Enhancement of alpha-adrenoceptor-mediated responses in prostate of testosterone-treated rat

The present study was undertaken to investigate the effect of testosterone on the alpha-adrenoceptor-mediated contractile responses in ventral lobe of rat prostate. Contractile responses to various alpha-adrenoceptor agonists (phenylephrine, A61603 (N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulphonamide), clonidine, guanfacine, ST587 ((2,3-dihydro-benzo[1,4]dioxin-2-ylmethyl)-[2-(2,6-dimethoxy-phenoxy)-ethyl]-amine) and xylazine) were tested in prostate strips obtained from control and testosterone (3 mg/kg, s.c. 5 days a week for 15 days-10 doses total)-treated rats. Dose-response curves for alpha-adrenoceptor agonists in testosterone-treated animals showed a leftward shift, indicating increased sensitivity of tissue to alpha-adrenoceptor agonists. To find the mechanism of increased sensitivity, K(A) value and receptor reserve of phenylephrine were estimated. Neither the K(A) value nor the receptor reserve of phenylephrine was altered in testosterone-treated rats. The concentration-occupancy curve for A61603 was shifted leftward and the K(A) value for A61603 decreased about four-fold. The K(B) value of 2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane (WB4101) was not altered, however, the K(B) value for prazosin was decreased approximately 5.5-fold. These findings indicate that the testosterone-mediated increase in sensitivity of prostate to alpha-adrenoceptor agonists is due to alterations in the alpha(1)-adrenoceptor pool.     

Potentiation of alpha-adrenoceptor-mediated responses following chronic beta-adrenoceptor stimulation in the rat heart.

1. Noradrenaline and isoprenaline were infused subcutaneously in rats by use of Alzet osmotic minipumps. The effects of catecholamine infusion on ventricular alpha- and beta-adrenoceptor density and also the responses of isolated cardiac tissues were compared with saline infusion. 2. Noradrenaline (1 mg kg-1) or isoprenaline (40 micrograms kg-1) infused for 3 days resulted in a desensitization of beta-adrenoceptor-mediated responses of isolated left atria and papillary muscles. Concentration-response curves to isoprenaline were shifted to the right in left atria whilst maximum responses were reduced in papillary muscles. Right atrial rate responses were not affected by infusions of catecholamines. 3. Infusions of either noradrenaline or isoprenaline resulted in a supersensitivity of alpha-adrenoceptor-mediated responses in isolated papillary muscles with leftward displacements of concentration-response curves to phenylephrine. 4. The density of both ventricular [3H]-dihydroalprenolol and [3H]-prazosin binding sites was reduced following noradrenaline infusion. Isoprenaline infusion reduced only the density of [3H]-dihydroalprenolol binding sites. 5. Noradrenaline infusion therefore 'down-regulates' both alpha- and beta-adrenoceptors in the rat heart but at the same time ventricular alpha-adrenoceptor-mediated responses are enhanced. Isoprenaline similarly enhances responses to phenylephrine and possible mechanisms for this phenomenon are discussed.     

Pre- and postjunctional protective effect of neocuproine on the nitrergic neurotransmitter in the mouse gastric fundus

