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Adenosine receptors were studied on heart cells grown in cultures by the radioligand binding technique. We used the hydrophilic A1 adenosine receptor radioligand [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX), to monitor the level of the receptors on intact cardiocytes. The binding showed high affinity (Kd = 0.13 nM) and the number of [3H]CPX binding sites (Bmax) was 23.1 fmol/dish (21 fmol/mg protein). The Ki of the agonists R-N6-(2-phenylisopropyl)-adenosine (R-PIA) and S-N6-(2-phenylisopropyl)-adenosine (S-PIA), and of the antagonists CPX and theophylline were 3.57, 49.0, 1.63 and 4880 nM, respectively. The number of adenosine receptors was very low during the first days in cultures (5 fmol/dish) and increased gradually until it reached a plateau on days 8–10. Treatment with norepinephrine or isoproterenol which accelerated the rate of contractions, induced up regulation of the receptors. Bmax increased 2–3-fold by application of norepinephrine for 4 days, while receptor affinity to the radioligand was unaffected. Lactate dehydrogenase (LDH) and creatine kinase (CK) activity increased only by 22 and 38%, respectively. Similarly, 3 days treatment with triiodothyronine (T3, 10−8 M), which also accelerated heart rate, increased the number of adenosine receptors by 56% without a significant change in the affinity of the receptors to [3H]CPX. Carbamylcholine (5 × 10−6 M), which reduced the rate of heart contractions, caused 26% down regulation while the affinity of the receptors remained unchanged. It is concluded that there is a linkage between the rate of cardiac contractions and the level of adenosine receptors. Thus, the level of adenosine receptors may respond to drug-induced chronic changes in cardiac contractile activity so as to restore conditions to normal (basal) contractions.
Adenosine modulates a variety of physiological functions in the heart. These actions are mediated by cell surface adenosine receptors, which can be classified into A1 and A2 subtypes , based upon their properties of activating (A2) or inhibiting (A1) adenylate cyclase activity, and according to the order of agonist potency [2,3].
In conditions of stress like hypoxia or ischemia, the concentration of adenosine in the extracellular fluid rises dramatically, mainly through the breakdown of ATP . In these conditions adenosine has therapeutic and protective effects on the heart [5,6]. Adenosine causes negative chronotropic, dromotropic and ionotropic effects on the cardiac tissue via A1 receptors [7,8]. Adenosine also acts as a coronary vasodilator via A2 receptors . These effects of adenosine occur also without prior catecholamine treatment . Stimulation that enhances the cAMP content (catecholamines, forskolin, amrinone), makes the heart more sensitive to adenosine [11-14]. In addition, adenosine causes hyperpolarization by activation of K+ channels  via G proteins [16, 17]. These K+ channels are the same channels that are stimulated by acetylcholine in the cardiac tissue [18-20].
Chronic treatment with an adenosine agonist or antagonist is capable of regulating the density of adenosine A1 receptors in the cardiac tissue [21-23]. Since these treatments affect the rate of heart contractions, we addressed the question of whether modulation of cardiac contractility contributes to regulating the synthesis of A1 adenosine receptors. We describe here the binding properties of adenosine receptors in intact rat cardiocytes grown in culture using [3H]CPX (DPCPX)‡, which has been reported to be an A1 selective antagonist . The radioligand binding technique has been used to study the possible mechanisms involved in up or down regulation of adenosine receptors as a result of treatment with agents affecting contractile activity. We show that NE or TH, which accelerate the heart rate of contractions, cause an elevation in the receptor level, whereas Carb, which attenuates the rate of contractions, reduces the level of adenosine receptors. Thus, our data indicates that contractile activity plays a major role in controlling the level of adenosine receptors in the heart.
