Exocytosis in secretory cells of rat lacrimal gland. Peroxidase release from lobules and isolated cells upon cholinergic stimulation

Oxidative Stress in the Protection and Injury of the Lacrimal Gland and the Ocular Surface: are There Perspectives for Therapeutics?

Oxidative stress (OS) is a major disruption in the physiology of the lacrimal functional unit (LFU). Antioxidant enzymes have dual protective activities: antioxidant and antimicrobial activities. Peroxidases have been indistinctly used as markers of the secretory activity of the LFU and implicated in the pathophysiology, diagnosis and treatment of dry eye disease (DED), even though they comprise a large family of enzymes that includes lactoperoxidase (LPO) and glutathione peroxidase (GPO), among others. Assays to measure and correlate OS with other local LFU phenomena have methodological limitations. Studies implicate molecules and reactions involved in OS as markers of homeostasis, and other studies identify them as part of the physiopathology of diseases. Despite these conflicting concepts and observations, it is clear that OS is influential in the development of DED. Moreover, many antioxidant strategies have been proposed for its treatment, including calorie restriction to nutritional supplementation. This review offers a critical analysis of the biological mechanisms, diagnostic outcomes, drug use, dietary supplements, and life habits that implicate the influence of OS on DED.

Phenylephrine increases tear cathepsin S secretion in healthy murine lacrimal gland acinar cells through an alternative secretory pathway

Little is known about the relationship between stimulation of lacrimal gland (LG) tear protein secretion by parasympathetic versus sympathetic nerves, particularly whether the spectrum of tear proteins evoked through each innervation pathway varies. We have previously shown that activity and abundance of cathepsin S (CTSS), a cysteine protease, is greatly increased in tears of Sjögren's syndrome (SS) patients and in tears from the male NOD mouse of autoimmune dacryoadenitis that recapitulates SS-associated dry eye disease. Beyond the increased synthesis of CTSS detected in the diseased NOD mouse LG, increased tear CTSS secretion in NOD mouse tears was recently linked to increased exocytosis from a novel endolysosomal secretory pathway. Here, we have compared secretion and trafficking of CTSS in healthy mouse LG acinar cells stimulated with either the parasympathetic acetylcholine receptor agonist, carbachol (CCh), or the sympathetic α₁-adrenergic agonist, phenylephrine (PE). In situ secretion studies show that PE significantly increases CTSS activity and protein in tears relative to CCh stimulation by 1.2-fold (***, p = 0.0009) and ∼5-fold (*, p-0.0319), respectively. A similar significant increase in CTSS activity with PE relative to CCh is observed when cultured LGAC are stimulated in vitro. CCh stimulation significantly elevates intracellular [Ca²⁺], an effect associated with increases in the size of Rab3D-enriched vesicles consistent with compound fusion, and subsequently decreases in their intensity of labeling consistent with their exocytosis. PE stimulation induces a lower [Ca²⁺] response and has minimal effects on Rab3D-enriched SV diameter or the intensity of Rab3D-enriched SV labeling. LG deficient in Rab3D exhibit a higher sensitivity to PE stimulation, and secrete more CTSS activity. Significant increases in the colocalization of endolysosomal vesicle markers (Lamp1, Lamp2, Rab7) with the subapical actin suggestive of fusion of endolysosomal vesicles at the apical membrane occur both with CCh and PE stimulation, but PE demonstrates increased colocalization. In conclusion, the α₁-adrenergic agonist, PE, increases CTSS secretion into tears through a pathway independent of the exocytosis of Rab3D-enriched mature SV, possibly representing an alternative endolysosomal secretory pathway.

Novel Insights Into Muscarinic and Purinergic Responses in Primary Cultures of Rat Lacrimal Gland Myoepithelial Cells

Purpose

The functional characteristics of receptors that regulate lacrimal gland myoepithelial cells are still somewhat unclear. To date, mainly muscarinic receptors have been of interest; however, further knowledge is needed regarding their expression and functional roles. For this purpose, primary cultures of rat lacrimal gland myoepithelial cells were established and examined functionally.

Methods

Rat lacrimal glands were excised, minced, and further digested, yielding mixtures of cells that were seeded in culturing flasks. After 4-6 weeks, primary monocultures of myoepithelial cells were established, verified by immunocytochemistry. The cells were stained for all muscarinic receptor subtypes (M1–M5) and examined functionally regarding intracellular [Ca²⁺] responses upon activation of muscarinic receptors. For methodological verification, purinergic functional responses were also studied.

Results

Expression of muscarinic receptor subtypes M2-M5 was detected, whereas expression of muscarinic M1 receptors could not be shown. Activation of muscarinic receptors by the non-selective muscarinic agonist methacholine (3 × 10⁻¹¹–10⁻³ M) did not cause a significant increase in intracellular [Ca²⁺]. However, activation of purinergic receptors by the non-selective purinergic agonist ATP (10⁻⁸–10⁻³ M) caused a concentration-dependent increase in intracellular [Ca²⁺] that could be blocked by the P2 antagonists PPADS and suramin.

Conclusions

Primary cultures of rat lacrimal gland myoepithelial cells were established that displayed a heterogeneous expression of muscarinic receptors. Purinergic functional responses demonstrated a viable cell population. Upon treatment with methacholine, no significant increase in intracellular [Ca²⁺] could be detected, indicating that cholinergic activation of myoepithelial cells occurs via other intracellular messengers or is dependent on interaction with other cell types.

α1-Adrenoceptors relate Ca(2+) modulation and protein secretions in rat lacrimal gland

Noradrenaline (NA) is a catecholamine with multiple roles including as a hormone and a neurotransmitter. Cellular secretory activities are enhanced by adrenergic stimuli as well as by cholinergic stimuli. The present study aimed to determine which adrenoceptors play a role in controlling intracellular calcium ion ([Ca(2+)]i) level in acinar cells of rat lacrimal glands. Expression of mRNA for adrenoceptor subtypes in the acinar cells was assessed using RT-PCR. All types except α2c, β1, and β3 were detected. NA induced a [Ca(2+)]i increase with a biphasic pattern in the acinar cells. Removal of extracellular Ca(2+) and use of Ca(2+)-channel blockers did not inhibit the NA-induced [Ca(2+)]i increases. In contrast, U73122 and suramin almost blocked these increases. The α1-adrenoceptor agonist phenylephrine induced a strong increase in [Ca(2+)]i. However, clonidine and isoproterenol failed to induce a [Ca(2+)]i increase. The peroxidase activity was quantified as a measure of mucin secretion. Ca(2+)-dependent exocytotic secretion of peroxidase was detected in rat lacrimal glands. The RT-PCR results showed that MUC1, MUC4, MUC5AC, MUC5B, and MUC16 were expressed in acinar cells. These findings indicated that NA activates α1-adrenoceptors, which were found to be the main receptors in Ca(2+)-related cell homeostasis and protein (including mucin) secretion in lacrimal glands.

Signaling Pathways of Purinergic Receptors and Their Interactions with Cholinergic and Adrenergic Pathways in the Lacrimal Gland

PURPOSE

Purinergic receptors play a key role in the function of the lacrimal gland (LG) as P1 purinergic receptors A₁, A_2A, and A_2B, P2X_1-7 receptors, and many of the P2Y receptors are expressed.

