抄録
As reported previously, aquatic animals , fish or amphibian, lack synchrony between heart rate variability (HRV) and respiration unlikely mammals and avian living on the ground directly exposed to gravity. We then speculate the HRV may be originated after moving in on the ground adapting cardiopulmonary functions against the gravity induced fluid movement. In this report, we tested a gravitational tolerance of cardiovascular system of fish (carp and goldfish) using reverse water immersion method changing the posture under air. The fish was restrained and kept maintaining respiration by circulating water at upright posture in water and then rapidly drained surrounded water. ECG and respiratory activities were monitored with implanted silver wire electrodes (0.1mm) in thoracic cavity and gill muscles respectively. The ECG analysis indicated that upright posture in air significantly elevated the heart rate indicating an ischemia while the respiration rate remained constant. At a horizontal posture after 10 min. exposure at upright position in air, the heart rate largely decreased beyond the normal level indicated a bradycardia. In the same time, the HRV drastically increased for more than 15 min. showing a strong arrhythmia. The results strongly suggested that a baroreflex did exist in fish cardiac system, but the response could not compensate the large blood shift induced by gravity. It was also suggested that the reflex may not be responsible for vasoconstriction like mammals. We consider the gravity exposure may play an essential role in evolutional developments of cardiovascular antigravity reflex. [J Physiol Sci. 2006;56 Suppl:S237]
