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Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS AND MATERIALS
RESULTS
DISCUSSION
REFERENCES
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Adenosine
A2a-receptor activation enhances shortening of isolated
cardiomyocytes. In the present study the effect of
A2a-receptor activation on the contractile performance of
isolated rat hearts was investigated by recording left ventricular
pressure (LVP) and the maximal rate of LVP development
(+dP/dtmax). With constant-pressure perfusion,
adenosine caused concentration-dependent increases in LVP and
+dP/dtmax, with detectable increases of 4.1 and
4.8% at 10
6 M and maximal increases of 12.0 and 11.1%
at 104 M, respectively. The contractile responses were
prevented by the A2a-receptor antagonists
chlorostyryl-caffeine and aminofuryltriazolotriazinyl-aminoethylphenol (ZM-241385) but were not affected by the
1-adrenergic antagonist atenolol. The adenosine
A1-receptor antagonist dipropylcyclopentylxanthine and
pertussis toxin potentiated the positive inotropic effects of
adenosine. The A2a-receptor agonists
ethylcarboxamidoadenosine and
dimethoxyphenyl-methylphenylethyl-adenosine also enhanced contractility. With constant-flow perfusion, 105 M
adenosine increased LVP and +dP/dtmax by 5.5 and
6.0%, respectively. In the presence of the coronary vasodilator
hydralazine, adenosine increased LVP and
+dP/dtmax by 7.5 and 7.4%, respectively.
Dipropylcyclopentylxanthine potentiated the adenosine contractile
responses with constant-flow perfusion in the absence and presence
of hydralazine. These increases in contractile performance were also
antagonized by chlorostyryl-caffeine and ZM-241385. The results
indicate that adenosine increases contractile performance via
activation of A2a receptors in the intact heart independent
of 1-adrenergic receptor activation or changes in coronary flow.
adenosine A1 receptor; 1-adrenergic receptor; myocardial contractility; constant-pressure-perfused heart; constant-flow-perfused heart; A1 antagonists; A2a antagonists; hydralazine
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS AND MATERIALS
RESULTS
DISCUSSION
REFERENCES
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ADENOSINE IS KNOWN TO
ELICIT multiple effects in the heart, including heart rate
reduction (37), vasodilation (3), and cardioprotection (1, 24, 27, 36), mediated by activation of adenosine A1 (24, 37), A2a
(3, 36, 40), and A3 (1, 27, 36)
receptors. One well-characterized effect of adenosine on myocardial
contractile performance is its ability to reduce the contractile and
metabolic responses elicited by 1-adrenergic stimulation
(8, 31). This antiadrenergic effect of adenosine is
mediated by A1 receptors, which activate the inhibitory
heterotrimeric G protein, Gi, reducing
1-adrenergic-elicited adenylyl cyclase activity, cAMP
accumulation, protein kinase A activity, and myocardial protein
phosphorylation (9, 13, 15, 30).
Adenosine also has a stimulatory influence on myocardial contractile performance elicited by activation of A2a receptors (11, 39, 40). This effect of adenosine is putatively mediated by activation of the stimulatory G protein, Gs, and involves presumably a cAMP-dependent pathway (11, 25, 40). However, a cAMP-independent pathway may also be involved (11, 26). Although the natural nucleoside adenosine has been shown to increase contractility of papillary muscles isolated from rats (23), guinea pigs (4), and dogs (6), it was not determined in these studies whether the increase in contractile performance was mediated by activation of A2a receptors. Some reports have suggested that A2a-receptor agonists do not increase the contractility of isolated ventricular myocytes from rats (32), rabbits (32), or guinea pigs (2, 32, 35). However, other reports indicate that A2a-receptor agonists increase contractile performance of rat (11, 40), chick embryonic (25, 26, 39), and human (34) ventricular myocytes. Thus, if the latter findings are correct, an increase in the contractility of the intact heart should also result via A2a-receptor-mediated mechanisms. The purpose of the present study was to investigate the role of A2a-receptor activation by adenosine in the enhancement of contractile performance of intact hearts isolated from rats. The results from this study indicate that adenosine increases ventricular contractile performance in the intact heart independently of adrenergic stimulation, perfusion pressure, and coronary flow and that this increase in contractility is prevented by A2a-receptor antagonists. These results further suggest a physiological role of the A2a receptor as a mediator of positive inotropy.
