Inhibitory actions of adenosine differ between ear and mesenteric arteries in the rabbit

The Adenosinergic System as a Therapeutic Target in the Vasculature: New Ligands and Challenges

Adenosine is an adenine base purine with actions as a modulator of neurotransmission, smooth muscle contraction, and immune response in several systems of the human body, including the cardiovascular system. In the vasculature, four P1-receptors or adenosine receptors—A₁, A_2A, A_2B and A₃—have been identified. Adenosine receptors are membrane G-protein receptors that trigger their actions through several signaling pathways and present differential affinity requirements. Adenosine is an endogenous ligand whose extracellular levels can reach concentrations high enough to activate the adenosine receptors. This nucleoside is a product of enzymatic breakdown of extra and intracellular adenine nucleotides and also of S-adenosylhomocysteine. Adenosine availability is also dependent on the activity of nucleoside transporters (NTs). The interplay between NTs and adenosine receptors’ activities are debated and a particular attention is given to the paramount importance of the disruption of this interplay in vascular pathophysiology, namely in hypertension., The integration of important functional aspects of individual adenosine receptor pharmacology (such as in vasoconstriction/vasodilation) and morphological features (within the three vascular layers) in vessels will be discussed, hopefully clarifying the importance of adenosine receptors/NTs for modulating peripheral mesenteric vascular resistance. In recent years, an increase interest in purine physiology/pharmacology has led to the development of new ligands for adenosine receptors. Some of them have been patented as having promising therapeutic activities and some have been chosen to undergo on clinical trials. Increased levels of endogenous adenosine near a specific subtype can lead to its activation, constituting an indirect receptor targeting approach either by inhibition of NT or, alternatively, by increasing the activity of enzymes responsible for ATP breakdown. These findings highlight the putative role of adenosinergic players as attractive therapeutic targets for cardiovascular pathologies, namely hypertension, heart failure or stroke. Nevertheless, several aspects are still to be explored, creating new challenges to be addressed in future studies, particularly the development of strategies able to circumvent the predicted side effects of these therapies.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Spontaneous electrical activity and associated changes in calcium concentration in guinea-pig gastric smooth muscle

Spontaneous electrical activity and internal Ca(2+) concentration ([Ca(2+)](i)) were measured simultaneously using conventional microelectrodes and fura-2 fluorescence, respectively, in isolated circular smooth muscle bundles of the guinea-pig gastric antrum. The smooth muscle bundles generated periodic slow potentials with accompanying spike potentials and associated transient increases in [Ca(2+)](i) (Ca(2+)-transients). Nifedipine abolished the spike potentials but not the slow potentials, and reduced the amplitude of associated Ca(2+)-transients. Caffeine, in the absence or presence of ryanodine, reduced resting [Ca(2+)](i) levels and abolished the slow potentials and associated Ca(2+)-transients. Depolarization elevated and hyperpolarization reduced resting [Ca(2+)](i) levels with associated changes in the frequency of slow potentials. The amplitude of Ca(2+)-transients changed in a bell-shaped manner with the membrane potential change. Slow potentials and associated Ca(2+)-transients were abolished if [Ca(2+)](i) levels were reduced by BAPTA-AM or if the internal Ca(2+) pump was inhibited by cyclopiazonic acid. 2-Aminoethoxy-diphenylborate (2-APB), a known inhibitor of inositol trisphosphate (IP(3))-mediated Ca(2+) release, also blocked slow potentials and Ca(2+)-transients. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP), a mitochondrial protonophore, depolarized the membrane, elevated [Ca(2+)](i) levels and abolished slow potentials and Ca(2+)-transients. Inhibition of mitochondrial ATP-sensitive K(+) channels by glybenclamide and 5-hydroxydecanoic acid (5-HAD) abolished slow potentials and Ca(2+)-transients, without altering the smooth muscle [Ca(2+)](i). It is concluded that in antrum circular muscles, the frequency of slow potentials is correlated with the level of [Ca(2+)](i). The slow potential is coupled to release of Ca(2+) from an internal store, possibly through the activation of IP(3) receptors; this may be initiated by the activation of ATP-sensitive K(+) channels in mitochondria following Ca(2+) handling by mitochondria.