Rat hearts (l–2 days old) were removed under sterile conditions and washed three times in PBS to remove excess blood cells. The hearts were minced to small fragments and then gently agitated in a solution of proteolytic enzyme-RDB (Ness-Ziona, Israel) prepared from a fig tree extract. The RDB was diluted 1:50 in PBS, at 25° for a few cycles of 10 min each, as described previously [25-27]. The supernatant suspensions containing dissociated cells, to which medium containing 10% horse serum (Biolab, Jerusalem, Israel) was added, were centrifuged at 150 g for 5 min. After centrifugation, the supernatant phase was discarded and cells were resuspended in high glucose (5 mg/mL) DMEM (Gibco, Uxbridge, U.K.) supplemented with 10% heat-inactivated horse serum and 2% chick embryo extract. The suspension of cells was diluted to 1.2 × 106 cells/mL and 1.5 mL were placed in 35 mm collagen-gelatin-coated plastic culture dishes. Cultures were incubated in a humidified atmosphere of 10% CO2, 90% air at 37°. The incubated cells were washed well 24 hr after plating to remove unattached cells. Confluent monolayers, which exhibit spontaneous contractions, developed in culture within 2 days. The growth medium was replaced every 3–4 days.
Intact cells were incubated at room temperature (22–25°) for 60 min, with various concentrations of [3H]CPX. 1 μg/mL dipyridamole, 0.2 U/mL adenosine deaminase in PBS, pH 7.4. Incubation was stopped by quick rinsing the cells five times with cold (4–10°) PBS. The cells were solubilized with 0.2 mL 0.5 N NaOH and neutralized upon addition of 0.1 mL 2 N Tris–HCl, pH 3.7. After addition of 3 mL of scintillation liquid radioactivity was determined using a βcounter. Non-specific binding of [3H]CPX was defined as the amount of radioactivity remaining after incubation with theophylline (5 mM). Specific [3H]CPX binding was calculated as the total radioactivity bound minus the non-specific binding (less than 20%).
For calculation of the association rate constant, k1, specifically bound [3H]CPX (0.6 nM) was determined as a function of time as indicated in the graph (Fig. 1).
In experiments studying the dissociation of [3H]-CPX binding, cells were incubated at 25° with 0.6 nM [3H]CPX to equilibrium (60 min). At time zero, the binding mixture was replaced by PBS (1 mL) containing theophylline (5 mM). The experiment was terminated as in the ligand binding method. koff was calculated from the slope of the plot.
For the competition experiments, cardiomyocytes were incubated in a mixture containing the competing drug at various concentrations and 0.6 nM [3H]CPX for 60 min at room temperature.
Number of beats/min were counted under a phase contrast microscope equipped with video motion detector system. Five dishes were monitored at random through a 25× objective.
Carb and Iso were dissolved in PBS. T3 was dissolved in 0.1 N NaOH and applied to the heart culture dishes to final concentration of 10 nM. Norepinephrine hydrochloride was dissolved in 0.1 mM of ascorbic acid. Dipyridamole was dissolved in 10% ethanol and applied to the cardiocytes to final concentration of 1 μM. Drugs and chemical used were from the Sigma Chemical Co. [3H]CPX (sp. act. 108.3 Ci/mmol) was purchased from Amersham International (Little Chalfont, Bucks, U.K.). R-PIA, S-PIA and CPX were obtained from Research Biochemicals International (Natick, MA, U.S.A.).
The time-course of specific [3H]CPX binding to intact cardiomyocytes grown in culture is illustrated in Fig. 1. The binding reached equilibrium within 7 min of incubation and was maintained for an additional 70 min. The rate constant for the pseudo first order association reaction (kob) was calculated to be 0.337/min (Fig. 1, inset).
The dissociation of [3H]CPX from the receptor binding sites was reversible and temperature dependent. At 25°, 50% of the ligand was dissociated from the receptors within 8 min (Fig. 2). Equilibrium was achieved after 20 min of dissociation. The dissociation rate constant (koff) was calculated to be 0.099/min (Fig. 2, inset).
