Download Heart rate modulates the slow enhancement of contraction due to

Am J Physiol Heart Circ Physiol
280: H2136–H2143, 2001.
Heart rate modulates the slow enhancement of
contraction due to sudden left ventricular dilation
PAULO JOSÉ FERREIRA TUCCI, NEIF MURAD, CLEVER LAND ROSSI,
ROBERTO JANZON NOGUEIRA, AND ORLANDO SANTANA, JR.
Department of Physiology, Universidade Federal de São Paulo, Escola Paulista
de Medicina, CEP 04023-900 São Paulo, Brazil
Received 25 September 2000; accepted in final form 29 November 2000
length-dependent activation; slow force response; Bowditch
effect; intact canine heart
HAS BEEN PREVIOUSLY REPORTED that contractile
strengthening due to a sudden myocardial stretch is
followed by a time-dependent enhancement of performance during the next few minutes. This pattern of
response to sudden stretch is essentially the same for
intact ventricles (32, 33, 44, 45) and unicellular (20, 47)
or multicellular (1, 4, 6, 10, 14, 21, 24, 26, 38, 42)
preparations and has been described for dog (32, 33, 44,
45), rat (4, 6, 20, 24), cat (1, 14, 26, 38), rabbit (10),
guinea pig (42, 47), and ferret (5, 12, 21) myocardium.
These data support the concept that the biphasic myocardial force response to stretch is a phenomenon of
cellular origin common to different mammalian species.
IT
Address for reprint requests and other correspondence: P. J. F.
Tucci, Rua Estado de Israel, 181/94, CEP: 04022-000 São Paulo,
Brazil (E-mail: [email protected]).
H2136
It is admitted that the immediate heightening of
contraction after a myocardial stretch is based on an
increase in myofilament Ca2⫹ sensitivity and potentiation of maximum Ca2⫹-activated force due to changes
in the myofibrillar force-Ca2⫹ relationship (4, 13, 18,
20, 24, 25, 28). Changes in myofilament Ca2⫹ responsiveness seem to be excluded as contributors to the
slow increase in contraction (13, 20, 24).
Time-dependent contraction strengthening seems to
entirely depend on an intracellular Ca2⫹ concentration
([Ca2⫹]i) increase (3, 13, 20, 21, 24). The mechanism of
[Ca2⫹]i increase during the slow response is still under
debate. Sufficient evidence has been accumulated to
rule out any relevant contribution of the sarcoplasmic
reticulum to the slow response (10, 13, 20, 24). Some
reports favor the idea that [Ca]i gain depends on L-type
Ca2⫹ channel transit intensification (1, 16, 24, 45);
however, this possibility was contested by others (14,
33). Participation of stretch-activated ion channels in
the [Ca]i elevation has been suggested (5, 16, 17, 30, 43,
48), and this possibility appears not to have been sufficiently tested until now. Todaka et al. (44), in isovolumic canine left ventricles, and Calaghan et al. (12), in
ferret papillary muscles, described a striking parallelism between [Ca2⫹]i and cAMP after myocardial
stretch, suggesting that changes in [Ca2⫹]i may be
primarily related to an increase in cAMP. A primary
change of intracellular Na⫹ concentration ([Na⫹]i) determining the increase of [Ca2⫹]i was previously theoretically predicted by Bluhm et al. (11). Alvarez et al.
(6), studying rat ventricular trabeculae, reported a
stretch-elicited intramyocardial paracrine mechanism,
by which sequential activation of ANG II, endothelin,
and Na⫹/H⫹ exchanger elevates [Na⫹]i. A [Ca2⫹]i increase should occur after the final action of the Na⫹/
Ca2⫹ exchanger.
