Robert M. Berne

Release of Adenosine from Ischemic Brain EFFECT ON CEREBRAL VASCULAR RESISTANCE AND INCORPORATION INTO CEREBRAL ADENINE NUCLEOTIDES

A threefold increase in tissue adenosine levels was produced by ischemia in the dog and the rat brain within 1 minute; adenosine levels increased further with longer periods of ischemia. Inosine and hypoxanthine were also increased by ischemia but to different degrees. The nucleosides and hypoxanthine appeared in cerebrospinal fluid during ischemia in the dog brain, and incubation of adenosine in normal cerebrospinal fluid failed to show the presence of degradative enzymes. Intra-arterially administered adenosine in the dog and the cat produced little or no increase in cerebral blood flow or the diameter of pial arterioles, respectively, when it was given in amounts that reduced arterial blood pressure. However, when it was applied topically to exposed pial arterioles of the cat, adenosine induced dilation that was roughly proportional to the dose used. When U-14C-adenosine was infused into the internal carotid arteries of the dog, no radioactivity was detectable in the cerebrospinal fluid and practically none appeared in the brain tissue. When labeled adenosine was added to dog cerebrospinal fluid, only a few counts appeared in the cerebral venous blood, whereas cerebral tissue was heavily labeled with 84–87% of the radioactivity in the form of adenine nucleotides. In the rat, after intravenous injection of labeled adenosine, the counts per gram of heart were twenty- to thirtyfold greater than those per gram of brain. These observations indicate that intra-arterially administered adenosine probably fails to cross the blood-brain barrier rapidly enough to influence cerebral blood flow but that it can be released from the ischemic brain into the cerebrospinal fluid and be reincorporated from the cerebrospinal fluid into brain nucleotides. Hence, adenosine can conceivably participate in the regulation of cerebral blood flow.

Longitudinal Gradients in Periarteriolar Oxygen Tension A Possible Mechanism For the Participation of Oxygen in Local Regulation of Blood Flow

The oxygen tension (Po2) on the external surface of arterioles between 8 and 100μ in diameter was measured with oxygen microcathodes (2 to 6μ diameter) in the suffused cheek pouch of hamsters and in the cremaster muscle of hamsters and rats anesthetized with pentobarbital and urethane. Intravascular Po2 was measured in 10 vessels and compared with extravascular Po2. Good agreement was found, with a mean difference of 1.4 ± 0.8 (SE) mm Hg. Significant longitudinal gradients were observed in periarteriolar Po2. Oxygen tension fell from 35 ± 4 mm Hg on the small arteries (ca. 80μ diameter) to 20 ± 3 mm Hg at the end of the terminal arterioles. These measurements were obtained with a suffusion solution Po2 of 39 ± 8 mm Hg, a tissue Po2 of 8 ± 2 mm Hg and femoral arterial blood Po2 of 69 mm Hg. When the suffusion solution Po2 was raised to 79 mm Hg, the resultant measurements were 42 ± 3 on the small arteries and 21 ± 3 mm Hg at the end of the terminal arterioles. Similar experiments were carried out while animals were breathing 95% oxygen and the Po2 of the cheek pouch suffusion solution was 39 mm Hg. Under these conditions, small artery Po2 was 152 ± 13 mm Hg and terminal arteriolar Po2 was 37 ± 9 mm Hg. Femoral artery blood Po2 was 427 ± 12 mm Hg. These data are consistent with the hypothesis that oxygen diffuses from the precapillary vessels and that intravascular Po2 falls progressively along the resistance vessels. This finding suggests a possible mechanism for the involvement of O2 in local regulation of blood flow.

Adenosine-sensitive ventricular tachycardia: evidence suggesting cyclic AMP-mediated triggered activity.

Catecholamine-induced triggered activity is thought to be caused by intracellular calcium overload mediated by elevation of intracellular cyclic AMP (cAMP). Although shown to occur in isolated preparations, evidence supporting its clinical existence has been lacking. Electrophysiologic studies were performed in four patients with structurally normal hearts who had exertionally related sustained ventricular tachycardia (VT). Programmed stimulation reproducibly initiated and terminated VT in all patients. Induction of tachycardia was also facilitated by infusion of isoproterenol. Adenosine, an endogenous nucleoside, whose only known electrophysiologic effect on ventricular myocardium and Purkinje fibers is antagonism of catecholamine-induced stimulation of intracellular cAMP production, reproducibly terminated all episodes of VT. The tachycardia was also terminated by intravenous verapamil and by the Valsalva maneuver and/or carotid sinus massage. Beta-Adrenergic receptor blockade with propranolol either terminated or prevented induction of VT during programmed stimulation or catecholamine challenge. Adenosine was also administered during VT to 14 patients whose arrhythmias fulfilled standard criteria for reentry, two of whom also had exercise-induced VT. Adenosine, at a dose (112.5 to 225 micrograms/kg iv) sufficient to cause either sinus slowing/arrest or ventriculoatrial block during ventricular pacing, failed to slow or terminate any episode of VT in these patients. Verapamil and autonomic modulation were also ineffective in this group of patients. Adenosine, verapamil, vagal maneuvers (acetylcholine), and beta-adrenergic receptor blockade are all known to decrease the slow-inward calcium current either directly by modulating calcium channels or indirectly by inhibiting production of cellular cAMP. Therefore the observation in this study that interventions that lower intracellular cAMP either terminate or prevent induction of VT in patients with structurally normal hearts and exercise-induced VT suggests that the mechanism of tachycardia may be cAMP-mediated triggered activity.

