Decrease in heart rates by artificial CO₂ hot spring bathing is inhibited by β₁-adrenoceptor blockade in anesthetized rats

Abstract

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 degrees C) with high-CO2 content ( approximately 1,000 parts/million; CO2-water). 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 degrees 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 approximately 30 min at room temperature (26 degrees 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 degrees C, but no differences were apparent at 35 degrees C. HR of rats in CO2-water was significantly slower than in tap water at 35 degrees C. Decreased HR in CO2-water was inhibited by infusion of atenolol (beta1-adrenoceptor 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.

Spinal cord transection inhibits HR reduction in anesthetized rats immersed in an artificial CO2-hot spring bath

“…In addition, the 4 difficulties associated with promptly generating sufficient quantities of CO 2 -water to fill a bathtub and maintaining high-CO 2 concentrations during the experiments also seem to have obstructed research progress in laboratories located far from natural springs. However, such difficulties can now be overcome using a novel apparatus to produce artificial CO 2 -water (Hashimoto and Yamamoto 2004). The CO 2 -water used in our previous study increased blood flow to immersed rat skin in a similar manner to that of CO 2 -hot spring baths on human skin, suggesting that the effects of the artificial CO 2 -water are comparable to those of natural CO 2 -hot spring water.…”

“…The CO 2 -water used in our previous study increased blood flow to immersed rat skin in a similar manner to that of CO 2 -hot spring baths on human skin, suggesting that the effects of the artificial CO 2 -water are comparable to those of natural CO 2 -hot spring water. Although the skin of rats does not become red, immersion in CO 2 -water does result in increased blood flow to the skin and decreased vascular resistance compared with immersion in tap-water (Hashimoto and Yamamoto 2004).…”

“…Animal preparation in detail was described elsewhere (Hashimoto and Yamamoto 2004). Briefly, the animals were anesthetized with urethane (1.39-1.60 g·kg -1 body weight, i.p.)…”

“…Water containing a high CO 2 concentration (965-1400 ppm, 35°C) was generated using an MRE-Spa 6 (Hashimoto and Yamamoto 2004). The animals were loosely fixed to plastic lattice-plates in the head-up position of about 30° to the horizontal in a polycarbonate bathtub (30 x 20 x 15 cm).…”

“…We previously reported that HR of rats was lower during immersion in CO 2 -water than in tap-water, and the lowered HR is probably achieved through the inhibition of sympathetic nerve activity rather to the facilitation of parasympathetic nerve activity (Hashimoto and Yamamoto 2004). Compared with efferent mechanisms, the afferent signal route that evokes a lowered HR remains unclear.…”