Injection Of Adenosine Research Articles

Pharmacological Effects of Adenosine on Canine Penile Erection.

We investigated the pharmacological effects of adenosine on canine penile erection in vivo. Intracavernous injection of adenosine induced a full erection, which was unaffected by hexamethonium and atropine. The erectile response induced by adenosine was augmented by intracavernous pretreatment with dipyridamole and markedly depressed by theophylline. The erectile response induced by cavernous nerve stimulation was also enhanced by dipyridamole and reduced by theophylline. The combination of acetylcholine and adenosine, when injected intracavernously, induced a full erection, and the effect of these drugs was not additive but synergistic. These results suggest that penile erection induced by adenosine is due to a direct relaxing effect on cavernous smooth muscle. We propose that adenosine plays a cooperative role with acetylcholine and also has a physiological role as a neuromodulator in canine penile erection.

Intracoronary adenosine causes angina pectoris like pain - an inquiry into the nature of visceral pain

The aim was to study the tentative role of adenosine as a messenger between myocardial ischaemia and angina pectoris. Adenosine was administered in serial doses of 0.1-20 mg either as an intravenous bolus, or intra-arterially over 10 s into the left coronary artery, the aorta and the iliac artery. Coronary sinus flow was determined by thermodilution. ECG was monitored continuously. The patient was not aware of which site or dose was used. After each injection, the start of, maximum, end, magnitude, and location of pain were noted. Six patients with angina pectoris referred for coronary angiography entered the study. After intracoronary adenosine injection in the absence of ischaemic ECG changes, a dose dependent degree of chest pain was experienced not different in quality or location from the patients' habitual angina pectoris. Adenosine into the aorta provoked pain in lower chest and upper abdomen, whereas injection into the iliac artery provoked pain in the ipsilateral leg. On intravenous injection equipotent doses of adenosine caused chest pain of the same degree and quality as after intracoronary injection. Immediately after intracoronary injection the coronary sinus blood flow started to increase, but the onset of chest pain was delayed. Onset of pain was earlier the higher the dose, the maximum dose resulting in onset after 18(SEM 2) s. Coronary sinus blood flow increased dose dependently after left coronary artery injection but following intravenous injection no further increase was seen beyond that induced by the lowest dose. We suggest that adenosine is an important messenger for the sensation of angina pectoris and the effect is not due to coronary steal leading to myocardial ischaemia.

Respiratory Response to Adenosine in Newborn Lambs Is Modified by Hypoxemia and by Heat Stress

A study was made to determine whether adenosine is involved in the regulation of breathing and whether the respiratory response to injections of adenosine and its analogue L-PIA (phenylisopropyl adenosine) is modified by moderate hypoxemia or by heat stress. Unanesthetized lambs with chronically implanted catheters were used for the investigation. Intravenous injections of adenosine caused an increase in the rate of breathing lasting for 5-30 s, except during heat stress, when the rate of breathing decreased. Injection of L-PIA also increased the rate of breathing, but the effects lasted longer than after injection of adenosine. During heat stress the response to injection of L-PIA varied. Both the arterial blood pressure and heart rate decreased transiently after intravenous injection of adenosine or L-PIA, while the central venous pressure increased. Adenosine might thus be part of a positive feedback system that stimulates breathing under normal environmental conditions with or without moderate hypoxemia. The respiratory responses to injection of adenosine during heat stress are different, which might indicate that under this condition the effects of adenosine on breathing are modified by its influence on temperature control.

Atrial natriuretic peptide (ANP) does not inhibit the basal vascular tone present in the in situ blood-perfused dog gracilis muscle

This study evaluated the effects of rat ANP(5-28) infusion into the blood-perfused dog gracilis muscle at concentrations ranging from 30 to 10,000 pg/ml. The vasculature of gracilis muscles from anesthetized beagle dogs was isolated and pump-perfused at constant flow with blood utilizing an extracorporeal circuit. Maximal vasodilatory capacity was determined by adenosine injection. ANP was infused into the arterial circuit to produce increasing arterial blood concentrations. Each infusion lasted 10 min. Systemic arterial pressure, central venous pressure, cardiac output and heart rate did not change during ANP infusion into the gracilis vasculature. ANP at arterial blood concentrations up to 10,000 pg/ml did not produce significant vasodilation although the vasculature showed pronounced vasodilation in response to adenosine. In vitro experiments showed that ANP had much less vasorelaxant activity in dog femoral artery and saphenous vein than in rabbit aorta. Therefore, rat ANP(5-28) at concentrations within and well above physiological and pharmacological ranges does not inhibit the basal vascular tone present in the innervated, blood-perfused dog gracilis muscle in situ.

