Muscarinic inhibition of acetylcholine release from a novel in vitro preparation of the guinea-pig trachea

An isolated preparation of the guinea-pig trachea is described which allows the simultaneous measurement of acetylcholine release and smooth muscle contraction. Incubation of the epithelium-free preparation with [3H]choline resulted in the formation of [3H]acetylcholine. Electrical stimulation caused the release of [3H]acetylcholine and a contractile response. Tetrodotoxin and omission of calcium from the medium abolished both the evoked release and contractions. The muscarinic agonists oxotremorine, carbachol and pilocarpine concentration-dependently inhibited the electrically evoked acetylcholine release and contracted the tracheal smooth muscle. Pre- and postsynaptic EC50 values for a given agonist were not different. Atropine (100 nmol/l) significantly facilitated the evoked acetylcholine release. A concentration of 10 nmol/l atropine did not change the evoked release but antagonized the inhibitory effect of oxotremorine. It is concluded that presynaptic muscarine autoreceptors inhibit the release of acetylcholine from parasympathetic nerves of the guinea-pig trachea.

Cholinergic agents structurally related to furtrethonium. 2. Synthesis and antimuscarinic activity of a series of N-[5-[(1'-substituted-acetoxy) methyl]-2-furfuryl]dialkylamines

In the first part of this study, devoted to the discovery of selective antimuscarinic agents, (+/-)- N-[5-[(1'-phenyl-1'-cyclohexylacetoxy)methyl]-2-furfuryl]dimeth yla mine (5a) proved to be at least 20 times more potent in the rat ileum and bladder than in guinea pig atria. Several (+/-)-N- [5-[(1'-substituted-acetoxy)methyl]-2-furfuryl]dialkylamine analogs of 5a were subsequently prepared. This involved exploration of the tertiary nitrogen substituents and modulation of the lipophilic side chain at position 5 of the furan ring, using the Hansch approach. A QSAR study was conducted to correlate activity with physicochemical properties of substituents. The possibility of describing all compounds in a single model indicates that variations of nitrogen and the lipophilic side chain contribute independently to activity. Compounds 5b, c,j, with bulky lipophilic substituents at the tertiary nitrogen, showed unprecedented selectivity between the two smooth muscle tissues, their antimuscarinic potency being from 10 to 90 times higher in the ileum than in the bladder. It is suggested that their interesting tissue selectivity is probably related to nonspecific phenomena involving the receptor environment, rather than real differences between the muscarinic receptors in the two tissues.

Comparative studies of the postjunctional activities of some very potent muscarinic agonists

This study was undertaken to determine the potencies of seven muscarinic agonists (methylfurtrethonium, dioxolane, oxathiolane, carbachol, muscarine, muscarone and oxotremorine) on the postjunctional muscarinic receptors of seven isolated preparations (guinea pig taenia-coli, ileum, jejunum, trachea and atria and rat jejunum and urinary bladder). The results indicate that the rank order of sensitivity of the preparations varies independently of the potency of the agonist used and it is almost the same for all the compounds with the exception of oxotremorine. Muscarone was the most potent compound in all the tissues. Intergroup comparisons in each preparation and the evaluation of the equieffective molar ratios relative to muscarone revealed that carbachol possesses a certain degree of cardioselectivity and oxathiolane, on the other hand, is much less active on the cardiac tissue than on the others. Oxotremorine is a peculiar compound endowed with cardioselectivity.

Effect of calcium antagonists on the response to noradrenaline in the whole and bisected rat vas deferens

1. The present study was carried out to look at the effect of different calcium antagonists on the response to noradrenaline in the whole and bisected rat vas deferens considering that the response consisted of three components (I) the phasic response (II) the tonic response and (III) the spikes (rhythmic contractions). 2. Nifedipine (3 x 10(-9)-1 x 10(-7) M) inhibited all the components at the same concentration range, verapamil (1 x 10(-7)-1 x 10(-5) M) inhibited the phasic and tonic response but not the rhythmic activity. This latter component, at a certain concentration range and especially in the prostatic portion was markedly potentiated. Diltiazem and flunarizine lay in an intermediate position. 3. Papaverine, a Ca2+ antagonist that acts mainly intracellularly, inhibited preferentially the tonic component; ryanodine was practically inactive. 4. Cromakalim inhibited only partially the phasic and tonic components but totally inhibited the rhythmic contractions. 5. These results can be explained by postulating two types of calcium channels opened by alpha-adrenoceptor stimulation. The first one is verapamil- and nifedipine-sensitive and allows the entry of Ca2+ directly available for the contraction and responsible for the phasic and partially responsible for the tonic component. The second channel is merely nifedipine-sensitive and allows the entry of Ca2+ trigger which can release Ca2+ from intracellular sites: the mobilized Ca2+ is able to sustain the tonic component and is the main one responsible for the rhythmic activity. There is the possibility that this second channel is associated with ATP-sensitive K+ channels.

