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Am J Physiol Heart Circ Physiol 306: H730–H737, 2014.
First published December 20, 2013; doi:10.1152/ajpheart.00831.2013.
Cardiac function of the naked mole-rat: ecophysiological responses to
working underground
Kelly M. Grimes,1 Andrew Voorhees,2 Ying Ann Chiao,3 Hai-Chao Han,2 Merry L. Lindsey,4
and Rochelle Buffenstein1
1
Department of Physiology and the Sam and Ann Barshop Institute for Aging and Longevity Studies, The University of Texas
Health Science Center at San Antonio, San Antonio, Texas; 2Department of Mechanical Engineering, The University of Texas
at San Antonio, San Antonio, Texas; 3Department of Pathology, University of Washington, Seattle, Washington; and
4
Mississippi Center for Heart Research, Department of Physiology and Biophysics, University of Mississippi Medical Center,
Jackson, Mississippi
Submitted 23 October 2013; accepted in final form 18 December 2013
Grimes KM, Voorhees A, Chiao YA, Han HC, Lindsey ML,
Buffenstein R. Cardiac function of the naked mole-rat: ecophysiological responses to working underground. Am J Physiol Heart Circ
Physiol 306: H730 –H737, 2014. First published December 20, 2013;
doi:10.1152/ajpheart.00831.2013.—The naked mole-rat (NMR) is a
strictly subterranean rodent with a low resting metabolic rate. Nevertheless, it can greatly increase its metabolic activity to meet the high
energetic demands associated with digging through compacted soils in
its xeric natural habitat where food is patchily distributed. We hypothesized that the NMR heart would naturally have low basal
function and exhibit a large cardiac reserve, thereby mirroring the
species’ low basal metabolism and large metabolic scope. Echocardiography showed that young (2– 4 yr old) healthy NMRs have low
fractional shortening (28 ⫾ 2%), ejection fraction (43 ⫾ 2%), and
cardiac output (6.5 ⫾ 0.4 ml/min), indicating low basal cardiac
function. Histology revealed large NMR cardiomyocyte cross-sectional area (216 ⫾ 10 ␮m2) and cardiac collagen deposition of 2.2 ⫾
0.4%. Neither of these histomorphometric traits was considered pathological, since biaxial tensile testing showed no increase in passive
ventricular stiffness. NMR cardiomyocyte fibers showed a low degree
of rotation, contributing to the observed low NMR cardiac contractility. Interestingly, when the exercise mimetic dobutamine (3 ␮g/g ip)
was administered, NMRs showed pronounced increases in fractional
shortening, ejection fraction, cardiac output, and stroke volume,
indicating an increased cardiac reserve. The relatively low basal
cardiac function and enhanced cardiac reserve of NMRs are likely to
be ecophysiological adaptations to life in an energetically taxing
environment.
naked mole-rat; cardiac reserve; ecophysiology; left ventricular function; echocardiography
that the naked mole-rat (NMR; Heterocephalus glaber), a hystricognath mouse-sized rodent, has
lead a strictly subterranean lifestyle in the xeric regions of
sub-Saharan Africa since the early Miocene epoch (26). Living
below ground, these rodents must meet all energy and nutrient
requirements from the consumption of underground roots and
tubers that they find while digging blindly through compacted
soils. Foraging for sparse and patchily distributed foods in this
arid environment is an energetically costly process, requiring
fivefold higher energy metabolism than at rest (30). The high
energetic demands of foraging below ground in a dry environment are thought to have contributed to the evolution of their
FOSSIL EVIDENCE REVEALS
Address for reprint requests and other correspondence: R. Buffenstein,
Barshop Institute, STCBM 2.2002, 15355 Lambda Drive, San Antonio, TX
78245 (e-mail: [email protected]).
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unique eusocial lifestyle, which increases the likelihood of
foraging success (19). Living communally in deep (⬃2 m)
underground colonies with up to 300 individuals (4), NMRs
must contend with atmospheric oxygen contents ranging from
as low as 4% in the nests to 20% in superficial foraging
burrows. Moreover, underground atmospheres can be very
humid and high in carbon dioxide (6). NMRs exhibit many
adaptations to this hostile milieu, including a low basal metabolic rate (9, 15), thermolability (9), and extreme tolerance to
hypercapnia and variable oxygen availability (12, 25, 35). This
harsh habitat at the same time provides substantial protection
from predation, disease, and climatic extremes, thus contributing to the low extrinsic mortality of this species (7, 11).
