Download Biphasic Changes in Maximum Relaxation Rate during Progressive

516
Biphasic Changes in Maximum Relaxation
Rate during Progressive Hypoxia in
Isometric Kitten Papillary Muscle and
Isovolumic Rabbit Ventricle
MARTIN G. ST. JOHN SUTTON, ERIK L. RITMAN, AND NORMAN F. PARADISE
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SUMMARY We studied the effects of graded hypoxia on mechanical performance of cardiac ventricular muscle by producing controlled, stepwise decreases in partial pressure of oxygen (Po2) in the
medium bathing the kitten papillary muscle preparation and in the perfusate of the Langendorffprepared rabbit heart. For kitten papillary muscle at 30°C and with stimulation rate at 30/min,
maximum rate of contraction (+dT/dtmM) and maximum rate of relaxation (—dT/dtm.*) were 184 ± 10
inN/mm2 per sec and 162 ± 12 mN/mm2 per sec, respectively, during control conditions with Po2 at 634
± 7 mm Hg. Step decreases in Po2 from 634 mm Hg produced decreases in steady state -dT/dtmu, that
were significantly greater than corresponding decreases in +dT/dtma», except at the lowest Po2. When
Po2 (mm Hg ± SE) was 411 ± 10, 218 ± 4, and 92 ± 3, steady state +dT/dtmM vs. -dT/dt™, (expressed
as % of pre-hypoxia control value ± SE) were: 97 ± 4 vs. 85 ± 7, 76 ± 5 vs. 59 ± 6, and 47 ± 5 vs. 28 ± 4,
respectively. When the lowest Po2 of 34 ± 6 mm Hg was achieved, considerable shortening of the
duration of the mechanical cycle occurred, and values for +dT/dtm«x and —dT/dtma, (expressed as % of
pre-hypoxia control value ± SE) of 28 ± 7 and 21 ± 7, respectively, were not significantly different.
Graded hypoxia similarly affected left ventricular isovolumic pressure developed by the coronary
perfused rabbit heart. In both preparations, changes in relaxation relative to changes in contraction
during progressive hypoxia were biphasic: decreases in maximum relaxation rate were disproportionately greater than decreases in maximum contraction rate with intermediate hypoxia, but the
proportionality was restored when severe hypoxia produced a decrease in cycle duration.
Circ Res 47: 516-524, 1980
SEVERAL studies of ventricular function of intact
animals and humans seem to demonstrate that
myocardial relaxation is slowed, or impaired, in
comparison to myocardial contraction during ischemia (Barry et al., 1974; Chesebro et al., 1976;
McLaurin et al., 1973; St. John Sutton et al., 1978).
However, results from studies of the effects of hypoxia on contraction and relaxation of isolated cardiac muscle preparations seem to differ from the
results obtained from these studies on intact animals. Hypoxia produced by changing the aerating
gas composition from 95% O2-5% CO2 to 95% N2-5%
CO2 was associated with a substantial decrease in
both force development and total duration of the
mechanical cycle of isolated cardiac tissue, but the
atmosphere of 95% N2-5% CO2 apparently did not
affect rate or duration of relaxation to any greater
From the Program in Physiology, Northeastern Ohio Universities
College of Medicine, Rootstown, Ohio and the Department of Physiology
and Biophysics, Mayo Foundation and Mayo Clinic, Rochester, Minnesota.
Supported in part by the Akron District Chapter of the American
Heart Association, National Institutes of Health (NIH) Grant HL04664
and by NIH Biomedical Research Development Grant 1406.
Address for reprints: Dr. Norman F. Paradise, Program in Physiology,
Northeastern Ohio Universities College of Medicine, Rootstown, Ohio
44272.
Received January 24, 1979; accepted for publication May 2, 1980.
extent than rate or duration of contraction (Bing et
al., 1976; Nakhjavan et al., 1971; Tyberg et al., 1970;
Weisfeldt et al., 1974). There exists the possibility,
however, that these on-off changes in oxygen partial
pressure (Po2) may mask effects of intermediate
degrees of hypoxia on the contraction-relaxation
cycle.
The present study was undertaken to elucidate
the effects of varying degrees of hypoxia on contraction and relaxation of mammalian ventricular
muscle. The major aim was to determine whether
graded hypoxia affects rates of relaxation differently than rates of contraction, as appears to be the
case for the intact heart during ischemia. Graded
and controlled reductions in Po 2 of the superfusate
bathing the isolated, isometrically contracting papillary muscle were produced in a stepwise fashion,
and subsequent changes in force development and
rate of force development were recorded. Similarly,
isovolumic pressure development was recorded
from the left ventricle of the Langerdorff-prepared
rabbit heart during progressive, stepwise decreases
in perfusate Po2. Analysis of the recorded data
permitted the effects of mild, moderate, and severe
degrees of hypoxia on relaxation and contraction
processes of cardiac ventricular muscle to be compared in two different preparations.
