Effects of a selective neuropeptide Y Y(1) receptor antagonist BIBP 3226 on double peaked vasoconstrictor responses to periarterial nerve stimulation in canine splenic arteries

Effects of Nifedipine on Renal and Cardiovascular Responses to Neuropeptide Y in Anesthetized Rats

Neuropeptide Y (NPY) acts via multiple receptor subtypes termed Y₁, Y₂ and Y₅. While Y₁ receptor-mediated effects, e.g., in the vasculature, are often sensitive to inhibitors of L-type Ca²⁺ channels such as nifedipine, little is known about the role of such channels in Y₅-mediated effects such as diuresis and natriuresis. Therefore, we explored whether nifedipine affects NPY-induced diuresis and natriuresis. After pre-treatment with nifedipine or vehicle, anesthetized rats received infusions or bolus injections of NPY. Infusion NPY (1 µg/kg/min) increased diuresis and natriuresis, and this was attenuated by intraperitoneal injection of nifedipine (3 µg/kg). Concomitant decreases in heart rate and reductions of renal blood flow were not attenuated by nifedipine. Bolus injections of NPY (0.3, 1, 3, 10 and 30 μg/kg) dose-dependently increased mean arterial pressure and renovascular vascular resistance; only the higher dose of nifedipine (100 μg/kg/min i.v.) moderately inhibited these effects. We conclude that Y₅-mediated diuresis and natriuresis are more sensitive to inhibition by nifedipine than Y₁-mediated renovascular effects. Whether this reflects a general sensitivity of Y₅ receptor-mediated responses or is specific for diuresis and natriuresis remains to be investigated.

Exhaustive swimming differentially inhibits P2X1 receptor- and α1-adrenoceptor-mediated vasoconstriction in isolated rat arteries

AIM

To investigate the effects of exhaustive swimming exercise on P2X1 receptor- and α1-adrenoceptor-mediated vasoconstriction of different types of arteries in rats.

METHODS

Male Wistar rats were divided into 2 groups: the sedentary control group (SCG) and the exhaustive swimming exercise group (ESEG). The rats in the ESEG were subjected to a swim to exhaustion once a day for 2 weeks. Internal carotid, caudal, pulmonary, mesenteric arteries and aorta were dissected out. Isometric vasoconstrictive responses of the arteries to α,β-methylene ATP (α,β-MeATP) or noradrenaline (NA) were recorded using a polygraph.

RESULTS

The exhaustive swimming exercise did not produce significant change in the EC(50) values of α,β-MeATP or NA in vasoconstrictive response of most of the arteries studied. The exhaustive swimming exercise inhibited the vasoconstrictive responses to P2X1 receptor activation in the internal carotid artery, whereas it reduced the maximal vasoconstrictive responses to α1-adrenoceptor stimulation in the caudal, pulmonary, mesenteric arteries and aorta. The rank order of the reduction of the maximal vasoconstriction was as follows: mesenteric, pulmonary, caudal, aorta.

CONCLUSION

Exhaustive swimming exercise differentially affects the P2X1 receptor- and α1-adrenoceptor-regulated vasoconstriction in internal carotid artery and peripheral arteries. The ability to preserve purinergic vasoconstriction in the peripheral arteries would be useful to help in maintenance of the basal vascular tone during exhaustive swimming exercise.

Neurogenic double-peaked vasoconstriction of human gastroepiploic artery is mediated by both alpha1- and alpha2-adrenoceptors