We have previously shown that aged NMRs, in keeping with
their low extrinsic mortality, display negligible senescence (8)
and less than half the age-dependent declines in diastolic
function of the left ventricle (LV) commonly seen in both mice
and humans (16). In this article, we provide a much broader
assessment of NMR cardiac function using in vivo and ex vivo
techniques, as well as a histological examination of ventricular
morphology. We questioned if cardiac function in young,
healthy NMRs would reflect the species’ subterranean lifestyle.
We hypothesized that the NMR would exhibit low basal
cardiac function and have an increased cardiac function capacity during high activity. The aim of this study was therefore to
assess the NMR heart in light of the species’ unique ecophysiological adaptations in order to establish the normal cardiac
parameters for this species under normoxic conditions. We also
assessed the same parameters in physiologically age-matched
C57BL/6J mice. In light of the limitations of two-species
comparative studies (13), this well-studied mouse strain was
used to confirm the reliability of our measurements relative to
published data and also to assist in the interpretation and
evaluation of the novel data from NMRs.
METHODS
Animals. The male and female NMRs (ages 2– 4 yr) used in this
study were second- or third-generation captive-born animals, descended from animals captured in Kenya in 1980. Animals were
maintained at the University of Texas Health Science Center at San
Antonio (UTHSCSA) in normoxic conditions in interconnected systems consisting of tubes and cages of varying sizes to approximate the
multichambered burrow and tunnel systems that NMRs inhabit in the
wild. NMRs were housed under climatic conditions approximating
their native habitat (30°C; ⱖ50% relative humidity). They met all
their nutrient and water needs through an ad libitum supply of fruit
0363-6135/14 Copyright © 2014 the American Physiological Society
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NAKED MOLE-RAT CARDIAC FUNCTION
and vegetables, supplemented with a protein- and vitamin-enriched
cereal (Pronutro; Bokomo, Rosendal, South Africa). The male and
female mice in this study, of the C57BL/6J strain (ages 3–5 mo), were
group housed at 25°C on a 12:12-h light-dark cycle and were given ad
libitum access to food and water. The ages selected for this study
allowed for physiological age matching between species such that
both were at equivalent percentages of maximum life span. This study
was approved by the Institutional Animal Care and Use Committee
(protocol 11042x) at UTHSCSA.
Heart rate and two-dimensional echocardiography. Heart rates
from awake, unanesthetized mice and NMRs were measured using a
modified blood pressure tail cuff (Hatteras Instruments, Cary, NC).
These measurements were acquired after a minimum of three training
sessions. For echocardiography, mice and NMRs were anesthetized
using 1–2% isoflurane in a 100% oxygen mix and placed in the supine
position with paws taped to electrodes on a temperature-controlled
electrocardiogram board. Heart rate, respiratory rate, and temperature
were monitored to ensure the data collected were physiologically
relevant. Measurements were acquired with the Vevo 2100 Imaging
System (VisualSonics, Toronto, ON, Canada) to obtain two-dimensional parasternal LV B-mode recordings at the long axis and M-mode
recordings at the short-axis midpapillary region according to the
recommendations set out by Lang et al. (24). Stress was induced by an
intraperitoneal injection of dobutamine (3 ␮g/g body mass) for each
animal. This dose was chosen because it was the minimum at which
heart rate and fractional shortening were maximally stimulated in both
species after dose-response experiments were conducted (data not
shown). Echocardiograms were recorded at baseline before injection
and 30 min after injection.
Biaxial tensile testing. To investigate the possibility of species LV
functional differences being due to differences in mechanics, biaxial
testing was performed. NMRs and mice were anesthetized with
isoflurane, and hearts were excised. LVs were separated from right
ventricles and atria and then horizontally sectioned and cut in half
along the circumference such that a midpapillary region of the free
wall was isolated. Samples were submerged in PBS at 25°C in a tissue
bath and mounted to racks connecting to actuators in a CellScale
BioTester (Waterloo, ON, Canada) that stretch tissue simultaneously
in the longitudinal and circumferential directions. Testing was conducted with samples following three different loading protocols: 20%
equibiaxial stretch over 30 s, 20% circumferential stretch and 10%
longitudinal stretch over 30 s, and 10% circumferential and 20%
longitudinal stretch over 30 s. To reduce viscoelastic effects, each
protocol was conducted slowly six times for tissue preconditioning,
and only the final cycle was considered for analysis. Forces and
displacements were determined from actuator output data, and Cauchy
stresses were calculated following a protocol described by Fomovsky
and Holmes (14).
