Apparent heterogeneity of cardiac A1 adenosine receptors as revealed by radioligand binding experiments on N-ethylmaleimide-treated membranes

Chronotropic and vasodilatory responses to adenosine and isoproterenol in mouse heart: effects of adenosine A1 receptor overexpression

1. Chronotropic and vasodilatory effects of adenosine receptor activation with 2-chloroadenosine (2-ClAdo) and beta-adrenoceptor activation with isoproterenol were studied in wild-type murine hearts and transgenic hearts overexpressing the A1 adenosine receptor. 2. Treatment of wild-type hearts with 2-ClAdo induced bradycardia (pEC50 6.4+/-0.2) and vasodilatation (pEC50 7.9+/-0.1; minimal resistance 2.2+/-0.2 mmHg/mL per min per g). The A1 receptor-mediated bradycardia was 20-fold more sensitive in transgenic hearts (pEC50 7.7+/-0.2), whereas coronary vasoactivity of 2-ClAdo was unaltered (pEC50 7.6+/-0.1). 3. beta-Adrenoceptor stimulation with isoproterenol increased heart rate (pEC50 8.5+/-0.2; maximal rate 594+/-23 b.p.m.) and produced vasodilation (pEC50 8.7+/-0.1; minimal resistance 1.7 +/-0.2 mmHg/ml, per min per g) in wild-type hearts. Treatment with 10 IU/mL adenosine deaminase increased the magnitude of the tachycardia (maximal rate 653+/-27 b.p.m.) without altering potency (pEC50 8.5+/-0.1). Antagonism of A1 receptors with 10nmol/L 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) produced a comparable increase in the magnitude of the chronotropic response (maximal rate 695+/-26b.p.m.) without altering potency (pEC50 8.3+/-0.1). 4. Isoproterenol-mediated vasodilatation was unaltered by transgenic A1 receptor overexpression. Overexpression of A1 receptors significantly reduced the maximal heart rate during beta-adrenoceptor stimulation by 35% (to 381 +/-28 b.p.m.) without altering potency (pEC50 8.4+/-0.2). At 10nmol/L, DPCPX increased the magnitude of the chronotropic response to isoproterenol in transgenic hearts (maximal heart rate 484+/-36 b.p.m.) without altering potency (pECs50 8.3+/-0.2). 5. The data show that transgenic A1 receptor overexpression selectively sensitizes the cardiovascular A1 receptor response and that A1 receptor activation by endogenous adenosine depresses the magnitude, but not potency, of the beta-adrenoceptor-mediated chronotropic response in mouse heart. The A1 receptor-mediated depression of beta-adrenoceptor responsiveness is non-competitive (reduced response magnitude with no change in sensitivity). This indicates that A1 receptor activation non-competitively inhibits effector mechanisms activated by beta-adrenoceptors (e.g. adenylate cyclase) and/or A1 receptors activate unrelated but opposing mechanisms. This inhibitory response may have physiological importance during periods of sympathetic stimulation of cardiac work.

The complex regulation of receptor-coupled G-proteins

Heterotrimeric G-proteins are associated with the cytoplasmic surface of the cell membrane as oligomeric structures. The oligomeric structures were deduced from a variety of studies including target (irradiation) analysis, hydrodynamic evaluation of detergent extracted material, and cross-linking of G-proteins in their membrane environment. From the functional mass determined by target analysis, it was estimated that one receptor (for glucagon) is associated with 8-10 units of Gs, the heterotrimeric G-protein that stimulates adenylyl cyclase. It is proposed that the receptor associates with each monomer of the chain via weak and strong binding forces that are dictated according to whether either GTP or GDP is bound to the alpha-subunits (weak forces) or, due to the hormone-induced release of the nucleotides during the exchange reaction, these subunits become transiently devoid of nucleotides (strong forces). The hormone-induced changes in type and degree of nucleotide binding allow for movement of the receptor along the oligomeric chain and filling of the nucleotide binding sites with the activating nucleotide, GTP. In this manner, the receptor catalytically activates Gs. It is suggested that the dynamic instability of the oligomeric chain produced by the asymmetric distribution of GTP and GDP along the chain results in release of a GTP-monomer from one end and association of a GDP-monomer at the opposite end. Adenylyl cyclase associates with the released GTP-monomer inducing a transient state of the coupled proteins. In a Mg-dependent fashion, hydrolysis of GTP occurs resulting in re-organization of the coupled proteins such that alpha and beta gamma interact with distinct domains of the cyclase molecule. The final state of the coupled process determines the degree of cyclase activity. Release of Pi from its binding site restores association of alpha and beta gamma to the GDP-bound form of the heterotrimer. The latter associates with the oligomeric structure of G-proteins to complete the cycle of events in the overall process of hormonal activation of the system.