The association constant (k1) was calculated from the equation k1 = kob − koff/([3H]CPX) according to Cheng and Prusoff , and was found to be 0.397/min/nM. Thus, the equilibrium dissociation constant determined kinetically from the ratio koff/k1 was estimated to be 0.25 nM.
To define the saturability of the ligand, intact cardiocytes (6 days old) were incubated at room temperature with various concentrations of [3H]CPX. For the non-specific binding the cardiocytes were incubated also with 5 mM theophylline. Figure 3 shows the relationship between the radioligand concentrations and the number of specific binding sites in the cultured cardiomyocytes. Maximal saturation of the antagonist occurred at a concentration of 0.2nM. Scatchard analysis  indicates that the maximum number of binding sites is 23.1 fmol/dish (21 fmol/mg protein) and the Kd for [3H]CPX is 0.13 nM (Fig. 3, inset), which is in adequate agreement with the kd value calculated from the kinetic experiments. The data best fits a straight line which indicates that there is one binding site on the receptor.
The agonists R-PIA and S-PIA, and the antagonists CPX and theophylline were examined in competition binding assays with [3H]CPX (Fig. 4). The inhibition constants (Ki) were calculated according to Bylund and Yamamura . Figure 4 indicates that CPX is the most A1 potent antagonist (Ki = 1.6 nM). After CPX, the agonist R-PIA (Ki = 3.57) is more potent than the agonist S-PIA (Ki = 49.0 nM) and the antagonist theophylline (Ki = 4880 nM).
[3H]CPX binding in cardiocytes was investigated during their development in culture. It was found that the number of A1 adenosine receptor binding sites increased with culture age, without reaching the plateau, whereas CK activity reached a plateau (0.70–0.75 U/dish) in 5-day-old cultures (Fig. 5).
Since NE and Iso stimulate the contraction of cardiomyocytes, we investigated whether this increase in the rate of heart contractions is associated with a change in the level of adenosine receptors. Thus, treatment with NE for 4 days caused an increase of 270% of [3H]CPX binding (Fig. 6). Scatchard analysis (Fig. 6, inset) shows that the affinity of the receptor for its ligand remained almost unchanged. There was no significant difference in protein content between control and NE-treated cells (1.08 ± 0.14 and 1.18 ± 0.18 mg/dish, respectively). However, in NE treated cells LDH and CK activity increased by 22 and 34%, respectively. The rate of heart contractions was doubled, whereas cell number was decreased by 17% (Table 1). The effect of catecholamines was time (Fig. 7) and dose dependent (Fig. 8). Figure 7 shows an increase of 30, 105 and 171% in [3H]CPX binding following 2, 3 and 4 days of NE treatment, respectively, and an increase of 41% after 2 days of Iso treatments. Figure 8 shows a dose–response of [3H]CPX binding following ISO or NE treatments. Increases of 20, 28 and 41% were achieved by 10−7, 10−6, 10−5 M of Iso treatments, respectively (Fig. 8). [3H]CPX binding was elevated by 36, 57 and 77% in 10, 20 and 100 μM, respectively, following 60 hr of NE treatment (Fig. 8).
Thyroid hormones like catecholamines stimulate the contraction of cardiomyocytes . Therefore, the effect of access thyroid hormones on [3H]CPX binding was studied. Three days of treatment with T3 increased the number of adenosine receptors by 56% (Fig. 9) without a significant change in the affinity of the receptors to [3H]CPX (Fig. 9, inset). The maximum number of binding sites (Bmax) in T3 treatment was 5.0 ± 0.23 fmol/dish and 3.2 ± 0.14 fmol/dish in the control (the experiment was carried out in multi wells and not in the ordinary 35 mm dishes). There were no significant differences between T3 and the control on the affinity of the receptors. The Kd was 0.11 nM (Fig. 9, inset). The dose effect on T3 on [3H]CPX binding is shown in Fig. 10. The maximum increase of adenosine receptors was obtained at 10 nM of T3. Higher concentrations than 100 nM causes toxic effects as revealed by a decrease in CK and LDH activity in the homogenates (data not shown). Protein content and cell number in T3-treated cells were similar to the control (data not shown).
