Tracheal submucosal gland serous cells stimulated in vitro with adrenergic and cholinergic agonists. A morphometric study

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Mucus secretion from individual submucosal glands of the ferret trachea

Mucus secretion from individual tracheal glands in adult ferrets was studied with time-lapse optical imaging of mucus droplets under an oil layer. Density of functional glands (determined by responses to 1 muM carbachol) was 1.5 +/- 0.3 per mm(2) (n = 6). Secretion rates (in pl.min(-1).gland(-1)) were as follows: 4.1 +/- 0.7 basal (unstimulated; n = 27, 669 glands), 338 +/- 70 to 10 microM forskolin (n = 8, 90 glands), 234 +/- 13 to 1 microM VIP (n = 6, 57 glands), 183 +/- 92 to 10 microM isoproterenol (n = 3, 33 glands), 978 +/- 145 to 1 microM carbachol (n = 11, 131 glands), and 1,348 +/- 325 to 10 muM phenylephrine (n = 7, 74 glands). The potency (EC(50), in microM) and efficacy (V(max), in pl x min(-1) x gland(-1)) were 7.6 (EC(50)) and 338 +/- 16 (V(max)) to forskolin, 1.0 (EC(50)) and 479 +/- 19 (V(max)) to VIP, 0.6 (EC(50)) and 1,817 +/- 268 (V(max)) to carbachol, and 3.7 (EC(50)) and 1,801 +/- 95 (V(max)) to phenylephrine. Although carbachol and phenylephrine were equally effective secretagogues, only carbachol caused contractions of the trachealis muscle. Synergy was demonstrated between 300 nM isoproterenol and 100 nM carbachol, which, when combined, produced a secretion rate almost fourfold greater than predicted from their additive effect. The dependence of fluid secretion on Cl(-) and HCO(3)(-) varied depending on the mode of stimulation. Secretion stimulated by VIP or forskolin was reduced by approximately 60% by blocking either anion, while carbachol-stimulated secretion was blocked 68% by bumetanide and only 32% by HEPES replacement of HCO(3)(-). These results provide parametric data for comparison with fluid secretion from glands in ferrets lacking CFTR.

Regulation of secretion from serous and mucous cells in the trachea

The physical properties of mucus and the efficiency of tracheal mucociliary clearance depend on maintenance of a balanced interaction among several epithelial cell types. Some of these cell types are specialized to perform ion and water transport, others to perform synthesis and secretion of macromolecules. Our studies have been aimed specifically at identifying the neural mechanisms regulating macromolecule secretion from two of these cell types, i.e. serous and mucous gland cells. Because these cells occur as part of a complex epithelium, it is difficult to monitor the properties and functions of each cell type individually. We have therefore relied principally on morphological methods, which can potentially focus on a single cell type within a heterogeneous tissue. Such studies, however, depend on the availability of visible markers (enzyme-labelled antibodies, radioligands, etc.), and many important aspects of gland cell function cannot be assessed morphologically. Two alternative approaches are therefore being developed: the isolation and segregation of gland cells according to type, and the production of monoclonal antibodies that recognize secretory products of individual cell types. These methods allow serous and mucous cells to be studied by biochemical as well as morphological methods.

Airway mucus: composition and regulation of its secretion by neuropeptides in vitro

Human and canine airway mucosa in vitro synthesizes and secretes mucus glycoprotein, proteoglycans and lipids which can be separated by density gradient ultracentrifugation in caesium bromide. In secretions from unstimulated explants, the small amount of mucus glycoprotein present is found in association with proteoglycans. 'Free' mucus glycoprotein of typical buoyant density is present only after stimulation of submucosal gland secretion by methacholine. Lipids are synthesized, at least in part, by the airway mucosa and occur in explant secretions as a viscoelastic gel, suggesting that they significantly influence the rheological properties of airway mucus. In addition to cholinergic and adrenergic secretomotor neurons, the airway mucosa is innervated by peptidergic fibres containing immunoreactivity to vasoactive intestinal peptide (VIP) and substance P (SP). In explants of non-bronchitic human airway, VIP inhibits baseline glycoprotein and lysozyme secretion; in canine airway mucosa, by contrast, VIP is a weak partial secretory agonist. SP is the most potent agonist of canine airway glycoprotein release described to date and appears to evoke secretion by a direct action on a stereospecific SP receptor rather than by inducing release of other endogenous secretagogues. VIP and SP have little effect on glycoprotein discharge by mucous and serous cells of the submucosal gland; SP appears to induce secretion by causing contraction of submucosal gland ducts. This may represent the most rapid way for delivering mucus into the airway in response to injury or irritation of airway epithelium.

Airway glandular development and stem cells

Submucosal glands in the lung play important roles in several hypersecretory lung disease processes, including chronic bronchitis, asthma, and cystic fibrosis. In this context, submucosal glands undergo abnormal growth and differentiation through processes that are poorly understood. To better understand the pathophysiological mechanisms that lead to submucosal gland hypertrophy and hyperplasia in the adult human lung, efforts have been made to dissect the molecular signals and cell types responsible for normal submucosal gland development in the airway. Such studies have revealed a close relationship between progenitor?stem cell phenotypes in the surface airway epithelia and submucosal glands, and thus it has been suggested that submucosal glands serve as a protective niche for surface airway epithelial stem cells. Furthermore, the pluripotent progenitor cells that exist in the surface airway epithelium, which have the capacity to differentiate into ciliated, secretory, intermediate, and basal cells, also have a developmental capacity for submucosal glands. This putative adult stem cell compartment of the airway epithelium has been the focus of research attempting to identify molecular markers for signaling pathways that control stem cell phenotypes and their capacity for proliferation and differentiation following airway injury.

Effects of electromagnetic radiation from a cellular telephone on epidermal Merkel cells

The number of reports on the effects induced by electromagnetic radiation (EMR) from cellular telephones in various cellular systems is still increasing. Until now, no satisfactory mechanism has been proposed to explain the biological effects of this radiation except a role suggested for mast cells. Merkel cells may also play a role in the mechanisms of biological effects of EMR. This study was undertaken to investigate the influence of EMR from a cellular telephone (900 MHz) on Merkel cells in rats. A group of rats was exposed to a cellular telephone in speech position for 30 min. Another group of rats was sham-exposed under the same environmental conditions for 30 min. Exposure led to significantly higher exocytotic activity in Merkel cells compared with the sham exposure group. This finding may indicate the possible role of Merkel cells in the pathophysiology of the effects of EMR.

