LYMPHATIC REGULATION OF HEMATOCRIT DURING HYPOXIA IN THE TOAD BUFO-WOODHOUSEI

Hypoxia rapidly increases hematocrit (Hct) in anuran amphibians by red ucing plasma volume, but the mechanism(s) mediating this response is u nknown. We tested the hypothesis that, during hypoxia, plasma volume i s 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 d ye from the dorsal lymph sac to the arterial blood, and flow through a n open LH cannula. We also tested whether splenic contraction or choli nergic nerves contribute to the hypoxia-induced changes. Graded hypoxi a between 21 and 4% O-2 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 l ymph 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 signifi cant effect on Hct and LHR. These results indicate that the LHs play a regulatory role in hypoxia-induced hemoconcentration.