GRADED HYPOXIA AND MYOCARDIAL RELAXATION/St. John
Methods
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Preparation
Papillary Muscles
Following the removal of hearts from chloroformanesthetized kittens (450-2200 g), the right ventricle was opened rapidly and submerged in oxygenated physiologic salt solution. Papillary muscles
were carefully excised and arranged to contract
isometrically by securing the severed tuft of the
ventricular insertion in a spring-loaded Perspex
clamp and tying the tendinous end of the muscle
directly to a glass rod which was connected to a
glass extension of an RCA 5734 mechano-electronic
transducer (manufacturer's specification for frequency response, 12,000 hertz). The muscle was
bathed in physiologic salt solution of the following
composition (mmol/liter): Na+, 135; K+, 5.0; Ca2+,
2.0; Mg2+, 1.0; Cl", 98; HC(V, 24; HPO4=, 1.0; SO4=,
1.0; CH3COO", 20; glucose, 10. An internal circulation within the bathing chamber (Blinks, 1965) was
created by bubbling gases through the solution.
During an initial 2- to 3-hour equilibration, muscle length was increased until force development
upon stimulation was maximal. Thereafter, muscle
length was kept constant. Muscles were stimulated
at 20/min during the equilibration period by unidirectional pulses applied through punctate electrodes (Blinks, 1965). Stimulus duration was 2 msec
and stimulus strength was slightly above threshold
(range: 1.0-1.5 V).
Rabbit Hearts
After heparinization (300 U/kg) of thoracotomized male rabbits (2-3 kg) under Nembutal anesthesia (40 mg/kg, iv), the aorta was cannulated and
the heart excised from the chest cavity. A small
incision in the left atrium was made to permit the
passage of a fluid-filled balloon through the mitral
valve into the left ventricle to record isovolumic
pressures. Perfusate with a composition identical to
that employed for the studies on papillary muscle
was delivered to the hearts at a constant rate of 35
ml/min (Gilson peristaltic pump, #HP16). After a
1-hour equilibration period, the atrioventricular
node was crushed and electrical pacing of the right
ventricle was initiated to maintain heart rate constant during the subsequent experimental procedures.
Procedures
Papillary Muscles
The bathing solution was aerated with combinations of gases issuing from two pressurized cylinders, one containing 95% O2-5% CO2 and the other
95% N2-5% CO2. During the 2- to 3-hour equilibration period, only the 95% O2-5% CO2 gas mixture
was employed. Oxygen partial pressure achieved in
superfusate during aeration with 95% O2-5% CO2
Suttonetal.
517
was approximately 635 mm Hg. Thereafter, the
oxygen tension in the bathing medium was reduced
in stepwise decrements by adjusting the relative
proportion of O2 and N2 bubbling through the chamber. In three experiments, each 10-minute stepwise
reduction in oxygen tension was followed by a return to control conditions for 15 minutes by restoring aeration with 95%.O2-5% CO2. The last stepwise
reduction in oxygen tension produced nearly complete anoxia as a consequence of aeration with the
95% N2-5% CO2 gas mixture only. Oxygen partial
pressure in superfusate during aeration with this
oxygen-deficient gas mixture was approximately 30
mm Hg. In a second series of 28 experiments, the
graded reductions in oxygen tension were produced
at 10-minute intervals without intervening periods
of reoxygenation. In a third series of six experiments, mechanical function was studied for prolonged periods during exposure to 95% O2-5% CO2
only. In a fourth series of four experiments, oxygen
tension was reduced to approximately one-third of
control in a single step and subsequent changes in
mechanical function were studied for 70 minutes. In
a fifth series of four experiments, the effects on
mechanical function of seven step decreases in oxygen tension to approximately one-third of control
with six intervening periods of reoxygenation were
evaluated.
The effect of decreasing oxygen tension on papillary muscle mechanical performance was studied
at several stimulation rates (range: 20-60/min) with
temperature at 30 ± 0.1 °C. Oxygen tension and pH
were measured using a Radiometer Copenhagen
BMS3 Mkll microsystem and a PHM73 pH/gas
monitor. Solution pH of 7.45 remained constant
throughout all experimental procedures and was
independent of the composition of the aerating
gases. After a change in composition of the aerating
gases, equilibration of the gases with the bathing
solution occurred within approximately 75 seconds.
