Submitted on July 31, 2003
Accepted on August 26, 2003
Decrease in heart rates by artificial CO2 hot spring bathing is inhibited by 
1-adrenoceptor blockade in anesthetized rats
Masaaki Hashimoto1* and Noriyuki Yamamoto2
1 Department of Physiology, Asahikawa Medical University School of Medicine, Asahikawa, Hokkaido, Japan
2 Japanese Red Cross Hokkaido College of Nursing, Kitami, Hokkaido, Japan
* To whom correspondence should be addressed. E-mail: mhashi{at}asahikawa-med.ac.jp.
To investigate the effects of carbon dioxide (CO2) hot spring baths on physiological functions, head-out immersion of urethane-anesthetized, fur-sheared male Wistar rats was performed. Animals were immersed in water (30 or 35 °C) with high CO2 content (ca. 1000 ppm, CO2-water). CO2-water for bathing was made using an artificial spa maker with normal tap water and high pressure CO2 from a gas cylinder. When a human foot was immersed for 10 min in the CO2-water at 35 °C, the immersed skin reddened, while skin color did not change in normal tap water at the same temperature. Arterial blood pressure, heart rate, underwater skin tissue blood flow and temperatures of the colon and immersed skin were continuously measured while animals were immersed in a bathtub water for about 30 min at room temperature (26 °C). Immersed skin vascular resistance, computed from blood pressure and tissue blood flow, was significantly lower in the CO2-water bath than in tap water at 30 °C, but no differences were apparent at 35 °C. Heart rate of rats in CO2-water was significantly slower than in tap water at 35 °C. Decreased heart rate in CO2-water was inhibited by infusion of atenolol (
1-adrenoceptor blocker), but was unaffected by atropine (muscarinic cholinoceptor blocker). Theses results suggest that bradycardia in CO2-hot spring bathing is caused by inhibition of the cardiac sympathetic innervation. This CO2-water maker should prove useful device for acquiring physiological evidence of balneotherapy.
