Atrial natriuretic factor inhibits adrenergic and purinergic neurotransmission in the rabbit isolated vas deferens

This study tests the hypothesis that atrial natriuretic factor (ANF) acts to inhibit neurotransmission in the rabbit vas deferens. The vas deferens is a unique model of autonomic neurotransmission in that it is composed of primarily nonvascular smooth muscle and has both a purinergic or twitch contraction and an adrenergic or phasic contraction associated with its response to electrical stimulation. In this study ANF was found to inhibit both adrenergic and purinergic neurotransmission in the rabbit vas deferens. ANF inhibited both the electrically induced phasic contraction and electrically induced norepinephrine release in a concentration-dependent manner over the ANF concentration range of 10(-10) to 10(-7) M. ANF at a concentration of 10(-7) M had no effect on norepinephrine-induced or ATP-induced contractions. Therefore, the neuromodulatory effect of ANF in the rabbit vas deferens appears to be prejunctional, on the release of the neurotransmitters norepinephrine and ATP from the nerve terminal and not postjunctional on the smooth muscle. Neither the alpha-2 antagonist rauwolscine nor the cyclooxygenase inhibitor indomethacin had any effect on the inhibitory effect of ANF on electrically induced twitch or phasic contractions. Additionally, ANF did not affect vasa deferentia prostaglandin E production. Therefore, the inhibitory neuromodulatory ANF effect is not mediated via alpha-2 adrenergic receptors or prostaglandin E production. The observed inhibitory neuromodulatory effects in this study may be involved in the hypotensive effects of ANF including natriuresis, diuresis and vasodilation.     

The contribution of cholinergic postganglionic neurotransmission to contractions of rabbit detrusor.

Field stimulation was used to elicit a contractile response in muscle strips from rabbit detrusor. The blockade of this response by tetrodotoxin (1 X 10(-7) M) ranged from 100% at 1 Hz to 86% at 40 Hz. At concentrations which produced strictly muscarinic antagonism (up to 4 X 10(-7) M) atropine depressed the frequency-response curve by about 42% at maximum but was much less effective at frequencies below 10 Hz. Similarly, treatment of the strips with hemicholinium-3 (5.2 X 10(-4) M) for 90 minutes in the presence of field stimulation at 60 Hz, depressed the frequency-response curve by 52% at maximum but produced less depression below 10 Hz. The hemicholinium-3-resistant response was neither depressed further by atropine (4 X 10(-7) M) nor potentiated by physostigmine (2 X 10(-6) M). Although hemicholinium-3 has antimuscarinic and anticholinesterase properties, these were found not to interfere with the tests for residual cholinergic transmission. Therefore, it was concluded that only part of the motor neurotransmission in rabbit detrusor is cholinergic. The remaining portion is predominant at frequencies below 10 Hz and is postulated to involve a chemical mediator other than acetylcholine.     

Mechanical trauma to bladder epithelium liberates prostanoids which modulate neurotransmission in rabbit detrusor muscle

Transitional epithelium of the rabbit urinary bladder has been implicated as a major site of prostanoid production and various studies have indicated that prostanoids have significant influences in muscle activity and neurotransmission in the bladder. We have examined the possibility that mechanical irritation of the epithelium could release diffusable substances which could influence neuromuscular function in the bladder. Epithelium was dissected from muscle strips of rabbit urinary bladder and the two components were incubated in separate chambers. Krebs' solution bathing epithelium was transferred to the bath in which the muscle was being field stimulated. Increases in the basal tension and spontaneous activity of the muscle as well as in the electrically evoked responses were observed after transfer and were related to the intensity of the irritation given the epithelium sample. The effects were mimicked by prostaglandins E1 E2, F2 alpha and I2 and the transfer effect was reduced significantly by pretreatment of the epithelium, but not the muscle, with indomethacin (10 microM) or ibuprofen (100 microM). Transfer of solution bathing-irritated epithelium also raised basal tension but not the maximum response to bethanechol. Finally, radioimmunoassay was used to demonstrate that irritation of epithelium samples caused the appearance of prostaglandin and 6-oxo-prostaglandin F1 alpha in the bathing medium and that this appearance was profoundly depressed by indomethacin (10 microM). It is plausible, therefore, that mechanical irritation of the bladder epithelium could result in changes in neuromuscular function in the underlying muscle layers and that these changes would be consistent with the symptoms associated with mechanical trauma of urothelium.     