Effects of beta-agonists on airway epithelial cells

beta-Adrenergic receptor (betaAR) agonists exert a variety of effects on airway epithelial cells. Among their best known actions is their ability to increase ciliary beat frequency, mediated by cyclic adenosine monophosphate (cAMP) production, stimulation of protein kinase A (PKA), and phosphorylation of an outer dynein arm light chain. Submucosal glands express betaARs, and beta-agonists may stimulate secretion of mucus from airways, although human data are controversial. beta-Agonists may also affect ion transport across epithelial cells by opening apical ion channels such as the cystic fibrosis transmembrane regulator. This effect, likely to occur in submucosal glands, can influence water fluxes across the airway epithelium and may have profound influences on mucus hydration. betaAR activation can increase intracellular calcium in some ciliated cells, thereby stimulating ciliary beating and possibly influencing transepithelial ion transport. betaAR-mediated activation of cAMP-dependent protein kinase accelerates epithelial cell migration, thereby enhancing epithelial wound repair. beta-Agonists reduce the ultrastructural damage seen with infection and potentiate secretion of certain cytokines from epithelial cells while inhibiting secretion of others. Finally, beta-agonists may have effects on airway epithelial cells that are mediated through betaARs but do not require cAMP production. The signaling mechanisms of some beta-agonist effects are not well understood but are important to our understanding of airway epithelial cell growth, differentiation, and function.

Differential effect of epidermal growth factor on serous and mucous cells in porcine airway submucosal gland

Using a patch-clamp technique, we found that the fresh porcine submucosal gland acinar cells contained two functionally distinct cell populations, i.e. physiologically relevant concentration of acetylcholine (ACh, 30 nM) induced two distinct patterns of electric response in tracheal gland acinar cells. One was characterized by an outstanding oscillatory Cl(-)-current activity, and the other was with poor Cl(-)-current response but with a comparable K(+)-current. We examined the effect of epidermal growth factor (EGF) on the ACh-induced electric responses in these cells. EGF affected only the latter (K(+)-prominent) cell type to potentiate significantly the ACh-induced K(+)-current. An immunohistochemistry revealed that the receptor for EGF was identified preferentially on the mucous, but not serous, cells. Genistein, one of the tyrosine-kinase inhibitors, abolished the augmentation effect of EGF on the ACh-induced current. Thus, we identified the serous cell with a Cl(-)-rich current in response to ACh and the mucous cell with a K(+)-dominant response. Moreover, EGF affected the mucous cells alone to potentiate the ACh-induced electric response. EGF may contribute to the pathophysiological alterations in chronic inflammatory airways both in morphological (mucous cell hypertrophy/hyperplasia) and functional (thick viscous hypersecretion) ways.

Ion transport by sheep distal airways in a miniature chamber

Ion transport and the electric profile of distal airways of sheep lungs were studied in a miniature polypropylene chamber with a 1-mm aperture. Small airways with an inner diameter < 1 mm were isolated, opened longitudinally, and then mounted as a flat sheet onto the 1-mm aperture where it was glued and secured with an O-ring. Both sides of the tissue were bathed with identical physiological solutions at 37 degrees C and oxygenated. Pooled data from 27 distal airways showed an inner airway diameter of 854 +/- 22 (SE) microm and a transepithelial potential difference (PD) of 1.86 +/- 0.29 mV, lumen negative. Short-circuit current (I(sc)) was 25 +/- 3.5 microA/cm(2), tissue resistance was 96 +/- 14 Omega, and conductance was 15.2 +/- 1.7 mS/cm(2). At baseline, amiloride-sensitive Na transport accounted for 51% of I(sc) (change in I(sc) = 9.7 +/- 2.6 microA/cm(2); n = 8 airways), corresponding to 0.36 microeq. cm(-2). h(-1). Treatment with 0.1 mM bumetanide did not reduce the I(sc) (n = 5 airways). Exposure to 1 microM Ca ionophore A-23187 raised the I(sc) by 9 microA/cm(2) (47%; P < 0.03; n = 6 airways). The latter effect was blunted by bumetanide. Carbachol at 1 microM provoked a biphasic response, an initial rapid rise in I(sc) followed by a decline (n = 3 airways). There was no significant increase in PD or I(sc) in response to isoproterenol or dibutyryl cAMP. The data suggest that Na absorption constitutes at least 50% of baseline transport activity. Cl or other anion secretion such as HCO(3) appears to be present and could be stimulated by raising intracellular Ca.

Submucosal gland secretions in airways from cystic fibrosis patients have normal [Na(+)] and pH but elevated viscosity

Fluid and macromolecule secretion by submucosal glands in mammalian airways is believed to be important in normal airway physiology and in the pathophysiology of cystic fibrosis (CF). An in situ fluorescence method was applied to measure the ionic composition and viscosity of freshly secreted fluid from airway glands. Fragments of human large airways obtained at the time of lung transplantation were mounted in a humidified perfusion chamber and the mucosal surface was covered by a thin layer of oil. Individual droplets of secreted fluid were microinjected with fluorescent indicators for measurement of [Na(+)], [Cl(-)], and pH by ratio imaging fluorescence microscopy and viscosity by fluorescence recovery after photobleaching. After carbachol stimulation, 0.1--0.5 microl of fluid accumulated in spherical droplets at gland orifices in approximately 3--5 min. In gland fluid from normal human airways, [Na(+)] was 94 +/- 8 mM, [Cl(-)] was 92 +/- 12 mM, and pH was 6.97 +/- 0.06 (SE, n = 7 humans, more than five glands studied per sample). Apparent fluid viscosity was 2.7 +/- 0.3-fold greater than that of saline. Neither [Na(+)] nor pH differed in gland fluid from CF airways, but viscosity was significantly elevated by approximately 2-fold compared to normal airways. These results represent the first direct measurements of ionic composition and viscosity in uncontaminated human gland secretions and indicate similar [Na(+)], [Cl(-)], and pH to that in the airway surface liquid. The elevated gland fluid viscosity in CF may be an important factor promoting bacterial colonization and airway disease.

