Electrical Potential Difference Across
Blood Vessels
By David H. Harshaw, Jr., M.D., Howard Ziskind, Roger Mazlen, B.S.,
and Philip N. Sawyer, M.D.
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• Over the past decade, several types of experiments characterizing the electrical properties of blood vessels have been carried out.1' 2
Some of these experiments were done to determine the possible existence, magnitude, and
polarity of the normal potential difference
across the aortic and vena caval wall using in
vivo and in vitro preparations.1"3 The first
group of experiments was done in the living
dog where saline-filled polyethylene tubes
were implanted across canine blood vessel
walls for periods up to several weeks. Measurement of the potential difference using these
catheters as salt bridges gave values as high
as 30 to 40 millivolts (mv.) with the intimal
side of the blood vessel ordinarily negative to
the adventitial side. We were able to determine, by mathematical treatment of the data
at that time, that the potential difference was
in large degree produced by tissues surrounding the aorta. This finding has since been
substantiated by additional experimental
work. Since then, in vitro experiments have
been developed to measure the transniural
potential difference across membranized aortic
wall. As the techniques of measurement improved, it became apparent that if an electrical potential existed across the exteriorized,
but otherwise uninjured, blood vessel, it was
quite amall.
This paper will describe a technique which
yielded measurements of the potential difference across the isolated canine aorta and vena
cava, reproducible to approximately 50 niicroFroin tlie Department of Surgery and Surgical
Kosearcli, State University of Now York, Downstate
Medical Center, Brooklyn, New York.
Supported in part by Grants H3879, C2 and C3,
from tlie National Heart Institute, National Institutes of Health, Bethesda, Maryland, and the American Heart Association.
Beceived for publication January 10, 1962.
360
volts (^V.). The measurements of the vascular transmural potential difference in these
experiments appears to be sufficiently close
to zero to be considered zero statistically.
Methods
The measurement of a steady-state direct current
voltage is relatively simple until one attempts to
measure voltages in the 1-mv. range, or smaller. At
about this point, previously insignificant factors
become important. Some of the factors which we
have had to consider during such measurements
include: (1) thermal electromotive forces in the
measuring equipment, including galvanometer, amplifier, contacts, and soldered connections; (2)
stray currents picked up from power lines; (3)
stjitic electricity arising from personnel moving
around the measuring equipment; (4) liquid junction and diffusion potentials within the solution;
(5) amplifier drift causing the zero or baseline
reading to change; and (6) natural variation and
instability of standard electrodes.
Because of these factors, an in vitro preparation was chosen. This, in itself, imposes certain
biological limitations on the measuring technique,
thereby restricting the degree of precision of measurement. However, these limitations are inherently
much smaller than those which occur during measurement of potential differences in vivo. The potential difference was measured across both aorta, and
vena cava from dogs. To exclude the effects of
anesthetic agents, the animals were sacrificed by
air embolization. The chest was opened, the thoracic aorta and vena cava were removed and the
specimens placed in a serum substitute5 at a pH
of 7.3 to 7.4. A serum substitute was chosen instead of blood or plasma since these tend to foam
when gas is bubbled through them.
The pH of this solution was adjusted by the
use of a bicarbonate buffer system. Various concentrations of CO2 were bubbled through the solution. The greater the surface area of solution exposed to the atmosphere, the higher the per cent
of CO2 needed in the gas mixture to keep the
solution pH at a constant level. A pH of 7.3 to
7.4 was obtained in our experimental apparatus
by using 15 to 17.5 per cent CO2 and 85 to 82.5
per cent 0 2 gas mixture. The temperature was kept
Circulation Retearoh. Volume XI, September ISSt
ELECTRICAL POTENTIAL DIFFERENCE
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constant by using an environment of warm ah'
about the cells. An "air bath" decreased static and
electric noise levels below those ordinarily present
in a water bath. The measuring cell in which the
tissue was placed is made of Lucite and is complotely shielded by the "air bath," which in this
series of experiments also served as a Faraday
Cage. The blood vessel was freed of perivascular
areolar tissue after it was removed from the animal. It was split lengthwise between intercostal
vessels to form a flat membrane. The membrane
was placed between the Lucite cells which were
then tightened around the tissues (fig. 1). Paired
3 N KC1 calomel cells wei-e placed across the membrane. Salt bridges connected the calomel cells to
the bathing solution on either side of the membrane. In turn, the calomel cells were connected to
a very sensitive chopper-type micro-voltmeter*
from which continuous readings were run. This
instrument in our experimental system is accurate
to ± 30 fj.v. Before and after measuring the potential difference across the blood vessel, control zero
readings were obtained on the calomel cells alone
using the same solution which was used to bathe
the tissues. This permitted one to correct for baselino drift duo to changes in the electrodes and the
agar salt bridges.