Tension (T) developed by the muscle and the
rate of change of tension (dT/dt), obtained by electronic differentiation of the tension signal, were
recorded continuously on paper at 2.5 mm/min
(Electronics for Medicine, Inc.). The frequency response of this recording system was flat for frequencies up to 100 hertz. Additionally, photographic
records of tension traces were obtained from a
Tektronix 7613 storage oscilloscope. Measured parameters included: (1) peak developed tension (PT),
(2) time-to-peak tension (TTP), (3) maximum rate
of contraction (-t-dT/dtw,), (4) maximum rate of
relaxation (—dT/dtmax), and (5) one-half relaxation
time (V2RT), the time during which tension fell from
its peak to a value midway between peak and resting levels.
At the end of each experiment, muscle length
and weight were measured. Cross-sectional area
(wr2) and diameter (2r) of each muscle were computed assuming a cylindrical shape and a specific
CIRCULATION RESEARCH
518
gravity of unity. Peak developed tensions were normalized by muscle cross-sectional areas and were
expressed in units of milli-Newtons per mm2 (mN/
mm 2 ). Mean diameter (mm ± SE) for all muscles
studied was 0.84 ± 0.03.
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Rabbit Hearts
Immediately prior to its arrival at the heart,
perfusate was passed through a membrane oxygenator (Travenol Laboratories, Inc., #5M0321) where
equilibration with gases was achieved. Step decreases in perfusate Po 2 were achieved in five hearts
by adjusting the proportion of O2 and N2 passing
through this gas exchange system. Progressive step
decreases in P02 were produced without intervening
periods of reoxygenation and each decrease in Po 2
was maintained for 15 minutes instead of 10 minutes. Po 2 of arterial perfusate was not measured
during the course of each experiment because the
removal of samples of arterial perfusate for analysis
would have necessitated a temporary reduction of
perfusate flow to the heart. However, at the termination of each experiment, the heart was removed
from the arterial line and the experimental protocol
was repeated so that the arterial perfusate Po2's
could be measured. Isovolumic pressure development of the left ventricle was recorded (Gould
Recorder, #2600) throughout the experimental procedures with temperature at 30°C. The frequency
response of the catheter-transducer (Gould Statham P23)-recording system was evaluated by producing full scale (50-mm) sinusodial pressure fluc-
VOL. 47, No. 4, OCTOBER 1980
tuations with a Multifunction Pressure Generator
(model MPG-30, Millar Instrument, Inc.) Changes
in recorded pulse amplitude were within 5% for
frequencies up to 13 hertz.
Statistical Analyses
Analysis of variance for single-factor experiments
having repeated measures and for two-factor experiments having repeated measures were employed,
and Newman-Keuls tests were used to assess the
statistical significance of differences between individual pairs of means (Winer, 1971).
Results
Effects of Graded Hypoxia on Mechanical
Function of Papillary Muscle
The changes in mechanical function of the isometric papillary muscle following a step decrease in
Po2 are illustrated in Figure 1. Mechanical function
of this muscle stabilized within 10 minutes of the
step decrease in Po2. The transient increases in PT,
-1-dT/dtmax, and —dT/dtmax occurring immediately
after the decrease in Po 2 and shown in Figure 1
were not always observed.
Photo-oscillographic recordings of steady state
beats following step decreases in P02 with intervening periods of reoxygenation are shown in Figure 2
for selected procedures from a single experiment.
There is a direct relationship between PT and Po2,
with greater decreases in PT being associated with
larger step decreases in Po2. It is apparent from the
180
46
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160
42
Q "V
Uj g
O \ 38
140
\
^*
Q
>
-J ^
Uj £
:*.
120
UJ
Q
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34
Trace 2
Uj
100
•3
f\
Tract 3
Po2 - §50
146 mm Hg
8
10
MINUTES
1 Transient changes in papillary muscle performance following a step decrease in Po2- A decrease in Po2
from 650 to 146 mm Hg (shown at time zero) produced a transient increase and subsequent decline in peak developed
tension (PT), maximum rate of contraction (+dT/dtTnaj), and maximum rate of relaxation (—dT/dtmaJ. Note the
proportionately greater effect of this level of hypoxia on —dT/dtmal than on either PT or +dT/dtmsiI. Arrows labeled
Trace 1, Trace 2, and Trace 3 designate times at which photographic traces of beats 1, 2, and 3, respectively, of inset
were recorded. Temperature was 30°C and stimulation rate was 50/min. Muscle diameter was 0.66 mm.
FIGURE
GRADED HYPOXIA AND MYOCARDIAL RELAXATION^. John Sutton et al.
519
in Figures 3-6. The modulating effects of Po 2 on
maximum
rate of relaxation and maximum rate of
02
Test
contraction are shown in Figure 3. Progressive decreases in Po 2 produced proportionately greater
decreases in steady state — dT/dtma* as compared to
+dT/dtmax except at the lowest Po2.