Protective effects of adenosine in myocardial ischemia.

Adenosine is released from the myocardium in response to a decrease in the oxygen supply/demand ratio, as is seen in myocardial ischemia; its protective role is manifested by coronary and collateral vessel vasodilation that increase oxygen supply and by multiple effects that act in concert to decrease myocardial oxygen demand (i.e., negative inotropism, chronotropism, and dromotropism). During periods of oxygen deprivation, adenosine enhances energy production via increased glycolytic flux and can act as a substrate for purine salvage to restore cellular energy charge during reperfusion. Adenosine limits the degree of vascular injury during ischemia and reperfusion by inhibition of oxygen radical release from activated neutrophils, thereby preventing endothelial cell damage, and by inhibition of platelet aggregation. These effects help to preserve endothelial cell function and microvascular perfusion. Long-term exposure to adenosine may also induce coronary angiogenesis.

Adenosine: electrophysiologic effects and therapeutic use for terminating paroxysmal supraventricular tachycardia

Adenosine was administered intravenously to 17 patients undergoing intracardiac electrophysiologic studies. At a mean dose of 179 +/- 88 micrograms/kg (+/- SD), adenosine suppressed sinus node automaticity and depressed atrioventricular (AV) nodal conduction. These effects were less than 20 sec in duration and were not influenced by muscarinic blockade with atropine (0.02 to 0.03 mg/kg). Adenosine at this dose had no effect on antegrade conduction over accessory pathways in patients with Wolff-Parkinson-White syndrome. No independent hemodynamic effects were observed. In six patients, adenosine was administered intravenously during stimulation-induced paroxysmal supraventricular tachycardia. In the five patients in whom the reentry loop of their tachycardia included the AV node, adenosine at a mean dose of 83 +/- 35 micrograms/kg (+/- SD) terminated their tachycardia within 20 sec after peripheral intravenous injection. The dose of adenosine required to terminate these tachycardias did not produce manifest sinus node suppression, and sinus rhythm promptly resumed in all patients. Adenosine did not terminate either supraventricular tachycardia due to intra-atrial reentry or atrial flutter, but did produce transient AV block during these arrhythmias. Our findings demonstrate that the human sinus and AV nodes are sensitive to physiologic doses of adenosine and that adenosine may be used safely and effectively to terminate acute episodes of supraventricular tachycardia that involve the AV node in the reentry pathway.

Diagnostic and therapeutic use of adenosine in patients with supraventricular tachyarrhythmias.

Adenosine has been shown to affect both sinus node automaticity and atrioventricular (AV) nodal conduction. The effects of increasing doses of intravenous adenosine were assessed in 46 patients with supraventricular tachyarrhythmias. Adenosine reliably terminated episodes of supraventricular tachycardia in all 16 patients with AV reciprocating tachycardia, in 13 of 13 patients with AV nodal reentrant tachycardia and in 1 of 2 patients with junctional tachycardia with long RP intervals. Adenosine produced transient high grade AV block without any effect on atrial activity in six patients with intraatrial reentrant tachycardia, four patients with atrial flutter, three patients with atrial fibrillation and in single patients with either sinus node reentry or an automatic atrial tachycardia. The dose of adenosine required to terminate episodes of supraventricular tachycardia was variable (range 2 to 23 mg). Side effects were minor and of short duration. These results demonstrate that adenosine is useful for the acute therapy of supraventricular tachycardia whenever reentry through the AV node is involved. When arrhythmia termination is not affected, atrial activity may be more readily analyzed during adenosine-induced transient AV block.

