Prejunctional Alpha 2-adrenoceptors Research Articles

Different sites of action for alpha 2-adrenoceptor antagonists in the modulation of noradrenaline release and contraction response in the vas deferens of the rat.

Rat vas deferens was prepared, loaded with [3H]noradrenaline, and superfused to measure the release of tritium in resting conditions and in response to electrical field stimulation. The alpha 2-adrenoceptor antagonists yohimbine, CH-38083 (7,8-(methylenedioxi)-14 alpha-hydroxyalloberbane HCl), and idazoxan increased the electrically induced release of tritium in a concentration-dependent manner, whereas noradrenaline and the alpha 2-adrenoceptor agonist xylazine exerted opposite effects. The inhibitory effect of noradrenaline on electrically induced tritium release was antagonized by yohimbine, CH-38083, and idazoxan. Of the alpha 2-adrenoceptor antagonists tested, yohimbine and CH-38083 reversed the xylazine-induced inhibition of tritium release, and idazoxan was found to be completely ineffective against xylazine. Idazoxan, yohimbine and CH-38083 antagonized the inhibitory effect of xylazine on electrical stimulation-induced contractions of the vas deferens, as was evidenced by the apparent pA2 values. We conclude from the present experiments that noradrenaline and xylazine inhibit noradrenaline release by acting on distinct prejunctional alpha 2-adrenoceptors and that the receptor subtype that responds to xylazine is insensitive to idazoxan. In addition, inhibition by xylazine of contractility but not of noradrenaline release was antagonized by idazoxan, suggesting that besides noradrenergic neurotransmission, other motor transmitter systems (purinergic) may also be involved in the inhibition by alpha 2-adrenoceptor antagonists of mechanical responses in the rat vas deferens.

Prejunctional opioid mu-receptors and adenosine A1-receptors on the sympathetic nerve endings of the rat tail artery interact with the alpha 2-adrenoceptors.

Experiments were designed to study the interaction between prejunctional alpha 2-adrenoceptors and both adenosine and opioid receptors at the postganglionic sympathetic nerve endings innervating the tail artery of the rat. Segments of this vessel were preincubated with [3H]-noradrenaline and then perfused/superfused with [3H]-noradrenaline-free medium. Their perivascular nerves were field stimulated with standard stimulation parameters: 24 pulses at 0.4 Hz, 0.3 ms, 200 mA. In some experiments, the stimulation parameters were adjusted in order to obtain similar reference release values despite the presence of a first release-modulating drug. The adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA; 0.3-10 mumol/l) and [D-Ala2,MePhe4,Glyol5]enkephalin (DAGO; 0.3-10 mumol/l) depressed the stimulation-evoked overflow of tritium in a concentration dependent manner. The release-inhibiting effect of both NECA and DAGO was enhanced in the presence of the alpha 2-adrenoceptor antagonist rauwolscine (3 mumol/1) while it was attenuated in the presence of the alpha 2-adrenoceptor agonist 5-bromo-6-[2-imidazolin-2yl-amino]-quinoxaline (UK-14,304; 0.1 mumol/l). These changes occurred both at standard and adjusted stimulation parameters. These results demonstrate that the prejunctional adenosine A1- and opioid mu-receptors interact with the prejunctional alpha 2-adrenoceptors. The level at which these interactions take place (receptors themselves or transduction mechanisms) as well as the physiological significance of the phenomenon remain to be determined.

Involvement of prejunctional alpha 2-adrenoceptor in bovine ciliary muscle movement.

The effects of adrenergic agents on bovine ciliary muscle were studied in vitro. Norepinephrine, phenylephrine, or isoproterenol did not affect the muscle tone. High concentrations of guanethidine (10(-5) M) partly inhibited the nerve-mediated contractions of the muscle. The nerve-mediated contractions also were inhibited in the following order: norepinephrine > epinephrine >> isoproterenol. The inhibition was blocked by yohimbine or phentolamine, but not by optimum concentrations of prazosin, moxisylyte, propranolol, and carteolol. Usual concentrations of phosphodiesterase inhibitors markedly inhibited the nerve-mediated and carbachol-induced contractions. These results suggest that an adrenergic alpha 2 receptor located at the nerve terminals inhibits the contraction of cholinergically innervated ciliary muscle. Adrenergic innervation seems to play an inhibitory role in the muscle tissue when the muscle tone is elevated.

