2-(4-(2-carboxyethyl)phenethylamino)-5--n-ethylcarboxamidoadenosine and Hypotension

Previously we have shown that stimulation of inhibitory A1 adenosine receptors located in the nucleus tractus solitarii (NTS) attenuates cardiopulmonary chemoreflex (CCR) evoked inhibition of renal, adrenal and lumbar sympathetic nerve activity and reflex decreases in arterial pressure and heart rate. Activation of facilitatory A2a adenosine receptors, which dominate over A1 receptors in the NTS, contrastingly alters baseline activity of regional sympathetic outputs: it decreases renal, increases adrenal and does not change lumbar nerve activity. Considering that NTS A2a receptors may facilitate release of inhibitory transmitters we hypothesized that A2a receptors will act in concert with A1 receptors differentially inhibiting regional sympathetic CCR responses (adrenal>lumbar>renal). In urethane/chloralose anesthetized rats (n=38) we compared regional sympathetic responses evoked by stimulation of the CCR with right atrial injections of serotonin 5HT3 receptor agonist, phenylbiguanide, (1-8μg/kg) before and after selective stimulation, blockade or combined blockade and stimulation of NTS A2a adenosine receptors (microinjections into the NTS of CGS-21680 0.2-20pmol/50nl, ZM-241385 40pmol/100nl or ZM-241385+CGS-21680, respectively). We found that stimulation of A2a adenosine receptors uniformly inhibited the regional sympathetic and hemodynamic reflex responses and this effect was abolished by the selective blockade of NTS A2a receptors. This indicates that A2a receptor triggered inhibition of CCR responses and the contrasting shifts in baseline sympathetic activity are mediated via different mechanisms. These data implicate that stimulation of NTS A2a receptors triggers unknown inhibitory mechanism(s) which in turn inhibit transmission in the CCR pathway when adenosine is released into the NTS during severe hypotension.

Topics: Adenosine; Adrenal Glands; Animals; Biguanides; Blood Pressure; Chemoreceptor Cells; Heart Atria; Heart Conduction System; Heart Rate; Hypotension; Kidney; Lumbosacral Region; Male; Microinjections; Models, Neurological; Phenethylamines; Rats; Rats, Sprague-Dawley; Receptors, Adrenergic, alpha-2; Reflex; Serotonin 5-HT3 Receptor Agonists; Solitary Nucleus; Sympathetic Nervous System; Triazines; Triazoles

The mechanisms responsible for vascular autoregulation in the brain during changes in mean arterial blood pressure are ambiguous. Potentially, adenosine, a purine nucleoside and potent vasodilator, may be involved as earlier studies have documented an increase in brain adenosine concentrations with cerebral ischemia and hypotension. Consequently, we tested the hypothesis that adenosine is involved in vasodilatation during hypotension within the autoregulatory range (>50 mm Hg) by exposing adenosine 2a receptor (A2aR) knockout and wild type (WT) mice to short (2 to 5 mins) periods of hypotension. We found that autoregulation was significantly (P<0.05) impaired by 29% in A2a knockout mice as compared with WT animals. Furthermore, the A2R antagonist (A2a>A2b:10-85>1), ZM-241385, in a dose (1, 5, 10 mg/kg, intraperitoneally)-related manner, attenuated autoregulation in WT mice. In knockout mice treated with ZM-2413585 (5 and 10 mg/kg), autoregulation was further impaired indicating that A2b receptors also participated in cerebral vasodilatation. Treatment with dipyridamole (1.0 mg/kg) that increases extracellular concentrations of adenosine improved autoregulation in the A2aR knockout mice. We would conclude that adenosine through both A2a and A2b receptors is involved in physiologic vascular regulation during hypotension even within the autoregulatory range.

Topics: Adenosine; Adenosine A2 Receptor Antagonists; Animals; Antihypertensive Agents; Blood Pressure; Cerebrovascular Circulation; Dipyridamole; Hypotension; Male; Mice; Mice, Knockout; Phenethylamines; Receptor, Adenosine A2A; Receptor, Adenosine A2B; Triazines; Triazoles; Vasodilation; Vasodilator Agents