METHODS

This review examines the current knowledge of purinergic receptors in the LG as well as the signaling pathways activated by these receptors.

RESULTS

These receptors are expressed on the acinar, ductal, and myoepithelial cells. Considerable crosstalk exists between the pathways activated by P2X₇ receptors with those activated by M₃ muscarinic or α_1D adrenergic receptors. The mechanism of the crosstalk between P2X₇ and M₃ muscarinic receptors differs from that of the crosstalk between P2X₇ and α_1D adrenergic receptors.

CONCLUSIONS

Understanding purinergic receptors and how they modulate protein secretion could play a key role in normal and pathological responses of the LG.

Calcium signaling in lacrimal glands

Lacrimal glands provide the important function of lubricating and protecting the ocular surface. Failure of proper lacrimal gland function results in a number of debilitating dry eye diseases. Lacrimal glands secrete lipids, mucins, proteins, salts and water and these secretions are at least partially regulated by neurotransmitter-mediated cell signaling. The predominant signaling mechanism for lacrimal secretion involves activation of phospholipase C, generation of the Ca(2+)-mobilizing messenger, IP3, and release of Ca(2+) stored in the endoplasmic reticulum. The loss of Ca(2+) from the endoplasmic reticulum then triggers a process known as store-operated Ca(2+) entry, involving a Ca(2+) sensor in the endoplasmic reticulum, STIM1, which activates plasma membrane store-operated channels comprised of Orai subunits. Recent studies with deletions of the channel subunit, Orai1, confirm the important role of SOCE in both fluid and protein secretion in lacrimal glands, both in vivo and in vitro.

Na(+)/K(+)-ATPase expression changes in the rabbit lacrimal glands during pregnancy

PURPOSE

To investigate the expressional changes of Na(+)/K(+)-ATPase subunits in the lacrimal glands (LG) of term pregnant rabbits.

METHODS

LG were obtained from term pregnant rabbits and age-matched female control rabbits for laser capture microdissection (LCM), real time RT-PCR, western blot, and immunofluorescence. The mRNA and proteins of α1, α2, β1, β2, and β3 subunits of Na(+)/K(+)-ATPase were detected and quantified.

RESULTS

Although only the mRNA for β3 from whole LG of pregnant rabbits was significantly different from that of normal controls, many mRNA levels for α1, α2, β1, β2, and β3 from acini and epithelial cells from various duct segments that were collected by LCM were significantly different from those of normal control rabbits. Western blots demonstrated that the expressions of all three β subunits were significantly higher in pregnant rabbits, while both α subunits remained unchanged during pregnancy. Interestingly, immunofluorescence results showed that the distribution patterns of all Na(+)/K(+)-ATPase subunits during pregnancy were similar to those of the control rabbits.

CONCLUSIONS

Changes were found in mRNA and protein expressions of Na(+)/K(+)-ATPase subunits in LG from term pregnant rabbits and these changes suggest a role in the pregnancy-related LG secretion changes and dry eye symptoms observed in these animals.

Changes of chloride channels in the lacrimal glands of a rabbit model of Sjögren syndrome

PURPOSE

To test the hypothesis that expressions of Na-K-2Cl cotransporter-1 (NKCC1), cystic fibrosis transmembrane conductance regulator (CFTR), and chloride channel 2 γ subunit (ClC2γ) in the lacrimal glands (LGs) of rabbits with induced autoimmune dacryoadenitis (IAD) are changed.

METHODS

LGs were obtained from adult female rabbits with IAD and age-matched female control rabbits. LGs were processed for laser capture microdissection, real-time reverse transcription-polymerase chain reaction, Western blot, and immunofluorescence.

RESULTS

In rabbits with IAD, messenger RNA (mRNA) abundance and protein expressions of NKCC1 and CFTR from whole LGs were significantly lower than those in controls. mRNA abundance of NKCC1, CFTR, and ClC2γ from rabbits with IAD was significantly different from that in acinar and ductal cells from controls. NKCC1 was localized to the basolateral membranes of all acinar and ductal cells, with weaker staining intensity in ductal cells, and the staining pattern from rabbits with IAD appeared similar to that from controls. CFTR was found as punctate aggregates in the apical cytoplasm of all acinar and ductal cells, with the intensity in ductal cells much stronger and no significant difference between controls and rabbits with IAD. ClC2γ was also localized to the apical cytoplasm as punctate aggregates of all acinar cells but not in ductal cells, and a similar staining pattern was observed in rabbits with IAD compared with control rabbits.

CONCLUSIONS

Our data demonstrated significant changes of mRNA and protein expressions of NKCC1, CFTR, and ClC2γ in rabbits with IAD, suggesting that these changes may contribute to the altered lacrimal secretion, particularly Cl transport, in rabbits with IAD.

Na(+)/K(+)-ATPase in the lacrimal glands of rabbits and its changes during induced autoimmune dacryoadenitis

PURPOSE

To test the hypothesis that expression of Na(+)/K(+)-ATPase subunits in the lacrimal glands (LGs) of rabbits with induced autoimmune dacryoadenitis (IAD) changes.

METHODS

LGs were obtained from adult female rabbits with IAD and age-matched female control rabbits. The LGs were processed for laser capture microdissection (LCM), real time RT-PCR, western blot, and immunofluorescence for the detection of mRNA and proteins of the α1, α2, β1, β2, and β3 subunits of Na(+)/K(+)-ATPase.

RESULTS

In the rabbits with IAD, mRNA levels of α1, β1, and β3 from whole LGs were significantly lower. In samples of acini and epithelial cells from various duct segments, collected by LCM, mRNA levels of α1, β1, β2, and β3 were significantly lower in the rabbits with IAD, although mRNA for α2 could not be detected. However, western blots demonstrated that all five subunits were significantly higher in the rabbits with IAD, although their distribution patterns were similar to those of the control rabbits, as demonstrated by immunofluorescence.

CONCLUSIONS

The data presented herein demonstrated significant changes in mRNA and protein expressions of Na(+)/K(+)-ATPase subunits in rabbits with IAD, suggesting that these changes may play a role in the pathogenesis of Sjögren's syndrome and altered LG secretion, as observed in these animals.

Duct system of the rabbit lacrimal gland: structural characteristics and role in lacrimal secretion

PURPOSE

To develop a nomenclature for the lacrimal duct system in the rabbit, based on the anatomic and structural characteristics of each duct segment, and to provide RT-PCR and immunofluorescence data to support the notion that the duct system plays important roles in lacrimal function.

METHODS

Paraffin-embedded lacrimal glands (LGs) were stained with hematoxylin and eosin (H&E) and evaluated with a stereomicroscope. Cryosections of LG were stained with cresyl violet, and acinar cells and ductal epithelial cells were isolated from each duct segment by laser capture microdissection (LCM). mRNA levels from these cells were analyzed by real-time RT-PCR. Standard protocol was followed for immunofluorescence detection of ionic transporters.

RESULTS

The lacrimal duct system was divided into six segments on the basis of morphologic characteristics: the intercalated, intralobular, interlobular, intralobar, interlobar, and main excretory ducts. Although the morphologic features change incrementally along the entire duct system, the gene expression of ionic transporters and aquaporins, including AE3, AQP4, AQP5, CFTR, ClC2gamma, KCC1, NHE1, NKAalpha1, NKAbeta1, NKAbeta2, NKAbeta3, and NKCC1 varied greatly among duct segments. Immunofluorescence results were generally in accordance with the abundance of mRNAs along the acinus-duct axis.