1. The contribution of postjunctional P2X receptors and subtypes of alpha-adrenoceptors to vasoconstrictor responses following periarterial electrical nerve stimulation (PNS, 30 s trains of pulses at a frequency of 2, 4 or 8 Hz) was investigated in human gastroepiploic arteries. 2. The vasoconstrictor response to PNS at a stimulation of 4 or 8 Hz was a two-peaked response, whereas at a frequency of 2 Hz it appeared only as a late peak. All vasoconstrictions evoked by PNS were abolished by phentolamine, a nonselective alpha-adrenoceptor inhibitor, but not by alpha,beta-methylene ATP, a P2X receptor-desensitizing agent. 3. The early peak to PNS at 4 or 8 Hz was abolished by prazosin, an alpha1-adrenoceptor antagonist, while the late one still remained, although it was markedly inhibited. The responses remaining after prazosin were blocked by rauwolscine. The vasoconstrictor response to PNS at 2 Hz was not affected by prazosin (0.1 microM), but was abolished by rauwolscine (0.1 microM), an alpha2-adrenoceptor antagonist. 4. OPC-28326 (10 microM), a newly developed vasodilator, which preferentially exerts its antagonistic actions on the alpha2B- and alpha2C-adrenoceptors, significantly reduced the noradrenaline-induced vasoconstriction in the absence or presence of prazosin. OPC-28326 had a greater inhibitory effect on the late peak evoked by PNS than the early one. The neurogenic responses remaining after OPC-28326 were abolished by prazosin. 5. The present results suggest that sympathetic vasoconstriction of the human gastroepiploic artery is mediated by both alpha1- and alpha2-adrenoceptors postjunctionally, but not by P2X receptors. The alpha2-adrenoceptors may be preferentially activated at a low frequency of stimulation, which induces a constriction more slowly than that by alpha1-adrenoceptors. The existence of alpha2-adrenoceptors may cause an enhancement of alpha1-adrenoceptor-induced responses.

Prejunctional AT(1) receptor subtype-dependent modification of neurotransmitter releases in canine isolated splenic arteries

1. The regulation by angiotensin II (Ang II) formed locally on nerve-stimulated purinergic and adrenergic components of double-peaked vasoconstrictions in the canine splenic artery and Ang II receptor subtypes involved were investigated. 2. The perfusion of the precursor angiotensin I (Ang I, 0.1-1 nm) did not affect the vasoconstrictor responses to noradrenaline (NA, 0.03-1 nmol) and adenosine 5'-triphosphate (ATP, 0.03-1 micromol). The second component vasoconstrictor response to nerve stimulation was dose dependently potentiated by Ang I (0.1-1 nm). The first peaked constriction was slightly, but insignificantly increased. The potentiating effects of Ang I were abolished by KRH-594 (10 nm), a selective AT(1) receptor antagonist, but not by PD 123319 (1-10 nm), an AT(2) receptor antagonist. KRH-594 (10 nm) or PD 123319 (10 nm) never affected the vasoconstrictions to either NA or ATP. 3. The treatment with KRH-594 (1-10 nm) produced a greater inhibition on the second peaked response than the first one, although both of them were dose dependently inhibited. PD 123319 (1-10 nm) did not affect the vasoconstrictor responses induced by nerve stimulation. 4. Inhibition of angiotensin-converting enzyme with 10 nm enalaprilat reduced the second peaked response, having no significant inhibition on the first peaked response. A higher dose of enalaprilat (100 nm) produced a greater inhibition of the second peak than the first one. It reduced the second peak by approximately 65%, while the first peak was decreased approximately 35%. After treatment with enalaprilat, Ang I (1 nm) failed to enhance the neuronal vascular response. Enalaprilat at doses used did not affect the vasoconstrictions to either NA or ATP. 5. The present results indicate that endogenously generated Ang II may produce a more marked potentiation of adrenergic transmission than purinergic transmission via activation of prejunctional AT(1) receptors.

Modification of transmitter release from periarterial nerve terminals by dipyridamole in canine isolated splenic artery

1. The aim of the present study was to determine the modulatory effects of dipyridamole on purinergic and adrenergic transmission in the canine isolated, perfused splenic artery. 2. Periarterial nerve electrical stimulation readily induced a double-peaked vasoconstriction consisting of an initial transient, predominantly P2X receptor-mediated constriction followed by a prolonged, mainly alpha1-adrenoceptor-mediated response. 3. Exposure of tissues to dipyridamole (0.1-1 micro mol/L) dose-dependently inhibited both the first and second peaks of the vasoconstrictor response at a low frequency of stimulation (1 Hz), whereas at an intermediate frequency of stimulation (4 Hz), the first peak of the response was depressed without any significant effect being observed on the second peak of constriction. 4. At a higher dose (1 micro mol/L) dipyridamole potentiated vasoconstrictor responses to noradrenaline (0.03-1 nmol). At any doses used, dipyridamole had no effect on the vasoconstrictor responses to ATP (0.03-1 micro mol). 5. Tyramine (0.01-0.3 micro mol) induced vasoconstriction in a dose-dependent manner. The dose-response curves for tyramine were shifted to the right following treatment with dipyridamole (0.1-1 micro mol/L). 6. The present results indicate that dipyridamole may inhibit purinergic and adrenergic transmission presynaptically, whereas postsynaptically dipyridamole may potentiate the adrenergic vascular constriction by inhibition of transmitter uptake.

