model was used to achieve core cooling, from a mildly hyperthermic state, while skin temperature was clamped. Our ability to achieve this is illustrated in Fig. 1. Hypothalamic temperature was driven from 39.3 1C (70.1) to 37.5 1C (70.1, Po0.05) over 25 min, while skin temperature did not differ significantly from its pre-experimental state (37.3 70.3 1C; P40.05). These data verify that we successfully clamped skin temperature while cooling the hypothalamus at a rate of 1.15 1C h1 100 g1 (70.11). In addition, mean arterial pressure was normal (80.46 714.17 mmHg) and increased throughout core cooling to 103.44 721.44 (Po0.05). High-amplitude oscillations were evident in all rats when mildly hyperthermic (39 1C) and the skin temp was clamped (3637 1C), and, as previously noted (Haley et al., 2004), consisted of a high-amplitude peak extending from a stable baseline (Fig. 2B). The characteristics of these oscillations are summarised in Table 1. Core cooling elicited a significant reduction in Q˙sk at 25 min (Po0.05; Table 1). This is illustrated for
ARTICLE IN PRESS
Thermode temperature (°C)
41
Body temperature (°C)
C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
39
(A)
39 37 35 33
*
31 29
(B)
38
* 37 Mean skin Hypothalamic Rectal
36 35 32
Tail temperature(°C)
781
* (C)
31 30
*
29
0
5
10
15
20
25
30
Time (min) Fig. 1. Abdominal thermode temperature (A), hypothalamic, rectal, mean skin (B) and tail temperature (C) during core heating (0–5 min) and core cooling (5–30 min; n=8). Results are mean with standard errors. * indicates a significant difference between 0 and 5 min (core heating) compared to 25–30 min (core cooling; Po0.05).
one animal in Fig. 2A. However, core cooling had minimal impact upon the characteristics of these oscillations, with the only significant change being a reduction in the oscillation peak height at 25 min (Po0.05; Table 1). Since, the oscillation amplitude did not change, the modification to the peak height merely reflected a downward displacement of the whole Q˙sk signal (Fig. 2A), and not a change in the characteristics of the oscillations themselves.
4. Discussion Our results show that increases in whole-body temperature, in combination with an elevated and constant skin temperature (37 1C), can reproducibly induce high-amplitude, cutaneous blood-flow oscillations in the rat tail. The principal finding of this study is that these oscillations appear not to be effected by a large reduction in core temperature (2 1C), when skin temperature is elevated and clamped. Our observations also imply that a common mechanism may be involved in both rats and humans, which appears to be initiated by an elevated skin temperature.
It is well established that reductions Q˙sk in are largely dependent on core temperature changes when wholebody skin temperature is stable (Owens et al., 2002; Raman et al., 1983; Rand et al., 1964). Thus, in the context of our study, central thermoafferent flow during core cooling would conflict with warm signals arising from the skin. The integration of these error signals, within the current rat model, would elicit a generalised reduction in cutaneous vascular conductance, as illustrated in our companion paper (Gordon et al., 2004). This result is consistent with those from our previous human studies, in which high-amplitude blood-flow oscillations were induced during rewarming from a prior cold exposure, despite the presence of a core temperature afterdrop (Haley et al., 2004). A significant reduction in the peak height of these oscillations was observed after 25 min of core cooling. Nevertheless, we believe this change did not reflect a modification to the characteristics of these oscillations (width, amplitude or frequency), but rather a downward displacement of the whole Q˙sk signal, perhaps reflecting a sympathetically induced vasoconstriction. Accordingly, while we cannot yet exclude a central, core temperature role, we interpret the results from both our human and animal studies to
ARTICLE IN PRESS C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
782
indicate that these oscillations may be largely independent on central thermal state, when the skin temperature is elevated. In the current project, we observed an increase in mean arterial pressure during core cooling, with no apparent affect on these high-amplitude oscillations. Such a change is consistent with a generalised elevation of vasoconstrictor drive, even though the skin temperature was clamped. We interpret this to indicate that
Skin blood flow (AU)
500 400 300 200 100 0 0
5
10
(A)
15
20
25
30
Time (min)
Skin blood flow (AU)
450 400 350 300 250 200 150 100 3.00
3.05
(B)
3.10
3.15
3.20
3.25
Time (min)
Fig. 2. A typical skin blood flow trace from one rat during core heating (0–5 min) and core cooling (5–30 min) in arbitrary units (AU), as measured on the ventral surface of the tail via laserDoppler flowmetry (A). The high-amplitude oscillations are also presented on a smaller time scale (B), for the same rat during whole-body heating (after approximately 35 min).