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METHODS AND MATERIALS |
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TOP
ABSTRACT
INTRODUCTION
METHODS AND MATERIALS
RESULTS
DISCUSSION
REFERENCES
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Preparation of Isolated Perfused Hearts
Male Sprague-Dawley rats (310 ± 7 g; Harlan Sprague Dawley, Indianapolis, IN) were initially anesthetized with pentobarbital sodium (40 mg/kg ip). The hearts were rapidly excised, immersed in ice-chilled physiological saline (PS), and immediately perfused via the aortic cannula with nonrecirculated PS at 37°C. A perfusion pump maintained a column of PS at a height of 88 cm in the perfusion apparatus, providing a constant coronary perfusion pressure of 65 mmHg. In constant-flow preparations, the perfusion pump delivered PS directly to the perfusion apparatus at a desired constant flow. The PS was prepared daily and contained (in mM) 120 NaCl, 4.7 KCl, 2.5 CaCl2, 25 NaHCO3, 1.2 MgSO4, 1.2 KH2PO4, and 10 glucose. The pH was maintained at 7.4 by gassing the PS with 95% O2-5% CO2. Coronary perfusion pressure was recorded by a pressure transducer connected via a side tube to the aortic perfusion cannula. Left ventricular pressure (LVP) was determined using a water-filled, latex balloon-tipped polyethylene cannula (1.5 mm ID) attached to a strain-gauge manometer (Micro-Med, Louisville, KY). The balloon (Hugo Sachs, Hugstetten, Germany) was inserted into the lumen of the left ventricle. The diastolic pressure was set at 5-10 mmHg and held constant. The hearts were paced at 5 Hz with a stimulator (model SD9, Grass Instruments, Quincy, MA) via platinum wire electrodes inserted into the right and left atria. The voltage was 10% above threshold, and the pulse duration was 5 ms. LVP, maximum rates of LVP development (+dP/dtmax) and relaxation (
dP/dtmax), heart rate, and end-diastolic
pressure were assessed continuously by an analog-to-digital converter
(Heart Performance Analyzer, Micro-Med, Louisville, KY) at a frequency
of 500 Hz. These data were averaged over 0.5-s intervals and stored on
a personal computer (Gateway 2000, North Sioux City, SD). A system of
dual outputs allowed the signal from the Heart Performance Analyzer to
be simultaneously recorded on a multichannel polygraph (model 7758A,
Hewlett-Packard, Waltham, MA). Coronary flow was determined
volumetrically. The hearts were allowed to stabilize for 
45 min
before an experimental protocol was begun. After the stabilization
period, hearts that did not have an LVP 75 mmHg were not utilized.
Experimental Protocols
In the first six protocols, hearts were constant-pressure perfused at 65 mmHg. This procedure resulted in coronary flows of 16 ± 1 ml/min (12 ± 1 ml · min1 · g wet wt1). In
protocols 7-9, hearts were constant-flow perfused,
unless otherwise indicated, at 16 ± 1 ml/min, resulting in a
perfusion pressure of 65 mmHg.
Protocol 1.
The effect of adenosine on left ventricular contractility was
determined. Adenosine was infused into the perfusion PS at 1% of the
coronary flow rate for 3 min yielding final PS adenosine concentrations
of 108-104 M. Concentrations were
administered randomly. The maximum contractile response (LVP,
+dP/dtmax, and
dP/dtmax) to each concentration of adenosine
was achieved within 1 min after the beginning of the infusion. Coronary
flow was determined, and coronary resistance was calculated by dividing
the perfusion pressure (65 mmHg) by the coronary flow.
Protocol 2.
The effects of the A2a-receptor antagonists
chlorostyryl-caffeine (CSC) and
aminofuryltriazolotriazinyl-aminoethylphenol (ZM-241385) on the
contractile response to adenosine were determined. The antagonists were
infused to produce a final concentration of 107 M in the
PS, and the contractile response to a 3-min administration of
105 M adenosine was determined periodically throughout
this infusion. The contractile response to a 3-min administration of
105 M adenosine was also determined periodically
throughout a 60-min period in the presence of 0.01% DMSO, the vehicle
for CSC and ZM-241385. Because of the lability of CSC, it was prepared
fresh immediately before use.