Synthesis and pharmacological investigation of the enantiomers of muscarone and allomuscarone

A strategy based on the use of (R)- and (S)-lactic ester as starting materials allowed the synthesis of the two enantiomers of muscarone [(-)-1 and (+)-1] and allomuscarone [(-)-5 and (+)-5] in greater than 98% enantiomeric excess. The compounds were examined for their ability to bind to membranes from cerebral cortex (M1), heart (M2), and salivary glands (M3) and to recognize affinity agonist states of the muscarinic receptors. The two pairs of enantiomers were also tested in five functional assays, and their muscarinic potency was determined. In both binding and functional tests, (-)-1 (2S,5S) and (-)-5 (2R,5S) were the eutomers of muscarone and allomuscarone, respectively. The eudismic ratio of muscarone, evaluated in the functional tests, spanned a range of 280-440. These values are substantially different from ones (2.4-10.1) reported in the literature. From a stereochemical point of view, muscarone behaves as muscarine and all other major muscarinic agonists; as a consequence, the hypotheses advanced to account for the anomalies of muscarone no longer have reason to exist.

Dopamine-induced relaxation of the guinea-pig isolated jejunum is not mediated through dopamine receptors

The possible involvement of specific dopamine receptors in the relaxing effect of dopamine in the guinea-pig isolated jejunum has been investigated. The relaxing effect of dopamine does not show the tachyphylaxis phenomenon and it is present in preparations from guinea-pigs pretreated with reserpine. These results indicate that dopamine has a direct action. Comparison of the effect of dopamine with those of other dopamine receptor agonists, i.e. apomorphine, bromocriptine and the DA1 selective fenoldopam, were made to calculate potency ratios. Since apomorphine, bromocriptine and fenoldopam were shown to relax the guinea-pig jejunum, partly behaving as indirectly acting agents, comparisons were made on reserpine-pretreated guinea-pigs. It has been found that apomorphine is 2.5 times, fenoldopam 3 times and bromocriptine 20 times more active than dopamine in relaxing the guinea-pig jejunum. The order of potency is different from that found in other dopamine receptors containing tissues. The effects of the dopamine receptor blockers, haloperidol and cis-alpha-flupenthixol and the DA1 selective blocker SCH 23390 on the relaxing effect of dopamine were also studied. The relaxing effect of dopamine was not reduced by haloperidol, cis-alpha-flupenthixol and SCH 23390. It is concluded that specific postjunctional dopamine receptors are not involved in the relaxing action of dopamine. Since dopamine is known to interact with alpha- and beta-adrenoceptors in a variety of tissues, the effects of the alpha-adrenoceptor blocker phentolamine and the beta-adrenoceptor blocker propranolol on the relaxing effect of dopamine were also studied. Noradrenaline has been used to check the responsiveness of the tissue. Phentolamine did not block the responses to dopamine and propranolol was able only to partially reduce responses to dopamine, at concentrations higher than those at which it antagonized noradrenaline. Mechanisms other than dopamine, alpha- or beta-receptor activation should be involved in the relaxing effect of dopamine in the guinea-pig jejunum.

Prejunctional effects of muscarinic agonists on 3H-acetylcholine release in the rat urinary bladder strip