Mechanisms of release of ATP from vascular purinergic nerves

1. The vasoconstrictor response to periarterial nerve electrical stimulation (PNS) and neurotransmission by ATP are discussed and illustrated, using canine isolated and perfused splenic arterial preparations. 2. The conditions for appearance of dominant purinergic constrictor response to PNS are discussed. 3. Modulation of the purinergic vasoconstrictor responses to PNS by several kinds of presynaptic receptor agonists and antagonists is reviewed. 4. Influences of purinergic responses to PNS by guanethidine, reserpine, tetrodotoxin (TTX) or omega-conotoxin GVIA (omegaCTX) are also reviewed. 5. Effects of imipramine and removal of the endothelium are discussed. 6. Evidence is presented for selective inhibition of purinergic responses to PNS by an adequate cold storage of the vessel. 7. The roles of ATP released by PNS in isolated canine splenic arteries are proposed.

Purinergic and adrenergic cotransmission in canine isolated and perfused gastroepiploic arteries

1. The vasoconstrictor responses of canine gastroepiploic artery to periarterial electrical nerve stimulation (PNS; 30 s trains of pulses at a frequency of 2, 4 or 8 Hz) were observed in a frequency dependent manner. The PNS-induced vasoconstrictions were abolished by tetrodotoxin (1 micromol/L) and mostly depressed but not completely by guanethidine (10 micromol/L). 2. Vasoconstrictor responses to administered noradrenaline were antagonized significantly by prazosin (0.1 micromol/L), an alpha1-adrenoceptor antagonist, but were not significantly affected by suramin (100 micromol/L), a P2 purinoceptor antagonist, or alpha,beta-methylene ATP (1 micromol/L), a P2X receptor desensitizing agent. Exogenous ATP-induced responses were clearly depressed by suramin or alpha,beta-methylene ATP, but were not significantly affected by prazosin. 3. The vasoconstrictor responses to PNS at a low frequency (2 and 4 Hz) of stimulation were markedly inhibited by suramin (100 micromol/L) and by alpha,beta-methylene ATP (1 micromol/L). The remaining responses after suramin or alpha,beta-methylene ATP were abolished by subsequent application of prazosin (0.1 micromol/L). At a high frequency (8 Hz) of stimulation, the vascular response was not significantly inhibited by suramin or alpha,beta-methylene ATP, but it was abolished by prazosin. 4. Injection of xylazine (0.3-30 nmol/L), an alpha2-adrenoceptor agonist, did not induce any clear vasoconstriction. The exposure of tissues to rauwolscine (0.1-0.3 micromol/L), an alpha2-adrenoceptor antagonist, dose-dependently increased PNS-induced vasoconstrictions at all frequencies tested. 5. The present results indicate that ATP acts as a cotransmitter with noradrenaline and is responsible for post-junctional vasoconstrictor responses at low frequencies of sitmulation, whereas the effect of noradrenaline is dominant at high-frequency stimulation in canine gastroepiploic artery. Prejunctional alpha2-adrenoceptor autoinhibition may modulate the release of either noradrenaline or ATP from sympathetic nerve terminals.

Effects of reserpine on nerve stimulation-induced constrictions in canine isolated splenic artery

1. Our previous studies have demonstrated that peri-arterial electrical nerve stimulation (PNS) of the canine splenic artery induces a double-peaked vasoconstriction consisting of an initial transient, dominantly P2X purinoceptor-mediated constriction, followed by a prolonged, mainly alpha1-adrenoceptor-induced response. In the present study, we examined the effects of reserpine on PNS-induced double-peaked responses. 2. The vasoconstrictor response to tyramine was abolished after reserpine treatment, but the responses to noradrenaline (NA) and ATP were not significantly modified. 3. The PNS-induced second peak vasoconstrictor responses were markedly reduced in reserpinized vessels, whereas the first peak vasoconstrictor responses were not so strongly influenced (i.e. they were not significantly affected at 1 Hz, but were significantly affected at 4 and 10 Hz). 4. All reserpine-resistant responses were unaffected by treatment with prazosin, but were abolished by subsequent application of alpha,beta-methylene ATP. The exposure of reserpine-treated tissues to NA almost completely restored tyramine-induced vasoconstriction and the second neurogenic peak vasoconstrictor response, but failed to affect the first neurogenic response. 5. The present results indicate that ATP and NA are cotransmitters responsible for the double-peaked vasoconstrictor responses of canine splenic artery. In addition, it is suggested that PNS causes NA release not only from intragranular NA storage sites, but also from tyramine-sensitive cytoplasmic sites.