Signal transduction of mucous secretion by bronchial gland cells

Bronchial glands, which consist of mucous and serous cells, are abundant in human airways, playing a major role in the airway secretion. Cl(-) secretion is accompanied by water transport to the lumen in the acinar cells of bronchial glands. Agonists that increase [Ca(2+)]i induce the Cl(-) secretion in bronchial glands. Ca(2+) release from a IP(3)-sensitive Ca(2+) pool at the apical portion stimulates and opens Ca(2+)-sensitive Cl(-) channels at the apical membrane, producing Cl(-) secretion in bronchial glands. K(+) channels at the basolateral membranes are Ca(2+)-sensitive and activated by Ca(2+) release from a cADPribose-sensitive Ca(2+) pool, maintaining the Cl(-) secretion in bronchial glands. Further, cADP ribose in concert with IP(3) induce [Ca(2+)]i oscillation, inducing Cl(-) secretion in bronchial glands. Some tyrosine kinases are involved in the Cl(-) secretion in bronchial glands. Mucous and serous cells in bronchial glands take part in mucin secretion and the secretion of defensive substances (glycoconjugates), respectively. [Ca(2+)]i oscillations are shown to play a central role in the exocytosis of secretory granules in serous cells of bronchial glands. Other signal transductions of mucin and glycoconjugates in airway gland cells remain to be studied, although agonists which increase [cAMP]i are also well known to induce mucin and glycoconjugate secretion from airway glands.

Cholinomimetic action of macrolide antibiotics on airway gland electrolyte secretion

We investigated the acute effects of erythromycin (EM) and its derivatives on ionic currents in airway glands from feline tracheae. Therapeutic concentrations of EM or clarithromycin (CAM) attenuated the whole cell currents evoked by ACh in a competitive manner. The maximally stimulated inward Cl- currents were reduced to 54 and 83% and the outward K+ currents to 55 and 84% of control values by EM and CAM, respectively, whereas the responses induced by phenylephrine, norepinephrine, caffeine, or ionomycin were unaffected by EM, CAM, or EM523, a synthetic derivative of EM. K+ channels in excised outside-out patches were not influenced by macrolides. Although therapeutic concentrations of macrolides showed no effect on the baseline currents, high concentrations of macrolides alone evoked currents mimicking the ACh response, which were abolished completely by atropine. We concluded that macrolides act as a partial agonist on cholinergic receptors, resulting in a reduction of Cl- secretion at pharmacological doses of the agents, which may exhibit a pronounced effectiveness on hypertrophied and/or cholinergically sensitized submucosal glands in pathological airways.

Hyperventilation-induced airway injury and vascular leakage in dogs: effects of alpha1-adrenergic agonists

alpha1-Adrenergic agonists inhibit hyperventilation-induced bronchoconstriction (HIB) in dogs. We tested the hypothesis that alpha-agonists inhibit HIB by reducing bronchovascular leakage and edema that theoretically could cause airway obstruction. Peripheral airways were isolated by using a bronchoscope; pretreated with either methoxamine (Mx), norepinephrine (NE), or saline aerosol; and then exposed to a 2,000 ml/min dry-air challenge (DAC) for 2 min. Colloidal carbon was injected before DAC and used to quantify bronchovascular permeability. Mx-, NE-, and vehicle-treated airways were prepared for morphometric analysis within 1 h after DAC. Light microscopy revealed that the 2-min DAC produced minimal bronchovascular leakage and little epithelial damage. However, pretreatment with either Mx or NE significantly enhanced dry air-induced bronchovascular hyperpermeability and mucosal injury. The increased damage associated with these alpha1-agonists implicates a protective role for the bronchial circulation. The fact that alpha1-agonists inhibit HIB suggests that neither dry air-induced leakage nor injury directly contributes to the development of airway obstruction. In addition, our data suggest that alpha-agonists attenuate HIB in part by augmenting hyperventilation-induced bronchovascular leakage and by replacing airway water lost during a DAC.

The effect of K+ channel openers on submucosal gland function and epithelial transport of the ferret trachea, in vitro

The effects of three K+ channel openers on lysozyme output from submucosal gland serous cells and epithelial albumin transport following maintained submaximal stimulation by the secretagogues methacholine and phenylephrine were examined in the ferret trachea in vitro preparation. The K+ channel openers Ro 31-6930, 2-(6-cyano-2,2-dimethyl-2H-1-benzopyran-4-yl)-pyridine 1-oxide (10 nM-10 microM), levcromakalim, BRL38227 (10 nM-10 microM) and pinacidil (100 nM-10 microM) produced a concentration dependent inhibition of (20 microM) methacholine-induced lysozyme output, with pD2 values of 7.64, 7.72 and 7.28 respectively. Ro 31-6930 (10 nM-10 microM), levcromakalim (10 nM-10 microM) and pinacidil (1 nM-10 microM) also produced a concentration dependent inhibition of (100 microM) phenylephrine-induced lysozyme output, with pD2 values of 7.64, 6.55 and 9.16 respectively. Furthermore, glibenclamide (1 microM) produced a modest attenuation of the K+ channel opener effects on secretagogue-induced lysozyme output. All three K+ channel openers failed to produce any significant change in either methacholine or phenylephrine-induced albumin outputs. The K+ channel openers exerted marked effects on airway secretion processes, suggesting that these compounds may have an antisecretory effect. The relevance of the use of the K+ channel openers in airway disease remains to be determined.

