The dependence of fluid secretion by mandibular salivary gland and pancreas on extracellular calcium

Ion Transport and Water Movement

Secretion of water and electrolytes in salivary glands occurs by a dual process involving the formation of a plasma-like, isotonic primary-secretion in salivary acini and its subsequent modification in salivary-ducts by the removal and addition of specific ions. The mechanisms underlying the formation of primary acinar secretion have been investigated with a number of experimental approaches such as electrophysiology, the measurement of ion transport in gland fragments and dispersed acinar cells, and the evaluation of the ionic requirements for secretion in isolated, perfused gland preparations. The accumulated evidence suggests that salivary secretion is formed by a complex interaction between passive and active ion movements across acinar cell membranes, resulting in the trans-acinar movement of CI and Na+ and, by the osmotic gradient which develops, of water. A major consequence of stimulation is the release of K + through Ca++ -and voltage-sensitive channels and its subsequent recycling back into the cells by ouabain- and furosemide-sensitive transport systems. This results in NaCl uptake across the basolateral cell membrane and the subsequent efflux of CI through luminal membrane channels, which also appear to be sensitive to cellular Ca + +. The rates of these various ion movements appear to be, therefore, closely linked and interdependent. Ductal modification of the primary secretion has been studied in microperfused duct preparations. The evidence likewise indicates that it involves interactions between complex conductance pathways in the luminal cell membrane and a Na, K pump present in the basolateral cell membrane and that it is under autonomic and hormonal control. Activation of ductal transport mechanisms results in NaCl reabsorption and KHCO3 secretion. Final saliva thus differs from primary secretion in electrolyte composition and, because water permeability is low in the duct epithelium, becomes hypotonic. Alterations in fluid and electrolyte secretion such as those observed in disease can result, therefore, from disturbances in one or more of these complex transport processes in acinar or duct cells.

Ca2+ dynamics in salivary acinar cells: distinct morphology of the acinar lumen underlies near-synchronous global Ca2+ responses

In salivary acinar cells, the pattern of the Ca2+ signals that regulates fluid and enzyme secretion has yet to be resolved, as there are conflicting reports in the literature. We have used a two-photon technique to directly visualize the acinar cell lumen in living fragments of exocrine tissue and simultaneously recorded agonist-induced changes in intracellular Ca2+. We show near-synchronous global Ca2+ responses in submandibular acinar cells, distinct from the typical apical to basal Ca2+ wave usually seen in rodent pancreatic acinar cells. In an effort to explain the basis of these near-synchronous global Ca2+ responses we used immunocytochemical experiments to localize luminal proteins and inositol trisphosphate receptors (InsP3Rs) in tissue fragments. Zona occludens 1 (ZO-1), a tight junction protein, shows that individual submandibular acinar cells are often nearly completely encircled by a narrow luminal structure. By contrast, in pancreatic fragments, ZO-1 staining shows short luminal branches terminating abruptly at the apical pole of single acinar cells. Co-immunostaining of InsP3Rs type 2 and type 3 showed them in the same region as ZO-1 in both exocrine tissues. Functional experiments showed that the near-synchronous global Ca2+ responses were still observed in the absence of extracellular Ca2+ and also in the presence of ryanodine. We conclude that the elaborate luminal region of submandibular cells leads to a hitherto unrecognized extensive distribution of InsP3Rs in a band around the cell and that this underlies the near-synchronous global Ca2+ response to agonists. We suggest that this may be a structural adaptation in submandibular cells to support the copious amounts of fluid secreted.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Visualization of the secretory process involved in Ca2+-activated fluid secretion from rat submandibular glands using the fluorescent dye, calcein

