Effects of anoxia on cerebral blood flow in the rat brain: evidence for a role of adenosine in autoregulation

The role of adenosine in cerebral vascular regulation during reductions in perfusion pressure

The adenosine antagonist caffeine was used to test the hypothesis that endogenous adenosine contributes to the regulation of cerebral blood flow during acute reductions in mean arterial blood pressure. Caffeine, administered intraperitoneally in 20 and 40 mg kg-1 doses, failed to alter the autoregulatory flow responses to marked reductions in arterial blood pressure, showing that adenosine is not essential for cerebrovascular regulation during severe hypotension.

Indomethacin and ibuprofen enhance anoxia-induced hyperemia in rat brain

Cerebral blood flow (CBF) in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of CBF over periods of several hours. Following intraperitoneal administration, two non-steroidal anti-inflammatory agents, indomethacin and ibuprofen (0.01 and 0.1 mg/kg) showed a tendency to increase resting CBF and the reactive hyperemia elicited by a brief (24 s) anoxic challenge was potentiated. A third agent, diclofenac sodium, was less effective, enhancing the hyperemic response only at the higher (0.1 mg/kg) dose level. The results indicate that indomethacin and ibuprofen may be of value in the treatment of diseases involving cerebrovascular insufficiency.

Localization of S-adenosylhomocysteine hydrolase and adenosine deaminase immunoreactivities in rat brain

Antisera against rat-liver S-adenosylhomocysteine hydrolase (SAH-hydrolase) and calf intestinal mucosal adenosine deaminase (ADA) were raised in rabbits and subsequently used to determine the distribution of the corresponding enzymes in rat-brain using the peroxidase-antiperoxidase immunohistochemical procedure. SAH-hydrolase antigenicity was prominent in the neocortex, hippocampal formation, cerebellum and olfactory tubercle. In the cerebellum, only those cells associated with the Purkinje layer possessed pronounced reactivity with anti-SAH hydrolase. The intense staining present could be correlated mainly with nuclei, the cytosol being stained less intensely. Weak ADA antigenicity was found throughout the brain, but strong antigenic reactivity was associated with neurones in the basal hypothalamus, superior colliculus and in nerve fibres in many regions. Many ADA antigenic neurones and fibres were seen in close proximity to blood vessels. The distribution of ADA antigenicity was also studied in cat and rabbit brain. In cat brain only general staining of tissue occurred with anti-ADA and no intensely stained regions comparable to those seen in rat brain were observed. Rabbit brain showed weak specifically stained neurones only in the superior colliculus. Enzyme assays were also performed to confirm immunohistochemical findings. There appears to be little in common between regions which stained intensely with anti-SAH hydrolase and anti-ADA respectively. The possible implications of these findings are discussed.

A dual function for adenosine 5'-triphosphate in the regulation of vascular tone. Excitatory cotransmitter with noradrenaline from perivascular nerves and locally released inhibitory intravascular agent

A dual function for adenosine 5'-triphosphate in the regulation of vascular tone is considered. Adenosine 5'-triphosphate can cause vasodilation, acting via P2-purinoceptors located on vascular endothelial cells to release an endothelium-derived relaxing factor which diffuses to the vascular smooth muscle and induces vasodilation. The main source of intraluminal adenosine 5'-triphosphate is likely to be endothelial cells, and its release can be measured during pathophysiological conditions such as ischemia and hypoxia, in amounts likely to be sufficient to activate endothelial P2-purinoceptors. Adenosine 5'-triphosphate can also be released during intravascular platelet aggregation and from intact and damaged vascular smooth muscle cells, and so may play a role in the complex physiological mechanisms controlling local vascular tone under normoxic conditions and during vessel injury. Evidence is also presented for adenosine 5'-triphosphate acting as an excitatory cotransmitter with noradrenaline from sympathetic perivascular nerves, to cause vasoconstriction via excitatory P2-purinoceptors located on vascular smooth muscle. The postjunctional mechanical and electrical responses of a number of blood vessels following perivascular nerve stimulation contain a component that is resistant to blockade of the alpha-adrenoceptor. This nonadrenergic response is mimicked by adenosine 5'-triphosphate and can be blocked by selective desensitization of the P2-purinoceptor by alpha,beta-methylene adenosine 5'-triphosphate. Vesicular storage of adenosine 5'-triphosphate and its release from sympathetic perivascular nerves has also been demonstrated. The functional significance of adenosine 5'-triphosphate acting intraluminally as a vasodilator and extraluminally as a vasoconstrictor neuronal agent in the control of vascular tone is discussed.