Omega conotoxin and prejunctional modulation of the biphasic response of the rat isolated urinary bladder to single pulse electrical field stimulation

1. Single pulse electrical field stimulation (EFS) produces a biphasic response of muscle strips of the rat isolated urinary bladder consisting of an early and a late contraction which were atropine-resistant and atropine-sensitive, respectively. Repeated application of desensitizing doses of the P2 purinoceptor agonist, alpha, beta-methylene ATP (mATP) inhibited the early response while leaving unaffected the late component. 2. Omega conotoxin (CTX, 0.1 microM) inhibited both the early and the late response either in control conditions or after enhancement by physostigmine (0.1 microM). The effect of CTX was, in both cases, more pronounced on the late than the early response to EFS. CTX (0.1 microM) failed to affect contraction produced by ATP or acetylcholine at concentrations (0.3 mM and 0.5 microM) which produced a response similar to that to EFS. 3. The effect of physostigmine was more intense for the late than the early response and was abolished by atropine. In the presence of CTX, physostigmine enhanced both the early and the late components of the mechanical response to EFS. 4. Nifedipine (0.1-1 microM) reduced to a similar extent both the early and late responses. Bay K 8644 (1 microM) produced a marked enhancement of the response to EFS, which, however, did not have a distinct late peak. In the presence of Bay K 8644, either atropine (3 microM) or tetrodotoxin (1 microM) had minor inhibitory effects indicating the myogenic origin of the response. 5. Neurokinin A (0.1-1 nM) enhanced both the early and late responses to EFS without affecting the contraction produced by exogenous acetylcholine or ATP. A consistent potentiation was evident also in the presence of CTX and for the early response, in the presence of atropine. Clonidine (3 microM) inhibited the response to EFS either in the absence or the presence of physostigmine. The inhibitory effect of clonidine, shown previously to depend upon activation of prejunctional alpha 2-adrenoceptors, was still observed in presence of CTX or atropine. 6. It is concluded that CTX-sensitive voltage dependent calcium channels play a more important role in determining the cholinergic rather than the non-cholinergic, putatively purinergic, component of the biphasic response of the rat bladder to single pulse EFS. The action of CTX is likely to be exerted on N-type rather than L-type (dihydropyridine-sensitive) calcium channels. Prejunctional modulation (enhancement by neurokinin A, inhibition by clonidine) occurs even in the presence of CTX-sensitive channels blockade.

Actions of 8-hydroxy-2-(N-dipropylamino) tetralin (8-OH-DPAT) at alpha 2-adrenoceptors.

1 We have examined the actions of the 5-HT1A-receptor ligand 8-OH-DPAT at alpha 2-adrenoceptor ligand binding sites in human platelet and rat kidney membranes and at functional pre- and postjunctional alpha 2-adrenoceptors in rat atrium and human saphenous vein, respectively. 2 8-OH-DPAT had low affinity for the alpha 2A ligand binding site of human platelet but showed 10 times higher affinity for the alpha 2B ligand binding site of rat kidney (K1 of 6.41, -log M). 3 In functional studies, 8-OH-DPAT had low potency as an antagonist of noradrenaline-induced contractions in human saphenous vein, with a KB of 5.40 (-log M). 4 In rat atria preincubated with [3H]-noradrenaline, 8-OH-DPAT potentiated stimulation-evoked overflow of tritium with an EC30 (concentration producing 30% increase in the stimulation-evoked overflow) of 6.37 (-log M). 5 It is concluded that 8-OH-DPAT shows selectively for the alpha 4B ligand binding site of rat kidney and for the functional prejunctional alpha 2-adrenoceptor of rat atrium, which resembles the alpha 2B ligand binding site.