A number of fundamental points related to the
[Ca2⫹]i gain elicited by stretch require more investigation for an unbiased understanding of the intimate
Ca2⫹ transit changes evoked by stretch. Accordingly,
there is no definitive evidence about whether 1) Ca2⫹
gain occurs during diastole, systole, or both, 2) L-type
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0363-6135/01 $5.00 Copyright © 2001 the American Physiological Society
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Tucci, Paulo José Ferreira, Neif Murad, Clever Land
Rossi, Roberto Janzon Nogueira, and Orlando Santana, Jr. Heart rate modulates the slow enhancement of
contraction due to sudden left ventricular dilation. Am J
Physiol Heart Circ Physiol 280: H2136–H2143, 2001.—In
isovolumic blood-perfused dog hearts, left ventricular developed pressure (DP) was recorded while a sudden ventricular
dilation was promoted at three heart rate (HR) levels: low (L:
52 ⫾ 1.7 beats/min), intermediate (M: 82 ⫾ 2.2 beats/min),
and high (H: 117 ⫾ 3.5 beats/min). DP increased instantaneously with chamber expansion (⌬1DP), and another continuous increase occurred for several minutes (⌬2DP). HR
elevation did not alter ⌬1DP (32.8 ⫾ 1.6, 33.6 ⫾ 1.5, and
34.3 ⫾ 1.2 mmHg for L, M, and H, respectively), even though
it intensified ⌬2DP (17.3 ⫾ 0.9, 20.7 ⫾ 1.0, and 26.8 ⫾ 1.2
mmHg for L, M, and H, respectively), meaning that the
treppe phenomenon enhances the length dependence of the
contraction component related to changes in intracellular
Ca2⫹ concentration. Frequency increments reduced the half
time of the slow response (82 ⫾ 3.6, 67 ⫾ 2.6, and 53 ⫾ 2.0 s
for L, M, and H, respectively), while the number of beats
included in half time increased (72 ⫾ 2.9, 95 ⫾ 2.9, and 111 ⫾
3.2 beats for L, M, and H, respectively). HR modulation of the
slow response suggests that L-type Ca2⫹ channel currents
and/or the Na⫹/Ca2⫹ exchanger plays a relevant role in the
stretch-triggered Ca2⫹ gain when HR increases in the canine
heart.
HEART RATE AND SLOW FORCE RESPONSE
METHODS
Experiments were performed on isolated hearts from male
dogs weighing 12–16 kg that were supported by the arterial
blood of a second dog. The support dog (19–23 kg) was
anesthetized with morphine hydrochloride (2 mg/kg im),
chloralose (60 mg/kg iv), and urethane (600 mg/kg iv), heparinized (500 U/kg iv), and mechanically ventilated. Maintenance doses of chloralose (6 mg/kg iv) plus urethane (60
mg/kg iv) and heparin (50 U/kg iv) were administered hourly.
Only data from experiments in which the mean arterial
pressure of the support dog remained stable at ⬎80 mmHg
were utilized. Arterial blood pH and gases were periodically
analyzed and corrected as needed (pH ⫽ 7.35–7.45, PO2 ⬎
100 mmHg, PCO2 ⫽ 35–45 mmHg) by addition of bicarbonate
or adjustment of ventilation.
A roller pump circulated the blood from the left femoral
artery of the support dog to a reservoir to perfuse the isolated
heart through a cannula fixed in the ascending aorta. The
reservoir was placed at a height sufficient to maintain a
perfusion pressure of 100 mmHg. The blood temperature,
measured in the perfusion cannula, was maintained at 37°C
by means of a heat exchanger. Coronary venous and thebesian flow returned from the isolated heart to the support dog
through catheters inserted into the pulmonary artery and
the apex of the left ventricle. Electrodes for epicardial electrocardiograms were sewn to the right ventricle.
Total atrioventricular block was induced by injecting 10%
formalin (0.6–2.0 ml) into the region of the atrioventricular
node through a right atrial incision, and the atriostomy was
closed by a purse-string suture. This procedure was performed during temporary occlusion of the coronary perfusion
line; cardiac ischemia occurred for only 50–170 s during this
period. Electrodes for artificial pacing were inserted into the
right ventricular anterior wall, and an artificial stimulator
was used to pace the heart at the lowest frequency that
maintained a regular rhythm. The mitral apparatus was
excised, and a soft distensible latex balloon mounted on a
cannula (18 mm diameter) was placed in the left ventricle. A
purse-string suture at the base of the left atrium was tightened around the cannula and held the balloon inside the left
ventricular cavity. The balloon was sufficiently compliant,
and its size was large enough so as not to contribute to
pressure over the range of volumes studied. A rubber stopper
traversed by two polyethylene catheters occluded the distal
end of the mitral cannula. One catheter (15 cm long, 0.3 cm
diameter) was attached to a P23 ID pressure transducer, and
the other (15 cm long, 0.3 cm diameter) was connected to a
stopcock that allowed the control of the amount of saline
inside the ventricular balloon. Left ventricular pressure was
determined by means of the transducer signal conditioner
(model 13-6615-50, Gould) of a Windograf recorder. Left
ventricular developed pressure (DP, i.e., systolic pressure
minus diastolic pressure) was used for the analysis of ventricular performance. Perfusion pressure, measured at the
level of the left ventricular midlevel cavity, was continuously
monitored.