Polarized release of lipid mediators derived from phospholipase A2 activity in a human bronchial cell line

The release of arachidonic acid (AA) and platelet activating factor (PAF) from airway epithelial cells may be an important mediating factor in lung physiological and inflammatory processes. The type of lung response may be determined by the directional release of AA and PAF. We used the human bronchial epithelial cell line, BEAS2B (S6 subclone; BEAS), to investigate the polarized release of AA and PAF from lung epithelial cells. BEAS, grown on Transwell filters, were prelabeled with either 3H-AA or 3H-lyso-PAF. 3H-AA products and 3H-PAF were analyzed by high performance liquid chromatography and thin layer chromatography, respectively. BEAS incubated with melittin (2-4 micrograms/ml for 15 min) had an increased release (compared to vehicle-incubated cells) of both free 3H-AA and 3H-PAF into the apical compartment but not into the basolateral compartment. Treatment of the BEAS cells with the phospholipase A2 (PLA2) inhibitor mepacrine (1 mM) prior to, and during, incubation with melittin inhibited the increase in 3H-AA and 3H-PAF release into the apical compartment by 65% and 100%, respectively. Exposure of BEAS cells to ozone (O3; 1.0 ppm for 15 min) increased the release of polar 3H-AA products as well as 3H-PAF into both the apical and basolateral compartments. Mepacrine did not significantly inhibit the O3-induced release of polar 3H-AA products or 3H-PAF into either the apical or basolateral compartments. These data suggest the direction of the release of 3H-AA (or 3H-AA products) and 3H-PAF is stimulus-specific and that PLA2 involvement in the release of the lipids is also dependent on the stimulus. The directional release of AA, AA products, and PAF may be important in the airways responses to various agonists and oxidants.

Bethanechol and a G-protein activator, NaF/AlCl3, induce secretory response in Paneth cells of mouse intestine

Paneth cells located at the bottom of intestinal crypts may play a role in controlling the bacterial milieu of the intestine. Using morphometry to clarify the secretory mechanism of the Paneth cells, we studied the ultrastructural changes in mouse Paneth cells produced following intra-arterial perfusion with Hanks' balanced salt solution containing a cholinergic muscarinic secretagogue (bethanechol), a neuroblocking agent (tetrodotoxin), or a G-protein activator (NAF/AlCl3). Bethanechol (2 x 10(-4) mol/l) induced Paneth-cell secretion. Many Paneth cells massively exocytosed their secretory material into the crypt lumen; the enhanced secretion caused degranulation and vacuole formation. However, tetrodotoxin (2 x 10(-6) mol/l) did not prevent the bethanechol-enhanced secretion by the Paneth cells. NaF (1 x 10(-2) mol/l) and AlCl3 (1 x 10(-5) mol/l) induced massive exocytosis of the Paneth cells; the exocytotic figures were similar to those observed in mice stimulated by bethanechol. G-protein activation was followed by a sequence of intracellular events, resulting in exocytosis.

Pirenzepine block of ACH-induced mucus secretion in tracheal submucosal gland cells

Muscarinic stimulation of mucus secretion, as measured by the release of [3H]glycoprotein, was studied in explants from the tracheal epithelium of weanling swine. The mucus glycoprotein secretion was transient, ceasing within the first 10 min of a continuous exposure to 100 microM ACh. Increasing the solution's osmotic pressure did not alter basal mucus glycoprotein secretion. Mucus glycoprotein secretion was inhibited by 2-10 microM PZP, indicating that the M3 muscarinic receptors mediate cholinergic stimulation of mucus production.

The effect of hydrogen peroxide on smooth muscle tone, mucus secretion and epithelial albumin transport of the ferret trachea in vitro

The effect of hydrogen peroxide (H2O2) was examined on baseline and on methacholine- and phenylephrine-stimulated smooth muscle tone, mucus volume and lysozyme outputs, and epithelial albumin transport of the ferret whole trachea in vitro. H2O2 (10 microM-10 mM) had no significant effect on tracheal smooth muscle tone but produced concentration-dependent increases in mucus volume, lysozyme and albumin outputs. The potential difference (P.D.) across the trachea was not changed by H2O2. Exposure of the trachea to H2O2 (1 mM) for 2 h reduced the smooth muscle contractions and lysozyme outputs due to methacholine (1 microM) and phenylephrine (10 microM). Methacholine-induced albumin output was significantly increased by H2O2 but that due to phenylephrine was not significantly affected. Exposure to H2O2 had no significant effect on the mucus volume output produced by methacholine or phenylephrine. Thus H2O2 directly stimulates submucosal gland secretion, including secretion from serous cells, and epithelial albumin transport across the ferret trachea but has no effect on tracheal smooth muscle tone. H2O2 reduces methacholine- and phenylephrine-induced smooth muscle contractions and serous cell secretion. H2O2 causes hyperresponsiveness of albumin output to methacholine but not to phenylephrine.

Reflex and chemical responses of tracheal submucosal glands in piglets

In adult animals, airway fluid secretion is enhanced reflexly via central nervous system pathways, and locally by mediators such as substance P. To evaluate the role of maturation on these regulatory mechanisms, we compared the effects of reflex stimulation and intravenous substance P administration on airway secretion in anesthetized, paralyzed and artificially ventilated piglets, 9 to 22 days of age, and older piglets all aged 10 weeks. Airway secretion was monitored by counting the hillocks appearing in the upper trachea in an exposed field of tracheal epithelium (1.2 cm2) coated with powdered tantalum. In younger animals, mechanical stimulation of the larynx had no discernible effect on tracheal submucosal gland secretion. Neither excitation of airway irritant receptors nor stimulation of pulmonary C-fiber receptors by capsaicin caused a significant increase of fluid secretion from tracheal submucosal glands. In addition, stimulation of peripheral chemoreceptors by ventilating animals with 12% O2 in N2, and 6% O2 in N2, failed to induce a substantial change in airway secretion, when compared with number of hillocks in the control period. Furthermore, administration of sodium cyanide had little or no effect on baseline secretion. In contrast, to the weak reflex responses in younger piglets electrical stimulation of the vagus nerve caused the number of hillocks to increase on average by 16.3 +/- 2.3 (P less than 0.01). In addition, local application of a pledget soaked in solution of methacholine caused the number of hillocks to increase by 32.1 +/- 5.2 (P less than 0.01). Intravenous administration of substance P also induced an augmentation in fluid secretion. Increase in concentration of substance P (10(-8), 10(-7), 10(-6), and 10(-5) M, 1 ml) was associated with a concomitant elevation in the number of activated submucosal glands (5.3 +/- 2.6, 10.0 +/- 4.4, 27.1 +/- 4.5, 41 +/- 5). In older piglets, stimulation of laryngeal mucosa, airway irritant receptors, as well as stimulation of pulmonary C-fiber receptors induced a significant increase in tracheal secretion, although stimulation of peripheral chemoreceptors had no effect on airway secretion. These data suggest that reflex responses of submucosal glands are weak during early postnatal development, however, tracheal submucosal glands do respond to exogenously administered cholinergic substances and tachykinin peptides.