The effects of nifedipine and felodipine on cerebral blood flow during anoxic episodes

Cerebral blood flow (CBF) in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of flow over periods of several hours. Brief periods of anoxia increase the rate of flow. The dihydropyridine calcium antagonists did not affect basal flow rate and depressed the increase in CBF elicited by anoxia. These findings may have significant implications for the therapeutic use of dihydropyridine calcium antagonists in brain ischaemia.

Nifedipine: more than a calcium channel blocker

Nifedipine exhibits a greater incidence of side effects than the other currently marketed calcium channel antagonists. In addition to those effects attributable to calcium channel blockade, nifedipine produces side effects similar to the effects of adenosine. It is probable that nifedipine exerts part of its physiological actions through potentiation of adenosine. Adenosine, an endogenous calcium channel blocker, modifies synaptic events throughout the nervous system and causes sedation, smooth and skeletal muscle relaxation, anticonvulsion, hypotension and hypothermia, all reversible by caffeine or theophylline administration. Nifedipine inhibits adenosine uptake from, and release into, the extracellular space and binds at an adenosine receptor. Both nifedipine and adenosine interact with benzodiazepine binding sites. Interaction between nifedipine and adenosine should be kept in mind when treating patients with nifedipine.

Cardiovascular effects of adenosine in man; possible clinical implications

The results summarized above indicate that adenosine is a physiologically relevant modulator of the cardiovascular system in man. The levels of adenosine are low during resting conditions, but may increase during conditions of oxygen and/or substrate deficiency. Already the basal concentration seems to be sufficient to affect regional flow in vital organs such as the heart. Several drugs may act by increasing the levels of adenosine or by influencing its receptors. In addition, adenosine may be used in many clinical situations as a vasodilator, antiaggregatory compound as well as an antiarrythmic agent. Its effect is easy to control due to the extremely short plasma half-life. The dose range for the clinical effects are summarized in Table 6. Both the physiological and pharmacological aspect of adenosine are subject to intense study in several laboratories.

The effects of lidoflazine and flunarizine on cerebral reactive hyperemia

Cerebral blood flow in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of flow over periods of several hours. The bi-fluorophenyl-piperazine derivatives, lidoflazine and flunarizine, enhanced the reactive hyperemia elicited by a brief (30 s) anoxic challenge. They did not alter resting cerebral blood flow rates. Verapamil, a potent calcium slow channel blocker, decreased resting flow rates but did not alter the duration of the reactive hyperemia. As lidoflazine and flunarizine are potent inhibitors of adenosine uptake, whereas verapamil is not, the results are consistent with the hypothesis that adenosine plays a significant role in cerebral vascular autoregulation.

Adenosine deaminase inhibitors enhance cerebral anoxic hyperemia in the rat

Cerebral blood flow in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of flow over periods of several hours. The adenosine deaminase inhibitors erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) (1.0-100 micrograms/kg) and deoxycoformycin (0.1-1 micrograms/kg) potentiated the reactive hyperemia elicited by a brief (24-s) anoxic challenge. Basal flow rate was unaltered by EHNA administration and slightly enhanced by deoxycoformycin. The results are consistent with the hypothesis that adenosine plays a significant role in cerebral vascular regulation and suggest that low doses of these deaminase inhibitors may be useful in the treatment of cerebral vascular insufficiency.