these oscillations appear not to be affected by global changes in vascular tone. However, some caution must be exercised since we did not measure vascular pressure at the site of the cutaneous blood flow measurements. That is, measurements from the femoral artery may not reflect intravascular pressure at the site of fibre optic probe (Fagrell, 1985). Nevertheless, since our rats were normotensive, we believe that our indices of cutaneous vascular conductance may be interpreted to reflect changes in local vascular tone, the cause of which we are yet to determine. We first observed these high-amplitude oscillations in cutaneous blood flow in non-acral skin (thigh), and later in acral regions (finger tip; Haley et al., 2004). We have now shown that these oscillations may be reproducibly induced in rat tail (acral) skin. It is well established that differences exist in the innervation of vascular smooth muscle for both types of skin (Johnson and Park, 1979; O’Leary et al., 1985; O’Leary and Johnson, 1989). Thus, the presence of the high-amplitude oscillations in both acral and non-acral regions may indicate that a common mechanism is possibly involved. We believe this to be a local phenomenon, which may possibly be dependent upon local tissue temperature. To test this possibility, we are currently using our open-loop rat model (Gordon et al., 2004) in combination with a local temperature clamp applied to the tail. It is recognised that anaesthesia alters thermoregulation, in particular the width of the thermoneutral (interthreshold) zone. Accordingly, we have considered the possibility that our anaesthetised rat model may confound data interpretation. However, (Malkinson et al., 1993) have shown urethane to be appropriate for thermal experiments, and that it does not elicit undesirable affects on the sympathetic nervous system. Since the high-amplitude oscillations were also evident in non-anaesthetised humans (Haley et al., 2004), we do not believe that it is an artefact of anaesthesia. To summarise, we conclude that whole-body heating reproducibly induces high-amplitude oscillations in both
Table 1 Tail skin blood flow (arbitrary units; AU) during deep-body heating (pre-heating) and cooling (core cooling; n=8) Skin blood flow variables
Average (AU) Baseline (AU) Peak Height (AU) Amplitude (AU) Width (ms) Frequency (Hz)
Core pre-heating
Core cooling
0–5 min
5–10 min
10–15 min
15–20 min
20–25 min
25–30 min
205 (33) 114 (28) 497 (88) 383 (91) 758 (26) 0.41 (0.05)
220 (39) 136 (39) 504 (94) 368 (103) 723 (40) 0.38 (0.05)
191 (32) 111 (29) 483 (100) 372 (102) 693 (45) 0.37 (0.05)
162 (30) 86 (18) 458 (102) 373 (102) 695 (46) 0.38 (0.07)
161 (30) 84 (18) 464 (94) 380 (94) 699 (40) 0.39 (0.07)
158 (30)* 79 (17) 459 (91)* 380 (90) 699 (38) 0.39 (0.07)
Data are 5 min means with standard errors (see text for details). * indicates a significant difference between heating and cooling (Po0.05).
ARTICLE IN PRESS C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
rat and human Q˙sk. Core cooling, while strong enough to significantly reduce cutaneous vascular conductance, did not elicit changes to the characteristics of these oscillations, while Tsk was elevated and clamped. Our observations from several studies imply that a common mechanism may be involved across species and skin regions, and this mechanism appears to be activated by an elevated skin temperature.