Distribution and physiological roles of ATP-sensitive K+ channels in the vertebrobasilar system of the rabbit

The effect of an opener (levcromakalim) and a blocker (glibenclamide) of ATP-sensitive K+ (KATP) channels was investigated in the vertebrobasilar system of the rabbit. Arterial tension and membrane potential were measured by the isometric tension recording method and the microelectrode technique, respectively. Glibenclamide (10(-6) mol/L) depolarized the membrane and potentiated the contraction to histamine in vertebral arteries. The sensitivity to the relaxant effects of levcromakalim was in the following descending order: vertebral > proximal basilar > distal basilar > superior cerebellar arteries. Vertebral arteries were approximately 50 times more sensitive to levcromakalim than were superior cerebellar arteries. The relaxation to levcromakalim was abolished by glibenclamide (10(-6) mol/L). Glibenclamide attenuated vasorelaxation to adenosine in proximal arteries (vertebral and proximal basilar) but not in superior cerebellar arteries. Levcromakalim (7 x 10(-8) mol/L) and adenosine (10(-5) mol/L) induced glibenclamide-sensitive membrane hyperpolarization in vertebral arteries but not in distal basilar arteries. These results suggest that KATP channels contribute to the determination of resting membrane potential and resting tone in vertebral arteries. Furthermore, there is a marked heterogeneity in the sensitivity to an opener of KATP channels, and the heterogeneity has a functional link to the mechanism underlying vasorelaxation to adenosine in the vertebrobasilar system of the rabbit.

Purinoceptor modulation of noradrenaline release in rat tail artery: tonic modulation mediated by inhibitory P2Y- and facilitatory A2A-purinoceptors

1. The effects of analogues of adenosine and ATP on noradrenaline release elicited by electrical stimulation (5 Hz, 2700 pulses) were studied in superfused preparations of rat tail artery. The effects of purinoceptor antagonists, of adenosine deaminase and of adenosine uptake blockade were also examined. Noradrenaline was measured by h.p.l.c. electrochemical detection. 2. The A1-adenosine receptor agonist, N6-cyclopentyladenosine (CPA; 0.1-100 nM) reduced, whereas the A2A-receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 3-30 nM) increased evoked noradrenaline overflow. These effects were antagonized by the A1-adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 20 nM) and the A2-adenosine receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX; 100 nM), respectively. The P2Y-purinoceptor agonist, 2-methylthio-ATP (1-100 microM) reduced noradrenaline overflow, an effect prevented by the P2-purinoceptor antagonist, cibacron blue 3GA (100 microM) and suramin (100 microM). 3. Adenosine deaminase (2 u ml-1), DMPX (100 nM) and inhibition of adenosine uptake with S-(p-nitrobenzyl)-6-thioinosine (NBTI; 50 nM) decreased evoked noradrenaline overflow. DPCPX alone did not change noradrenaline overflow but prevented the inhibition caused by NBTI. The P2Y-purinoceptor antagonist, cibacron blue 3GA (100 microM) increased evoked noradrenaline overflow as did suramin, a non-selective P2-antagonist. 4. It is concluded that, in rat tail artery, inhibitory (A1 and P2Y) and facilitatory (A2A) purinoceptors are present and modulate noradrenaline release evoked by electrical stimulation. Endogenous purines tonically modulate noradrenaline release through activation of inhibitory P2Y and facilitatory A2A purinoceptors, whereas a tonic activation of inhibitory A1 purinoceptors seems to be prevented by adenosine uptake.

Evoked noradrenaline release in the rabbit ear artery: enhancement by purines, attenuation by neuropeptide Y and lack of effect of calcitonin gene-related peptide

1. Adenosine (30 microM) and its analogues 5'-N-ethylcarboxaminoadenosine (5 and 30 microM) and L-phenylisopropyladenosine (5 and 30 microM), potentiated the evoked but not spontaneous release of tritiated noradrenaline in the rabbit central ear artery. 2. Prejunctional inhibition of the evoked but not spontaneous release of tritiated noradrenaline by 100 nM neuropeptide Y is greater at 2 min than at 10 min after superfusion of the peptide. 3. Calcitonin gene-related peptide (2.63 to 263 nM) did not affect the evoked or spontaneous release of tritiated noradrenaline in this preparation. 4. These results are discussed in terms of prejunctional modulation of sympathetic transmission in the rabbit central ear artery.