The central feature of fluid and electrolyte secretion by salivary acinar cells is transepithelial Cl- movement as a driving force for the secretion. However, little is known about the membrane localization and regulation by agonists of various anion channels. To characterize the anion transport and fluid secretion, we visualized the secretory process induced by the cholinergic agonist, carbachol (CCh), using the anionic fluorescent dye, calcein, under a confocal laser scanning microscope. The fluorescence of calcein loaded into the isolated acini was spread diffusely throughout the cytoplasm and was less intense in the secretory vesicles which occupied the apical pole. Cytoplasmic calcein was released into intercellular canaliculi just after the addition of CCh, depending upon a rise in [Ca2+]i by Ca2+ release from intracellular stores. Thereafter, the formation of watery vacuoles connected with intercellular canaliculi was visualized in the calcein-loaded acini, depending upon external Ca2+. Both the calcein release and vacuole formation were inhibited by suppressing the Ca(2+)-activated K+ efflux. The calcein release was also affected by the external anion substitution, suggesting that calcein is released through an anion channel. In the isolated, perfused glands, CCh-induced fluid secretion was sustained in two phases, whereas the loaded calcein was initially and transiently released into the saliva. By revealing the [Ca2+]i dependence and sensitivities to channel blockers, our results suggest that the initial phase of CCh-induced fluid secretion was evoked in association with the release of the organic anion, calcein, and the late phase of fluid secretion, during which calcein is less permeable, was associated with the formation of watery vacuoles. Thus, the anion channels possessing the distinct property of anion permeation may be activated in the initial phase and late phase. These results indicate that the anionic fluorescent dye, calcein, is useful for visualizing the process of Ca(2+)-dependent fluid secretion, and for clarifying the relation between fluid secretion and anion transport.

Inhibition by thiocyanate of muscarinic-induced cytosolic acidification and Ca2+ entry in rat sublingual acini

Thiocyanate (SCN-) plays a critical part in an oral antimicrobial system by acting as a substrate for peroxidases. Salivary glands concentrate SCN- from blood up to 5 mM in saliva; however, the influence of SCN- on salivary acinar-cell function is unknown. The present study examined the effects of SCN- on the regulation of cytosolic pH (pHi) and free Ca2+ concentration ([Ca2+]i) in rat sublingual mucous acini using the pH- and Ca(2+)-sensitive fluorescent indicators, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein and fura-2, respectively. SCN- induced a concentration-dependent inhibition of the carbachol-stimulated cytosolic acidification (K1/2, approx. 1.4 mM SCN-). Cytosolic pH recovery from an acid load was not changed by substitution of Cl- by SCN-, suggesting that Na+/H+ exchange activity was not affected by SCN-. SCN- did not alter the initial carbachol-stimulated increase in [Ca2+]i; however, the sustained [Ca2+]i increase was inhibited by > 65% (K1/2, approx. 1.0 mM SCN-). Furthermore, SCN- prevented the carbachol-stimulated Mn2+ influx, indicating that it inhibits the divalent-cation entry pathway. Consistent with decreased Ca2+ mobilization being involved in the blockade of the agonist-induced acidification by SCN-, only total replacement of Cl- with SCN- significantly inhibited the acidification induced by the Ca2+ ionophore ionomycin. The permeability to SCN- through the Ca(2+)-dependent Cl- channels was 5.2-fold higher than the permeability to Cl-. These results suggest that inhibition of the agonist-induced cytosolic acidification by high-concentration SCN- may be mediated by both competitive inhibition of HCO3- efflux and by blockade of Ca2+ influx.

Activation of protein kinase C does not cause desensitization in rat and rabbit mandibular acinar cells

We have examined whether activation of protein kinase C by phorbol esters decreases the responsiveness of rat and rabbit mandibular, and rat lacrimal, acinar cells to muscarinic stimulation. Intracellular free calcium concentration ([Ca2+]i) was measured in isolated single acini and cell clusters by fura-2 microspectrofluorimetry. Accumulation of inositol phosphates was measured in acinar cell suspensions. All three cell types showed very similar changes in [Ca2+]i in response to acetylcholine (ACh), although mobilization of Ca2+ required somewhat higher ACh concentrations in rat lacrimal acinar cells than in mandibular acinar cells. There was no evidence for different dose dependencies of the peak and plateau phases of the [Ca2+]i response. The ACh-evoked [Ca2+]i increase in rabbit mandibular acinar cells exhibited desensitization, since it declined in magnitude when cells were stimulated repeatedly with a maximal dose of agonist. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) rapidly and irreversibly decreased the ACh-evoked [Ca2+]i signals in rat lacrimal acinar cells and reduced ACh-stimulated inositol phosphate accumulation. This inhibitory effect of TPA was most marked in cells stimulated with low doses of ACh, implying that TPA treatment shifted the ACh dose response curve to higher ACh concentrations. In contrast to the results obtained with lacrimal acinar cells, TPA had no effect on the [Ca2+]i and inositol phosphate responses to ACh in either rat or rabbit mandibular acinar cells. These results suggest that, although ACh-evoked [Ca2+]i signals, and hence presumably the stimulus-response coupling machinery, are very similar between different acinar cell types, acinar cells show marked differences in their sensitivity to phorbol esters. The insensitivity of mandibular acinar cell [Ca2+]i signals to TPA also suggests that the secretory tachyphylaxis observed in perfused rat and rabbit mandibular salivary glands is unlikely to be a consequence of negative feedback mediated by protein kinase C.

