![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
Vol. 288, Issue 2, 805-813, February 1999
, F. U.
Müller,Institut für Pharmakologie und Toxikologie, Westfälische Wilhelms-Universität Münster, Germany
 |
Abstract |
|---|
 Top
 Abstract
 Introduction
Materials and methods
Results
Discussion
References
|
|---|
Diadenosine tetraphosphate (AP4A) is an endogenous compound and exerts diverse physiological effects in animal systems. However, the effects of AP4A on inotropy in ventricular cardiac preparations have not yet been studied. The effects of AP4A on force of contraction (FOC) were studied in isolated electrically driven guinea pig and human cardiac preparations. Furthermore, the effects of AP4A on L-type calcium current and [Ca]i were studied in isolated guinea pig ventricular myocytes. In guinea pig left atria, AP4A (0.1-100 µM) reduced FOC maximally by 36.5 ± 4.3%. In guinea pig papillary muscles, AP4A (100 µM) alone was ineffective, but reduced isoproterenol-stimulated FOC maximally by 29.3 ± 3.4%. The negative inotropic effects of AP4A in atria and papillary muscles were abolished by the A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentylxanthine. In guinea pig ventricular myocytes, AP4A (100 µM) attenuated isoproterenol-stimulated L-type calcium current and [Ca]i. In human atrial and ventricular preparations, AP4A (100 µM) alone increased FOC to 158.3 ± 12.4% and 167.5 ± 25.1%, respectively. These positive inotropic effects were abolished by the P2-purinoceptor antagonist suramin. On the other hand, AP4A (100 µM) reduced FOC by 27.2 ± 7.4% in isoproterenol-stimulated human ventricular trabeculae. The latter effect was abolished by 1,3-dipropyl-cyclopentylxanthine. In summary, after beta adrenergic stimulation AP4A exerts negative inotropic effects in animal and human ventricular preparations via stimulation of A1-adenosine receptors. In contrast, AP4A alone can exert positive inotropic effects via P2-purinoceptors in human ventricular myocardium. Thus, P2-purinoceptor stimulation might be a new positive inotropic principle in the human myocardium.
| |
Introduction |
|---|
|
Top
Abstract
Introduction
Materials and methods
Results
Discussion
References
|
|---|
Diadenosine
polyphosphates are a group of adenine dinucleotides that consist of two
adenosine molecules linked via their 5' position by a chain of two or
more phosphates (abbreviated APnAs, where
n = 2-6 represents the number of phosphates in the
connecting chain). APnAs are synthezised during
protein synthesis (for review see Plateau and Blanquet, 1992
) and are
found in both prokaryotes and eukaryotes. In mammalian tissue,
APnAs have been identified in hepatocytes,
platelets, adrenal chromaffin cells, and brain synaptosomes (for review
see Hoyle, 1990; Pintor and Miras-Portugal, 1995). Several lines of
evidence support the notion that APnAs can exert
pleiotropic physiological effects (for review see Hoyle, 1990; Baxi and
Vishwanatha, 1995).
APnAs are stored in secretory granules of
adrenomedullar chromaffin cells and are exocytotically released into
the vascular system by the action of secretagogues (Pintor et al.,
1991). Furthermore, APnAs are stored in the dense
secretory granules of platelets and they are released during platelet
aggregation. Thereafter, they can exert different effects on other
platelets. For instance, AP3A induces platelet
aggregation, whereas AP4A can inhibit ADP-induced platelet aggregation (for review see Baxi and Vishwanatha, 1995). But
APnAs, once released, can also act on smooth
muscle cells: APnAs stimulate contraction of
nonvascular smooth muscle preparations, e.g.,
AP4A, AP5A and
AP6A elicit pronounced contractions in vesica urinaria, taenia caeci, and vas deferens (Stone, 1981; Hoyle et al.,
1989). Moreover, diadenosine polyphosphates act on vascular smooth
muscle preparations: AP5A and
AP6A exert vasoconstrictory effects in
endothelium-intact preparations, e.g., isolated rat aortic rings
(Schlüter et al., 1994), isolated perfused rat mesenteric arterial beds (Ralevic et al., 1995), and isolated human umbilical arteries (Davies et al., 1995). In contrast, AP4A
exerts both vasoconstrictory and vasodilatory effects. In rat
mesenterial preparations without endothelium,
AP4A leads to vasoconstriction, whereas in
endothelium-intact preparations, AP4A exerts
vasodilatory effects (Busse et al., 1988). Moreover,
AP4A dilates endothelium-intact coronary vessels
from rabbit, pig, and dog hearts (Pohl et al., 1991; Kitakaze et al.,
1995; Nakae et al., 1996). These data indicate that
APnAs are a new class of endogenous regulators of
cardiovascular function whose importance is currently being
appreciated. We have shown that AP6A exerts
negative inotropic effects in the human myocardium via
A1-adenosine receptors (Vahlensieck et al.,
1996). However, as mentioned above, the length of the polyphosphate
chain of APnAs can greatly alter its
physiological function. Hence, we hypothesized that the cardiac effects
of AP4A and other APnAs might differ from those of AP6A. Moreover, we
tested whether the inotropic effects of AP4A and
AP2A might differ in human compared with animal
cardiac preparations and tried to elucidate functionally which
receptors and mechanisms are involved.