Stress and displacement data were fitted to a two-dimensional
stress-strain relationship based on a four-parameter Fung-type strain
energy density equation:
W ⫽ 1 ⁄ 2c共eQ ⫺ 1兲
Q ⫽ b1E2␪ ⫹ b2Ez2 ⫹ b3E␪Ez .
(1)
Specifically, it yields a two-dimensional orthotropic nonlinear stressstrain relationship (27):
␴␪ ⫽ 共1 ⫹ 2E␪兲共b1E␪ ⫹ b3Ez兲ceQ
␴z ⫽ 共1 ⫹ 2Ez兲共b2Ez ⫹ b3E␪兲ceQ ,
(2)
where W is the strain energy density; E␪ and Ez are the circumferential
and longitudinal components of the Green strain, respectively; and c,
b1, b2, and b3 are material constants determined by fitting Eq. 2 to the
experimental data. To directly compare the mechanical properties
under physiological conditions, the stretch ratios under diastolic
pressures were calculated using the Law of Laplace and diastolic
dimensions.
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Histological analyses. After tensile testing, the LV midpapillary
slices were fixed in 10% zinc formalin and embedded in paraffin. Two
5-␮m-thick sections were collected every 50 ␮m through the thickness of the sample. For each pair, one of the slides was stained with
hematoxylin and eosin (H&E) for cardiomyocyte alignment and the
other with picrosirius red (PSR) for collagen density. For alignment
and collagen analyses, slides were imaged at three locations along the
circumferential midline of the specimen at ⫻10 magnification. Image
processing to calculate alignment and density was conducted using
custom Matlab-based codes (version 7.14; The MathWorks, Natick,
MA). Briefly, cardiomyocyte alignment was calculated using a gradient-based edge-finding routine. Collagen density was calculated by
counting the percentage of pixels in the sample that were positively
stained with PSR.
Tissue from a separate population of animals was taken for cardiomyocyte cross-sectional area measurements. For these, animals were
euthanized with isoflurane anesthesia, and hearts were flushed with
cardioplegic solution before excision to arrest cardiomyocytes in
diastole. LV midpapillary regions were isolated as before but here
fixed and embedded without circumferential sectioning before being
stained with H&E. To calculate cardiomyocyte area, 30 myocytes per
section were randomly scanned at ⫻40 magnification according to
methods previously described such that quantifiable cardiomyocytes
were rounded and had central nuclei (28). Areas were quantified using
NIS-Elements imaging software (Nikon, Melville, NY).
Statistical analyses. Five male and five female NMRs in addition to
five male and five female mice were used for in vivo cardiac
physiology comparisons and echocardiographic assessments. For all
remaining analyses, two additional animals of each sex and species
were included to provide sufficient tissue for the various morphometric measurements undertaken. Samples were randomly selected for
histological evaluations, with the number of animals per experiment
denoted in all figure legends. For all assessments, sex differences were
evaluated using unpaired t-tests. Since no sex differences were evident
in either species, male and female data were combined. Data are
expressed as means ⫾ SE. Unpaired t-tests (with Welch’s corrections
if the variances were significantly different) were used to compare
values between species for basic measurements, echocardiography,
biaxial testing, collagen deposition, and cardiomyocyte cross-sectional area. Linear regression was used to determine the rotation of
cardiomyocytes through the thickness of the samples. Samples with
correlation coefficients ⬍0.5 were excluded from the data set because
they may have been damaged during biaxial testing. A circular
statistics test based on the Rayleigh’s test was used to determine if
cardiomyocytes had an alignment that was significantly different from
a random uniform distribution (38). GraphPad Prism 5 (GraphPad
Software, San Diego, CA) or Matlab were used for all statistical
calculations.
RESULTS
Observed and predicted species differences in basic cardiac
parameters. NMRs had both larger body and heart masses
compared with mice, but given the interspecies body size
differences, the ratio of heart mass to body mass was significantly smaller for NMRs (Table 1). Allometric analyses based
on the species’ respective average weights revealed that both
species had heart weights lower than predicted (32). Average
unanesthetized NMR heart rate was less than half that of the
mouse and approximately half the rate predicted for a 45-g
mammal. In contrast, the mice had a 35% higher heart rate than
predicted by mass (5). NMR cardiac output, in keeping with
the low heart rate, was also less than half that of the mouse and
less than half that predicted by mass. Conversely, mouse
cardiac output was much higher than its allometrically predicted value (36).