CONCLUSIONS

Most LG research has focused on the acinar cells, with relatively little attention being paid to the lacrimal ducts. The lack of knowledge regarding the lacrimal ducts was so profound that a precise nomenclature had not been established for the duct system. The present data establish a nomenclature for each segment of the lacrimal duct system and provide evidence that ducts play critical roles in lacrimal secretion.

Neural regulation of lacrimal gland secretory processes: relevance in dry eye diseases

The lacrimal gland is the major contributor to the aqueous layer of the tear film which consists of water, electrolytes and proteins. The amount and composition of this layer is critical for the health, maintenance, and protection of the cells of the cornea and conjunctiva (the ocular surface). Small changes in the concentration of tear electrolytes have been correlated with dry eye syndrome. While the mechanisms of secretion of water, electrolytes and proteins from the lacrimal gland differ, all three are under tight neural control. This allows for a rapid response to meet the needs of the cells of the ocular surface in response to environmental conditions. The neural response consists of the activation of the afferent sensory nerves in the cornea and conjunctiva to stimulate efferent parasympathetic and sympathetic nerves that innervate the lacrimal gland. Neurotransmitters are released from the stimulated parasympathetic and sympathetic nerves that cause secretion of water, electrolytes, and proteins from the lacrimal gland and onto the ocular surface. This review focuses on the neural regulation of lacrimal gland secretion under normal and dry eye conditions.

Magnesium–calcium signalling in rat parotid acinar cells: effects of acetylcholine

This study investigated the effects of extracellular Mg(2+) ([Mg(2+)](o)) on basal and acetylcholine (ACh)-evoked amylase secretion and intracellular free Ca(2+) ([Ca(2+)](i)) in rat parotid acinar cells. In a medium containing 1.1 mM [Mg(2+)](o), ACh evoked significant increases in amylase secretion and [Ca(2+)](i). Either low (0 mM) or elevated (5 and 10 mM) [Mg(2+)](o) attenuated ACh-evoked responses. In a nominally Ca(2+) free medium, elevated [Mg(2+)](o) attenuated basal and ACh-evoked amylase secretion and [Ca(2+)](i). In parotid acinar cells incubated with either 0, 1.1, 5 or 10 mM [Mg(2+)](o), ACh evoked a gradual decrease in [Mg(2+)](i). These results indicate that the ACh-evoked Mg(2+) efflux is an active process since Mg(2+) has to move against its gradient. Either lidocaine, amiloride, N-methyl-D: -glucamine, quinidine, dinitrophenol or bumetanide can elevate [Mg(2+)](i) above basal level. In the presence of these membrane transport inhibitors, ACh still evoked a decrease in [Mg(2+)](i) but the response was less pronounced with either [Na(+)](o) removal or in the presence of either amiloride or quinidine. These results indicate marked interactions between Ca(2+) and Mg(2+) signalling in parotid acinar cells and that ACh-evoked Mg(2+) transport was not dependent upon [Na(+)](o).

Mechanism of Exocrine Pancreatic Insufficiency in Streptozotocin-Induced Type 1 Diabetes Mellitus

Diabetes mellitus (DM) is a major health problem at present affecting about 180 million people worldwide. DM is associated with many metabolic abnormalities in the body including the indigestion of carbohydrates leading to malnutrition and weight loss. In this article we investigate the cellular and molecular mechanisms of exocrine pancreatic insufficiency in streptozotocin (STZ, 60 mg kg(-1), i.p.)-induced DM in male rats compared to healthy age-matched controls. Either electrical field stimulation (EFS) or cholecystokinin octapeptide (CCK-8, 10(-8) M) can elicit large and significant (P < 0.05) increases in amylase output from pancreatic segments compared to basal secretion. Insulin (10(-6) M) alone has no significant effect on amylase output compared to basal but it enhanced the secretory responses to either EFS or CCK-8. When rats were rendered diabetic with STZ, either EFS or CCK-8-evoked amylase output was significantly (P < 0.01) decreased compared to the responses obtained with either EFS or CCK-8 alone in healthy age-matched control pancreas. In addition, CCK-8 can elicit large dose-dependent release of amylase in age-matched control and diabetic acinar cells with significantly (P < 0.05) reduced responses in diabetic acinar cells. CCK-8 evoked a large rapid increase in peak cytosolic free calcium concentration ([Ca2+]c) followed by a decrease to a plateau phase in age-matched control fura-2-loaded pancreatic acinar cells. These responses were significantly (P < 0.05) decreased in STZ-induced diabetic acinar cells. In the presence of 10(-6) M insulin, CCK-8 evoked a much larger increase in the Ca2+ transient compared to the response obtained with CCK-8 alone. These effects were significantly (P < 0.01) inhibited in STZ-induced diabetic acinar cells. Similarly, in zero extracellular Ca2+ [Ca2+]ĉ, the CCK-8-evoked [Ca2+]c was significantly (P < 0.05) reduced in both diabetic and age-matched control acinar cells, but with more pronounced reduction in diabetic acinar cells. CCK(A) receptor mRNA levels remained unchanged in diabetic rat acinar cells compared to age-matched healthy control. In contrast, amylase mRNA was significantly (P < 0.05) reduced in diabetic acinar cells compared to control. The results indicate that reduced amylase secretion in response to either EFS or CCK-8 in the diabetic pancreas may be due to reduced [Ca2+]c and gene expression for amylase and not to the gene expression of CCK(A) receptor in pancreatic acinar cells.

Increased expression of advanced glycation end-products and their receptor, and activation of nuclear factor kappa-B in lacrimal glands of diabetic rats

AIMS/HYPOTHESIS

To assess the involvement of the AGE-specific receptor (AGER, also known as RAGE) axis and nuclear factor kappa-B (NFKB, also known as NF-kappaB) activation in the development of lacrimal gland and tear film dysfunction in diabetes, the present study evaluated: (1) lacrimal gland and tear film alterations in diabetic rats; and (2) the expression of AGE, AGER and NFKB in ocular tissues of normoglycaemic and diabetic rats.

MATERIALS AND METHODS

Diabetes was induced in male Wistar rats with intravenous streptozotocin. Tear secretion parameters were measured and NFKB expression was evaluated in lacrimal glands of control and diabetic rats by western blot. Immunohistochemistry with confocal microscopy was used to assess AGE, AGER and NFKB expression in lacrimal glands of both groups.

RESULTS

Lacrimal gland weight and tear film volume were lower in diabetic than in control rats (p=0.01 and 0.02, respectively). IL1B and TNF concentrations in tears were higher in diabetic than in control rats (p=0.007 and 0.02, respectively). NFKB protein was identified in rat cornea, conjunctiva and lacrimal glands. AGE, AGER and NFKB expression were greater in lacrimal glands of diabetic than in those of control rats.

CONCLUSIONS/INTERPRETATION

Diabetes induces significant alterations in rat lacrimal gland structure and secretion. The higher expression of AGE, AGER and NFKB in lacrimal glands of diabetic rats suggests that these factors are involved in signalling and in subsequent inflammatory alterations related to dry eye in diabetes mellitus.

