Cardiorespiratory responses of the toad (Bufo marinus) to hypoxia at two different temperatures

Central vascular blood flows and ventilation were measured in conscious toads (Bufo marinus) at 15 and 25 degrees C. The animals were exposed to hypoxia (Fi(O)(sum)=0.10 and 0.05, where Fi(O)(sum) is the fractional oxygen concentration of inspired air) at both temperatures. In addition, the cardiorespiratory responses to hypercapnia (Fi(CO)(sum)=0.05) and atropine injection (5 mg kg(-)(1); 7.4 micromol kg(-)(1)) were studied at 25 degrees C. At 25 degrees C, systemic blood flow ( q_dot (sys)) exceeded pulmocutaneous blood flow ( q_dot (pc)), indicating a large net right-to-left shunt ( q_dot (pc)/ q_dot (sys) was 0.39). q_dot (pc)/ q_dot (sys) was reduced significantly to 0.22 at 15 degrees C. At both temperatures, q_dot (pc) increased significantly during hypoxia (from 26.2 to 50.8 ml min(-)(1 )kg(-)(1) at 25 degrees C and from 11. 2 to 18.9 ml min(-)(1 )kg(-)(1) at 15 degrees C), whereas q_dot (sys) changed little (from 77.2 to 66.2 ml min(-)(1 )kg(-)(1) at 25 degrees C and from 54.3 to 50.1 ml min(-)(1 )kg(-)(1) at 15 degrees C). As a result, the net right-to-left shunt was greatly reduced, while total cardiac output remained almost unaffected. The ventilatory response was more pronounced during hypercapnia but, since q_dot (pc) and q_dot (sys) were affected similarly, there was no change in the shunt pattern. In undisturbed toads at 25 degrees C, atropine injection increased q_dot (pc) and eliminated the net right-to-left shunt. This is consistent with the known vagal innervation of the pulmonary artery. The present study shows that the cardiac right-to-left shunt that prevails in undisturbed and resting toads is reduced with increased temperature and during hypoxia. These findings are consistent with the general view that the cardiac right-to-left shunt is regulated and reduced whenever oxygen delivery is compromised or metabolic rate is increased.