Investigation of the selectivity of alpha, beta-methylene ATP in inhibiting vascular responses of the rat in vivo and in vitro

The dual nature of extracellular ATP as a concentration-dependent platelet P2X1 agonist and antagonist

Patient groups subject to higher occurrence of stroke (e.g., people with diabetes, cystic fibrosis, pulmonary hypertension) have reduced release of ATP from their erythrocytes (ERYs) when subjected to flow-induced deformation or pharmacological stimuli. These same groups also have platelets that are more adhesive in comparison to controls. Here we show platelet aggregation, and inhibition of that aggregation, is affected by free Ca(2+) entering the platelet through the ATP-gated P2X1 receptor. The addition of ATP (10 microM) increased the platelet NO by 26.7 +/- 7.7%. This value was decreased significantly to below basal levels in the presence of NF 449 (p < 0.001), an inhibitor of the P2X1 receptor on the platelet. Aggregation profiles measured in the presence of ATP revealed that when the P2X1 receptor was blocked, or when the measurements were performed in Ca(2+) free buffer, platelet aggregation was nearly eliminated. Our findings employing standard aggregation measurements suggest that ATP behaves as a platelet inhibitor below 1.6 x 10(-19) moles ATP per platelet; however, above this value, ATP behaves as a platelet activator. These findings suggesting a dual nature of ATP with regard to platelet behavior were confirmed by passing platelets over endothelial cells that were coated in the channels of a microfluidic device. Importantly, it was determined that ERY-derived ATP release was a major determinant of platelet adhesion to the endothelium. These findings may have implications in anti-platelet drug design as most current therapies focus on the inhibition of P2Y-type receptors. Moreover, through the use of microfluidic technologies, we have provided in vitro evidence for a possible relationship between ERY properties and platelet behavior in vivo.

Both extracellular ATP and shear stress regulate the release of nitric oxide in rat caudal artery

1. To elucidate the physiological role of nitric oxide (NO) in regulating vascular tone, the effects of NG-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, on the vasoconstrictor response to noradrenaline (NA) in rat caudal artery was examined. 2. NG-Nitro-L-arginine methyl ester significantly potentiated the NA-induced increase in perfusion pressure in the perfused caudal artery, but did not affect the NA-induced contraction in caudal artery ring preparations. In addition, an increase in perfusion pressure mechanically produced by a stepwise increase in flow rate was not affected by L-NAME. 3. Noradrenaline evoked a significant increase in the release of endogenous ATP and its metabolites from the perfused artery, whereas increased perfusion pressure as a result of increased flow rate did not evoke release of endogenous ATP. 4. In the presence of exogenously applied ATP, L-NAME significantly potentiated the increase in perfusion pressure produced by increased flow rate. 5. These results indicate that perfused vascular tone is regulated by endogenous NO and suggest that extracellular ATP may participate in the synthesis and release of NO by shear stress in endothelial cells in the rat caudal artery.

Evidence for purinergic neurotransmission in the urinary bladder of pithed rats

The purpose of this study was to investigate the contribution of adenosine-5'-triphosphate (ATP) to segmental (L6-S2) spinal electrical stimulation evoked increases in intra-vesical pressure in pithed rats. Exogenous ATP and substance P produced dose-dependent increases in intra-vesical pressure (ED10 mmHg (dose required to elicit 10 mmHg increase in intra-vesical pressure)= 1.7 mg/kg and 1.1 microg/kg, i.v., respectively). Desensitisation (or antagonism) of P2x purinoceptors with alpha,beta-methylene ATP (alpha,beta-meATP; 30 microg/kg per min, i.v.) or pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 10 mg/kg, i.v.) significantly (p < 0.05) antagonized the intra-vesical pressure responses to ATP (> 8 and 3.6-fold increase in ED10 mmHg, respectively) but had no significant effect on intra-vesical pressure responses to substance P. Spinal stimulation evoked frequency-dependent increases in intra-vesical pressure (EF20 mmHg (frequency required to produce 20 mmHg increase in intra-vesical pressure) = 3.4 Hz). Blockade of muscarinic cholinoceptors and adrenoceptors with atropine (3 mg/kg, i.v.), propranolol (3 mg/kg, i.v.) and phentolamine (10 mg/kg, i.v.) produced marginal attenuation of the intra-vesical pressure responses to spinal stimulation indicating a major non-adrenergic non-cholinergic (NANC) component in the overall response. The NANC responses were significantly (p < 0.05) antagonized by alpha,beta-meATP (30 microg/kg per min, i.v.) and PPADS (10 mg/kg, i.v.) (> 2.6-fold increase in EF20 mmHg), consistent with involvement of a purinergic neurotransmitter, presumably ATP. Comparative studies in young (4-6 months) and old (21-23 months) Fischer rats revealed no age-dependent changes in the relative contribution of the cholinergic and purinergic systems, with the latter being the dominant one. These findings suggest that purinergic neurotransmission, presumably mediated by ATP acting via P2x purinoceptors, represents a major component of excitatory innervation to the urinary bladder in pithed rats.