Thromboxane receptor agonists enhance adrenergic neurotransmission in rabbit isolated mesenteric arteries

Two thromboxane receptor agonists, carbocyclic thromboxane A2 (CTA) and 9,11-dideoxy-11,9-epoxymethano prostaglandin F2 alpha (U46619), were examined for their influence on basal force and electrically induced force of isolated superior mesenteric arterial rings from rabbits. Both CTA and U46619 contracted the arterial rings and augmented contractile responses to electrical stimulation (4-20 Hz). The CTA and U46619 also augmented contractile responses to norepinephrine (P less than .05) and enhanced norepinephrine release in response to a 16 Hz electrical stimulation (P less than .05). The increases in basal force, neurogenic responses and norepinephrine release were prevented by the thromboxane receptor antagonist, (1S-[1-alpha,2-beta(5Z),3-beta, 4-alpha])-7-[3- [(2- [(phenylamino)carbonyl]hydrazino]methyl]-7- oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid. The U46619 was more potent than CTA and had a more consistent effect on norepinephrine release. These results are consistent with the hypothesis that thromboxane receptor agonists augment adrenergic neurogenic responses via thromboxane receptors. The mechanism of the enhanced neurogenic responses involved both pre- and postjunctional effects of the thromboxane receptor agonists.     

Pharmacological characterization of muscarinic receptors implicated in rabbit detrusor muscle contraction and activation of inositol phospholipid hydrolysis in rabbit detrusor and parotid gland

In the present study, we evaluated the pharmacological characteristics of the functional muscarinic receptors implicated in rabbit detrusor contraction and coupled to inositol phospholipid turnover in rabbit detrusor and parotid gland. The selectivity of several muscarinic antagonists for detrusor vs. salivary gland muscarinic receptors was also examined. The affinities for the muscarinic m1-, m2- and m3-receptor subtypes were determined using membranes from human cloned receptors expressed in CHO-K1 cells using [3H]-N-methyl scopolamine as a radioligand. Anti-muscarinic activity was determined in isolated rabbit detrusor by measuring the displacement of the contractile response to carbachol, and in rabbit detrusor and rabbit parotid by measuring the displacement of inositol phospholipid hydrolysis (total inositol phosphate accumulation) to carbachol. A significant correlation was found between the potencies to antagonize carbachol-induced rabbit detrusor contraction (pK(B)) and the affinities (pKi) for the m3-receptor subtype (r = 0.93, P = 5 x 10(-6)). Lower, but significant, correlations [0.88 (P = 6.3 x 10(-5)), 0.72 (P = 4.6 x 10(-3))] were obtained with m1- or m2-receptor subtypes, respectively. Each muscarinic antagonist tested displayed similar potency to antagonize carbachol-stimulated inositol phospholipid hydrolysis in rabbit detrusor and parotid (r = 0.96, P = 8 x 10(-3)). A significant correlation was found between the potencies to antagonize carbachol-stimulated inositol phospholipid hydrolysis (pK(B)), determined in rabbit detrusor and rabbit parotid, and the affinities (pK(i)) for the m3-receptor subtype [r = 0.96 (P = 0.01), 0.99 (P = 5 x 10(-5)), respectively] and for the m1-receptor subtype [r = 0.98 (P = 3.5 x 10(-3)), 0.94 (P = 0.02), respectively] but not for the m2-receptor subtype [r = 0.33, 0.57, ns, respectively]. In each in vitro assay, methoctramine (preferential M2 selective antagonist) and pirenzepine (preferential M1 selective antagonist) were slightly potent. We suggest that the muscarinic receptor implicated in the response to carbachol in rabbit detrusor and parotid gland corresponds to the M3-subtype. None of the muscarinic antagonists studied in rabbit tissues displayed preferential affinity for the detrusor.     