Dexamethasone increases the expression of adenosine A(3) receptors and augments degranulation in response to their activation in the rat basophilic leukemia cell line, RBL-2H3. We have studied the effects of dexamethasone on mast cell activation induced by A(3) receptor stimulation in vivo. Administration of the A(3) receptor agonist APNEA [N(6)-2-(4 aminophenyl)ethyladenosine; 10-30 microg kg(-1) i.v.] to anesthetized Sprague-Dawley rats induced falls in blood pressure. Pretreatment with dexamethasone (1 mg kg(-1), i.p., -24 h) blocked the hypotensive response to APNEA but not those induced by the A(1) receptor agonist N(6)-cyclopentyladenosine, the A(2A) receptor agonist 2-[p-(2-carboxyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine, or the mast cell degranulating agent compound 48/80 (100-300 microg kg(-1), i.v.). APNEA (10 and 30 microg kg(-1), i.v.) and compound 48/80 (100 and 300 microg kg(-1), i.v.) increased plasma histamine concentrations dose dependently. Pretreatment with dexamethasone significantly inhibited the increases induced by the lower doses of each compound. APNEA induced degranulation of mast cells in thymus but not in skin or skeletal muscle, whereas compound 48/80 induced degranulation in each tissue. Pretreatment with dexamethasone inhibited APNEA-induced degranulation of mast cells in the thymus and slightly, yet significantly, reduced degranulation induced by compound 48/80. Thus, in contrast to the findings in RBL-2H3 cells in vitro, in the whole animal, dexamethasone down-regulates the response of the mast cell to A(3) receptor activation. The qualitatively similar effects on compound 48/80 suggest that dexamethasone suppresses mast cell responsiveness by modulating site(s) downstream from the adenosine A(3) receptor, possibly at the level of the G(i) family of trimeric GTP-binding proteins.

Topics: Adenosine; Animals; Apnea; Blood Pressure; Cell Degranulation; Dexamethasone; Heart Rate; Histamine; Hypotension; Male; Mast Cells; Phenethylamines; Purinergic P1 Receptor Antagonists; Rats; Rats, Sprague-Dawley; Receptor, Adenosine A3; Receptors, Purinergic P1

1. Adenosine produced a biphasic lowering of the mean BP with a drastic bradycardic effect at the highest doses. The first phase hypotensive response was significantly reduced by the nitric oxide (NO) synthase inhibitor L-NAME. 2. The A(2a)/A(2b) agonist NECA produced hypotensive and bradycardic responses similar to those elicited by adenosine, which were not significantly modified by the A(2b) antagonist enprofylline. 3. The A(2a) agonist CGS 21680 did not significantly influence basal HR while induced a hypotensive response antagonized by the A(2a) selective antagonist ZM 241385, and reduced by both L-NAME and the guanylate cyclase inhibitor methylene blue. 4. The A(1) agonist R-PIA showed a dose-dependent decrease in BP with a drastic decrease in HR at the highest doses. The A(1) selective antagonist DPCPX significantly reduced the bradycardic activity and also the hypotensive responses obtained with the lowest doses while it increased those obtained with the highest ones. 5. The A(1)/A(3) agonist APNEA, in the presence of the xanthinic non-selective antagonist 8-pSPT, maintained a significant hypotensive, but not bradycardic, activity, not abolished by the histamine antagonist diphenhydramine. 6. The selective A(3) agonist IB-MECA revealed a weak hypotensive and bradycardic effect, but only at the highest doses. 7. In conclusion, in the systemic cardiovascular response to adenosine two major components may be relevant: an A(2a)- and NO-mediated hypotension, and a bradycardic effect with a consequent hypotension, via atypical A(1) receptors. Finally, an 8-pSPT-resistant hypotensive response not attributable to A(3) receptor-stimulation or to release of histamine by mastocytes or other immune cells was observed.

Topics: Adenosine; Adenosine-5'-(N-ethylcarboxamide); Animals; Blood Pressure; Diphenhydramine; Dose-Response Relationship, Drug; Enzyme Inhibitors; Guinea Pigs; Heart Rate; Hypotension; Male; Methylene Blue; NG-Nitroarginine Methyl Ester; Nitric Oxide; Nitric Oxide Synthase; Phenethylamines; Purinergic P1 Receptor Agonists; Purinergic P1 Receptor Antagonists; Receptors, Purinergic P1; Theophylline; Triazines; Triazoles; Vasodilator Agents; Xanthines