Neuroeffector mechanisms involved in the regulation of dog splenic arterial tone

It has been recognized that sympathetic neurons release several transmitters but mainly adenosine 5'-triphosphate (ATP), noradrenaline, and neuropeptide Y (NPY). Recently, we reported that periarterial nerve electrical stimulation (PNS) produced biphasic vasoconstrictions consisting of an initial transient, predominantly P2X-purinoceptor-mediated constriction followed by a prolonged, alpha(1)-adrenoceptor-mediated one in canine isolated splenic arteries. In this article, we tried to analyze the effects of several selective key drugs that influence the PNS-induced responses, and we functionally showed sympathetic transmitter releasing mechanisms by pharmacological analysis using purinergic, adrenergic, and NPYergic agonists and antagonists.

Interaction between neuropeptide Y Y1 receptors and alpha1B-adrenoceptors in the neurovascular junction of canine splenic arteries

Previous study has demonstrated that periarterial electrical nerve stimulation (30-s trains of pulses at a frequency of 1 or 4 Hz) induces a double-peaked vasoconstriction consisting of an initial transient, predominantly P2X-receptor-mediated constriction followed by a prolonged, mainly alpha(1)-adrenoceptor-mediated response in the isolated canine splenic artery. Treatment with 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378, 0.1 micromol/l), a selective alpha(1D)-adrenoceptor antagonist, produced a slight but significant inhibition of the second peaked responses. A marked inhibition of second peaked responses was obtained by exposure of the tissues to chloroethylclonidine (60 micromol/l), an alpha(1B)- and alpha(1D)-adrenoeptor antagonist. Neither BMY 7378 nor chloroethylclonidine affected the first peaked vasoconstrictor responses. [Leu(31),Pro(34)]Neuropeptide Y (10-30 nmol/l), a selective neuropeptide Y Y(1) receptor agonist, enhanced the second peaked responses in the presence of BMY 7378 but failed to enhance the responses in the presence of chloroethylclonidine. The results indicate that the postjunctional alpha(1B)-adrenoceptor subtype is likely coupled to neuropeptide Y Y(1) receptors responsible for the cooperation of the sympathetic adrenergic and neuropeptide Yergic transmission in the canine splenic artery.

Pharmacological analysis of functional neurovascular transmission in canine splenic arteries: role of neuropeptide Y

1 The effects of neuropeptide Y (NPY) upon the isolated vasculature are reviewed. 2 The vasconstrictor responses to periarterial nerve stimulation (PNS) and neurotransmission by noradrenaline (NA) and ATP are discussed and illustrated using canine isolated perfused splenic artery. 3 Modulation of the vascular responses to PNS by NPY via pre- and post-junctional NPY Y2 and Y1 receptors is discussed. 4 Evidence is presented for different alpha1-adrenoceptor subtypes mediating vasoconstriction to exogenous and endogenously released NA and their different locations in the neurovascular junction and extrajunctional regions. 5 Activation of NPY Y1-receptors potentiates sympathetic nerve activated alpha1-adrenoceptor vasoconstriction. The proposal that the postjunctional alpha1B adrenoceptor may be linked to the NPY Y1-receptor and is responsible for co-operation between sympathetic and NPYergic interactions in the vasculature is discussed.