Protocol. Eleven preparations were studied. After the
equilibration period (30 min), the deflated volume of the
balloon was determined for each heart. The deflated volume
was defined as the minimal volume of saline in the balloon
that allowed a stable level of diastolic pressure and a positive
pressure without artifacts during contraction. The deflated
volume was associated with a negative diastolic pressure in
all cases. In each study, left ventricular pressure was recorded while the balloon was at the deflated volume and after
sudden expansion until the highest final level of peak systolic
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Ca2⫹ channels, nonspecific stretch-activated ion channels, and Na⫹/Ca2⫹ exchanger are structures effectively involved in the slow response, and 3) one or more
mechanisms are involved in the [Ca2⫹]i increase consequent to stretch. In addition, it is not fully understood to what extent the experimental conditions of
cellular preparations act on subcellular functions,
causing results that are not applicable to a bloodperfused, normal-temperature intact heart. Pragmatically, it may be stated that no point has been sufficiently validated to fully explain the [Ca2⫹]i gain that
follows myocardial stretch.
Despite the recognized dependence of the slow response on changes in Ca2⫹ kinetics and although
rhythm decisively influences Ca2⫹ transit, there has
been no systematic evaluation of the behavior of the
slow enhancement of contraction after a sudden stretch
when stimulation frequency varies.
Intimate mechanisms concerned with force-frequency relationships are reasonably well established.
Steady-state and transient changes in stimulus frequency are associated with changes in [Ca]i that parallel the force change. It is admitted (2) that, in most
mammals, heart rate (HR) elevation within the physiological range is accompanied by contractile enhancement dependent on an increase in [Ca]i due to accentuation of the reverse action of the Na⫹/Ca2⫹
exchanger. In addition, recent evidence has shown that
slow Ca2⫹ channel currents are upregulated by increased stimulation rates (8, 9, 29, 31, 39, 40, 49).
Previous studies on isolated myocells have suggested
some kind of interaction between rhythm of contraction and slow force response. Hongo et al. (20) reported
that 44% of isolated rat ventricular myocytes bathed in
1 mM Ca2⫹-Tyrode solution showed the slow response
at a stimulation rate of 0.5 Hz. When stimulation
frequency was increased to 1 Hz, the occurrence of the
slow response was increased to 60%. On the other
hand, White et al. (47) reported that reduction of stimulation rate of single guinea pig ventricular myocytes
seems to increase the probability of producing the slow
increase. Lakatta and Jewell (26) described an inverse
relation between slow response half-time (T/2) and
frequency of stimulation and a dependence of the number of beats needed to complete T/2 on stimulation rate
in cat papillary muscles. These data support the idea
that stimulation rhythm may interfere with the timedependent myocardial response to a sudden stretch.
The objective of the present study was to describe the
influence of HR on the slow increase of left ventricular
contraction after a sudden dilation. Isolated isovolumic
blood-perfused dog hearts were subjected to sudden
dilation at three different levels of HR. We found that
the intensity of time-dependent contraction enhancement after a sudden dilation is proportional to stimulation frequency. In addition, the time course of the
slow response and the number of beats needed to complete T/2 shifted in opposite directions: an increase in
stimulation frequency reduced T/2 and raised the number of beats included in T/2.
H2137
H2138
HEART RATE AND SLOW FORCE RESPONSE
RESULTS
The HRs at which ventricular dilations were studied
were 52 ⫾ 1.7 (low), 82 ⫾ 2.2 (intermediate), and 117 ⫾
3.5 (high) beats/min.
Ventricular response to a sudden dilation reproduced the pattern previously described (44, 45) (Fig. 1).
Ventricular volume expansion was promptly succeeded
by an instantaneous increase in DP followed by a
Fig. 1. Slow time base records of 1 experiment, representative of developed
pressure (DP) changes after a sudden
ventricular dilation (arrowhead) at 3 levels of heart rate (HR). HR elevation
clearly intensified and shortened the
slow response.
continuous increase in mechanical performance for
several minutes. Immediately after intensification of
the instantaneous contraction, there was a short period
of modest decline of performance, described and discussed elsewhere (45). Sudden ventricular dilation frequently provoked bursts of premature beats. Experiments in which this results in the evaluation of
immediate (⌬1DP) and time-dependent (⌬2DP) heightening of performance that was hazardous were excluded from analysis.