1. Electrical field stimulation (EFS) of non-adrenergic non-cholinergic nerves of the mouse gastric fundus induced frequency-dependent transient relaxations which were mimicked by nitric oxide (NO), added as acidified NaNO(2). The NO donors S-nitrosocysteine, S-nitrosoglutathione, SIN-1 and hydroxylamine induced sustained concentration-dependent relaxations. The NO synthase blocker L-nitro arginine (L-NOARG; 300 microM) abolished the relaxations to EFS without affecting the relaxations to NO. 2. The copper(I) chelator neocuproine (10 microM) enhanced the relaxations to EFS and NO but inhibited those to S-nitrosocysteine and S-nitrosoglutathione. Neocuproine potentiated the relaxations to SIN-1, which releases NO extracellularly, without affecting the relaxations to hydroxylamine, which releases NO intracellularly. 3. The potentiating effect of neocuproine on the relaxations to EFS was more pronounced after inhibition of catalase with 3-amino-1,2,4-triazole (1 mM) but not after inhibition of Cu/Zn superoxide dismutase (SOD) with diethyl dithiocarbamic acid (DETCA, 1 mM). The potentiating effect of neocuproine on relaxations to NO was not altered by 3-amino-1,2,4-triazole or DETCA treatment. 4. The relaxations to EFS were significantly inhibited by the oxidants hydrogen peroxide (70 microM) and duroquinone (10 microM) but only after inhibition of catalase with 3-amino-1,2,4-triazole or after inhibition of Cu/ZnSOD with DETCA respectively. 5. Our results suggest that neocuproine can act as an antioxidant in the mouse gastric fundus and that both catalase and Cu/ZnSOD protect the nitrergic neurotransmitter from oxidative breakdown. Since inhibition of catalase but not inhibition of Cu/ZnSOD potentiated the effect of neocuproine on relaxations to EFS without affecting the relaxations to NO, catalase may protect the nitrergic neurotransmitter mainly at a prejunctional site whereas Cu/ZnSOD protects at a postjunctional site.     

Purinoceptors mediating relaxation and spasm in the rat gastric fundus.

1. The relaxant and spasmogenic effects of purines and analogues were studied in longitudinal strips of rat gastric fundus to characterize the purinoceptors involved. Classification was studied by use of agonist potency orders and of antagonists in circumstances where the influence of confounding factors was reduced. In general tone was raised by carbachol (0.1 microM). 2. Adenosine produced relaxation and was potentiated by nitrobenzylthioinosine (NBTI, 0.3 and 30 microM), an adenosine-uptake inhibitor. 8-Sulphophenyl-theophylline (8-SPT, 30 microM), a selective P1-purinoceptor antagonist, antagonized adenosine and 5'-N-ethylcarboxamidoadenosine (NECA), a selective agonist at P1-purinoceptors. 3. At resting tone, adenosine 5'-triphosphate (ATP) induced a small, phasic relaxation followed by a maintained spasm. When tone was raised by carbachol, ATP induced a larger relaxation followed by a smaller spasm. NBTI did not potentiate ATP, nor did 8-SPT antagonize ATP, suggesting that ATP does not act directly or indirectly at P1-purinoceptors. 4. With raised tone, and in the presence of indomethacin (10 microM) and 8-SPT (30 microM), 2-methylthio ATP (2-MeSATP) and ATP produced relaxations followed by spasms while alpha,beta-methylene ATP (alpha,beta-MeATP) induced only relaxation; all responses were concentration-dependent. The compounds had similar slopes and maxima for relaxation and spasm. The rank orders of potency were 2-MeSATP much greater than alpha,beta-MeATP greater than ATP for relaxation and 2-MeSATP much greater than ATP for spasm.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pre- and post-junctional supersensitivity: Differentiation by intraventricular infusions of norepinephrine and methoxamine

Intraventricular infusions of methoxamine, an adrenergic agonist that has a relatively low affinity for the presynaptic uptake mechanism, produces a significant dose-dependent increase in locomotor activity comparable to the increase elicited by infusion of equimolar doses of norepinephrine (NE). The behavioral responsiveness to infusion of both NE and methoxamine was more than doubled 3 weeks after pretreatment with 6-hydroxydopamine (6-OHDA). These results indicate that the increased responsiveness to NE induced by 6-OHDA is due to enhanced postsynaptic receptor sensitivity rather than to a loss of presynaptic uptake inactivation.     

Relaxant effect of capsaicin in the rat gastric fundus.

The effect of capsaicin was studied in precontracted longitudinal muscle strips of the rat gastric fundus. Capsaicin induced a relaxation in the concentration range 10(-7)-10(-6) M. The relaxation induced by 10(-6) M capsaicin was completely prevented by extrinsic denervation of the stomach. The adrenoceptor antagonists phentolamine and propranolol did not influence the effect of capsaicin while hexamethonium potentiated it; this potentiation was not observed with another nicotinic receptor antagonist trimethaphan. Tetrodotoxin did not have a consistent effect as it reduced the capsaicin-induced relaxation in some but not all tissues. The peptidase trypsin consistently reduced the action of capsaicin but vasoactive intestinal polypeptide (VIP) antiserum, desensitization to calcitonin gene-related peptide (CGRP), and CGRP antiserum had no influence. The neuropeptide involved in the relaxant effect of capsaicin in the rat gastric fundus has thus still to be determined.     

