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
HYPOTHESIS
ARTERIAL BAROREFLEX
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.
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.
Effects of sodium nitroprusside injection
A
12:46:30.000 AM
20
0
Arterial Pressure (kPa)
6
5
4
3
2
Pa (kPa)
Piriformis (mV)
40
0
2
4
6
8
Mean Arterial Pressure (kPa)
Piriformis EMG
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).
500
0
-500
500
LYMPH MUSCLE ACTIVITY
0
-500
Sodium Nitroprusside
Heart Rate (bpm)
80
60
40
20
0
-20
46:30
47:30
3
48:00
48:30
49:00
49:30
50:00
50:30
51:00
1 min
Effects of phenylephrine injection
B
Pa (kPa)
Piriformis (mV)
1000
500
0
-500
Gracilis (mV)
Gracilis EMG
500
0
-500
HR (BPM)
-1000
1:01:00
1:01:30
1:02:00
1:02:30
4
1:03:00
1:03:30
1:04:00
1:04:30
1:05:00
1:05:30
1 min
Lymph Muscles in Cane Toads
Lung
Lymph hearts
S
G
A
T
P
Ilium
Femur
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.
A
G
Lung
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)
SNP
0.20
0.15
0.10
0.05
0.00
Heart Rate (bpm)
80
60
40
20
0
-20
Control
Phenylephrine
Piriformis EMG
1500
1000
B
0.1
Arterial Pressure (kPa)
8
6
4
2
*
0.2
LabChart Window
-1000
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).
0.3
0.0
Muscle Burst Frequency (min -1)
BACKGROUND
Lymph movement in cane toads
47:00
Muscle Burst Frequency (min -1)
HR (BPM)
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
Lunglymph 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.
Gracilis (mV)
Gracilis EMG
METHODS
Lung
60
LabChart Window
1000
A
Baroreflex Responses
Heart Rate (beats min-1)
ABSTRACT
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.
Control
Phe
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.
CONCLUSIONS
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.