Protocol 3.
The effect of the A1-receptor antagonist
dipropylcyclopentylxanthine (DPCPX) at 107 M on the
adenosine-elicited contractile response was assessed. The antagonist
was administered 5 min before and during a 3-min exposure to
105 M adenosine.
Protocol 4.
The effects of the A2a-receptor agonists
ethylcarboxamido-adenosine (NECA) and
dimethoxyphenyl-methylphenylethyl-adenosine (DPMA) at 106
M and carboxyethylphenethyl-aminoethylcarboxamido-adenosine
(CGS-21689) at 107-105 M on left
ventricular contractility were determined. The agonists were
administered for 5 min.
Protocol 5. The effect of pertussis toxin on the adenosine-elicited contractile responses was assessed. Rats received activated pertussis toxin (25 µg/kg ip; Research Biochemicals, Natick, MA) 48 h before initiation of experiments. The toxin was activated by incubation in 50 mM KH2PO4 (pH 7.5), 1 mg/ml ovum albumin, and 250 mM dithiothreitol at 30°C for 18 h.
Protocol 6.
The effect of the -adrenergic antagonist atenolol (106
M) on the contractile responses elicited by 105 M
adenosine and 109 M isoproterenol (a
1-adrenergic agonist) was determined. The antagonist was
infused for 5 min, and adenosine or isoproterenol was administered
during the final 3 min.
Protocol 7.
The effect of 105 M adenosine on left ventricular
contractile performance was determined under constant-flow conditions.
This was considered because, during constant-pressure perfusion, the vasodilation elicited by adenosine may have resulted in an increased flow that, on its own, could have augmented contractility. After achieving steady state under constant-pressure perfusion, the hearts
were switched to constant-flow perfusion at their natural flow rate of
16 ± 1 ml/min. Subsequently, adenosine was administered at
108-104 M for 3 min, and LVP,
+dP/dtmax, and coronary perfusion pressure were recorded.
Protocol 8.
The effect of the A1-receptor antagonist DPCPX
(107 M) on the adenosine-elicited contractile response
was assessed under constant-flow conditions. The antagonist was
administered 5 min before and during a 3-min exposure to
105 M adenosine.
Protocol 9.
Hydralazine, a known vasodilator (33), was employed in
constant-flow experiments to vasodilate the hearts so that the addition of adenosine would contribute little to the total vasodilation. Pretreatment of hearts with 104 M hydralazine attenuated
the reduction of coronary perfusion pressure (~1-5 mmHg)
mediated by adenosine-induced vasodilation. Some hearts were also
treated with 107 M DPCPX 5 min before and during the
3-min administration of 105 M adenosine to prevent the
possible involvement of A1 receptors. The contractile
response to adenosine was determined before and after treatment of the
hearts with 107 M CSC for 60 min or 107 M
ZM-241385 for 5 min and then after CSC or ZM-241385 was allowed to wash
out from the heart for an additional 30 min.
Animals
The animals were maintained and used in accordance with recommendations in the Guide for the Care and Use of Laboratory Animals [Institute of Laboratory Animals Resources, National Research Council (NIH Publication), vol. 25, no. 28, revised 1996] and the guidelines of the Institutional Animal Care and Use Committee of the University of Massachusetts Medical School.Materials
Adenosine (Boehringer Mannheim, Indianapolis, IN) and NECA (Research Biochemicals, Natick, MA) were prepared as stock solutions of 102 M and 3 × 103 M, respectively, in
PS or deionized water (MilliQ water system, Millipore, Bedford, MA).
CSC (Research Biochemicals) was prepared as a stock solution of
103 M in 50% DMSO, and isoproterenol (Sigma Chemical,
St. Louis, MO) was prepared as a stock solution of 102 M
in 0.1% (wt/vol) sodium metabisulfite. CGS-21680, DPMA, DPCPX (Research Biochemicals), and ZM-241385 (Tocris Cookson, Ballwin, MO)
were prepared in stock solutions of 102 M in DMSO. All
solutions were further diluted in PS or water before infusion.