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Table 1. Interspecies comparison of mice and NMRs: observed values and their relative percentage of values predicted
allometrically
%Predicted Value
Body mass, g
Heart mass, mg
Heart:body mass, %
Nonanesthetized heart rate, beats/min
Cardiac output, ml/min
Mouse
NMR
Significance
26.1 ⫾ 1.3
130 ⫾ 10
0.50 ⫾ 0.02
704 ⫾ 11
14.9 ⫾ 0.7
45.6 ⫾ 3.3
190 ⫾ 10
0.41 ⫾ 0.03
256 ⫾ 8
6.5 ⫾ 0.4
0.0002
0.0004
0.007
⬍0.0001
⬍0.0001
Mouse
NMR
80
68
135
149
57
43
All values are means ⫾ SE. NMR, naked mole-rat. Sample sizes are n ⫽ 10 (5/sex) for each species. Significance is indicated by P values. The following
equations were used for predicted values, where Mb denotes body mass: heart mass, 0.0058 Mb(kg)0.98 ⫻ 1000000 (32); heart rate, 10⫺0.251 log Mb(g) ⫹ 3.072 (5);
and cardiac output, 187 Mb(kg)0.81 (36).
Assessment of basal LV dimensions and function. Pronounced species differences in LV dimensions and function
were evident under basal conditions. M-mode echocardiograms (Fig. 1) revealed that NMRs displayed larger LV wall
thicknesses than mice in diastole (Table 2). However, it must
be noted that NMRs have significantly larger hearts overall
compared with mice (Table 1). Conversely, end-diastolic dimension was not different between species, but end-systolic
dimension in the NMR was significantly larger. All LV function parameters (fractional shortening, ejection fraction, cardiac output, and stroke volume) were significantly lower in the
NMR than the mouse, indicating reduced cardiac function in
the NMR under basal conditions.
LV dimensions and function under exercise-like stress. Given
these results, it was important to determine whether the low
cardiac function of young, healthy NMRs under nonstressed
conditions was indicative of an underlying unhealthy heart
condition or was a species-specific trait. If the former was true,
NMR cardiac function under stress would also be compromised (37). Since dobutamine is routinely used to induce an
exercise-like stress response, treatment with this drug was also
used to evaluate the cardiac reserve of the species. Doseresponse data revealed that intraperitoneal injection of dobutamine at 3 ␮g/g body mass elicited maximal inotropic and
chronotropic responses in both species. End-systolic dimension
with dobutamine was no longer significant between species,
indicative of a greater increase in NMR cardiac function (Table
2). Furthermore, NMR ejection fraction and stroke volume
were no longer significantly lower than the mouse parameters
with dobutamine treatment.
Fig. 1. Representative M-mode echocardiography shows that mice have greater left ventricular (LV) contractility at baseline and after dobutamine-induced
cardiac stress than do naked mole-rats (NMR) either at baseline or after dobutamine. Echocardiogram time scale is in seconds.
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Table 2. Species differences in LV dimensions and function at baseline and under stress conditions
Baseline
Heart rate, beats/min
Average diastolic wall thickness, mm
LV end-diastolic dimension, mm
LV end-systolic dimension, mm
Fractional shortening, %
Ejection fraction, %
Cardiac output, ml/min
Stroke volume, ␮l
Dobutamine
Mouse
NMR
Significance
Mouse
NMR
Significance
461 ⫾ 14
0.77 ⫾ 0.03
3.67 ⫾ 0.08
2.27 ⫾ 0.12
39 ⫾ 2
57 ⫾ 3
14.9 ⫾ 0.7
34.4 ⫾ 1.4
229 ⫾ 10
0.87 ⫾ 0.02
4.03 ⫾ 0.24
2.93 ⫾ 0.24
28 ⫾ 2
43 ⫾ 2
6.5 ⫾ 0.4
26.0 ⫾ 1.4
⬍0.0001
0.0198
N.S.