Transgenic A1 adenosine receptor overexpression increases myocardial resistance to ischemia

Activation of myocardial A1 adenosine receptors (A1AR) protects the heart from ischemic injury. In this study transgenic mice were created using the cardiac-specific alpha-myosin heavy chain promoter and rat A1AR cDNA. Heart membranes from two transgene positive lines displayed approximately 1,000-fold overexpression of A1AR (6,574 +/- 965 and 10,691 +/- 1,002 fmol per mg of protein vs. 8 +/- 5 fmol per mg of protein in control hearts). Compared with control hearts, transgenic Langendorff-perfused hearts had a significantly lower intrinsic heart rate (248 beats per min vs. 318 beats per min, P < 0. 05), lower developed tension (1.2 g vs. 1.6 g, P < 0.05), and similar coronary resistance. The difference in developed tension was eliminated by pacing. Injury of control hearts during global ischemia, indexed by time-to-ischemic contracture, was accelerated by blocking adenosine receptors with 50 microM 8-(p-sulfophenyl) theophylline but was unaffected by addition of 20 nM N6-cyclopentyladenosine, an A1AR agonist. Thus A1ARs in ischemic myocardium are presumably saturated by endogenous adenosine. Overexpressing myocardial A1ARs increased time-to-ischemic contracture and improved functional recovery during reperfusion. The data indicate that A1AR activation by endogenous adenosine affords protection during ischemia, but that the response is limited by A1AR number in murine myocardium. Overexpression of A1AR affords additional protection. These data support the concept that genetic manipulation of A1AR expression may improve myocardial tolerance to ischemia.

Isolation and characterization of an avian A1 adenosine receptor gene and a related cDNA clone

We have isolated the gene for a chick A1 adenosine receptor along with a cDNA that codes for the same adenosine receptor. The cDNA clone was isolated from both adipose tissue and heart cDNA libraries and encodes a 324-amino acid protein with 80% identity with mammalian A1 adenosine receptors. Transient expression of the cDNA in human embryonic kidney (HEK) 293 cells shows that it encodes a protein that binds [3H]CCPA (2-chloro-N6-[cyclopentyl-2,3,4,5-3H]cyclopentyladenosine, a specific agonist radioligand, with a KD of 5.6 +/- 2.4 nM. Cyclic AMP measurements in HEK 293 cells co-transfected with the chick cDNA and a cDNA for a luteinizing hormone/choriogonadotropin receptor shows that A1 adenosine receptor agonists antagonize the cyclic AMP-elevating effect of bovine luteinizing hormone. Two partial genomic clones were isolated. The first contains 5'-untranslated sequence including a putative promoter region which does not contain a TATA box, an intron and the first third of the coding sequence of the A1 adenosine receptor cDNA. The coding sequence of this partial genomic clone terminates at a second intron. The second partial genomic clone contains the rest of the coding sequence and the 3'-untranslated elements in a single exon. Thus the chick A1 adenosine receptor gene contains one intron in the 5'-untranslated region and a minimum of one intron in the coding sequence.

Further characterization of the binding of the adenosine receptor agonist [3H]CGS 21680 to rat brain using autoradiography

2-[p-(2-carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadeno sine (CGS 21680) is considered a selective ligand for adenosine A2A receptors, which are known to be enriched in striatum and olfactory tubercle. We have investigated the characteristics of [3H]CGS 21680 binding in several brain regions using quantitative autoradiography. In agreement with previous data the radioligand was found to label the caudate-putamen, accumbens nucleus, olfactory tubercle and globus pallidus, but also many other structures, e.g. cerebral and cerebellar cortex, hippocampus, thalamus and some brainstem nuclei, were labelled. Cortical and striatal binding of [3H]CGS 21680 was unaltered by high concentrations of the adenosine transport inhibitor dipyridamole or the phosphodiesterase inhibitor rolipram but was displaced by 1,3-diethyl-8-phenylxanthine, the A2 selective adenosine antagonist CP 66,713, and the A2A selective agonist SHA 118. These three agents were approximately equipotent in striatum, cortex and hippocampus. The A2 selective agonist CV 1808 was a 4-5 times more potent displacer in cortex and hippocampus than in the striatum. [3H]CGS 21680 binding was strongly magnesium-dependent in all the studied brain regions, in contrast to the binding of adenosine A1 agonists. The binding of [3H]CGS 21680 to cerebral cortex and hippocampus, but not the binding to striatum, was displaced by the adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine in nanomolar concentrations. The present study provides evidence that in cerebral cortex and hippocampus, most of the [3H]CGS 21680 binds to a receptor site that is distinct from the striatal A2A receptor and the classical adenosine A1 receptor and may represent a hitherto unrecognized binding site.

Regulation of adenosine receptors in cultured heart cells

A1 adenosine receptors were studied on heart cells grown in cultures by the radioligand binding technique. Treatments with agents that accelerated heart rate for 3-4 days, caused an increase in the level of adenosine receptors. Treatments that attenuated heart rate, reduced the level of the receptors. Thus, the cardiac cells respond to environmental conditions affecting heart contraction so as to restore the basal rate of heart activity.

Characterization of adenosine receptors in intact cultured heart cells

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 x 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.