Attempts to uncover subtypes of alpha-adrenoceptors and associated mechanisms by using sequential administration of blocking drugs

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.

Vasopressor nerve responses in the pithed rat, previously identified as α2-adrenoceptor mediated, may be α1D-adrenoceptor mediated

Responses to pressor nerve stimulation in the pithed rat have been variously described as mediated, at least in part, by α(2)-adrenoceptors and by α(1A) and α(1D)-adrenoceptors. We have examined the subtypes of α-adrenoceptor involved in rises in diastolic blood pressure in the pithed rat preparation produced by vasopressor nerve stimulation with 10 pulses at 1 Hz or 20 pulses at 5 Hz. Vasopressor nerve responses to 1 Hz stimulation were markedly inhibited by the α(1A)-adrenoceptor antagonist RS 100329 (0.1mg/kg) and by the α(1D-)adrenoceptor antagonist BMY 7378 (0.1mg/kg). The α(2)-adrenoceptor antagonist yohimbine (0.1mg/kg) significantly increased pressor nerve responses to 1 Hz stimulation, but yohimbine (1mg/kg) significantly reduced pressor nerve responses. However, following BMY 7378 (0.1mg/kg), yohimbine (1mg/kg) did not produce any further inhibition of pressor nerve responses to 1 Hz stimulation. The α(2A)-adrenoceptor antagonist BRL 44408 (1mg/kg) did not reduce pressor responses to 1 Hz stimulation. BMY 7378 produced much less inhibition of pressor nerve responses to 5 Hz stimulation, whereas RS 100329 produced similar inhibition of 1 Hz and 5 Hz responses. Yohimbine (0.1 and 1mg/kg) did not significantly affect pressor nerve responses to 5 Hz stimulation. In conclusion, pressor nerve responses in the pithed rat involve both α(1A) and α(1D)-adrenoceptor, but there is no clear evidence for the involvement of α(2)-adrenoceptors.

Dual control of local blood flow by purines

The potent and widespread vascular actions of purine nucleotides and nucleosides have long been recognized. A dual function for ATP in the regulation of vascular tone is considered. ATP acts as an excitatory cotransmitter with noradrenaline from sympathetic perivascular nerves, to cause vasoconstriction via P2X-purinoceptors located on vascular smooth muscle. In contrast, ATP can act via P2Y-purinoceptors located on vascular endothelial cells to release EDRF, which diffuses to the vascular smooth muscle and produces vasodilatation. The main source of intraluminal ATP is likely to be endothelial cells, and its release can be measured during conditions such as changes in flow and hypoxia, in amounts sufficient to activate endothelial P2Y-purinoceptors. In some vessels, ATP acts directly on P2Y-purinoceptors located in the vascular smooth muscle to produce vasodilatation; the possibility that the origins of this ATP are nonsympathetic purinergic or sensory-motor nerves is discussed. ATP 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, during changes in blood flow and during vessel injury.

Presynaptic alpha 2-autoinhibition in a vascular neuroeffector junction where ATP and noradrenaline act as co-transmitters

1. alpha 2-Autoinhibition of transmitter release was investigated in the largest rami caecales of the rabbit ileocolic artery. Vasoconstriction, elicited by electrical field stimulation or by exogenous agonists, was measured as an increase in perfusion pressure. 2. Short periods of electrical stimulation elicited monophasic vasoconstriction, whereas longer periods (greater than 10 s) produced biphasic vasoconstriction. Prazosin had no significant effect on the first component of the biphasic vasoconstriction elicited by electrical stimulation, but did reduce the second component at higher frequencies. alpha, beta-Methylene ATP significantly attenuated the first component whilst the second component was relatively resistant. 3. The alpha 2-adrenoceptor antagonist yohimbine did not change responses evoked by very short pulse trains (less than 2 s) but enhanced responses to longer pulse trains. When vasoconstriction was biphasic, both phases were potentiated by yohimbine. 4. The results indicate that the vasoconstriction elicited by brief trains of sympathetic nerve impulses is mainly or exclusively mediated by ATP, whereas at longer pulse trains a noradrenergic component comes into play. The potentiation produced by yohimbine is due to interruption of presynaptic alpha 2-adrenoceptor-mediated autoinhibition of transmitter release. The autoinhibition affects both purinergic and adrenergic components of sympathetic neurotransmission.