Norepinephrine-sensitive, phenoxybenzamine-resistant receptor sites associated with contraction in rabbit arterial but not venous smooth muscle: possible role in adrenergic neurotransmission

The possibility that not all contractions of rabbit blood vessels to norepinephrine (NE) are mediated through alpha adrenoceptors sensitive to phenoxybenzamine (PBZ) was investigated. Dose-response curves (DRCs) to NE were made in the absence and presence of PBZ pretreatment which minimized the contribution of alpha adrenoceptors. In all arteries studied (saphenous, renal, femoral and central ear arteries), after PBZ-treatment, NE produced biphasic DRCs. The initial component of these DRCs corresponded to doses of NE which in the absence of PBZ were supramaximal. Under conditions of our experimentation the plateau-phase usually occurred at between 5 and 40% of the pre-PBZ maximal response to NE. The second phase occurred with further additions of NE, and achieved a mean of 72 (+/- 4)% of the pre-PBZ maximal contraction to NE. The latter component presumably represented contractions mediated through low-affinity sites for NE which were insensitive to doses of PBZ sufficient to alkylate alpha adrenoceptors. In veins (saphenous and inferior vena cava), we found no evidence for such sites. Our results are discussed in light of current ideas of adrenergic neurotransmission in vascular smooth muscle as proposed by Hirst and Neild (1980a) and others who suggest that response to high concentrations of neuronally released NE occur through PBZ-resistant receptors termed gamma adrenoceptors located exclusively at the postsynaptic membrane. We were able to demonstrate PBZ-resistant, low-affinity sites for NE contractions in the femoral artery, a vessel with very sparse adrenergic innervation, and conclude that such sites for NE are present in a number of arteries (and not veins) irrespective of their innervation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adenosine receptor antagonism attenuates angiotensin II effects on adrenergic neurotransmission in the rabbit isolated vas deferens

Angiotensin II augments adrenergic neurotransmission in the rabbit isolated vas deferens and suppresses purinergic neurotransmission. This study tests the hypothesis that angiotensin II augments adrenergic neurotransmission by depressing the neuronal release of ATP, resulting in suppressed formation of the inhibitory neuromodulator, adenosine or a related purine. Exogenous ATP added to the vasa deferentia increased adenosine formation and depressed adrenergic neurotransmission thus providing indirect support for the hypothesis. The adenosine receptor antagonist, 8-(sulfophenyl)theophylline (10 and 100 microM) depressed responses to exogenous adenosine and ATP but did not alter contractile responses to nerve stimulation or exogenously administered norepinephrine thus indicating that endogenous adenosine had no basal influence upon neurotransmission. However, the 8-(sulfophenyl)theophylline reduced angiotensin II effects on both adrenergic neurogenic contractions and evoked norepinephrine release. Additionally, the augmentation of adrenergic neurogenic contractions by angiotensin II was enhanced in the presence of ATP. These results are consistent with an ATP involvement in angiotensin effects on adrenergic neurotransmission and contrary to the initial hypothesis, suggest that purines enhance adrenergic neurotransmission in the presence of angiotensin II.     

Correlation between phosphatidylinositol labeling and contraction in rabbit aorta: effect of alpha-1 adrenergic activation

Activation of rabbit aortic strips with alpha adrenergic agonists increased the labeling (with [32P]Pi) of phosphatidylinositol (PI) and phosphatidic acid and contracted the vascular preparations in dose-related fashion. Epinephrine, norepinephrine and methoxamine produced maximal effects, whereas clonidine behaved as partial agonist and B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazole-[5,4-d] azepin dihydrochloride) was almost without activity in the two experimental models used. Phenylephrine was a full agonist in producing contraction, but failed to elicit the maximal increase in PI labeling. The EC50 values to produce contraction of aortic strips were lower for all agonists than those required to increase the incorporation of radioactive phosphate into PI, but there was a good correlation between the two sets of data. The increased PI labeling and contraction of aortic strips induced by epinephrine were antagonized by prazosin and yohimbine in dose-related fashion, but the first alpha blocker was about three orders of magnitude more potent than the second in antagonizing the two effects. The present results indicate that both stimulation of PI labeling and contraction are mediated through activation of alpha-1 adrenoceptors in rabbit aorta.     