The inhibitory effects of some muscarinic agonists on tritiated acetylcholine release evoked by field stimulation were investigated in the rat urinary bladder strip. The acetylcholine stores of the preparation were labelled with 3H-choline. Electrical field stimulation caused an outflow of tritium, reflecting the release of 3H-acetylcholine. The release of 3H-acetylcholine was decreased in a concentration-dependent manner by all the agonists tested: oxotremorine, muscarone, muscarine, carbachol and methylfurtrethonium. On the contrary, only muscarine and muscarone enhanced the basal efflux of tritium in a concentration-dependent fashion. Concentration-response curves were determined both at 2 Hz and at 1 Hz by using intermittent administration of the drugs. Maximal depression in release (by 78-82%) was observed in experiments at 1 Hz. A similar inhibition was obtained at 2 Hz frequency only when a low concentration of calcium (0.6 mM) in the medium was used. Oxotremorine was the most potent among the tested compounds with the same intrinsic activity as the other drugs. In contrast to the other agonists investigated, oxotremorine showed in about 10-fold greater potency at pre- than at postjunctional muscarine receptors in the rat urinary bladder. This difference might depend either on heterogeneity of muscarine receptors or on different mechanism(s) relating to the transducing properties of receptors at the pre- and postjunctional level. A comparison between the relative prejunctional potencies in the rat urinary bladder and in the guinea pig myenteric plexus (data from the literature) suggests that prejunctional muscarine receptors are similar in these tissues. Furthermore, the findings obtained with a low concentration of calcium in the medium may support the view that intraneuronal availability of calcium plays a significant role in modulating the prejunctional negative feed-back mechanism in the rat urinary bladder.

Determination of dissociation constants and relative efficacies of some potent muscarinic agonists at postjunctional muscarinic receptors

This study was undertaken to determine dissociation constants (KA and relative efficacies (er) of seven muscarinic agonists (methylfurtrethonium; dioxolane, oxathiolane, carbachol, muscarine, muscarone and oxotremorine) in three isolated tissues (guinea-pig ileum and atria and rat urinary bladder). The rank order of affinities (-log KA) of the various compounds varied depending on the tissue used. er values for the different agonists did not differ significantly from each other in any of the three tissues, except that the er of muscarine in the guinea-pig ileum was higher than those of the other compounds and that of oxotremorine in the rat urinary bladder was lower than those of the other agonists. Comparisons among tissues show that KA and er values were the same in different tissues for some compounds (muscarone, muscarine and methylfurtrethonium), while significant differences were found for the other compounds. This suggests the existence of a discrete receptor population recognized by some but not all agonists. For oxotremorine er as well as -log KA, is greater in atria than in smooth muscle: these factors combine to determine the cardioselectivity of this compound which can now ascribed to receptor selectivity.

Profiles of the response to noradrenaline in the whole and bisected rat vas deferens

1. The present study was carried out to look at the differences between the whole and bisected rat vas deferens in response to cumulative and non-cumulative administration of noradrenaline in terms of sensitivity and maximum response. An effort was made to quantify the parameters characterizing the various components of the mechanical response to single concentrations of the agonist. 2. The sensitivity, as measured by the -log EC50 value for noradrenaline, was significantly lower in the prostatic portion than in the whole vas deferens and epididymal portion. The maximum contractions reached by the epididymal and the prostatic half were 65.03 +/- 10.43% and 13.84 +/- 5.28% of the maximal contraction evoked by noradrenaline in the whole rat vas deferens. 3. Various concentrations of noradrenaline were tested as single concentrations in the three preparations. These components of the response to noradrenaline were individualized, i.e. an early rapid phasic component followed by a second slower tonic component with spikes superimposed. The time-course of these components, as well as their ratios, were different depending on the concentration of agonist tested and on which half of the vas deferens was considered. 4. The importance of oxidation of exogenous noradrenaline as well of the neuronal and extraneuronal uptake mechanisms was considered. EDTA was devoid of any action. Cocaine enhanced potency and the maximum contraction of noradrenaline and all the parameters related to the different phases of the contraction were increased. Hydrocortisone at the highest concentration employed (2 X 10-5 M) was able to reduce significantly the -log EC50 value of the cumulative concentration-response curve to noradrenaline and both the phasic and tonic components of the contradiction to noradrenaline (3 X 10-6 M). 5. In conclusion, our evaluation of the parameters of the mechanical response of the whole and bisected vas deferens to noradrenaline rationalize the, often contradictory, findings of the current literature. Chiefly, the attainment of a careful description of the features of the response to noradrenaline can offer a dependable approach to the study of the underlying mechanisms of the three components.

