Relaxation of the isolated coronary artery by inosine: noninvolvement of the adenosine receptor

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Thymoquinone-induced relaxation of isolated rat pulmonary artery

AIM OF THE STUDY

The effect of thymoquinone (TQ), the main constituent of the volatile oil of black seed (Nigella sativa L. family Ranunculaceae), on the isolated rat pulmonary arterial rings was investigated.

MATERIALS AND METHODS

Isolated rat pulmonary arterial rings were precontracted with phenylephrine and concentration-response curves to TQ were constructed. The effects of different receptors antagonists or enzyme inhibitors were examined.

RESULTS

TQ caused a concentration-dependent decrease in the tension of the pulmonary arterial rings precontracted by phenylephrine. The effects of TQ were not influenced by pretreatment of the rings with propranolol (a non-selective beta-blocker), atropine (a non-selective blocker for muscarinic receptors), theophylline (an adenosine receptor antagonist), indomethacin (a cyclooxygenase inhibitor), L-NAME (a NO synthase inhibitor), methylene blue (an inhibitor of soluble guanylyl cyclase) and nifedipine (a Ca(2+) channel blocker). The effects of TQ were significantly potentiated by bosentan (an ET(A)/ET(B) receptor antagonist). The effects of TQ were slightly abolished by pretreatment of the rings with glibenclamide (a non-selective blocker of ATP-sensitive K+ channels). TQ totally abolished the pressor effects of serotonin and phenylephrine on the isolated rat pulmonary arterial rings.

CONCLUSION

The results of the present study suggest that TQ-induced relaxation of the precontracted pulmonary artery is probably mediated, at least in part, by activation of ATP-sensitive potassium channels and possibly by non-competitive blocking of serotonin, alpha1 and endothelin receptors.

Protective mechanisms of inosine in platelet activation and cerebral ischemic damage

OBJECTIVE

Inosine is a naturally occurring nucleoside degraded from adenosine. Recent studies have demonstrated that inosine has potent immunomodulatory and neuroprotective effects. In the present study, we further investigated the inhibitory effects of inosine on platelet activation in vitro and in vivo, as well as in attenuating middle cerebral artery occlusion (MCAO)-induced focal cerebral ischemia in rats.

METHODS AND RESULTS

Inosine concentration-dependently (0.5 to 6.0 mmol/L) inhibited platelet aggregation stimulated by agonists. Inosine (1.5 and 3.0 mmol/L) inhibited phosphoinositide breakdown, [Ca+2]i, and TxA2 formation in human platelets stimulated by collagen (1 microg/mL). In addition, inosine (1.5 and 3.0 mmol/L) markedly increased levels of cyclic guanylate monophosphate (GMP) and cyclic GMP-induced vasodilator-stimulated phosphoprotein Ser157 phosphorylation. Rapid phosphorylation of a platelet protein of molecular weight 47,000 (P47), a marker of protein kinase C activation, was triggered by collagen (1 microg/mL). This phosphorylation was markedly inhibited by inosine (3.0 mmol/L). Inosine (1.5 and 3.0 mmol/L) markedly reduced hydroxyl radical in collagen (1 microg/mL)-activated platelets. In in vivo studies, inosine (400 mg/kg) significantly prolonged the latency period of inducing platelet plug formation in mesenteric venules of mice, and administration of 2 doses (100 mg/kg) or a single dose (150 mg/kg) of inosine significantly attenuated MCAO-induced focal cerebral ischemia in rats.