ATP, alpha,beta-methylene ATP and suramin as tools for characterization of vascular P2x receptors in the pithed rat

1. In pithed rats, the blood pressure effects of ATP, alpha,beta-methylene ATP (mATP), alpha-adrenoreceptor agonists and of electrical stimulation of the thoracolumbar sympathetic outflow were studied in the absence and presence of mATP, suramin and adrenoreceptor antagonists. 2. ATP elicited an initial rise in mean blood pressure followed by a decrease and a second increase. mATP produced a short-lived increase in blood pressure, whereas equieffective doses of noradrenaline, methoxamine and B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(5,4-d)-azepine) produced a more prolonged, biphasic pressor response. 3. In the presence of high doses of prazosin, rauwolscine plus propranolol, the initial vasopressor and the vasodepressor effect to ATP were not affected, whereas the delayed vasopressor response to ATP, the vasopressor response to electrical stimulation and even more so that to noradrenaline were suppressed. 4. Suramin, which by itself produced a short-lived decrease, followed by a persistent increase in blood pressure, decreased the pressor responses to ATP (initial phase), to mATP and to electrical stimulation without affecting the fall and second rise in blood pressure elicited by ATP and the pressor response to noradrenaline. 5. Desensitization of P2x receptors by a low dose of mATP abolished the initial vasopressor response to ATP but failed to affect the subsequent blood pressure effects of ATP as well as the pressor responses to noradrenaline and electrical stimulation. A high dose of mATP, in addition, decreased the vasopressor responses to noradrenaline, methoxamine, B-HT 920 and electrical stimulation; the delayed effects of ATP on blood pressure were not changed. 6. The electrically induced increase in blood pressure subsequent to administration of high doses of prazosin, rauwolscine plus propranolol was diminished by suramin and by the low and high dose of mATP. 7. The present study suggests that under certain circumstances ATP, which, added exogenously, has a triphasic effect on mean blood pressure, contributes to the electrically induced vasopressor response by activation of P2x receptors.

Selective blockade by nifedipine of 'purinergic' rather than adrenergic nerve-mediated vasopressor responses in the pithed rat

1. Nifedipine can attenuate pressor responses to sympathetic nerve stimulation both in the presence and in the absence of alpha-adrenoceptor blocking agents. 2. In the presence of alpha,beta-methylene ATP, nifedipine produces only a small attenuation of the vasopressor response. 3. Nifedipine attenuates the vasopressor response produced by intravenous bolus administration of alpha,beta-methylene ATP. 4. The results suggest that the purinergic component of the vasopressor response to stimulation of the sympathetic outflow in the rat is subject to blockade by nifedipine, whereas the alpha-adrenoceptor-mediated response to co-transmitter noradrenaline is relatively resistant.

Blockade of vasopressor and vas deferens responses by alpha,beta-methylene ATP in the pithed rat

1. The pressor responses produced by the intravenous administration of alpha,beta-methylene ATP were tachyphylactic. 2. alpha,beta-Methylene ATP can attenuate pressor responses to sympathetic nerve stimulation both in the presence and in the absence of alpha-adrenoceptor blocking agents. 3. alpha,beta-Methylene ATP has no effect on the pressor responses produced by bolus injections of noradrenaline. 4. In the presence of alpha-adrenoceptor blocking agents, alpha,beta-methylene ATP further attenuates contractions of the vas deferens produced by nerve stimulation. 5. The results, together with previous data, suggest that the vasopressor response to stimulation of the sympathetic outflow in the rat is partly purinergic and partly alpha-adrenergic and that this occurs as co-transmission. The same applies to rat vas deferens, confirming in vitro data. The pithed rabbit had an alpha-blocker-resistant vasopressor nerve-mediated response but this was resistant to alpha,beta-methylene ATP.