Direct cutaneous hyperalgesia induced by adenosine

The intradermal injection of adenosine produces a dose-dependent decrease in mechanical nociceptive threshold in the hindpaw of the rat that is not attenuated by elimination of indirect pathways for the production of hyperalgesia. Adenosine-induced hyperalgesia is mimicked by the A 2-agonists, 5′-(N-ethyl)-carboxamido-adenosine and 2-phenylaminoadenosine but not by the A 1-agonist, N 6-cyclo-pentyladenosine and antagonized by the adenosine A 2-receptor antagonist, PD 081360-0002 but not by the A 1-antagonist, 1,3-dipropyl-8-(2-amino-4-chlorphenyl)xanthine. The latency to onset of adenosine and 2-phenylaminoadenosine hyperalgesia is similar to that produced by prostaglandin E 2, a directly acting hyperalgesic agent but shorter than that produced by leukotriene B 4, which acts indirectly. 2-Phenylaminoadenosine hyperalgesia is prolonged by rolipram, a phosphodiesterase inhibitor. Both 2-phenylaminoadenosine and prostaglandin E 2 hyperalgesia are antagonized by the A 1-agonist N 6-cyclo-pentyladenosine and the mu-agonist, [ d-Ala 2, NMe-Phe 4, Gly-ol]enkephalin. However, 1-acetyl-2-(8-chloro-10,11-dihydrodibenz[b,f]oxazepine-10-carbonyl)hydrazine, a prostaglandin-receptor antagonist, inhibits prostaglandin E 2 (Taiwo and Levine, Brain Res. 458, 402–406, 1988) but not 2-phenylamino-adenosine hyperalgesia and PD 081360-0002, the adenosine receptor antagonist, inhibits 2-phenylamino-adenosine but not prostaglandin E 2 hyperalgesia. These data suggest that adenosine is a directly acting agent that produces hyperalgesia by an action at the A 2-receptor and that this hyperalgesia is mediated by the cAMP second messenger.

放射線障害防護薬剤に関する研究(第28報)放射線障害に対する核酸構成物質の防護効果

The effects of various nucleic acid constitutional compounds, base, nucleoside and nucleotide on lethality and skin injury induced by soft X-irradiation were studied. The survival effect was determined by use of survival days after irradiation of lethal dose of 70 kVp, 2100R and the protective effect on skin injury was determined by use of degrees on skin injury after 30 kVp, 1100R soft X-irradiation. As a result of these studies, the survival effect was observed by single injection of inosine at 120, 60 and 5 min before irradiation and three times injection after irradiation. And the other nucleic acid constitutional compounds had no effect on survival. Protective effect of skin injury was observed by single injection of adenosine, guanosine, inosine, 5'-adenosine monophosphate (5'-AMP), 5'-guanosine monophosphate (5'-GMP) and 5'-inosine monophosphate (5'-IMP) before irradiation. Protective effect of skin injury by three time injection before irradiation were revealed in adenosine, inosine, 5'-AMP and 5'-IMP. For the investigation of superoxide anions and hydroxyl radicals scavenge activities, the relationship between radical scavenge activities and protective effect of radiation by using various nucleosides was not observed.

Chest pain and oesophageal pressure relationships in man following an intravenous bolus of adenosine.