Nitric oxide and cGMP mediate alpha1D-adrenergic receptor-Stimulated protein secretion and p42/p44 MAPK activation in rat lacrimal gland

PURPOSE

To determine whether alpha(1)-adrenergic receptors use the nitric oxide (NO)/cGMP pathway to stimulate protein secretion by rat lacrimal gland.

METHODS

Identification and cellular location of endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) were determined by Western blot and immunofluorescence techniques, respectively. Rat lacrimal gland acini were isolated by collagenase digestion, and protein secretion stimulated by phenylephrine, an alpha(1)-adrenergic agonist, was measured with a fluorescence assay system. Acini were preincubated with inhibitors for 20 minutes before addition of phenylephrine (10(-4) M). NO and cGMP were measured in response to phenylephrine stimulation. Activation of p42/p44 MAPK was determined by Western blot analysis with an antibody against phosphorylated (active) p42/p44 MAPK.

RESULTS

eNOS and nNOS were both present in lacrimal gland. eNOS appeared to be localized with caveolae, whereas nNOS was present in the nerves surrounding the acini. Inhibition of eNOS with N(G)-nitro-l-arginine methyl ester (l-NAME; 10(-6) M) completely inhibited phenylephrine-stimulated protein secretion, whereas the inactive isomer d-NAME and inhibition of nNOS with S-methyl-l-thiocitrulline did not. Phenylephrine increased NO production in a time- and concentration-dependent manner, but the increase was abolished by the alpha(1D)-adrenergic receptor inhibitor BMY-7378. Inhibition of guanylate cyclase with oxadiazoloquinoxalin (ODQ) also inhibited phenylephrine-induced protein secretion, whereas phenylephrine caused a 2.2-fold increase in cGMP. In addition, preincubation with l-NAME and ODQ inhibited phenylephrine-stimulated p42/p44 MAPK activation.

CONCLUSIONS

alpha(1D)-Adrenergic agonists stimulate eNOS to produce NO, leading to production of cGMP by guanylate cyclase, to transduce the extracellular signal through the cell and stimulate protein secretion in rat lacrimal gland.

Differential effects of the EGF family of growth factors on protein secretion, MAPK activation, and intracellular calcium concentration in rat lacrimal gland

The purpose of this study was to investigate the expression of the EGF family of growth factors and EGF receptor subtypes (ErbB1-4) present in lacrimal gland and determine the effects of these growth factors on different functions of rat lacrimal gland. RT-PCR was used to detect mRNA expression in the lacrimal gland of selected members of the EGF family of growth factors, namely EGF, transforming growth factor alpha (TGF-alpha), heparin-binding EGF (HB-EGF), and heregulin. The presence of ErbB receptors was investigated by immunofluorescence microscopy and western blot analysis. The effects of EGF, TGF-alpha, HB-EGF, and heregulin on protein secretion from lacrimal gland acini were examined using a fluorescent assay for peroxidase, a marker of protein secretion. Fura-2 tetra-acetoxymethyl ester was used to measure the effects of the growth factors on intracellular [Ca2+] ([Ca2+]i) in acini. MAPK activation in acini by these growth factors was also examined by western blot analysis using antibodies specific to phosphorylated p42/44 MAPK and total p42 MAPK. Rat lacrimal gland expressed EGF, TGF-alpha, HB-EGF, and heregulin mRNA, and all four ErbB receptors were present in the lacrimal gland as detected by western blot analyses. ErbB 1 and ErbB2 were located in basal and lateral membranes of acinar and ductal cells. The location of ErbB3 could not be determined while ErbB4 was found in ductal cells. Heregulin (10(-7) m) significantly increased protein secretion in lacrimal gland acini whereas all growth factors tested significantly increased [Ca2+]i at 10(-7) m. TGF-alpha (10(-9) m), heregulin (10(-7) m), EGF (10(-7) m), and HB-EGF (10(-7) m) significantly increased the amount of phosphorylated MAPK in lacrimal gland acini. We conclude that all members of the EGF family of growth factors studied are synthesised in rat lacrimal gland, could activate all four ErbB receptors that are present in this tissue, and differentially activate lacrimal gland functions.

Streptozotocin-induced type 1 diabetes mellitus alters the morphology, secretory function and acyl lipid contents in the isolated rat parotid salivary gland

Diabetes mellitus (DM) is associated with numerous conditions including hypo-secretion of digestive enzymes. This study investigated the morphology, secretory function (alpha-amylase release) and acyl lipid contents in the isolated parotid gland of STZ-induced diabetic and age-matched control rats in order to provide insights into diabetes-induced salivary insufficiency. The techniques employed included light microscopy, colourimetric and gas chromatography (GC) analysis, respectively. Diabetes mellitus was induced in adult male Wistar rats by a single intraperitoneal (IP) injection of streptozotocin (STZ) (60 mg per kg body weight). Control animals were injected with a similar volume of citrate buffer. The animals were tested for DM 4 days after STZ injection and 2 months later when they were humanely killed for the experiment. The morphological results showed diabetic parotid glands to be extensively infiltrated with lipid droplets of various magnitudes, whereas glands from control animals display normal structure with the absence of lipid droplets. The analysis of parotid secretory function revealed a significant (p < 0.05) dose-dependent decrease in alpha-amylase release in response to noradrenaline (NA) in STZ-treated glands when compared to age-match control parotid glands. Furthermore, the levels of acyl lipids (16:0, 16:1, 18:0 and 18:1) in diabetic parotid glands was significantly (p < 0.01) reduced compared to control glands, along with a reduced ratio of 16:1/16:0. The results indicate DM can elicit changes in the morphology, secretory function and acyl fatty acid quantity in the isolated rat parotid gland.

Effects of ageing on morphology, amylase release, cytosolic Ca2+ signals and acyl lipids in isolated rat parotid gland tissue

Xerostomia (oral dryness sensation) is due to dryness of the oral cavity and it is more prevalent in the elderly. This study investigated the effect of ageing on parotid gland structure and function of control (2-6 months) and aged (12, 16-18 and 22-24 months) rats employing light microscopic, colorimetric, gas chromatographic and microspectrofluorimetric methods to investigate the morphological changes of the parotid glands, amylase release, endogenous lipid distribution and cytosolic free calcium levels, respectively. When compared to controls, age-related changes were apparent in glands obtained from rats aged 16-18 and 22-24 months, which included reduced acinar cell distribution, enlarged parotid ducts with fatty and connective tissue and mast cell infiltrations. Parotid acini from 12, 16-18 and 22-24-month-old glands showed significant (p < 0.05) age-related decreases in amylase release, compared to controls when challenged with acetylcholine (ACh). No change in basal calcium signals was observed in parotid acini from 2-6 to 16-18-month-old-animals. However, stimulation of 16-18-month-old parotid acini with 10(-5)M ACh resulted in a significant (p < 0.001) decrease in both peak and plateau phases of the cytosolic Ca2+ signal when compared to control. Gas chromatography of de novo and essential acyl lipids revealed no changes in the amount of either acyl lipid group in glands obtained from 2-6 to 22-24-month-old animals. Lipid analysis of phospholipid associated acyl chains showed a higher relative proportion of linoleic acid in older glands. The results reveal that ageing is associated with marked and distinct morphological changes including infiltrations of lipids and mast cells of the parotid gland and decreases in amylase release and cytosolic Ca2+ signals.

