Periarterial Electrical Nerve Stimulation Research Articles

The Preferential Inhibitory Effect of Olmesartan, a New Angiotensin II Type 1 Antagonist, on Sympathetic Nerve Terminals in Isolated Canine Splenic Artery

Effects of olmesartan (RNH-6270: (5-methyl-2-oxo-1,3-dioxolen-4-yl)methoxy-4-(1-hydroxy-1-methylethyl)-2-propyl{4-[2-(tetrazol-5-yl)-phenyl]phenyl}methylimidazol-5-carboxylase, an active form of olmesartan medoxomil (CS-866)) was investigated in isolated, perfused canine splenic arterial preparations. Neither exogenous noradrenaline- nor ATP-induced vasoconstrictor responses were modified by treatment with the used concentrations of olmesartan (1 – 100 nM). A high concentration of 10 nM angiotensin II caused a potentiation of either noradrenaline- and ATP-induced constrictions, although 1 nM angiotensin II did not induce any potentiating effects for these responses. These potentiations were inhibited by olmesartan in a concentration-related manner. Periarterial nerve electrical stimulation (PNS) readily induced a biphasic constriction consisting of an initial P2X purinoceptor-mediated vasoconstriction followed by a prolonged mainly α1-adrenoceptor-mediated response. PNS-induced 1st and 2nd peaked responses were significantly inhibited by olmesartan in a concentration-related manner. With a low concentration of 1 nM angiotensin II, which did not induce any vascular effects by itself, PNS-induced responses were markedly enhanced. The enhanced responses were inhibited by olmesartan. It is concluded that endogenous angiotensin II exerts its stimulating action on the releases of ATP and noradrenaline from the periarterial sympathetic nerve terminal, and olmesartan has an inhibitory property on angiotensin II-induced potentiation of endogenous ATP- and noradrenaline-induced responses.

Neurogenic double‐peaked vasoconstriction of human gastroepiploic artery is mediated by both α1‐ and α2‐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.

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.

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.

Interaction between neuropeptide Y Y 1 receptors and α 1B-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 α 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 μmol/l), a selective α 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 μmol/l), an α 1B- and α 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 α 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.

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.

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 .ALPHA.1 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.

Antagonistic Interaction Between BIIE 0246, a Neuropeptide Y Y2-Receptor Antagonist, and ωy-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°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 α, β-methylene ATP and the 2nd one, by prazosin. These responses were consistently inhibited by ω-conotoxin GVIA (ω-CTX), whereas they were facilitated by BIIE 0246, a neuropeptide Y (NPY) Y2-receptor antagonist. The ω-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.

Effects of L-765,314, a Selective and Potent α1B-Adrenoceptor Antagonist, on Periarterial Nerve Electrical Stimulation-Induced Double-Peaked Constrictor Responses in Isolated Dog Splenic Arteries

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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.

NPY as a periarterial sympathetic transmitter in isolated vessel

Double-peaked vasoconstrictor responses to periarterial nerve electrical stimulation (PNS) were readily induced in the condition of 30-s trains of pulses in isolated, perfused canine splenic artery, using the cannula insertion technique. P2X purinoceptors have been shown to be involved mainly in the 1st peaked response and alpha 1 adrenoceptors mostly in the 2nd one. Administrated NPY induced no direct vasoconstriction or only a slight vasoconstriction, and it inhibited the double-peaked vasoconstriction in a dose-related manner. A small dose of NPY Y1-receptor agonist, L-P NPY, readily potentiated the 2nd peaked response to PNS, and an increasing dose of LP-NPY did both the 1st and 2nd peaked responses. The 2nd peaked response was significantly inhibited by treatment with chloroethylclonidine (CEC), an alpha 1B-adrenoceptor antagonist, but the 1st peaked response was not influenced. On the other hand, an alpha 1A-antagonist, WB4101, rather potentiated the 2nd peaked response, although it did not modify the 1st one. Administered NA-induced constrictions were consistently inhibited by WB4101 but not by CEC. From these results, it is concluded that NPY inhibits the release of ATP and NA from periarterial nerve terminals by activation of presynaptic NPY Y2 receptors, and it potentiates PNS-induced vasoconstrictions via alpha 1B-adrenoceptors by activation of postjunctional NPY Y1 receptors in the canine splenic artery.

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.