| |
Materials and Methods |
|---|
|
Top
Abstract
Introduction
Materials and methods
Results
Discussion
References
|
|---|
Rate and Force of Contraction (FOC) Experiments.
Experiments
were performed as described previously (Bokník et al., 1997).
In brief, right atria, left atria, and right papillary muscles were
isolated from hearts of reserpinized (5 mg/kg, 16 h before
sacrifice) male guinea pigs (300-400 g). Reserpinization was performed
to exclude effects of endogenous catecholamines and for reasons of
comparability with our previous work (Brückner et al. 1985;
Vahlensieck et al., 1996). Furthermore, isolation of left atria
and left papillary muscles from hearts of male rats, ventricular strips
from 1-day-old neonatal rats (Gombosová et al., 1998) and left
atria from murine hearts was performed.
), human atrial and
ventricular preparations at 0.5 Hz (Steinfath et al., 1992); the
voltage was about 10 to 20% greater than the threshold. Preparations
were allowed to contract until a stable mechanical recording was
reached (at least 30 min) before 0.2 U/ml adenosine deaminase (ADA) was
added for additional 30 min. ADA was used to exclude the possibility
that APnAs were degraded to adenosine. ADA
converts adenosine to inactive inosine. In the presence of 0.2 U/ml
ADA, the negative inotropic effect of 1 mM adenosine was abolished
(data not shown).
Furthermore, in some experiments stability of
AP4A in the bathing solution before and at the
end of experiments was checked via high-pressure liquid
chromatography (Heidenreich et al., 1995). There was no
degradation of AP4A and adenosine was not
detectable in the bathing solution. The lower limit of detection for
adenosine was 5 µM.
In concentration-response experiments, AP4A or
AP2A was added cumulatively, each step every 10 min. In single-concentration experiments, the tested
APnA was applied for 10 min. In experiments with
antagonists, preparations were preincubated with the selective A1-adenosine receptor antagonist
1,3-dipropyl-cyclopentylxanthine (DPCPX; 0.3 µM) for 30 min or 1 mM
suramin for 60 min. DPCPX (0.3 µM) applied for 30 min did not
affect contractility (Von der Leyen et al., 1989).
Isolation of Guinea Pig Ventricular Myocytes.
Guinea pig
ventricular myocytes were isolated by a collagenase/protease digestion
of Langendorff-perfused hearts (37°C, 52 mm Hg) as recently described
by Vahlensieck et al. (1996). In brief, hearts were perfused for 5 min
with calcium-free solution A (135 mM NaCl, 4 mM KCl, 0.3 mM
NaH2PO4, 1 mM MgCl2, 10 mM HEPES, 10 mM dextrose, pH 7.4), before enzymes were added to this
solution at a flow-independent dose rate of 1.4 mg/min
collagenase (type 1; Worthington Biochemical, Freehold, NJ) and 0.6 mg/min protease (type XIV; Sigma Chemical Co., Deisenhofen, Germany)
over a period of 5 min. Afterwards the hearts were perfused for 10 min
with enzyme-free solution A containing 0.2 mM calcium. Cells were
harvested after mincing the hearts with fine scissors, gentle agitation of the tissue, and filtering through a nylon mesh.
Measurement of L-Type Calcium Current (ICaL).