The uptake of radiolabelled precursors of mucus glycoconjugates by secretory tissues in the feline trachea

1. We gave one of three radiolabelled precursors of mucus glycoconjugates ([3H]proline, [3H]glucose and [35S]sulphate) into the tracheas of anaesthetized cats for 3 h. In other cats [35S]sulphate was given by intravenous injection. 2. After a further 2 h, tracheas were removed and fixed. Serial actions were cut and alternate sections were stained with Haematoxylin and Eosin or prepared as unstained autoradiographs. Points on submucosal gland and surface epithelium were chosen with a grid on photomicrographs of the stained sections. Absorbance, which is proportional to autoradiographic grain density, was estimated on corresponding points on unstained autoradiographs by flying-spot microdensitometry. 3. With [3H]proline as precursor, the grain densities were greater over surface epithelium than over submucosal gland. With [3H]glucose, grain densities were greater over the surface epithelium in three cases, equal in one and greater over submucosal gland in the last. [35S]Sulphate, given either into the tracheal segment or intravenously, yielded grain densities that were greater over the submucosal glands than over surface epithelium. 4. The areas of submucosal gland the surface epithelium were estimated by point counting and the total content of radioactivity in the two structures estimated by multiplying mean absorbance by area. Ratios of the total radiolabel in surface epithelium to that in submucosal gland were consistently high when [3H]proline was the precursor and low with [35S]sulphate, given by either route. [3H]Glucose gave intermediate ratios. 5. Secretions washed from the trachea were subjected to gel-exclusion chromatography. Washings from tracheas labelled with [3H]proline contained some molecules eluting in the void volume of a Sepharose CL-4B column (suggesting a relative molecular mass of greater than 10(6) Da), but more of the radiolabel eluted in three peaks in the partially included volume. Density gradient ultracentrifugation of the void volume material gave radiolabelled peaks at densities of approximately 1.60 and 1.50 g ml-1, consistent with glycosylated proteins, as well as less dense material (less than 1.30 g ml-1), probably proteins with little or no glycosylation. 6. We discuss the justification of using these radiolabelled precursors to give relatively selective labelling of secretory products from submucosal gland and surface epithelium.

Bronchial hyperresponsiveness to methacholine in patients with impaired left ventricular function

To elucidate the pathogenesis of bronchospasm in congestive heart failure, we studied 23 patients with chronic impairment of left ventricular function due to coronary artery disease or dilated cardiomyopathy. In 21 of them we found marked bronchial hyperresponsiveness to methacholine. The mean dose (+/- SD) of methacholine that elicited a 20 percent decrease in the forced expiratory volume in one second (FEV1) was 421 +/- 298 micrograms, nearly the same as in patients with symptomatic asthma. In contrast, there was no bronchial response to methacholine in 9 of 10 patients who had coronary artery disease but normal left ventricular function. Administration of the bronchodilator albuterol led to a partial (43 percent) reversal of the methacholine-induced bronchial obstruction. In 12 patients, pretreatment with the alpha-adrenergic agonist methoxamine (10 mg by inhalation), a potent vasoconstrictor, fully prevented the methacholine-induced decrease in FEV1. The protective effect of methoxamine was blocked by the alpha-adrenergic antagonist phentolamine in all six patients who received this agent. We conclude that bronchial hyperresponsiveness to cholinergic agonists is frequent in patients with impaired left ventricular function and may contribute to the wheezy dyspnea commonly observed in such patients. The bronchoconstriction may be mediated at least in part by dilatation of the bronchial vessels.

Characterization of beta-adrenergic receptors in cultured bovine tracheal gland cells

We characterized the beta-adrenergic receptors that mediate secretory responses to isoproterenol in cultured bovine tracheal submucosal gland cells. Previous studies have shown that these cells have morphological and biochemical features characteristic of serous cells. Isoproterenol, epinephrine, and norepinephrine each stimulated the secretion of 35SO4-labeled macromolecules from these cultured serous cells with a rank order of potency (isoproterenol greater than epinephrine greater than norepinephrine) consistent with the presence of beta 2-adrenergic receptors. These functional studies were supported by radioligand-binding studies using [I125]-iodocyanopindolol (125I-CYP) to identify beta-adrenergic receptors. 125I-CYP binding to membrane particulates prepared from cultured serous cells was saturable and of high affinity (equilibrium dissociation constant 20 +/- 3 pM; mean +/- SE, n = 6) and was antagonized stereoselectively by propranolol. Adrenergic agonists competed for 125I-CYP-binding sites with a rank order of potency characteristic of the beta 2-adrenergic receptor subtype. A specific beta 2-adrenergic receptor antagonist, ICI 118.551, competed for a single class of 125I-CYP-binding sites with high affinity (inhibition constant 1.8 +/- 0.3 nM, n = 3). We concluded that the secretory response of cultured tracheal gland cells to isoproterenol is a response mediated by beta-adrenergic receptors of the beta 2 subtype.

The transport of albumin across the ferret in vitro whole trachea

1. The whole trachea of the ferret has been isolated in vitro in an organ bath and used to study the transport of bovine serum albumin (BSA) and two dextrans (70,000 and 9000 Da) from external buffer solution to air-filled lumen, assessed by fluorescent-labelled tracers. 2. In control conditions, when mucus secretion was not stimulated by drugs, the concentration of albumin in the lumen was over half that in the buffer, and about six times greater than those of the two dextrans. 3. Methacholine and phenylephrine caused large increases in mucus secretion and albumin output and decreases in albumin concentration. The responses were proportional to drug concentration. We concluded that albumin output is increased but diluted with submucosal gland secretion. 4. Salbutamol caused a small increase in mucus secretion and large increases in output and concentration of albumin. The concentration of albumin became greater than that in the external buffer medium. The responses were proportional to concentration of salbutamol. 5. Histamine increased mucus secretion and albumin output and concentration. 6. None of the four drugs increased the output of dextran-70,000. Methacholine and phenylephrine increased the output of dextran-9000, but to a far less extent than for albumin. 7. Cooling the trachea and buffer to 4 degrees C almost abolished the stimulation of mucus and albumin outputs due to methacholine. 8. Increasing the concentration of albumin external to the trachea did not proportionally increase albumin secretion, the logarithmic relationship suggesting saturation of an active transport system. 9. We conclude that albumin is secreted by active transport into the tracheal lumen, and that the rate of transport can be augmented by salbutamol to build up a higher concentration in the lumen than in the external buffer.