Synthesis and pharmacological investigation of stereoisomeric muscarines

The synthesis of the eight stereoisomers of muscarine has been efficiently accomplished by utilizing the two enantiomers of lactic esters as starting material. The synthetic strategy is based on a SnCl4-catalyzed addition of allyltrimethylsilane to O-protected lactic aldehydes followed by an iodocyclization process. All the final derivatives possess an enantiomeric excess higher than 98%. The four pairs of enantiomers bound to M1, M2, and M3 muscarinic receptor subtypes in membranes from cerebral cortex, heart, and salivary glands, respectively, and recognized heterogeneous states of the receptors. Of the eight isomers, only natural muscarine (+)-1 recognized three affinity states of the M2 receptor. The compound was also the only one to show selectivity in the binding study, demonstrating 37- to 44-fold higher affinity for the M2 than for the M1 or M3 receptors. In addition, the compounds were tested in functional assays on isolated guinea pig atria (M2 receptors) and ileum (mixed population of M2 and M3 receptors) and their muscarinic potencies were determined. Among the eight isomers, again only (+)-1 enantiomer was found to be very active on both tissues. Its potency was more than two orders of magnitude higher than that of its enantiomer (-)-1 as well as the other six isomers. The eudismic ratios (E.R.) deduced from the two functional tests were 324 and 331.

Cromakalim blocks the purinergic response evoked in rat vas deferens by single-pulse electrical stimulation

The present study was carried out to look at the influence of the K+ channel opener cromakalim, compared with suramin and prazosin, on the contractile response evoked by single-pulse field stimulation and exogenous agonists in epididymal and prostatic portions of rat vas deferens. In the epididymal portion suramin abolished the first phase of the response to single shock, prazosin deeply affected the second phase and a combination of both antagonists almost completely abolished both phases. Cromakalim was able to inhibit in a concentration-dependent manner the first purinergic phase (pD2 = 5.90 +/- 0.11), leaving practically unaffected the second, adrenergic phase. This inhibitory effect of cromakalim on the electrically evoked response was counteracted by glibenclamide. Cromakalim and prazosin, but not suramin, affected the response to exogenous noradrenaline. Suramin but not cromakalim was able to antagonize responses to alpha, beta-methylene-ATP. In the prostatic portion because of a less clear discrimination between adrenergic and purinergic phases of the electrically evoked response, the picture was less clear although the trend was identical. Cromakalim was not able to antagonize the response to ATP. It is concluded that in rat vas deferens cromakalim inhibits purinergic transmission by acting prejunctionally.

Effect of cromakalim on the purinergic and cholinergic transmission in the rat detrusor muscle

Contraction of the rat detrusor muscle is mediated by cholinergic and purinergic mechanisms. The present study was carried out to look at the influence of cromakalim, compared with atropine, suramin and nifedipine on the contractile response evoked by single shock and exogenous agonists (carbachol and ATP) in rat urinary bladder. Cromakalim was able to inhibit only partially the response to carbachol and profoundly affected the response to exogenous ATP. Atropine suppressed the response to carbachol and was inactive versus ATP. Suramin was inactive versus carbachol and was able to antagonize the response to ATP. Nifedipine proved to be a non-competitive antagonist versus carbachol (pD2 = 7.66 +/- 0.05) and deeply affected the response to ATP. Cromakalim inhibited only partially the first, purinergic, phase of the electrically evoked response but was able to inhibit in a concentration-dependent manner the second, cholinergic, phase (logIC50 = 6.87 +/- 0.05). Nifedipine blocked both the phases. Atropine blocked partially only the second phase. Suramin inhibited the first phase but, at least partially, also the second one. The combination of atropine and suramin enhanced the inhibition of the second phase. The antagonistic effect of suramin on the second phase could indicate an overlap of the purinergic and cholinergic components. The comparison between pre- and postjunctional effects indicates that cromakalim acts on purinergic transmission predominantly postjunctionally. On the contrary, the action on cholinergic transmission seems to occur mainly at prejunctional level. This conclusion can be relevant in view of the claimed importance of K+ channel openers in the treatment of urinary disorders.

Effects of cromakalim (BRL 34915) on mechanical responses of rat vas deferens to noradrenaline and naphazoline

The contractile responses produced by noradrenaline were compared to those produced by naphazoline in the rat isolated vas deferens, a preparation which is usually quiescent. The responses to noradrenaline were biphasic: prominent spontaneous contractile activity was superimposed on contractions produced by naphazoline. After removal of naphazoline the rat vas deferens displayed periodic spontaneous activity. The effects of cromakalim and verapamil were compared on contractions induced by noradrenaline and naphazoline and on spontaneous activity induced by exposure to naphazoline. Cromakalim (1-10 x 10(-7) M) shifted to the right the cumulative concentration-response curve of naphazoline but not that of noradrenaline; at the same range of concentrations, cromakalim inhibited naphazoline-induced periodic spontaneous activity. Verapamil (3-30 x 10(-8) M) flattened in a dose-dependent manner the concentration-response curves of noradrenaline and naphazoline; higher concentrations of verapamil (3-30 x 10(-6) M) were required to modify periodic spontaneous activity. It was concluded that naphazoline but not noradrenaline can block K+ channel(s) and that this property can represent an important difference between imidazolines and phenethylamines.