Modulation of peripheral adrenergic neurotransmission by methionine-enkephalin

The isolated perfusion cat spleen prelabeled with [3H]norepinephrine was used to study the effects of morphine and Met-enkephalin on the exocytotic release of norepinephrine from sympathetic adrenergic neurons after nerve stimulation. The overflow of endogenous norepinephrine, total 3H and dopamine-beta-hydroxylase (DBH) from cat spleens was measured during postganglionic stimulation of the splenic nerve. Perfusion of spleens with Met-enkephalin (10(-8)-10(-5) M) produced a dose-dependent decrease in the release of endogenous norepinephrine upon nerve stimulation. These changes were paralleled by significant dose-dependent Met-enkephalin-induced decreases in the nerve stimulation-mediated release of total 3H, DBH and in the perfusion pressure. However, perfusion of spleens with morphine (10(-7)-10(-4) M) produced no significant changes in the release of endogenous norepinephrine, total 3H or DBH after nerve stimulation at 5 Hz. Morphine (10(-7)-10(-4) M) also had no significant effects on the contraction of the splenic capsule. Perfusion of spleens with naloxone (10(-6) M), a pure narcotic antagonist, did not alter the release of endogenous norepinephrine, total 3H, DBH or perfusion pressure. However, perfusion with naloxone (10(-6) M) plus Met-enkephalin (10(-6)-10(-5) M) antagonized the inhibitory effects of Met-enkephalin. These findings support the hypothesis that the opiate receptor population in peripheral tissues are heterogenous and that Met-enkephalin depresses exocytotic release of norepinephrine by interacting with a specific presynaptic opiate receptor.     

Role of calcitonin gene-related peptide-containing nerves in the vascular adrenergic neurotransmission.

The role of calcitonin gene-related peptide (CGRP)-containing nerves in adrenergic vasoconstrictor response to periarterial nerve stimulation (PNS) was investigated in mesenteric vascular beds isolated from rats, which were perfused with Krebs' solution (5 ml/min). In perfused mesenteric vascular beds, PNS (1-12 Hz, for 30 sec) caused a frequency-dependent increase in the perfusion pressure, which was prevented by 50 nM prazosin and abolished by 300 nM tetrodotoxin and 5 microM guanethidine. Bolus infusion of norepinephrine (NE, 0.5 and 1 nmol) also produced a pressor response which was blocked by 50 nM prazosin. In the preparation treated with 500 nM capsaicin for 20 min, pressor responses to PNS of 1 to 8 Hz were potentiated significantly. The pressor responses to NE infusion also were enhanced in both the preparations treated with and without capsaicin. In the capsaicin-untreated preparation contracted by 7 mM methoxamine in the presence of 5 microM guanethidine, PNS (1-12 Hz) caused a frequency-dependent vasodilation, which was abolished by 300 nM tetrodotoxin. However, no vasodilator response to PNS was observed in the preparation treated with capsaicin. In the contracted preparation, bolus infusion of rat CGRP (10 and 100 pmol) produced a marked long-lasting vasodilation which mimicked the PNS-evoked vasodilation, whereas neither bolus infusion of substance P (1 and 10 nmol), neurokinin A (1 and 10 nmol) nor neurokinin B (1 and 10 nmol) produced relaxation. In the preparation labeled with [3H]NE, the PNS (4 Hz)-evoked 3H release was not altered after capsaicin treatment, whereas the pressor response to PNS was potentiated significantly.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of adrenergic neurotransmission in canine tibial artery after exposure to 5-hydroxytryptamine in vitro

Blood vessels may be exposed to 5-hydroxytryptamine when platelets aggregate and release vasoactive substances at sites of damage or disease. The functional consequences were studied of exposing the dog tibial artery for 2 hr to 5-hydroxytryptamine (10(-6) M) in vitro. During the exposure, unmetabolized 5-hydroxytryptamine was accumulated by the cocaine-sensitive amine uptake mechanism of tibial artery adrenergic nerves. After exposure to [3H]-5-hydroxytryptamine, transmural electrical stimulation caused the release of the tritiated indoleamine which was blocked by tetrodotoxin. After a 1-hr washout of rings of tibial artery exposed previously to 5-hydroxytryptamine, contractions in response to transmural electrical stimulation were depressed, whereas the response to exogenously added norepinephrine was unaffected. That the decreased response to electrical stimulation after exposure to 5-hydroxytryptamine was due to decreased release of norepinephrine from adrenergic nerves was demonstrated in strips of the artery preincubated in [3H]norepinephrine. The inhibition of [3H]norepinephrine release after exposure to 5-hydroxytryptamine, was blocked by the serotonergic antagonist, methiothepin, but not by the alpha adrenergic antagonist, phentolamine, suggesting that serotonergic receptors mediate the inhibition. The inhibition of [3H]norepinephrine release also was prevented by blocking adrenergic neuronal uptake with cocaine before exposure to 5-hydroxytryptamine. These results suggest that 5-hydroxytryptamine is accumulated and released by tibial artery adrenergic nerves as a cotransmitter. In so doing, the indoleamine inhibits adrenergic neurotransmission in the tibial artery by its action at prejunctional serotonergic receptors.     

