Rat Anococcygeus Muscle Research Articles

Prejunctional action of the Indian red scorpion venom on adrenergic transmission in the rat anococcygeus muscle: M. C. E. Gwee, 1L. S. Cheah, 1P. Gopalakrishnakone, 2P. T. H. Wong1 and K. S. Y. Low1 (Venom and Toxin Research Group, Departments of 1Pharmacology, and 2Anatomy, National University of Singapore, Kent Ridge, Singapore 0511)

Prejunctional action of the Indian red scorpion venom on adrenergic transmission in the rat anococcygeus muscle: M. C. E. Gwee, 1L. S. Cheah, 1P. Gopalakrishnakone, 2P. T. H. Wong1 and K. S. Y. Low1 (Venom and Toxin Research Group, Departments of 1Pharmacology, and 2Anatomy, National University of Singapore, Kent Ridge, Singapore 0511)

Effects of ethanol and other aliphatic alcohols on NO-mediated relaxations in rat anococcygeus muscles and gastric fundus strips.

1. In anococcygeus muscles, ethanol (20-500 mM) slightly increased the tone and inhibited relaxations elicited by nitrergic nerve stimulation (0.5-5 Hz) in a concentration-dependent manner. 2. Other aliphatic alcohols decreased the tone but had inhibitory effects similar to ethanol on stimulation-induced relaxations, the EC50 (mM) values being: methanol 280, ethanol 80, propan-1-ol 20, propan-2-ol 55, propan 1,2-diol 135, butan-1-ol 120, butan-2-ol 15 and pentan-1-ol 3. 3. Relaxations induced by sodium nitroprusside (SNP, 10 nM) were inhibited by ethanol (20-500 mM) in a concentration-dependent manner and by propan-2-ol (100 mM). Relaxations induced by NO (1 microM) were inhibited by high concentrations of ethanol (200-300 mM) and by propan-2-ol (100 mM). 4. In gastric fundus strips, ethanol (60-200 mM) did not affect the resting tone but inhibited NO-mediated relaxations elicited by low frequency (1 Hz) field stimulation and reduced the initial relaxation by high frequency field stimulation (10 Hz) and by SNP (50 nM). The relaxant action of isoprenaline (10 nM) was not reduced although it was slightly slower in onset. Other aliphatic alcohols tested decreased the tone and inhibited relaxations elicited by field stimulation. 5. Acetaldehyde (1-10 mM) inhibited relaxations elicited by field stimulation and SNP in both the rat anococcygeus muscles and gastric fundus strips. The tone of gastric fundus strips was decreased by acetaldehyde but it was transiently increased in anococcygeus muscles. 6. The dehydrogenase inhibitors, 4-methylpyrazole (100 MicroM) and disulfiram (100 MicroM) did not significantly affect the inhibition by ethanol of nitrergic nerve stimulation-induced relaxations in anococcygeus muscles, which suggests that ethanol acts directly.7. The inhibition of NO-mediated relaxations by alcohols is attributed to sequestration of NO to form nitroso-alcohols.

Inhibition by ethacrynic acid of NO-mediated relaxations of the rat anococcygeus muscle.

1. The effects of ethacrynic acid were studied on relaxations elicited by nitric oxide (NO), the NO-donors sodium nitroprusside (SNP) and glyceryl trinitrate (GTN), nitrergic nerve stimulation and the NO-independent agent papaverine in isolated preparations of rat anococcygeus muscles. 2. Ethacrynic acid (100 mumol/L) produced complete relaxation of partially contracted anococcygeus muscles, but the tone recovered after the ethacrynic acid was washed out. Following exposure to ethacrynic acid, the relaxant responses to NO, SNP, GTN and nitrergic nerve stimulation were abolished or markedly reduced; however, the response to papaverine was only slightly reduced. 3. The presence of 3 mmol/L L-cysteine during the period of exposure to ethacrynic acid prevented the inhibition of the relaxing effects of SNP, GTN and nitrergic nerve stimulation almost completely, but did not affect the slight reduction in responses to papaverine. 4. The addition of L-cysteine (3 mmol/L) after incubation with ethacrynic acid did not significantly affect the inhibited responses to SNP and GTN; however, the inhibited responses to nitrergic nerve stimulation were slightly but significantly increased. 5. The results suggest that endogenous sulphydryl groups are required for the actions of NO, NO-donating drugs and the nitrergic transmitter in the rat anococcygeus muscle and possibly for the synthesis or release of the nitrergic transmitter.