In contrast to the disproportionate effects of
graded hypoxia on -f-dT/dtmax and — dT/dtmax, progressive decreases in Po 2 produced nearly proportional changes in steady state PT and +dT/dtmax
over the entire range of Po2's studied (Fig. 4). Figure
5 shows the effects of progressive, step reductions
in Po2 on both steady state peak developed tension
and time-to-peak tension (TTP). Superimposed
tracings of steady state beats recorded from a single
muscle are shown in the inset of Figure 5. It is
apparent from this set of tracings that decreases in
Po 2 did not affect the total duration of the contraction-relaxation cycle until measured Po2 reached 29
mm Hg. At this lowest Po2, there was a considerable
shortening in the total duration of the mechanical
FIGURE 2 Effect on tension twitches of progressive recycle (lowest trace in inset of Figure 5). This obserductions in oxygen tension with intervening 15-minute
vation is representative of all of the experimental
periods of reoxygenation. Upper traces are steady state
observations of this study. The duration of the
control beats recorded at the end of 15-minute periods of
reoxygenation with 95% O2-0% N2-5% CO?.. Lower traces mechanical cycle decreased substantially only when
an atmosphere of 95% N2-5% CO2 was used to aerate
are steady state test beats recorded 10 minutes after
the bathing medium.
each step decrease in Po2. Partial pressures of oxygen
(mm Hg) measured during the control and test periods
The data in Figure 6 show that steady state one(Po2 control/Po 2 test) are reported to the right of each
half relaxation time (V&RT) was unaltered by
panel. Peak developed tension (mN/mm2) of control
changes in Po 2 between 634 ± 7 mm Hg (V&RT =
beats (upper traces) vs. test beats (lower traces) were 56.2
236 ± 30 msec) and 92 ± 3 mm Hg (V2RT = 240 ±
vs. 48.6 (panel A), 55.7 vs. 42.7 (panel B), 54.8 vs. 28.6
21 msec). However, there was a significant decrease
(panel C), and 54.3 vs. 21.6 (panel D). Experimental
in '/2RT when Po 2 was 34 ± 6 mm Hg. This decrease
conditions: temperature, 30° C; stimulation rate, 30/min;
was related to the considerably shortened duration
muscle, diameter, 0.93 mm.
of the mechanical cycle associated with the lowest
achievable Po2.
Data from experiments performed to assess the
data in Figure 2 that decreases in TTP were assostability of the preparation in the absence of deciated with the decreases in Po2. However, the total
creases in Po 2 are reported in row 1 of Table 1. The
duration of the contraction-relaxation cycle was
6% increase in tension development after 70 minutes
affected minimally by the changes in Po 2 shown in
indicated that the changes in mechanical function
this figure. Furthermore, the effects of 10-minute
of the papillary fibers during the 70 minutes of
intervals of hypoxia, whether mild, moderate, or
progressive hypoxia (Figs. 3-6) were attributable to
severe, are nearly completely reversible, as indithe decreases in Po2 only, and not to a spontaneous
cated by the restoration of tension development
deterioration of the preparation. Changes in PT
(upper trace in each panel of Figure 2) during each
following the onset of a sustained, constant level of
15-minute period of reoxygenation with the 95% O2hypoxia and repetitive bouts of hypoxia to the same
0% N2-5% CO2 gas mixture. The same sets of
level with intervening periods of reoxygenation are
changes in mechanical function were observed
shown in rows 2 and 3, respectively, of Table 1.
when progressive hypoxia was produced without
Overall, papillary muscle function remained relaintervening periods of reoxygenation (compare Figtively stable under these experimental conditions.
ure 2 with inset in Figure 5). The effects of graded
In contrast, decreases in PT during progressive step
hypoxia on mechanical function were independent
decreases in Po 2 (Figs. 4 and 5) were considerably
of the protocol employed. Therefore, most of the
larger. Data in Figure 4 show that PT was 87 ± 6,
experiments performed in this study followed the
73 ± 6, and 54 ± 6% of pre-hypoxia control when
protocol for producing progressive hypoxia without
Po 2 was 317 ± 8, 218 ± 4, and 131 ± 3 mm Hg,
intervening periods of reoxygenation.
respectively. Thus, the decreases in mechanical
The effects of graded hypoxia on mechanical
function reported in Figures 3-6 can be attributed
function of seven papillary muscles (mean equivaalmost exclusively to the decreases in Po 2 and not
lent diameter, 0.84 ± 0.07 mm) stimulated at 30/
to long-term, cumulative effects of hypoxia.