α2‐Adrenoceptor blocking profile of SK&F 104078: further evidence for receptor subtypes

1. The ability of the putative, selective post-junctional alpha 2-adrenoceptor antagonist, SK&F 104078 to antagonize the effects of structurally-diverse agonists at pre-junctional alpha 2-adrenoceptors in the guinea-pig ileum and rat vas deferens in vitro and in the rat heart in vivo, and at post-junctional alpha 2-adrenoceptors in the rabbit ear vein in vitro, was examined. Results obtained with SK&F 104078 were compared with those obtained with yohimbine. 2. Xylazine and B-HT933 each caused a concentration-dependent inhibition of the field-stimulation-evoked twitch responses of the guinea-pig ileum and rat vas deferens. SK&F 104078 did not antagonize either agonist in the guinea-pig ileum and exerted only minimal blocking activity against xylazine in the rat vas deferens. In contrast, SK&F 104078 competitively antagonized B-HT933 in the rat vas deferens (pA2 = 6.45). Yohimbine competitively antagonized both agonists in each tissue (pA2 values ranged between 7.46 and 7.88). 3. In the pithed rat xylazine and B-HT933, injected intravenously, caused a dose-dependent reduction in the tachycardia elicited by stimulation of the cardiac preganglionic sympathetic nerves. SK&F 104078 (10 mg kg-1, i.v.) caused a 20-30 fold rightward displacement of the dose-response curve to xylazine, but did not affect responses to B-HT933. Yohimbine (1 mg kg-1, i.v.) antagonized both agonists to a similar degree. 4. In the rabbit ear vein xylazine, B-HT933, noradrenaline and UK 14304 elicted vasoconstrictor responses. Prazosin was without effect, but in contrast, SK&F 104078 was a competitive antagonist of each of the agonists (pA2 values ranged between 6.63 and 6.72). Yohimbine also competitively antagonized each of the agonists in this preparation (pA2 values ranged between 7.81 and 8.07). 5. SK&F 104078 was also a competitive antagonist (pA2 = 6.20) against noradrenaline at post-junctional alpha 1-adrenoceptors in the rabbit aorta. 6. These data show that SK&F 104078 is a competitive antagonist at post-junctional alpha l- and alpha 2-adrenoceptors. Its antagonist potency at pre-junctional alpha 2-adrenoceptors is agonist- and tissuedependent. Yohimbine does not discriminate between pre- and post-junctional alpha 2-adrenoceptors. The findings are discussed in terms of the possible existence of subclasses of OC2-adrenoceptors.

Local application of drugs to sympathetic nerve terminals: an electrophysiological analysis of the role of prejunctional α‐adrenoceptors in the guinea‐pig vas deferens

1. Focal extracellular recording techniques were used to study the effects of clonidine, yohimbine and tyramine on the intermittent transmitter release mechanism in the guinea-pig vas deferens in vitro. Drugs were applied locally to the varicosities located within the recording electrode. Statistical methods were used to determine whether noradrenaline (NA) acts locally to inhibit secretion from the same or a closely related release site (local regulation) on an impulse-to-impulse basis. 2. The alpha-adrenoceptor agonist, clonidine, inhibited transmitter release, an effect reversed by the alpha-adrenoceptor antagonist, yohimbine. Yohimbine alone increased action potential-evoked transmitter release, findings consistent with the idea that transmitter release is regulated through prejunctional alpha-adrenoceptors. 3. The indirectly acting sympathomimetic, tyramine, powerfully inhibited evoked transmitter release, an effect reversed by both yohimbine and phentolamine. The inhibitory effects of tyramine were greatly reduced in tissues taken from animals pretreated with reserpine. Clonidine powerfully inhibited transmitter release in reserpinized tissues showing that prejunctional alpha-adrenoceptors were functionally intact. The inhibitory effects of tyramine on transmitter release are therefore mediated indirectly through the release of endogenous NA. 4. Paradoxically, when transmitter release from a small population of variscosities on a single nerve fibre was studied in the absence of alpha-adrenoceptor antagonists, no evidence was found for local regulation of transmitter release. 5. The intermittent character of the transmitter release process makes it difficult to envisage how impulse-to-impulse regulation could occur. Furthermore, it is unlikely that NA will accumulate to any appreciable extent in the vicinity of the secreting varicosity. 6. The pharmacological evidence clearly supports the view that NA released from sympathetic nerve terminals by nerve impulses modulates subsequent transmitter release. However, the evidence does not support the view that released NA acts locally to inhibit secretion from recently activated varicosities.

