Pre- and postjunctional alpha-adrenoceptor-mediated effects of prazosin, methoxamine and 6-fluoronoradrenaline in blood-perfused canine skeletal muscle in situ

Pre- and postjunctional effects of the alpha 1-selective adrenoceptor antagonist prazosin and the alpha 1- and alpha 2-selective adrenoceptor agonists methoxamine and 6-fluoronoradrenaline, respectively, were studied in skeletal muscle in situ. Prazosin reduced the vasoconstriction and enhanced the overflow of endogenous noradrenaline elicited by sympathetic nerve stimulation (1-4 Hz, 2 min); the threshold concentration was 10-100 times lower for postjunctional than for prejunctional alpha-adrenoceptor blockade. The enhancement of noradrenaline overflow by prazosin was not inversely frequency-dependent, as shown elsewhere for alpha 2-adrenoceptor antagonists. Thus, different mechanisms may be involved. Inhibition of prostaglandin synthesis by diclofenac did not alter the stimulation-evoked noradrenaline overflow, indicating a minor importance of prostaglandin-mediated transjunctional mechanisms in the modulation of noradrenaline overflow. Methoxamine and 6-fluoronoradrenaline elevated the basal vascular tone and, at higher concentrations, reduced the stimulation-evoked noradrenaline overflow. Methoxamine was 20 times more selective than 6-fluoronoradrenaline for postjunctional receptors. Our results are compatible with a pre- and postjunctional localization of alpha 2-adrenoceptors and a predominantly, but not exclusively, postjunctional localization of alpha 1-adrenoceptors. The postjunctional selectivity for prazosin was less marked than previously reported from in vitro studies. Hence, care should be taken when extrapolating in vitro findings to the more complex in vivo situation.     

Prejunctional beta 2-adrenoceptor-mediated enhancement of noradrenaline release in skeletal muscle vasculature in situ

Prejunctional beta-adrenoceptor-mediated modulation of sympathetic neurotransmission was studied in desipramine-pretreated canine blood-perfused gracilis muscle. Overflow of endogenous noradrenaline (NA) to venous plasma and vasoconstriction reflect pre- and postjunctional events in this in vivo model. The nonselective beta-adrenoceptor agonist isoprenaline (15 nM in arterial plasma) and the beta 2-selective agonist rimiterol (50 nM) caused similar vasodilatation (35-40% increase in vascular conductance, p less than 0.01), enhancement of nerve stimulation-evoked vasoconstriction (+20-25%, p less than 0.01), and NA overflow (+13%, p less than 0.05). Isoprenaline, 3 nM, evoked vasodilatation but did not alter NA overflow. Blockade of beta 2-adrenoceptors by ICI 118,551 increased basal vascular tone (+9%, p less than 0.01) and reduced nerve stimulation-evoked vasoconstriction (-7%, p less than 0.05), but failed to alter NA overflow significantly (-12%, p = 0.12). ICI 118,551 blocked all responses to the beta-adrenoceptor agonists. Thus, prejunctional beta 2-adrenoceptor-mediated facilitation of sympathetic neurotransmission could be demonstrated in vivo, but the effects were modest. Previous experiments, however, have demonstrated a considerably larger influence of alpha-adrenoceptor-mediated prejunctional modulation in this model. Hence, prejunctional modulation via beta-adrenoceptors appears to be of subordinate importance when compared with the alpha-adrenoceptor-mediated inhibitory mechanism under these physiological conditions.     

Prejunctional alpha 2-adrenoceptors modulate stimulation-evoked norepinephrine release in rabbit lateral saphenous vein

Segments of rabbit lateral saphenous vein prelabelled with [3H]noradrenaline were perfused and superfused with physiological salt solution. Tritium overflow evoked by transmural nerve stimulation (3 Hz for 2 min) was abolished by tetrodotoxin (1 microM). The selective alpha 2-adrenoceptor agonist UK 14,304 inhibited stimulation-evoked 3H-overflow in a concentration-dependent manner, with an IC50 of 71 nM. In contrast, the alpha 2-adrenoceptor agonist B-HT 933 had no effect on 3H-overflow in concentrations up to 10 microM. The selective alpha 2-adrenoceptor antagonists idazoxan and SKF 86466, as well as the non-selective alpha-antagonist phentolamine, facilitated the nerve stimulation evoked 3H-overflow, with an order of potency of idazoxan greater than or equal to phentolamine greater than SK&F 86466. Prazosin (100 nM) had little effect on 3H-overflow. These findings suggest that stimulation-evoked neurotransmitter release is modulated via prejunctional alpha 2-adrenoceptors.     