Pentobarbital sodium was obtained from Abbott Laboratories (North
Chicago, IL), and all other salts, glucose, and solvents were of
certified grade (Fisher Scientific, Boston, MA, or J. T. Baker,
Phillipsburg, NJ).
Statistical Treatments
Contractile data (LVP and ±dP/dtmax) were recorded via the Heart Performance Analyzer immediately before each treatment with adenosine, NECA, DPMA, CGS-21680, or vehicle and during the peak responses to the agents. In some studies the concentration-dependent effects of adenosine on all three indexes of left ventricular contractility (LVP, +dP/dtmax, anddP/dtmax) were determined. These studies
indicated that the effect of adenosine on contractile performance is
adequately assessed through measurements of LVP and
+dP/dtmax. Therefore, in most studies, LVP and
+dP/dtmax were utilized as the indexes of
contractility. Values are means ± SE. Statistical analysis was
performed by randomized block ANOVA, which allowed the contractile
responses to each treatment to be compared with a control value
generated from the same heart. P < 0.05 was accepted
as indicating a statistically significant difference.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS AND MATERIALS
RESULTS
DISCUSSION
REFERENCES
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Adenosine Enhances Contractile Performance With Constant-Pressure Perfusion
Adenosine at 105 M increased LVP,
+dP/dtmax, and dP/dtmax
within 3 min (Fig. 1). On termination of
the adenosine administration, the contractile variables returned to
control levels within 5 min. Concentration-response curves revealed
increases for LVP, +dP/dtmax, and
dP/dtmax of 4.1, 4.8, and 4.6% at
106 M adenosine, increases of 8.0, 8.7, and 9.4% at
105 M adenosine, and maximum increases of 12.0, 11.1, and
14.3% at 104 M adenosine, respectively (Fig.
2). The hearts demonstrated
contractile responses to 3-min infusions of adenosine every 10 min for
90 min. Repeated infusion of the vehicle for adenosine (PS or water) did not significantly change contractile performance.
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Although 106-104 M adenosine
significantly increased contractility of the constant-pressure-perfused
hearts, only 104 M adenosine significantly increased
coronary flow and decreased coronary resistance (Fig.
3). With 104 M adenosine,
the increase in coronary flow was 8.5% and the decrease in coronary
resistance was 7.5%. Thus, 106-105 M
adenosine produced an increase in the contractile variables without
significantly affecting coronary flow or resistance.
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Adenosine-Elicited Increase in Contractile Performance Is Prevented by A2a-Receptor Antagonists
The A2a-receptor antagonists CSC and ZM-241385, when administered alone at 107 M, did not significantly affect
LVP and +dP/dtmax (Fig.
4). However, both antagonists prevented
the 8.8 and 8.1% increase in LVP and +dP/dtmax,
respectively, caused by 105 M adenosine. A 60-min,
continuous infusion of CSC was required to prevent the
adenosine-elicited contractile response, whereas only a 5-min infusion
of ZM-241385 was required for the same inhibitory effect. A 60-min
infusion of 0.001-0.005% DMSO, the vehicle for CSC and ZM-241385,
did not alter the contractile response to adenosine.
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A1-Receptor Antagonist, DPCPX, Potentiates the Adenosine-Elicited Increase in Contractile Performance
DPCPX was used in some studies to ascertain whether A1-receptor stimulation could affect the adenosine-elicited response. In the absence of DPCPX, 105 M adenosine increased LVP by 9.5% and
+dP/dtmax by 9.8% (Fig. 5). However, in the presence of
107 M DPCPX, the adenosine-elicited increases in LVP and
+dP/dtmax were significantly potentiated
to 15.2 and 15.4%, respectively. DPCPX alone did not affect the
basal level of LVP or +dP/dtmax.
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A2a-Receptor Agonists, NECA and DPMA, Increase Left Ventricular Contractile Performance
As with adenosine, NECA and DPMA at 106 M increased
LVP and +dP/dtmax (Fig. 5). NECA increased LVP
by 4.5% and +dP/dtmax by 4.2%. DPMA increased
LVP and +dP/dtmax by 4.1 and 3.5%,
respectively. The A2a-receptor agonist CGS-21680 at
107-105 M did not
increase LVP and +dP/dtmax (data not shown).
Furthermore, at these concentrations, CGS-21680 did not significantly
increase coronary flow. Infusion of the appropriate vehicle (DMSO or
water) did not affect LVP or +dP/dtmax.