Nitric oxide synthase inhibitors 7- and 6-nitroindazole relax smooth muscle in vitro

7-Nitroindazole induced concentration-dependent relaxation of precontracted rabbit aorta, dog middle cerebral artery, rat anococcygeus muscle and rat stomach fundus. Relaxations to 7-nitroindazole in rabbit aorta were unaffected by nitric oxide synthase blockade or endothelial removal. 6-Nitroindazole also caused concentration-dependent relaxation in dog middle cerebral artery and rabbit aorta, being equipotent with 7-nitroindazole in both tissues. These data suggest that indazole derivatives can induce an endothelium- and nitric oxide synthase-independent relaxation of smooth muscle in vitro.

Nitric oxide-mediated neurotransmission is attenuated in the anococcygeus muscle from diabetic rats.

The effect of STZ-induced diabetes of 8-weeks duration was examined on nitric oxide-mediated neurotransmission in the rat anococcygeus muscle. In the presence of noradrenergic blockade and raised tissue tone, relaxant response to nerve stimulation (0.5-5 Hz, for 10 s), sodium nitroprusside (5 and 10 nmol/l) and nitric oxide (1 and 3 mumol/l) were significantly reduced in anococcygeus muscles from diabetic rats compared to responses from control rats (p < 0.05). In contrast, relaxations to papaverine (3 and 10 mumol/l) were not reduced in tissues from diabetic rats. The nitric oxide synthesis inhibitor NOLA (100 mumol/l) abolished relaxant responses to nerve stimulation but had no effect on responses to any of the relaxant agents used. Exposure to NOLA at 10 mumol/l reduced stimulation-induced relaxations; this reduction was significantly greater in tissues from the diabetic group than from the control group (p < 0.05), probably as a consequence of the smaller relaxant responses in muscles from diabetic rats. Contractile responses to nerve stimulation (1-10 Hz, for 10 s), but not noradrenaline (0.03-30 mumol/l), were significantly greater in anococcygeus muscles from diabetic rats than from control rats (p < 0.05). NOLA (100 mumol/l) significantly enhanced stimulation-induced contractions (p < 0.05), however the enhancement was significantly less in tissues from diabetic rats (p < 0.05). The results suggest that STZ-induced diabetes impairs smooth muscle reactivity to nitric oxide in the rat anococcygeus muscle.

Prejunctional action of the venom from the Indian red scorpion Mesobuthus tamulus on adrenergic transmission in vitro