Pharmacological comparison between the nitrergic responses produced by intramural nerve stimulation and exogenous NO-donors in rat gastric fundus.

To investigate whether the nitrergic nerve-mediated smooth muscle relaxation is caused by authentic nitric oxide (NO) and is mediated via guanosine 3':5'-cyclic monophosphate (cyclic GMP), we compared the response to electrical field stimulation of nitrergic nerve (EFS) with other NO-related responses in rat gastric fundus strips. EFS, sodium nitroprusside (SNP), S-nitroso-N-acetylpenicillamine (SNAP), and acidified NaNO2 and inducible NO synthase (iNOS)-mediated NO all produced relaxation and elevated cyclic GMP level in rat fundus strips. However, the basal and stimulated cyclic GMP levels were significantly lower than the basal level in aorta (40+/-4 pmol/g wet tissue). Methylene blue and 6-anilino-5,8-quinolinedione (LY83583), both known as soluble guanylyl cyclase inhibitors and O2- generators that scavenge NO, reduced the elevation of cyclic GMP level by all stimuli and inhibited the relaxations only in response to NaNO2 and iNOS-mediated NO but not to the other stimuli. These results suggest that in the rat gastric fundus strips the relaxations induced by not only nitrergic nerve but also SNP and SNAP are not associated with cyclic GMP production, in contrast to the relaxations mediated by authentic NO.     

Pharmacological characterization of the postjunctional beta-adrenoceptors in the rat gastric fundus

In order to characterize the postjunctional beta-adrenoceptors in the rat gastric fundus, we studied the influence of beta-agonists and beta-antagonists on methacholine-contracted fundus strips. The mixed beta-agonist isopropylnoradrenaline and the beta 2-selective agonist fenoterol had a concentration-dependent relaxing effect and at higher concentrations completely inhibited the methacholine-induced tone. The reputedly beta 1-selective agonist prenalterol only produced about 50% inhibition and another reputedly beta 1-selective agonist, tazolol, had almost no relaxing effect. The beta-antagonists propranolol (beta 1 + beta 2), practolol (beta 1), H35/25 (beta 2) and ICI 118,551 (beta 2) all shifted the concentration-response curves for isopropylnoradrenaline and fenoterol in a parallel way to the right, but the slope of the Schild plot was not significantly different from 1 only for the antagonism of isopropylnoradrenaline by H35/25. The relaxing effect of prenalterol was only clearly antagonized by ICI 118,551. The results suggest that postjunctional beta 1- and beta 2-adrenoceptors are present in the rat gastric fundus.     

Interactions of nitric oxide synthase inhibitors and dexamethasone with alpha-adrenoceptor-mediated responses in rat aorta.

1. The effects of NG-nitro-L-arginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA), their D-isomers, and dexamethasone on noradrenaline (NA)-induced contractions and antagonism by alpha-adrenoceptor antagonists, have been investigated in rat isolated thoracic aortic rings with/without endothelium. 2. NA produced concentration-dependent contractions of isolated aortic rings with EC50 values of 2.41 +/- 0.54 (n = 21) and 28.00 +/- 8.50 (n = 25) nM for endothelium-denuded and -intact preparations respectively. Acetylcholine (ACh) relaxed NA-precontracted rings with intact, but not those denuded of endothelium. 3. Treatment with L-NAME (1-30 microM), or L-NMMA (10-500 microM), but not their D-isomers, resulted in an endothelium-dependent enhancement of NA-induced contractions. Pre-treatment, in vitro, with 0.5 microM dexamethasone neither directly potentiated, nor influenced L-NAME-induced potentiation of NA-mediated contractions in endothelium-intact rings; however, dexamethasone pretreatment reduced EC50 values for NA, and also prevented L-NAME-induced potentiation, in denuded rings equilibrated for 5 h under resting tension. 4. In both intact and denuded rings, phentolamine, prazosin and WB 4101 shifted NA concentration-response curves to the right; L-NAME, and also L-NMMA, but not their D-isomers, reversed the blockade as indicated by significant decreases in NA dose-ratios. In denuded rings, reversal by L-NAME or L-NMMA was prevented following pretreatment with dexamethasone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of S-nitrosothiols and nitrate tolerance in the rat gastric fundus.