After titration of maximum tolerable i.v. bolus dose of adenosine, this dose was given to seven volunteers (20-42 years), instrumented with a three-lumen oesophageal pressure catheter with recording sites at the levels of the stomach, the lower oesophageal sphincter (LOS) and the oesophagus. In addition to continuous pressure recordings, chest pain was estimated continuously by a 10-graded category-ratio scale. Baseline resting pressures were 8.4 (1.9) mmHg in the stomach, 1.6 (1.7) mmHg in the oesophagus and 20 (2.6) mmHg in the LOS resulting in a net LOS pressure of 11 (+/- 1.3) mmHg. Following injection of adenosine which provoked transient chest pain with a rated maximum of 5.4 (1.0), resting oesophageal pressure did not change while net LOS pressure decreased to -1.3 (1.9) mmHg (P less than 0.0001). Adenosine injection did not affect swallowing-induced peristaltic contractions of the oesophagus and LOS although the peristaltic wave was delayed (P less than 0.05). Thus, chest pain evoked by adenosine cannot be caused by spastic oesophageal contractions. Adenosine may have a relaxing effect on the LOS but does not block its normal reactions to swallowing.

The effect of adenosine and adenosine analogues on methylxanthine-induced hypercalciuria in the rat.

The chronic effects of dietary caffeine, theophylline, and theobromine on urinary calcium excretion were investigated in the adult male rat. The effect of acutely administered adenosine and adenosine analogues on methylxanthine-induced hypercalciuria was concurrently investigated. When rats were fed equimolar amounts of theobromine, caffeine, and theophylline, it was found that urinary calcium excretion increased; on day 7 values were increased over controls (p less than 0.05) by 54, 146, and 208%, respectively. On day 20, an injection of adenosine reduced calcium excretion in methylxanthine-treated rats to levels not different from control values. In another series of experiments, theophylline was fed to a group of rats to establish hypercalciuria. Three adenosine analogues N6-(L-2-phenylisopropyl)adenosine (R-PIA), N6-(D-2-phenylisopropyl)adenosine (S-PIA), and 5'-(N-ethylcarboxamido)adenosine (NECA) with different adenosine receptor specificities (A2,A1, and weakly A2, respectively) were administered to different groups of theophylline-fed and control rats, and their calcium excretions were measured. It was found that the order of efficacy of the analogues in reducing calcium excretion was NECA greater than R-PIA greater than S-PIA, which is consistent with the receptor being A2. A proportion of the methylxanthine-induced hypercalciuria may be due to adenosine antagonism.

Intravitreous injection of adenosine or its agonists causes breakdown of the blood-retinal barrier.

Pigmented rabbits were anesthetized and given an intravitreous injection of 0.1 mL of a test substance or vehicle alone. Vitreous fluorophotometry was performed before injections and at various time points after injections. Compared with pretreatment scans, vehicle-injected eyes showed no change in intravitreous fluorescein sodium leakage at 6 and 24 hours after injection. Injection of adenosine (10(-2) mol/L) resulted in fluorescein leakage that was significantly greater than that which occurred in control eyes at 6 hours after injection, but returned to baseline at 24 hours. This effect was significantly attenuated by an adenosine receptor antagonist (BW-A1433U), suggesting that it was mediated by specific adenosine receptors. A nonselective adenosine receptor agonist, N-ethylcarboxamidoadenosine, and two relatively A1 selective receptor agonists, N6-cyclopentyladenosine and N6-phenylisopropyladenosine, also caused dose-dependent intravitreous fluorescein leakage. The relative order of potency was N-ethylcarboxamidoadenosine much greater than N6-phenylisopropyladenosine, which was greater than N6-cyclopentyladenosine, implicating A2 adenosine receptors. Intravitreous injection of dipyridamole, an adenosine uptake inhibitor, caused enhanced fluorescein leakage, presumably from extracellular accumulation of endogenous adenosine. The results of this study suggest that adenosine may be a mediator of blood-retinal barrier breakdown.

The role of adenylate cyclase in the D-2 modulation of the release of dopamine: a microdialysis study in the striatum of the rat : M. Santiago and B.H.C. Westerink. Centre for Pharmacy, University of Groningen, Groningen, The Netherlands