Prejunctional action of the venom from the Indian red scorpion Mesobuthus tamulus on adrenergic transmission in vitro. Toxicon 32, 201–209, 1994.—Venom (RSV) from the Indian red scorpion, Mesobuthus tamulus ( Buthus tamulus), can cause increased peripheral sympathetic activity with consequent enhancement of adrenergic responses. We have therefore investigated the effects of RSV on adrenergic transmission in the rat isolated anococcygeus muscle. The effects of phentolamine (5 μM), tetrodotoxin (2 μM), guanethidine (5 μM), desipramine (1 μM) and reserpine pretreatment in vivo (5 mg/kg s.c. × 24 hr and 5 mg/kg i.p. × 3 hr) on the contractile responses of the rat anococcygeus muscle to RSV (1.5 μg/ml), field stimulation, noradrenaline (NA, 1 μM or 3 μM) and tyramine (15 μM) were compared. The contractile responses to RSV and to field stimulation were completely blocked by phentolamine, tetrodotoxin, guanethidine and reserpine pretreatment, but the responses were potentiated by desipramine. The contractile responses to tyramine were completely blocked by phentolamine, reserpine pretreatment as well as desipramine. The responses to NA were completely blocked by phentolamine, but were potentiated by gunethidine, desipramine and reserpine. Relatively low concentrations (0.1 μg/ml × 4) of RSV which did not produce any observable increase in tone of the anococcygeus muscle, potentiated the contractile responses of the anococcygeus muscle to field stimulation, but not the responses to exogenous NA; 4-aminopyridine (25 μM × 2) also potentiated the muscle responses to field stimulation. HPLC measurements revealed only very low concentrations (0.10 ± 0.03 μmol/g venom) of NA in RSV. Thus, the adrenergic agonist action of RSV in the rat isolated anococcygeus muscle can be attributed to the involvement of some prejunctional mechanism(s) of action that stimulates the release of neurotransmitter which differs from the indirect action mediated by tyramine.

Analysis of bradykinin-induced relaxations in the rat isolated anococcygeus muscle

The present study investigates the mechanism(s) of action of relaxations induced by bradykinin and by electrical field stimulation (EFS) in isolated rat anococcygeus muscle, where contractile tone has been elevated with clonidine. Bradykinin, EFS, and the bradykinin B 1 receptor agonist, des-Arg 9-bradykinin, produced quantitatively and qualitatively similar relaxations. Bradykinin B 1 receptor antagonist, [des-Arg 9, Leu 8]-bradykinin (1 μM), attenuated the relaxation responses of bradykinin B 1 receptor agonist and inhibited bradykinin and EFS-induced relaxation responses. Bradykinin B 2 receptor antagonist, [β-(2-thienyl)-Ala 5,8, D-Phe]-bradykinin (1 μM), significantly inhibited the relaxation responses of bradykinin, EFS, and bradykinin B 1 receptor agonist. Methylene blue (30 μM) and N-methylhydroxylamine (1 mM) significantly inhibited the bradykinin- and EFS-induced relaxation responses. The relaxation responses of bradykinin and EFS were not affected by captopril (5 μM), superoxide dismutase (100 U/ml), and catalase (100 U/ml). Nitric oxide synthase inhibitor, L-N G-nitro-arginine (L-NOARG, 30 μM), significantly inhibited the EFS- and bradykinin-induced relaxation responses. L-arginine (100 μM) reversed the inhibitory effect of L-NOARG on the relaxation responses of EFS and bradykinin. In addition, L-arginine potentiated the relaxation responses of EFS and bradykinin. The data of the present study suggests that bradykinin, similar to EFS, generates an endogenous nitrate, probably nitric oxide, which subsequently activates guanylate cyclase and relaxes the rat anococcygeus muscle.

Modulation of acetylcholine-induced contractions of the rat anococcygeus muscle by activation of nitrergic nerves.

1. Acetylcholine-induced contractions of the rat isolated anococcygeus muscle were blocked by atropine (0.1 microM), slightly enhanced by hexamethonium (0.1 mM) and tetrodotoxin (1 microM), but little affected by prazosin (0.1 microM). 2. In the presence of the alpha 2-adrenoceptor agonist, UK14304, which raised the tone of the muscle, acetylcholine had a biphasic effect consisting of an initial relaxation followed by a contraction. 3. Atropine (0.1 microM) enhanced the relaxant component and abolished the contractile component of the response, whereas tetrodotoxin, omega-conotoxin GVIA or hexamethonium abolished or greatly reduced the relaxant component. 4. The nitric oxide synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) increased acetylcholine-induced contractions in the absence of UK14304 and markedly reduced the relaxant component to acetylcholine in the presence of UK14304. The effects of L-NAME were annulled by L-arginine (300 microM). 5. The results suggest that acetylcholine acts concurrently on muscarinic receptors of the smooth muscle to cause contraction and nicotinic receptors of nitrergic nerves to cause relaxation. The observed response is the resultant of these two opposing effects and depends also on the prevailing tone.