min and with temperature at 30°C are summarized
P
02
P
Control
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CIRCULATION RESEARCH
520
VOL. 47, No. 4, OCTOBER 1980
100
100
I
c
o
O
+
a
\
dT/dtmat
50
50
a
x
O
*P < 0.05
Mean ± SE
\
o
200
400
600
mmH
9
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3 Effects of stepwise decreases in Po2 on maximum rate of contraction and maximum rate of relaxation of
isometric papillary muscle. Decreases in P02 were produced without intervening periods of reoxygenation. Mean steady
state values (expressed as % of pre-hypoxia control ± SE) of maximum rate of contraction, +dT/dtma%, and maximum
rate of relaxation, —dT/dtmal, are plotted against mean P02. Standard errors of mean values of P02 are shown by the
horizontal bars. During control conditions with mean Po2 of 634 ± 7 mm Hg, mean +dT/dtmal and mean —dT/dtmta
were 184 ± 10 and 162 ± 12 mN/mm2 per sec, respectively. Decreases in Po2 produced significant decreases in both
+dT/dtmal. and —dT/dtmal (P < 0.001). Furthermore, decreases in —dT/dtmBX were significantly greater than decreases
in +dT/dtma% (P < 0.001), and the interaction between Po2 and ±dT/dtmaI was significant (P < 0.001). Mean values of
+dT/dtmBI designated by asterisks (*) are significantly greater than corresponding mean values of —dT/dtma% (P <
0.01). These data show that decreases in —dT/dtmaz are larger than corresponding decreases in +dT/dtmal at
intermediate Po2's but not at the lowest achievable Po2.
FIGURE
Effects of Graded Hypoxia on Mechanical
Function of Rabbit Left Ventricle
The tracings in Figure 7 illustrate the effects of
step decreases in perfusate Po 2 on isovolumic pressure developed by the left ventricle of the rabbit
lOOr
heart. Decreases in perfusate Po 2 produced decreases in both peak left ventricular pressure development and time-to-peak pressure development.
Substantial shortening of the duration of the mechanical cycle occurred only at the lowest Po 2 studied. Data from five experiments are summarized in
nlOO
Control
Q 2
3*
o
50
50
• Peak Tension
o
+dT/dlmo,
UJ
0
O
Mean ± SE
UJ
600
200
400
PQ
mm Hg
FIGURE 4 Comparison of tension development and rate of tension development during stepwise reductions in PO2Mean steady state values ofpeak developed tension (PT) and maximum rate of contraction (+dT/dtma%) are expressed
as % of pre-hypoxia control at each P02 level studied. During control conditions with P02 at 634 ± 7 mm Hg, PT was
46.7 ± 3.2 mN/mm2 and +dT/dtmiI was 184 ±10 mN/mm2 per sec. Standard errors of mean values of P02 are not
shown, but are the same as in Figure 3.
GRADED HYPOXIA AND MYOCARDIAL RELAXATION/S*. John Sutton et al.
521
600
Peak Tension
40
Time-to-Peak Tension
$
1>-
400
rn
620
488
388
298
217
138
98
29
20
^
200
3
(6
600
400
200
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" o 2 . mm Hg
5 Effects of step decreases in P02 on peak developed tension (PT) and time-to-peak tension development
(TTP). TTP decreased only moderately with step decreases in Po2 between 634 ± 7 and 218 ± 4 mm Hg, but thereafter
the decreases were pronounced. Inset: Superimposed tracings of steady state beats illustrating relation between tension
development and Po2. Po2's associated with tracings are listed to the right. Upper trace was recorded with Po2 at 620
mm Hg, and progressively lower traces were recorded at correspondingly lower Po2's. Data shown in the inset were
obtained from a muscle with diameter of 0.88 mm.
FIGURE
Table 2. Decreases in developed pressure (row b)
and time-to-peak pressure development (row d)
during graded hypoxia were similar to the decreases
in PT and TTP observed in the papillary muscle
during graded hypoxia (compare rows b and d of
Table 2 with Figure 5). Changes in +dP/dtmax and
—dP/dtmax are reported in rows e and f. Progressive
hypoxia produced significant decreases in both
4-dP/dUax and -dP/dt m a x (P < 0.001). In addition,
decreases in —dP/dtmax were significantly greater
than decreases in +dP/dt max (P < 0.05) and the
interaction between Po 2 and ±dP/dtmax was significant (P < 0.001). Differences between corresponding values of -l-dP/dtmax and -dP/dtmax were statistically significant when Po 2 (mm Hg) was 313 ± 10
r
FIGURE 6 Effect of stepwise reductions in Po2 on onehalf relaxation time (lART). xkRT of 162 ± 7 msec
achieved at the lowest Po2of34±6 mm Hg was significantly less than mean values of lhRT obtained at each
of the other Po2's (P < 0.01). No other pairs of mean
values were found to be statistically significantly different. Therefore severe hypoxia, but not intermediate hypoxia, produced a significant shortening of V2RT.