Adenosine is known to cause pain when injected intravenously or intra-arterially. We have conducted a double-blind placebo-controlled study by injecting adenosine intradermally in 6 healthy subjects (5 male, 1 female; age: 27–34 years). Pain was assessed using the visual analogue scale. The intradermal injection of 2 μmol of adenosine produced pain significantly greater than normal saline after 15 sec (T0) (29 ± 13 vs. 7 ± 6 mm, P = 0.004), 1 min after T0 (13 ± 9 vs. 0 ± 0 mm, P = 0.002) and 2 min after T0 (4.5 ± 5 vs. 0 ± 0 mm, P < 0.05). There was evidence of hyperalgesia to mechanical and heat stimuli at the injection site (primary hyperalgesia). There was no evidence of mechanical hyperalgesia in the cutaneous area surrounding the injected site (secondary hyperalgesia). In all cases the intradermal injection of adenosine produced local hyperemia (mean surface are: 147 ± 69 mm2) which was absent after placebo injection. The pre-injection of bamiphylline, a rather selective antagonist of A1 adenosine receptors, differently from placebo, completely suppressed the adenosine-induced pain after 15 sec (T0) (15 ± 10 vs. 0 ± 0 mm, P = 0.002) and 1 min after T0 (9 ± 7 vs. 0 ± 0 mm, P = 0.002). No anesthesia to heat, cold and mechanical stimuli was detected at the bamiphylline site. The adenosine-induced erythematous area was wider at the bamiphylline pre-injected site than at the placebo pre-injected site (173 ± 114 vs. 119 ± 85 mm2). We conclude that adenosine is an algogenic substance able to activate not only visceral but also cutaneous nociceptors. Furthermore our results suggest that the activation of nociceptors is at least partially mediated by A1 adenosine receptors.

Effects of defolliculation on membrane current responses of Xenopus oocytes.

1. Catecholamines, adenosine, gonadotrophins, vasoactive intestinal peptide (VIP) and E-series prostaglandins all elicit K+ currents in follicle-enclosed Xenopus oocytes. Evidence suggests that cyclic nucleotides act as intracellular messengers in the activation of this K+ conductance. Muscarinic agonists and some divalent cations (e.g. Co2+, Mn2+, Ni2+ and Cd2+) elicit slow oscillatory Cl- currents, which are activated through hydrolysis of inositol phospholipids and mobilization of intracellular calcium by inositol phosphates. 2. We investigated whether these membrane current responses were generated in the oocyte itself or in enveloping follicular cells which are coupled to the oocyte by gap junctions. Oocytes were defolliculated, either enzymatically using collagenase, or by manual dissection combined with rolling over poly-L-lysine-coated slides. Removal of follicular cells was checked using scanning electron microscopy. Membrane current responses of defolliculated oocytes were compared with responses seen in follicle-enclosed oocytes taken from the same ovary. 3. The K+ responses evoked by all the various hormones/neurotransmitters were either drastically reduced (greater than 90%) or abolished by defolliculation. K+ currents generated by the adenylate cyclase activator forskolin and by intraoocyte injection of adenosine 3',5'-cyclic monophosphate (cyclic AMP), or guanosine 3',5'-cyclic monophosphate were similarly reduced in defolliculated oocytes. In contrast, oscillatory Cl- currents to acetylcholine and divalent cations were selectively preserved through defolliculation. 4. Injection of cyclic AMP (1-20 pmol) into defolliculated oocytes had little or no effect on oscillatory Cl- currents elicited by ACh. However, the calcium-dependent transient Cl- current, activated by depolarization of the oocyte membrane, was consistently potentiated (100-900%) by injections of cyclic AMP (1-10 pmol). 5. These experiments suggest that cyclic nucleotide-activated K+ currents arise essentially in follicular cells and are monitored within the oocyte through electrical coupling by gap junctions. Oscillatory Cl- responses evoked by ACh and divalent cations are produced largely or wholly in the oocyte itself.

Intracellular distribution and chemical forms of arsenic in rabbits exposed to arsenate

Inhibition of the methylation of arsenic in rabbits by ip injection of periodate-oxidized adenosine (PAD) prior to an iv injection of 74As-arsenate (AsV; 0.4 mg As/kg body wt) caused a marked increase in the retention of 74As in both the cellular organelles and the soluble fractions of liver and kidney. One day after exposure, almost 30% of the arsenic in the liver and about 40% of the arsenic in the kidney was recovered in the nuclear fraction. In the liver nuclei, the inhibition of the methylation increased the 74As content of the insoluble fraction and most of this arsenic was protein-bound. The major part of the soluble intranuclear 74As was in the form of AsIII, formed by reduction of the administered AsV. In the liver, PAD also caused a pronounced increase in the 74As content of the microsomal fraction. In the kidneys, where most of the arsenic was present as AsV, there was a marked accumulation of arsenic in the mitochondria.