Pharmacology of neuropeptide Y receptor antagonists. Focus on cardiovascular functions

Neuropeptide Y is one of the most abundant mammalian neuropeptides identified to date. The possible actions of neuropeptide Y, that is co-localized and released with noradrenaline, as a sympathetic co-transmitter has attracted much attention during the last decade. In recent years, several non-peptide antagonists with high subtype selectivity for neuropeptide Y receptors have been introduced. With them, the status of neuropeptide Y as a sympathetic transmitter has been established, and so have profound cardiovascular effects mediated by neuropeptide Y Y(1) and Y(2) receptors. Significant release of neuropeptide Y occurs especially upon stronger sympathetic activation, and recent data suggest that the importance of neuropeptide Y seems enhanced in stress-related cardiovascular disorders. The true significance of neuropeptide Y has thus started to unfold, owing to the presence of the first generation of selective neuropeptide Y receptor antagonists. This review concerns the pharmacology of these agents, what we have learnt from them, and might find out in the future.

Antagonistic interaction between BIIE 0246, a neuropeptide Y Y2-receptor antagonist, and omega-conotoxin GVIA, a Ca2+ channel antagonist, in presynaptic transmitter releases in dog splenic arteries

Isolated dog splenic arteries were perfused with Krebs-Henseleit solution at 37 degrees C, using the cannula inserting method. Periarterial nerve electrical stimulation (10-V amplitude; 1-ms duration; 30-s trains of pulses; 1, 4 and 10 Hz) readily caused double peaked vasoconstrictions, i.e., 1st peaked response was mostly inhibited by alpha,beta-methylene ATP and the 2nd one, by prazosin. These responses were consistently inhibited by omega-conotoxin GVIA (omega-CTX), whereas they were facilitated by BIIE 0246, a neuropeptide Y (NPY) Y2-receptor antagonist. The omega-CTX-induced blocking effects of transmitter release were significantly antagonized by BIIE 0246. It is possible that the NPY Y2 receptor activity may partially be linked to presynaptic Ca2+ channels.

The direct stimulation of Gi proteins by neuropeptide Y (NPY) in the rat left ventricle

Neuropeptide Y (NPY) is a neuropeptide with high distribution in the cardiovascular system of mammals, where it modulates heart and vessel contractility. In the rat heart, the presence of at least three different NPY receptor subtypes has been hypothesised. Notwithstanding this, receptor activation might not be the only mechanism responsible for the complex cardiac effects of the peptide. In this study, we investigated the effect of NPY on the GTPase activity of G-proteins in the rat left ventricle as a possible alternative mechanism of action for the peptide in the rat heart. Our results show that NPY, but also the neuropeptide fragment (18-36) (NPY (18-36)), stimulated the basal, spontaneous GTPase activity of ventricle membranes only when it was measured under the condition of an absence of Mg2+. This stimulation was resistant to BIBP3226 a non-peptidergic antagonist at Y1 receptors, but it was significantly reduced in membranes treated with selective antibodies against the Gialpha subunits. NPYs effect was concentration-dependent with a maximum of activity at 10nM. At this concentration, NPY (and NPY 18-36) was able to inhibit forskolin (FSK)-induced cyclic adenosine-5'-monophosphate (cAMP) elevation in rat left ventricle slices. Our results assess that NPY in the rat heart is able to activate the GTPase activity of Gi proteins, in a receptor-independent way.

Neuropeptide Y inhibits double peaked vasoconstrictor responses to periarterial nerve stimulation primarily through prejunctional Y2 receptor subtype in canine splenic arteries

1 The effects of BIIE 0246, a novel and non-peptide neuropeptide Y (NPY) Y2 receptor antagonist on sympathetic vasoconstriction of the canine splenic artery were investigated. 2 The vasoconstrictor response to periarterial electrical nerve stimulation was described to be a double peaked vasoconstriction consisting of an initial transient, dominantly P2X purinoceptor-mediated constriction followed by a prolonged, mainly alpha1 adrenoceptor-induced response. 3 BIIE 0246 at a concentration of 0.1-1 microM dose-dependently potentiated double peaked constrictions at low frequencies (1 and 4 Hz), whereas at high frequency (10 Hz), it failed to affect these responses. BIIE 0246 (1 microM) also enhanced double peaked responses even in the presence of rauwolscine (0.1 microM). NPY (13-36) (1-100 nM), a selective Y2 receptor agonist reduced these two peaked responses in a dose-related manner. The vasoconstriction to noradrenaline (0.1-10 nmol) or adenosine triphosphate (0.01-1 micromol) was not significantly influenced by either 1 microM BIIE 0246 or 100 nM NPY (13-36). Exposure of tissues to 1 microM BIIE 0246 almost completely prevented the suppression of double peaked constrictions by NPY (13-36) (10 nM) or by NPY (10 nM). 4 We conclude that NPY inhibits sympathetic purinergic and adrenergic vasoconstrictions through an activation of prejunctional Y2 receptor subtype in the neurovascular junction of the canine splenic artery.