Mode of antagonism of methoctramine, AF-DX 116 and hexahydrosiladifenidol in guinea-pig left atrium and ileum: comparison of Schild and resultant analysis

1. Methoctramine, AF-DX 116 and hexahydrosiladifenidol (HHSiD) are the muscarinic antagonists most widely used to study muscarinic receptor subtypes. 2. The present study was undertaken to examine the mode of antagonism of these compounds in guinea-pig left atrium and ileum by comparison of the Schild and resultant analysis. With this method the effect of various concentrations of the test antagonist on the antagonism produced by specific concentrations of a reference antagonist was measured and the equilibrium dissociation constant of the test antagonist-receptor complex estimated. Atropine was used for comparative purposes and scopolamine as the reference antagonist. 3. At the cardiac level the affinity values obtained by Schild and resultant analysis for methoctramine and AF-DX 116, as for atropine, are very similar: these results indicate that the two cardio-selective antagonists and the non-selective antagonist, atropine, bind at a common site with the reference antagonist scopolamine. The resultant plot for the ileo-selective HHSiD has a slope considerably less than unity: this finding might indicate that this antagonist binds to a site different from that of scopolamine and it should be considered like an allosteric antagonist. 4. At the ileal level the affinity values obtained by Schild and resultant analysis are identical for the ileo-selective antagonist HHSiD as for atropine but not for methoctramine and AF-DX 116. This indicates a mutual binding site with scopolamine for HHSiD and atropine but not for the two cardio-selective antagonists. However, it is worth emphasizing that the difference between affinity values obtained by Schild and resultant analysis is seen when relatively high concentrations are required: a dual mode of interaction (both competitive and allosteric) could be involved.

Inhibition by glibenclamide of the effects of cromakalim on responses of rat vas deferens to naphazoline

Cromakalim has been shown to inhibit naphazoline-induced contractions and spontaneous activity induced by exposure to naphazoline in the rat isolated vas deferens. Glibenclamide 10(-6) M blocked both these effects of cromakalim. Our data add to the list of data derived mainly from experiments on vascular smooth muscle; they suggest that the same glibenclamide-sensitive K+ channel is present in vascular and non-vascular smooth muscle and that it may be involved in the relaxant actions of cromakalim.

Differential sensitivity to prazosin and yohimbine blockade of tyramine and noradrenaline

Previous researches demonstrated that in the rat vas deferens the effects of noradrenaline released by tyramine were more easily affected than those induced by exogenous noradrenaline by the non selective alpha-blockers, phentolamine and dihydroergocristine. The investigation has now been extended to the effects of selective alpha-blockers, prazosin and yohimbine with the aim to see whether the major sensitivity of tyramine to alpha-blockade correlates with the type of receptor activated. The results obtained with the two antagonists which selectively block alpha 1 and alpha 2 receptors strongly resemble each other and those previously obtained with the non selective alpha-adrenoceptor blockers. Thus, the peculiar sensitivity to alpha-blockade of noradrenaline released by tyramine with respect to exogenous noradrenaline does not seem to be dependent on the type (alpha 1 or alpha 2) of receptor involved.

Action of papaverine and verapamil on norepinephrine and calcium evoked contraction of vascular and non vascular smooth muscle

On isolated rat vas deferens both papaverine and verapamil show a non competitive antagonism against norepinephrine and a competitive antagonism against Ca2+. Verapamil is nearly 100 times more active than papaverine. On the main pulmonary artery and on the thoracic aorta of the rabbit, verapamil shows a competitive antagonism against both norepinephrine and Ca2+ but it is more effective (almost 100 times) against Ca2+ than norepinephrine. Papaverine also shows a competitive antagonism against norepinephrine but a non competitive antagonism against Ca2+. The contrasting results obtained on rat vas deferens and rabbit vessels might be due to: 1) mechanism(s) of action of the agonist; 2) properties of the biological object as far concerns receptor activation and mechanism(s) of excitation-contraction coupling; 3) mechanism(s) of action of spasmolytic drugs. All these factors act in a cooperative way in determining the quality and the quantity of the observed responses.