Effects of age on morphology, protein synthesis and secretagogue-evoked secretory responses in the rat lacrimal gland

This study investigated changes in the morphology and protein synthesis and protein and peroxidase secretion due to peptidergic and aminergic stimulation from rat lacrimal gland acinar cells of 3-5, 9, 12, 20 and 24 month old rats. There was a marked reduction in the presence of Golgi apparatus in the acinar cells of glands from the 24 month old rats coupled to dilatation and degeneration of rough endoplasmic reticulum, when compared to that in the acinar cells of glands from 3-5 and 12 month old rats. Following incorporation of tritiated leucine for 360 min (6 h), the amount of newly synthesised protein in acinar cells of the 12 month old rats was significantly (p < 0.01) higher than that in the acinar cells of 3-5 month old animals. However, at 20 months the amount of newly synthesised protein in these acinar cells was significantly (p < 0.01) reduced to less than that in acinar cells of both the 3-5 and 12 month old animals. Immunohistochemical and immunofluorescence studies identified the presence of substance P (SP), vasoactive intestinal peptide (VIP), histamine and 5-hydroxytryptamine (5-HT) in the lacrimal glands of 3-5 month old rats. Stimulation by either SP, VIP, histamine or 5-HT resulted in significant increases in total protein output and peroxidase release from acinar cells of the 3-5 month old rats. However, all responses to the secretagogues were reduced with ageing from 3-5 to 24 months of age. The results indicate that ageing is associated with alteration in the ability of acinar cells to synthesise and secrete proteins.

Regulatory pathways in lacrimal gland epithelium

Tears are a complex fluid that continuously cover the exposed surface of the eye, namely the cornea and conjunctiva. Tears are secreted in response to the multitude of environmental stresses that can harm the ocular surface such as cold, mechanical stimulation, physical injury, noxious chemicals, as well as infections from various organisms. Tears also provide nutrients and remove waste from cells of the ocular surface. Because of the varied function of tears, tears are complex and are secreted by several different tissues. Tear secretion is under tight neural control allowing tears to respond rapidly to changing environmental conditions. The lacrimal gland is the main contributor to the aqueous portion of the tear film and the regulation of secretion from this gland has been well studied. Despite multiple redundencies in pathways to stimulate secretion from the lacrimal gland, defects can occur resulting in dry eye syndromes. These diseases can have deleterious effects on vision. In this review, we summarize the latest information regarding the regulatory pathways, which control secretion from the lacrimal gland, and their roles in the pathogenesis of dry eye syndromes.

cDNA and genomic cloning of lacritin, a novel secretion enhancing factor from the human lacrimal gland

Multiple extracellular factors are hypothesized to promote the differentiation of unstimulated and/or stimulated secretory pathways in exocrine secretory cells, but the identity of differentiation factors, particularly those organ-specific, remain largely unknown. Here, we report on the identification of a novel secreted glycoprotein, lacritin, that enhances exocrine secretion in overnight cultures of lacrimal acinar cells which otherwise display loss of secretory function. Lacritin mRNA and protein are highly expressed in human lacrimal gland, moderately in major and minor salivary glands and slightly in thyroid. No lacritin message or protein is detected elsewhere among more than 50 human tissues examined. Lacritin displays partial similarity to the glycosaminoglycan-binding region of brain-specific neuroglycan C (32 % identity over 102 amino acid residues) and to the possibly mucin-like amino globular region of fibulin-2 (30 % identity over 81 amino acid residues), and localizes primarily to secretory granules and secretory fluid. The lacritin gene consists of five exons, displays no alternative splicing and maps to 12q13. Recombinant lacritin augments unstimulated but not stimulated acinar cell secretion, promotes ductal cell proliferation, and stimulates signaling through tyrosine phosphorylation and release of calcium. It binds collagen IV, laminin-1, entactin/nidogen-1, fibronectin and vitronectin, but not collagen I, heparin or EGF. As an autocrine/paracrine enhancer of the lacrimal constitutive secretory pathway, ductal cell mitogen and stimulator of corneal epithelial cells, lacritin may play a key role in the function of the lacrimal gland-corneal axis.

Quantitation of rat lacrimal secretion: a novel sandwich ELISA with high sensitivity

Modulation of lacrimal acinar cell tear secretion may involve multiple factors acting both in subtle and pronounced ways. Functional screens of recombinant protein products arising from gene array technologies, or protein fractions derived from lacrimal conditioned media or extracellular matrix, will require a highly sensitive assay capable of monitoring tear protein secretion by small replicate cultures. To improve significantly on current methods, a rat- and mouse-specific sandwich ELISA was developed. For this purpose, chickens and rabbits were immunized with serum-free secretion media from carbachol and VIP-stimulated rat lacrimal acinar cell cultures. Immune sera were characterized by ELISA, Western blotting and immunohistochemistry, and subsequently optimized for use in a sandwich ELISA. Both antisera detected a wide range of different rat tear proteins, and immunostained only the secretory granule-rich juxtalumenal region in sections of rat lacrimal gland. Chicken, but not rabbit, antiserum cross-reacted with rabbit and human tears. In sandwich ELISA, capture with purified chicken immunoglobulin fraction and detection with rabbit antiserum detected as little as 1 ng ml-1 tear protein in 10,000-fold diluted rat secretion medium--a level of sensitivity 8000 times greater than the rat tear peroxidase assay. Such specificity and sensitivity greatly reduce the quantity of media needed for assay, and makes feasible functional screens for scarce factors that may influence lacrimal secretory processes, and in turn possibly play a role in human lacrimal insufficiency syndromes.

Role of laminin-1, collagen IV, and an autocrine factor(s) in regulated secretion by lacrimal acinar cells

Adhesion to novel basement membrane component BM180 in the presence of laminin-1 promotes stimulus-secretion coupling in lacrimal acinar cells [G. W. Laurie, J. D. Glass, R. A. Ogle, C. M. Stone, J. R. Sluss, and L. Chen. Am. J. Physiol. 270 (Cell Physiol. 39): C1743-C1750, 1996]. The identity of the active laminin-1 site and the possibility that other promoters of coupling are present in the acinar cell microenvironment were probed by use of different substrates, media, neutralizing antibodies and cell numbers. Regulated peroxidase secretion was unaffected by basement membrane coat concentration and was detectable at reduced levels in serum-free medium. Anti-laminin-1 antibodies, particularly against sites in the beta1 and gamma1 chains, but not alpha1 chains, partially suppressed regulated secretion, as did an anti-collagen IV antibody. Without effect were RGD peptide and antibodies against entactin, the beta1-integrin subunit, and several growth factors. Increasing cell number in serum-free medium revealed an unknown, serum-maskable, secretion-enhancing activity with a remarkable specificity for regulated secretion. Stimulus-secretion coupling, therefore, appears to be modulated by several extracellular factors whose relative contributions remain to be determined.

