Decrease in heart rates by artificial CO2 hot spring bathing is inhibited by 1-adrenoceptor blockade in anesthetized rats

Hashimoto, Masaaki, and Noriyuki Yamamoto. Decrease in heart rates by artificial CO2 hot spring bathing is inhibited by 1-adrenoceptor blockade in anesthetized rats. J Appl Physiol 96: 226–232, 2004. First published August 29, 2003; 10.1152/japplphysiol.00812.2003.—To investigate the effects of carbon dioxide (CO2) hot spring baths on physiological functions, head-out immersion of urethane-anesthetized, fursheared male Wistar rats was performed. Animals were immersed in water (30 or 35°C) with high-CO2 content ( 1,000 parts/million; CO2water). CO2-water for bathing was made by 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, whereas skin color did not change in normal tap water at the same temperature. Arterial blood pressure, heart rate (HR), underwater skin tissue blood flow, and temperatures of the colon and immersed skin were continuously measured while animals were immersed in a bathtub of water for 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. HR of rats in CO2-water was significantly slower than in tap water at 35°C. Decreased HR in CO2-water was inhibited by infusion of atenolol ( 1adrenoceptor blocker), but it 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 a useful device for acquiring physiological evidence of balneotherapy.