Differential effects of nifedipine on nerve-mediated and noradrenaline-evoked contractions of rat anococcygeus muscle

In rat anococcygeus muscle the inhibitory effect of nifedipine (0.01, 0.1, 1.0 and 10 μM) was determined on adrenergic twitches in response to electrical field stimulation (trains of 4 pulses, 0.1 ms pulse duration, 10 Hz) and on twitch-matching contractions evoked by noradrenaline. Nifedipine concentration-dependently reduced the neurogenic twitch with an IC 50 of 0.083 μM. Nifedipine reduced the noradrenaline-evoked contraction to a markedly lesser degree (IC 50 > 10 μM). The difference in the magnitude of inhibition of electrically evoked twitch and twitch-matching noradrenaline-evoked contraction was statistically significant at every concentration of nifedipine. It is concluded that inhibition of the twitch by nifedipine involves some other mechanism(s) in addition to its Ca 2+ channel blocking property in smooth muscle.

Methamphetamine antagonistic property of (+)- and (−)-4-phenyltetrahydroisoquinoline in rat anococcygeus muscle

The antagonistic effects of (+)- and (−)-4-phenyl-1,2,3,4-tetrahydroisoquinoline (4PTIQ) on methamphetamine in the rat anococcygeus muscle were compared with those of cocaine and nomifensine. Methamphetamine contracted the anococcygeus muscle through the release of norepinephrine from noradrenergic nerve terminals. (+)-4PTIQ inhibited the methamphetamine-induced contractions more strongly than cocaine and nomifensine. (+)-4PTIQ had no potentiating effects on exogenous norepinephrine-induced contraction, which was considered to be an index of amine neuronal uptake blockade. On the other hand, (−)-4PTIQ, cocaine and nomifensine produced a significant leftward shift of the norepinephrine concentration-response curve, i.e. they showed a strong blocking effect on amine neuronal uptake. These results suggest that the inhibitory effects of (+)-4PTIQ on the action of methamphetamine are mediated by a mechanism other than inhibition of amine neuronal uptake.

EFFECTS OF HYDROXOCOBALAMIN AND HAEMOGLOBIN ON NO‐MEDIATED RELAXATIONS IN THE RAT ANOCOCCYGEUS MUSCLE

1. The effects of hydroxocobalamin (Vitamin B12a) on relaxations produced by nitric oxide (NO), some NO-donating compounds and nitrergic nerve stimulation in isolated preparations of the rat anococcygeus muscle were compared with the effects of haemoglobin. 2. Hydroxocobalamin (30 mumol/L) significantly reduced relaxations induced by NO (0.1-3 mumol/L) and sodium nitroprusside (SNP; 0.01-0.3 mumol/L) but did not affect relaxations induced by glyceryl trinitrate (GTN; 0.01-1 mumol/L), S-nitrosocysteine (0.1-0.3 mumol/L) or stimulation of nitrergic nerves. A higher concentration of hydroxocobalamin (100 mumol/L) slightly reduced nitrergic nerve stimulation-induced relaxations. 3. Haemoglobin (10 mumol/L) blocked relaxation induced by NO and reduced relaxations induced by SNP, GTN, S-nitrosocysteine and nitrergic nerve stimulation. 4. When nitrergic nerve stimulation-induced relaxations had been partially reduced by the NO synthase inhibitor L-NAME (5-10 mumol/L), hydroxocobalamin had only a weak and transient inhibitory effect. 5. Noradrenergic contractions induced by field stimulation were not affected by hydroxocobalamin (30 mumol/L), but were enhanced by haemoglobin (10 mumol/L). 6. The results suggest that the transmitter released from nitrergic nerves in anococcygeus muscles resembles NO-releasing compounds such as S-nitrosocysteine and GTN but not SNP or free NO.

