Regulation of muscarinic acetylcholine receptors and contractility of guinea pig vas deferens

Muscarinic receptor trafficking

Knowledge of the mechanisms responsible for the trafficking of neurotransmitter receptors away from the cell surface is of obvious importance in understanding what regulates their expression and function. This chapter will focus on the mechanisms responsible for the internalization and degradation of muscarinic receptors. There are both receptor subtype-specific and cell-type specific differences in muscarinic receptor trafficking. Studies on muscarinic receptor trafficking both in cells in culture and in vivo will be described, and the potential physiological consequences of this trafficking will be discussed.

Role of noradrenaline on the expression of the Na+/K+-ATPase alpha2 isoform and the contractility of cultured rat vas deferens

Rat vasa deferentia were cultured for 3 days in Dulbecco's modified Eagle's medium in the absence or presence of 1 microM noradrenaline (NA) to investigate if the lack of NA release is the key factor to explain the selective reduction of the Na(+)/K(+)-ATPase alpha(2) isoform previously observed after in vivo denervation of this organ (Quintas et al., Biochem Pharmacol 2000;60:741-7). The lack of effects of the indirect sympathomimetic tyramine and the neuronal amine uptake blocker cocaine on NA curves indicated that cultured organs were denervated completely. Organ culture induced supersensitivity, expressed as a 6.3-fold increase of pD(2) and a 42% elevation of maximal contraction for NA but not for Ba(2+). Western blotting indicated that the level of the alpha(1) isoform of Na(+)/K(+)-ATPase was unchanged after organ culture, but the alpha(2) isoform was down-regulated drastically to levels that were barely detectable. The addition of NA to the culture medium did not prevent the reduction of alpha(2) expression although it did impede NA supersensitivity (in fact a 4-fold decrease of pD(2) and a 32% reduction of maximal response were observed after incubation in the presence of NA). A striking reduction of L-type Ca(2+) channel expression also was observed, indicated by an 85% decrease of [3H]isradipine binding sites. These data suggest that NA is a trophic factor relevant to the control of muscle contraction, mediated by alpha(1)-adrenoceptors, but not to the expression of either Na(+)/K(+)-ATPase or the L-type Ca(2+) channel.

Effects of cholinergic agents on anorectal physiology

BACKGROUND

Despite their potential therapeutic benefit, the effects of cholinergic agents on anal function have been poorly investigated.

AIM

To analyse the effects of neostigmine and atropine on anorectal responses to rectal isobaric distension.

METHODS

This was a placebo-controlled, randomized, double-blind crossover study, performed in 12 healthy volunteers who received intravenously, on 3 separate days, neostigmine, atropine or the placebo. During each day of the experiment, seven pressure steps (ranging from 1 to 31 mmHg) in three different protocols of rectal isobaric distension (phasic, stepwise and tonic) were applied using an electronic barostat. Manometric responses of the anal canal, adaptative volumes and perception scores of the rectum were recorded.

RESULTS

During stepwise distension, a significant drug effect was encountered at the anal level. No drug effect was observed on the other investigated parameters (rectal volumes and rectal perception scores) or for the other modes of distension. Compared to placebo, neostigmine significantly decreased pressures at the upper level of the anal canal for both recto anal inhibitory reflex and mean resting pressures. In contrast, atropine significantly increased pressures at the lower part of the anal canal but did not modify upper anal pressures.

CONCLUSION

The present study suggests that cholinergic effects result more from an indirect action on intermediate neurotransmitters and rectal myenteric neurons, than from a direct action on anal targets.

Changes in muscarinic acetylcholine receptors in the isolated duodenum from repeatedly cold-stressed rats and the effect of neurotropin

In rats repeatedly cold-stressed by specific alternation of rhythm in environmental temperature (SART-stressed rats), the contractile response to acetylcholine (ACh) of the isolated duodenum was remarkably decreased, whereas the contractile responses to K+, Ba2+ and Ca2+ were comparable to those in non-stressed rats. The amount of [3H]quinuclidinyl benzilate in the duodenum of SART-stressed rats was about 50% of that in non-stressed rats, but the KD value remained unchanged. Long-term administration of hexamethonium prevented the changes in SART-stressed rats. The daily treatment with Neurotropin, an extract isolated from inflamed rabbit dermis inoculated with vaccinia virus, dose-dependently prevented the changes in SART-stressed rats. However, Neurotropin had no effect on the ACh-induced decrease in muscarinic ACh receptor (m-ACh.R) in cultured vas deferens of guinea pig. These results suggest that down-regulation of m-ACh.R in duodenum by SART stress may be associated with enhanced activity in the parasympathetic center. Moreover, Neurotropin is thought to prevent the down-regulation of m-ACh.R throughout the central nervous system.