References Fagrell, B., 1985. Dynamics of skin microcirculation in humans. J. Cardiovasc. Pharmacol. 7 (Suppl 3), S53–S58. Gordon, C.J., Haley, C.D., McLennan, P.L., Tipton, M.J., Mekjavic´, I.B., Taylor, N.A.S., 2004. An open-loop model for investigating mammalian thermosensitivity. J. Thermal Biol. 2004, Submitted. Haley, C.D., Zeyl, A., Taylor, N.A.S., Jenkins, A.J., 2004. Novel, high-amplitude blood-flow oscillations in vasodilating human skin. J. Thermal Biol. 2004, Submitted.
783
Johnson, J.M., Park, M.K., 1979. Reflex control of skin blood flow by skin temperature: role of core temperature. J. Appl. Physiol, 47, 1188–1193. Malkinson, T.J., Veale, W.L., Cooper, K.E., 1993. Experimental characterization and applications of an anesthetized animal model for thermoregulatory investigations. Biomed. Sci. Instrum. 29, 369–376. O’Leary, D.S., Johnson, J.M., 1989. Baroreflex control of the rat tail circulation in normothermia and hyperthermia. J. Appl. Physiol. 66, 1234–1241. O’Leary, D.S., Johnson, J.M., Taylor, W.F., 1985. Mode of neural control mediating rat tail vasodilation during heating. J. Appl. Physiol. 59, 1533–1538. Owens, N.C., Ootsuka, Y., Kanosue, K., McAllen, R.M., 2002. Thermoregulatory control of sympathetic fibres supplying the rat’s tail. J. Physiol. 543, 849–858. Raman, E.R., Roberts, M.F., Vanhuyse, V.J., 1983. Body temperature control of rat tail blood flow. Am. J. Physiol. 245, R426–R432. Rand, R.P., Burton, A.C., Ing, T., 1964. The tail of the rat, in temperature regulation and acclimization. Canad. J. Physiol Pharmacol. 43, 257–267.
Journal of Thermal Biology 29 (2004) 779–783 www.elsevier.com/locate/jtherbio
Investigating high-amplitude oscillations in rat tail skin blood flow during core heating and cooling Cassandra D. Haley, Christopher J. Gordon, Nigel A.S. Taylor, Arthur B. Jenkins Department of Biomedical Science, University of Wollongong, Wollongong, NSW 2500, Australia
Abstract In a separate paper, we describe high-amplitude oscillations in human skin blood flow (Q˙sk). Using an open-loop model in rats, we independently modulated and clamped hypothalamic and skin temperatures. Central heating reliably induced these high-amplitude oscillations in tail Q˙sk, which occurred at 0.4170.03 Hz spanning 758.1725.7 ms, and were comprised of high-amplitude peaks (496.8787.6 AU) arising from a stable baseline (114.1727.6 AU). Central cooling significantly reduced Q˙sk, but not the amplitude, the frequency, width or baseline of the oscillations. These observations indicate that such high-amplitude oscillations are not primarily mediated via central thermal state. Instead, we believe these oscillations to be turned on by an elevated skin temperature. r 2004 Elsevier Ltd. All rights reserved. Keywords: Body temperature; Laser-Doppler flowmetry; Skin temperature; Rats; Tail skin blood flow; Vasomotion
1. Introduction High-amplitude oscillations in human, non-acral skin blood flow (Q˙sk) during external heating have been reported and described in a companion paper (Haley et al., 2004). These oscillations appeared to be of cardiovascular origin (0.4 Hz), they were of uniform amplitude and wavelength, and could be reliably induced during whole-body heating (Haley et al., 2004), indicating a probable association with temperature regulation. We hypothesised these oscillations could be driven by threshold-dependent changes in skin temperature, and were not related to core temperature changes. To test this hypothesis, we required indepenCorresponding author. Tel.: +61-242-214081; fax: +61242-214096. E-mail address: [email protected] (C.D. Haley).
dent control over both core and skin temperatures, and for this we have turned to an animal model. In another companion paper (Gordon et al., 2004), we have described an open-loop model for investigating mammalian thermosensitivity. This model, developed for use in rats, permitted the modification of deep-body and hypothalamic temperature while independently clamping skin temperature. Since the rat tail is a primary avenue for heat loss, and since its vascular control is analogous to that for human acral skin, this combination was ideally suited to our needs (O’Leary et al., 1985). To test the null hypothesis, that high-amplitude oscillations were not caused by core cooling, we heated (40 1C) then cooled the deep tissues, including the hypothalamus (1.1570.11 1C h1 100 g1), while skin temperature was clamped (36–37 1C). Skin blood flow was measured, using laser-Doppler flowmetry, for evidence of high-amplitude oscillations.
