Lymphatic regulation of hematocrit during hypoxia in the toad Bufo woodhousei

Hypoxia rapidly increases hematocrit (Hct) in anuran amphibians by reducing plasma volume, but the mechanism(s) mediating this response is unknown. We tested the hypothesis that, during hypoxia, plasma volume is reduced by impaired lymph heart (LH) function, decreasing lymph flow into the circulation. In Bufo woodhousei, we measured the effects of hypoxia on Hct, lymph heart rate (LHR), LH pressure, the movement of dye from the dorsal lymph sac to the arterial blood, and flow through an open LH cannula. We also tested whether splenic contraction or cholinergic nerves contribute to the hypoxia-induced changes. Graded hypoxia between 21 and 4% O2 produced graded increases in Hct (P < 0.0001) and decreases in LHR (P = 0.01). Hypoxia reduced the rate of increase in arterial Evans blue concentration after injection into the dorsal lymph sac (P = 0.041) and decreased flow through an open LH cannula (P < 0.012). Hypoxia increased Hct and reduced LHR similarly in control, splenectomized, and sham-splenectomized toads. Atropine had no significant effect on Hct and LHR. These results indicate that the LHs play a regulatory role in hypoxia-induced hemoconcentration.

The aerobic cost of saltatory locomotion in the fowler's toad (Bufo woodhousei fowleri)

Studies of kangaroos suggest that hopping provides energy savings during locomotion at high speeds, although studies of small mammals suggest that hopping is no more economical than running. To obtain comparative data on anurans, we exercised Fowler's toads (Bufo woodhousei fowleri, 25.8 g) on treadmills at speeds ranging from 0.09 to 0.63 km h-1 while measuring oxygen consumption (VO2), endurance or hop kinematics. The toads walked at slow speeds and hopped at fast speeds. Steady-state VO2 (VO2,ss) increased linearly with speed to a maximum (VO2, max) of 1.17 ml O2 g-1 h-1 at 0.27 km h-1 and was nine times the average pre-exercise VO2. The maximum rate of oxygen consumption during treadmill exercise was comparable to VO2,max previously reported for less natural exercise regimes. At speeds greater than or equal to 0.27 km h-1, VO2,ss was independent of speed. At speeds less than or equal to 0.36 km h-1, toads moved for over 1h, but endurance decreased sharply at higher speeds. Hop rate, hop length, hop height and angle of take-off increased with speed. Hopping in B.w. fowleri was not less costly than running in other animals of similar body size and was inefficient at converting metabolic to mechanical energy. The present study suggests that hopping in toads, as in small mammals, is not economical during sustained locomotion and is most important during short bursts of high-intensity activity.