NADPH-diaphorase reactivity in nerves supplying the rat anococcygeus muscle

NADPH-diaphorase histochemistry was used to establish the nature of neurons and nerve fibers in the rat anococcygeus muscle. NADPH-diaphorase-reactive nerve cell bodies were found in ganglia in the caudal dorsal fascia of the muscle and within the muscle itself. The reactive nerve cells had a relatively smooth surface and a single axon. They formed bundles of positive nerve fibers which entered the muscle and branched further to form a network extending towards the cranial end. In addition, bundles of NADPH-diaphorase-reactive fibers of extrinsic origin joined the loose plexus of local bundles and microganglia. The NADPH-diaphorase-reactive neurons supplying the rat anococcygeus muscle provide a neuroanatomical basis for the non-adrenergic non-cholinergic relaxation of the muscle mediated by nitric oxide.

Dual action of ranitidine at cholinergic sites in the rat isolated anococcygeus muscle.

1. The effects of ranitidine on contractile responses of the rat isolated anococcygeus muscle to acetylcholine (ACh) and carbachol (CCh) were investigated. 2. Four consecutive cumulative concentration-response curves (CRCs) for ACh or CCh were constructed in the absence or presence of ranitidine. The mean EC50 values were determined from the individual CRCs and used for the calculation of the corresponding EC50 ratios. 3. Ranitidine (0.01, 0.05 and 0.5 mM) produced a progressively leftward shift of the ACh curves; the corresponding EC50 ratios obtained were 3.2 +/- 0.4, 5.7 +/- 0.5 and 9.4 +/- 1.4. Neostigmine 0.5 microM alone or in combination with ranitidine 0.1 mM produced very marked leftward shifts of the ACh curves (EC50 ratios 68.2 +/- 14.3 and 56.3 +/- 9.6 respectively). In contrast, ranitidine (0.05, 0.5 and 2 mM) produced a rightward and parallel shift of the CCh curves (EC50 ratios 1.0 +/- 0.1, 3.4 +/- 0.32 and 12.5 +/- 1.7 respectively). 4. Ranitidine can therefore mediate a dual action at cholinergic sites in the rat isolated anococcygeus muscle, actions which can be attributed to its inherent anticholinesterase and antimuscarinic activity. The relatively marked anticholinesterase action of ranitidine was rather surprising in view of the limited cholinergic innervation present in the rat anococcygeus muscle.

Epithelium-dependent responses of serotonin in a co-axial bioassay system

Serotonin (10 −6–10 −4 M) produced relaxations in a concentration-dependent (at 10 −6 and 10 −5 M concentrations) manner followed by a contraction (at 10 −4 M concentration) in a co-axial systems, which consisted of guinea-pig trachea as a donor organ for epithelial derived-relaxing factor(s) and phenylephrine-precontracted rat anococcygeus muscle as assay tissue. Serotonin produced a concentration-dependent contraction only in precontracted rat anococcygeus muscle mounted alone or mounted co-axially within epithelium-denuded trachea. Indomethacin (10 −6 M) significantly inhibited the initial relaxations (from 25.1 ± 7.8 to 7.8 ± 5.0% and from 35.6 ± 8.7 to 10.4 ± 8.3% at 10 −6and 10 −5M concentrations of serotonin), but did not affect the contraction. Imipramine (10 −8 M) and hydrocortisone (3 × 10 −5 M) reduced the initial relaxations (from 20.5 ± 1.6 to 3.8 ± 1.5% and from 32.1 ± 6.4 to 18.9 ± 3.9% at 10 −6M and 10 −5M concentrations of serotonin, respectively) and also converted the serotonin (10 −4 M)-induced contraction to a relaxation. In the co-axial system with trachea from guinea-pigs previously sensitized with i.p. injected egg-ovalbumin, the serotonin-induced biphasic response was converted to a contractile response only after ovalbumin challenge. Histopathologic changes were observed in the epithelium of challenged tracheas taken from sensitized guinea-pigs and alterations of serotonin-induced epithelium-dependent responses were attributed to the morphological and/or functional damage of tracheal epithelium caused by ovalbumin challenge. In the modified co-axial system, phenylephrine-induced contractions faded quickly when the rat anococcygeus muscle was mounted in epithelium-intact guinea-pig trachea, and the percentage fade was significantly higher (92.3 ± 2.8%) than that obtained when the anococcygeus muscle was mounted in epithelium-denuded trachea (56.9 ± 8.4%) or when it was mounted alone (44.6 ± 7.7%). Our results suggest that guinea-pig tracheal epithelium is capable of modulating the responsiveness of rat anococcygeus muscle to serotonin by affecting the basal or stimulated release of some inhibitory mediators.