Blockade of cholinergic receptors by an irreversible antagonist, propylbenzilylcholine mustard (PrBCM), in the rat cerebral cortex causes deficits in passive avoidance learning

Studies were made on the effects of blockade of muscarinic acetylcholine (mACh) receptors in the rat cerebral cortex on learning and memory assessed by performance of a step-through passive avoidance task. Bilateral injection of propylbenzilylcholine mustard (PrBCM) into both the frontal and parietal cortex at doses of 2.25 X 4 to 22.5 X 4 micrograms decreased mACh receptors dose-dependently, as assessed by [3H]quinuclidinyl benzilate binding studies. When the training trial of a step-through passive avoidance task was performed 24 h after injection of 7.5 X 4 to 22.5 X 4 micrograms PrBCM into the frontal and parietal cortex, and then a retention test was made 24 h after the training trial, the treated rats showed shorter latencies than controls. In contrast, injection of PrBCM into the occipital cortex had no significant effect on performance in the test. These results confirm the notion that cholinergic neurotransmission in the cerebral cortex, especially the frontoparietal cortex, is important in learning and memory. The effects of injection of PrBCM (22.5 X 4 micrograms) into the frontoparietal cortex on 3 postulated phases of the learning and memory process (i.e. registration, retention and recall) were also examined. When PrBCM was injected 24 h before the training trial, no retention of the task was observed 14 days after the training trial. However, when PrBCM was injected 24 h after the training trial, retention of the task 14 days after the training trial was not affected. When PrBCM was injected 3-24 h after the initial training trial, the latencies in the retention test examined 24 h later were shorter than those of control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist-induced muscarinic acetylcholine receptor down-regulation in intact rat brain cells

Intact brain cells were prepared by dissociating whole adult rat brains without the cerebellum using a sieving technique. It has been found that preincubation of these cells with the muscarinic acetylcholine receptor agonist, carbamylcholine, results in a significant reduction in the specific binding of [3H]N-methylscopolamine to the receptors after the agonist was washed away. This agonist-mediated receptor down-regulation increased with prolongation of the exposure period to the agonist, and a steady state was achieved after 3 h at 37 degrees C. This effect of agonist was concentration-dependent, reaching a 30-35% decline in subsequent ligand binding upon preincubation with 1 mM carbamylcholine for 3 h. Carbamylcholine-induced receptor down-regulation was not apparent when exposure to the agonist was performed at 15 degrees C. In addition, it was abolished when the receptors were blocked by atropine. The decline in [3H]N-methylscopolamine binding induced by agonist was reflected as a significant reduction in the receptor density with no change in receptor affinity, suggesting that 'true' receptor down-regulation takes place. Moreover, when the receptors were labeled with the lipophilic antagonist [3H]quinuclidinyl benzilate instead of the hydrophilic ligand [3H]N-methylscopolamine, the magnitude of the observed receptor down-regulation was significantly lower in case of the former than the latter. This suggests that exposure of intact brain cells to muscarinic agonists might induce a slight degree of accumulation of receptors in intracellular sites before the receptors are actually degraded. These results are discussed in relation to previous findings regarding muscarinic receptor regulation in clonal cell lines.

Recovery of the muscarinic cholinergic receptor from its down-regulation in cultured smooth muscle

The recovery of the muscarinic cholinergic receptor (mAChR) from its down-regulation by long-term exposure to ACh was investigated. This was done to obtain information about regulation of the mAChR. Exposure of guinea-pig vas deferens to 30 microM ACh for 24 h decreased the amount of mAChR to 30% of the initial level, as measured with L-[3H]quinuclidinyl benzilate (QNB). The amount of mAChR was restored to 190% of its prewithdrawal level within 48 h of removal of ACh, without change in the KD value for L-[3H]QNB. This restoration was entirely dependent on protein synthesis. The half-life of the receptor was calculated to be 69 h. The recovery of mAChR was blocked by treatment with antimicrotubular agents, carboxylic ionophores, or 5 mM EGTA, which affect membrane protein synthesis. However, cytochalasin B and cyclic nucleotide derivatives had no effect. These data indicate that the recovery of mAChR was due to new synthesis of mAChR. The findings suggested that microtubules and the Golgi apparatus were involved in the biosynthesis of mAChR and that extracellular Ca2+ was necessary for the synthesis. Unlike the case with nicotinic AChR, the synthesis did not seem to be increased by cAMP or high extracellular Ca2+.