Pertussis Toxin Potentiated the Adenosine-Elicited Increase in Contractile Performance
Because the A1-receptor antagonist DPCPX potentiated the contractile response of adenosine, pertussis toxin was used as an alterative approach to eliminate the influence of A1-receptor stimulation, which is manifest via inhibitory G protein action. Treatment with pertussis toxin increased the 106, 105, and 104 M
adenosine-elicited increases in LVP from 6.0, 9.5, and 13.7% to 10.4, 15.9, and 21.0%, respectively (Fig. 6).
The increases in +dP/dtmax caused by these
adenosine concentrations were enhanced from 6.0, 9.8, and 14.0% to
10.4, 16.4, and 18.0%, respectively. Thus pertussis toxin, like DPCPX,
potentiated the adenosine-elicited increases in the contractile
variables by 31-74%.
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Adenosine-Elicited Contractile Response Is Unaffected by the
1-Adrenergic Receptor Antagonist Atenolol
5 M adenosine-elicited 10.4-10.8% increases in
these contractile variables (Fig. 7).
However, the adrenergic antagonist did prevent the 17.8 and 18.3%
increases in LVP and +dP/dtmax,
respectively, caused by the -adrenergic agonist isoproterenol at
109 M. Isoproterenol was employed at this concentration
because it produced a contractile response similar in magnitude to that
caused by 105 M adenosine. Isoproterenol at
108 M produced a twofold increase in contractility (data
not shown), as previously reported for a perfused rat heart preparation
(9).
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Adenosine Increased Contractile Performance With Constant-Flow Perfusion
In constant-flow-perfused hearts having a coronary flow of 16 ± 1 ml/min, 105 M adenosine increased LVP,
+dP/dtmax, and dP/dtmax
by 5.5, 6.0, and 6.5%, respectively (Fig.
8). At 104 M adenosine the
increases were 8.7, 8.9, and 9.4%, respectively, for these contractile
variables. The coronary perfusion pressure was only significantly
decreased by 5.2% at 104 M adenosine. This indicated
that the increase in the contractile variables at 105 M
adenosine was most likely independent of a change in perfusion pressure.
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A1-Receptor Antagonist DPCPX Potentiates the Adenosine-Elicited Contractile Performance Increase in Constant-Flow-Perfused Hearts
In constant-flow-perfused hearts, DPCPX potentiated the 105 M adenosine-induced increases in LVP and
+dP/dtmax (Fig.
9). DPCPX potentiated the
adenosine-elicited increases in LVP and
+dP/dtmax from 5.6 and 5.9% to 8.8 and 8.6%,
respectively. The potentiation was similar to that observed in the
constant-pressure-perfused hearts (Fig. 5).
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Adenosine Elicits an Increase in Contractile Performance in the Maximally Dilated Constant-Flow-Perfused Hearts
In constant-flow-perfused hearts, hydralazine, a vasodilator, was administered at 104 M to maximally
dilate the coronary vasculature. The coronary flow increased and ranged
from 18 to 20 ml/min to maintain a perfusion pressure of 65 mmHg. In
the presence of hydralazine, 105 M adenosine increased
LVP and +dP/dtmax by 7.5 and 7.4%, respectively (Fig. 10). In the presence of DPCPX,
the adenosine-induced increases in these contractile variables were 12 and 11.2%, respectively. CSC at 107 M prevented these
increases in LVP and +dP/dtmax. After a 30-min washout of CSC, 105 M adenosine in the absence of DPCPX
increased LVP and +dP/dtmax by 6.8 and 6.7%,
respectively. ZM-241385, like CSC, at 107 M prevented the
adenosine-induced increase in LVP and +dP/dtmax (data not shown). The adenosine-elicited contractile response also
returned on washout of ZM-241385. Overall the above results indicate
that the adenosine-elicited increase in contractility of the perfused
heart is independent of changes in coronary flow.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS AND MATERIALS
RESULTS
DISCUSSION
REFERENCES
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Adenosine Enhancement of Contractile Performance
The present study indicates that A2a-receptor activation increases left ventricular contractile performance in intact hearts isolated from rats. Adenosine increased contractility in a concentration-dependent fashion, starting at 106 M (Fig.