Results
The potential difference Avas measured
across a total of 10 aortae and 10 venae
cavae for periods as long as four hours. Potential difference measurements were recorded
every five minutes both for the tissue and the
control zero readings. A graphical plot of
the control readings against time showed that
the variation followed a straight line. The
zero baseline was then computed by the
method of least squares. For every blood
vessel reading, the computed control reading
for the same time was then subtracted giving
a calculated potential difference across the
blood vessel at that moment. In table 1, the
average value of the potential difference across
each blood vessel is recorded along with the
number of measurements which that average
represents. The overall average is also re-
361
AOAO M L T
MtCKJES
FIGURE 1
A line drmving of a Lucite cell winch has been
separated into tiuo half-cells. The tissue is either
aorta or vena cava which has been stripped of its
periva'scular tissue and cut lengthwise to form a
membrane. The blood vessel is placed over an
opening of 1-cm.1 area, thus separating the compartments of each half-cell.
t-test for significant deviation of the average
potential difference value from zero for each
blood vessel wall segment was carried out. It
was found both for aorta and A'ena cava that
the measurements were not significantly different from zero at the 0.1 per cent level.
Discussion
Two points require further clarification.
The first question which arises is whether
the tissues were adequately oxygenated to
allow normal metabolism. This has been discussed elsewhere,4 but in brief if one uses
Warburg's equation,0 one can compute the
oxygen requirement for aorta and vena cava:
d =
o X D/A.
For oxygenated bathing solutions on each side
of the sheet of tissue, where d is the thickness
of sheet of tissue or membrane in centimeters,
Co is oxygen tension in bathing solution in
atmospheres, D equals 1.64 X 10"8 cm.3 O2/
min./cm.- at 37 C, and A equals
8
Qoi (in mm. /hr./mg. dry weight) X mg. wet tissue
1,000 mm.ycm.8 X 60 miu./h r. X wet weight/dry weight
and wet weight/dry weight ratio is 5.0 for
corded for aorta and vena cava, together with
the artery.7 The maximum qO? for the aorta
the average deviations of these values. A
and vena cava is 0.62 and 0.59 mm.8/hr./rng./
dry weight, respectively,4'8 and the average
•Keithley Instruments, Cleveland, Ohio.
Circulation Research, Volume XI. September 196£
362
HABSHAW, ZISKDTD, MAZLEN, SAWYER
TABLE 1
Average Value of the Potential Difference Across the Aorta and Vena Cava
Aorta*
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Experiment
number
14
15
16
17
18
22
23
24
25
26
Number of
determinations
of potential
Number of
determinations
of lero
Average
potential
difference (/iv.)
Average error
of potential
difference {fiv.)
25
23
35
29
24
14
20
17
19
21
25
13
13
30
2S
26
26
38
33
30
-101
± 28
± 25
±103
± 20
± 87
± 33
± 33
± 65
± 58
± 45
+109
— 75
+ 71
+ 91
+ 32
- 23"
— 9
-137
— 69
'Polarity is given with respect to the intima.
Mean potential difference = —11.2 MV.
Average error of mean potential difference = ±71.2 Mv.; P value = 5.20. The possibility
that the mean potential difference ia different from zero equals 0.0007.
Vena cavat
Experiment
number
Number of
determinations
of potential
Number of
determinations
of xero
Average
potential
difference (jiv.)
Average error
of potential
difference ((tv.)
20
21
27
28
30
31
32
33
34
35
21
14
21
13
19
19
18
12
15
18
21
25
31
26
30
31
27
23
29
32
+ 120
+ 32
+ 87
+ 35
±21
±25
±31
±26
±30
±31
±27
±23
±29
±32
— 23
-131
- 18
— 33
+ 18
+ 32
tMenn potential difference = +11.8 /iv.
Average error of mean potential difference = ±50.7 n\r.; P value = 4.02.