0306-4565/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jtherbio.2004.08.055
ARTICLE IN PRESS 780
C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
2. Methods Eight male Wistar rats (494.8 716.0 g) were studied; housed at 21.6 71.1 1C on a 12-h light–dark cycle, and fed rat chow and water ad libitum. Following surgical procedures (see below), rats were heated internally, via a water-perfused abdominal thermode (50 1C), to a hypothalamic temperature (Thy) of approximately 39.5 1C, to induce the high-amplitude oscillations. Mean skin temperature ðT sk Þ was then clamped at approximately 37 1C via the water-perfused suit. This temperature was chosen since our human studies indicate that a skin temperature threshold of about 36 1C, was required to induce these oscillations. Core temperature was then rapidly reduced (core cooling), via perfusion of the abdominal thermode with water at 20 1C (40 min), with T sk remaining clamped at 37 1C. Mean arterial pressure was also collected from a pressure tranducer in the femoral artery (n=4, Gordon et al., 2004). The Animal Ethics Committee of the University of Wollongong approved all experimental procedures. 2.1. Surgical procedures As described in our companion paper (Gordon et al., 2004), 1 week prior to experimentation rats underwent implantation of an indwelling stainless steel guide cannula into the hypothalamus, to allow subsequent continuous measurement of hypothalamic temperature (Thy). On experimental days, following cannulation of the trachea, to enable mechanical ventilation (60 breaths min1), a 12 cm2 silicone elastomer, water-perfused thermode was placed into the abdominal cavity immediately above the aorta and inferior vena cava, to control central temperature (Gordon et al., 2004). Rats were carefully transferred to a water-perfused suit, to enable the clamping of skin temperatures throughout the trials. The water-perfused suit consisted of two sheets of silicone elastomer, with silicone tubing imbedded at 1 cm intervals, and it completely covered the dorsal and ventral surfaces of the rat, excluding the tail. The tail was placed on an inclined ramp (351) to facilitate venous drainage and was covered by specially designed perspex tubing, to minimise the effects of ambient temperature, but also to allow the collection of tail Q˙sk, through a small window. 2.2. Measurements Hypothalamic, rectal (Tre: 6 cm), skin (T sk : mean of abdomen and back), and tail temperatures (Tt: ventral surface next to the site of laser-Doppler measurement) temperatures were measured using calibrated thermocouples (custom made 21-gauge copper and constantan encased in teflon; Physitemp Instruments Inc. Clifton, NJ, USA). The temperature of the abdominal thermode
was also recorded from a thermocouple secured (during surgery) to the surface in contact with the greater blood vessels of the abdomen (Physitemp Instruments Inc. Clifton, NJ, USA). Temperature data were collected at 0.2 Hz. Tail Q˙sk was estimated at the ventral surface of the tail (1.5 cm from base of tail), using laser-Doppler flowmetry (TSI Laserflo BPM2, Vasamedics Inc., St. Paul, MN, USA), and collected at 20 Hz. Mean arterial pressure was collected from a pressure transducer connected to an amplifier (Onspot, Wollongong, Australia), and collected on a computer at 200 Hz. Mean arterial pressure was calculated using the formula: ((systolicdiastolic)/3)+diastolic. 2.3. Data analysis Data were analysed in 5-min blocks, commencing from the last 5 min of baseline, and prior to core cooling (0–5 min; pre-heating), and for five consecutive blocks during core cooling (5–30 min). Skin blood flow data were analysed first to provide a simple mean flow across the 5-minute blocks. When high-frequency oscillations were detected, data were fractionated to isolate baseline and peak data; then averaged separately. Finally, the peaks were analysed to determine oscillation frequency, the width of each peak and the peak amplitude (peak height minus baseline flow). Data are reported as means with standard errors. Statistical comparisons were made between the last 5 min of core heating and each of the core cooling blocks using a paired t-test. Alpha was set at 0.05.