Prejunctional inhibition of non-adrenergic non-cholinergic transmission in the rat anococcygeus muscle

The muscarinic agonist McN-A-343 did not affect the tone or nitroprusside-induced relaxations of the rat anococcygeus, but inhibited non-adrenergic non-cholinergic (NANC) relaxations. Atropine, pirenzepine and gallamine blocked the McN-A-343 inhibition of NANC relaxations. Agonists of alpha 1-(methoxamine) or alpha 2-adrenoceptors (clonidine, rilmenidine) increased tone and reduced NANC relaxations. Carbachol also increased tone but produced a greater reduction of NANC relaxations. The findings suggest that activation of prejunctional muscarinic receptors on NANC terminals inhibits transmitter release, but there is no evidence for prejunctional alpha-adrenoceptors.     

7-OH-DPAT-induced inhibition of norepinephrine release in PC12 cells

The purpose of this study was to investigate mechanisms of suppression of norepinephrine release by 7-OH-DPAT, a dopamine D(2)/D(3) receptor agonist, in PC12 cells pretreated with nerve growth factor (NGF). 7-OH-DPAT caused inhibition of basal and K(+)-evoked norepinephrine release, which could be blocked by pretreatment with raclopride, a D(2)/D(3) receptor antagonist. Moreover, dopamine D(2) and D(3 )receptors were identified by immunocytochemistry. Expression of D(2), D(3), and D(4) mRNAs and their proteins were detected using RT-PCR and immunoblotting. Furthermore, 7-OH-DPAT produced no change in cGMP levels; however, 7-OH-DPAT inhibited forskolin-stimulated cAMP accumulation that was antagonized by pretreatment with raclopride. In addition, 7-OH-DPAT inhibited carbachol-induced Ca(2+) transient, conversely, 7-OH-DPAT had no effect on 4-aminopyridine-induced Ca(2+) transient. Taken together, suppression of cAMP accumulation and calcium mobilization by 7-OH-DPAT is involved in the inhibition of norepinephrine release through activation of dopamine D(2)/D(3) receptors.     

A method for recording smooth muscle and vascular responses of the blood-perfused dog trachea in situ.

1 A method is described for measuring responses of dog tracheal musculature and vasculature in situ. 2 The upper two thirds of the trachea was perfused with blood through both cranial thyroid arteries at a constant pressure. The blood flow through the arteries was measured with an electromagnetic flowmeter. The response of the tracheal musculature was measured as a change in pressure in a water-filled cuff inserted into the trachea via the mouth. Drugs were injected close-arterially. 3 Acetylcholine produced dose-dependent increases in blood flow rate (vasodilatation) and in tracheal intraluminal pressure (tracheal constriction). These responses were antagonized by atropine. 4 Isoprenaline produced vasodilatation which was blocked by propranolol. Adrenaline and noradrenaline caused vasocontriction which was blocked by phentolamine. 5 All three catecholamines produced a decrease in tracheal intraluminal pressure (tracheal dilatation). The tracheal dilatation in response to adrenaline and noradrenaline was converted to constriction by propranolol. The tracheal constriction thus unmasked was abolished specifically by phentolamine. 6 From these results it is concluded that the tracheal musculature and vasculature contain muscarinic receptors, and excitatory alpha- and inhibitory beta-adrenoceptors. In the tracheal musculature beta-adrenoceptors predominate over alpha-adrenoceptors; the reverse is true in the tracheal vasculature.     