Tracheal gland mucous cells stimulated in vitro with adrenergic and cholinergic drugs

To determine the responsiveness of tracheal mucous cells to adrenergic and cholinergic stimulation, we analyzed changes in their structure induced by neurotransmitter-like agonists. Ferret tracheal rings were exposed for 30 min in vitro to one of the following: phenylephrine, isoproterenol, or bethanechol (all at 10(-5) M), in the presence of absence of appropriate antagonists. Electron microscopy and morphometric analysis revealed that the volume density of mucous cells (Vvmc, i.e. the space occupied by mucous cells in the submucosa) significantly decreased, and the surface density of mucous cell apical membrane (Svam) increased in response to isoproterenol and bethanechol but not to phenylephrine. In metabolic labeling experiments, the morphological changes were accompanied by secretagogue-evoked release of 35S-labeled macromolecules. Taken together, these data suggest that tracheal mucous cells secrete 35S-labeled macromolecules in response to beta-adrenergic and muscarinic agonists by an exocytotic process that involves a reduction in cell size.

The effects of peptide histidine isoleucine and neuropeptide Y on mucus volume output from the ferret trachea

1. The effects of peptide histidine isoleucine (PHI) and neuropeptide Y (NPY) were examined on the mucus volume output produced by methacholine and phenylephrine in the ferret whole trachea in vitro. 2. Sustained application of methacholine (5 microM) or phenylephrine (20 microM) produced a maintained volume output of mucus from the trachea. Both these agonists also increased the output of lysozyme (a marker for serous cell secretion). 3. PHI inhibited the maintained mucus volume output produced by methacholine but had no effect on that due to phenylephrine. The output of lysozyme produced by methacholine or phenylephrine was not significantly changed by PHI. 4. NPY enhanced the volume output of mucus produced by methacholine or phenylephrine; however, the rate of output of lysozyme in mucus produced by both agonists was reduced by NPY. 5. We suggest that PHI has no effect on serous cell secretion but inhibits secretion from another source, possibly mucous cells. NPY inhibits serous cell secretion but has a stronger stimulant action on secretion from another source, again possibly mucous cells. 6. PHI and NPY may be important physiological modulators of mucus volume output in the ferret trachea.

Effect of live and heat-killed bacteria on the secretory activity of Paneth cells in germ-free mice

Germ-free mice were given live or heat-killed facultative anaerobes, and the ultrastructure of ileal Paneth cells was quantitatively examined with special reference to secretory granules showing a bipartite substructure (central core and peripheral halo). After administering live or heat-killed bacteria, there was a decrease in the area occupied by the cores of secretory granules in Paneth cells, and exocytosed core material was observed in the crypt lumen. There were no changes in the area occupied by the halo of secretory granules. None of the examined Paneth cells phagocytosed bacteria. It is concluded that certain bacteria may affect the secretion of antibacterial agents contained in the secretory granules of Paneth cells.

Airway responsiveness to intravenous and inhaled acetylcholine in the guinea pig after cigarette smoke exposure

Exposure of conscious guinea pigs to cigarette smoke results in bronchial hyperresponsiveness. To examine the mechanisms involved, we measured airway responses to increasing doses of intravenous or inhaled acetylcholine in guinea pigs exposed to cigarette smoke (n = 20) or to air (n = 20). After exposure the guinea pigs were anesthetized, paralyzed, and studied in a pressure-sensitive body plethysmograph while ventilated through a tracheostomy. Two and 6 puffs of an aerosol of increasing concentrations (0.05 to 500 micrograms/ml) of acetylcholine were delivered via the tracheostomy. Intravenous acetylcholine was delivered in boluses of 0.1 ml of increasing concentrations (0.5 to 50,000 micrograms/ml) via a catheter in an external jugular vein. Pulmonary resistance (RL), dynamic compliance (Cdyn), and heart rate (HR) were measured at baseline (after aerosolized or intravenous saline) and after each dose of acetylcholine. The peak responses to both inhaled and intravenous acetylcholine were rapid in onset (less than 15 s), short-lived (3 to 4 breaths), and were noncumulative. The baseline RL, Cdyn, and HR were not different in the smoke and air exposure groups. In the intravenous acetylcholine group, there were no differences in RL, Cdyn, and HR responses between the air and smoke exposure groups. In the inhaled acetylcholine group, the dose-response curve was shifted to the left (p less than 0.05) and reached a higher maximal response (p less than 0.01) after smoke exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

The actions of methacholine, phenylephrine, salbutamol and histamine on mucus secretion from the ferret in-vitro trachea

Methacholine, phenylephrine and histamine produced highly significant and salbutamol significant increases in the rate of mucus secretion from the ferret trachea. Methacholine, phenylephrine and histamine all produced highly significant increases in the rate of output of lysozyme, but the concentration of lysozyme in the mucus was significantly increased only by phenylephrine. Salbutamol produced no significant change in the output of lysozyme, and the concentration of lysozyme in the mucus was significantly decreased. It is concluded that methacholine, phenylephrine and histamine are potent stimulators of serous cell secretion whereas salbutamol has only a weak secretory action on these cells. Methacholine, histamine and salbutamol probably stimulate secretion from mucous cells as well as from serous cells. The increase in the concentration of lysozyme produced by phenylephrine may be due to stimulation of a fluid reabsorption mechanism.