Mechanism of resistance to alpha-adrenergic receptor antagonists of renal nerve stimulation-induced vasoconstriction at low frequencies

To determine why renal vasoconstriction elicited by periarterial nerve stimulation (RNS) at lower frequencies (< 4 Hz) is resistant to alpha-adrenergic receptor blockade in the rat kidney, we reevaluated the effect of alpha-receptor antagonists on the vasoconstrictor response to norepinephrine (NE) and to RNS and on the release of adrenergic transmitter. The alpha-receptor antagonist prazosin (PZ) at 0.2 and 7 nM reduced the vasoconstrictor response to NE, and 2.4 microM PZ abolished it. PZ (0.2 or 7 nM) reduced RNS-induced vasoconstriction without altering the fractional tritium overflow. PZ (2.4 microM) enhanced fractional tritium overflow and reduced the vasoconstrictor response to RNS at 2-10 Hz, but not at 0.5 or 1 Hz. The effect of 0.2 nM PZ to reduce RNS-induced vasoconstriction was reversed by increasing the concentration to 2.4 microM. Corynanthine (COR; 2.6 microM), a preferential alpha-receptor blocker, or phenoxybenzamine (PBZ; 30 nM) abolished the vasoconstrictor response to NE but only partially reduced response to RNS and enhanced the fractional tritium overflow. Rauwolscine (RW; 2.5 nM), a preferential alpha 2-receptor antagonist, did not alter the vasoconstrictor response to NE but potentiated RNS-induced vasoconstriction and fractional tritium overflow. RW (7.7 microM) inhibited NE-induced vasoconstriction but potentiated the vasoconstrictor response to RNS and fractional tritium overflow. PZ (7 nM) abolished the potentiation by RW and reduced the vasoconstrictor response to RNS. These data suggest that a component of RNS-induced vasoconstriction in the rat kidney is attributable to co-release of a nonadrenergic transmitter with NE. The diminished effect of alpha-receptor antagonists at higher concentrations (e.g., PZ 2.4 microM) to reduce RNS-induced vasoconstriction is caused by their prejunctional action to enhance co-release of the nonadrenergic transmitter.

Sympathetic vascular control of the laryngeo-tracheal, bronchial and pulmonary circulation in the pig: evidence for non-adrenergic mechanisms involving neuropeptide Y

Neuropeptide Y (NPY) and noradrenaline (NA) are co-stored in sympathetic perivascular nerves of the airway mucosa and lung. THe superior laryngeal, bronchial and pulmonary vascular responses were therefore studied in anaesthetized pigs after systemic injections of NPY and NA and after stimulation (2 or 10 Hz, 15 V, 5 ms) of the cranial and caudal portions of hte cervical sympathetic trunk or the stellate ganglia. NPY and NA increased vascular resistance, suggesting vasoconstriction in all three vascular beds. Stimulation of the cervical sympathetic trunk in the cranial direction caused clear-cut vasoconstriction and a decrease in the superficial blood flow in the laryngeal and tracheal circulation supplied by the superior laryngeal artery. This vascular response may be related to release of NA at 2 Hz and possibly also NPY at 10 Hz, since a remaining vasoconstrictor response at 10 Hz was present in reserpinized preganglionically transected pigs when tissue content of NA but not NPY was depleted. The decrease in superficial blood flow in the tracheal mucosa on sympathetic stimulation was absent after reserpine, however. Stimulation of the cervical sympathetic trunk in caudal direction provoked vasoconstriction in the bronchial and pulmonary vascular beds in control pigs. The basal tone of these two vascular beds was not influenced on electrical stimulation after reserpine pretreatment, however, suggesting involvement of NA and possibly aslo NPY, which were both depleted by reserpine. Electrical stimulation of the stellate ganglia also evoked reserpine-sensitive vasoconstriction in both the bronchial and pulmonary vascular beds. The left stellate ganglion dominated the vasomotor response in the bronchial circulation, whereas the right side mainly influenced the pulmonary circulation and the heart.