CONCLUSIONS

Platelet aggregation contributes significantly to MCAO-induced focal cerebral ischemia. The most important findings of this study suggest that inosine markedly inhibited platelet activation in vitro and in vivo, as well as cerebral ischemia. Thus, inosine treatment may represent a novel approach to lowering the risk of or improving function in thromboembolic-related disorders and ischemia-reperfusion brain injury.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Comparative study of the contractile activity evoked by ATP and diadenosine tetraphosphate in isolated rat urinary bladder

The present study was designed to investigate the effect and possible mechanism(s) of action of ATP and diadenosine tetraphosphate (AP(4)A) on the isolated rat urinary bladder rings. ATP ( 0.1- 1 x 10(-3)M) or AP(4)A ( 0.01- 0.1 x 10(-3)M) produced contractions of the isolated bladder rings in a concentration-dependent manner. The contraction-induced by AP(4)A in the bladder rings was approximately ten times more potent than that produced by ATP. Addition of ATP prior to addition of AP(4)A or vice versa desensitized bladder tissue to the second agonist with great reduction in the contraction produced. Electrical field stimulation (EFS, 40 V, 0.5 ms, 2 Hz) produced contraction (79.8 +/-7.1 g tension x g(-1)tissue) in the bladder rings that can be greatly reduced by prior addition of ATP or AP(4)A. Theophylline, a P(1)-purinoceptor antagonist, significantly reduced the contraction-induced by AP(4)A and did altered that produced by ATP in bladder rings. Atropine, a non-selective muscarinic receptor antagonist, or indomethacin, a cyclo-oxygenase inhibitor, significantly suppressed the contractions of the bladder rings to ATP or AP(4)A. Similarly, nifedipine, an l -type Ca(2+)channel blocker, significantly attenuate the contractions induced by ATP and AP(4)A in the isolated rat urinary bladder rings. In conclusion, the results of the present study show that ATP, AP(4)A, and EFS evoked contractions in the rat urinary bladder rings and that the contractions induced by AP(4)A was more potent than that produced by ATP. Furthermore, the contractions evoked by ATP or AP(4)A were Ca(2+)-dependent and mediated at least in part through one of the cyclo-oxygenase products. Also, the present results suggested the involvement of the P(1)-purinoceptor in mediating the contractions evoked by AP(4)A but not ATP in the bladder rings.

Effect of adenosine on pulmonary circulation of rabbits

The effect and mechanism of action of adenosine on the pulmonary circulation of rabbits were studied. Adenosine (10(-5)-10(-3) M) produced a concentration-dependent decrease in pulmonary arterial tension of precontracted pulmonary arterial rings. Removal of endothelium (denuded) augmented the adenosine-induced vasodilation in the pulmonary arterial rings. Theophylline (5 x 10(-5) M), an adenosine receptor antagonist, reduces the vasodilation induced by adenosine in intact and denuded rings. Pretreatment of the pulmonary rings with the cyclooxygenase inhibitor indomethacin (5 x 10(-6) M) significantly attenuated the adenosine-induced relaxation in denuded but not in the intact pulmonary arterial rings. Methylene blue (5 x 10(-5) M), a guanylate cyclase inhibitor, significantly reduced the relaxation induced by adenosine in both the intact and the denuded arterial rings. Adenosine significantly attenuated the pressor responses of serotonin and acetylcholine in the intact and denuded rabbit's pulmonary arterial rings. The results of this study indicate that adenosine induces pulmonary vasodilation and that functional endothelium is not required to evoke this dilation. In addition, guanylate cyclase activity and the generation of cGMP is essential for adenosine to induce vasodilation in the rabbit lung. Furthermore, the results of this study may suggest that adenosine could be used to reduce the severity of pulmonary hypertension and possibly pulmonary edema.

Inosine--a natural modulator of contractility and myocardial blood flow in the ischemic heart?