Decidual cells in the human ovary at term: II. Morphometric analysis of cytoplasmic processes and organelles

Morphometric analysis of human ovarian decidual cells was performed with a Videoplan computer, and mean values were established for the area and perimeter of cellular processes and organelles. Two-hundred forty electron micrographs representing 160 cells were analyzed. The mean decidual cell area was 218.7 microns2, of which 34.5 microns2 was occupied by the nucleus (15.8% of the cytoplasmic area); the nucleus contained 1.74 micron2 of nucleolar material (0.8%). The endoplasmic reticulum occupied 13.63 microns2 (6.2%). Mitochondria occupied 7.3 microns2 (3.3%) and the Golgi network 5.49 microns2 (2.5%). Decidual secretory bodies occupied 0.91 micron2 (0.42%) and cytoplasmic processes 1.89 micron2 (0.94%). The remainder of the cytoplasm, containing inclusions and cytoskeleton, represented 71% of the cell area. Perimeter measurements indicated an average decidual cell was surrounded by 87.8 microns of plasma membrane. The mean nuclear membrane measured 28.3 microns (representing 32.3% of the plasma membrane, pm, or 4.1% of total cellular membranes, cm). Outer mitochondrial membranes measured 156.6 microns (178% pm, 23.5% cm); endoplasmic reticulum membranes measured 350.3 microns (400% pm, 52.6% cm); Golgi membrane measured 30.77 microns (35% pm; 4.5% cm) and membrane surrounding secretory bodies measured 9.8 microns (11.2% pm; 1.4% cm). A mean of 280 secretory bodies per ovarian decidual cell was calculated. The plasma membranes of evaginated cytoplasmic processes represented 22.3% of the total pm (19.6 microns or 2.9% cm). A mean of seven such processes was observed per 87.8 microns of plasma membrane (160/cell). These morphometric data provide a baseline for comparisons of human ovarian decidual cells with uterine decidua, in vivo and in vitro, as well as with decidual cells of other species.

The effect of vasoactive intestinal peptide on smooth muscle tone and mucus secretion from the ferret trachea

The effect of vasoactive intestinal peptide (VIP) was examined on the smooth muscle contraction and mucus secretion produced by methacholine and phenylephrine in the ferret whole trachea in vitro. VIP (0.5 to 800 nM) produced a concentration-dependent relaxation of the ferret trachea contracted by methacholine (1 microM) and phenylephrine (10 microM). The concentration-response curves for methacholine- and phenylephrine-induced contractions were both shifted to the right by VIP (0.1 microM). Methacholine-induced secretion was inhibited in a concentration-dependent manner by VIP, whereas that due to phenylephrine was enhanced. The concentration-response curve for methacholine-induced secretion was shifted to the right by VIP, whereas the curve for phenylephrine was shifted to the left. Methacholine produced a concentration-dependent increase in the rate of output of lysozyme from the ferret trachea with no corresponding increase in the concentration of lysozyme in the mucus. Phenylephrine produced a concentration-dependent increase in the rate of output and in the concentration of lysozyme. VIP (0.1 microM) significantly increased the concentration of lysozyme in the mucus produced by methacholine with no increase in the rate of lysozyme output. However, the rate of lysozyme output due to phenylephrine was significantly increased by VIP (0.1 microM) with no increase in concentration. We suggest that VIP inhibits secretion from mucous cells stimulated by methacholine, and enhances the secretion produced by phenylephrine from serous cells.

Autoradiographic studies of the distribution of 35sulfate label in ferret trachea: effects of stimulation

Na2(35)SO4 is among the common isotopic mucin precursors used to label airway secretory cells and monitor their active discharge in response to drugs. Previous work has established that 35S is taken up by multiple cell types in the trachea, yet there is no direct evidence linking 35S release with secretory activity by any specific cell type. In this study, we have used autoradiography to identify the sites of uptake and release of 35S in ferret trachea. Confirming work performed in other species, we found uptake sites include surface epithelium (ciliated and goblet cells), submucosal glands (serous and mucous cells) and cartilage. Extending these findings using a "pulse-chase" protocol, we found that 35S turns over very rapidly in ciliated but not submucosal gland cells or cartilage. Specific grain density over epithelium declined from 0.12 +/-0.006 grains/micron 2 immediately after the pulse to 0.05 +/- 0.004 grains/micron 2 at 4 h. In contrast, corresponding figures for the glands and cartilage showed no spontaneous loss of label during the same period. At 4 h, when the epithelium contained very little 35S label, we exposed tracheal rings in vitro to neurotransmitter receptor agonists (bethanechol, phenylephrine, and isoproterenol; all at 10(-5) M). Counts in the medium (determined by scintillation spectrometry) increased 2-3 times in response to each agonist. These increases were prevented by preincubating tracheal rings with appropriate antagonists. Autoradiography showed that stimulated glands contained many fewer silver grains than untreated or antagonist-blocked glands. In contrast, neither cartilage nor epithelium showed decreased labeling after stimulation. These results indicate that (a) sulfated glycoconjugates turn over rapidly in the tracheal epithelium and may account for much sulfated material spontaneously released into organ culture medium and into the tracheal lumen, and (b) 4 h after Na2(35)SO4 incubation, the major source of 35S-labeled macromolecules released from ferret trachea by neural agonists is the submucosal glands.

Neuropeptides degranulate serous cells of ferret tracheal glands

To determine whether serous or mucous cells in tracheal submucosal glands respond to the neuropeptides substance P (SP) and vasoactive intestinal peptide (VIP), we studied the peptide-induced changes in gland cell morphology accompanying release of 35SO4-labeled macromolecules from tracheal explants of ferrets. Explants were labeled for 1 h in medium containing 35SO4 and washed for 3.5 additional hours. Base-line secretion in the absence of drugs declined between 1.5 and 3.5 h after the pulse. Between 2.5 and 3.5 h, the average percent change in counts per minute recovered per sample period was not significantly different from zero (P greater than 0.3; n = 6). Substance P (10(-5) M) and VIP (2 X 10(-6) M) added 4 h after labeling each increased greatly the release of 35SO4-labeled macromolecules (SP, 219%; VIP, 180%) above base line. Bethanechol, a muscarinic-cholinergic agonist (10(-5) M), increased secretion by an average of 142% above base line (each effect, P less than 0.05; n = 6 each). Light and electron microscopy of the control tissues showed glands with narrow lumens and numerous secretory granules. Glands treated with SP or VIP had enlarged lumens and the serous cells were markedly degranulated. These phenomena were documented by morphometry and suggest that SP and VIP cause secretion from glands at least partially by stimulating exocytosis from serous cells.

