HOAGLAND, T.M.*; HANCOCK, T.V.; HILLMAN, S.S.: Systemic Vascular Compliance in Two Amphibians

The mean circulatory filling pressure (MCFP), vascular compliance (VC) and unstressed blood volume (USBV) were measured in Bufo marinus and Rana catesbeiana, using in vivo methods. These values are integral for the understanding of venous function and cardiovascular homeostasis, and have been determined in mammals and fish but never in amphibians. In the present experiments, systemic arch arterial (P_SA) and ventral abdominal venous (P_VEN) pressures were measured in unanesthetized amphibians before and during mechanical occlusion of the conus arteriosus, while total blood volume was incrementally manipulated. Mechanical occlusion of the conus arteriosus (zero-flow condition) rapidly (<10 s) dropped P_SA and increased P_VEN (equals MCFP). Capacitance curves were constructed by varying total vascular volume and then measuring MCFP during zero-flow cardiac output conditions. These capacitance curves enable the extrapolation of the USBV and determination of VC. The MCFP, VC and USBV were 8.6 � 0.3 mmHg, 6.4 � 0.4 ml^.mmHg^-1.kg^-1, 18.0 ml^.kg^-1 (24.3% of total vascular volume) and 9.3 � 0.3 mmHg, 3.8 � 0.3 ml^.mmHg^-1.kg^-1 and 17.2 ml^.kg^-1 (31.1% of total vascular volume) for B. marinus and R. catesbeiana, respectively. These results show that important measures of venous function are readily obtained in amphibians and the more terrestrial cane toad has a greater VC and lower USBV than the aquatic frog; these adaptations could be advantageous in the move to terrestriality.