Age-related changes in morphology and secretory responses of male rat lacrimal gland

This study investigates the differences in the outward appearance and morphology of lacrimal glands, the morphology within the lacrimal acinar cells and the secretion of protein from acinar cells of young (3-5 months) and aged (20 and 24 months) male rats. The appearance of the glands, as seen by the naked eye, differed between the three age-groups. The lacrimal gland of young animals was a smooth pink tissue, while the tissue from aged animals appeared lobular and white in colour, thought to result from infiltration of fatty/connective tissue. Glands from 24 month old animals had a more pronounced lobular appearance than the glands from 20 month old animals. Light microscopy studies revealed that as the animals aged there was evidence of progressive morphological changes. These changes included thickening of the connective tissue sheath, chronic inflammation with increased infiltration by mast cells, patchy destruction of ductal and vascular tissues, enlargement of lacrimal ducts, luminal swelling of the acini, and changes in acinar type. Electron microscopy (EM) studies revealed the presence of 3 types of acini in the rat lacrimal gland: acini which contained only protein secretory granules (serous acini), acini which contained protein and mucous secretory granules (seromucous acini), and acini which contained only mucous secretory granules (mucous acini). In young glands the majority of acini were serous with a few seromucous acini and even fewer mucous acini. In aged glands there were significant reductions in serous acini (ANOVA; P < 0.01) when compared to the young glands. In 20-month-old glands, there were marked increases in the percentage occurrence of seromucous acini, while in 24 month old glands, there were large increases in the relative number of mucous acini. Qualitative EM studies demonstrated that the typical acini from young glands contained numerous protein secretory granules. Ageing was associated with a progressive loss of protein (serous) secretory granules. Furthermore, marked changes and patchy destruction of the endoplasmic reticulum and Golgi apparatus were observed in acini of glands from aged rats when compared to acini of glands from young rats. Measurement of total protein output from acini revealed a significant (Student's t-test, P < 0.05) decrease in protein secretion from aged glands compared to glands from young animals. These results suggest that not only is there considerable structural damage, chronic inflammation and mast cell infiltration to the lacrimal gland with ageing, but also possible redifferentiation of acini from serous to seromucous and then to mucous acini. Furthermore, the results also suggest a reduction or an inability of the acini to synthesise and to secrete protein from glands of aged animals compared to glands of young rats. All of these changes appear to occur more rapidly as the rats mature between 20 and 24 months. These findings provide a morphological basis to explain the phenomenon of reduced tear/protein secretion with ageing.

PKA inhibitor, H-89, affects the intracellular transit of regulated secretory proteins in rat lacrimal glands

We tested the effect of H-89, a protein kinase A (PKA) inhibitor, on the intracellular transit of the regulated secretory proteins in rat lacrimal glands. We show that H-89, by itself, induces the secretion of newly synthesized proteins trafficking in its presence but not of proteins already stored in the mature secretory granules. This secretion does not depend on the presence of extracellular Ca2+. The proteins released are identical to those secreted after cholinergic stimulation or under the action of the ionophore A-23187, but the secretion level is approximately 40% lower. The effect of H-89 seems to be due to PKA inhibition because other protein kinase inhibitors (calphostin C, chelerythrine, H-85) do not induce secretion. We further show that H-89 does not modify the rate of glycoprotein galactosylation but induces the secretion of newly galactosylated glycoproteins. Finally, we used a "20 degrees C block" procedure to show that H-89 affects a trans-Golgi network (TGN) or post-TGN step of the secretory pathway. Our results demonstrate that, in lacrimal cells, H-89 affects the intracellular trafficking of secretory proteins, suggesting a role for PKA in this process.

"BM180": a novel basement membrane protein with a role in stimulus-secretion coupling by lacrimal acinar cells

Regulated secretion requires the developmental coupling of neuronal or hormonal stimuli to an exocytotic response, a multistep pathway whose appearance may be linked with cellular adhesion to the newly formed exocrine cell basement membrane. We screened for adhesion-associated coupling activity using lacrimal acinar cells and have identified "BM180", a novel basement membrane protein enriched in guanidine HCl extracts of lacrimal and parotid exocrine secretory glands. BM180 resides primarily in a previously inexamined lower molecular-mass basement membrane peak (peak 2) that contains cell adhesion activity inhibitable with the anti-BM180 monoclonal antibody 3E12. Removal of peak 2 by gel filtration or preincubation of basement membrane with 3E12 decreased regulated peroxidase secretion by one-half without affecting constitutive secretion or the amount of cellular peroxidase available for release. Adding back peak 2 restored regulated secretion in a dose-dependent and 3E12-inhibitable manner and suggested a synergistic relationship between BM180 and laminin 1. BM180 has a mobility of 180 and 60 kDa in the absence or presence of dithiothreitol, respectively, and shows no immunological identity by competitive enzyme-linked immunosorbent assay with laminin 1, collagen IV, entactin, fibronectin, BM-40, perlecan, or vitronectin. We propose that BM180 is an important resident of certain glandular basement membranes where it interacts with the cell surface, thereby possibly signaling the appearance of a transducing element in the stimulus-secretion coupling pathway.

EGF receptor mRNA and protein in rat lacrimal acinar cells: evidence of its EGF-dependent phosphotyrosilation

This study was designed to demonstrate the presence of the epidermal growth factor (EGF) receptor in the rat exorbital lacrimal gland. EGF receptor gene transcription was demonstrated 1) by reverse transcription-polymerase chain reaction analysis of lacrimal gland and acinar cells from RNA with a set of specific primers deduced from the rat EGF receptor sequence and 2) by Northern blot analysis of rat lacrimal gland mRNA. Lacrimal acinar cell preparations contain a low but detectable amount of specific 125I-EGF binding sites and efficiently internalize the ligand on binding at 37 degrees C. A sheep polyclonal antibody, directed against the human EGF receptor, detects a protein of 170 kDa by Western blot analysis of membrane proteins of the whole gland. This protein can be immunoprecipitated by the same antibody from whole gland membrane proteins as well as from solubilized acinar cells. Incubations of acinar cells in the presence of EGF results in an increased content of tyrosine-phosphorylated residues in immunoprecipitated 170-kDa protein. Taken together, these results demonstrate for the first time both EGF receptor gene transcription and protein expression in a lacrimal tissue, i.e., the rat exorbital lacrimal gland. These results also suggest a specific cellular location of the EGF receptor in a cell population contained in acinar cell preparations.

Fluid phase endocytosis by isolated rabbit lacrimal gland acinar cells

It is well known that lacrimal gland acinar cells retrieve secretory vesicle membrane constituents from their apical plasma membranes after stimulated exocytosis of secretory proteins. There have also been indications of a recycling traffic involving the basal-lateral plasma membranes. In an effort to document this traffic, determine how it is regulated, and discern whether it involves more than one intracellular compartment, we studied internalization of the fluid phase marker, Lucifer Yellow, and its relationship to protein release in acinar cells isolated from rabbit lacrimal glands. Loading of intracellular vesicles was apparent with fluoresence microscopy. Stimulation with carbachol increased both the rate of internalization and the intracellular volume equilibrating with extracellular fluid, suggesting the loading of two compartments. A carbachol concentration of 10 microM increased uptake by 80% during 20-min incubations at 37 degrees C. Increasing the carbachol concentration to 1 mM reduced the response by 50%, and it appeared to do so by decreasing the intracellular volume accessible to extracellular fluid, rather than the rate of endocytosis. Carbachol affected protein release differently, increasing it by 50% at 10 microM and 80% at 1 mM. Acceleration of endocytosis by 10 microM carbachol was transient, becoming negligible after 60 min. Vasoactive intestinal peptide (VIP) and isoproterenol increased internalization 35% and 25% respectively; neither reduced uptake at the highest concentrations tested; and only isoproterenol significantly affected protein secretion. Combinations of VIP and carbachol exerted synergistic effects on both fluid phase internalization and protein release. Steady-state uptake at 18 degrees C in the presence of 10 microM carbachol was equal to uptake at 37 degrees C in the absence of carbachol, suggesting a temperature block in the pathway to at least one endocytic compartment. Decreasing the temperature to 18 degrees C eliminated the inhibitory action of excessive carbachol, suggesting that the compartment whose loading was impaired by excessive carbachol was positioned distal to the temperature block. Carbachol accelerated release of marker from preloaded cells, indicating that it stimulated recycling between the plasma membranes and endocytic compartments. This effect was maximal at a concentration of 10 microM and unchanged with increasing concentrations. In accord with the hypothesis that traffic into and out of a certain compartment was particularly dependent on stimulation, a fraction of the marker taken up by optimally stimulated cells at 37 degrees C was retained unless carbachol or VIP was present in the efflux medium.(ABSTRACT TRUNCATED AT 400 WORDS)