Purinergic receptors on insulin-secreting cells

The insulin secreting B cell is fitted with the two types of purinergic receptors: P2 (for ATP and/or ADP) and P1 (for adenosine). The activation of P2 purinoceptors by ATP or ADP evokes a biphasic stimulation of insulin secretion from isolated perfused rat pancreas; this stimulation is dose-dependent between 10(-6) and 10(-4) M. Non hydrolysable structural analogues are also effective, and the relative potency of various agonists (2-methylthio ATP >> ATP = ADP = alpha, beta-methylene ATP >> AMP) gave evidence for a P2y purinoceptor subtype. Proposed mechanisms include both an increased Ca2+ uptake and an increased intracellular Ca2+ mobilization via the hydrolysis of polyphosphoinositides. ATP (or ADP) potentiates physiological insulin-secreting agents (glucose and acetylcholine) and P2 purinoceptors could play a physiological role in the stimulation of insulin secretion. The activation of P1 purinoceptors (adenosine receptors) decreases insulin secretion. Using structural analogues of adenosine, the receptor was characterized as an A1 subtype; it is coupled to a pertussis toxin sensitive G protein and it inhibits adenylate cyclase. It is of physiological relevance that the B cell has the two types of purinoceptors with opposite effects. Recently, a metabolically stable structural analogue of ADP, adenosine-5'-0-(2-thiodiphosphate) or ADP beta S, has been described as a potent secretory agent, effective at nanomolar concentrations on isolated perfused rat pancreas. In vivo, this substance is able to increase insulin secretion and to improve glucose tolerance after IV administration in rats and oral administration in dogs. Furthermore in streptozotocin-induced diabetes. ADP beta S retains its insulin secreting effects. These results suggest that P2y purinoceptors could be a new target for antidiabetic drugs.

Pharmacological actions of the coenzymes NAD(H) and NADP(H) on the rat anococcygeus muscle

1. The pharmacological actions of the oxidized and reduced forms of nicotinamide-adenosine dinucleotide (NAD, NADH) and nicotinamide-adenosine dinucleotide phosphate (NADP, NADPH) were studied on rat isolated anococcygeus muscles. 2. The actions of the two nucleotides were different, but there were no apparent qualitative differences between the oxidized and reduced forms of each. 3. In fully relaxed anococcygeus muscles, NADP(H) produced transient contractions that were subject to desensitization, but NAD(H) had no effect. 4. NADP(H) slightly enhanced contractions elicited by noradrenergic nerve stimulation. In contrast, noradrenergic contractions were inhibited by NAD(H). NADH reduced the stimulation-induced release of noradrenaline, but enhanced contractions elicited by exogenous noradrenaline. 5. In anococcygeus muscles partly contracted with guanethidine, NAD(H) produced a further sustained increase in tone; in contrast, NADP(H) mainly produced transient relaxations to which there was immediate desensitization. 6. Relaxations of anococcygeus muscle elicited by nitrergic nerve stimulation were not affected by NAD. In contrast, NADP(H) reduced them. 7. The actions of NAD(H) were generally the same as those of adenosine and can be attributed to activation of P1-purinoceptors since they were blocked by the selective antagonist 8-sulphophenyltheophylline. 8. The actions of NADP resembled those of the P2-purinoceptor agonist ATP to some extent, but there were some differences. As suggested by others, NADP may act on a unique receptor.