0.0239
0.0002
0.0008
⬍0.0001
0.001
600 ⫾ 16
0.77 ⫾ 0.03
3.34 ⫾ 0.09
1.66 ⫾ 0.08
52 ⫾ 2
79 ⫾ 1
21.7 ⫾ 0.9
34.9 ⫾ 1.3
306 ⫾ 13
0.88 ⫾ 0.02
3.74 ⫾ 0.21
2.08 ⫾ 0.18
46 ⫾ 2
75 ⫾ 2
11.5 ⫾ 1.1
37.2 ⫾ 3.6
⬍0.0001
0.0152
N.S.
N.S.
0.0345
N.S.
⬍0.0001
N.S.
All values are means ⫾ SE. LV, left ventricular. Sample sizes are n ⫽ 10 (5/sex) animals for each species. Significance is indicated by P values; N.S. denotes
P ⬎ 0.05.
Determination of species-specific responses to dobutamine
further elucidated NMR heart function. There was no difference in chronotropic responses to dobutamine, as shown by the
similar percentage change in heart rate from baseline to 30 min
after dobutamine treatment (Fig. 2A). The NMR inotropic
response, however, was significantly higher than that of the
mouse: NMR fractional shortening increased by 72 ⫾ 10%
compared with 34 ⫾ 4% in the mouse (Fig. 2B). The NMR
increase (76 ⫾ 8%) in ejection fraction was about twofold
greater than that of the mouse (41 ⫾ 7%; Fig. 2C). Percent
change in cardiac output was again significantly higher in the
NMR (77 ⫾ 12%) compared with the mouse (45 ⫾ 4%; Fig.
2D). Following the same trend, NMRs greatly increased their
stroke volume (42 ⫾ 9%), whereas mice did not (2 ⫾ 3%; Fig.
2E). The stroke volume data are in keeping with previous
findings that this measurement does not change with dobutamine treatment in mice (37). Together, these results indicate
that the NMR heart did not display any apparent dysfunction,
suggesting that the observed low basal cardiac function was a
natural healthy phenotype for this species. Furthermore, NMRs
possess an enhanced cardiac reserve such that the work of the
NMR heart can be increased substantially beyond what is
necessary for the animal’s basic functions of living.
Ventricular mechanical properties. Predicted circumferential wall stresses as calculated by the Law of Laplace were not
different between species in both systolic and diastolic states
(Fig. 3A). The lack of a species difference in systolic stress
indicates that reduced systolic loading is likely not a means of
limiting myocardial wear and tear. Biaxial tensile testing under
physiologically relevant pressures and stresses showed no
differences in circumferential or longitudinal stretch ratios
under estimated diastolic conditions, indicating similar passive
mechanical properties (Fig. 3B).
Histological examination of ventricular composition. To
further probe the mechanisms for species differences in contractility and LV structure, we set out to determine cellular
differences in LV structure. The cross-sectional area of cardiomyocytes was increased in NMRs (216 ⫾ 10 vs. 178 ⫾ 7
mm2) (Fig. 4). Interestingly, it was more difficult to find
myocytes properly aligned for quantification in NMR tissue
than in that of mice. This may be evidence that NMR cardiomyocytes are less longitudinally arranged than those of mice.
Fig. 2. Percent changes in key cardiac function parameters: heart rate (HR), fractional shortening (FS), ejection fraction (EF), cardiac output (CO), and stroke
volume (SV). A: mouse HR increased similarly to that of NMRs. However, NMRs exhibited significantly greater percent changes in FS (B), EF (C), CO (D),
and SV (E). Overall, these data may be evidence of a greater cardiac reserve in the NMR. All values are means ⫾ SE. Sample sizes are n ⫽ 10 (5/sex) animals
for each species. N.S. denotes P ⬎ 0.05.
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Fig. 3. No significant difference in LV mechanics between
species. A: circumferential wall stress was estimated using individual dimensions determined from echocardiography and the
group average pressures according to the Law of Laplace. No
differences were found. B: the deformation of the tissue at end
diastole was predicted from the mechanical properties measured
by biaxial testing and the estimated diastolic stresses. Circumferential and longitudinal stretch ratios were found to be similar,
and no differences were found between groups. All values are
means ⫾ SE. Sample sizes are n ⫽ 8 (4/sex) animals per species.
Cardiomyocyte alignment is known to vary linearly through
the thickness of the wall. However, it is also dependent on the
size of the heart such that smaller hearts have greater myocardial fiber rotation to develop greater torsion (17, 29). In the present
study, NMR LV tissue showed a more overall circumferential
alignment of cardiomyocytes than that of the mouse due to
reduced rotation through the thickness of the myocardium (Fig.