Inotropic effects of HR elevation were observed in
the three analyzed conditions: 1) when the balloon was
deflated, 2) immediately after sudden ventricular dilation, and 3) when DP attained the final values after
slow contractile enhancement. In the deflated condition, DP increased (P ⬍ 0.001) from 12.8 ⫾ 1.4 mmHg
at the low HR to 15.0 ⫾ 1.4 mmHg at the intermediate
HR and 17.9 ⫾ 1.6 mmHg at the high HR, and the
same behavior was observed for DP values immediately after dilation (45.6 ⫾ 2.2, 48.6 ⫾ 2.0, and 52.2 ⫾
1.8 mmHg for low, intermediate, and high HR, respectively, P ⬍ 0.001) and for the final DP values (62.9 ⫾
2.5, 69.3 ⫾ 2.3, and 79.0 ⫾ 1.9 mmHg for low, intermediate, and high HR, respectively, P ⬍ 0.001). When
the inotropic effects of HR elevation were analyzed
with the DP values in deflated, dilated, and final conditions considered as relative values of DP observed at
low HR, a previously described pattern of myocardial
response to the frequency-resting length interplay was
observed. Indeed, it has been previously established
(26, 41) that the Bowditch effect induces a more pronounced contractile stimulation at shorter than at
longer muscle length, and this peculiarity was observed in the present experiments (Fig. 2B). In Fig. 2,
DP values at intermediate and high HR were taken as
relative values of DP obtained at low HR, taken as 1.
With HR elevations, relative DP rose more strikingly
in the deflated than in the dilated condition, showing
that the Bowditch effect was more prominent at
shorter than at longer length.
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pressure was reached. Experiments were conducted by promoting the same extent of ventricular dilation at three levels
of HR.
For each study, in the first stage of the protocol, the HRs to
be used and the volume of saline to be added to the balloon
were previously determined. The lowest HR that allowed a
regular control of heart rhythm was used as the low HR.
Thereafter, the stimulation frequency was elevated in steps
(⬃20 beats/min) to confirm that increments of ⬃60 beats/min
increased left ventricular systolic pressure. The upper limit
tested was defined as the highest value to be used as high
HR, and an intermediate value was chosen as the intermediate HR. With the HR settled at the high value, the maximal
volume of saline to be used during the experiment was
defined, taking into account that peak systolic pressure did
not exceed perfusion pressure (100 mmHg). The difference
between maximal volume and deflated volume of the balloon
corresponded to the volume of saline to be utilized in dilating
the left ventricle. The ventricular volume was returned to the
deflated condition, and a new 10-min equilibrium period was
allowed to elapse. Thereafter, the heart was paced in a
randomized manner at one of the three frequencies previously determined. After an equilibration period (⬃4–5 min),
left ventricular pressure was continuously recorded, and ventricular dilation was produced by manually injecting the
predetermined volume of saline into the balloon. Thereafter,
the stimulation frequency was successively changed to the
two other defined frequencies, and the ventricular responses
to dilation were again analyzed. Between interventions, the
preparations were allowed to stabilize for 10 min at the
deflated volume.
Statistical analyses. Values are means ⫾ SE. Repeatedmeasures ANOVA complemented by the Student-NewmanKeuls test was used for comparison of parameter changes.
P ⬍ 0.05 was considered statistically significant.
HEART RATE AND SLOW FORCE RESPONSE
H2139
Fig. 2. A: DP measured when the balloon was in the
deflated condition, immediately after ventricular dilation, and at the final values after the slow response at
low (L: 52 ⫾ 1.7 beats/min), intermediate (M: 82 ⫾ 2.2
beats/min), and high (H: 117 ⫾ 3.5 beats/min) HR.
Values are means ⫾ SE. *P ⬍ 0.05 vs. L; #P ⬍ 0.05 vs.
M. B: data from A normalized so that DP ⫽ 1.0 (relative
DP) at L when the balloon was deflated, immediately
after ventricular dilation, and at the final values after
the slow response. Relative increments of pressure attained at M and H were more elevated in the deflated
condition. *P ⬍ 0.05 vs. immediately; #P ⬍ 0.05 vs.
final.
