No abstract

Anurans from terrestrial environments have an enhanced ability to maintain mean arterial blood pressure (Pm) through lymph mobilization in response to desiccation or hemorrhage compared with semiaquatic or aquatic species. Because short term blood pressure homeostasis is regulated by arterial baroreceptors, we compared baroreflex function in three species of anurans that span a range of environments, dehydration tolerance and an ability to maintain Pm with dehydration and hemorrhage. The cardiac limb of the baroreflex loop was studied using pharmacological manipulation of Pm with phenylephrine and sodium nitroprusside (20-200μgkg(-1)), and the resulting changes in heart rate (fH) were quantitatively analyzed using a four-parameter sigmoidal logistic function. Resting Pm in the aquatic species, Xenopus laevis, was 3.6±0.3kPa and was significantly less (P<0.005) than for the semiaquatic species, Lithobates catesbeianus (4.1±0.2kPa), or the terrestrial species, Rhinella marina (4.7±0.2kPa). The maximal baroreflex gain was not different among the three species and ranged from 12.1 to 14.3beatsmin(-1)kPa(-1) and occurred at Pm ranging from 3.0 to 3.8kPa, which were slightly below the resting Pm for each species. Mean arterial blood pressures at rest in the three species were near the saturation point of the baroreflex curve which provides the animals with a greater fH response range to hypotensive, rather than hypertensive, changes in Pm. This is consistent with the hypothesis that arterial baroreceptors are key sensory components that allow anurans to maintain Pm possibly by mobilization of lymphatic return in response to hypotension.

Anurans from different environments vary with respect to lymph mobilization capacity which is linked to blood pressure/volume status. We hypothesized that anurans from different environments might also vary in their ability to regulate blood pressure and heart rate through the baroreflex. We altered mean arterial blood pressure (MAP) pharmacologically and measured changes in heart rate (HR) to determine baroreflex gain. Experiments were conducted at 20 °C with three species of anurans from terrestrial (cane toad, Rhinella marina), semi‐aquatic (North American bullfrog, Lithobates catesbeianus) and aquatic (African clawed frog, Xenopus laevis) environments. MAP was increased with phenylephrine (Phe; 20–200 μg/kg, i.v.) and decreased with sodium nitroprusside (SNP; 20–200 μg/kg, i.v.). Heart rate ranged from 27 ± 3 to 31 ± 2 beats min−1 (P>;0.05), but MAP was significantly higher in R. marina (5.3 ± 0.2 kPa) compared with X. laevis (3.6 ± 0.2 kPa) or L. catesbeianus (4.2 ± 0.2 kPa). The baroreflex gain (ΔHR/ΔMAP) ranged from was −6.5 ± 0.5 to −7.2 ± 0.8 beats min−1 kPa−1 but was not significantly different. These data indicate that baroreflex gain does not vary in three species representing a wide environmental range, but the more terrestrial species (cane toad) operates at a higher arterial blood pressure suggesting that baroreflex resetting has occurred in this species. Supported by NSF‐IOS 0843082 and HHMI.