Prevention of supersensitivity-like phenomena in rat vas deferens by colchicine

Rat vas deferens has been shown to become supersensitive to alpha-adrenergic agonists on brief treatment with epinephrine (Epi-treatment). Epi-treatment increased the contractile response and the number of alpha-adrenoceptors in the tissue. The application of colchicine (2.5 mM) during Epi-treatment prevented the supersensitivity-like phenomena and increase in number of alpha-adrenoceptors. Vinblastine (10 microM) also counteracted the effect of Epi-treatment but cytochalasin B (100 microM) and strychnine (2.5 mM) did not. It is suggested that the supersensitivity-like phenomena induced in rat vas deferens by Epi-treatment were due to a change in microtubular components of the membrane.

Specific and non-specific desensitization of guinea-pig ileal smooth muscle

The effects of cis-2-methyl-4-dimethylaminomethyl-1-3-dioxolane methiodide (CD), a muscarinic agonist, histamine, substance P and K+-stimulation on the mechanical responses, Ca2+-dependence and desensitization in guinea-pig ileal longitudinal smooth muscle have been studied. The mechanical responses to all four stimulants are highly dependent upon extracellular Ca2+(Ca2+EXT) and are blocked by the Ca2+ channel antagonist nicardipine. The tonic (slow) components of response are more dependent on Ca2+EXT and are more sensitive to nicardipine (IC50 values 5.0 X 10(-8) - 2.5 X 10(-9)M) than are the phasic (fast) components of response. Tissue exposure to CD (5 X 10(-7)M, 10 min) or histamine (3 X 10(-6)M and 3 X 10(-4)M, 10 min) produces short term nonspecific desensitization but substance P (5 X 10(-8)M, 10 min) produces only specific desensitization. K+-induced responses neither desensitize nor are desensitized. Desensitization is concentration- and time-dependent for both specific and nonspecific processes. Nonspecific desensitization is protected by elevation of K+ concentration (5.36mM) in the incubating medium, by dithiothreitol and by inhibitors (mepacrine,p-bromophenacyl bromide and phenylgloxal) of phospholipase A2 and is potentiated by mellitin, an activator of phospholipase A2. Desensitization produced by the muscarinic agonist CS is protected by Gpp(NH)p (10(-4)M), but histamine-induced desensitization is unaffected. There is no loss of muscarinic receptors, measured by [3H]QNB binding following tissue exposure to low concentrations of CD (5.0 X 10(-7)M) for up to 72 h. However, an apparent loss of receptors (20-30%) is measured following 10-90 min exposure of tissue to 10(-3)M CD. It is suggested that contractions of guinea-pig ileal longitudinal smooth muscle elicited by CD, histamine, substance P or K+ mobilize a common pool of Ca2+ through a Ca2+ channel antagonist (nicardipine) sensitive pathway. However, the existence of short term nonspecific desensitization (CD and histamine), specific desensitization (substance P) or no desensitization (K+ stimulation) indicates that significant differences exist in the pathways linking initial stimulus to mechanical response. The ability of elevated K+ to protect against nonspecific desensitization suggest that post stimulus membrane hyperpolarization may represent one contributing component to nonspecific desensitization. Products of phospholipid degradation may also contribute to desensitization since inhibitors or activators of phospholipase A2 prevented or potentiated respectively, nonspecific desensitization.

Affinity of nortriptyline to muscarinic receptors in the bladder and ileum of man and guinea-pig

The antagonism by nortriptyline of carbachol- or urecholine-induced contractions was studied in strips of ileum and bladder derived from man and guinea-pig. Analyses of the results by the dose ratio method (Schild plots) showed significant differences in the affinities of the relevant muscarinic receptors to the antagonist: The Ki values in microM were as follows: Human ileum, 0.938; human bladder, 0.298; guinea-pig ileum, 0.159; guinea-pig bladder, 0.333 and 0.453. In man, the higher affinity of the drug to the receptors in the bladder than to those in the ileum may be of consequence in its therapeutic application as an antienuretic agent.