Table 1. Left ventricular contractile enhancement
induced by sudden dilation
⌬DP
⌬1DP
⌬2DP
L
M
H
50.1 ⫾ 2.1
32.8 ⫾ 1.6
17.3 ⫾ 0.9
54.3 ⫾ 1.9*
33.6 ⫾ 1.5
20.7 ⫾ 1.0*
61.1 ⫾ 1.3*†
34.3 ⫾ 1.2
26.8 ⫾ 1.2*†
Values are means ⫾ SE in mmHg. L, M, and H, low, intermediate,
and high heart rate; ⌬DP, total increase of developed pressure (DP)
that followed sudden left ventricular dilation; ⌬1DP, immediate
increase of DP induced by ventricular dilation; ⌬2DP, slow increase
of DP after sudden ventricular dilation. * P ⬍ 0.05 vs. L; † P ⬍ 0.05
vs. M.
nied HR elevations (0.34 ⫾ 0.01, 0.38 ⫾ 0.01, and
0.43 ⫾ 0.02 for low, intermediate, and high HR, respectively, P ⬍ 0.001; Fig. 3C).
With all data sets taken into account, the time to
reach half-maximal ⌬2DP (T/2) ranged from 45 to 96 s,
and 58–126 heartbeats were needed to complete T/2. In
every case, increments in stimulation frequency shortened the slow response time course (82 ⫾ 3.6, 67 ⫾ 2.6,
and 53 ⫾ 2.0 s for low, intermediate, and high HR,
respectively, P ⬍ 0.001; Fig. 4A), even though they
increased the number of heartbeats needed to complete
the slow response T/2 (72 ⫾ 2.9, 95 ⫾ 2.9, and 111 ⫾ 3.2
beats for low, intermediate, and high HR, respectively,
P ⬍ 0001; Fig. 4B). Lakatta and Jewell (26) described
a rectangular hyperbolic correlation between T/2 and
frequency stimulation in two of four cat papillary muscles in which the xy product is a constant (i.e., y ⫽ a/x),
suggesting that T/2 could be entirely determined by the
number of heartbeats. In our data, a nonsignificant
correlation (r ⫽ 0.5469, P ⬎ 0.05) was found for this
fitting. For our results, we found a significant correlation (r ⫽ 0.8756, P ⬍ 0.0001; Fig. 5) for a rectangular
hyperbola of the following type: y ⫽ ab/(b ⫹ x), where
the xy product is not a constant. This result indicates
that T/2 only partially depends on the number of heartbeats.
DISCUSSION
The left ventricular systolic response to a sudden
dilation observed in the present study reproduced the
pattern previously described for myocardial stretch of
intact ventricle (44, 45) and unicellular (20, 47) or
multicellular preparations (1, 4, 6, 10, 12, 14, 21, 24,
38).
The present results should be analyzed from the
viewpoint of a confirmation of force-interval relationships described for similar experimental conditions. It
has been said that inotropism shows a flat or only a
modest response to HR increases in conscious animals
(7, 15, 19, 34). However, in isolated preparations, the
inotropic effect of force-frequency relationships is de-
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Table 1 summarizes the characteristics of ventricular performance heightening induced by sudden ventricular dilation at the three HR levels. The total
(⌬1DP ⫹ ⌬2DP) increase of DP (⌬DP) was intensified by
increased HR, as indicated by a significant difference
(P ⬍ 0.001) observed in all comparisons among the
values obtained for low, intermediate, and high HR:
50.1 ⫾ 2.1, 54.3 ⫾ 1.9, and 61.1 ⫾ 1.3 mmHg, respectively.
A remarkable difference was detected with respect to
the influence of HR rise on immediate and slow contraction increases: although the treppe phenomenon
does not influence the immediate pressure increase
evoked by ventricular expansion, a clear intensification
of the slow response was promoted by HR elevation.
The values for ⌬1DP at low, intermediate, and high HR
(32.8 ⫾ 1.6, 33.6 ⫾ 1.5, and 34.3 ⫾ 1.2 mmHg, respectively) were not significantly different (Fig. 3A),
whereas ⌬2DP was augmented by HR elevation in
every experiment, resulting in a significant difference
(P ⬍ 0.001; Fig. 3A) for mean values at low, intermediate, and high HR (17.3 ⫾ 0.9, 20.7 ⫾ 1.0, and 26.8 ⫾
1.2 mmHg, respectively). In addition, ⌬2DP values
were significantly correlated (r ⫽ 0.7028, P ⬍ 0.001)
with the respective HR values (Fig. 3B). This kind of
behavior for ⌬1DP and ⌬2DP resulted in a progressive
increment of the ⌬2DP contribution to ⌬DP, as can be
inferred by the increase in ⌬2DP/⌬DP that accompa-
H2140
HEART RATE AND SLOW FORCE RESPONSE
scribed as a more prominent phenomenon (2). In our
experiments, a positive inotropic effect of HR elevation
consistently occurred in all experiments within the
entire frequency range utilized, and, as previously established (41), systolic enhancement was more pronounced at shorter than at longer lengths.