The 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 constriction (first peak) followed by a prolonged response (second peak) in the isolated, perfused canine splenic artery. At low frequencies (1 and 4 Hz), a neuropeptide Y (NPY) Y(1) receptor antagonist BIBP 3226 (0.1-1 microM) produced a dose-dependent inhibitory effect on the second peak, but did not modify the first peak. At a high frequency (10 Hz), 1 microM BIBP 3226 induced a slight, but significant inhibition on both the first and second peaked responses. At a low frequency (1 Hz), the first peak was not influenced by blockade of alpha(1)-adrenoceptors or NPY Y(1) receptors with prazosin (0.1 microM) or BIBP 3226 (1 microM), respectively, but abolished by P2X receptor desensitization with alpha,beta-methylene ATP (alphabeta-m ATP, 1 microM). At a high frequency (10 Hz), the first peak was mostly inhibited by alphabeta-m ATP and partially by prazosin and BIBP 3226. On the other hand, the second peak at a low frequency was largely decreased by BIBP 3226 and partially by prazosin and alphabeta-m ATP, whereas at a high frequency, it was largely attenuated by prazosin and partially by alphabeta-m ATP and BIBP 3226. The results suggest that at a low frequency, the firstly transient constriction of double peaked responses is mainly induced via an activation of P2X-receptors, whereas at a high frequency, it is mostly mediated by the P2X-receptors, and partially by alpha(1)-receptors and NPY Y(1)-receptors. The secondary prolonged vasoconstriction at frequencies used is predominantly mediated via both alpha(1)-receptor and NPY Y(1) receptor activations, and in part by P2X-receptors. Furthermore, an activation of NPY Y(1) receptors may play an important role in evoking the prolonged vasoconstrictor response to longer pulse trains of stimulation at a low frequency, whereas an alpha(1)-adrenoceptor activation exerts a main vasomotor effect for the prolonged response at a high frequency.

Periarterial Electrical Nerve Stimulation-Induced Adrenergic Vasoconstriction Inhibited by Adrenergic α1B-Receptor Blockade but Not by α1A-Blockade

The periarterial electrical nerve stimulation at a frequency of 4 Hz (30-s trains of pulses) induced a double-peaked vasoconstriction in the canine splenic artery. The treatment with chloroethylclonidine (CEC, 60 microM) markedly inhibited the second-peaked constriction, whereas it produced an insignificant effect on the first-peaked response. The vasoconstriction to noradrenaline (NA, 1 nmol) was not significantly influenced by 60 microM CEC. On the other hand, WB 4101 (1 microM) consistently abolished the vascular response induced by NA (1 nmol), but rather potentiated the double-peaked constriction. The results indicate that neuronal NA may junctionally exert its vasoconstrictor effect via an activation of postjunctional alpha1B-receptors, whereas exogenous NA may extrajunctionally activate alpha1A-receptors for its vascular action in the canine splenic artery.

ATP as a periarterial sympathetic transmitter in isolated vessel

Double-peaked vasoconstrictor responses to periarterial nerve electrical stimulation (PNS) were readily induced in the condition of 30-s trains of pulses in isolated, perfused canine splenic artery, using the cannula insertion technique. P2X Prinoceptors have been shown to be involved mainly in the 1st peak response and alpha 1-adrenoceptors mostly in the 2nd one. A small dose of TTX did not affect the 1st peak, but strongly inhibited the 2nd one, although larger doses of TTX diminished either the 1st or 2nd response. A small dose of guanethidine also inhibited only the 2nd one. The 1st one was inhibited by prolonged cold storage of the vessel. On the other hand, omega CT produced parallel inhibitory effects on the 1st and 2nd peaked responses. A low pulse number of ES only caused the release of purinergic transmitters from sympathetic nerve terminals. The possible role as a transmitter of ATP is discussed in periarterial sympathetic transmission.

Effects of Neuropeptide Y on Double-Peaked Constrictor Responses to Periarterial Nerve Stimulation in Isolated, Perfused Canine Splenic Arteries

The periarterial electrical nerve stimulation readily induced a double-peaked vasoconstriction in the isolated, perfused canine splenic artery. P2X-Purinoceptors have previously been shown to be involved mainly in the 1st-phase response and alpha 1-adrenoceptors, mostly in the 2nd-one. The dose used of neuropeptide Y (NPY) (0.01-0.1 microM) given into the preparation caused a slight but insignificant vasoconstriction. The treatment with NPY at concentrations of 0.01-0.1 microM produced a parallel inhibition on the 1st- and 2nd-phase responses following nerve stimulation at the frequencies used (1-10 Hz) in a dose-dependent manner. The vasoconstrictor responses to administered ATP (0.01-1 mumol) or noradrenaline (0.03-3 nmol) were slightly but not significantly potentiated by 0.1 microM NPY. The results indicate that NPY predominantly exerts a prejunctionally inhibitory modulation on the purinergic and adrenergic transmission in peripheral sympathetic nerves innervating the canine splenic artery.

Pharmacological analysis of vasoconstrictor responses to periarterial purinergic nerve stimulation.