Cholinesterases and choline acetyltransferase in the ductus deferens of the guinea-pig

Several authors have reported that longitudinal and circular muscle layers of the guinea-pig ductus deferens possess a rich adrenergic innervation. Cholinergic innervation has been doubted by several authors, especially its presence in the longitudinal muscle layer. Acetylcholinesterase and butyrylcholinesterase were demonstrated, by electron-microscopic examination of both muscle layers of the guinea-pig ductus deferens, to be localized on the axolemma of the nerve endings and in smooth-muscle fibres, on sarcolemma, in the intracellular caveolae and in the intercellular space. Activity of cholinesterases and choline acetyltransferase was measured by the radiometric method and was found in both muscle layers. The activity of butyrylcholinesterase was higher than that of acetylcholinesterases in the homogenates of the whole ductus deferens and in the longitudinal muscle layer. In the circular muscle layer, the activity of acetylcholinesterase was higher than the activity of butyrylcholinesterase. In both muscle layers, we also found choline acetyltransferase, the activity being stronger in the circular layer. The localization of cholinesterases in smooth-muscles in the same places as the calcium and muscarinic acetylcholine receptors is discussed, together with the possibility that the enzyme is in some way involved in the excitation-contraction mechanism of smooth muscle.

Inhibition of agonist-induced degradation of muscarinic cholinergic receptor by quinacrine and tetracaine--possible involvement of phospholipase A2 in receptor degradation

Muscarinic cholinergic receptors (mAChR) are degraded on the addition of agonists through energy- and temperature-dependent processes, probably with clustering and endocytosis. Pretreatment of guinea-pig vas deferens with 0.5 mM quinacrine or 5 mM tetracaine, phospholipase A2 (PLase A2) inhibitors, inhibited the ACh-induced degradation of mAChR in the smooth muscle and kept mAChR on the surface membrane, while cocaine and procaine were not effective. On pretreatment with quinacrine or tetracaine the PLase A2 activity in the smooth muscle decreased continuously during culture without change in the contractile response of the tissue. Pretreatment with cocaine and procaine which had no significant effect on the down regulation of mAChR did not inhibit PLase A2 activity. However, activation of PLase A2 by long-term culture of the muscle with ACh and formation of endogenous inhibitor of PLase A2 were not observed under our experimental conditions. The participation of PLase A2 in the agonist-induced degradation of mAChR is discussed in the light of these findings.

Effect of glucocorticoids on alpha-2 adrenoceptors in vas deferens of reserpinized rat in organ culture

The changes in alpha adrenoceptors in vasa deferentia of reserpinized rats in organ culture were examined by binding studies with 3H-clonidine, 3H-yohimbine and 3H-prazosin. On organ culture for 1 day, the 3H-clonidine binding sites decreased by 1.07 pmol/g tissue and after 2 days no binding sites were detectable. 3H-Yohimbine binding sites also decreased during culture, but in smaller extent than 3H-clonidine binding sites. On the other hand, 3H-prazosin binding sites showed no apparent change in amount during culture. Addition of 0.1 mM hydrocortisone or dexamethasone to the culture medium inhibited the decrease in 3H-clonidine binding sites (1.7 pmol/g tissue), and the effect of glucocorticoid was blocked by the inhibitors of protein synthesis, cycloheximide and puromycin. However, hydrocortisone showed no effect on the 3H-yohimbine and the 3H-prazosin binding sites. An anti-endocytotic agent, an anti-microtubular agent and protease inhibitors had no effects on the decrease of 3H-clonidine binding sites during culture. These results suggest that the amount of alpha-2 adrenoceptors can change rapidly while alpha-1 adrenoceptors are stable and that glucocorticoids are important in regulation of conformation of alpha-2 adrenoceptor through synthesis of certain protein(s).

