1. Skeletal lymph muscles are effectors of the arterial baroreflex in the cane toad (Rhinella marina) Matthew Wall, Undergraduate Research Fellowship Faculty Mentor: Michael Hedrick Department of Biological Sciences, University of North Texas, Denton, TX A number of skeletal muscles play an important role in regulating lymph flux in frogs and toads by changing the compliance of lymph sacs moving lymph toward the lymph hearts. The lymph hearts pump lymph into the circulatory system to maintain blood pressure and blood volume. Because lymph mobilization in anurans is dependent upon blood volume/ pressure status, we hypothesized that lymph muscles, like cardiac and lymph heart effectors, are also regulated by the baroreflex. We tested this hypothesis by measuring muscle activity with electromyography while increasing mean arterial blood pressure (MAP) with phenylephrine (Phe; 0.02-1.0 mg/kg, i.v.) and decreasing MAP with sodium nitroprusside (SNP; 0.02-1.0 mg/kg, i.v.). Decreasing MAP with SNP resulted in a 275% increase in lymph muscle contraction frequency while increasing MAP with Phe had no effect on muscle burst frequency. These results suggest that lymph muscles are regulated by MAP and are effectors of the baroreflex. Supported by NSF-IOS 0843082. CONCLUSIONS REFERENCES Drewes, R.C., Hedrick, M.S., Hillman, S.S. and Withers, P.C. (2007). Unique role of skeletal muscle contraction in vertical lymph movement in amphibians. J. Exp. Biol. 210: 3931-3939. Hillman, S.S., Drewes, R.C., Hedrick, M.S. and Withers, P.C. (2010). Interspecific comparisons of lymph volume and lymphatic fluxes: Do lymph reserves and lymph mobiliazation capacities vary in anurans from different environments? Physiol. Biochem. Zool. 84: 268-276. Lung BACKGROUND Lung HYPOTHESIS Because lymph muscles are important for lymph movement, and blood volume/pressure homeostasis, lymph muscle activity is also regulated by the arterial pressure and the baroreflex. Effects of sodium nitroprusside injection METHODS Cane toads (N=5) were anesthetized with buffered tricaine (MS222) and outfitted with cannulae in the sciatic artery (PE 50) and ventral abdominal vein (PE 90) to measure arterial pressure and inject drugs, respectively. Muscle activity was measured using electromyography (EMG) using teflon-coated stainless steel wires that were sewn directly into lymph muscles. Intravenous injections of sodium nitroprusside (SNP) and phenylephrine (Phe) were used to decrease and increase mean arterial pressure, respectively. Muscle EMG activity was measured in the m. piriformis, m. gracilis minor and m. sphincter ani cloacalis before and after altering blood pressure. Fig. 3. Effects of A) sodium nitroprusside (SNP) and B) phenylephrine (Phe) to decrease and increase arterial pressure, respectively. Note increases in lymph muscle activity in m. Piriformis and m. Gracilis minor with reductions in mean arterial pressure with SNP injection. There is no change in lymph muscle activity with Phe injection to increase pressure. ABSTRACT Lung G A S T P G Ilium Lymph hearts Femur A Lymph Muscles in Cane Toads A B Fig. 2. Lymph muscles in cane toads depicted in a ventral view (A) and posterior view (B). Key: A=Abdominal crenator; G=Gracilis minor; T=Tensor fasciae latae; P=piriformis; S=Sphincter ani cloacalis (from Drewes et al. 2007) Lymph movement in cane toads Fig. 1. Direction of lymph movement in toads. Lymph must move from limbs and ventral part of the animal to the lymph hearts located near the spinal column near the dorsal surface (From Hillman et al. 2010). Effects of phenylephrine injection ARTERIAL BAROREFLEX Arterial Pressure (kPa) Piriformis EMG Gracilis EMG Heart Rate (bpm) 1 min Arterial Pressure (kPa) Piriformis EMG Gracilis EMG Heart Rate (bpm) Fig. 4. Arterial baroreflex in four toads. Each symbol represents a different animal. Mean arterial pressure was changed using SNP and Phe (0.02 – 1.0 mg/kg). Fig. 5. Summary of effects of SNP and Phe injection on lymph muscle burst frequency in five toads. Reductions in MAP with SNP (see Fig. 3A) caused a significant increase in burst frequency, while increases in MAP with Phe (see Fig. 3B) showed no change in burst frequency. B A LYMPH MUSCLE ACTIVITY Lymph muscle activity in cane toads appears to be regulated by changes in mean arterial pressure and is especially sensitive to hypotension. These data support the hypothesis that lymph muscle activity is an effector of the arterial baroreflex.