Mechanism of the apparent parasympathetic inhibition of adenosine induced heart rate slowing in the dog.

The inhibitory action of intravenously administered adenosine on the sinoatrial (SA) node is not expressed in the conscious dog in the presence of normal vagal tone. After pharmacological or surgical parasympathetic blockade, however, adenosine exerts a powerful negative chronotropic effect. In order to determine the reason why this action of adenosine is blocked by the intact parasympathetic nervous system, we measured the chronotropic effects of adenosine while applying a constant cholinergic stimulus to the SA node. In conscious dogs during a systemic infusion of acetylcholine at a rate sufficient by itself to inhibit the SA node, intravenous adenosine injections caused further dose dependent reductions in heart rate. In anaesthetised, vagotomised dogs, intravenous adenosine caused similar negative chronotropic effects with or without concomitant electrical stimulation of the vagus nerve. As an example, 5 mumol.kg-1 adenosine reduced heart rate by 22 (SEM 4)% from a baseline heart rate of 172(10) beats.min-1; when heart rate was lowered to 66(1) beats.min-1 by electrical vagal stimulation, this dose of adenosine reduced heart rate by 36(8)%. Propranolol had no effect on these responses. We conclude that there is no direct cholinergic inhibition of the negative chronotropic action of adenosine on the canine SA node but rather that the inhibitory action of systemically administered adenosine on the SA node is simply masked by the withdrawal of vagal tone in response to the arterial hypotension resulting from this mode of adenosine administration.

Modulation of a steady-state Ca2+-activated, K+ current in tail sensory neurons of Aplysia: role of serotonin and cAMP

1. The modulation of membrane currents by serotonin (5-HT) was studied in isolated clusters of tail sensory neurons. Serotonin was applied by micropressure ejection onto the somata of sensory neurons voltage-clamped at fixed holding potentials. The range of holding potentials tested in this study was selected to produce a steady-state Ca2+-activated K+ current (IK,Ca). Serotonin induced an inward shift in the holding current associated with a decrease in slope conductance. 2. Intracellular injection of adenosine 3',5'-cyclic monophosphate (cAMP) mimicked the response to 5-HT and induced an inward current associated with a decrease in slope conductance. The responses to 5-HT and cAMP had similar voltage dependencies and both responses were due to an apparent decrease in K+ current. Responses to cAMP were markedly reduced when generated at the peak of a response to 5-HT. The nonsummation of the maximal current responses indicated that 5-HT and cAMP utilize a common, saturable mechanism. 3. In contrast to the consistent decrease in steady-state K+ conductance elicited by cAMP, injection of guanosine 3',5'-cyclic monophosphate (cGMP) produced variable responses. In most cells, cGMP induced outward shifts in holding current that were associated with an increase in slope conductance. 4. Several lines of evidence indicated that IK,Ca contributed to the holding current at the level of membrane potentials that were examined. Inward shifts in holding current associated with a decrease in slope conductance were produced in the presence of agents that block Ca2+ channels, such as Co2+, Cd2+ or Ni2+ and by replacement of extracellular Ca2+ with Ba2+. Reducing the concentration of cytoplasmic Ca2+ through intracellular injection of EGTA had similar effects. Furthermore, inward shifts in holding current were produced by 5 mM tetraethylammonium chloride (TEA), which is known to block IK,Ca in neurons of Aplysia. This concentration of TEA also attenuated the outward current produced in response to direct intracellular injection of Ca2+. 5. Serotonin appears to modulate the IK,Ca that contributes to the steady-state holding current. The same manipulations that block the steady-state IK,Ca (see above) also attenuated the response to 5-HT. Furthermore, K+ currents activated by intracellular injection of Ca2+ were attenuated by 5-HT. 6. These results indicate that the changes in holding current produced by 5-HT are mediated, at least in part, by cAMP. In addition, it appears that 5-HT modulates a steady-state calcium-activated K+ current in addition to the previously described S-current (40, 58) and delayed K+ current (8, 9).(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine‐induced depolarizing responses associated with negative slope conductance in LB‐cluster neurones of Aplysia.