2). Only the highest concentration of adenosine (104 M)
significantly increased coronary flow (Fig. 3). The increases in
contractility elicited by adenosine were prevented by the
A2a-receptor antagonists CSC (14, 19) and
ZM-241385 (22, 29) at 107 M (Fig. 4).
Because these compounds are primarily A2a-receptor antagonists, the present findings would suggest that the contractile responses are probably the result of A2a-receptor
stimulation. In addition, the A2a-receptor agonists NECA
and DPMA increased left ventricular contractile performance, further
indicating that the increase is mediated by A2a-receptor
activation (Fig. 5). The increase in left ventricular contractile
performance mediated by adenosine was not antagonized by the
-adrenergic antagonist atenolol, indicating that the contractile
response was not mediated by -adrenergic stimulation (Fig. 7).
The antagonism of the adenosine-elicited contractile response by
CSC required 60 min of continuous CSC treatment to completely develop. However, the antagonism of the adenosine-elicited contractile response was observed within 5 min after the administration of ZM-241385. The reason for this difference in the time required to
achieve A2a-receptor blockade is not known.
Adenosine Contractile Effects With Constant-Pressure and Constant-Flow Perfusion
The adenosine-elicited increase in contractile performance was presumably not caused by increased coronary flow elicited by adenosine-induced vasodilation, because it was also observed under constant-flow perfusion conditions (Fig. 8). An adenosine-elicited contractile response was observed after pretreatment of hearts with the coronary vasodilator hydralazine (Fig. 10). Moreover, hydralazine dilated the vascular bed, thereby reducing the vasodilatory effect of 104 M adenosine (Fig. 8). This prevented any reduction in
perfusion pressure caused by adenosine. The increase in ventricular
contractility elicited by adenosine under constant-flow perfusion
conditions was antagonized by CSC and ZM-241385, further indicating
that this increase in contractility is mediated by activation of the A2a receptor.
The adenosine-elicited contractile responses were somewhat greater with constant-pressure than with constant-flow perfusion. This small difference in the contractile responses could be a flow-dependent component related to the Gregg phenomenon (7, 16) in constant-pressure-perfused hearts. The flow-dependent component is assumed to be small, because only the largest concentration of adenosine administered caused a significant increase in coronary flow. Thus the positive inotropic responses elicited by adenosine are independent of coronary flow changes.
Potentiation of Adenosine by Pertussis Toxin and A1-Receptor Antagonism
Pertussis toxin, which is known to prevent A1-receptor-mediated events in the heart (18, 20) by ADP-ribosylation of Gi protein, potentiated the adenosine-elicited increase in contractile performance. This suggests that, in the absence of pertussis toxin, adenosine is also activating myocardial A1 receptors, which appear to oppose the A2a-receptor-elicited contractile effects of adenosine. Moreover, in the presence of the A1-receptor antagonist DPCPX (17), the contractile response to adenosine was greater than in the absence of the antagonist. This also indicates that with blockade of A1 receptors the A2a-receptor-mediated action is enhanced. The interaction of A1 receptors and A2a receptors has been observed previously where the use of A2a-receptor antagonists was reported to enhance the antiadrenergic action of A1-receptor agonists in perfused hearts (28).Myocardial Actions of Adenosine and Adenosine Analogs
The results from this study suggest that A2a-receptor activation increases contractility in the intact heart, as previously reported for ventricular myocytes isolated from rats (11, 40), human myocardium (34), and chick embryos (25, 26, 39). In addition, these results confirm previous studies that indicate that stimulation of adenosine receptors increases contractility in isolated papillary muscles from rats (23), guinea pigs (4), and dogs (6). However, other investigators have shown that adenosine or A2-receptor agonists do not increase contractility in isolated ventricular myocytes from rats (32), rabbits (32), or guinea pigs (2, 32, 35). Utilizing papillary muscle isolated from guinea pigs, Stein et al. (35) reported that the A2a-receptor agonist CGS-21680 does not increase myocardial contractility. In addition, Shryock et al. (32) and Felsch et al. (12) reported that the A2a-receptor agonists CGS-21680, methylphenylethoxy-adenosine (WRC-0090), and NECA do not increase the contractile performance of hearts isolated from guinea pigs. It has also been shown that adenosine does not increase contractility of rat ventricular strips (5). The reason for these differences is not known, but several suggestions can be offered. First, the effect of A2a-receptor stimulation on myocardial contractile performance may be lost in studies where the animals were stressed before experimentation. During