Difference in the potency of alpha 2-adrenoceptor agonists and antagonists between the pithed rabbit and rat

The subtypes of alpha-adrenoceptors which mediate pressor responses to sympathomimetic agonists or to nerve stimulation in pithed rabbits have been classified according to the effects of 'selective' antagonists and a comparison has been made, for the alpha 2-subtype, with corresponding responses in the rat. In the rabbit the dose-response curve for phenylephrine was shifted to the right in parallel by prazosin (1 mg kg-1) and was unaffected by rauwolscine (1 mg kg-1). The dose-response curve for noradrenaline was shifted to the right by prazosin (1 mg kg-1) and was shifted to a smaller extent by rauwolscine (1 mg kg-1) or imiloxan (10 mg kg-1). After rauwolscine, prazosin produced a rightward shift larger than when given alone. After prazosin, rauwolscine produced a rightward shift larger than when given alone. The responses to pressor nerve stimulation at low frequencies (less than 1 Hz) could be reduced by prazosin, rauwolscine or imiloxan but those at a higher frequency could be reduced only by prazosin. These results indicate that the responses to noradrenaline or to nerve stimulation are mediated by both alpha 1- and alpha 2-adrenoceptors. Low doses or frequencies have a proportionately greater component which is alpha 2. Responses to noradrenaline after prazosin (1 mg kg-1), were sufficiently sensitive to rauwolscine to be considered as predominantly alpha 2. A comparison was therefore made of such responses in the rat and rabbit. They were produced by a lower dose per unit body weight in the rat whereas this was less marked for the alpha 2-adrenoceptor agonist guanabenz. In the rabbit they were more susceptible to blockade by rauwolscine but were less sensitive to Wy 26703 than in the rat. This demonstrates that the alpha 2-adrenoceptors mediating pressor responses in vivo, like those in other tissues in vitro, are different in rat and rabbit, with regard to antagonists.

Differential effects of alpha-beta-methylene ATP on responses to nerve stimulation in SHR and WKY tail arteries

The effects of alpha,beta-,methylene-adenosine triphosphate, (alpha,beta-methylene ATP, a P2-receptor desensitising agent) have been evaluated on vasoconstrictor responses elicited by exogenous agonists or electrical field stimulation in isolated perfused SHR or WKY tail arteries and on tritium release elicited by electrical field stimulation in SHR-tail arteries pre-labeled with 3H-noradrenaline. Exposure to alpha,beta-methylene ATP (0.1 mumol/l) significantly inhibited vasoconstrictor responses to electrical field stimulation in SHR tail arteries. These inhibitory effects were not further increased at a higher concentration of alpha,beta-methylene ATP (1 mumol/l). In WKY tail arteries, alpha,beta-methylene ATP (1 mumol/l) failed to significantly inhibit vasoconstrictor responses to electrical stimulation. In SHR tail arteries prelabelled with 3H-noradrenaline, alpha,beta-methylene ATP (1 mumol/l) did not inhibit the stimulation evoked release of tritium. However, at this concentration, alpha,beta-methylene ATP significantly antagonized the vasoconstrictor responses of SHR tail arteries induced by exogenous ATP (1 mumol/l), beta,gamma-methylene ATP (30 mumol/l), a stable agonist at P2-receptors, or 60 mmol/l KCl. These effects of alpha,beta-methylene ATP on contractile responses to KCl were not observed in WKY-tail arteries. In tail arteries obtained from reserpine pretreated SHR, despite a 85-95% decrease in endogenous noradrenaline tissue content, the vasoconstrictor responses induced by periarterial field stimulation were greatly diminished, but not abolished. These residual responses to periarterial field stimulation were not antagonized by prazosin (0.1 mumol/l), but were practically abolished by the addition of alpha,beta-methylene ATP (1 mumol/l). In tail arteries from WKY rats pretreated with reserpine, exposure to prazosin (0.1 mumol/l) further reduced the residual responses elicited by electrical field stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)