1. Periarterial electrical nerve stimulation at a low frequency (1 Hz) readily induced a vasoconstrictor response of the canine splenic artery in a pulse number-related manner (1-30 pulses of trains). The vasoconstrictor response to trains of up to 10 pulses at 1 Hz of stimulation appeared to be monophasic, whereas it became clearly distinguished into two phases at a longer train of 30 pulses. 2. The monophasic vasoconstrictor responses to trains of 1, 3 or 10 pulses were not modified by an alpha1-adrenoceptor blocking agent, prazosin (0.1 microM), but were completely inhibited by the P2X receptor desensitization with alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-methylene ATP; 1 microM). The 1st phase of vasoconstriction induced by a train length of 30 pulses was not influenced by the treatment with prazosin, but was abolished by alpha,beta-methylene ATP. The 2nd phase response was markedly inhibited by prazosin, and the remaining response of this phase was blocked by alpha,beta-methylene ATP. 3. Rauwolscine (0.3 microM), an alpha2-adrenoceptor antagonist, enhanced the vasoconstrictor responses to trains of 1, 3 or 10 pulses. Particularly at 10 pulses of electrical stimulation, the vasoconstrictor responses were significantly potentiated. The blockade of neuronal uptake of noradrenaline with imipramine (1 microM) did not affect the vasoconstrictor responses to trains of 1, 3 or 10 pulses. 4. It is concluded that short pulse trains of stimulation at a low frequency may selectively activate a purinergic component of sympathetic cotransmission, and the prejunctional alpha2-adrenergic feedback mechanism may tonically participate into the modulation of ATP release. Imipramine-sensitive neuronal uptake mechanism may not play an important role in regulating vascular responses to periarterial purinergic nerve stimulation.

Adrenergic-purinergic interactions on vasoconstrictor responses to periarterial electric nerve stimulation in canine splenic arteries.

1. Periarterial nerve electrical stimulation caused a double peaked vasoconstriction in isolated perfused canine splenic arterial preparations. At low frequencies (1-3 Hz), the 1st peak responses were significantly inhibited by alpha,beta-methylene ATP. On the other hand, at high frequencies (8-10 Hz), the responses were not completely inhibited by alpha,beta-methylene ATP but the remaining response was abolished by an additional treatment of prazosin. 2. Concerning the 2nd peak responses, at low frequencies (1-3 Hz), the response was mostly suppressed by alpha,beta-methylene ATP, but at high frequencies (6-10 Hz), the response was not significantly modified by it, although the remaining responses were completely blocked by prazosin. Thus, at high frequencies an adrenergic and purinergic interaction may exist presynaptically, to prevent the inhibition by alpha,beta-methylene ATP. 3. At 1 Hz, rauwolscine, an alpha2-adrenoceptor antagonist, caused a potentiation of electrical stimulation-induced responses (both 1st and 2nd peaked responses) which were inhibited by prazosin, and the remaining ones were abolished by alpha,beta-methylene ATP. On the other hand, at 10 Hz, rauwolscine did not cause any potentiation of the double peaked responses. 4. The biphasic responses at 1 Hz were strongly inhibited by exogenously applied ATP, and its inhibition was reversed in part by a P1 receptor antagonist 8-phenyltheophylline (8-PT). On the other hand, the biphasic vasoconstrictions at 10 Hz were only slightly depressed by ATP, and a subsequent administration of 8-PT produced a partial recovery of the 1st phase response but not that of the 2nd one. 5. From these results, it is concluded that (1) at low frequencies the double peaked responses are mostly mediated via P2X receptor, presynaptic P1 receptors may also modulate the release of ATP, and presynaptic alpha2-adrenergic mechanism may tonically participate in the release of noradrenaline (2) at high frequencies the responses are mostly mediated via alpha1-adrenoceptors and presynaptic P2 receptors may exert its action to inhibit the release of noradrenaline from adrenergic nerve terminals.

Effects of prolonged cold storage on double peaked vasoconstrictor responses to periarterial nerve stimulation in isolated canine splenic arteries.

1. P2X-Purinoceptors and alpha1-adrenoceptors have previously been shown to involve in the double peaked vasoconstrictor responses to periarterial electrical nerve stimulation in the isolated and perfused canine splenic artery. The present study made an attempt to investigate effects of prolonged cold storage (7 days at 4 degrees C) on vasoconstrictor responses to periarterial electrical nerve stimulation, tyramine, noradrenaline and adenosine 5'-triphosphate (ATP) in the isolated canine splenic artery. 2. The periarterial nerve stimulation (1-10 Hz) readily causes a double peaked vasoconstriction in the non-stored preparations. After cold stored for 7 days, the double peaked vasoconstriction was still recognized, although the response became significantly smaller. The first phase was decreased relatively greater than the second phase by the cold storage. 3. In the cold stored preparations, the dose-response curve for tyramine was shifted to the right in a parallel manner. Prazosin almost completely inhibited tyramine-induced vasoconstriction but alpha,beta-methylene ATP failed to influence the response to tyramine. 4. The vasoconstrictor responses to noradrenaline and ATP were not significantly modified by the prolonged cold storage. 5. From these results, it is concluded that the functions of sympathetic co-transmission of purinergic components might be influenced more than that of adrenergic components in the cold storage canine splenic artery.