Novel inhibition of contractility by wortmannin in skeletal muscle

1. The effects of wortmannin and 2-(4-morpholinyl)-8-phenyl-1[4H]-benzopyran-4-one ({"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002), inhibitors of phosphatidylinositol 3-kinase, on the contractile responses of murine skeletal muscle were studied. Wortmannin (10–100 μM) suppressed twitch and tetanic contraction evoked by field stimulation of diaphragm without causing elevation of muscle tone. The inhibition was quasi-irreversible with IC₅₀∼15 μM. In contrast, {"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002 increased twitch responses and elevated muscle tone.
2. Wortmannin reversibly depressed the maximal slope of action potential upstroke by ∼40% and inhibited the membrane depolarization and spontaneous burst of action potential induced by crotamine, a polypeptide toxin that activates the Na⁺ channel of skeletal muscle.
3. Wortmannin inhibited contractures evoked by high K⁺, ryanodine and caffeine, but potentiated the contracture induced by rapamycin, which binds to myoplasmic FK506 binding protein, an immunophilin closely associated with the ryanodine receptor. The contractures elicited by cardiotoxin, which disrupts the integrity of sarcolemma and thereby elevates `myoplasmic'' Ca²⁺ level, were suppressed only slightly.
4. In placed left atrium and ventricular strip, wortmannin and {"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002 produced a positive inotropic effect.
5. The results suggest that, in addition to depressing the Ca²⁺ mobilization from sarcoplasmic reticulum, wortmannin exerts a novel inhibitory action on the excitation-contraction coupling in skeletal muscle but not in cardiac muscle.

     

Blockade of autoreceptor-mediated inhibition of norepinephrine release by atipamezole is maintained after chronic reuptake inhibition

The role of alpha(2)-adrenergic autoreceptor desensitization in the delayed onset of antidepressant efficacy of selective norepinephrine (NE) reuptake inhibitors is unclear. Using the alpha(2)-antagonist yohimbine, we showed previously that chronic treatment with desipramine (DMI) did not alter autoreceptor-mediated inhibition of NE release in the cortex. However, yohimbine may have non-selective effects that could confound this interpretation. Thus, using microdialysis, we measured acute effects of the highly selective alpha(2)-antagonist atipamezole on NE release in the prefrontal cortex following chronic DMI treatment, after 0-8 d washout. Atipamezole induced a similar elevation of extracellular NE in all treatment groups, indicating no change in autoreceptor function. Further, the effect was most rapid in DMI-treated rats with 0- and 2-d washout, suggesting that autoreceptor-mediated inhibition was most prominent when NE levels were highest. This provides further evidence that autoreceptor-mediated inhibition of NE neurotransmission remains functional after chronic DMI treatment, arguing against the hypothesis that desensitization of alpha(2)-autoreceptors accounts for the delayed onset of action of selective NE reuptake inhibitors.     

Angiotensin II type 2 receptor-mediated inhibition of norepinephrine release in isolated rat hearts

We investigated whether endogenous and exogenous angiotensin II (Ang II) regulates norepinephrine (NE) release from cardiac sympathetic nerves via both Ang II type 2 receptors (AT2Rs) and Ang II type 1 receptors (AT1Rs). Using isolated rat hearts, sympathetic nerves were electrically stimulated. Ang II with PD-123319 (AT2R antagonist) but not Ang II alone produced a significant increase in nerve stimulation-induced NE overflow, which was abolished by the addition of AT1R antagonist losartan. In contrast, NE overflow was markedly decreased by losartan with or without Ang II. This decrease was abolished by the combination with PD-123319, nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (NOARG), icatibant (bradykinin B2 receptor antagonist), or PKSI-527 (kininogenase inhibitor). CGP-42112A (AT2R agonist) suppressed nerve stimulation-induced NE overflow in the same way as the combination of Ang II and losartan, and this suppression was abolished by PD-123319, NOARG, icatibant, or PKSI-527. There were significant increases in NOx (NO2/NO3) contents in coronary effluent under conditions where NE overflow was suppressed. Ang II seems to function as an inhibitory modulator of cardiac noradrenergic neurotransmission via AT2Rs and well-known AT1R-mediated stimulatory actions. The inhibitory mechanism may involve local bradykinin production, its B2 receptor activation, and NO as a downstream effector.     