1. Current- and voltage-clamp methods were used to evaluate the intracellular and ionic mechanisms involved in dopamine-induced slow depolarizations recorded from neurones of the LB cluster in the abdominal ganglion of Aplysia kurodai. 2. In voltage-clamped cells, dopamine induced a slow inward current that, over the range studied (-40 to -110 mV), decreased in amplitude with hyperpolarization of the cell, but failed to invert when the cell was hyperpolarized beyond the reversal potential for K+,(E)K. 3. Bathing the ganglion in 3-isobutyl-1-methylxanthine (IBMX) caused a significant increase in the dopamine response. 4. Most of the responses to dopamine were markedly augmented in Ca2+-free media, but were depressed in Na+-free media. 5. An intracellular injection of cyclic adenosine 3',5'-monophosphate (cyclic AMP) into the same cell type produced an inward current which, like the response to dopamine, diminished in amplitude with hyperpolarization of the cell. 6. Like the dopamine response, the cyclic AMP response increased in the presence of IBMX, was enhanced in Ca2+-free media, was depressed in Na+-free media, and was unaffected by changes in external potassium. 7. In a few cells, although the cyclic AMP-induced responses disappeared in Na+-free media, the dopamine-induced slow inward current responses did not. However, these Na+-free resistant responses disappeared completely in Na+- and Ca2+-free media. 8. It was concluded that most of the dopamine-induced inward current responses were produced by an increase in permeability, mainly to Na+, triggered by a receptor-controlled increase in intracellular cyclic AMP.

P 1-purinoceptors in the cerebrovascular bed of the goat in vivo

The possible existence and function of specific P 1-purinoceptors in the cerebrovascular bed of the unanesthetized goat have been investigated. Blood flow to one cerebral hemisphere (cerebral blood flow) was measured by means of an electromagnetic flow probe previously implanted around the ipsilateral internal maxillary artery. The injection of adenosine, AMP, ADP and ATP (3–30 μg) directly into the internal maxillary artery increased cerebral blood flow and decreased cerebrovascular resistance in a dose-dependent manner. Continuous infusion of 8-phenyltheophylline (8-PT), 100 μg/min, into the internal maxillary artery did not alter the resting cerebral blood flow or the cerebrovascular resistance, but significantly inhibited the cerebral vasodilation induced by adenosine, AMP, ADP and, to a lesser degree, ATP. The acetylcholine- and histamine-induced cerebral vasodilation was unaffected by 8-PT. These results indicate that adenosine, AMP, ADP and, at least in part, ATP increase cerebral blood flow by acting on specific P 1-purinoceptors located in the cerebrovascular wall. These P 1-purinoceptors do not appear to be tonically activated under physiological conditions.

Myocardial risk area and peak gray level measurement by contrast echocardiography: Effect of microbubble size and concentration, injection rate, and coronary vasodilation

Contrast agents were injected via the intracoronary route in eight dogs during two-dimensional echocardiographic imaging to determine the influence of microbubble size and concentration, injection rate, and coronary vasodilation on risk area and peak gray level measurement. At an injection rate at 13 cc/sec, the average background-subtracted peak gray level intensity of hand-agitated diatrizoate meglumine/diatrizoate sodium was significantly ( p < 0.01) higher than that of hand-agitated diatrizoate meglumine/diatrizoate sodium + 0.9% saline, sonicated diatrizoate meglumine/diatrizoate sodium, and sonicated 70% sorbitol. These differences were abolished by the use of 38 cc/sec injection rates and intracoronary injection of adenosine. Perfusion area determinations as assessed by planimetry were unaffected by the contrast agent used, the injection rate, or by intracoronary administration of adenosine. We conclude that risk area measurement by the ultrasound contrast technique is not affected by varying contrast agents, injection rates, or vasodilation. However, peak gray level intensity is variable among contrast agents and may result in variability of time-activity curve analysis.

Purinergic receptors in the brainstem mediate hypotension and bradycardia.