M3 muscarinic acetylcholine receptor coupling to PLC in rat exorbital lacrimal acinar cells

This study was designed to characterize the muscarinic acetylcholine receptor (mAChR) subtype present in rat exorbital lacrimal gland as well as its biochemical coupling. The nonselective muscarinic antagonist [N-methyl-3H]scopolamine ([3H]NMS) binds with high affinity to a homogeneous population of binding sites in both membranes [dissociation constant (Kd) = 82.3 +/- 3.2 pM] and acinar cell (Kd = 170.3 +/- 20 pM) preparations. Muscarinic antagonist inhibition of [3H]NMS binding is homogeneous with the following order of potency: atropine > or = 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) > pirenzepine > 11-([2-(diethylamino)-ethyl]-1-piperidinyl)-acetyl- 5,11-dihydro-6H-pirido[2,3-b]1,4,benzo diazepine-6-one (AFDX 116). Both the affinity of the selective antagonists 4-DAMP, pirenzepine, and AFDX 116 and Northern blot analysis of lacrimal gland mRNAs show a single mAChR population of the M3 subtype. Muscarinic agonist inhibition of [3H]NMS binding displays both high (approximately 20%)- and low-affinity sites (approximately 80%). Both the receptor occupancy and the stimulation by agonists or the inhibition by antagonists of the accumulation of [3H]inositol phosphate were examined under identical conditions with respect to tissue preparations (acinar cells) and buffer (Krebs-Ringer). Results demonstrate 1) the efficient coupling of the M3 mAChR subtype with the phosphatidylinositol (4,5))bisphosphate-specific phospholipase C activity and 2) that the efficacy of a muscarinic agonist is dependent on its structure. Lastly, comparison of the agonists affinity and potency to trigger the [3H]inositol phosphate accumulation suggests that the occupation of the high-affinity agonist binding state of the M3 mAChR was involved in the cellular response.

High density of Ca(2+)-dependent K+ and Cl- channels on the luminal membrane of lacrimal acinar cells

Tight-seal whole-cell recording and Ca2+ imaging were simultaneously performed on cell clusters or individual acinar cells of rat lacrimal glands during application of the secretagogue acetylcholine. Activation of Ca(2+)-dependent K+ and Cl- currents was selectively followed as a function of time by placing the cell potential near the equilibrium potential for Cl- or for K+ ion, respectively. Upon acetylcholine application to cell clusters, K(+)- and Cl(-)-selective currents displayed a distinctive initial rise ("hump"). At this time, there was only a small elevation of Ca2+ concentration, [Ca2+]i, that was restricted to the luminal end of acinar cells. A quantitative analysis of Ca2+ and current signals during the hump suggested that the luminal membrane contained high densities of K(+)- and Cl(-)-selective channels, roughly 10 times higher than those found in the basolateral domain. Distinct luminal and basolateral membrane domains were preserved in isolated cells, but with less contrasted densities than in cell clusters. The results suggest that Ca(2+)-dependent K+ channels are implicated not only in the transfer of salt from the blood compartment to the interior of acinar cells, as commonly accepted, but also in the electrolyte secretion from the cell interior to the acinar lumen.

Subcellular gradients of intracellular free calcium concentration in isolated lacrimal acinar cells

The spatial distribution of intracellular free calcium concentration ([Ca2+]i) was measured in small clusters of isolated rat lacrimal acinar cells by imaging the fluorescence of the Ca(2+)-sensitive dye fura-2. In the absence of extracellular Ca2+, stimulation with acetylcholine (ACh) caused an increase in [Ca2+]i, due to release of intracellular Ca2+ stores, which was maximal at the luminal pole of the cell. In contrast, the organellar Ca(2+)-ATPase inhibitor 2,5-di(tert-butyl)-hydroquinone caused an increase in [Ca2+]i, which was most marked in the basolateral region of the cell. When the cells were stimulated with ACh in a medium containing Ca2+, the gradients of [Ca2+]i (with [Ca2+]i most elevated at the luminal pole) were maintained for the duration of agonist stimulation. The possible implications of these results concerning the location and identity of intracellular Ca2+ stores, and the location of the sites that underlie agonist-stimulated Ca2+ influx, are considered. In particular, it seems likely that intracellular inositol-1,4,5-trisphosphate (InsP3) binding sites may be concentrated in the luminal region of the cell. It is not clear, however, whether this implies that there is a distinct luminally located InsP3-sensitive organelle.

Structure and function of non-enzymatically dissociated lacrimal gland acini

Lacrimal gland acini were isolated utilizing a non-enzymatic dissociation procedure. This method resulted in the rapid isolation of an enriched population of acini from rat lacrimal gland with a complete absence of interlobular and rare occurrence of intralobular duct epithelium. The isolated acini had high viability and retained good histological organization. Alkaline phosphatase activity was present in the myoepithelial cells and capillaries associated with the periphery of the acini, indicating a relatively undisturbed basal surface. Secretion of peroxidase by the isolated acini in response to cholinergic and alpha-adrenergic stimulation indicated that the cell surface receptors in this preparation were retained and were physiologically functional. Thus, we report a dissociation protocol that eliminates enzymatic digestion, but results in a relatively enriched acinar preparation that is appropriate for the assessment of cellular biochemistry and physiology of lacrimal exocrine function of the acini.

Beta-adrenoceptor control of peroxidase synthesis in nasal glands

Endogenous peroxidase activity was observed in the cisternae of the rough-surfaced endoplasmic reticulum (r-ER), including the nuclear envelope, Golgi complex, and secretory granules of the cat's nasal glands. A single injection of propranolol resulted in a decrease of peroxidase-positive secretory granules, an increase of electron-lucent granules, and no change in the level of peroxidase activity in the r-ER and Golgi complex at either 5 or 9 hours after the injection. Continuous administration of propranolol over 7 hours led to the disappearance of peroxidase activity from the r-ER and secretory granules. These data would indicate that propranolol inhibits only the synthesis of peroxidase. Twenty minutes after a single injection of isoproterenol, the rate at which the granule content containing the peroxidase reaction product was discharged into the acinar lumen increased sharply. However, 60 minutes after the injection, the secretion rate of peroxidase-positive granules decreased. The same experiment then was repeated, but this time before the isoproterenol was injected, propranolol was administered to the cats. This time no change in peroxidase activity appeared in the acinar cells. It was concluded, therefore, that the stimulation of beta-receptors enhances the synthesis of peroxidase.