1. The relaxant responses of S-nitroso-L-cysteine (CysNO), S-nitroso-N-acetyl-D,L-penicillamine (SNAP), S-nitroso-N-acetyl-L-cysteine (SNAC) and S-nitrosoglutathione (GSNO) in the rat gastric fundus (forestomach) were studied and compared to the relaxant responses obtained in response to nitric oxide (NO) and electrical field stimulation (EFS, 10 s strains) of non-adrenergic non-cholinergic (NANC) nerves. 2. CysNO (10(-7)-3 x 10(-4) M) caused transient relaxation of the precontracted rat gastric fundus, comparable to the response to NO (10(-6)-10(-4) M) and EFS. SNAP, SNAC and GSNO elicited more sustained relaxations. 3. The cyclic GMP-specific phosphodiesterase inhibitor, zaprinast (3 x 10(-5) M) increased the relaxant effect of CysNO, SNAP and GSNO while the NO-synthase inhibitor, NG-nitro-L-arginine (L-NOARG, 3 x 10(-4) M) had no influence. 4. In the presence of LY 83583 (10(-5) M), which releases superoxide anions, the relaxant response to NO and CysNO was decreased, whereas that to all other stimuli was unaltered. The inhibitory effect of LY 83583 on CsNO-induced relaxations was prevented by superoxide dismutase (SOD, 1000 u ml-1). 5. Tissues incubated for 1 h with 5.5 x 10(-4) M nitroglycerin (GTN) became tolerant to GTN. In this condition, the relaxant response to 10(-5) M NO was maintained, while the relaxations by EFS (8 Hz) and 3 x 10(-5) M SNAP were significantly decreased. The reduction of the response to the other S-nitrosothiols was not significant.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relaxant effects of alpha-adrenoceptor agonists in the rat isolated gastric fundus

In rat gastric fundus preparations with tone raised by the addition of barium chloride or carbachol, and in the presence of propranolol (2 microM) to prevent beta-adrenoceptor mediated effects, the adrenoceptor agonists noradrenaline, adrenaline, alpha-methylnoradrenaline, isoprenaline, cirazoline and phenylephrine all caused concentration-related relaxant responses. Relaxations to the catecholamines were poorly antagonized by prazosin (0.01-1 microM) resulting in the slopes of Schild plots being less than unity, low pA2 values for prazosin against the catecholamines and a clear relaxant effect of the catecholamines even in the presence of 1 microM prazosin. The prazosin-resistant relaxations were unaffected by higher concentrations of prazosin (2 microM) and propranolol (30 microM) or by further additions of idazoxan (1 microM) or haloperidol (30 microM). The relaxations were not due to a non-specific effect of the catechol nucleus since neither dihydroxyphenylethylene glycol (DOPEG) nor dihydroxyphenylacetic acid (DOPAC) produced relaxant effects at concentrations up to 300 microM. In contrast to the results with the catecholamines, prazosin was a potent antagonist of the relaxant effect of cirazoline and phenylephrine, although the antagonism was difficult to quantify due to a lowering of the slope of the concentration response curves to cirazoline and phenylephrine with the higher concentrations of prazosin (0.1 and 1.0 microM). In conclusion postjunctional relaxatory effects of catecholamines in the rat gastric fundus are mediated partly via alpha 1-adrenoceptors and partly via an atypical adrenoceptor.     