Differential effects of drugs interacting with autonomic transmitters on responses of rat vas deferens to field stimulation

1. Frequency-response curves (0.1-30 Hz) were obtained in the epididymal portion of rat vas deferens. At low frequencies (0.1-1 Hz), the parameters evaluated were the first twitch and the fourth twitch at each frequency. The responses to trains of stimuli at intermediate (2-5 Hz) and high (10-30 Hz) frequencies were biphasic consisting of phase I (the first rapid phase of tetanus) and of phase II (the secondary slowly developing one). 2. Prazosin inhibited the first and the fourth twitch but not when the frequency was < 1 Hz. Suramin inhibited the first twitch while substantially depressing the fourth one. The combination of prazosin and suramin almost completely abolished all the twitches evoked by a train of stimuli at low frequencies. Nifedipine left almost unaltered the first twitch while markedly depressing the fourth one, especially at relatively high frequency (1 Hz). Verapamil was devoid of any inhibitory action. Papaverine depressed the first twitch while only at the highest concentration used (1 x 10(-4) M) markedly inhibited the fourth one. Chloroethylclonidine (CEC) depressed the first twitch and increased the fourth. 3. When intermediate (2-5 Hz) and high (10-30 Hz) frequencies are considered, prazosin and suramin partially inhibited both phase I and phase II, while in combination they almost completely abolished both phases. Nifedipine and verapamil selectively suppressed phase II, leaving phase I unaffected. Papaverine completely abolished both phase I and phase II. CEC was able to completely abolish phase I but increased phase II. 4. These results suggest that the response to the first twitch of a train at low frequency is prevailingly noradrenergic, prazosin-sensitive, while when the twitches are close enough (i.e. at 1 Hz) a summation of stimuli takes place and a predominant purinergic component, both suramin- and nifedipine-sensitive, becomes evident. 5. At high frequencies, both phases are due to the concomitant release of noradrenaline and adenosine triphosphate (ATP). The noradrenergic component of phase I is nifedipine-insensitive and CEC-sensitive, resembling the pharmacological profile of the endogenously released noradrenaline by single pulse, while that of phase II, nifedipine-sensitive and CEC-insensitive, is similar to that produced by exogenously applied noradrenaline.

Influence of lithium pretreatment and of cooling on the responsiveness of the rat isolated jejunum and urinary bladder to muscarinic agonists

1. The aim of the present study was to see whether contractile responses induced by muscarinic agonists in the rat jejunum and urinary bladder were differently affected by procedures that mainly influence the steps following agonist-receptor interaction. Thus, the effects of ex vivo lithium pretreatment (6.8 mmol kg-1 i.p. for 3 days) and in vitro cooling from 37 degrees C to 20 degrees C) on the contractile responses to full and partial agonists, carbachol, oxotremorine, muscarine and pilocarpine were studied. 2. Lithium pretreatment did not affect muscarinic responses on the urinary bladder. It significantly reduced responses to carbachol and oxotremorine but not to muscarine and pilocarpine on the rat jejunum. 3. A decrease of the bath temperature from 37 degrees C to 20 degrees C potentiated responses to carbachol, muscarine and oxotremorine and abolished those to pilocarpine in the urinary bladder. The same lowering of the bath temperature potentiated responses to carbachol, did not affect those to muscarine and to oxotremorine and reduced those to pilocarpine in the jejunum. 4. Together the findings indicate that differences exist in the stimulus-response coupling induced by muscarinic agonists between the two tissues and that, in a given tissue, differences exist among agonists in their ability to activate the agonist-receptor-transducer complex.

Formation and release of [3H]acetylcholine in the rat urinary bladder strip

The relationship between different frequencies of loading stimulation and [3H]acetylcholine (ACh) formation and release from nerve terminals has been investigated in extratrigonal strips of the urinary bladder of the rat. An increase in frequency (0.2, 0.4 and 0.8 Hz) for the 30 min incubations with [3H]choline produced an enhancement of storage of [3H]ACh from 19.5 to 34% of total tritium content in the tissue. Higher frequencies (1.6 and 3.2 Hz) failed to increase storage further on. The [3H]choline content did not vary significantly. Electrical field stimulation at 2 Hz (360 shocks) produced a release of tritium. The evoked outflow was higher when the strip was loaded at 0.8 Hz than at the other frequencies tested. Both [3H]ACh and [3H]choline were measured in the perfusate of strips preloaded at 0.8 Hz. Most of the induced outflow was found to be [3H]ACh, as in previous experiments carried out using 0.2 Hz as a loading frequency. The findings suggest that in the rat urinary bladder strip loading at 0.8 Hz is suitable for increasing the formation and the resulting release of [3H]ACh during electrical stimulation.