Pharmacological characterization of voltage-sensitive Ca 2+ channels in autonomic nerves

Hololena curta venom a potent inhibitor of voltage sensitive Ca 2+ channels and neurotransmitter release in mammalian brain, and synthetic funnel web spider toxin an inhibitor of P channels, were examined for their activity on autonomic nerves. Hololena curta (0.5 to 5.0 μg venom protein/ml) potently inhibited motor responses of the cholinergic guinea pig ileum myenteric plexus and the adrenergic rat anococcygeus muscle. Synthetic funnel web spider toxin was inactive at concentrations up to 100 μM. Hololena curta inhibited K +, and electrically evoked release of tritium from labeled superfused tissues. Furthermore, K +-contracted rat aorta was not relaxed by Hololena curta thereby precluding effects of Hololena curta on postjunctional L type smooth muscle Ca 2+ channels. The pattern of effects of Hololena curta on peripheral autonomic nerves was similar to the N channel inhibitor ω-conotoxin GVIA. These results suggest that Hololena curta venom constituents block Ca 2+ channels in peripheral autonomic nerves. The study failed to establish the presence of functional P type Ca 2+ channels on these peripheral autonomic nerves and further suggests that N type channels may be exclusively responsible for supplying the Ca 2+ necessary for neurotransmitter release in these nerves.

Actions and interactions of NG-substituted analogues of L-arginine on NANC neurotransmission in the bovine retractor penis and rat anococcygeus muscles.

1. The effects and interactions of a series of NG-substituted analogues of L-arginine known to inhibit nitric oxide synthase were examined on non-adrenergic, non-cholinergic (NANC) neurotransmission in the bovine retractor penis (BRP) and rat anococcygeus muscles. 2. Treatment of BRP muscle strips with either NG-nitro L-arginine (L-NOARG: 0.1-10 microM) or NG-nitro L-arginine methyl ester (L-NAME; 0.1-100 microM) produced a concentration-dependent blockade of NANC relaxation: blockade was complete at the highest concentration of each. 3. Pretreatment with L-arginine (1-10 mM) had no effect on NANC relaxation by itself, but inhibited, in a concentration-dependent manner, the subsequent ability of both L-NOARG (0.1-300 microM) and L-NAME (0.1-1 mM) to produce blockade. L-Arginine (1-10 mM) reversed established submaximal blockade of NANC relaxation induced by L-NOARG (1 microM) or L-NAME (1 microM), but had little effect on maximal blockade induced by these agents. 4. In contrast to L-NOARG and L-NAME, NG-monomethyl L-arginine (L-NMMA; 1 microM-1 mM) had no effect by itself on NANC relaxation of the BRP. L-NMMA (0.1-1 mM) did, however, like L-arginine, inhibit, in a concentration-dependent manner, the subsequent ability of both L-NOARG (0.1-1 mM) and L-NAME (0.1-3 mM) to produce blockade, but was more potent. As with L-arginine, L-NMMA (0.1-1 mM) reversed established submaximal blockade of NANC relaxation induced by L-NOARG (1 microM) or L-NAME (1 microM), but had little effect on maximal blockade induced by these agents. 7. These findings suggest a complex series of interactions between L-arginine and certain of its NG-substituted analogues that are commonly used to inhibit nitric oxide synthase. The most striking new finding is that L-NMMA does not block NANC relaxation in the BRP, but acts with greater potency than the endogenous substrate, L-arginine, to inhibit the blockade induced by L-NOARG or L-NAME.Even on rat anococcygeus where L-NMMA acts as a partial blocker of NANC relaxation, further blockade by L-NOARG is inhibited.