Freshly isolated cardiomyocytes were plated in Petri dishes that served
as recording chambers (volume ~1 ml) on the stage of an inverted
microscope (Leica, Köln, Germany). Electrophysiological experiments were performed as described previously (Vahlensieck et al.,
1996). In brief, solution A supplemented with 2 mM calcium served as
the extracellular solution and recording pipettes (soft glass coated
with Sylgard, 1.5-2.5 M
) were filled with 80 mM K-aspartate, 50 mM
KCl, 10 mM KH2PO4, 0.5 mM MgCl2, 3 mM MgATP, 5 mM HEPES, 1 mM EGTA, pH 7.4. ICaL were elicited
by voltage steps from a holding potential of 
40 mV to a test
potential of +10 mV for 300 ms, applied every 10 s. Current was
recorded by an L/M-PC-amplifier (LIST-Electronic, Darmstadt, Germany)
connected to a 486 computer, which was equipped with the ISO2 software
(version 1.2; MFK, Niedernhausen, Germany). Currents were evaluated as the difference between peak inward current and the current level at the
end of the test pulse. Series resistance was compensated to the maximum
possible extent, using the feedback circuitry of the amplifier.
Measurement of Cytosolic Calcium [Ca]i.
Freshly isolated myocytes were placed onto a Petri dish on the stage of
a modified inverted microscope (Diaphot 200; Nikon, Tokio, Japan). The
cells were incubated with solution B (135 mM NaCl, 4.8 mM KCl, 0.3 mM
NaH2PO4, 1 mM MgCl2, 2 mM
CaCl2, 10 mM HEPES, and 10 mM dextrose, pH 7.4)
additionally containing cell-permeant indo-1/AM (25 µM; Molecular
Probes, Eugene, OR) and 2.5% of the nonionic surfactant Pluronic F-127
(20% in DMSO; Molecular Probes). After 3 min the cells were perfused
with solution B (0.8 ml/min) for at least 20 min to allow the washout
of the extracellular dye and intracellular indo-1 deesterfication. The
myocytes were electrically stimulated via platinum wire electrodes with
a frequency of 1 Hz. The experiments were performed at room temperature
to minimize loss of the Ca++ indicator from the cells
(Spurgeon et al., 1990).
Chemicals.
APnAs (n = 2-5)
purchased from Sigma Chemical Co. were purified chromatographically
before use as described previously (Heidenreich et al., 1995). DPCPX
(Research Biochemicals International, Natick, MA), ADA (Boehringer
Mannheim, Germany), (±)-isoprenaline-HCl (Boehringer Ingelheim), and
suramin sodium (ICN Biomedicals, Eschwege, Germany) were used as
described. All other chemicals were of analytical or best commercial
grade available. Deionized and twice distilled water was used throughout.
Statistical Analyses. The experimental data given in text and figures are means ± S.E.M. of n experiments. Each n represents one muscle or one cell that was treated only one time with the respective testing protocol, i.e., there were no replicates included into the data presented.
The significance of differences of contractile responses were estimated by means of two-sided Student's t test for unpaired observations. Data of multiple groups were compared using one-way analysis of variance followed by Bonferroni's correction for multiple comparisons. A p value less than .05 was regarded as significant. In guinea pig cardiac preparations no plateau effects were achieved within the concentrations investigated. Thus, IC20 values were determined. The IC20 value is the concentration that produces a reduction to 80% of the prestimulation value. The IC20 values were determined by linear interpolation.| |
Results |
|---|
|
Top
Abstract
Introduction
Materials and methods
Results
Discussion
References
|
|---|
Effects of APnAs in Animal Cardiac Preparations. In isolated spontaneously beating guinea pig right atria, AP4A and AP2A (0.1-100 µM) exerted concentration-dependent negative chronotropic effects starting at 10 µM, reducing the frequency maximally by 33.7 ± 6.1% (n = 8, Fig. 1) and 48.9 ± 12.8% (n = 4, Fig. 1), respectively. The IC20 value amounted to 8 µM (AP4A) and 16 µM (AP2A). These effects were abolished by 0.3 µM DPCPX (Table 1). The negative chronotropic effects were reversible upon washout.
|
|
|
|
). Now,
AP2A and AP4A were tested
and thus, for comparison, the effects of AP3A and
AP5A (each 100 µM) on frequency and FOC in
isolated guinea pig cardiac preparations were studied as well.
AP3A and AP5A reduced
frequency in spontaneously beating right atria by 18.6 ± 2.9%
and 26.4 ± 10.8% (n = 3, Table 1), respectively.