Adenosine acts at many sites to modulate neuronal activity. The purpose of this study was to investigate a possible role for adenosine as a neuromodulator of brainstem cardiovascular control. Microinjections of adenosine (0-2.3 nmol) were made stereotaxically into various brainstem sites. Injection of adenosine into the nucleus tractus solitarii (NTS) produced dose-related decreases in heart rate and systolic and diastolic blood pressures. Maximal changes occurred 90 seconds after injection. Injection into the area postrema also produced decreased heart rate and systolic and diastolic blood pressures. No significant effect occurred following injection into the C1 area. Adenosine 5'-triphosphate and its analogue, beta, gamma-methylene adenosine 5'-triphosphate also produced dose-related and potent vasodepressor and bradycardia effects in the NTS. Injection of 1,3-dipropyl-8-p-sulfophenylxanthine (0.92 nmol), a potent adenosine receptor antagonist, produced no effect itself, but abolished for 45 minutes the actions of further injections of adenosine and adenosine 5'-triphosphate (but not L-glutamate) in both the NTS and area postrema. Thus, NTS and area postrema injections of adenosine decrease blood pressure and heart rate in anesthetized normotensive rats through adenosine receptors located in these areas. These findings support a role for endogenous adenosine as a central modulator in cardiovascular control.

Regulation of cAMP-stimulated ion current by intracellular pH, Ca2+, and calmodulin blockers

1. Iontophoretic injection of adenosine 3',5'-cyclic monophosphate (cAMP) into identified neurons elicited a slow transient Na+ current whose amplitude and duration were sensitive to altered intracellular pH (pHi), calmodulin blocking drugs, depolarization, and manipulations of internal and external Ca2+. 2. Intracellular acidification between resting pHi to several tenths of a pH unit increased the amplitude of the cAMP-stimulated current and prolonged its duration. 3. Intracellular alkalinization of similar magnitude also increased the amplitude and duration of the current response. The effects of alkalinization were somewhat labile. In cells alkalinized by NH4+-containing salines, washout of NH4+ with normal saline caused acidification and further enhanced the cAMP current response. The immediacy of the increase and the dual acid/basic sensitivity of the response suggest an accommodative process whereby the responsiveness of the cell to cAMP adapts to a maintained pHi. 4. The calmodulin blockers trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide increased the amplitude and duration of the current response. Phorbol ester activators of Ca2+/phospholipid-dependent kinase had no effect on the current. 5. Periods of depolarization preceding tests significantly reduced current response amplitude. This effect was dependent on saline Ca2+ and was blocked by Co2+. 6. Intracellular injection of the Ca2+ chelator ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N',-tetraacetic acid also augmented the amplitude and duration of the current response. 7. The above effects are consistent with a possible common site of action on cAMP degradation. This interpretation is consistent with previous evidence for pH-sensitive and Ca2+/calmodulin-dependent cAMP phosphodiesterase activity in Pleurobranchaea nervous tissue.

Cyclic nucleotide potentiation of muscarinic responses in neuroblastoma cells

The role of cyclic nucleotides in modulating acetylcholine-induced and dopamine-induced responses was examined with cultured neuroblastoma N1E-115 cells by means of intracellular recording techniques. Acetylcholine-induced muscarinic hyperpolarization and muscarinic depolarization were potentiated by bath application of a dibutyryl analog of adenosine 3′,5′-phosphate (cyclic AMP) or phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone. Dibutyryl cyclic AMP did not affect the resting membrane potential and membrane resistance. Acetylcholine-induced nicotinic depolarization was unaffected by dibutyryl cyclic AMP or phosphodiesterase inhibitors. Intracellular pressure injection of cyclic AMP caused a potentiation of muscarinic hyperpolarization and muscarinic depolarization without marked change in the resting membrane potential. Nicotinic depolarization and dopamine depolarization were not affected by cyclic AMP injection. Among the possible metabolites of cyclic AMP, injection of adenosine potentiated muscarinic hyperpolarization, but did not change nicotinic depolarization and dopamine depolarization. Injection of guanosine 3′,5′-phosphate (cyclic GMP) potentiated muscarinic hyperpolarization and muscarinic depolarization without effect on nicotinic depolarization and dopamine depolarization. We conclude that cyclic AMP and cyclic GMP enhance muscarinic responses in neuroblastoma cells. It is suggested that synaptic transmission in the nervous system may be modulated postsynaptically by changes in intracellular cyclic nucleotide levels.