(column 4, row g), 220 ± 7 (column 5, row g) and
135 ± 6 (column 6, row g), but not statistically
significant at the lowest Po2 (column 7, row g).
Thus, —dP/dtmax was depressed to a greater extent
than +dP/dtmax by intermediate degrees of hypoxia,
but the proportionality between —dP/dtmax and
-t-dP/dtmax was restored during severe hypoxia.
Comparison of the data shown in Figure 7 and
Table 2 with the data shown in Figures 3-6 demonstrates that step decreases in perfusate Po 2 produced effects on mechanical performance of the
isolated, coronary perfused rabbit heart which resembled closely the effects of decreases in superfusate P02 on mechanical performance of the isolated,
superfused papillary fiber.
Discussion
Step reductions in superfusate Po 2 from approximately 650 to 75 mm Hg were associated with
several characteristic alterations in the mechanical
function of the kitten papillary fiber. A decrease in
steady state PT accompanied each step decrease in
Po2. TTP also decreased with step decreases in Po2,
although the duration of the contraction-relaxation
cycle remained nearly invariant in this Po 2 range.
Thus, since cycle duration remained constant but
TTP decreased, there was a decrease in the duration of the upstroke, or contraction phase, and a
concomitant increase in the duration of the downstroke, or relaxation phase, of the cycle. The disproportionately greater effects of decreases in Po 2
on -dP/dtmax than on -1-dP/dtmax (Fig. 3) are a
reflection of this set of changes in mechanical function. These findings were observed consistently in
CIRCULATION RESEARCH
522
VOL. 47, No. 4, OCTOBER 1980
TABLE 1 Stability of Isolated, Superfused Papillary Muscle Preparation
Peak tension development (% of control) at
Condition
40 min
70 min
Omin
lOmin
1. Constant Po 2 of 632 ± 5 mm Hg
100
102 ± 1
2. Sustained decrease in Po 2 from
636 ± 8 to 226 ± 8 mm Hg
(n= 4)
100
78 ± 8
73 ± 10
71 ± 9
3. Ten-minute episodes of hypoxia
(Po2 = 232 + 4 mm Hg) with
intervening 15-minute periods of
reoxygenation (Po2 = 626 ± 10
mm Hg) (re = 4)
100
84 ± 6
81 ± 6
73 ± 7
105 ± 1
106 ± 2
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Control values for peak tension development (mN/mm2 ± SE) for conditions 1, 2, and 3 were 40.5 ± 4.2, 51.2 ±
6.3, and 40.2 ± 8.4, respectively. Values reported in row 3 were measured after the first (10-minute), fourth (40minute), and seventh (70-minute) step decrease in Po2. Tension development during the reoxygenation period
immediately preceding the seventh step decrease in Po 2 (70 minutes, row 3) of 39.5 ± 7.8 mN/mm2 was not
statistically significantly different than tension development immediately preceding the first step decrease in Po2
(0 min) of 40.2 ± 8.4 mN/mm2. Experimental conditions: stimulation rate, 30/min; temperature, 30°C; mean muscle
diameters (mm ± SE): (1) 0.88 ± 0.13, (2) 0.90 ± 0.11, and (3) 0.74 ± 0.06.
both the kitten papillary fiber preparation and in
the coronary perfused rabbit heart. These data
indicate that, compared to contraction, myocardial
relaxation processes are apparently impaired to a
greater extent with moderate reductions in oxygen
supply.
Under conditions produced by aeration of the
superfusate or perfusate with 95% N2-5% CO2 (measured Po 2 of approximately 30 mm Hg), the duration of the contraction-relaxation cycle shortened
considerably in both kitten papillary muscle and
coronary perfused rabbit heart preparations. Under
150
120/min, 30° C
PO2
628
3E
100
SEUJ
>%
50
LUSF
400 msec
FIGURE 7 Relation between perfusate Po2 and steady
state isovolumic pressure developed by the left ventricle
of rabbit heart. Peak steady state pressure development
(systolic pressure minus diastolic pressure) during control with perfusate Po2 at 628 mm Hg was 106 mm Hg
(upper trace) and with perfusate Po2 at 36 mm Hg was
19 mm Hg (lower trace). Heart rate was maintained
constant throughout the experimental procedures by
electrical stimulation of the right ventricle. Not shown
by these superimposed tracings are increases in diastolic
pressure of 4 and 11 mm Hg when P02 was 140 and 36
mm Hg (lower two traces), respectively.
these conditions of lowest Po2, V2RT of kitten papillary muscle twitches decreased significantly and
values of +dT/dtmax and —dT/dtmax, when compared to their respective control values, were not
statistically significantly different (Fig. 3). Changes
in +dP/dtmax and —dP/dtmax following the onset of
aeration of rabbit heart perfusate with 95% N2-5%
CO2 (rows e-g in column 7 of Table 2) were similar
to the corresponding changes in +dT/dt,nax and
—dT/dtmax recorded from kitten papillary muscle.