The effects of clonidine, guanfacine and phenylephrine on the excitatory and inhibitory responses of the rat anococcygeus muscle.

The effects of clonidine, guanfacine and phenylephrine on twitch responses and the basal tone of the rat anococcygeus muscle were investigated. Clonidine (10(-9)-3 x 10(-8) M) and guanfacine (10(-9)-10(-7) M) inhibited the twitch responses with the same potency, whereas phenylephrine (10(-9)-10(-7) M) was found ineffective. The inhibitory effect of clonidine and guanfacine was antagonized by yohimbine. Higher concentrations of clonidine and guanfacine increased the muscle tone and elicited inhibitory responses during field stimulation. Phenylephrine at concentrations greater than 10(-7) M also increased the muscle tone but induced biphasic responses. Clonidine (10(-7)-3 x 10(-5) M), guanfacine (3 x 10(-7)-3 x 10(-5) M) and phenylephrine (3 x 10(-7)-10(-5) M) caused concentration-dependent increases in the basal tone. The order of potency of these agonists in increasing the basal tone was clonidine > guanfacine > phenylephrine. Both yohimbine (10(-8)-10(-5) M) and prazosin (10(-9)-10(-7) M) antagonized these tonic contractions. Prazosin was found to be 39-, 122- and 83-fold more potent than yohimbine in antagonizing clonidine, guanfacine and phenylephrine-induced tonic contractions, respectively. Clonidine and guanfacine inhibited twitch responses through stimulation of presynaptic alpha-2 adrenoceptors. Postsynaptic alpha-1 adrenoceptors seem responsible for the contractile effects of clonidine, guanfacine and phenylephrine in the rat anococcygeus muscle.

Effect of neuropeptide Y on adrenergic and non-adrenergic, non-cholinergic responses in the rat anococcygeus muscle.

1. The effects of neuropeptide Y (NPY) were examined on adrenergic and non-adrenergic, non-cholinergic (NANC) neurotransmission in the rat anococcygeus muscle. 2. NPY (0.1-0.3 microM) greatly potentiated the contractile responses induced by field stimulation. Prazosin (0.1 microM) completely abolished the stimulation-induced responses either in the absence or presence of NPY. 3. NPY (0.1-0.3 microM) enhanced only the contractile responses to low doses of noradrenaline (NA, 0.003-0.01 microM). Responses to tyramine were unaffected by the same concentrations of NPY. 4. In superfused anococcygeus, previously loaded with [3H]-NA, NPY (0.1-0.3 microM) failed to modify the basal, as well as the stimulation-evoked, release of tritium at 2 and 4 Hz. 5. NANC relaxations induced by electrical stimulation were significantly reduced, in a concentration-related manner, by 0.1-0.3 microM NPY. 6. L-NG-nitro-arginine (L-NOARG, 30 microM) enhanced the stimulation (0.25-1 Hz)-induced motor responses. In the presence of L-NOARG (30 microM), NPY (0.1 microM) did not modify the motor responses induced by field stimulation (0.25-0.5 Hz). L-Arginine did not reverse the NPY-induced potentiation of stimulation-induced motor responses. 7. The relaxations of anococcygeus muscle induced by sodium nitroprusside (SNP, 0.01-0.3 microM) were diminished by NPY (0.1-0.3 microM). 8. Our study suggests that NPY, at concentrations devoid of contractile effect, potentiates the motor responses of rat anococcygeus muscle as a consequence, at least in part, of the inhibition of NANC relaxing responses by a different mechanism from L-NOARG.