A remarkable feature of our results was that the HR
increase did not affect immediate heightening of contraction due to ventricular dilation, but the systolic
influence of HR was limited to the slow response. These
data agree with the present concept that immediate
contractile strength after stretching is due to changes
in physical factors and in the myofibrillar force-Ca2⫹
relationship (4, 13, 18, 20, 24, 25, 28) and with the fact
that there is no report that the treppe phenomenon
alters these factors. In contrast, force-frequency rela-
Fig. 4. A: time for half of the slow response (T/2) at L,
M, and H. *P ⬍ 0.05 vs. L; #P ⬍ 0.05 vs. M. B: number
of heartbeats needed to complete T/2 at L, M, and H.
*P ⬍ 0.05 vs. L; #P ⬍ 0.05 vs. M. Symbols, individual
values; solid horizontal lines, means; dotted horizontal
lines, SE.
tionships and slow response to a sudden stretch are
thought to be entirely linked to changes in Ca2⫹ kinetics. On this basis, the concept of the isolated influence
of HR on the slow response fully agrees with present
concepts about the changes of length-tension relationships elicited by a sudden myocardial stretch.
On the other hand, a greater slow force response
elicited by the inotropic effect of HR calls close attention to the reason why this action contrasts with the
influence on the time-dependent response reported for
other inotropic interventions acting on the degree of
myocell activation before stretch. Indeed, the more
marked slow increase obtained by stimulation frequency was a singular action compared with results
reported in studies with muscles that were spontaneously close to full activation (4) and in studies on the
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Fig. 3. A: immediate (⌬1DP) and slow (⌬2DP) increases of DP after sudden ventricular dilation at L, M, and H.
Values are means ⫾ SE. *P ⬍ 0.05 vs. L; #P ⬍ 0.05 vs. M. HR elevations did not affect the immediate increase but
potentiated the slow increase. B: slow increase (⌬2DP) plotted as a function of HR. Intensity of the slow response
is linearly correlated with the stimulation frequency. C: relative contribution of slow increase of contraction to total
pressure elevation (⌬2DP/⌬DP) after sudden ventricular dilation at L, M, and H. Symbols, individual values; solid
horizontal lines, means; dotted horizontal lines, SE. *P ⬍ 0.05 vs. L; #P ⬍ 0.05 vs. M.
HEART RATE AND SLOW FORCE RESPONSE
effect of Ca2⫹ concentration in the bath (4, 5, 20, 36,
47), ␤-adrenergic agonists (44), or a Ca2⫹ agonist devoid of adrenergic action (44) on the slow increase. In
all these investigations it was reported that increased
basal contractility inhibits or abolishes the slow contractile response to stretch. An opposite behavior was
observed in our results for the treppe phenomenon.
The inotropic effect of HR elevation induced a slow
response intensification. Therefore, in interpreting our
results, it is necessary to take into account an inotropic
action of HR elevation that not only augments [Ca]i in
the basal (deflated) condition but also renders the myocell prone to intensifying the [Ca]i increase due to
stretch proportionally to HR increments.
The hyperbolic relation (y ⫽ a/x) suggested by
Lakatta and Jewell (26) for T/2 and HR is of interest.
We found a significant correlation for a rectangular
hyperbola in which the xy product is not a constant [y ⫽
ab/(b ⫹ x)], indicating that T/2 and HR are inversely
correlated, but the phenomenon depends only partially
of the number of heartbeats. The possible heartbeat
dependence of the slow response seems to be a consequential finding, because this kind of association reinforces the concept that excitation-contraction events
are a relevant mechanism for intracellular Ca2⫹ gain
after a stretch.
Our results can be understood if we take into consideration two previously described mechanisms that
may possibly affect Ca2⫹ kinetics in our experiments:
1) upregulation of Ca2⫹ channel currents (ICaL) by
frequency of stimulation and 2) Na⫹/Ca2⫹ exchanger
as the basis for a slow force response.