In electrically driven left atria, AP3A and
AP5A reduced FOC by 44.8 ± 4.9%
(n = 4, Table 1) and 39.3 ± 6.1%
(n = 4, Table 1), respectively. In papillary muscles,
AP3A and AP5A alone had no
effect on FOC but reduced isoproterenol-stimulated FOC by 17.5 ± 2.2% (n = 8, Table 1) and 13.6 ± 1.3%
(n = 4, Table 1), respectively. Analysis of variance revealed no significant difference comparing the effects of the different APnAs. Thus AP2A,
AP3A, AP4A,
AP5A, and AP6A (Vahlensieck et al., 1996) exerted qualitatively and quantitatively similar negative
chronotropic and inotropic effects in the guinea pig myocardium. For a
synopsis see Table 1.
Others previously investigated the effects of
AP4A on rat vascular tissue, e.g., mesenteric
arterial bed and aortic rings (Schlüter et al., 1994; Ralevic et
al., 1995). Hence, for comparison we tested the effects of
AP4A on rat cardiac preparations. The results
were similar to those in the guinea pig myocardium. In rat left atria,
AP4A (100 µM) reduced FOC by 22.1 ± 2.1%
(n = 5). In rat papillary muscles,
AP4A alone did not change FOC significantly (n = 8, data not shown). However, after isoproterenol
stimulation (10 nM), AP4A reduced FOC by
13.9 ± 5.9% (n = 4). The effects of, for
instance, beta adrenergic stimulation are larger in adult than in neonatal cardiac preparations. Hence, we tested whether the
inotropic effects of AP4A change postnatally.
AP4A (100 µM) alone did not change FOC
significantly (n = 3), whereas in
isoproterenol-stimulated neonatal ventricular preparations,
AP4A reduced FOC by 17.7 ± 2.7%
(n = 4). Thus, similar effects were noted in isolated
ventricular preparations from neonatal and adult rats.
In the murine heart the presence of a new unique
AP4A receptor has been shown (Hilderman et al.,
1991; Walker et al., 1993). However, it has not been tested whether
this receptor couples to inotropy. Thus, we investigated the effects of
AP4A in isolated murine left atria.
AP4A (100 µM) reduced FOC by 27.7 ± 1.7%
(n = 5). This effect was blocked by 0.3 µM DPCPX
(n = 6). In contrast, binding of
AP4A to the novel AP4A
receptor was not affected by adenosine receptor agonists or antagonist
like theophylline (Hilderman et al., 1991). Thus, the negative
inotropic effect of AP4A in murine atria was
apparently mediated via DPCPX-sensitive
A1-adenosine receptors and not via this novel
AP4A receptor.
Effects of AP4A on ICaL. Application of 100 µM AP4A alone to isolated guinea pig ventricular myocytes did not affect the amplitude of ICaL (n = 4, data not shown). Measurements were performed in four individual cells obtained from three guinea pig hearts. Stimulation with 10 nM isoproterenol increased the calcium current to 433.0 ± 55.5% of control (n = 4, Fig. 4B). Additional application of 100 µM AP4A attenuated the isoproterenol-stimulated calcium current to 153.1 ± 12.9% of control (n = 4). Measurements were performed in four individual cells obtained from two guinea pig hearts. A typical experiment illustrating the original time course of the amplitude of the calcium current through L-type calcium channels is depicted in Fig. 4A.
|
Effects of AP4A on [Ca]i. Application of 100 µM AP4A alone to isolated guinea pig ventricular myocytes did not affect the [Ca]i (n = 4, data not shown). Measurements were performed in four individual cells obtained from two guinea pig hearts. Stimulation with 3 nM isoproterenol increased the calcium concentration to 198.1 ± 24.6% of control (n = 12, Fig. 5B). Additional application of 100 µM AP4A attenuated the stimulated calcium transient to 148.6 ± 13.5% of control (n = 12). Measurements were performed in 12 individual cells obtained from five guinea pig hearts. A typical experiment illustrating the time course of the amplitude of the free [Ca]i is depicted in Fig. 5A.
|
Effects of AP4A in Human Cardiac Preparations. All subsequent experiments were performed with 100 µM APnAs based on the findings reported above in animal preparations. In contrast to all animal atrial prepreparations studied, 100 µM AP4A alone increased FOC to 158.3 ± 12.4% in isolated electrically driven human atrial trabeculae carneae (n = 9, Fig. 6). This effect was blocked by the P2-purinoceptor antagonist suramin (1 mM, n = 4, Fig. 6B). Suramin itself in the concentration used, i.e., 1 mM, did not change FOC (data not shown).
|
|
; Schmitz et al., 1992). However, carbachol (10 µM) did not
reduce the positive inotropic effect of 100 µM
AP4A (n = 4, data not shown).