5A). Furthermore, NMRs show reduced rotation of fiber angle
despite having cardiomyocyte arrangement at the epicardial surface that is comparable to that of mice (Fig. 5B).
Interstitial collagen content was quantified because collagen
provides structural support for cardiomyocytes and contributes
to the mechanical properties of the LV. NMRs exhibited a
nonsignificant trend for increased interstitial collagen deposition (P ⫽ 0.054) as evidenced by PSR staining (Fig. 6).
DISCUSSION
We questioned if cardiac function and structure in young
NMRs at their most robust stage of health would be reflective
of the ecophysiological adaptations of the species. We predicted that the NMR would exhibit low basal cardiac function
that could be greatly increased under conditions of high activity. In effect, the NMR’s cardiac function would mirror its low
basal metabolism and large metabolic scope, since digging in
its natural environment is a very energetically costly process
(30). We report for the first time that NMRs exhibit unique
cardiac traits pertaining to LV form and function. NMRs
displayed low baseline cardiac function (Table 2). This suggests that NMRs are capable of maintaining their basal physiological needs at a low level of cardiac function, especially
with respect to mice. Nevertheless, NMRs are capable of
markedly ramping up their cardiac function in response to
treatment with an exercise mimetic.
Species lifestyle explains low cardiac function in the NMR.
Supporting our hypothesis, we found that NMRs have much
lower heart rate, fractional shortening, ejection fraction, cardiac output, and stroke volume at baseline compared with mice
(Tables 1 and 2). Lower cardiac function under basal conditions in the NMR likely reflects the lower metabolic requirements associated with ecophysiological adaptations to life
underground (15). Gas exchange in the burrows of the NMR is
poor, because air movement is largely restricted to diffusion
through soil. The reduced oxygen availability is further exacerbated by the large number of animals, microorganisms, and
plant roots respiring together. NMRs have adapted to such
forbidding conditions by evolving extreme tolerance of variable oxygen atmospheres, particularly hypoxia (6). For example, NMR brain slices maintain synaptic activity for three times
longer than mouse slices kept under identical anoxic conditions
(25). Low levels of oxygen consumption and consequent low
resting metabolic rates, which are 66 –75% of those expected
allometrically in the NMR (9, 15), are also considered an
adaptive trait to life in a low-oxygen environment (31). With
this low resting metabolism, the NMR’s low cardiac function
at baseline is likely adequate to meet the basal energy needs of
this animal.
Additionally, NMRs have high hematocrit levels, and their
hemoglobin has a higher affinity for oxygen compared with
aboveground-dwelling mammals (6, 20). Even after being
housed under normoxic conditions in captivity for more than
30 years, NMRs maintain hematocrits of ⬃50%. Logically,
with this greater oxygen uptake per heartbeat, the NMR heart
Fig. 4. NMR LVs had significantly larger cardiomyocyte cross-sectional areas. A: representative images at ⫻40 magnification of hematoxylin and eosin
(H&E)-stained mouse and NMR LV midwall sections with highlighted cells that are considered suitable for quantification. B: a graph of cross-sectional area
showing 21% greater area in NMRs. All values are means ⫾ SE. Sample sizes are n ⫽ 6 (3/sex) animals per species. Scale bars are 200 ␮m.
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Fig. 5. Decreased cardiomyocyte fiber rotation in NMR LVs. A: representative images of cardiomyocyte alignment taken at ⫻10 magnification and stained with
H&E. The line represents the circumferential direction, and the dashed arrow points in the mean fiber direction. At the epicardial and endocardial surfaces, NMR
cardiomyocytes were aligned more circumferentially compared with those of mice. For each species, all images are from the same animal. B: a comparison of
myocyte orientation shows reduced rotation of NMR myocytes through the LV wall. All values are means ⫾ SE. Scale bars are 100 ␮m.
should not have to pump as hard to maintain adequate oxygen
and nutrient supply to other tissues, especially under normoxic
conditions. Low resting body temperature (⬃32–35°C), stemming from both reduced metabolism and lack of an insulating
pelage, could also promote low NMR cardiac function by
decreasing cardiac output. A decrease in temperature by 1°C
can cause a 2% increase blood viscosity (1, 21). This, coupled
with the already higher hematocrit levels, means the NMR
heart must pump against a much more viscous fluid than those
of aboveground-dwelling mammals.