No influence of prejunctional α2‐adrenoceptors on the effects of nicotine and tyramine in guinea‐pig atria

1. Guinea-pig left atria were stimulated at a rate of 0.5 Hz and preloaded with 10 microCi 7-[3H]noradrenaline. In the presence of cocaine, 3 x 10(-6) mol l-1, and atropine, 10(-7) mol l-1, noradrenaline release was stimulated twice in each atrium, either by electrical field stimulation (one pulse, 0.2 ms 30 V, during each refractory period for 5 min) or by addition of either nicotine, 3 x 10(-5) mol l-1, or tyramine 10(-5) mol l-1. 2. The release of radioactivity caused by field stimulation and the associated positive inotropic effect were almost abolished by 3 x 10(-8) mol l-1 tetrodotoxin. Pretreatment with 3 x 10(-7) mol l-1 idazoxan increased the effects of field stimulation, while they were attenuated in the presence of clonidine, 3 x 10(-7) mol l-1. 3. The inotropic effect and the release of radioactivity caused by nicotine were greatly attenuated by tetrodotoxin, but were not significantly influenced by idazoxan or clonidine. 4. The release of radioactivity caused by tyramine and the inotropic effect of this drug were resistant to tetrodotoxin pretreatment and were not modified by idazoxan or clonidine. 5. It is concluded that nicotine releases noradrenaline by initiating a propagated action potential. But activation of prejunctional alpha 2-adrenoceptors does not seem to attenuate the transmitter release by limiting the spread of the action potential in the prejunctional sympathetic neuronal network.

Pharmacological profile of the novel .ALPHA.-adrenoceptor antagonist KT-611(naftopidil).

The pharmacological profile of a new alpha-adrenoceptor antagonist, KT-611 (naftopidil), was studied in vitro. In the dog mesenteric and carotid arteries and in the rabbit, guinea pig and rat thoracic aortae, KT-611 competitively inhibited alpha 1-adrenoceptor-mediated contractions induced by noradrenaline with pA2 values ranging from 6.73 to 8.15. KT-611 also inhibited the postjunctional alpha 2-adrenoceptor-mediated contractions in the dog saphenous vein (pA2 = 6.77) or dog basilar artery. However, the responses mediated through prejunctional alpha 2-adrenoceptors (rat vas deferens), beta-adrenoceptors (rat atria), muscarinic receptors (guinea pig ileum) and 5-HT2 receptors (dog mesenteric artery) were little affected by KT-611. KT-611 also inhibited the sympathetic adrenergic contraction evoked by electrical transmural stimulation in the dog mesenteric artery, and the inhibition was not relieved upon repetitive washing for 1 hour with the drug-free solution. 3H-prazosin and 3H-clonidine binding to the rat cortex membranes was inhibited by KT-611 with pKi values of 7.69 and 5.75, respectively. These results suggest that KT-611 is an alpha 1-adrenoceptor antagonist with a weak antagonistic activity to postjunctional alpha 2-adrenoceptors.

Role of cyclic AMP in the prejunctional α2-adrenoceptor modulation of noradrenaline release from the rat tail artery

Experiments were designed to evaluate the effect of cyclic AMP on the electrically-induced release of noradrenaline from vascular sympathetic nerve terminals. The possible implication of the inhibition of adenylate cyclase in the negative feed-back control by prejunctional alpha 2-adrenoceptors of neurotransmitter release was also investigated. Rat isolated tail arteries were preincubated with [3H]-noradrenaline; the preparations were subsequently perfused/superfused with [3H )-noradrenaline-free medium and their perivascular nerves were field stimulated with 24 pulses at 0.4 Hz (0.3 ms, 200 mA). 2 compounds known to enhance the intracellular concentration of cyclic AMP, namely the membrane permeant analogue 8-Br-cAMP (10-300 mumol/l) and forskolin (0.3-10 mumol/l), an activator of adenylate cyclase, concentration-dependently enhanced the stimulation-evoked tritium overflow. The 1,9-dideoxy derivative of forskolin, which does not stimulate adenylate cyclase, was ineffective. Exposure to the cyclic AMP phosphodiesterase inhibitor rolipram 30 mumol/l produced a moderate increase (about 20%) in tritium overflow. However, in the presence of rolipram the facilitatory effect of forskolin was significantly more pronounced than in its absence. Whereas 8-Br-cAMP produced a slight concentration-dependent enhancement of the stimulation-induced vasoconstriction, forskolin and rolipram depressed it. The alpha 2-adrenoceptor agonist B-HT 933 (3-30 mumol/l) concentration-dependently inhibited the tritium overflow. The effect of B-HT 933 30 mumol/l was slightly, but significantly reduced in the presence of 8-Br-cAMP 100 and 300 mumol/l, but was not changed in the presence of forskolin 3 mumol/l. The facilitatory effect of rauwolscine 1 mumol/l was enhanced in the presence of 8-Br-cAMP 100 mumol/l. During perfusion with 8-Br-cAMP 100 mumol/l, the current strength and frequency were decreased to 150 mA and 0.2 Hz, respectively in order to obtain similar amounts of tritium overflow to those observed in the absence of the cyclic AMP analogue with the initial stimulation parameters. Under these conditions, the inhibition of the overflow by B-HT 933 30 mumol/l and the facilitation by the alpha 2-adrenoceptor antagonist rauwolscine 1 mumol/l were unaltered as compared to controls under initial stimulation conditions. It is concluded that, in the rat tail artery, the terminals of perivascular sympathetic nerves are endowed with an adenylate cyclase system. Cyclic AMP is able to modulate noradrenaline release, but does not appear to play a role in the initiation of the release process itself. In addition, the results do not support the hypothesis that prejunctional alpha 2-adrenoceptors depress noradrenaline release through the inhibition of adenylate cyclase.