Severe hypoxia did not appear to have the disparate
effects on contraction and relaxation processes that
were observed when tissues were studied at slightly
higher Po2's. Thus, the effects of aeration with 95%
N2-5% CO2 on mechanical function of two different
heart muscle preparations at 30°C shown in this
study and in previous studies at about 30° C (Bing
et al., 1976; Nakhjavan et al., 1971; Parmley and
Sonnenblick, 1969; Tyberg et al., 1970; Weisfeldt et
al., 1974) are not representative of the sets of
changes occurring in mechanical performance of
the myocardium in response to decrements in P02
ranging between approximately 650 and 75 mm Hg.
However, the pattern of myocardial relaxation following onset of severe hypoxia may be temperature
dependent. Frist and coworkers (1978) studied the
kitten papillary fiber stimulated at a rate of 12/min
and found that a step change in aeration from 95%
O2-5% CO2 to 95% N2-5% CO2 produced a shortening
of V6RT at 29°C, which is similar to the findings of
the present study, but a prolongation of M>RT was
observed at 38°C. There exists the possibility that
intermediate degrees of hypoxia affect contraction
and relaxation processes in the normothermic range
differently than at 30°C.
Delivery of oxygen to the cells of the cylindrically
shaped papillary muscle occurs by diffusion from
the external solution. According to the formulation
of Hill (1928), total tension development by the
papillary muscle under a given set of experimental
GRADED HYPOXIA AND MYOCARDIAL RELAXATION/St. John Sutton et al.
523
TABLE 2 Effect of Step Decreases in Perfusate Oxygen Partial Pressure (P02) on Mechanical Function of Rabbit
Left Ventricle (30° C, 120/min, n = 5)
a. P02 (mm Hg)
b. Developed pressure
1
2
3
4
5
6
7
627 ± 14
110 + 5
528 ± 13
100 ± 6
414 ± 10
85 ± 7
313 ± 10
68±5
220 + 7
50±5
135 ±6
34 ±4
31 + 8
20 + 3
9± 1
8± 1
8± 1
8± 1
10 + 2
13 ±4
20 ± 7
179 + 7
179 + 6
89 ±6
89 ± 3
173 ± 5
77 ± 5
73 ± 5
NS
151 ± 7
53 ± 3
38±3
P < 0.01
130 ± 6
41 ±4
26 ±2
P < 0.01
111 ± 7
25 + 4
19 + 2
NS
162 ± 5
66 ± 5
55 ±2
P < 0.01
(mm Hg)
c. Diastolic pressure
d.
e.
f.
g-
(mm Hg)
TTP (msec)
+dP/dU» (%)
- d P / d U « (%)
Statistical significance
(row e vs. row f)
100*
lOOf
NS
NS = not significant.
• Mean pre-hypoxia control value was 1132 + 105 mm Hg/sec.
f Mean pre-hypoxia control value was 657 ± 25 mm Hg/sec.
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conditions may be considered to be the sum of the
force developed by the fraction of cells contracting
anaerobically within the core of the muscle and the
force developed by the fraction of cells which are
oxygenated adequately and contracting aerobically
peripherally. To test the extent to which this twocompartment model might be applicable to the
interpretation of the data of the present study, the
effects of progressive decreases in Po 2 on mechanical function of the left ventricle of the capillary
perfused rabbit heart were investigated. P02 modulation of left ventricular isovolumic pressure development was found to be similar to the P02 modulation of papillary muscle isometric tension development. Since diffusion distances for oxygen in the
capillary perfused heart preparation presumably
correspond to intercapillary distances and ordinarily are considerably snorter than distances for oxygen diffusion in the isolated papillary muscle, it is
unlikely that the decline in force development of
the papillary muscle results simply from changes in
the relative contribution of an anaerobic set of cells
and an aerobic set of cells. The two-compartment
model emerging from Hill's formulation (Hill, 1928)
may not, therefore, completely explain the modulating effects of Po2 changes on papillary muscle
function that were observed in the present study.
This conclusion is in harmony with data reported
by Frezza and Bing (1976) which showed that force
development by the rat papillary muscle was modulated by changes in Po 2 between 550 and 450 mm
Hg, even though there was no evidence for the
existance of an anaerobic core within the fiber in
this range of values of P02.