Continuous measurement of cell volume changes in perfused rat salivary glands by proton NMR

Changes in intracellular and extracellular water content have been measured in perfused rat salivary glands by repetitive application of an inversion recovery (IR) pulse sequence. The relaxation reagent Gd-DTPA (10 mM) was included in the perfusate so that the intracellular and extracellular water proton signals could be distinguished by their different longitudinal relaxation times. Changes in water content in response to altered perfusion pressure and perfusate osmolarity were determined at 30-s intervals and indicated a clear separation of the intracellular and extracellular components. Using a modification of the IR pulse sequence, changes in intracellular water content were also measured at 6-s intervals. With this time resolution, differences in the rates of cell shrinkage in response to hyperosmotic perfusates and the secretomotor agonist acetylcholine were observed. The results suggest that this approach offers a relatively noninvasive method for studying cell volume regulation in intact, perfused tissues and organs.

Inhibitors of the intracellular Ca2+ release mechanism prevent muscarinic-induced Ca2+ influx in rat sublingual mucous acini

The effects of inhibitors of the intracellular Ca2+ release mechanism on divalent cation fluxes were examined in acinar cells loaded with the Ca(2+)-sensitive, Mn(2+)-quenchable dye, fura-2. TMB-8 and dantrolene (DTL) dramatically inhibited the carbachol (CCh)-stimulated increase in [Ca2+]i and Mn2+ influx. These agents do not directly inhibit divalent cation entry since addition of TMB-8 or DTL after CCh stimulation did not block Mn2+ influx. TMB-8 did not influence the [Ca2+]i increase or the Mn2+ influx produced by thapsigargin. These results indicate that TMB-8 and DTL do not interfere with divalent cation influx by inhibiting a step distal to depletion of the intracellular Ca2+ pool. TMB-8 and DTL did not significantly influence the muscarinic-stimulated production of inositol trisphosphate (IP3) and inositol tetrakisphosphate (IP4), although TMB-8, but not DTL, did decrease the CCh-stimulated 1,4,5-IP3 levels approximately 55%. The above results directly demonstrate that the filling state of the intracellular Ca2+ store primarily regulates the Ca2+ entry mechanism in sublingual mucous acinar cells.

Membrane potential regulates Ca2+ uptake and inositol phosphate generation in rat sublingual mucous acini