Both tension in the LV wall and the velocity of contraction
are important contributors to myocardial oxygen consumption
(3). Based on our findings of low cardiac function, the tension
and contraction velocity should be low in the NMR heart. Thus
it is plausible that the low cardiac function serves as an
energy-saving strategy for the NMR heart. Furthermore, a
lower heart rate prolongs the diastolic filling period, and basal
oxygen consumption of the heart is 20% of that during systole
(3). Therefore, a lower resting heart rate may promote overall
metabolic efficiency in the NMR. A strategy that limits basal
Fig. 6. Similar LV interstitial collagen deposition in NMRs and mice. A: representative images at ⫻10 magnification of picrosirius red-stained (PSR) mouse and
NMR LV midwall sections with arrows pointing to collagen fibers. B: a graph showing collagen density is not significantly different between species. All values
are means ⫾ SE. Sample sizes are n ⫽ 8 (4/sex) animals per species. Scale bars are 100 ␮m.
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energy expenditure is thus adaptive given both the NMR’s
harsh underground living conditions and its energetically
costly foraging process.
Increased cardiac reserve in the NMR. Despite our knowledge of reduced basal metabolic requirements in the NMR, it
was important to ascertain whether the NMR heart was truly
“idling” at a lower basal level or functionally compromised due
to pathological reasons. We thus used a ␤-adrenergic receptor
agonist, dobutamine, to mimic exercise-induced cardiac stress,
since it was not possible to undertake echocardiography in
conscious animals running on a treadmill. Dobutamine, like
exercise, induces an increase in both heart rate and contractility
and is commonly used as a stress test to determine cardiac
functional reserve. If NMR heart function was compromised,
dobutamine stress would be unable to induce greater contractility (37).
It is doubtful that the NMR heart is less healthy than that of
the mouse, because dobutamine treatment caused a greater
increase in cardiac function in NMRs. LV end-systolic dimensions were similar between species with dobutamine treatment,
whereas they were significantly greater in NMRs at baseline
(Table 2). This is evidence of a greater increase in cardiac
function in the NMR that is further supported by the echocardiographic measurements of LV function. NMRs show significantly higher percentage changes in fractional shortening,
ejection fraction, cardiac output, and stroke volume compared
with mice (Fig. 2). Still, both species exhibit similar chronotropic stimulation with dobutamine, because heart rate changes
were not significantly different. Despite this, the NMR undergoes a near doubling of most parameters of its cardiac function
under exercise-like conditions, unlike the mouse. This is evidence of a large cardiac reserve, which may allow NMRs to
meet the obligatory high energy needs associated with burrowing (30) despite having low basal cardiac function.
Having low basal metabolism allows NMRs to be economical with their energy expenditure. In sub-Saharan Africa, the
NMR’s food sources are sparse and the soil is densely compacted. The energetic cost of digging to find food in this
environment is thus extremely high, causing metabolic rates to
increase more than fivefold from resting levels (30). Collectively, adaptations to a burrowing lifestyle and hypoxic environment likely contribute to the low basal cardiac function and
heart rate of NMRs and provide a reason why the NMR
displays so large an increase in cardiac reserve.
Pathology unlikely despite low NMR cardiac function. Diminished basal NMR cardiac function (Table 2) was accompanied by morphological traits commonly associated with
cardiac pathology in both mice and humans (10). Histological
assessments revealed significantly greater cardiomyocyte
cross-sectional area and a trend, albeit not significant (P ⫽
0.054), toward increased LV collagen content in NMRs relative to mice (Figs. 4 and 6). Both increased cell size and
increased collagen deposition are commonly seen in LV dysfunction and are associated with cardiomyocyte cell death and
fibroblast proliferation (22). Nonetheless, it is unlikely that
these histomorphometric features allude to a pathological state
in this species, for the young NMRs examined in the present
study displayed increased cardiac reserve. Cardiac reserve is
commonly diminished in disease states (23). Increased cardiomyocyte cross-sectional area, rather, is likely to be a hallmark
of the species, since NMRs have larger hearts and greater LV
wall thicknesses than mice. Although increased collagen limits
LV relaxation capabilities (22), the trend toward increased LV
collagen deposition in young NMRs is not detrimental (Fig. 6),
because we have seen no difference in diastolic relaxation
between species (Fig. 3B).