The energetic role of inosine (INO) remains controversial. The aim of the present study was first to test whether endogenous INO consumption/production correlates with regional myocardial contractile performance and second to test whether locally increased levels of INO influence contractility and blood flow in severely ischemic myocardium. Fentanyl-anesthetized dogs with implanted sonomicrometry crystals and independently perfused left anterior descending coronary arteries were studied. Two relatively load-independent indexes of regional myocardial contractility derived from left ventricular pressure-segment length loops were used: the regional stroke work-end-diastolic segment length relationship (Wr/L(ed)) and the end-systolic pressure-segment length relationship (Plv/L(es)). Very good correlations between myocardial contractile performance (as measured by the slope of the regional Wr/L(ed) relationship) and endogenous INO consumption/production under both nonischemic and ischemic conditions were found. Ischemia severely depressed contractility, significantly shifting rightward the Wr/L(ed) and Plv/L(es) relationships. INO infused into the left anterior descending bypass, in a concentration of 600 to 800 mumol/L, partially restored contractile performance as evidenced by a significant leftward displacement of both relationships. Wr, measured at a common maximum L(ed), increased significantly by 61 +/- 5%. Border-zone collateral flow (microspheres) increased by 35 +/- 7% within the endocardial segments and by 34 +/- 9% in the epicardial segments, but no increase in flow in the ischemic region was measureable. With the current emphasis on recanalization with thrombolytic therapy and considering the apparent safety of INO, this naturally occurring nucleoside might prove to be a useful adjunctive agent in the treatment of acute myocardial ischemia.

Nucleotide precursors modify the effects of isoproterenol. Studies on heart function and cardiac adenine nucleotide content in intact rats

In closed-chest rats, isoproterenol (ISO, 25 mg/kg), 5 hours after subcutaneous administration, increased heart rate by 53%, left ventricular (LV) dP/dtmax by 80%, and cardiac output by 37%. LV systolic pressure (LVSP, -10%), mean arterial pressure (MAP, -12%), and total peripheral resistance (TPR, -36%) were diminished. In separate experiments, continuous intravenous infusion of adenine (50 mg/kg/hr) for 5 hours reduced heart rate (-11%), LVSP (-16%), MAP (-20%), TPR (-33%), and LV dP/dtmax (-20%). Cardiac output was increased (+20%). Inosine has been shown to have similar effects, except for a decline in cardiac output. Adenine (50 mg/kg/hr) attenuated the ISO-induced increase in heart rate and LV dP/dtmax and aggravated the decline in LVSP, MAP, and TPR. The increase in cardiac output was not changed. Inosine (200 mg/kg/hr) modified the ISO effects to a similar extent. Ribose (200 mg/kg/hr) added to the adenine infusion did not have functional effects. However, it aggravated the modifying influence of inosine on LVSP, LV dP/dtmax, and MAP. ISO reduced the cardiac ATP content (mumol/g) from a control value of 5.02 +/- 0.06 (n = 12) to 3.51 +/- 0.13 (n = 10). Adenine (3.56 +/- 0.21, n = 7) and ribose (3.64 +/- 0.11, n = 9) alone did not affect it, but inosine attenuated it (4.33 +/- 0.08, n = 8). Adenine and inosine in combination with ribose abolished the ISO-induced ATP decline (5.18 +/- 0.23, n = 7, and 4.76 +/- 0.10, n = 8, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of adenosine deaminase and adenosine deaminase complexing protein in rabbit heart. Implications for adenosine metabolism

The distribution of adenosine deaminase and adenosine deaminase complexing protein in rabbit heart has been compared using immunohistochemical staining procedures. Sections (4-5 microns) of tissue fixed in Clarke's solution or paraformaldehyde and embedded in paraffin were stained by the peroxidase anti-peroxidase method for adenosine deaminase or complexing protein, using affinity purified antibodies. Staining for adenosine deaminase and complexing protein was observed in the central myocardium of all heart chambers. Adenosine deaminase was detected in endothelial cells of blood vessels and adjacent pericytes. The nuclei of arteries stained heavily for adenosine deaminase, whereas those of venules and small veins, although positive, stained much more lightly. The cytoplasm of blood vessel endothelial cells and smooth muscle cells of the tunica media were also weakly positive for adenosine deaminase. Endothelial cells of the endocardium and epicardium did not stain. Randomly distributed mononuclear inflammatory cells and interstitial connective tissue fibroblasts were also negative for adenosine deaminase. These results raise the possibility that endothelial cells containing adenosine deaminase could serve as a metabolic barrier preventing the free exchange of plasma and interstitial adenosine. Positive staining for complexing protein was restricted to blood vessel endothelial cells, especially cytoplasmic processes. Colocalization experiments carried out with biotinylated primary antibodies indicate that some vessels are positive for both adenosine deaminase and complexing protein. This is the first experimental evidence of possible in situ association of adenosine deaminase and complexing protein.