Neurotransmitters and pre‐ and post‐junctional receptors involved in the vasoconstrictor response to sympathetic nerve stimulation in rat tail artery

The study was prompted by the need to re-evaluate, in view of the complexity of the evidence in the literature, the relative roles of different sympathetic transmitters and receptors in the contractile response of the tail artery of adult normotensive rats to electrical field stimulation. By the pharmacological approach employed, noradrenaline and adenosine 5'-triphosphate appeared to contribute to this response; the possible roles of other putative transmitters such as neuropeptide Y could not be examined due to lack of specific antagonists. Noradrenaline, clearly the main mediator, exerted both excitatory and inhibitory effects, acting in part via different receptors depending on the stimulus parameters. Thus, yohimbine and prazosin (alpha 2- and alpha 1-adrenoceptor antagonists, respectively) were about equally effective as inhibitors of the noradrenaline-mediated contractile response to stimulation with short trains and/or at low frequency, but the response caused by long trains and/or high-frequency stimulation was much more strongly inhibited by prazosin than by yohimbine. As expected, yohimbine enhanced the [3H]noradrenaline overflow response to long but not to short stimulus trains, presumably because in the latter case the noradrenaline concentration in the relevant biophase was too low to activate the pre-junctional alpha 2-adrenoceptors. Finally, propranolol, an unselective beta-adrenoceptor antagonist, enhanced the neurogenic contraction, indicating that noradrenaline restricts this response by effects via post-junctional beta-adrenoceptors. Adenosine triphosphate appeared to exert dual, excitatory as well as inhibitory, post-junctional effects. Thus, the P2x-purinoceptor desensitizing agent, alpha, beta-methylene adenosine triphosphate, abolished the initial phase, but enhanced the amplitude of the neurogenic contraction, without affecting the nerve stimulation-induced overflow of [3H]noradrenaline. The results indicate that noradrenaline and adenosine triphosphate, the main mediators of the neurogenic contraction of this preparation, act in a more complex fashion than earlier thought; they argue against a significant direct contribution by other putative transmitters but do not exclude that such agents may act indirectly as modulators of this response.

Functional evidence for heterogeneity of peripheral prejunctional α2‐adrenoceptors

1. We have examined the potencies of a series of alpha 2-adrenoceptor antagonists in functional studies of prejunctional alpha 2-adrenoceptors in rat atrium and vas deferens, and compared potencies with affinities for the alpha 2A-ligand binding site of human platelet and the alpha 2B-site of rat kidney. 2. Antagonist potency in rat atrium was expressed as an EC30 (concentration producing 30% increase in the stimulation-evoked overflow of tritium in tissues pre-incubated with [3H]-noradrenaline). Antagonist potency in rat vas deferens was expressed as a pA2 or KB at antagonizing the inhibition by the alpha 2-adrenoceptor agonist xylazine of the isometric twitch to a single stimulus, or as an EC30. 3. In ligand binding studies, Ki values were obtained for the displacement by alpha-adrenoceptor antagonists of [3H]-yohimbine binding to human platelet or rat kidney membranes. 4. In functional studies, three antagonists (ARC 239, prazosin and chlorpromazine) distinguished between prejunctional alpha 2-adrenoceptors of rat atrium (EC30) and rat vas deferens (pA2) and showed 49, 12 and 7 times higher potency in rat atrium, respectively. ARC 239 was also 17 times more potent in rat atrium than rat vas deferens when EC30 values were compared. 5. The correlation of affinity for the alpha 2A-site of human platelet was better with prejunctional potency in rat vas deferens than rat atrium. 6. The correlation of affinity for the alpha 2B-site of rat kidney was better with prejunctional potency in rat atrium than rat vas deferens. 7. It is concluded that prejunctional alpha 2-adrenoceptors of rat vas deferens and rat atrium differ, and these receptors may resemble the alpha 2A- and alpha 2B-ligand binding sites, respectively.

