Abstract
Like humans, the heart rate (HR) of anesthetized rats immersed in CO2-water is lower than that when immersed in tap water at the same temperature. To investigate the afferent signal pathway in the mechanism of HR reduction, Wistar rats were anesthetized with urethane and then the spinal cord was transected between T4 and T5. The animals were immersed up to the axilla in a bathtub of tap-water (CO2 contents: 10–20 mg·l−1) or of CO2-water (965–1,400 mg·l−1) at 35°C while recording HR, arterial blood pressure, and arterial blood gas parameters (PaCO2, PaO2, pH). Arterial blood gas parameters did not change during immersion, irrespective of CO2 concentration of the bath water, whereas the HR was reduced in the CO2-water bath. The inhalation of CO2-mixed gas (5 % CO2, 20 % O2, 75 % N2) resulted in increased levels of blood gases and an increased HR during immersion in all types of water tested. The HR reduction observed in sham transected control animals immersed in CO2-water disappeared after subsequent spinal cord transection. These results show that the dominant afferent signal pathway to the brain, which is involved in inducing the reduced HR during immersion in CO2-water, is located in the neuronal route and not in the bloodstream.
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Acknowledgments
We are grateful to Dr. M Shibata (Yamanashi Institute of Environmental Sciences) for critical reading of the manuscript and to Mitsubishi Rayon Engineering (Tokyo, Japan) for providing the apparatus for producing artificial CO2-hot spring water (MRE-Spa, laboratory model). This study was funded in part by a JSPS Grant-in-Aid for Scientific Research (B)#16390057 to M.H. and a JSPS Grant-in-Aid for Young Scientists (B)# 17700478 to N.Y.
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Yamamoto, N., Hashimoto, M. Spinal cord transection inhibits HR reduction in anesthetized rats immersed in an artificial CO2-hot spring bath. Int J Biometeorol 51, 201–208 (2007). https://doi.org/10.1007/s00484-006-0055-6
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DOI: https://doi.org/10.1007/s00484-006-0055-6