In salivary acinar cells, muscarinic-induced fluid secretion is associated with a 1,4,5-IP3 induced increase in the cytosolic free Ca2+ concentration ([Ca2+]i), which in turn activates Ca(2+)-dependent K+ and Cl- channels that modulate the membrane potential. In the present study the influence of the membrane potential on [Ca2+]i and inositol phosphates was monitored in rat sublingual mucous acini. Depolarization induced by switching from 5.8 mM extracellular K+ ([K+]e) to 116 mM [K+]e resulted in a transient increase in the [Ca2+]i measured using the Ca2+ sensitive fluorescent indicator Fura-2. This initial rapid (t1/2 approximately 5 s) increase (approximately 3-fold) in [Ca2+]i was dependent on extracellular Ca2+, insensitive to nifedipine, and followed by establishment of a 'new' resting [Ca2+]i, approximately 35% higher than the level in physiological [K+]e. Depolarization also induced a significant rise in the resting cellular inositol trisphosphate (IP3) and inositol tetrakisphosphate (IP4) contents, but not 1,4,5-IP3 content. Stimulation with 10 microM carbachol (CCh, a muscarinic agonist) produced a biphasic increase in [Ca2+]i, the initial transient phase due to mobilization of Ca2+ from an intracellular pool, and a sustained phase mediated by an influx of Ca2+. Membrane depolarization had no effect on the initial phase, while, the sustained increase in [Ca2+]i was eliminated. The CCh-enhanced quench of the Fura-2 signal by Mn2+ (an index for divalent cation entry) was reversibly inhibited by depolarization. The enhanced Mn2+ uptake induced by inhibiting microsomal Ca(2+)-ATPase with thapsigargin was similarly inhibited by membrane depolarization, consistent with the effect of depolarization primarily acting on the Ca2+ entry pathway and not on receptor coupling. Depolarization did not alter the initial CCh-induced increases in IP3, IP4 or 1,4,5-IP3 content, or the sustained increase in 1,4,5-IP3, whereas, depolarization significantly blunted (> 70%) the sustained, CCh-induced generation of IP3 and IP4. The membrane potential, therefore, appears to modulate Ca2+ activated fluid secretion by controlling the driving force for Ca2+ entry via a depletion-activated Ca2+ entry pathway. Inositol phosphate metabolism is also influenced by the membrane potential, but this effect apparently plays a minor role in regulating [Ca2+]i since 1,4,5-IP3 levels were unchanged by depolarization.

Effects of acetylcholine, isoprenaline and forskolin on electrolyte and protein composition of rabbit mandibular saliva

1. The major purpose of this study was to investigate cellular regulation of the ductal transport processes in salivary glands which act to modify the electrolyte composition of primary saliva and cause it to become hypotonic. This was achieved using an isolated mandibular gland preparation by observing the effect of different stimuli on the electrolyte composition of saliva secreted at the same flow rate, on the assumption that these stimuli do not influence primary saliva composition. The effects of the same stimuli on the volume of primary fluid secretion and on protein secretion were also observed. Proteins were measured in total and as individual components after their separation by high-performance liquid chromatography. 2. Acetylcholine was used as a 'Ca2+-mobilizing' agonist (i.e. one which both elevates intracellular Ca2+ concentration and activates protein kinase C). Isoprenaline was initially used to elevate intracellular cyclic AMP concentration but was subsequently abandoned in favour of forskolin. 3. Acetylcholine was a very potent stimulus of primary fluid secretion. By contrast, isoprenaline and forskolin were essentially without effect, even when superimposed on acetylcholine stimulation. 4. As judged by saliva electrolyte composition, increasing the concentration of acetylcholine enhanced ductal absorption of Na+ and Cl- and secretion of K+ (and presumably HCO3-). Forskolin had the opposite effect: when superimposed on submaximal acetylcholine stimulation it caused saliva concentrations of Na+ and Cl- to remain high and K+ low (i.e. it inhibited ductal transport processes). The inhibitory effect of forskolin on ductal transport could be overcome by increasing the concentration of acetylcholine, and vice versa. 5. Acetylcholine, isoprenaline and forskolin each increased salivary protein secretion, although the kinetics of secretion differed. The spectrum of proteins secreted in response to the three stimuli was the same. The relative proportions of the individual proteins was influenced by the strength of stimulation (i.e. the proportions at high total protein output differed from those at low total protein output) but not apparently by the nature of the stimulus. 6. Thus, the three major secretory processes in the rabbit mandibular salivary gland respond differently to the two major signal transduction mechanisms. For primary fluid secretion, Ca2+ is stimulatory and cyclic AMP almost without effect; for ductal transport, Ca2+ is stimulatory and cyclic AMP inhibitory; and for protein secretion both Ca2+ and cyclic AMP are stimulatory.

Ca2+ not cyclic AMP mediates the fluid secretory response to isoproterenol in the rat mandibular salivary gland: whole-cell patch-clamp studies