Effects of CRL 41034 on the adrenergic neuroeffector interaction in the canine saphenous vein

Experiments were designed to determine the effect of CRL 41034, a buflomedil analogue, on the adrenergic responsiveness of canine veins. Rings of saphenous vein (without endothelium) were suspended for isometric tension recording in modified Krebs-Ringer bicarbonate solution at 37 degrees C. CRL 41034 produced a concentration-dependent inhibition of the contractions evoked by the alpha adrenergic agonists norepinephrine, phenylephrine and UK 14304 which was insensitive to the blockade of neuronal uptake by cocaine. CRL 41034 was more potent in inhibiting the concentration-dependent contractions evoked by UK 14304 than those by phenylephrine and the antagonism it caused against the response to UK 14304 fulfilled the criteria for competitivity. CRL 41034, at 10(-5) M significantly depressed, and at 10(-4) M abolished the contractions induced by electrical stimulation of the adrenergic nerves and those evoked by the indirect sympathomimetic amine tyramine. Strips of canine saphenous vein were superfused after incubation with [3H] norepinephrine. During sympathetic nerve activation, CRL 41304 increased the stimulation-evoked overflow of [3H] norepinephrine and 3-methoxy-4-dihydroxyphenylglycol; in the presence of rauwolscine the compound only increased the stimulation-evoked overflow of 3,4-dihydroxyphenylglycol. These experiments suggest that the major vascular effects of CRL 41034 in canine veins are blockade of alpha 2-adrenoceptors on vascular smooth muscle, and inhibition of prejunctional alpha 2-adrenoceptors on adrenergic nerve endings.

CHANGES IN PREJUNCTIONAL POTENCY OF B‐HT 933 DURING AGING IN THE RAT VAS DEFERENS

1. Age-related changes in prejunctional alpha 2-adrenoceptors were examined in the rat vas deferens using pharmacological techniques. 2. B-HT 933 (1 x 10(-8) - 1 x 10(-6) mol/L) caused a concentration-dependent inhibition of isometric contractions (tetrodotoxin-sensitive) induced by stimulation with single field-stimulus pulses, in both the epididymal and prostatic regions of rat vas deferens. The concentration-response curve to B-HT 933 was shifted to the right with age in the prostatic regions of the vas deferens. 3. In high concentrations (10(-6) - 3 x 10(-4) mol/L), B-HT 933 caused concentration-dependent enhancement of the contractile response to stimulation and evoked spontaneous contractile activity. No significant difference in this postjunctional activity occurred with age in either the prostatic or epididymal regions of the vas deferens. 4. Schild analysis revealed no significant differences in pA2 values for the antagonisms of the prejunctional inhibitory effect of B-HT 933 by rauwolscine in either the prostatic or epididymal regions of vas deferens between young and old rats. 5. These results could be interpreted as a decrease in alpha 2-adrenoceptor number with age. The more marked decrease in the prejunctional inhibitor potency of B-HT 933 in prostatic regions of vas deferens with aging may be due to a smaller receptor reserve in this region of the vas deferens.

Pharmacology of noradrenaline and neuropeptide tyrosine (NPY)‐mediated sympathetic cotransmission