Release of Adenosine by the Normal Myocardium in Dogs and Its Relationship to the Regulation of Coronary Resistance

The evidence supporting the hypothesis that adenosine is the mediator of metabolic regulation of coronary blood flow was obtained from experiments characterized by myocardial hypoxia. If adenosine serves the role of physiological regulator of coronary blood flow, it must also be released by the normal heart. Experiments designed to study this question were performed on 15 open-chest dogs in which adenosine was sought in perfusates of the epicardial surface of the well-oxygenated heart. The pericardial space was perfused with warm (37°C) Tyrodes or Krebs-Henseleit solutions (400 to 1200 ml over 1 to 3 hours), and the perfusates were analyzed for adenosine. With a normal myocardial oxygen supply, adenosine was present in the perfusates in a concentration of 3.1 ± 0.5 x 10>-8M. Partial asphyxia, induced by reducing pulmonary ventilation, significantly (P ≤ 0.02) increased the adenosine concentration of the perfusates to 5.4 ± 0.8 x 10-8M. In four dogs the normal pericardial fluid was found to contain adenos...

Increases in Cerebral Interstitial Fluid Adenosine Concentration during Hypoxia, Local Potassium Infusion, and Ischemia

This study used the brain dialysis technique to test the hypothesis that the adenosine concentration of cerebral interstitial fluid increases during situations in which cerebral oxygen supply is inadequate for oxygen demand. Sealed 300-μm hollow dialysis fibers were implanted in the caudate nucleus of pentobarbital-anesthetized rats and perfused at 2 μl/min with artificial cerebrospinal fluid. In vitro tests indicated the recovery of adenosine, inosine, and hypoxanthine from the external medium to be ∼20% at 2 μl/min and close to 100% at 0.1 μl/min. Three in vivo interventions were tested: hypoxia/hypotension (Pao2 = 41.9 mm Hg; MABP = 42.8 mm Hg; n = 9), local potassium infusion (n = 4), and cerebral anoxia/ischemia (n = 10). These interventions produced 10-, 4-, and 30-fold increases in perfusate adenosine concentration, respectively, as well as increases in perfusate concentrations of inosine and hypoxanthine. A separate group of rats (n = 9) perfused at 0.1 μl/min yielded estimates of cerebral interstitial fluid adenosine, inosine, and hypoxanthine concentrations of 1.26, 3.30, and 7.19 μM, respectively. These results are consistent with the adenosine hypothesis for the regulation of CBF.

Brain adenosine production in the rat during 60 seconds of ischemia.

In rats, cerebral perfusion pressure was altered abruptly by aortic transection to determine the production by ischemic brain of adenosine and its metabolites, inosine and hypoxanthine. Brain samples were obtained after 0, 5, 10, 15, 30, and 60 seconds of ischemia. Also measured were ATP, ADP AMP, phosphocreatine (PCr), lactate, and pyruvate. Blood pressure was monitored continuously, and arterial Po2, PcO2, and pH were measured just prior to induction of ischemia. Adenosine was elevated to 2.30 ± 0.31 (Sti) nmol/g at 5 seconds from a control value of 0.96 ± 0.07. A significant elevation of adenosine continued to 60 seconds (5.50 ± 1.24). Furthermore, inosine showed a progressive upward trend during the entire 60 seconds of ischemia, whereas no change in hypoxanthine occurred between the moment of transection (31.81 ± 2.01 nmol/g) and 60 seconds of ischemia (34.72 ± 2.93). PCr decreased by 1.24 fimol/g within the first 5 seconds. After the onset of hypotension, significant changes did not occur in AMP and ADP until 30 seconds, and in ATP and pyruvate until 60 seconds after aortic transection; lactate was elevated by 10 seconds. The rapid rise of cerebral adenosine within 5 seconds after the onset of ischemia supports a role for adenosine in the regulation of cerebral blood flow. Ore Res 45: 486-492, 1979

Effect of Ischemia on Adenine Nucleotides in Cardiac and Skeletal Muscle

Rabbit skeletal muscle contains 56% more ATP and 225% more creatine phosphate than rabbit cardiac muscle. With ischemia the per cent reduction in ATP and creatine phosphate is less in skeletal than cardiac muscle and ADP and AMP decrease in skeletal and increase in cardiac muscle, IMP is present in skeletal muscle and increases with ischemia, whereas it is absent in cardiac muscle but appears with ischemia. Adenosine, inosine, and hypoxanthine are absent in skeletal and cardiac muscle. Ischemia results in the appearance of adenosine, inosine, and hypoxanthine in cardiac muscle but only the latter two compounds appear in ischemic skeletal muscle. Although complete ischemia is required to demonstrate the presence of adenosine in cardiac muscle, the appearance of this nucleoside is nevertheless consistent with the hypothesis that under physiological conditions quantities of adenosine, undetectable by present methods, may play a role in the regulation of coronary blood flow.