It has been suggested that the smaller cardiomyocyte size
and lower collagen content seen in aged Ames dwarf mouse
hearts are beneficial and allow this mutant strain to achieve
greater longevities compared with wild-type mice (18). Although this premise may hold true within a species, it does not
do so across species of disparate longevities. Indeed, we have
observed traits in NMR hearts opposite to those of the Ames
dwarf mice despite the fact that the extraordinary longevity of
NMRs is seven- to eightfold greater than the maximum life
span of Ames dwarf mice (2, 8, 11). Furthermore, we have
previously shown that NMRs display attenuated age-related
declines in cardiac diastolic function (16). These histomorphometric traits must then reflect species-specific qualities that do
not hamper evolutionary fitness or hinder the NMR from
meeting its physiological requirements. Interestingly, exceptionally high cardiac collagen deposition (13–18%) does not
keep the Burmese python from markedly boosting its metabolism after a large meal. This high collagen deposition is
drastically different from the normal 1–2% commonly found in
most mammalian hearts, and yet this reptile does not display
cardiac dysfunction (33). On the contrary, the Burmese python
is capable of extreme physiological cardiac hypertrophy, which
is accompanied by a 3.3-fold postprandial augmentation of
cardiac output (33, 34). Clearly, the python’s cardiac function
is not hindered by such high collagen. It is instead a unique
nondetrimental cardiac feature of the species like those we
have described presently in the NMR.
The lower basal cardiac function observed in NMRs might
reflect species differences in cardiomyocyte fiber rotation.
Mice have much greater fiber rotation in their LVs compared
with NMRs (Fig. 5). This promotes increased contractile efficiency, because greater fiber rotation is linked to improved
torsion, allowing a heart to better eject blood with each contraction (29). However, mice also have significantly smaller
cardiomyocyte cross-sectional area and smaller diastolic wall
thickness in their overall smaller hearts (Fig. 4, Table 1). It is
possible that the greater fiber rotation is a compensation for the
thinner walls and fewer cardiomyocytes in the smaller mouse
heart (17). In contrast, the reduced fiber rotation contributes to
lower contractility in the NMR heart. However, low contractility is in line with the low-energy, basal state in which NMRs
exist until they have to meet the high energy demands of
burrowing. Despite this, low basal contractility does not necessarily limit mechanical wear and tear on the NMR heart,
because the mathematically predicted systolic stress does not
differ between species (Fig. 3A). However, the species’ reduced heart rate may limit the effects of cyclic loading, thereby
reducing cardiac tissue fatigue.
In summary, this first study characterizing the hearts of
young NMRs has revealed reduced baseline cardiac function
yet enhanced cardiac reserve, in keeping with the species’
inherent ecophysiology. Assessing these unique cardiac features of the NMR was an essential step in establishing the
groundwork for understanding how the heart of this novel
species responds to aging and other stressors. Future studies
will assess NMR cardiac function with age and explore how
AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00831.2013 • www.ajpheart.org
NAKED MOLE-RAT CARDIAC FUNCTION
this species may employ protective mechanisms in its heart to
withstand both oxidative stress and the vagaries of time.
ACKNOWLEDGMENTS
We gratefully acknowledge Kaitlyn Lewis and Miranda Orr for editorial
support. Karl Rodriguez and Megan Smith helped with tissue harvesting and
animal care.
GRANTS
This work was supported primarily by American Heart Association Grantin-Aid 12030299 (to R. Buffenstein), National Science Foundation CAREER
Award 0644646 (to H.-C. Han), and National Institutes of Health Grants
HL095852 and HHSN 268201000036C (N01 HV-00244) for the San Antonio
Cardiovascular Proteomics Center from the National Institutes of Health (to
M. L. Lindsey).
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
AUTHOR CONTRIBUTIONS
K.M.G., A.V., H.-C.H., M.L.L., and R.B. conception and design of research; K.M.G., A.V., and Y.A.C. performed experiments; K.M.G., A.V., and
Y.A.C. analyzed data; K.M.G., A.V., H.-C.H., M.L.L., and R.B. interpreted
results of experiments; K.M.G. and A.V. prepared figures; K.M.G. and R.B.
drafted manuscript; K.M.G., A.V., Y.A.C., H.-C.H., M.L.L., and R.B. edited
and revised manuscript; K.M.G., A.V., Y.A.C., H.-C.H., M.L.L., and R.B.
approved final version of manuscript.
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