Core skeletal muscle ryanodine receptor calcium release complex

The core skeletal muscle ryanodine receptor (RyR1) calcium release complex extends through three compartments of the muscle fibre, linking the extracellular environment through the cytoplasmic junctional gap to the lumen of the internal sarcoplasmic reticulum (SR) calcium store. The protein complex is essential for skeletal excitation‐contraction (EC)‐coupling and skeletal muscle function. Its importance is highlighted by perinatal death if any one of the EC‐coupling components are missing and by myopathies associated with mutation of any of the proteins. The proteins essential for EC‐coupling include the DHPR α_1S subunit in the transverse tubule membrane, the DHPR β_1a subunit in the cytosol and the RyR1 ion channel in the SR membrane. The other core proteins are triadin and junctin and calsequestrin, associated mainly with SR. These SR proteins are not essential for survival but exert structural and functional influences that modify the gain of EC‐coupling and maintain normal muscle function. This review summarises our current knowledge of the individual protein/protein interactions within the core complex and their overall contribution to EC‐coupling. We highlight significant areas that provide a continuing challenge for the field. Additional important components of the Ca²⁺ release complex, such as FKBP12, calmodulin, S100A1 and Stac3 are identified and reviewed elsewhere.     

Prejunctional GABA-B inhibition of cholinergic,neurally-mediated airway contractions in guinea-pigs

GABA is a known inhibitory neurotransmitter in the CNS. Recent studies have also demonstrated the presence of GABA in peripheral tissue, including lung. To delineate a role for GABA in lung, the effect of GABA and selective GABA agonists and antagonists on neuronally-induced airway contractions in guinea pigs were studied. In vitro, tracheal contractions induced by electrical field stimulation (EFS) were inhibited by tetrodotoxin and atropine indicating that the contractions were mediated by neuronal release of acetylcholine. The contractions caused by EFS, but not those by exogenous acetylcholine, were inhibited by GABA (EC₅₀ = 4.5 μM) and the selective GABA-B agonist baclofen (EC₅₀ = 9 μM), but not by the GABA-A agonist, muscimol. The inhibitory effect of baclofen was not affected by the GABA-A antagonist, bicuculline, but was significantly reversed with the GABA-B antagonists, 3-aminopropylphosphonic acid (3-APPA) (pA₂ = 4.5) and 2-hydroxysaclofen (pA₂ = 4.1).In vivo, vagal nerve stimulation (5 V, 20 Hz, 0.5 ms, 5 s) in anesthetized, mechanically ventilated guinea-pigs caused cholinergic-dependent bronchospasms that were inhibited by intravenous GABA (3 and 10 mg/kg) and baclofen (1–10 mg/kg), but not by muscimol. The inhibitory effects of GABA and baclofen against vagal bronchospasm were blocked by 3-APPA (5 mg/kg, iv), but not by bicuculline. Responses to the GABA-B agonists were unaltered after the treatment of animals with phentolamine or propranolol to block α-adrenergic and β-adrenergic receptors, respectively. Bronchospasm due to intravenous methacholine was also unchanged by GABA and baclofen. These results demonstrate that GABA inhibits cholinergic bronchoconstriction in guinea-pigs and does so by a mechanism involving inhibitory prejunctional GABA-B receptors.     

Adenylyl cyclase-dependent inhibition of myocardial norepinephrine release by presynaptic adenosine A1-receptors

Adenosine A1-receptor-mediated inhibition of exocytotic norepinephrine (NE) release from sympathetic nerve endings has been implicated as an endogenous cardioprotective mechanism. So far, the intraneuronal signal transduction underlying the adenosine A1-receptor-elicited inhibition of NE release is not known. In the present study, we determined in isolated Langendorff-perfused rat hearts the role of inhibitory G-proteins and of adenylyl cyclase (AC) on NE release after pharmacologic adenosine A1-receptor activation. NE release was induced by electrical field stimulation and was assessed in the coronary effluent by high-performance liquid chromatography. Adenosine A1-receptor activation with 2-chloro-N6-cyclopentyladenosine (CCPA) decreased NE release by approximately 50% in hearts from both untreated and pertussis toxin-pretreated rats. In hearts from untreated rats, suppression of NE release in response to CCPA was completely abolished by the cell-permeable AC inhibitor 9-(tetrahydro-2'-furyl)adenine (SQ 22536). Direct activation of AC with forskolin increased NE release by approximately 20%. In the presence of forskolin, stimulation of adenosine A1-receptors with CCPA or inhibition of AC with SQ 22536 decreased NE release to baseline. These findings suggest a Gi-protein-independent but AC-dependent inhibition of NE release following adenosine A1-receptor activation.     