Thus, the positive inotropic effect of AP4A in
human ventricular preparations is not beta
adrenoceptor-mediated and probably not due to phosphodiesterase inhibition.
In contrast to the effects of AP4A alone, 100 µM AP4A reduced FOC in isoproterenol (3 µM)-stimulated human ventricular trabeculae by 27.2 ± 7.4%
(n = 6, Fig. 8). The
negative inotropic effect was abolished in the presence of 0.3 µM
DPCPX (n = 5, Fig. 8B). Qualitatively similar results
were obtained with lower isoproterenol concentrations (0.03 µM; data
not shown).
|
). For clarity, a synopsis is presented as Table 2.
|
| |
Discussion |
|---|
|
Top
Abstract
Introduction
Materials and methods
Results
Discussion
References
|
|---|
A crucial finding of the present investigation is that AP4A exerts both positive and negative inotropic effects in human cardiac preparations, whereas in animal cardiac preparations only negative inotropic effects were observed. This exemplifies the difficulties in extrapolating animal studies to human pharmacology. However, it is not known whether AP4A exerts positive inotropic effects in the nonfailing human myocardium as well.
The experiments in guinea pig cardiac preparations present evidence
that the negative inotropic effects of AP4A are
mediated via A1-adenosine receptors. Adenosine
causes a negative chronotropic effect by slowing of the pacemaker rate
in the sinus node (West et al., 1987) and in left atria a direct
negative inotropic effect due to an activation of a potassium outward
current (Wang and Belardinelli, 1994; Brückner et al., 1985).
Like adenosine or the selective A1-adenosine
receptor agonist
()-N6-phenylisopropyladenosine
(R-PIA), AP4A reduced force and rate of contraction in guinea pig atrial preparations. These effects like
those of R-PIA (Von der Leyen et al., 1989) were abolished by the A1-adenosine receptor antagonist DPCPX
(0.3 µM). Of note, AP4A exerted a positive
inotropic effect in the presence of DPCPX. We speculate that this
effect is due to stimulation of P2-purinoceptors. However, further experiments have to clarify whether this speculation holds true.
In guinea pig papillary muscles, R-PIA does not increase
FOC, whereas after stimulation with cAMP-increasing agents like
beta adrenoceptor agonists, additionally applied
R-PIA reduces FOC (Schmitz et al., 1985). Similarly,
AP4A alone did not change FOC in guinea pig
papillary muscles, but exerted negative inotropic effects after
isoproterenol stimulation. Again, like for R-PIA (Von der
Leyen et al., 1989), these contractile effects were abolished by DPCPX.
The reduction of isoproterenol-stimulated FOC by adenosine or
R-PIA is accompanied by an inhibition of the current through L-type calcium channels (Brückner et al., 1985; Kato et
al., 1990) and a reduction of the free
[Ca]i (Fenton et al., 1991). AP4A markedly attenuated the amplitude of
isoproterenol-stimulated ICaL and the
isoproterenol-stimulated free [Ca]i. Thus, all
direct effects of AP4A in guinea pig cardiac
preparations are consistent with stimulation of
A1-adenosine receptors.
In some vascular physiological systems APnAs
exert opposite effects dependent on their polyphosphate chain length.
Thus, we hypothesized that APnAs have different
effects on myocardial contractility as well. However, all of the
APnAs (n = 2-5) investigated
here, as well as AP6A (Vahlensieck et al., 1996),
exerted negative inotropic effects in guinea pig cardiac preparations.
This is in agreement with a previous report in guinea pig left atria
(Hoyle et al., 1996). However, the effects of
APnAs in ventricular preparations had not been
investigated before.
For comparison, the effects of AP4A on cardiac
inotropy were studied in mouse and rat cardiac preparations. In mouse
left atria AP4A exerted negative inotropic
effects. Although a special AP4A-binding protein
in the murine heart has been identified (Hilderman et al., 1991;
Walker et al., 1993), at least the inotropic effects in murine atria
are DPCPX sensitive and hence apparently
A1-adenosine receptor mediated. In rat cardiac
preparations AP4A exerted negative inotropic
effects in left atria and in isoproterenol-stimulated papillary
muscles. Thus, in all animal species tested, i.e., guinea pig, rat, and
mouse, AP4A exerted only negative inotropic effects.