The synthesis and pharmacological profile of a series of neuroprotective adenosine agonists are described. Novel A(1) agonists with potent central nervous system effects and diminished influence on the cardiovascular system are reported and compared to selected reference adenosine agonists. The novel compounds featured are derived structurally from two key lead structures: 2-chloro-N-(1-phenoxy-2-propyl)adenosine (NNC 21-0041, 9) and 2-chloro-N-(1-piperidinyl)adenosine (NNC 90-1515, 4). The agonists are characterized in terms of their in vitro profiles, both binding and functional, and in vivo activity in relevant animal models. Neuroprotective properties assessed after postischemic dosing in a Mongolian gerbil severe temporary forebrain ischemia paradigm, using hippocampal CA1 damage endpoints, and the efficacy of these agonists in an A(1) functional assay show similarities to some reference adenosine agonists. However, the new compounds we describe exhibit diminished cardiovascular effects in both anesthetized and awake rats when compared to reference A(1) agonists such as (R)-phenylisopropyladenosine (R-PIA, 5), N-cyclopentyladenosine (CPA, 2), 4, N-[(1S,trans)-2-hydroxycyclopentyl]adenosine (GR 79236, 26), N-cyclohexyl-2'-O-methyladenosine (SDZ WAG 994, 27), and N-[(2-methylphenyl)methyl]adenosine (Metrifudil, 28). In mouse permanent middle cerebral artery occlusion focal ischemia, 2-chloro-N-[(R)-[(2-benzothiazolyl)thio]-2-propyl]adenosine (NNC 21-0136, 12) exhibited significant neuroprotection at the remarkably low total intraperitoneal dose of 0.1 mg/kg, a dose at which no cardiovascular effects are observed in conscious rats. The novel agonists described inhibit 6, 7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate-induced seizures, and in mouse locomotor activity higher doses are required to reach ED(50) values than for reference A(1) agonists. We conclude that two of the novel adenosine derivatives revealed herein, 12 and 5'-deoxy-5'-chloro-N-[4-(phenylthio)-1-piperidinyl]adenosine (NNC 21-0147, 13), representatives of a new series of P(1) ligands, reinforce the fact that novel selective adenosine A(1) agonists have potential in the treatment of cerebral ischemia in humans.

Topics: Adenosine; Animals; Blood Pressure; Brain; Brain Ischemia; Cardiovascular System; Cell Line; Gerbillinae; Guinea Pigs; Hypotension; Mice; Mice, Inbred Strains; Neuroprotective Agents; Protein Binding; Purinergic P1 Receptor Agonists; Rats; Rats, Inbred Strains

1. The adenosine A3 receptor agonist, N6-2-(4-aminophenyl)ethyladenosine (APNEA) induces hypotension in the anaesthetized rat. The present experiments were carried out to explore the role of mast cells in the response. 2. Intravenous injection of APNEA (1-30 micrograms kg-1 to rats in which the A3 receptor-mediated response had been isolated by pretreatment with 8-(p-sulphophenyl) theophylline (8-SPT)), induced dose-related falls in blood pressure accompanied at higher doses by small falls in heart rate. Responses to the mast cell degranulating agent, compound 48/80 (10-300 micrograms kg-1, i.v.) were qualitatively similar to those to APNEA. 3. Pretreatment with sodium cromoglycate (0.25-20 mg kg-1, i.v.) induced dose-related, although incomplete, blockade of the hypotensive responses to APNEA. At 20 mg kg-1, sodium cromoglycate also inhibited the cardiovascular response to compound 48/80 but had no effects on those to the selective A1 receptor agonist, N6-cyclopentyladenosine (CPA) or the selective A2A receptor agonist, 2-[p-(2-carboxyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680). Lodoxamide (0.01-20 mg kg-1) also blocked selectively but incompletely the response to APNEA. 4. The cardiovascular responses to compound 48/80 (10-300 micrograms kg-1, i.v.) were markedly suppressed in animals which had received repeated doses of the compound by the intraperitoneal route. Similarly APNEA was essentially devoid of cardiovascular activity in such preparations. In contrast, responses to CPA were similar in animals treated repeatedly with compound 48/80 to those obtained in control animals. 5. Plasma and serum histamine concentrations were markedly increased associated with the pronounced hypotensive responses induced by intravenous injections of APNEA (30 or 100 microg kg-1) in the presence of 8-SPT, or compound 48/80 (300 microg kg-1).6. Taken together the data implicate the mast cell in a key role in adenosine A3 receptor-mediated hypotension in the anaesthetized rat.

Topics: Adenosine; Animals; Antihypertensive Agents; Apnea; Cromolyn Sodium; Dose-Response Relationship, Drug; Hypotension; Injections, Intravenous; Male; Mast Cells; Phenethylamines; Rats; Rats, Sprague-Dawley; Receptors, Purinergic P1; Serotonin; Theophylline