EFFECTS OF DRY SEASON DORMANCY ON OXYGEN-UPTAKE, HEART-RATE, AND BLOOD PRESSURES IN THE TOAD, BUFO-PARACNEMIS

The cardiodynamic consequences of dry season dormancy in ectothermic v ertebrates is not well known. Our hypothesis was that dormancy would r educe cardiac activity. We therefore determined oxygen uptake and card iovascular function in aestivating toads, Bufo paracnemis, native to S ao Paulo State, Brazil. Specimens were collected and kept in the labor atory under controlled temperature and light regimes. We compared oxyg en uptake, heart rate, blood pressure, rate-pressure product (RPP), an d blood gases in toads during aestivation (dry winter season) and thei r early active season (spring). Oxygen uptake of winter toads at 25 de grees C was considerably lower than that of spring toads (winter: 24.0 +/- 1.8 ml/(kg.h); early spring: 44.4 +/- 5.1 ml/(kg.h); mean +/- SE; same in the following). A seasonal dichotomy was also observed at 15 degrees C although the differences was less pronounced (15.8 +/- 1.8 m l/(kg.h) winter; 23 +/- 2.1 ml/(kg.h) early spring). Chronic arterial cannulation permitted measurements of cardiodynamic variables without any undesired change in VO2. Heart rates of winter toads were signific antly lower than those of early spring animals at both experimental te mperatures (25 degrees C: winter 25 +/- 1.4 beats/min.; early spring: 35.2 +/- 5.1 beats/min. 15 degrees C: winter 15, 4 +/- 1.8 beats/min.; early spring: 23.9 +/- 2.1 beats/min). Systemic, diastolic and mean a rterial pressures decreased slightly but not significantly during aest ivation. We conclude that: (1) Bufo paracnemis downregulates metabolic rate during the dry season and (2) heart rate is also downregulated w ith little change of blood pressure. While the energetics of these res ponses are probably beneficial for survival during aestivation, the un derlying biochemical mechanisms remain obscure. (C) 1997 Wiley-Liss, I nc.