In contrast to the animal preparations, in human atrial and ventricular
trabeculae carneae AP4A alone exerted pronounced
positive inotropic effects. This effect is not consistent with
stimulation of A1-adenosine receptors, because
adenosine alone does not affect FOC in human ventricular preparations
(Böhm et al., 1985).
Of note, the positive inotropic effect of AP4A
was accompanied by prolonged contractile time parameters. In contrast,
beta adrenoceptor stimulation (e.g., by isoproterenol)
shortens contraction time, probably by phosphorylation of a protein in
the sarcoplasmic reticulum (phospholamban) and disinhibition of the
enzyme that mediates the Ca++ uptake into the
sarcoplasmic reticulum (Koss and Kranias, 1996). Moreover, the positive
inotropic effects of AP4A were not attenuated by
the beta adrenoceptor antagonist propranolol. In contrast, a
prolongation of time parameters is seen after alpha
adrenoceptor stimulation or due to calcium sensitizers (Neumann et al.,
1996). However, the positive inotropic effect was not sensitive to
prazosin, thus alpha adrenoceptor agonism is not likely
involved. Moreover, because propranolol and prazosin failed to abolish
the positive inotropic effect of AP4A, this
effect is not due to catecholamine release. It is conceivable that
AP4A could sensitize the myofilaments to
Ca++. However, AP4A is a
very polar compound and it is thus unlikely to permeate passively
through the cell membrane. Moreover, the positive inotropic
effects of AP4A were blocked by the
P2-purinoceptor antagonist suramin. Thus, the
positive inotropic effect of AP4A is most
probably due to P2-purinoceptor agonism. To the
best of our knowledge, this is the first evidence that stimulation of P2-purinoceptors can increase FOC in human
myocardial preparations. It needs to be elucidated whether
P2X or P2Y receptors are
involved. Regrettably, no selective high-affinity antagonists for the
receptor subtypes are presently available. We failed to block the
effects by the P2Y receptor antagonist basilene
blue, which exerted inotropic effects of its own at the high
concentrations studied (100 and 500 µM). It is tempting to speculate
that the P2 receptor involved is coupled to
phospholipase C as previously demonstrated for the alpha1 adrenoceptor in the human
heart (Kohl et al., 1989), which could explain why
AP4A prolongs contractile time parameters.
Although in guinea pig myocardium all APnAs
exerted qualitiatively similar negative inotropic effects, in human
cardiac preparations other APnAs, i.e.,
AP2A (this study) and AP6A
(Vahlensieck et al., 1996) had only marginal or no positive inotropic
effect. This indicates that polyphosphate chain length plays a role for receptor agonism in the human heart.
In addition to the positive inotropic effects,
AP4A exerted negative inotropic effects in human
ventricular myocardium after beta adrenergic stimulation.
These negative inotropic effects were mediated via DPCPX-sensitive
A1-adenosine receptors as in guinea pig
ventricular preparations. A1-adenosine receptors
have been clearly identified and functionally studied in the human atrium and ventricle (Böhm et al., 1985, 1989).
One could argue that the effects of AP4A in human
myocardium were observed at relatively high concentrations. However, we speculate that similar concentrations could be achieved in vivo. The
concentration of AP4A stored in granules of
bovine adrenomedullar chromaffin cells was about 6 mM (Rodriguez del
Castillo et al., 1988). Although the concentration of
AP4A in human adrenomedullar tissue has not yet
been determined, it is conceivable that AP4A will
be stored and released from human chromaffin granules as well,
providing high concentrations in the circulation. In addition AP4A might be provided by local release
mechanisms: AP4A is stored in human platelets and
is released during platelet aggregation with local extracellular
concentrations exceeding 100 µM (Flodgaard and Klenow, 1982;
Lüthje et al., 1987). Thus, during thrombus formation high
concentrations are present locally. Moreover,
AP4A is present in the human cardiac tissue (H. Schlüter, unpublished observations) and might be produced and
released from cardiomyocytes. Even higher concentrations might be
achieved under conditions of stress. APnAs appear
after heat shock or exposure to a wide variety of oxidants with
severalfold increased concentrations (Bonaventura and Cashon, 1992).