Inosine causing insulin release and increased myocardial uptake of carbohydrates relative to free fatty acids in dogs

The aim of the present study was to determine the effect of i.v. inosine on myocardial substrate uptake and function in the in situ dog heart. Inosine was infused i.v. at a rate of 5 mg kg min-1 in eight closed-chest pentobarbital anaesthetized dogs. Inosine caused a 46% decrease (P less than 0.01) in plasma free fatty acids (FFA), a 15% decrease (P less than 0.05) in plasma glycerol, an 18% decrease (P less than 0.05) in plasma glucose and a 46% increase (P less than 0.01) in blood lactate. This was associated with a 55% decrease (P less than 0.01) in myocardial FFA uptake and a 72% increase in lactate uptake, while glucose uptake remained unchanged. These metabolic changes were associated with a five-fold increase (P less than 0.05) in arterial insulin. Inosine caused an 18% increase (P less than 0.01) in myocardial blood flow without changing MVO2. There was a 33% increase (P less than 0.01) in LV dP/dtmax, a decrease in LVEDP from 4.9 +/- 0.9 (mean +/- SEM) to 0.9 +/- 0.3 mmHg (P less than 0.05) and a 24% decrease (P less than 0.01) in systemic vascular resistance. Inosine caused a transient 38% increase (P less than 0.05) in pulmonary vascular resistance. In conclusion, in addition to a positive inotropic effect and vascular effects inosine was found to cause release of insulin and to shift myocardial metabolism towards increased uptake of carbohydrates relative to FFA.

Evidence that the intracellular effects of adenosine in the guinea-pig aorta are mediated by inosine

Previous studies have demonstrated that high concentrations of adenosine interact with both a cell surface receptor and with an intracellular site to evoke relaxation of the guinea-pig aorta. The intracellular action of adenosine was investigated in the present study. The purine sensitive 'P-site' did not appear to be involved since other P-site agonists did not consistently evoke relaxation. A major interaction with intracellular S-adenosylhomocysteine hydrolase also appeared unlikely since 1-homocysteine had only minor effects on adenosine-evoked responses. Inhibition of adenosine deaminase attenuated responses evoked by high concentrations of adenosine. The deaminated metabolite of adenosine, inosine, also evoked aortic relaxation. These responses were mediated solely via an intracellular site since they were blocked by an inhibitor of nucleoside-facilitated diffusion but were unaffected by an adenosine receptor antagonist. These results indicate that a major part of the intracellular effect of adenosine is mediated by its deaminated metabolite inosine.

Mechanisms of the contractile effect induced by uridine 5-triphosphate in canine cerebral arteries

This study was performed to elucidate mechanisms responsible for the contraction of isolated canine cerebral arteries induced by uridine 5'-triphosphate (UTP) and to ascertain whether UTP given intracisternally causes cerebral arterial constriction. The latter was proven arteriographically to be the case. In vitro, UTP (10(-4)M) and UDP were similar in potency, produced sustained contractions, and were more effective than other pyrimidine nucleotides or uridine. Unlike serotonin (5-HT), UTP was not antagonized by cinanserin and failed to cause constriction of mesenteric arteries. Adenosine similarly antagonized 5-HT and UTP. The Ca2+ antagonist nimodipine abolished contractions caused by high K+ but only incompletely antagonized 5-HT or UTP. On the other hand, procedures that hyperpolarize the cell membrane (low K+ followed by K+) abolished tonic contractions induced by UTP. Hyperpolarization prior to UTP (with or without nimodipine) did not, however, prevent the occurrence of a phasic contraction. Papaverine or lanthanum antagonized this phasic response. It was concluded that UTP selectively affects cerebral arteries, may initiate contraction by releasing membrane bound Ca2+, depolarizes the cell membrane to open receptor operated and potential sensitive calcium channels, but does not inhibit the electrogenic Na-pump nor specifically antagonize the vasodilator adenosine.