Purinergic inhibition of adrenergic transmission in rabbit blood vessels.

The effect of ATP and its congeners on the adrenergic neuroeffector transmission was evaluated in isolated blood vessels of the rabbit. ATP, ADP, AMP and adenosine inhibited the contractile response of the portal vein to adrenergic nerve stimulation, with a threshold concentration of the order of 0.1 muM and ED50 of about 1 microM. These agents, but not papaverine, inhibited the nerve stimulation-induced response in preference to the norepinephrine- or serotonin-induced response in the portal and saphenous veins and pulmonary and ear arteries. In the portal vein labeled with [3H]norepinephrine, ATP diminished the nerve stimulation-induced efflux of tritiated material. This nucleotide also reduced the KCl-induced tritium efflux but not the tyramine induced-efflux in the [3H]norepinephrine-labeled thoracic aorta. ATP had no significant effect on the uptake of [3H]norepinephrine in the portal vein, ear artery and thoracic aorta. Indomethacin and theophylline partially blocked the inhibitory action of ATP on the neurogenic constrictor response in some of the ear artery and saphenous vein preparations. Desipramine, atropine, propanolol, haloperidol and 2,2'-pyridylisatogen, a blocking agent against ATP in the taenia coli, were without such antagonistic effect. The results are consistent with a proposed negative feedback modulator role of ATP or a related purine compound in adrenergic transmission.     

Low-level N-methyl-D-aspartate receptor activation provides a purinergic inhibitory threshold against further N-methyl-D-aspartate-mediated neurotransmission in the cortex.

N-methyl-D-aspartate (NMDA) is 33 times more potent at releasing adenosine than it is at releasing [3H]norepinephrine from slices of rat parietal cortex. Consequently, maximal adenosine release occurs at levels of NMDA receptor activation which release little norepinephrine. The potential modulatory role of the adenosine released during NMDA receptor activation on NMDA-evoked [3H]norepinephrine release was investigated. The A1-selective agonist R-(-)N6-(2-phenylisopropyl)adenosine (10 microM) decreased 100 microM NMDA-evoked [3H]norepinephrine release by 27%; this was reversed by the P1 antagonist 8-phenyltheophylline (8-PT, 10 microM), indicating that NMDA-evoked norepinephrine release from cortical slices is susceptible to purinergic modulation. On the other hand, 8-PT had no effect on [3H]norepinephrine release evoked by 100 microM NMDA, suggesting that endogenous adenosine, released during NMDA receptor activation, does not modulate [3H]norepinephrine release. However, [3H]norepinephrine release precedes adenosine release, so that the released adenosine may not be temporally available to modulate [3H]norepinephrine release. Pretreatment with a concentration of NMDA (10 microM) which releases substantial endogenous adenosine, but very little [3H]norepinephrine decreased subsequent 100 microM NMDA-evoked [3H]norepinephrine release by 52%. 8-PT partially reversed this inhibition, indicating that prereleased adenosine, acting at P1 purinoceptors, modulated subsequent NMDA-evoked [3H]norepinephrine release. These results suggest that adenosine, released during submaximal NMDA receptor activation, may provide an inhibitory threshold which must be overcome in order for other NMDA-mediated processes to proceed maximally.     