Selective opioid antagonist effects on opioid-induced inhibition of release of norepinephrine in guinea pig cortex

Opioid agonists with selectivity for mu, delta and kappa-receptors have each been shown to inhibit the K+-stimulated release of [3H]norepinephrine (NE) from slices of guinea pig cortex maintained in vitro. In order to provide further evidence that each of these types of opioid receptor can regulate the release of NE in this tissue, experiments with receptor-type selective opioid antagonists have been conducted. In initial experiments, the selectivity of the antagonists for specific types of opioid receptors in the cortex of the guinea pig in an incubation medium of the same composition as that used for release studies was confirmed. The delta-receptor selective antagonist, ICI 174,864, prevented the inhibitory actions of the delta-selective agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE), but had little effect on the inhibitory actions of the mu-selective agonist, Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO), or the kappa-selective agonist, U-50,488H. In contrast, the kappa-selective antagonist, nor-binaltorphimine (nor-BNI) prevented the inhibitory actions of U-50,488H, but had little effect on the inhibitory actions of DPDPE or DAMGO. The greater potency of the partially mu-selective antagonist, naloxone, in reversing the effects of DAMGO relative to those of DPDPE or U-50,488H was confirmed. These results support the conclusion that mu- delta- and kappa-opioid receptors each exert a negative regulatory effect on the stimulated release of NE in the cortex of the guinea pig.     

The effect of stimulus intensity on alpha-adrenoceptor-mediated feedback control of noradrenaline release.

1 Mouse vas deferns stimulated transmurally (2.5 Hz, 2-32 V, 40-620 mA, FOR 45 s) responded with a twitch and a secondary contraction. Both responses were abolished by cinchocaine and were voltage-dependent. 2 In tissues previously incubated with (3H)-(--)-noradrenaline, stimulation also produced an increase in tritium overflow from the tissue. Phentolamine increased tritium overflow by 19% at high stimulus intensities (30 V, 600 mA) and by 130% at low stimulus intensities (11 V, 200 mA). 3 It is concluded that alpha-adrenoceptor-mediated feedback control of noradrenaline release is more marked at low stimulus intensities and that this is compatible with a role for calcium ions in this control mechanism.     

Prejunctional muscarinic receptor modulation of noradrenaline release from sympathetic neurones in rabbit aorta

The prejunctional muscarinic modulation of stimulation-evoked release of 3H-noradrenaline from sympathetic neurones in rabbit aorta was examined. The role of transmitter uptake, alpha-adrenoceptor blockade, stimulation frequency and endothelium on the modulation was investigated. Rings of aorta were incubated with (-)-3H-noradrenaline and subsequently subjected to electrical-field stimulation. Fractional 3H-overflow was determined by liquid scintillation counting. Acetylcholine (10(-8)-3 x 10(-6) M) added cumulatively, reduced the stimulation-evoked 3H-overflow up to 80%. The effect of acetylcholine was the same in intact and endothelium-free aorta. The inhibitory effect of acetylcholine was inversely related to the frequency of stimulation (1-10 Hz). The maximal inhibition (%) was 80 (1 Hz), 53 (3 Hz) and 14 (10 Hz). The inhibitory effect of acetylcholine (10(-6) M) and carbachol (10(-5) M) reached a maximum 15 min. after addition and then remained almost constant. Cocaine (3 x 10(-5) M) did not alter the effect of acetylcholine. Desipramine (10(-6) M) and corticosterone (4 x 10(-5) M) attenuated the inhibition seen with low concentrations (10(-8)-10(-7) M) of acetylcholine. The acetylcholine-induced inhibition was antagonized by desipramine. Cocaine plus corticosterone attenuated the inhibition seen with high concentrations (10(-6)-3 x 10(-6) M) of acetylcholine. Rauwolscine (10(-6) M) enhanced the maximal inhibitory effect of acetylcholine. We conclude that the inhibitory effect of acetylcholine on 3H-overflow from rabbit aorta preloaded with 3H-noradrenaline is (1) inversely related to stimulation frequency; (2) independent of endothelium; (3) unaffected by neuronal and extraneuronal transmitter uptake; (4) that cocaine is not a prejunctional muscarinic antagonist; (5) that cocaine, but not desipramine, is suited as a neuronal uptake inhibitor in studies of prejunctional muscarinic receptor subtypes; and (6) and that there is an inverse interaction between prejunctional alpha2-adrenoceptors and muscarinic receptors.     