We have performed whole-cell patch-clamp studies on dispersed secretory cells of the rat mandibular gland to determine how beta-adrenergic stimulation causes fluid secretion. When the pipette contained a high K+ solution, the resting membrane potential averaged -33 mV +/- 1.1 (SEM, n = 34) and the clamped cell showed strong outward rectification. We monitored K+ and Cl- currents for periods of 15 min by recording the currents needed to clamp the cell potential at 0 and -80 mV, respectively. Isoproterenol (1-2 mumol/l) caused increases in the clamp current at 0 mV (the K+ current) and at -80 mV (the Cl- current) in about 80% of cases, although the responses were variable in size and time-course; the responses were indistinguishable from those induced by acetylcholine or the Ca2+ ionophore, A23187. The alpha-adrenergic antagonist, phentolamine (1-2 mumol/l), had no effect on the response, but the beta-adrenergic antagonist, propranolol (10 mumol/l), blocked it completely. The isoproterenol response could not be mimicked by application to either surface of the cell membrane, of cyclic AMP (100 mumol/l), forskolin (1 or 20 mumol/l) or cholera toxin (2.5 micrograms/ml). However, increasing the Ca2+-chelating capacity of the pipette solution by raising its EGTA concentration from the customary 0.5 to 20 mmol/l, blocked the response to isoproterenol, suggesting that beta-adrenergic agonists activate Cl- and K+ channels by raising cytosolic Ca2+. Since neomycin, which blocks phospholipase C, blocked the action of isoproterenol without impairing the cell responsiveness to A23187, it appears that isoproterenol, like muscarinic agonists, increased cytosolic Ca2+ via the phosphatidylinositol cycle.

Evidence for apical chloride channels in rabbit mandibular salivary glands. A chloride-selective microelectrode study

Double-barrelled, chloride-selective microelectrodes were used to study mandibular gland acinar cells at rest and during cholinergic stimulation. At rest, intracellular chloride activity was five times the expected equilibrium activity. During sustained stimulation with acetylcholine, chloride activity fell to three times the expected equilibrium activity. Thus, the gradient for chloride exit was reduced in the stimulated cell. These results lead to the conclusion that stimulation increases the permeability of the acinar cell to chloride. Experiments in which extracellular chloride was removed provided evidence that the permeability increase was due to opening of chloride channels located principally in the apical membrane of the acinar cell.

Ion transport and water movement

Secretion of water and electrolytes in salivary glands occurs by a dual process involving the formation of a plasma-like, isotonic primary secretion in salivary acini and its subsequent modification in salivary ducts by the removal and addition of specific ions. The mechanisms underlying the formation of primary acinar secretion have been investigated with a number of experimental approaches such as electrophysiology, the measurement of ion transport in gland fragments and dispersed acinar cells, and the evaluation of the ionic requirements for secretion in isolated, perfused gland preparations. The accumulated evidence suggests that salivary secretion is formed by a complex interaction between passive and active ion movements across acinar cell membranes, resulting in the trans-acinar movement of Cl- and Na+ and, by the osmotic gradient which develops, of water. A major consequence of stimulation is the release of K+ through Ca++- and voltage-sensitive channels and its subsequent recycling back into the cells by ouabain- and furosemide-sensitive transport systems. This results in NaCl uptake across the basolateral cell membrane and the subsequent efflux of Cl through luminal membrane channels, which also appear to be sensitive to cellular Ca++. The rates of these various ion movements appear to be, therefore, closely linked and interdependent. Ductal modification of the primary secretion has been studied in microperfused duct preparations. The evidence likewise indicates that it involves interactions between complex conductance pathways in the luminal cell membrane and a Na, K pump present in the basolateral cell membrane and that it is under autonomic and hormonal control. Activation of ductal transport mechanisms results in NaCl reabsorption and KHCO3 secretion. Final saliva thus differs from primary secretion in electrolyte composition and, because water permeability is low in the duct epithelium, becomes hypotonic. Alterations in fluid and electrolyte secretion such as those observed in disease can result, therefore, from disturbances in one or more of these complex transport processes in acinar or duct cells.

36Cl fluxes in dispersed rat submandibular acini: effects of Ca2+ omission and of the ionophore A23187