Suramin: a selective inhibitor of purinergic neurotransmission in the rat isolated vas deferens

The effects of the putative P2 purinoceptor antagonist suramin on contractile responses of the rat isolated vas deferens to electrical field stimulation and exogenously applied drugs (alpha,beta-methylene ATP and noradrenaline) were investigated. Suramin was devoid of agonist activity in the rat vas deferens. The response of the vas deferens to single pulse field stimulation was characteristically biphasic with a large first component peaking between 260-300 ms after the stimulus followed by a second smaller component peaking at about 650 ms post-stimulus. Suramin (100 nM-1 mM) selectively impaired the first, purinergic phase of the response to single pulse field stimulation but was without effect on the second, noradrenergic component. The response of the vas deferens to trains of electrical field stimuli (10 Hz for 10 s) was also biphasic. Suramin (1 microM-1 mM) reduced the first (less than 1 s) phase of the response by 30%, the second (greater than 5 s) plateau phase by 50% and inhibited the intermediate (2-4 s) phase by 80%. Dose-contact periods of 20 or 30 min respectively were sufficient to achieve equilibration of the inhibitory effects of suramin against the responses to single pulse field stimulation or trains of pulses. Following 30 min incubation with 1 mM suramin, the remaining first and second phase components of the response to trains of pulses were impaired and subsequently abolished by the alpha 1-adrenoceptor antagonist WB4101 establishing their noradrenergic origin. Suramin (300 microM) abolished responses of the vas deferens to alpha,beta-methylene ATP but was without effect on those to noradrenaline. Suramin (30 microM) induced a rightward shift in the concentration-response relationship to alpha,beta-methylene ATP that was associated with a significant 40% increase in the maximum response, but did not modify responses to noradrenaline. The inhibitory effects of suramin (3-300 microM) on responses of the vas deferens to approximate EC50 concentrations of alpha,beta-methylene ATP were reversible on repeated washout for 40-60 min. These results reveal suramin to be a useful pharmacological tool for the study of purinergic neurotransmission in rodent vasa deferentia.

Norepinephrine and ATP are synergistic in the mouse vas deferens preparation

Norepinephrine (NE) and ATP are thought to be neurotransmitters involved with contractions of the mouse vas deferens (MVD) in vitro. The EC50 values of exogenously administered NE and ATP were 5.16 +/- 1.37 and 315.32 +/- 221.18 microM, respectively. Contractions of MVD induced by electrical field stimulation were blocked by alpha,beta-methylene adenosine-5'-triphosphate (purinergic desensitizer) and not by prazosin (alpha 1-adrenoceptor antagonist), suggesting that ATP is the predominant transmitter in this preparation. However, because the EC50 value for ATP was over 100-fold greater than that for NE, we performed isobolographic analysis comparing NE and ATP actions separately and together. Our results demonstrated a synergistic interaction of ATP and NE. At all ratios of ATP:NE examined, exogenous NE enhanced contractile responses to ATP. These data indicate that the co-transmitters, NE and ATP, in the MVD interact postjunctionally in a synergistic manner.

Dual regulation of pancreatic vascular tone by P2X and P2Y purinoceptor subtypes

The effects of ATP on the pancreatic vascular bed of the rat were studied under resting tone. ATP exerted two different effects depending on the concentration used: a slight vasodilatation in the 1.65-49.5 microM range which was statistically significant only at 16.5 microM and a concentration-related vasoconstriction in the 495-4 950 microM range. Theophylline, a P1 purinoceptor antagonist, did not modify the vasodilator effect of ATP. The existence of two P2 purinoceptor subtypes (P2y and P2x) in our preparation may be responsible for the dual effect of ATP. The P2y antagonist 2,2'pyridylisatogen (PIT) used at 5 microM, revealed a vasoconstrictor effect of ATP 165 microM, a concentration without effect per se. Furthermore, the transient vasoconstrictor effect of ATP 495 microM was changed into a long-lasting one in the presence of PIT. On the other hand, the blockade of P2x purinoceptors by the desensitizing agent, alpha,beta-methylene ATP, increased the vasodilator effect of ATP 16.5 microM. In conclusion, two subtypes of P2 purinoceptor do exist on the pancreatic vascular bed: P2y inducing vasodilatation and P2x inducing vasoconstriction. At vascular resting tone, the effect observed with ATP therefore depends on the concentration used and on the balance between P2y/P2x purinoceptors.