Furthermore, in open-chest dogs, AP4A was found
in the coronary venous blood during ischemia and reperfusion, whereas
it was not detected under nonischemic conditions (Kitakaze et al.,
1995).
Providing that high concentrations can occur, is there any conceivable
physiological role for AP4A in the human heart?
At least in open-chest dogs administration of
AP4A before and after sustained ischemia
reduced infarct size (Node et al., 1995). Thus, AP4A might mediate cardioprotection against
ischemia and reperfusion injury in humans. Recently, it has been
demonstrated that adenosine preconditioned human myocardium against
ischemia in vivo (Leesar et al., 1997). Thus,
AP4A might serve as preconditioning substance as well.
In patients with heart failure, endogenous catecholamine levels are
increased (Daly and Sole, 1990), and similar to adenosine, AP4A might attenuate the effects of sympathetic
overstimulation. Finally, under appropriate conditions,
AP4A might increase cardiac inotropy. Thus, the
design of P2-purinoceptor agonists might offer a
novel modality for therapy of end-stage heart failure.
| |
Acknowledgments |
|---|
We gratefully acknowledge the expert technical assistance of H. Sickler. This work was supported by Deutsche Forschungsgemeinschaft and Deutsche Akademie der Wissenschaften.
| |
Footnotes |
|---|
Accepted for publication August 4, 1998.
Received for publication March 23, 1998.
1
Current address: Klinik und Poliklinik für
Thorax-, Herz-, Gefächirurgie, Westfälische
Wilhelms-Universität Münster, Germany.
2 Current address: Medizinische Klinik I, Marienhospital Herne, Ruhr-Universität Bochum, Germany.
3 Current address: Klinik für Thorax- und Kardiovaskuläre Chirurgie, Heinrich Heine-Universität Düsseldorf, Germany.
Send reprint requests to: Dr. med. Ute Vahlensieck, Institut für Pharmakologie und Toxikologie, Westfälische Wilhelms-Universität Münster, Domagkstr. 12, D-48129 Münster, Germany.
| |
Abbreviations |
|---|
APnAs, diadenosine polyphosphates;
AP4A, diadenosine tetraphosphate;
DPCPX, 1,3-dipropyl-cyclopentylxanthine;
R-PIA, ()-N6-phenylisopropyladenosine;
FOC, force
of contraction;
ICaL, L-type calcium current;
[Ca]i, intracellular calcium concentration.
| |
References |
|---|
|
Top
Abstract
Introduction
Materials and methods
Results
Discussion
References
|
|---|
AP4A and AP5Afrom perfused bovine adrenal medulla and isolated chromaffin cells.
Life Sci
48:
2317-2324[Medline].AP4A and AP5Ain bovine adrenal medulla: presence in chromaffin granules.
J Neurochem
51:
1696-1703[Medline].This article has been cited by other articles:
|
|
 |
|
  A. L. Garcia-Villalon, L. Monge, N. Fernandez, A. Salcedo, R. Narvaez-Sanchez, and G. Dieguez Coronary response to diadenosine pentaphosphate after ischaemia-reperfusion in the isolated rat heart Cardiovasc Res, February 1, 2009; 81(2): 336 - 343. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 U. Gergs, P. Boknik, W. Schmitz, A. Simm, R.-E. Silber, and J. Neumann A positive inotropic effect of ATP in the human cardiac atrium Am J Physiol Heart Circ Physiol, April 1, 2008; 294(4): H1716 - H1723. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Luo, V. Jankowski, N. Gungar, J. Neumann, W. Schmitz, W. Zidek, H. Schluter, and J. Jankowski Endogenous Diadenosine Tetraphosphate, Diadenosine Pentaphosphate, and Diadenosine Hexaphosphate in Human Myocardial Tissue Hypertension, May 1, 2004; 43(5): 1055 - 1059. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. Vassort Adenosine 5'-Triphosphate: a P2-Purinergic Agonist in the Myocardium Physiol Rev, April 1, 2001; 81(2): 767 - 806. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
 |
 |
|
 |
 |
 |
, n = 4) and AP4A
(
, n = 8) on frequency in isolated spontaneously
beating guinea pig right atria. Ordinate: frequency in percent of
predrug values (Ctr), predrug values amounted to 154 ± 12 beats
per minute; abscissa: APnAs concentration in M, vertical
lines show S.E.M. Asterisks denote significant differences versus
Ctr.