Control of mucoglycoprotein output from the rabbit nose

A method for the collection of rabbit nasal washings to analyse outputs of mucous glycoproteins is described. The radiolabels sodium [35S]sulphate and [3H]glucose are bound to the glycoproteins. The release of bound 35S and 3H was enhanced by cervical sympathetic nerve stimulation. Adrenoceptor agonists (phenylephrine, dobutamine and salbutamol) given I.V. increased the output of 35S, and the last two drugs increased the output of 3H. The blocking effects of thymoxamine and propranolol on these responses are described. Pilocarpine (given I.V. or intranasally) produced large increases in 35S release; histamine had little effect. Irritants (ammonia and cigarette smoke) and diluted serum or plasma, given intranasally, produced large increases in 3H output, and sometimes enhanced 35S release. Radiolabelled nasal washings fractionated on Sepharose CL-4B gel chromatography columns formed two peaks, with most of the radioactivity in the high molecular weight (mucous glycoprotein) peak.

Tracheobronchial epithelium of the sheep: II. Ultrastructural and morphometric analysis of the epithelial secretory cell types

In a light microscopic study we have described the morphology and distribution of six distinct, granule-containing cells in the tracheobronchial epithelium of sheep lung. We designed the present study to determine qualitatively and quantitatively whether these six cell types differ in ultrastructural morphology. Cell height varied from 30.6 micron for mucous cell M1 to 9.6 micron for Clara cells. Cell width varied from 21.2 micron for M1 to 9.3 micron for Clara cells. Nuclear dimensions ranged from 7.5 micron in M3 to 4.0 micron in M1 and M2. Mucous cell M1 had electron-dense granules (1.5 micron in diameter); M2, electron-lucent granules (1.6 micron); M3, nucleated electron-lucent granules (0.51 micron); M4, cored granules (1.1 micron); serous (SC) and Clara cells (CC), electron-opaque granules (0.58 micron and 0.37 micron). The volume fraction of the cell occupied by granules was 63% in M1 and M2, M4 39%, SC 23%, CC 5%, and M3 4.5%. Smooth endoplasmic reticulum was observed only in M3 (33.8%) and CC (49%). Granular endoplasmic reticulum (GER) was most abundant in SC (21%) and least plentiful in M4 (2.2%). We conclude that mucous cells M3 and M4 and serous and Clara cells differ from each other and from M1 and M2 cells. Mucous cells M1 and M2 differ from each other only in amount of GER and secretory granule appearance.

Monoclonal antibodies as probes for unique antigens in secretory cells of mixed exocrine organs

In the past, it has been difficult to identify the secretory product and control mechanisms associated with individual cell types making up mixed exocrine organs. This report establishes the feasibility of using immunological methods to characterize both the biochemical constituents and regulatory mechanisms associated with secretory cells in the trachea. Monoclonal antibodies directed against components of tracheal mucus were produced by immunizing mice with dialyzed, desiccated secretions harvested from tracheal organ culture. An immunofluorescence assay revealed that of the total 337 hybridomas screened, 100 produced antibodies recognizing goblet cell granules; 64, gland cell granules; and 3, antigen confined to the ciliated apical surface of the epithelium. The tracheal goblet cell antibody described in this report was strongly cross-reactive with intestinal goblet cells, as well as with a subpopulation of submandibular gland cells, but not with cells of Brunner's glands or the ciliated cell apical membrane. The serous cell antibody was not cross-reactive with goblet, Brunner's gland, or submandibular cells, or the ciliated cell apical membrane. The antibody directed against the apical membrane of ciliated cells did not cross-react with gland or goblet cells or the apical membrane of epithelial cells in the duodenum. Monoclonal antibodies, therefore, represent probes by which products unique to specific cells or parts of cells in the trachea can be distinguished. The antibodies, when used in enzyme immunoassays, can be used to quantitatively monitor secretion by individual cell types under a variety of physiological and pathological conditions. They also provide the means for purification and characterization of cell-specific products by immunoaffinity chromatography.

Mapping of adrenergic receptors in the trachea by autoradiography

We investigated the distribution of adrenergic receptors in ferret trachea using autoradiography. [3H]Dihydroalprenolol, used to identify beta-adrenoceptors, revealed a high density of specific binding sites over surface epithelium and submucosal glands, with less labelling of smooth muscle. [3H]prazosin labelling showed that alpha 1-receptors were numerous in glands and epithelium, but sparse in smooth muscle. Comparison of adrenergic receptor densities in tracheal sections from the same animals showed a rank order for submucosal glands of alpha 1 greater than beta, for epithelium beta greater than alpha 1 and for smooth muscle beta greater than alpha 1. Within the submucosal glands, alpha 1- and beta-adrenergic receptors were differentially distributed, with alpha 1-receptors being significantly more numerous over serous than mucous cells and beta receptors being significantly more numerous over mucous than serous cells. This technique provides insight into adrenergic regulation of airway function and should be useful in investigations of how relative receptor densities may be altered in disease.

Ionic mechanisms in secretagogue-induced morphological changes in rat parotid gland

When 10(-5) M carbachol was added to parotid tissue slices incubated in buffer containing Ca++, watery vacuoles were formed in the cells. The percent volume density of vacuoles, as measured from 0.5-micron sections, increased from 0.64 +/- 0.15 SE (n = 7) to 3.09 +/- 0.99 (n = 5) in 10 min and, finally, to 7.27 +/- 1.88 (n = 4) in 30 min. In electron micrographs, most of the vacuoles appeared to arise from a location near the Golgi apparatus. Condensation of nuclear chromatin and a conformational change in mitochondria were also noted immediately after stimulation. The percent volume density values returned to basal levels with the addition of either 5 mM EGTA or 10(-6) M atropine after the addition of carbachol. Nuclei and mitochondria returned to normal configurations. In the presence of either 1 mM ouabain or high K+, or in the absence of added Ca++, carbachol failed to induce vacuole formation. However, low Na+ medium did not prevent the formation of vacuoles due to carbachol. Ultrastructural changes in nuclei and mitochondria were consistently associated with the appearance of vacuoles. Since both high K+ and ouabain blocked vacuole formation, it is unlikely that Na+ or K+ movements were important for the response. Rather, receptor-activated Ca++ influx, which is likely to be inhibited by depolarizing agents (such as high K+ or ouabain), is probably the more important factor in vacuole formation and other concomitant ultrastructural changes.