Nitrergic and purinergic interplay in inhibitory transmission in rat gastric fundus

1 This study was undertaken to analyse the involvement of ATP in non-adrenergic non- cholinergic (NANC) relaxation and possible interplay between nitrergic and purinergic systems in rat gastric fundus. 2 Experiments were performed in vitro on strips of longitudinal muscle from rat gastric fundus, recording the mechanical activity as changes in isometric force. In addition, NO release induced by different experimental conditions was assayed. 3 Under NANC conditions in serotonin-precontracted strips, electrical field stimulation (EFS) elicited a tetrodotoxin (TTX)-sensitive relaxation accompanied by nitric oxide (NO) release. This effect was antagonized by pretreatment with the NO synthase antagonist Nomega-nitro-L-arginine (L-NA) or by desensitization of purinergic receptors. Purinergic desensitization was also able to further antagonize the residual EFS-induced relaxation remaining after L-NA treatment. Exogenously applied NO [delivered as sodium nitroprusside (SNP)] or ATP (and related purines) induced concentration-dependent, TTX-insensitive relaxant responses. ATP also induced the release of NO. A reduction in the responses to ATP was observed in the presence of L-NA. In contrast, SNP-induced relaxation remained unchanged after desensitization of purinergic receptors. Finally, apamin, a blocker of the small conductance Ca2+ -dependent K+ channels, reduced the amplitude of the muscular relaxation evoked by either EFS, ATP or SNP. 4 In conclusion, this study provides evidence that in rat gastric fundus, ATP is one of the inhibitory transmitters released from NANC intramural neurones acting directly on the muscle, through receptors coupled to apamin-sensitive Ca2+ -dependent K+ channels and, indirectly, through the stimulation of NO production.     

Analysis of the alpha-adrenoceptor-mediated, and other, components in the sympathetic vasopressor responses of the pithed rat.

The vascular receptors activated following sympatho-adrenal stimulation were determined by analysing the effects of 'selective' antagonists on the vasopressor response to spinal sympathetic nerve activation in the pithed rat. The net vascular response to adrenal stimulation was a balance between alpha-adrenoceptor-mediated vasoconstriction and beta-adrenoceptor-mediated vasodepression. Part of the alpha-adrenoceptor-mediated response was 'prazosin-sensitive' (alpha 1) and the remainder was abolished by rauwolscine (alpha 2). As with adrenal stimulation, direct sympathetic nerve stimulation of the vasculature evoked pressor responses which were partly resistant to prazosin. Rauwolscine only partly blocked the prazosin-sensitive component. Reserpine pretreatment led to smaller responses than prazosin plus rauwolscine. Thus, the response resistant to alpha-adrenoceptor antagonists could be mediated, in part, by adrenoceptors distinct from alpha-adrenoceptors, as currently defined. alpha, beta-Methylene ATP reduced the nerve-mediated pressor response after alpha-adrenoceptor blockade or reserpine pretreatment but not in drug-free controls. The results suggest that stimulation of the adrenal medulla can produce a vasopressor response which consists of summating alpha 1- and alpha 2-adrenoceptor-mediated components, and is identical to the effect of injected adrenaline. In contrast, the response to vasopressor nerve stimulation appears to be essentially mediated by alpha 1-adrenoceptors, with a facilitatory influence from alpha 2-adrenoceptors. A further response obtained after alpha-adrenoceptor blockade may contain a purinergic component and another which is adrenergic but not mediated by stimulation of alpha-adrenoceptors.     

L-citrulline recycling by argininosuccinate synthetase and lyase in rat gastric fundus

The aim of this study was to investigate in rat gastric fundus whether L-citrulline, the co-product in the nitric oxide (NO) biosynthesis catalyzed by neuronal nitric oxide synthase (nNOS), can be converted back to the nNOS substrate L-arginine. Immunohistochemistry showed that argininosuccinate synthetase and argininosuccinate lyase, that mediate transformation of L-citrulline to L-arginine in the ureum cycle in hepatocytes, co-localize with nNOS. In longitudinal smooth muscle strips, L-arginine as well as L-citrulline (10(-3) M) was capable of completely respectively partially preventing the N(G)-nitro-L-arginine methyl ester (L-NAME) (3 x 10(-5) M)-induced inhibition of electrically induced nitrergic relaxations, whereas D-citrulline (10(-3) M) was not. The L-citrulline-mediated prevention of the L-NAME-induced inhibition was reduced by L-glutamine (3 x 10(-3) M), the putative L-citrulline uptake inhibitor, and by succinate, an argininosuccinate lyase inhibitor. The results demonstrate that the L-citrulline recycling mechanism is active in rat gastric fundus. Recycling of L-citrulline might play a role in providing sufficient amounts of nNOS substrate during long-lasting relaxations in gastric fundus after food intake.     