Pharmacological and physiological aspects for neuropeptide Y (NPY) and noradrenaline (NA) cotransmission have been studied in the peripheral sympathetic nervous control of blood vessels, heart, spleen and vas deferens. NPY coexists with NA in large dense cored vesicles and is released compared to NA mainly upon high frequency stimulation or strong reflex sympathetic activation. NPY release is inhibited via prejunctional alpha-2 adrenoceptors and adenosine receptors but facilitated by angiotensin II or beta-receptor activation. NPY exerts prejunctional inhibitory actions on both NA and NPY release, enhances the vasoconstrictor effect of NA and evokes potent, long-lasting vasoconstriction. Specific receptor mechanisms for NPY exist at both the pre- and postjunctional levels; a large amidated C-terminal portion of NPY is necessary for receptor binding, inhibition of cyclic AMP formation and vasoconstrictor effects. Denervation results in supersensitivity for both NA and NPY-evoked vasoconstriction. Reserpine pretreatment is associated with depletion of NA as well as NPY; the effect on NPY is entirely dependent on an intact nerve activity. Reserpine treatment combined with preganglionic denervation depletes NA by 99% while NPY levels are maintained intact. The characteristic appearance of the nerve stimulation evoked vasoconstrictor response with a high correlation to NPY outflow after reserpine treatment, suggests that NPY may be involved as a transmitter in a variety of vascular beds. NPY-synthesis in ganglia seems to be regulated by nicotinic receptor activity; secondary stimulation by eg reserpine stimulates and nicotine antagonists decrease NPY-synthesis. Many classical pharmacological agents including guanethidine, clonidine, yohimbine, angiotensin II, nicotine and desipramine influence NPY release. A complex interplay therefore seems to occur at both the pre- and postjunctional levels of transmission for the classical transmitter NA and the coexisting peptide NPY, creating a great diversity of chemical signalling potential.

Responses of blood vessels in the rabbit knee to acute joint inflammation.

These experiments examined the responses of articular blood vessels in the rabbit knee to induction of acute joint inflammation by intraarticular injection of 4% kaolin suspension. This produced an inflammatory response, which became evident about four hours after injection. Electrical stimulation of the nerve supply to the knee joint before induction of inflammation produced a biphasic response--an initial vasoconstriction during the stimulation phase followed by dilatation after stimulation stopped. These responses were followed up for eight hours and it was noted that the constrictor response became progressively greater, producing a further 19% decrease in blood flow during nerve stimulation about three hours after the injection of kaolin. The sensitivity of postjunctional alpha adrenoceptors, however, showed still greater increase in the inflamed joint as close intraarterial injection of 10(-6) M adrenaline produced an additional 30% reduction in blood flow four hours after kaolin injection compared with the control response. Possibly, the smaller enhancement of the constrictor response to nerve stimulation in the inflamed joint may reflect sensitisation of prejunctional alpha adrenoceptors in addition to the effects exerted on postjunctional alpha adrenoceptors by the inflammatory process. The dilator response also increased over eight hours, though this rise was less marked. These findings indicate that even over the limited time span of the experiments, significant alterations occurred in factors which influence the calibre of articular blood vessels.

Cardiovascular Effects of SK&F 104078, a Novel a-Adrenoceptor Antagonist, in Normotensive and Hypertensive Rats

SK&F 104078 is a novel alpha-adrenoceptor antagonist derived from the 3-benzazepine alpha 2-adrenoceptor antagonist SK&F 86466. SK&F 104078 will block both alpha 1- and vascular postjunctional alpha 2-adrenoceptors but does not block most prejunctional alpha 2-adrenoceptors. Intravenous (i.v.) administration of SK&F 104078 decreased blood pressure (BP) in both spontaneously hypertensive and DOCA-salt hypertensive rats. SK&F 104078 potentiated the hypotensive response to tilt in anesthetized spontaneously hypertensive rats (SHR). Although SK&F 104078 had no effect on BP in normotensive rats, the tilt-induced decrease in BP in these animals was also potentiated. In this regard, SK&F 104078 resembled the selective alpha 1-adrenoceptor antagonist prazosin, rather than the alpha 2-adrenoceptor antagonists rauwolscine or SK&F 86466. Oral administration of SK&F 104078 had no significant effect on BP in SHR unless extremely high doses were administered. This was consistent with low plasma concentrations of SK&F 104078 observed after oral administration. After i.v. administration, the clearance of SK&F 104078 from plasma was 123 ml/min/kg, the steady-state volume of distribution was 17 L/kg, and the fraction excreted unchanged in urine was less than 1%. The low oral bioavailability of SK&F 104078 did not appear to be due to high first-pass oxidative metabolism, since pretreatment of SHR with the suicide substrate inhibitor of cytochrome P-450, 1-aminobenzotriazole (ABT), did not result in increased oral efficacy. SK&F 101253, a close structural analogue of SK&F 104078, was an effective antihypertensive when administered orally. Comparison of the stability of SK&F 101253 and SK&F 104078 in acid media showed SK&F 104078, but not SK&F 101253, to be rapidly degraded.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of catecholamines on cholinergically and non-cholinergically mediated contractions of guinea-pig isolated bronchial muscle