Different sensitivities to alpha1 adrenoceptor blockade on periarterial sympathetic nerve-induced constriction by low and high frequencies in canine isolated splenic arteries

The periarterial nerve electrical stimulation at 4 and 10 Hz induced a monophasic vasoconstriction of the canine splenic artery in a pulse number-related manner (1-30 pulses). The responses at 4 Hz were not significantly affected by 0.1 microM prazosin, but abolished by 1 microM alpha, beta-methylene ATP. Prazosin (0.1 microM) partially but significantly inhibited responses at 10 Hz, and the remaining responses were blocked by 1 microM alpha, beta-methylene ATP. It indicates that the monophasic vasoconstrictor response to short pulses of stimulation at a low frequency is mediated by P2X-receptors, whereas the response at a high frequency may be due to activation of not only P2X-receptors but also alpha1 adrenoceptors.

Separate modulation of neuropeptide Y1 receptor on purinergic and on adrenergic neuroeffector transmission in canine splenic artery

Our previous study demonstrated that the vasoconstrictor responses to trains of up to 10 pulses at 1 Hz of stimulation appeared to be purinergic monophasic, whereas a longer train of 30 pulses induced a biphasic vasoconstriction consisting of an initial, transient purinergic constriction followed by a prolonged adrenergic response. Neuropeptide Y (NPY) at doses of 0.01 and 0.1 microM produced a dose-dependent inhibition on the monophasic and biphasic vasoconstrictor responses to nerve stimulation. The treatment with Leu31 Pro34 neuropeptide Y (LP-NPY) (0.03 microM) did not affect the monophasic responses to short pulse trains of stimulation. However, LP-NPY markedly potentiated the second phase response to 30 pulse trains of stimulation, although it did not modify the first one. The LP-NPY-induced potentiation was abolished by BIBP 3226 (1 microM), a selective NPY Y1 receptor antagonist. The results indicate that the activation of NPY Y1 receptors may enhance the prolonged adrenergic vasoconstriction but not the transient purinergic response in the canine splenic artery.

Existence of different alpha(1)-adrenoceptor subtypes in junctional and extrajunctional neurovascular regions in canine splenic arteries

The present study attempted to characterize the alpha(1)-adrenoceptor subtypes mediating vasoconstrictor responses to administered and nerve stimulation-evoked noradrenaline (NA) release in the isolated and perfused canine splenic artery. A previous study demonstrated that periarterial electrical nerve stimulation (30 s trains of pulses at a frequency of 1, 4 or 10 Hz) induced a double peaked vasoconstriction consisting of an initial transient, predominantly P2X-purinoceptor-mediated constriction followed by a prolonged, mainly alpha(1)-adrenoceptor-mediated response in the canine splenic artery. The effects of alpha(1)-adrenoceptor subtype antagonists on neuronally-mediated second peaked vasoconstrictions were analysed. BMY 7378 (10 - 100 nM), a selective alpha(1D)-adrenoceptor antagonist produced a dose-dependent inhibition of the second peak responses at all frequencies used. BMY 7378 (100 nM) reduced these responses by approximately 30%. Exposure of tissues to chloroethylclonidine (CEC, 60 microM), a selective alpha(1B)-adrenoceptor antagonist attenuated the second peak response by approximately 60%, even in the presence of BMY 7378 (100 nM). On the other hand, WB 4101 (100 nM), a selective alpha(1A)-adrenoceptor antagonist potentiated nerve-stimulation-evoked double peaked vasoconstrictions, especially at low frequencies (1 and 4 Hz). Vasoconstrictor responses to administered NA were dose-dependently antagonized by WB 4101 (10 - 100 nM), but were not significantly affected by either BMY 7378 (10 - 100 nM) or by CEC (60 microM). The present results indicate that NA released from sympathetic nerves may junctionally exert its vasoconstrictor effect via activation of postjunctional alpha(1B)- and in part alpha(1D)-adrenoceptors, whereas exogenous NA extrajunctionally activates alpha(1A)-adrenoceptors to produce its vascular action in canine splenic arteries.