Comparative effects of a selective adenosine A2 receptor agonist, CGS 21680, and nitroprusside in vascular smooth muscle

CGS 21680 (2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethyolcarboxamidoa denosine) is an adenosine agonist that has been reported recently to bind selectively to adenosine A2 receptors in rat brain. This adenosine agonist, and the parent compound NECA (5'-N-ethylcarboxamidoadenosine), were found to be potent vasorelaxants of prostaglandin F2 alpha (PGF2 alpha) precontracted porcine coronary smooth muscle with EC50s of 4.5 and 9.7 nM, respectively. Schild analysis of the inhibition of CGS 21680, NECA and 2-chloroadenosine induced relaxation of the porcine coronary artery by CGS 15943 (9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-C]quinazolin-5-amine), an A2 receptor antagonist, yielded identical pA2 values for the antagonist (approximately 9.3). This indicates that the same receptor mediates the effects of these three adenosine agonists. NECA and CGS 21680 were equipotent in most vascular preparations except in the canine coronary artery. Porcine coronary arterial rings contracted with PGF2 alpha were relaxed by NECA or CGS 21680 as well as by nitroprusside; those contracted with KCl (40 mM) were relaxed only by nitroprusside. In rabbit aorta, contractions induced by phenylephrine or PGF2 alpha were inhibited by nitroprusside but not by NECA or CGS 21680. Thus, the adenosine A2 receptor agonists, NECA and CGS 21680, are potent vasorelaxants that display regional vascular and species variations that differ from those of nitroprusside.

Actions of dipyridamole on endogenous and exogenous noradrenaline in the dog mesenteric vein

1. In the isolated mesenteric vein of the dog, dipyridamole inhibited both the excitatory junction potential (e.j.p.) and the slow depolarization evoked by perivascular nerve stimulation, to 60-70% of control, with no change in the postjunctional membrane potential. These inhibitory actions of dipyridamole were not modified by 8-phenyltheophylline or phentolamine, suggesting that the inhibition did not involve either the actions of endogenous adenosine or the prejunctional alpha-autoregulation mechanism. 2. Dipyridamole did not produce any detectable effects on either the facilitation process of the e.j.ps or the postjunctional membrane depolarization produced by exogenously applied noradrenaline (NA). 3. Dipyridamole reduced the outflow of both the NA and the 3,4-dihydroxyphenylglycol (DOPEG) evoked by perivascular nerve stimulation to below 10% of control, the effect being much greater than that of exogenously applied adenosine (to about 90% of the control). 4. Exogenously-added NA was degraded by incubation with a segment of the vein. Dipyridamole itself produced degradation of NA and accelerated the NA-induced degradation. By contrast, pyrogallol, but not pargyline or imipramine, prevented the NA-induced degradation. 5. It is suggested that dipyridamole degrades NA directly, and also indirectly through activation of catechol-O-methyl transferase, with no alteration of the activity of monoamine oxidase or of the uptake mechanisms of NA into nerve terminals.

Thiamin transport by human erythrocytes and ghosts

Thiamin transport in human erythrocytes and resealed pink ghosts was evaluated by incubating both preparations at 37 or 20 degrees C in the presence of [3H]-thiamin of high specific activity. The rate of uptake was consistently higher in erythrocytes than in ghosts. In both preparations, the time course of uptake was independent from the presence of Na+ and did not reach equilibrium after 60 min incubation. At concentrations below 0.5 microM and at 37 degrees C, thiamin was taken up predominantly by a saturable mechanism in both erythrocytes and ghosts. Apparent kinetic constants were: for erythrocytes, Km = 0.12, 0.11 and 0.10 microM and Jmax = 0.01, 0.02 and 0.03 pmol.microliter-1 intracellular water after 3, 15, and 30 min incubation times, respectively; for ghosts, Km = 0.16 and 0.51 microM and Jmax = 0.01 and 0.04 pmol.microliter-1 intracellular water after 15 and 30 min incubation times, respectively. At 20 degrees C, the saturable component disappeared in both preparations. Erythrocyte thiamin transport was not influenced by the presence of D-glucose or metabolic inhibitors. In both preparations, thiamin transport was inhibited competitively by unlabeled thiamin, pyrithiamin, amprolium and, to a lesser extent, oxythiamin, the inhibiting effect being always more marked in erythrocytes than in ghosts. Only approximately 20% of the thiamin taken up by erythrocytes was protein- (probably membrane-) bound. A similar proportion was esterified to thiamin pyrophosphate. Separate experiments using valinomycin and SCN- showed that the transport of thiamin, which is a cation at pH 7.4, is unaffected by changes in membrane potential in both preparations.