It has been shown (8, 9, 29, 31, 39, 40, 49) that
elevation of stimulation rate in frog and mammalian
myocardium induces an ICaL potentiation consisting of
a moderate increase of peak current amplitude and a
marked slowing of the inactivation kinetics by changing gating properties. Moreover, ICaL were potentiated
by frequency stimulation in a manner quite interesting
for the understanding of our results: there is a striking
proportionality between frequency stimulation and
ICaL potentiation. In human atrial myocytes, ICaL was
potentiated in a gradually progressive manner with
increasing heart stimulation rates in a range as wide
as 0.3–5 Hz (39).
Interestingly, it is recognized that ICaL amplification
promoted by stimulus frequency is favored by stimulation of cAMP production (8, 9, 31, 39, 40, 49). In effect,
it has been shown that cAMP elevations secondary to
␤-adrenergic agonists (8, 9, 31, 39, 40, 49) or nonspecific maneuvers (31) are capable of amplifying the ICaL
upregulation due to stimulation rate. According to the
reports of Todaka et al. (44) and Calaghan et al. (12)
that cAMP and [Ca]i rise at the same time after myocardial stretch, we are tempted to suggest that ICaL
upregulation by the treppe phenomenon and its amplification by a cAMP increase possibly acted concurrently in our experiments, promoting intensification of
the slow response by an increase in HR.
The potentiation of ICaL by frequency stimulation
can hardly explain the amplification of the HR slow
response observed in our experiments if we admit that
the time-dependent intact ventricle heightening of contraction elicited by sudden stretch is linked to accentuation of L-type Ca2⫹ channel inflow, as previously
suggested in some reports (1, 27, 45). Some authors
observed no influence of the Ca2⫹ channel blocker
verapamil on the slow response. Chuck and Parmley
(14), studying cat papillary muscles, reported that verapamil does not alter the slow response to a sudden
stretch. Lew (33) studied the effect of verapamil on the
slow force response in the in situ dog heart and reported that verapamil did not affect the volume load
length-pressure shift verified before the drug administration. In contrast, in the blood-perfused dog heart, we
(45) previously showed that Ca2⫹ channel blockade
promoted by verapamil infusion reduced the relative
contribution of the slow response to the total pressure
increase (⌬2DP/⌬DP) from 0.35 to 0.19 and increased
T/2 from 55 to 67 s. Such results clearly support the
view that, in blood-perfused intact isovolumic canine
heart, transsarcolemmal L-type Ca2⫹ channel influx
plays a relevant role in the stretch-induced slow
heightening of contraction of the canine myocardium.
Recently, Alvarez et al. (6) reported that the slow
force response to stretch can be inhibited by the blocker
of the AT1 angiotensin receptor losartan, the endothelin receptor BQ-123, and the Na⫹/H⫹ exchanger amiloride. These authors proposed that the slow force response results from a cascade of events initiated by the
release of ANG II elicited by stretch that successively
stimulates endothelin and the Na⫹/H⫹ exchanger. The
increase in [Na⫹]i thus promoted will allow that reversal action of Na⫹/Ca2⫹ exchanger terminates to increase force by elevating [Ca]i. It seems conceivable
that the slow force enhancement due to the Bowditch
effect observed by us could be understood if this mech-
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Fig. 5. T/2 plotted as a function of HR showing the inverse relation
between the time course of the slow increase and stimulation frequency. Dashed curve (P ⬎ 0.05) is a hyperbola showing the relation
expected if the slow increase were purely beat dependent, i.e., frequency ⫻ half time ⫽ constant. Solid curve (P ⫽ 0.0001) is the fitting
for a hyperbola in which frequency ⫻ half time is not constant [y ⫽
ab/(b ⫹ x)]. Note that T/2 values are plotted as minutes (cf. Fig. 4A,
where T/2 values are plotted in seconds).
H2141
H2142
HEART RATE AND SLOW FORCE RESPONSE
The authors thank Airton Andrade Santos for skillful technical
assistance in surgical preparation. The authors especially acknowledge Clovis de Araujo Peres for help with statistical analysis.
This work was done during the tenure of National Research
Council Grant 300.692/80-3(NV) and Fundação de Amparo à Pesquisa do Estado de São Paulo Grant 99/04533-4.
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