Effects of purinergic and adrenergic antagonists in a rat model of painful peripheral neuropathy

In previous studies, pain behaviors produced in the spinal nerve ligation rat model of neuropathic pain were partly reduced by surgical lumbar sympathectomy. However, systemic injection of phentolamine, an alpha-adrenoceptor blocker, was not effective in reducing pain behaviors, at least in the Sprague-Dawley strain of rats. This suggests that sympathectomy removes not only adrenoceptor function but also other factors that must contribute importantly to the generation of neuropathic pain behaviors. Since the purinergic substance adenosine 5'-triphosphate (ATP) is known to be co-released with norepinephrine (NE) from the sympathetic nerve terminals, we hypothesized that ATP might be involved in the sympathetic dependency of neuropathic pain. The present study tested this hypothesis by examining the effects of systemic injection of an adrenoceptor blocker (phentolamine), a purinoceptor blocker (suramin), and a combination of these two on behavioral signs of mechanical allodynia in the spinal nerve ligation model of neuropathic pain. The results of the present study showed two novel findings. First, the mechanical hypersensitivity (allodynia) resulting from the L5/6 spinal nerve ligation can be reduced either by sympathetic block accomplished by application of a local anesthetic or by surgical sympathectomy of the L2-L6 sympathetic ganglia. Second, suramin (at 100 mg/kg, i.p.) can reduce mechanical hypersensitivity in neuropathic rats when given in combination with 5 mg/kg of phentolamine. This effect was observed in a subset of neuropathic rats, and the drug responses were consistent in repeated treatments within the animal group. Neither phentolamine nor suramin changed the mechanical sensitivity of neuropathic rats when given alone. The data suggest that the purinergic substance ATP is co-released with NE from sympathetic nerve terminals and these two are together involved, at least in part, in the maintenance of the sympathetically dependent component of pain behaviors in some neuropathic rats.     

Differential effects of the isomers of tetramisole on adrenergic neurotransmission in cutaneous veins of dog.

Clinical observations indicate that dexamisole and levamisole, the isomers of tetramisole, cause mood elevation. Their effects on smooth muscle cells and adrenergic nerves were investigated in strips of dogs' saphenous veins. Dexamisole (2.5 X 10(-6) to 4 X 10(-5) M) augmented the contractile response to norepinephrine but depressed that to tyramine; cocaine inhibited the augmentation of the norepinephrine response. Levamisole (10(-5) M) did not alter the response to norepinephrine, but augmented that to tyramine. At 1.6 X 10(-4) M dexamisole, more than levamisole, depressed the responses to norepinephrine, tyramine and acetylcholine. Activation by K+ ions was not affected by the isomers. Preparations, incubated with 3H-norepinephrine, were mounted for superfusion, tension recording and determination of 3H-norepinephrine and metabolites in the superfusate. Dexamisole and levamisole augmented the 3H-norepinephrine overflow during nerve stimulation; levamisole decreased the efflux of deaminated metabolites. During tyramine-induced contractions, dexamisole depressed and levamisole augmented the efflux of 3H-norepinephrine; they reduced the appearance of metabolites. The increases in 3H-norepinephrine caused by the isomers during nerve stimulation were not seen after phenoxybenzamine. Dexamisole, more than levamisole, inhibited tissular uptake of 3H-norepinephrine. Levamisole, more than dexamisole, inhibited monoamine oxidase activity in vein homogenates. These interferences with release and disposition of norepinephrine may be related to the antidepressant properties of the tetramisole isomers.     

Specific enhancement of norepinephrine-induced contraction in rat veins after beta adrenergic antagonists.

Propranolol markedly increased the norepinephrine-induced maximal force in circular smooth muscle of the rat portal, mesenteric, renal and, to a lesser extent, femoral veins without affecting aortic or mesenteric artery responses to norepinephrine. Furthermore, two other beta receptor antagonists, practolol and N-isopropylmethoxamine, specifically enhanced maximal venous responses to norepinephrine. Contractions to norepinephrine, but not to serotonin, were increased by propranolol only in veins, even after the vasodilator, papaverine. The ability of propranolol to enhance norepinephrine-induced contraction in these rat veins paralleled the effectiveness of isoproterenol to relax such tissues. In addition, beta receptor antagonists enhanced the response of veins to the field stimulated release of norepinephrine from sympathetic nerves. These data support the conclusion that beta adrenergic stimulation modulates norepinephrine-induced constriction in certain rat veins but not in the aorta or mesenteric artery.