Selectivity profile of the α2-adrenoceptor antagonist efaroxan in relation to plasma glucose and insulin levels in the rat

The effects of efaroxan (RX 821037A; 2-[2-(2-ethyl-2,3-dihydrobenzofuranyl)]-2-imidazoline HCI) at α 1- and α 2-adrenoceptors were investigated in isolated tissues, pithed rats and conscious rats. In isolated tissues, efaroxan competitively antagonised the inhibitory effects of p-aminoclonidine in the electrically stimulated (0.1 Hz) rat vas deferens (pA 2 = 8.89) and the contractile effects of phenylephrine on the rat anococcygeus muscle (pA 2 = 6.03). Efaroxan had a selectivity ration ( α 2/ α 1) of 724 compared to a value of 182 for idazoxan. In pithed rats, the i.v. doses of efaroxan (μmol/kg) producing 2-fold shifts in dose-response curves for UK-14,304 at prejunctional cardiac α 2-adrenoceptors and postjunctional vascular α 2-adrenoceptors, and for cirazoline at postjunctional vascular α 1-adrenoceptors, were 0.05, 0.13 and 2.96, respectively. In conscious fasted rats, prazosin (5 mg/kg p.o.) increased resting glucose levels and exacerbated the hyperglycaemic effectsof UK-14,304 and adrenaline. In contrast, efaroxan (1–5 mg/kg p.o.) had little effect on resting plasma glucose but markedly antagonised the hyperglycaemic actions of UK-14,304 and adrenaline. Efaroxan increased resting plasma insulin levels and markedly potentiated the rise in insulin levels produced by adrenaline; this latter effect was prevented by the co-administration of propranolol. These results demonstrate that efaroxan is a potent and selective 2-adrenoceptor antagonist and provide further support for the involvement of α 2-adrenoceptors in glucose homeostasis.

Activation of noradrenergic and nitrergic mechanisms in the rat anococcygeus muscle by nicotine.

1. Nicotine (10 mumol/L) produced rapidly developing but transient contractions of anococcygeus muscle isolated from rats. The magnitude of the response varied considerably between preparations. Tachyphylaxis occurred, such that no response was elicited by the same or a larger concentration in the continued presence of 10 mumol/L nicotine. 2. Contractions produced by nicotine were not affected by atropine, but were abolished by Hexamethonium and the alpha-adrenoceptor antagonists prazosin and phentolamine. Contractions were absent in the anococcygeus muscles of rats pretreated with reserpine. 3. The alpha 2-adrenoceptor agonist UK14304, or guanethidine, raised the tone of the anococcygeus muscle, and converted responses to field stimulation and nicotine to relaxations. Nicotine-induced relaxations were more pronounced in the presence of UK14304 than guanethidine. 4. Relaxations produced by nicotine (1-18 mumol/L) were transient, and tachyphylaxis occurred. When precautions were taken to avoid tachyphylaxis, concentration-response curves could be constructed. The relaxations elicited by nicotine were abolished or greatly reduced by hexamethonium, tetrodotoxin or omega-conotoxin GVIA. 5. The nitric oxide synthase inhibitor L-NG-nitroarginine methyl ester (90 mumol/L) enhanced contractile responses to field stimulation and nicotine, and markedly reduced relaxations elicited by field stimulation and nicotine in the presence of UK14304. These relaxations were restored by L-arginine (270 mumol/L). 6. The results suggest that nicotine acts on nicotinic receptors of noradrenergic and nitrergic nerve terminals in the rat anococcygeus muscle, resulting in the release of noradrenaline and nitric oxide respectively.