the course of this study, it was found that adenosine-induced contractile responses were reduced or absent in hearts isolated from hyperexcited rats compared with hearts from tranquil rats. Second, the effect of A2a-receptor stimulation on myocardial contractility might also vary depending on the underlying level of arrhythmia. In our study a contractile response to adenosine was not detectable in hearts that displayed a high level of ventricular arrhythmias during experimentation. In these hearts the contractile response to adenosine was comparable to, or smaller than, the oscillations in baseline contractility elicited by these arrhythmias. Third, the variable effects of A2a-receptor stimulation on myocardial contractility observed in previous studies might be related to the experimental system utilized for measuring contractility. It is possible that our experimental system, which utilized an analog-to-digital converter and computer software for assessing left ventricular contractility, facilitated the resolution of the small increases in contractility elicited by adenosine. Other studies that relied solely on polygraphic recordings of myocardial contractility might have missed this response. Fourth, the lack of a contractile response to A2a-receptor stimulation in some reports could also be due to differences in preparations such as those that are inherent in ventricular myocyte isolation. Finally, the effect of A2a-receptor activation on myocardial contractility might vary depending on the specific A2a-receptor agonist utilized. It was found here that the A2a-receptor agonists NECA and DPMA produced smaller increases in contractility than adenosine (Fig. 5). However, the A2a-receptor agonist CGS-21680 (11, 21) did not cause a significant increase in contractility or coronary flow. It is likely that different A2a-receptor agonists may have varying degrees of A2a-receptor availability or activation/transduction efficiencies in the intact heart.Mechanism of Adenosine Action
The positive inotropic action of adenosine reported here appears to occur via A2a receptors. Agonists for this receptor have been reported in cardiac preparations to mediate responses via cAMP-dependent and -independent mechanisms (11, 26), possibly involving Ca2+ (26) but having only a minimal effect on intracellular Ca2+ transients (38). The mechanism(s) involved requires further study.In conclusion, the present study shows that A2a-receptor
activation increases contractility of the intact heart isolated from rats. This increase in contractility is independent of activation of
-adrenergic receptors or the increase in coronary flow mediated by
adenosine-induced vasodilation. Although the adenosine
A2a-receptor-elicited positive inotropic response is rather
small in magnitude, it may be important in providing contractile
support of the mechanically depressed hypoperfused heart in which
interstitial adenosine levels are increased (10).
Furthermore, A2a-receptor activation has been demonstrated
to attenuate the antiadrenergic action of adenosine mediated by
A1 receptors (28). Thus inhibition of the
antiadrenergic effect of adenosine coupled with its intrinsic direct
positive inotropic effect suggests that stimulation of A2a
receptors is likely to be capable of enhancing and maintaining
myocardial contractility. The role of A2a receptors in
modulation of heart contractile function should be addressed further.
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ACKNOWLEDGEMENTS |
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This publication was made possible by National Institutes of Health Grants AG-11491 and HL-22828. Its contents are solely the responsibility of the authors and do not necessarily represent the official view of the National Institutes of Health.
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FOOTNOTES |
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A preliminary report of this work has been presented in abstract form (FASEB J 13: A110, 1999).
Address for reprint requests and other correspondence: J. G. Dobson, Jr., Dept. of Physiology, 55 Lake Ave. North, University of Massachusetts Medical School, Worcester, MA 01655-0127 (E-mail: James.Dobson{at}umassmed.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 4 January 2000; accepted in final form 17 April 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS AND MATERIALS
RESULTS
DISCUSSION
REFERENCES
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1.
Armstrong, S,
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Adenosine receptor specificity in preconditioning of isolated rabbit cardiomyocytes: evidence of A3 receptor involvement.
Cardiovasc Res
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Behnke, N,
Muller W,
Neumann J,
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Scholz H,
and
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Significant increase from the Ado
value.
) and presence (
) of
pertussis toxin. Values are means ± SE for 5 hearts.
* Statistically significant increase from zero Ado.