Mechanical responses to catecholamines in the longitudinal muscle of guinea-pig gastric fundus.

1. In the longitudinal muscle of guinea-pig gastric fundus, adrenaline and phenylephrine (1-30 microM) both produced a slow contraction preceded by a relaxation. The slow contraction was strongly inhibited by prazosin (0.1 microM), but only weakly by yohimbine (1 microM), suggesting main contribution of alpha 1-adrenoceptors. 2. Most of the slow contraction was blocked by meclofenamate or indomethacin (0.1-0.3 microM). Both these drugs also inhibited spontaneously generated muscle tone. In some preparations, obtained from the apical fundus, a small contraction remained in the presence of meclofenamate. 3. During contraction induced by prostaglandin E2, adrenaline produced sustained relaxation and phenylephrine often transient relaxation, in the presence of meclofenamate. The transient relaxation, but not the sustained relaxation, was suppressed by prazosin. 4. In the presence of prostaglandin E2 (5 nM), after treating with phenoxybenzamine (30 microM) for 30 min, isoprenaline and adrenaline produced concentration-dependent relaxation, with IC50 s of 3.9 nM and 64 nM, respectively. Propranolol shifted these concentration-response curves to the right, with apparent pA2 s of 8.15 and 7.34, respectively. 5. It is suggested that in the fundic longitudinal muscle, adrenaline-induced contraction is mediated mainly by an increase in endogenous prostaglandin production through activation of alpha 1-adrenoceptors and that adrenaline produces transient relaxation through alpha 1-adrenoceptors and sustained relaxation through beta-adrenoceptors. The beta-adrenoceptors in the longitudinal muscle are more sensitive to adrenaline and isoprenaline than those in the circular muscle.     

Nicotinic acetylcholine receptor blocking effect of guanethidine in the rat gastric fundus

1. The aim of this study was to examine the mechanism of impaired platelet-mediated endothelium-dependent vasodilation in diabetes. Exposure of human platelets to high glucose in vivo or in vitro impairs their ability to cause endothelium-dependent vasodilation. While previous data suggest that the mechanism for this involves increased activity of the cyclo-oxygenase pathway, the signal transduction pathway mediating this effect is unknown. 2. Platelets from diabetic patients as well as normal platelets and normal platelets exposed to high glucose concentrations were used to determine the role of the polyol pathway, diacylglycerol (DAG) production, protein kinase C (PKC) activity and phospholipase A2 (PLA2) activity on vasodilation in rabbit carotid arteries. 3. We found that two aldose-reductase inhibitors, tolrestat and sorbinil, caused only a modest improvement in the impairment of vasodilation by glucose exposed platelets. However, sorbitol and fructose could not be detected in the platelets, at either normal or hyperglycaemic conditions. We found that incubation in 17 mM glucose caused a significant increase in DAG levels in platelets. Furthermore, the DAG analog 1-oleoyl-2-acetyl-sn-glycerol (OAG) caused significant impairment of platelet-mediated vasodilation. The PKC inhibitors calphostin C and H7 as well as inhibitors of PLA2 activity normalized the ability of platelets from diabetic patients to cause vasodilation and prevented glucose-induced impairment of platelet-mediated vasodilation in vitro. 4. These results suggest that the impairment of platelet-mediated vasodilation caused by high glucose concentrations is mediated by increased DAG levels and stimulation of PKC and PLA2 activity.     

Lead effects on non-adrenergic non-cholinergic relaxations in the rat gastric fundus.