3. Results The open-loop model was used to achieve core cooling, from a mildly hyperthermic state, while skin temperature was clamped. Our ability to achieve this is illustrated in Fig. 1. Hypothalamic temperature was driven from 39.3 1C (70.1) to 37.5 1C (70.1, Po0.05) over 25 min, while skin temperature did not differ significantly from its pre-experimental state (37.3 70.3 1C; P40.05). These data verify that we successfully clamped skin temperature while cooling the hypothalamus at a rate of 1.15 1C h1 100 g1 (70.11). In addition, mean arterial pressure was normal (80.46 714.17 mmHg) and increased throughout core cooling to 103.44 721.44 (Po0.05). High-amplitude oscillations were evident in all rats when mildly hyperthermic (39 1C) and the skin temp was clamped (3637 1C), and, as previously noted (Haley et al., 2004), consisted of a high-amplitude peak extending from a stable baseline (Fig. 2B). The characteristics of these oscillations are summarised in Table 1. Core cooling elicited a significant reduction in Q˙sk at 25 min (Po0.05; Table 1). This is illustrated for
ARTICLE IN PRESS
Thermode temperature (°C)
41
Body temperature (°C)
C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
39
(A)
39 37 35 33
*
31 29
(B)
38
* 37 Mean skin Hypothalamic Rectal
36 35 32
Tail temperature(°C)
781
* (C)
31 30
*
29
0
5
10
15
20
25
30
Time (min) Fig. 1. Abdominal thermode temperature (A), hypothalamic, rectal, mean skin (B) and tail temperature (C) during core heating (0–5 min) and core cooling (5–30 min; n=8). Results are mean with standard errors. * indicates a significant difference between 0 and 5 min (core heating) compared to 25–30 min (core cooling; Po0.05).
one animal in Fig. 2A. However, core cooling had minimal impact upon the characteristics of these oscillations, with the only significant change being a reduction in the oscillation peak height at 25 min (Po0.05; Table 1). Since, the oscillation amplitude did not change, the modification to the peak height merely reflected a downward displacement of the whole Q˙sk signal (Fig. 2A), and not a change in the characteristics of the oscillations themselves.
4. Discussion Our results show that increases in whole-body temperature, in combination with an elevated and constant skin temperature (37 1C), can reproducibly induce high-amplitude, cutaneous blood-flow oscillations in the rat tail. The principal finding of this study is that these oscillations appear not to be effected by a large reduction in core temperature (2 1C), when skin temperature is elevated and clamped. Our observations also imply that a common mechanism may be involved in both rats and humans, which appears to be initiated by an elevated skin temperature.
It is well established that reductions Q˙sk in are largely dependent on core temperature changes when wholebody skin temperature is stable (Owens et al., 2002; Raman et al., 1983; Rand et al., 1964). Thus, in the context of our study, central thermoafferent flow during core cooling would conflict with warm signals arising from the skin. The integration of these error signals, within the current rat model, would elicit a generalised reduction in cutaneous vascular conductance, as illustrated in our companion paper (Gordon et al., 2004). This result is consistent with those from our previous human studies, in which high-amplitude blood-flow oscillations were induced during rewarming from a prior cold exposure, despite the presence of a core temperature afterdrop (Haley et al., 2004). A significant reduction in the peak height of these oscillations was observed after 25 min of core cooling. Nevertheless, we believe this change did not reflect a modification to the characteristics of these oscillations (width, amplitude or frequency), but rather a downward displacement of the whole Q˙sk signal, perhaps reflecting a sympathetically induced vasoconstriction. Accordingly, while we cannot yet exclude a central, core temperature role, we interpret the results from both our human and animal studies to
ARTICLE IN PRESS C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
782
indicate that these oscillations may be largely independent on central thermal state, when the skin temperature is elevated. In the current project, we observed an increase in mean arterial pressure during core cooling, with no apparent affect on these high-amplitude oscillations. Such a change is consistent with a generalised elevation of vasoconstrictor drive, even though the skin temperature was clamped. We interpret this to indicate that
Skin blood flow (AU)
500 400 300 200 100 0 0
5
10
(A)
15
20
25
30
Time (min)
Skin blood flow (AU)
450 400 350 300 250 200 150 100 3.00
3.05
(B)
3.10
3.15
3.20
3.25
Time (min)
Fig. 2. A typical skin blood flow trace from one rat during core heating (0–5 min) and core cooling (5–30 min) in arbitrary units (AU), as measured on the ventral surface of the tail via laserDoppler flowmetry (A). The high-amplitude oscillations are also presented on a smaller time scale (B), for the same rat during whole-body heating (after approximately 35 min).