Contractions of amphibian Wolffian duct in response to acetylcholine, norepinephrine, and arginine vasotocin

The regulation of sperm transport through the Wolffian duct of male amphibians is poorly understood. These experiments were conducted using rough-skinned newts (Taricha granulosa) to determine if Wolffian ducts are capable of contracting in vitro and, if so, to characterize the contractile responses to acetylcholine (ACh), norepinephrine (NE), and neurohypophysial hormones. Dose-response curves for NE and ACh, which were prepared by measuring isometric contractions, are similar to those reported for mammalian vas deferens. For NE, the minimum effective dose and ED50 were found to be 1 X 10(-5)M and 4.17 X 10(-5)M, respectively. For ACh, the minimum effective dose was 3.2 X 10(-8)M and the ED50 was 1.37 X 10(-5)M. Alpha-adrenoreceptors appear to mediate the contractile responses to NE because phentolamine (10(-5)M) blocked or attenuated the response to NE (10(-6)M, 10(-5)M or 10(-4) M). Beta-adrenoreceptors appear to mediate relaxation because dichloroisoproterenol (10(-5)M) enhanced the response to 10(-5)M NE. The contractile response to three neurohypophysial hormones were also investigated. Arginine vasotocin was more effective in eliciting contractions than oxytocin. The effect of lysine vasopressin was intermediate between arginine vasotocin and oxytocin. These experiments demonstrate that amphibian (Taricha) Wolffian ducts contract in vitro in response to neurotransmitters and neurohypophysial hormones. The contractile response to neurotransmitters occurs in a dose-dependent manner; the response to neurohypophysial hormones is hormone specific.

Desensitization of the alpha adrenergic receptor system in guinea pig vas deferens

Desensitization induced by alpha adrenergic (alpha-Ad) stimulation was investigated in organ cultured vas deferens of guinea pig. Brief exposure (1-2 min) of the muscle to noradrenaline (NA) caused short-term desensitization to both NA and acetylcholine (ACh), but not to high K+. After removing the agonist this desensitization completely disappeared within 15 min. Prolonged exposure to NA (i.e., cultured with NA for 3-24 hr) elicited long-term desensitization to NA, ACh and K+ (50 mM), but it did not change the maximal contraction by high K+ (154 mM). After removing NA from the culture medium the response to the agonist was restored to normal within 24 hr, but not within 15 min. The number and affinity of alpha-Ad and muscarinic ACh receptors, which were measured by the binding of 3H-WB4101 and 3H-QNB, respectively, were not changed in the muscle during these treatments. Moreover, long-term desensitization, but not short-term desensitization, was depressed by the concomitant presence of cycloheximide. The possible mechanisms of desensitization were discussed in comparison with those of various receptor systems.

Mechanism of agonist-induced degradation of muscarinic cholinergic receptor in cultured vas deferens of guinea-pig

The mechanism of agonist-induced degradation of muscarinic cholinergic receptors (mAChRs) was examined by means of [3H]QNB binding by organ-cultured guinea-pig vas deferens. Long-term exposure to ACh decreased the surface mAChR. This so-called 'down regulation' of mAChR was significantly inhibited by anti-endocytotic drugs. Anti-microtubular agents also strongly inhibited the decrease of mAChR remaining on the cell surface. Ammonium chloride and protease inhibitors, which are known to inhibit lysosomal enzymes, had little effect on the decrease and no increase in intracellular mAChR could be detected under these conditions. Chloroquine blocked the decrease in mAChR remaining on the cell surface. Based on these findings, the involvement of clustering and endocytosis in mAChR degradation were considered as possibilities. Furthermore, contraction of the smooth muscle to ACh in long-term desensitization was also examined in relation to the number and nature of the receptors. When the muscles were cultured with ACh and chloroquine or vinblastine, there were indications that the surface mAChR and the contractile system were uncoupled.

Receptor-activated and energy-dependent decrease of muscarinic cholinergic receptors in guinea-pig vas deferens

Organ cultures of guinea-pig vasa deferentia were used for studying both the decrease in muscarinic cholinergic receptor (mAChR) concentration induced by muscarinic agonists and the simultaneous decrease in contraction to ACh. After a lag period the decrease followed first-order kinetics and was completely blocked by atropine. The decrease was dependent not only on the extent of occupation of mAChR by ACh, but was also related to the efficacy of agonists as deduced from the contractions. Moreover the amount of mAChR was not affected by other contractile agents, norepinephrine and high K+ even under conditions in which mAChR was occupied by atropine or pilocarpine. These findings indicate that the decrease in the amount of mAChR mediated by muscarinic receptors in vas deferens was due to the extent of both receptor occupation and receptor activation by agonists. Furthermore it was shown that the decrease involved energy- and temperature-dependent processes and that cyclic nucleotides did not regulate the quantitative level of mAChR.