Competitive inhibition of acetylcholinesterase by bretylium: possible mechanism for its induction of norepinephrine release

The antiarrhythmic drug bretylium tosylate competitively inhibits acetylcholinesterase activity. The Ki values for the inhibition of the purified enzyme (from electric eel), and acetylcholinesterase activity of crude rat ventricular and cortical homogenates were 6 X 10(-5), 3 X 10(-5), and 8 X 10(-5) M respectively. These values are close to the concentrations of the drug known to induce norepinephrine release from cardiac adrenergic presynaptic vesicles. It is suggested that inhibition of acetylcholinesterase activity by bretylium induces norepinephrine release through the effect of accumulated acetylcholine on nicotinic receptors in adrenergic nerve terminals.     

Prejunctional angiotensin receptors involved in the facilitation of noradrenaline release in mouse tissues.

The effect of angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) on the electrically induced release of noradrenaline was studied in preparations of mouse atria, spleen, hippocampus, occipito-parietal cortex and hypothalamus preincubated with [3H]-noradrenaline. The prejunctional angiotensin receptor type was investigated using the non-selective receptor antagonist saralasin (AT1/AT2) and the AT1 and AT2 selective receptor antagonists losartan and PD 123319, respectively. In atrial and splenic preparations, angiotensin II (0.01 nM-0.1 microM) and angiotensin III (0.01 and 0.1 nM-1 microM) increased the stimulation-induced overflow of tritium in a concentration-dependent manner. Angiotensin IV, only at high concentrations (1 and 10 pM), enhanced tritium overflow in the atria, while angiotensin-(1-7) (0.1 nM-10 microM) was without effect in both preparations. In preparations of hippocampus, occipito-parietal cortex and hypothalamus, none of the angiotensin peptides altered the evoked overflow of tritium. In atrial and splenic preparations, saralasin (0.1 microM) and losartan (0.1 and 1 microM), but not PD 123319 (0.1 microM), shifted the concentration-response curves of angiotensin II and angiotensin III to the right. In conclusion, in mouse atria and spleen, angiotensin II and angiotensin III facilitate the action potential induced release of noradrenaline via a prejunctional AT1 receptor. Only high concentrations of angiotensin IV are effective in the atria and angiotensin-(1-7) is without effect in both preparations. In mouse brain areas, angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) do not modulate the release of noradrenaline.     

Reversed response to histamine following local anaphylaxis in blood-perfused calf trachea in situ

Tracheas of calves were perfused via the tracheal artery in situ. An endotrcheal tube recorded tracheal muscle contractions. Carbachol, histamine or antigen caused tracheal vascular depressor responses. Carbachol caused contraction; histamine, 2-methylhistamine and antigen relaxed the tracheal muscle. Following antigen challenge, vascular responses remained unchanged; however endotracheal relaxations to histamine were reversed (i.e. contractions). Allergenic ‘challenge’ impairs H₂-receptors (airway relaxation), thus diminishing a compensatory mechanism which may relate to hyperresponsiveness characteristic of asthma.     

Effects of adenosine on adrenergic neurotransmission; Prejunctional inhibition and postjunctional enhancement

Summary The action of adenosine on adrenergic neuroeffector transmission was studied in the rabbit kidney in vitro and in situ, in the canine subcutaneous adipose tissue in situ and in the guinea-pig vas deferens in vitro. In the kidney, adenosine (0.1–10 M) caused a concentration-dependent increase in vascular resistance and in vasoconstrictor responses to nerve stimulation and administered noradrenaline. In the adipose tissue, adenosine also increased the vasoconstrictor responses but it decreased vascular resistance.In all three tissues studied adenosine significantly and reversibly depressed noradrenaline release evoked by nerve stimulation in a concentration-dependent manner. This effect of adenosine was not altered by phenoxybenzamine which blocked all vasoconstrictor responses and diminished the rise in vascular resistance by adenosine in the kidney. It is concluded that adenosine affects adrenergic neuroeffector transmission by two discrete mechanisms, prejunctional inhibition and postjunctional enhancement.