Carb is known as an inhibitor of contractile force and heart rate . Therefore we examined the influence of Carb on adenosine receptors. It was found that [3H]CPX binding was decreased by 26% following 72 hr of Carb treatment (Fig. 11), whereas the rate of contractions was reduced from 91 ± 16 to 30 ± 18 beats/min. Scatchard analysis (Fig. 11, inset) indicates that the Bmax decreased from 25.5 fmol/dish to 18.7 fmol/dish. Kd levels were unchanged significantly (0.16 and 0.17 nM in control and Carb-treated cells). The Carb influence on adenosine receptors was dose dependent (Fig. 12). [3H]CPX binding was reduced by 2, 17,45 and 53% following 84 hr of 10−3, 10−4, 10−5 and 10−6 M of Carb treatment, respectively. Protein content and cell number in Carb-treated cells were similar to that of the control (0.8 mg/dish and 82 × 104 cells/dish, respectively).
We have characterized and quantified the surface A1 adenosine receptors in intact cardiomyocytes grown in cultures, using the antagonist [3H]CPX. The binding of [3H]CPX is saturable, reversible and of high affinity, The dissociation constant of this ligand is Kd = 0.13 nM (Fig. 3). This value is lower than that obtained by Liang , who found in the membranes of chick atrial cultured cardiomyocytes to be 2.1 nM. The difference may be related to species differences, or due to differences in experimental conditions: intact cells versus a membrane preparation. When both of these differences were eliminated, like in the Martens experiment, who measured adenosine receptors on intact rat cultured ventricutar cells, then the Kd was 0.48 nM, which is closer to our results . A recent study using [3H]CPX binding to bovine cardiac membranes  reported a Kd of 0.12 nM, which is similar to our study.
The Bmax value in chick atria1 myocytes was 26.2 fmol/mg protein . Considering that this value was obtained in membranes preparation, it is higher than our Bmax(20–30 fmol/mg. protein). We have calculated the number of receptors per cell, assuming that the receptor distribution is uniform among the cells, to be 14–15 × 103 receptors/cardiomyocyte. This number is lower than the number of adenosine receptors obtained by Martens  in adult heart cells (40,000). This difference may reflect age dependence, as can be seen in Fig. 5, that during the development of the cultures the receptor level continues to increase with time. The number of adenosine receptors is about 10–20 times lower than the muscarinic acetylcholine receptors on the same cells , or in cardiac atrium . The number of adenosine A1 receptors is similar to the number of β adrenoreceptors, e.g. 24,000 .
Estimates of adenosine A1 receptor density in cardiac cell membranes are in the range of 15–30 fmol/mg protein in rat , pig , guinea pig [22,37], chick embryos [21,36], bovine  and diseased human cardiac preparations . There are differences in the density of atria1 adenosine receptors (A1) according to the species: guinea pig > rat > rabbit . This may explain the different sensitivity of the AV node to adenosine which has been reported as: guinea pig > man > rabbit > dog [37,40].
The present study shows an increase in the [3H]-CPX binding (Bmax) with no significant change in the Kd upon exposure of the cardiomyocytes to NE or Iso for 48–96 hr. Although this treatment also caused an elevation in CK and LDH activities (Table 1), nevertheless, the increase in [3H]CPX binding is 7–8-fold higher than the elevation of these enzymes. Thus, we can regard this stimulation by catecholamines as specific for A1 receptors.
The increase in the level of adenosine receptors following catecholamine treatment might be reflected by increased sensitivity to adenosine linked to a functional response. These responses include negative chronotropic, dromotropic and ionotropic effects on the heart [7,8]. A1 adenosine receptors in the heart also appear to mediate the attenuation of the positive inotropic effects of catecholamine [12, 13, 41-45]. Thus, the increased level of adenosine receptors following catecholamine treatment might act as an inhibitory feedback to protect heart cells from excessive contractions.