these oscillations appear not to be affected by global changes in vascular tone. However, some caution must be exercised since we did not measure vascular pressure at the site of the cutaneous blood flow measurements. That is, measurements from the femoral artery may not reflect intravascular pressure at the site of fibre optic probe (Fagrell, 1985). Nevertheless, since our rats were normotensive, we believe that our indices of cutaneous vascular conductance may be interpreted to reflect changes in local vascular tone, the cause of which we are yet to determine. We first observed these high-amplitude oscillations in cutaneous blood flow in non-acral skin (thigh), and later in acral regions (finger tip; Haley et al., 2004). We have now shown that these oscillations may be reproducibly induced in rat tail (acral) skin. It is well established that differences exist in the innervation of vascular smooth muscle for both types of skin (Johnson and Park, 1979; O’Leary et al., 1985; O’Leary and Johnson, 1989). Thus, the presence of the high-amplitude oscillations in both acral and non-acral regions may indicate that a common mechanism is possibly involved. We believe this to be a local phenomenon, which may possibly be dependent upon local tissue temperature. To test this possibility, we are currently using our open-loop rat model (Gordon et al., 2004) in combination with a local temperature clamp applied to the tail. It is recognised that anaesthesia alters thermoregulation, in particular the width of the thermoneutral (interthreshold) zone. Accordingly, we have considered the possibility that our anaesthetised rat model may confound data interpretation. However, (Malkinson et al., 1993) have shown urethane to be appropriate for thermal experiments, and that it does not elicit undesirable affects on the sympathetic nervous system. Since the high-amplitude oscillations were also evident in non-anaesthetised humans (Haley et al., 2004), we do not believe that it is an artefact of anaesthesia. To summarise, we conclude that whole-body heating reproducibly induces high-amplitude oscillations in both
Table 1 Tail skin blood flow (arbitrary units; AU) during deep-body heating (pre-heating) and cooling (core cooling; n=8) Skin blood flow variables
Average (AU) Baseline (AU) Peak Height (AU) Amplitude (AU) Width (ms) Frequency (Hz)
Core pre-heating
Core cooling
0–5 min
5–10 min
10–15 min
15–20 min
20–25 min
25–30 min
205 (33) 114 (28) 497 (88) 383 (91) 758 (26) 0.41 (0.05)
220 (39) 136 (39) 504 (94) 368 (103) 723 (40) 0.38 (0.05)
191 (32) 111 (29) 483 (100) 372 (102) 693 (45) 0.37 (0.05)
162 (30) 86 (18) 458 (102) 373 (102) 695 (46) 0.38 (0.07)
161 (30) 84 (18) 464 (94) 380 (94) 699 (40) 0.39 (0.07)
158 (30)* 79 (17) 459 (91)* 380 (90) 699 (38) 0.39 (0.07)
Data are 5 min means with standard errors (see text for details). * indicates a significant difference between heating and cooling (Po0.05).
ARTICLE IN PRESS C.D. Haley et al. / Journal of Thermal Biology 29 (2004) 779–783
rat and human Q˙sk. Core cooling, while strong enough to significantly reduce cutaneous vascular conductance, did not elicit changes to the characteristics of these oscillations, while Tsk was elevated and clamped. Our observations from several studies imply that a common mechanism may be involved across species and skin regions, and this mechanism appears to be activated by an elevated skin temperature.
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