The actions of catecholamines on the responses evoked by electrical field stimulation or by acetylcholine and substance P in guinea-pig bronchial strip chain have been examined. Electrical field stimulation evoked a biphasic contraction, consisting of a cholinergically-mediated fast contraction followed by a non-cholinergically-mediated slow contraction. All catecholamines tested caused a concentration-dependent reduction in the height of the biphasic contraction, where non-cholinergic contractions were more potently inhibited. The inhibitory effect of isoprenaline was largely prevented by propranolol (2 microM) alone, whereas those of noradrenaline and adrenaline were prevented by treatment with both propranolol (2 microM) and yohimbine (2 microM). The inhibitory effect of dopamine was unaffected either by propranolol (2 microM), yohimbine (2 microM) or haloperidol (10 microM). Submaximal contractions of bronchial muscle evoked by exogenous acetylcholine (2 microM) or substance P (0.2 microM) were also inhibited by catecholamines, except dopamine, but the effects were antagonized by propranolol (2 microM) alone. The results suggest that in guinea-pig isolated bronchial muscle, catecholamines can inhibit both cholinergic and non-cholinergic excitatory neurotransmissions not only by postjunctional beta-adrenoceptors but also by prejunctional alpha 2-adrenoceptors.

Involvement of .ALPHA.2-adrenergic receptors in the vagal reflex-induced tracheal constriction.

The effects of clonidine on the vagal reflex-induced tracheal constriction have been investigated in anesthetized, paralyzed, and artificially ventilated mongrel dogs. The cervical trachea was transected in situ into two parts. Responses of the tracheal musculature were measured as changes in the intratracheal pressure on an air-filled balloon introduced into the rostral side of the transected trachea. Reflex tracheal constriction was induced by afferent electrical stimulation at the central cut end of the vagus nerve. Drugs were injected or infused close intraarterially (i.a.) into the bilateral cranial thyroid arteries in such a way that each drug was applied just to the rostral trachea. The reflex tracheal constriction was abolished by a close i.a. infusion of 3 microM atropine. The magnitude of the reflex tracheal constriction was slightly reduced by a close i.a. infusion of 10 microM clonidine and was significantly reduced by the infusion of clonidine at a concentration of 100 and 300 microM. The response to 100 microM clonidine was antagonized by a close i.a. infusion of 1 microM yohimbine. The tracheal constriction induced by i.a. injection of 5.5 nmol acetylcholine was unaffected by infusion of 10, 100 and 300 microM clonidine. The vagal reflex-induced tracheal constriction seems to be inhibited by stimulation of prejunctional alpha 2-adrenoceptors.

No evidence for differences between pre‐ and postjunctional α2‐adrenoceptors in the periphery

1. We have compared prejunctional alpha 2-adrenoceptors in rat and guinea-pig vas deferens and rat and guinea-pig atria with postjunctional alpha 2-adrenoceptors in human saphenous vein and human platelets employing the antagonists yohimbine and SK&F 104078 and other alpha 2-adrenoceptor antagonists. 2. Yohimbine was approximately 10 times more potent prejunctionally than SK&F 104078 at antagonizing the inhibition by the alpha 2-adrenoceptor agonist xylazine of stimulation-evoked contractions in rat and guinea-pig vas deferens, and at increasing stimulation-evoked release of tritium in rat and guinea-pig atria pre-incubated with [3H]-noradrenaline. 3. Yohimbine was approximately 10 times more potent postjunctionally than SK&F 104078 at antagonizing contractions to noradrenaline in human saphenous vein and at displacing [3H]-yohimbine binding in human platelet membranes. 4. For the antagonists yohimbine, SK&F 104078, prazosin, phentolamine, CH 38083 and urapidil, there was a significant correlation between prejunctional potency in rat vas deferens atrium and postjunctional potency in human platelet, although the correlation was improved by the omission of prazosin. 5. We have no evidence for differences between functional pre- and postjunctional alpha 2-adrenoceptors in the periphery, although these functional receptors may differ from the ligand binding site in the human platelet.