The possibility that the mean potential difference is different from zero equals 0.0035.
thickness of canine aorta and vena cava is
0.10 cm. and 0.030 cm., respectively. The
weight of exposed aorta and vena cava is less
than 1.0 and 0.1 Gm., respectively. Using this
data, the partial pressure of oxygen required
to maintain this respiration is, at 38 C, 0.1570
atmospheres for aortae and 0.00143 atmospheres for venae cavae.
Substituting these values in Henry's Law
and converting the moles of O2 per liter of water at 37 C. to liters of 0 2 per liter of water at
standard temperature and pressure (S.T.P.),
the calculated partial pressure required to
oxygenate the aorta and vena cava equals
3.671 X 10-3 and 3.304 X 10-"' L. of O2
per L. water, respectively. Water saturated
with 82.5 per cent O2 at 1 atmosphere will
contain 0.7999 X Kh3 moles of O2/L., or 1.766
X 10-2 L. O2/L. water at S.T.P. Sendroy9 has
determined that plasma dissolves oxygen 0.9
as well as water. Krebs serum substitute solution undoubtedly dissolves O2 better than
plasma since it lacks the protein which interferes with oxygen solution. Therefore, Krebs
solution will contain at least 1.5984 X 10"L. O2/L. solution. This is approximately 20
and 2,000 times the amount of O2 needed for
maximum calculated respiration of aorta and
vena cava, respectively.
The second problem relates to the type of
Circulation Research, Volume XI, September
lQ6t
ELECTRICAL POTENTIAL DLFFEBENOE
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salt bridge used between the bathing solution
and the calomel cells. The type used in this
experiment contains agar and a solution very
similar to the serum substitute except that it
lacks bicarbonate and organic ions. Each of
these salt bridges forms two liquid junctions,
and these are a. potential source of error.
However, it was felt that this was more acceptable than having a KCl-type salt bridge
which has one less liquid junction and less
polarization. It was feared that the concentrated KCl would diffuse out of the salt bridge
and raise the K concentration of the bathing
solution to toxic levels.
The significance of a zero potential difference across the blood vessel has yet to be explained. It could represent lack of activity
in an inert type of tissue whose sole function
is to serve as a muscle-encased conduit for
blood. On the other hand, it may represent a
manifestation of a subtle electrochemical
mechauisni which prevents the development
of a large potential difference across itself
despite ion concentration differences, "injury
currents," etc. Recently, in a series of several hundred experiments, small net ion fluxes
of Na+ and Cl~ have been reported across a.
canine aorta and vena cava.4 Heretofore, demonstration of a net ion flux across a membrane
has been accompanied by a potential difference across that membrane. The net ion fluxes
measured by Sawyer et al.4 should give a
potential difference across the membrane of
about 50 mv. This would have been easily
detected by our potentionietric technique, and
the fact that it was not means that, despite
the differential flux, the Cl~ ion activity on
both sides of the membrane tends to remain
the same. This then suggests that some additional mechanism is operating, preventing ion
Circulation Research, Volume XI, September 196S
363
activity gradients from developing. The answer awaits further experiments now in
progress.
Summary
A technique for measuring the potential
difference across blood vessel wall in vitro is
described. It was found that the potential
difference across canine blood vessel wall in
vitro is roughly 0 ± 50 /xv. under the conditions of this experiment.
References
1. SAWYER, P. N., AND PATE, J. W.: Eloctric poten-
tial differences across the normal aorta and
aortic grafts in dogs. Am. J. Pliysiol. 175:
113, 1953.
2. SAWYER, P. N., AND PATE, J. W.: Bioelectric
phenomena as an, etiologic factor in intravascular thrombosis. Am. J. Physiol. 175: 103,
1953.
3. SAWYER, P. N., AND PATE, J. "W.: Study of
electrical potential differences across the normal aorta and aortic grafts of dogs. Bethesda,
Naval Medical Eesearch Institute, B«search
Beport, Project NM 007 0S1.10.06, vol. IT,
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8. KIRK, J. E., EFFERSOE, P. G., AND CHIANG, S. E.:
Bflte of respiration and glycolysis by human
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Electrical Potential Difference Across Blood Vessels
David H. Harshaw, Jr., Howard Ziskind, Roger Mazlen and Philip N. Sawyer
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Circ Res. 1962;11:360-363
doi: 10.1161/01.RES.11.3.360
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Copyright © 1962 American Heart Association, Inc. All rights reserved.
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