Mean diameter of all papillary muscles used in
the present study was 0.84 ± 0.03 mm. Most of the
muscles with diameters less than 0.84 mm did not
exhibit decreases in PT, +dT/dtmax, or -dT/dtma*
following the first step decrease in Po 2 from about
650 to 550 mm Hg. Therefore, these thinner muscles
were likely to have been oxygenated adequately in
the control state during aeration with 95% O2-5%
CO2 gas mixture. Muscles with diameters exceeding
0.84 mm generally exhibited decreases in PT, + d T /
dtmax, and —dT/dWax after the first step decrease in
P02. The decrease in mechanical function of these
thicker muscles associated with the first step decrease in P02 suggests that they may not have been
oxygenated adequately during aeration with 95%
02-5% CO2. However, the responses of these thicker
muscles to progressive hypoxia were the same as
the responses of the thinner muscles, viz., there was
a progressive shortening of the duration of contraction phase and a simultaneous lengthening of the
duration of relaxation phase. Thus, although an
anaerobic core may have been present in some of
the thicker muscles under control conditions, its
presence did not appear to affect the mechanical
responses to step reductions in P02, except that the
changes in mechanical function occurred at higher
Po2's in the thicker muscles, compared to the thinner muscles.
The effects of graded hypoxia on mechanical
function of cardiac ventricular muscle observed in
the present study may be related, in part, to decreases in the duration of the action potential
(McDonald and MacLoed, 1971) with subsequent
decreases in the levels of calcium stored at release
sites within the cardiac cell (Wood et al., 1969).
Additionally, altered calcium transport by sarcoplasmic reticulum (Lee et al., 1975) may explain
partially the observed mechanical responses to
graded hypoxia. Since, however, the sets of biochemical changes associated with varying degrees
of hypoxia are complex, these and other possible
explanations remain speculative.
Acknowledgments
We greatly appreciate the advice and assistance of Dr. George
S. Malindzak, Jr., Scott Shorten, and Fred M. Wolf during the
course of this study.
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Angiotensin II Increases Electrical Coupling
in Mammalian Ventricular Myocardium
KENT HERMSMEYER
SUMMARY Electrical measurements of current flow in ventricular myocardium immersed in silicone
oil showed that angiotensin II increases the cell-to-cell spread of current within seconds. The increases
in current spread and conduction velocity occur without any changes in resting membrane potential or
maximum rate of rise of the action potential. The concentration range was 10 nM to 10 /IM, with an ED50
of 100 nM for angiotensin exposures lasting about 10 seconds. The largest effects were an apparent
decrease in resistance through the cellular pathway to 50% of control and a 40% increase in conduction
velocity, which returned to control in about 15 minutes. Continuous or repeated exposure to angiotensin
caused desensitization to appear. These effects were found with or without denervation by 6-hydroxydopamine and /?-adrenergic blockade by 1 J»M propranolol in calf, pig, sheep, and rabbit ventricular
myocardium. Therefore, angiotensin appears to increase electrical conduction rapidly and directly in
cardiac muscle by decreasing resistance through the cellular pathway. Circ Res 47: 524-529, 1980
THE EFFECT of angiotensin II on the mammalian
ventricular myocardium is to increase maximum
tension development (Koch-Weser, 1964). This positive inotropic effect is direct, rather than resulting
from norepinephrine release (Fowler and Holmes,
1964; Koch-Weser, 1965). Unlike norepinephrine,
From the Physiologisches Institut, Universitat Bern, Bern, Switzerland.
Address for reprints: Dr. K. Hermsmeyer, Department of Pharmacology, University of Iowa BSB, Iowa City, Iowa 52242.
Supported by Grant HL 16328 and Research Career Development
Award HL00O73 from the National Institutes of Health, by the Roche
Research Foundation for Scientific Exchange and Biomedical Collaboration with Switzerland, and by the Schweizerische Stiftung fur Kardiologie.
Received December 17, 1979; accepted for publication May 8,1980.
angiotensin II does not induce arrhythmias, suggesting differences in the mechanism of action
(Koch-Weser, 1964). There is evidence to suggest
that angiotensin II can cause increased contraction
and a prolongation of the cardiac action potential
by enhancement of calcium influx under certain
conditions (Freer et al., 1976).
This report suggests another direct action of
angiotensin (II) on cardiac muscle. It has been
found (unpublished observations) that angiotensin
improves the synchrony of contraction of large
sheets of interconnected myocardial cells in tissue
culture. The synchrony was increased by a decrease
in coupling resistance that averaged 2-fold at its
Biphasic changes in maximum relaxation rate during progressive hypoxia in isometric kitten
papillary muscle and isovolumic rabbit ventricle.
M G Sutton, E L Ritman and N F Paradise
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Circ Res. 1980;47:516-524
doi: 10.1161/01.RES.47.4.516
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