The effect of lead exposure on non-adrenergic non-cholinergic (NANC) relaxations in rat gastric fundus was evaluated in this work. Wistar rats were divided into four groups: The control group received tap water and the three other received 0.008% of lead acetate in their drinking water for 15, 30 and 120 days. NANC relaxations induced by electrical field stimulation (0.5-8 Hz, 1 ms, 60 V) of gastric fundus strips was inhibited in all groups treated with lead. The strips from groups, control and 120 days of lead treatment (LEAD 120), were incubated with L-NOARG (100 microM). The presence of this blocker did not produce any additional inhibition. Sodium nitroprusside (10(-10)-10(-6) M) and 8-Br-GMPc (3 x 10(-8)-3 x 10(-4) M) produced dose-dependent relaxations in strips of both groups control and LEAD 120, however, in the LEAD 120, the potencies were significantly reduced from 7.32 +/- 0.05 to 6.40 +/- 0.09 (n = 5) and 4.26 +/- 0.06 to 3.69 +/- 0.05 (n = 5), respectively. Our data suggest that the chronic exposure to lead inhibits NANC relaxations probably by modulating NO release from NANC nerves and/or by interacting with intracellular transducer mechanisms in rat gastric fundus.     

Enhancement effects of nicotine on neurogenic contractile responses in rabbit gastric fundus

Nicotine, a nicotinic acetylcholine receptor agonist, plays a role in the modulation of neurotransmitter release following nerve stimulation in both the central and the peripheral nervous system. Nitric oxide and prostaglandins modulate the release of various neurotransmitters in different tissues. We aimed to investigate the effects of nicotine on neurogenic contractile responses via nicotinic acetylcholine receptors and, if a change occurred, to investigate the effects of N(W)-nitro-L-arginine methyl ester (L-NAME) and indomethacin on this change in rabbit gastric fundus. Electrical field stimulation (EFS)-evoked contractile responses were recorded from gastric fundus strips obtained from rabbits with an isometric force displacement transducer. Nicotine was applied to preparations at varying concentrations. Then, the effects of hexamethonium, cadmium (Cd(2+)), indomethacin, and L-NAME were tested on the EFS-evoked contractions in the presence of nicotine. Nicotine-induced transient neurogenic contractions in a dose-dependent manner. Cd(2+) and hexamethonium inhibited nicotine-induced transient neurogenic contractions, but indomethacin and L-NAME produced no effect. In conclusion, nicotine increased EFS-evoked contractile responses, possibly by facilitating neurotransmitter release from nerve terminals by a mechanism dependent on the influx of Ca(2+) from voltage-gated Ca(2+) channels via activation of nicotinic acetylcholine receptors in isolated rabbit gastric fundus. Endogenous nitric oxide and prostaglandins do not play a physiological role in the regulation of this neurotransmitter release.     

Peptide histidine isoleucine-like immunoreactivity release from the rat gastric fundus.

1. Longitudinal muscle strips from the rat gastric fundus were subjected to in vitro electrical field stimulation (EFS) under non-adrenergic non-cholinergic (NANC) conditions to study the release of peptide histidine isoleucine-like immunoreactivity (PHI-LI) and the correlation between PHI-LI release and NANC relaxation. 2. Different radioimmunoassay (RIA) systems employing C-terminal- and N-terminal-specific anti-PHI sera were used to determine the relative contributions of PHI and its C-terminally extended forms, peptide histidine glycine (PHI-Gly) and peptide histidine valine [PHV(1-42)], to the PHI-LI released by the rat gastric fundus. 3. In the presence of atropine (1 microM) and guanethidine (5 microM), EFS (120 mA, 1 ms, 0.25-32.0 Hz, trains of 2 min) induced frequency-dependent relaxations of 5-hydroxytryptamine (3 microM) pre-contracted strips. 4. EFS at frequencies of 8-32 Hz evoked significant increases in PHI-LI outflow. The increases in PHI-LI outflow evoked by 16-Hz EFS were abolished by tetrodotoxin (3 microM) and by a calcium-free medium, indicating an active release process from intramural nerves. 5. The EFS-induced release of PHI-LI measured with the N-terminal-specific antiserum was significantly greater than that detected with the C-terminal-specific antisera. 6. Sephadex G-25 gel permeation chromatographic analysis was performed on the PHI-LI release in response to 32-Hz EFS. A C-terminal-specific antiserum revealed one peak co-eluting with the rat PHI standard. When PHI-LI was measured with the N-terminal-specific antiserum, two peaks were found that co-eluted with the rat PHV(1-42) and rat PHI-Gly/PHI standards, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)