Transmembrane fluxes of 36Cl were investigated in dispersed acini of the rat submandibular gland in Ca2+-containing and Ca2+-free media and also in the presence of the divalent cation ionophore A23187. In Ca2+-replete medium, a time-dependent uptake of tracer resulted in a steady state 36Cl content of 8.5 +/- 0.3 nmol/mg protein in 3-5 min. This uptake was reduced 32% by 1 mM furosemide and 27% by 1 microM acetylcholine. In the presence of Ca2+, the ionophore (10(-5) M) reduced tracer uptake 36% and prevented further effects of either acetylcholine or furosemide. Both acetylcholine and A23187 caused a rapid net efflux of 36Cl from tracer-preloaded acini in Ca2+-containing medium (37% and 20%, respectively). When Ca2+ was omitted from the incubation medium, basal 36Cl uptake in the absence of added test substance was the same as in Ca2+-containing medium but was not affected by acetylcholine, while it was still reduced 29% by furosemide. Addition of acetylcholine to preloaded acini in Ca2+-free medium caused only a transient and unsustained 36Cl efflux but subsequent addition of Ca2+ produced a 36% reduction in tracer content. The ionophore caused a net 36Cl efflux in Ca2+-containing medium (24% decrease in 36Cl content) but had no effect in Ca2+-free medium. Subsequent addition of Ca2+ resulted in a 27% net efflux of tracer. The calmodulin inhibitor trifluoperazine caused a 14% increase in 36Cl uptake but did not cause 36Cl efflux from preloaded cells or modify acetylcholine-induced efflux of tracer from these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of calcium and calmodulin in release of kallikrein and tonin from rat submandibular gland

We investigated the role of calcium and calmodulin as intracellular mediators of kallikrein and tonin release induced by norepinephrine (NE). We studied the secretion rate of kallikrein and tonin from submandibular gland of rat in response to NE in the presence or absence of calcium, two calcium blockers, and four different calmodulin antagonists. Submandibular gland slices were incubated in vitro, and glandular kallikrein and tonin secreted into the incubation medium were determined by direct radioimmunoassays and expressed as nanograms per minute per milligram tissue. NE (10(-5) and 10(-4) M) increased the kallikrein secretion from the control value of 8.2 +/- 2.6 to 134.9 +/- 41.4 (P less than 0.05) and to 191.2 +/- 62.7 (P less than 0.05), and the release of tonin from a basal rate of 3.5 +/- 0.6 to 51.5 +/- 9.1 (P less than 0.05) and to 64.4 +/- 13.7 (P less than 0.05). The deletion of calcium and addition of EGTA into the incubation medium significantly attenuated the secretion of kallikrein and tonin induced by NE. Nifedipine, at concentrations which inhibit voltage-dependent calcium channels, did not affect the release of kallikrein and tonin, and only a high concentration (10(-4) M) reduced the release. TMB-8, a blocker of intracellular calcium, had no effect either. Phenothiazines, triflupromazine (10(-6) M) and trifluoperazine (10(-4) M), decreased significantly the kallikrein release elicited by 10(-5) M NE.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of muscarinic cholinergic receptors in the submandibular gland of the rat

The specific muscarinic ligand [3H]quinuclidinyl benzilate ([3H]QNB) was used to label acetylcholine receptors in the submandibular gland of the rat. Specific binding of [3H]QNB increased linearly with tissue concentration in the range of 0.02-0.3 mg of protein/ml. Kinetic analysis of [3H]QNB binding revealed the presence of a single population of high affinity binding sites, with a dissociation constant of 87.2 pM and a Hill coefficient of 0.95. The binding was saturable and the receptor density was 214 fmol/mg of protein. The rate constants at 37 degrees C for association and dissociation of the [3H]QNB-receptor complex were 5.98 X 10(-8) M-1 X min-1 and 6.6 X 10(-3) X min-1, respectively. The ratio k-1/k+1 gave a Kd value of 11.1 pM, similar to the Kd value (13.1 pM) determined by kinetic parameters when extrapolated at infinitely low receptor concentration. Muscarinic antagonists displaced [3H]QNB from muscarinic receptors with a Hill coefficient near to 1.0. Displacement curves for muscarinic agonists and for the atypical antagonist pirenzepine had Hill values significantly less than one. In the presence of 0.1 mM GPP(NH)P, the potency of agonists but not antagonists in displacing [3H]QNB binding decreased 2 to 3-fold. The [3H]QNB binding site was sensitive to the inhibitory effect of various sulfhydryl reagents. Repeated treatments of rats with an acetylcholinesterase inhibitor led to a decreased density of muscarinic receptors in the submandibular gland. This alteration was specific for the muscarinic recognition site and was paralleled by a reduced sensitivity to carbachol.