Differential blockade of alpha,beta-methylene-ATP, noradrenaline and electrically evoked contractions of the rat vas deferens

The present study was carried out compare the effects of nifedipine, verapamil, papaverine and chloroethylclonidine (CEC) on the electrically-induced contractions and on the contractions produced by exogenous noradrenaline and alpha, beta methylene-ATP in the epidydimal portion of rat vas deferens. Nifedipine inhibited in a concentration-dependent fashion the purinergic component (phase I) of the electrically evoked response. Verapamil (10(-7) M-10(-5) M) did not inhibit significantly or even potentiated phase I of the contractile response to single-pulse field stimulation but inhibited the response to alpha,beta methylene-ATP. Papaverine left unaffected phase I but depressed the response to alpha,beta methylene-ATP. CEC significantly potentiated the purinergic component of the electrically-evoked response and the response induced by alpha,beta methylene-ATP. Nifedipine was devoid of any inhibitory action on the noradrenergic component (phase II) of the response to single shock. Verapamil at the highest concentration used (10(-5) M) was able to partially inhibit phase II. Papaverine abolished in a concentration-dependent manner the noradrenergic component of the response to single shock. CEC abolished the phase II of single shock while was devoid of any inhibitory action on exogenous noradrenaline. The implications of these differences among the compounds studied in the present work are discussed in relation to roles of calcium channels.

The influence of verapamil and papaverine on the calcium- and epinephrine-induced responses of isolated guinea-pig atria

The effects of papaverine and verapamil on Ca2+ - and epinephrine evoked responses were investigated on spontaneously beating atria of the guinea-pig. Papaverine showed a non competitive antagonism against epinephrine but was ineffective against Ca2+; verapamil antagonized both the action of epinephrine and Ca2+ in a different manner but at the same concentrations. The pharmacological prerequisite to classify an anti-Ca2+ substance as a blocker of Ca2+ entry (that is the ability to selectively antagonize the Ca2+ -induced contraction relative to those of norepinephrine in vascular smooth muscle) was not met by the results obtained in the cardiac tissue. The behaviors of verapamil and papaverine with regard to Ca2+ response suggest another criterion for classifying a substance as a Ca2+ entry blocker based on the ability or inability to inhibit the response of the atrial preparation to Ca2+.

Potentiating actions of tranylcypromine and moclobemide on the sympathomimetic activity of dopamine

Ex vivo investigations were carried out to study the potentiating actions of the MAOIs moclobemide and tranylcypromine on peripheral alpha- and beta-adrenergic effects of dopamine. Isolated rat vas deferens and atria were used. Preliminary experiments showed that both moclobemide and tranylcypromine did not affect the response to noradrenaline, while they enhanced the response to tyramine. By comparison of the responses to dopamine in control and reserpinized rats, it was shown that dopamine acts in a direct and indirect manner on both the tissues tested. Two concentrations (20, 40 mg/kg, i.p.) of moclobemide were tested and administered at different time periods (2, 5 or 24 h) before the animals were sacrificed. Two concentrations (2, 20 mg/kg, i.p.) of tranylcypromine were tested at different intervals of time (2, 24 h). The results obtained pointed out that: (1) Moclobemide 20 mg/kg potentiated the alpha-adrenergic response to dopamine only 2 h after administration while 40 mg/kg exerted a potentiation which was evident not only 2 but also 5 h after administration. Both doses were ineffective 24 h after administration. (2) Moclobemide 20 and 40 mg/kg potentiated the beta-adrenergic response to dopamine. The effect was present 2 and 5 h after administration but it was absent 24 h after. (3) Tranylcypromine 20 mg/kg potentiated the alpha-adrenergic response to dopamine: this effect was present after 2 h from pretreatment and was still evident after 24 h. The lower concentration of tranylcypromine (2 mg/kg) enhanced the response to dopamine only after 24 h from pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)