Many of the physiological and biochemical responses characteristic of hyperthyroidism are similar to those induced by adrenergic stimulation. We have previously demonstrated that thyroxine induces an increase in contraction rate in cultured heart cells . Therefore, we analysed the effect of TH on adenosine receptors. When cultured heart cells were exposed to T3 for 72 hr, a 56% increase in the number of adenosine receptors was observed (Figs 9 and and10).10). The increase in the number of adenosine receptors is not accompanied by a significant change in binding affinity, nor by a change in total protein content. This increase in adenosine receptors is probably a result of increased receptor synthesis, since the protein synthesis inhibitor, cycloheximide (5 μg/mL) prevented T3 stimulation (data not shown). Its worth mentioning that the opposite result was obtained in adipocytes, in which T3 reduced the level of adenosine receptors by 65% .
Again we find that factors that accelerated the rate of heart contractions brought about an increase in the number of adenosine receptors, which increase the cardiac cell sensitivity to the attenuating effect of adenosine. This is probably the mechanism by which heart cells adapt to stress and compensate for excessive contractions. Further support for this mechanism of adaptation is demonstrated in the experiments with Carb. This transmitter, which like adenosine reduces the heart rate, caused a 28% reduction of adenosine receptors following 2–3 days of treatment (Fig. 12).
It is worth mentioning that similar results were obtained by Liang and Donovan , in which sustained exposure of cultured chick myocytes to adenosine agonists produced a 40% decrease in the force of contractions, accompanied by a 40% reduction in the receptor number.
It is possible that the changes obtained on the level of the A1 receptors are related to cAMP and not to cardiac contractions. Indeed, cAMP or theophylline caused an increase of A1 receptors (data not shown). However, these drugs also accelerated the rate of cardiac contractions. Thus, it is impossible to distinguish between these two possibilities. On the other hand, when direct measurements of cAMP level were performed following Iso treatment, it caused an elevation of the level of cAMP during the first 30 min. But the values of cAMP returned to normal in the cardiomyocytes after 24 and 48 hr. Furthermore, when the cells were treated with T3, for 48 hr, the basal level of cAMP was 10–12% higher than the control, insignificant difference . These results support our proposal that contractile activity plays a major role in regulation of adenosine receptor’s synthesis although the possibility that cAMP exerts a second messenger in this process is not excluded.
We have previously demonstrated, that any treatment which stimulates heart contractions or inhibits heart contractions causes a reduction in the level of muscarinic acetylcholine receptors . We reasoned that we could not get up regulation in the muscarinic receptor level; either because we did not find the required experimental conditions for the induction process, or the cardiac cells used were saturated with these receptors and no further up regulation was possible . Now, when we make comparison between these two receptors, which have similar functions on the heart, we find that: (A) the ratio of muscarinic to A1 receptors is 10: 1; (B) under the same experimental conditions, such as TH treatment, the muscarinic receptors are down regulated , whereas A1 adenosine receptors are up regulated (Figs 9 and and10).10). Thus, a different mechanism probably regulates the biosynthesis of these two receptors, but only the A1 receptors respond to environmental conditions (stress) so as to restore the basal rate of heart contractions.
We are indebted to Mrs T. Zinman and A. Isaac for their valuable technical assistance. This work was partially supported by funds from Israeli Ministry of Health and from Otto Meyerhoff Drug Receptor Center, at Bar-Ilan University.
‡Abbreviations: Carb, carbamylcholine; CK, creatine kinase; [3H]CPX, [3H]-8-cyclopentyl-1, 3-dipropylxanthine; DMEM, Dulbecco’s modified Eagle Medium; Iso, isoproterenol; LDH, lactate dehydrogenase; NE, norepinephrine; RDB, fig tree extract; R-PIA, R-N6-(2-phenylisopropyl)-adenosine; S-PIA, S-N6-(2-phenylisopropyl)-adenosine; TH, thyroid hormones; T3 tri-iodothyronine.