The initiation of calcium release following muscarinic stimulation in rat lacrimal glands

1. Acinar cells were isolated from rat lacrimal glands, and the Ca2+ release response of these cells was studied using two experimental approaches. In one approach, changes in Ca2+ concentration, Cai2+, were monitored by measuring Ca2(+)-dependent Cl- currents using tight-seal whole-cell recording. Alternatively, such changes were measured as a fluorescence signal in cells loaded with Fura-2. 2. Following bath application of ACh (0.5 microM), the cell current recorded at -60 mV was unchanged for ca 0.8 s, then rose in a biphasic manner. The initial phase of the current rise ('hump') took different appearances depending on the cell studied, and it sometimes stood out from the main part of the response as a partially isolated transient. 3. In cells which had been loaded with Fura-2, Cai2+ was found to rise abruptly following a silent period. The delay was larger if ACh (0.2-0.5 microM) was applied in a depolarizing isotonic K+ saline than if it was applied in the normal saline. In addition, the maximum of the Cai2+ response was reduced with depolarizing stimulating solutions. This indicates that membrane potential modulates the Cai2+ response. 4. Responses to 5 microM-ACh, a saturating agonist concentration, were almost identical in K+ saline and in normal saline. 5. If the cell potential was hyperpolarized, the delay of the ACh-induced current became shorter. 6. Breaking into an acinar cell with a pipette containing an elevated Ca2+ concentration (0.1-1 mM) led to a transient activation of Ca2(+)-induced currents during the first seconds of whole-cell recording. These transients were obtained more reliably if the transition to the whole-cell mode was achieved by applying a sharp pulse of potential ('zapping') rather than by applying suction to the pipette compartment. At -60 mV, the transients elicited with the former method by 0.5 mM-Ca2+ had a time-to-peak near 0.6s and an amplitude varying between 10 and 600 pA. With 0.1 mM-Ca2+, similar transients were also observed, but a number of cells failed to respond. Calcium-induced transients were blocked if cells were previously loaded with 50 microM-Ruthenium Red. 7. Performing the same experiments with inositol trisphosphate (InsP3, 20 microM) in the pipette solutions also led to early transient Ca2(+)-induced currents. Amplitudes, times-to-peak and 20-80% transition times were similar for 0.5 mM-Ca2+ and 20 microM-InsP3 stimulations.(ABSTRACT TRUNCATED AT 400 WORDS)

Signal transduction and control of lacrimal gland protein secretion: a review

Proteins in lacrimal gland fluid are secreted primarily by the acinar cells. Secretory proteins are synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, stored in secretory granules, and released upon a change in the cellular level of second messenger. The second messenger level is controlled by a process termed signal transduction. Agonists, primarily neurotransmitters in the lacrimal gland, bind to receptors in the basolateral membrane of secretory cells. This interaction activates enzymes in the membrane that cause production of second messengers. It has been hypothesized that second messengers stimulate secretion by activating specific protein kinases to phosphorylate proteins important for secretion. In the lacrimal gland, cholinergic agonists stimulate protein secretion. They act by activating phospholipase C to break down phosphatidylinositol bisphosphate into 1,4,5-inositol trisphosphate (1,4,5-IP3) and diacylglycerol (DAG). 1,4,5-IP3 causes release of Ca2+ from intracellular stores. This Ca2+, perhaps in conjunction with calmodulin, activates specific protein kinases that may be involved in secretion. DAG activates protein kinase C which stimulates protein secretion. alpha 1-Adrenergic agonists also stimulate lacrimal gland protein secretion. These agonists use a pathway that is separate from that utilized by cholinergic agonists and vasoactive intestinal peptide (VIP). The specific pathway has not been identified but may be DAG and protein kinase C. VIP, beta-adrenergic agonists, alpha-melanocyte stimulating hormone, and adrenocorticotropic hormone are lacrimal gland secretagogues. They activate adenylate cyclase to produce cAMP. cAMP stimulates protein kinase A, which perhaps causes protein secretion. Thus, three separate cellular pathways stimulate lacrimal gland protein secretion. Cholinergic agonists and VIP also stimulate lacrimal gland fluid secretion, and the same signal transduction pathways utilized by these agonists to stimulate protein secretion are most likely used for electrolyte and water secretion.

Evidence for an anion exchanger in the mouse lacrimal gland acinar cell membrane

Anion exchange transport in the mouse lacrimal gland acinar cell membrane was studied by measuring the intracellular H+ (pHi) and Cl- (aCli) activities with double-barreled ion-selective microelectrodes. In a HCO3- -free solution of pH 7.4 (HEPES/Tris buffered), pHi was 7.25 and aCli was 33 mM. By an exposure to a HCO3- (25 mM HCO3-/5% CO2, pH 7.4) solution for 15 min, aCli was decreased to 25 mM, and pHi was transiently decreased to about 7.05 within 1 min, then slowly relaxed to 7.18 in 15 min. Intracellular HCO3- concentration [HCO3-]i, calculated by the Henderson-Hasselbalch's equation, was 11 mM at 1 min after the exposure and then slowly increased to 15 mM. Readmission of the HCO3(-)-free solution reversed the changes in aCli and pHi. The intracellular buffering power was about 40 mM/pH. An addition of DIDS (0.2 mM) significantly inhibited the rates of change in aCli, pHi, and [HCO3-]i caused by admission/withdrawal of the HCO3- solution and decreased the buffer value. Replacement of all Cl- with gluconate in the HCO3- solution increased pHi, and readmission of Cl- decreased pHi. The rates of these changes in pHi were reduced by DIDS by 32-45% but not by amiloride (0.3 mM). In the HCO3- solution, a stimulation of intracellular HCO3- production by exposing the tissue to 25 mM NH4+ increased aCli significantly. While in the HCO3(-)-free solution or in the HCO3- solution containing DIDS, exposure to NH4+ had little effect on aCli.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of aldosterone and ADH on intracellular electrolytes in the toad urinary bladder epithelium

Quantitative electron microprobe analysis was employed to compare the effects of aldosterone and ADH on the intracellular electrolyte concentrations in the toad urinary bladder epithelium. The measurements were performed on thin freeze-dried cryosections utilizing energy dispersive x-ray microanalysis. After aldosterone, a statistically significant increase in the intracellular Na concentration was detectable in 8 out of 9 experiments. The mean Na concentration of granular cells increased from 8.9 +/- 1.3 to 13.2 +/- 2.2 mmol/kg wet wt. A significantly larger Na increase was observed after an equivalent stimulation of transepithelial Na transport by ADH. On average, the Na concentration in granular cells increased from 12.0 +/- 2.3 to 31.4 +/- 9.3 mmol/kg wet wt (5 experiments). We conclude from these results that aldosterone, in addition to its stimulatory effect on the apical Na influx, also exerts a stimulatory effect on the Na pump. Based on a significant reduction in the Cl concentration of granular cells, we discuss the possibility that the stimulation of the pump is mediated by an aldosterone-induced alkalinization. Similar though less pronounced concentration changes were observed in basal cells, suggesting that this cell type also participates in transepithelial Na transport. Measurements in mitochondria-rich cells provided no consistent results.

Acetylcholine-induced Na+ influx in the mouse lacrimal gland acinar cells: demonstration of multiple Na+ transport mechanisms by intracellular Na+ activity measurements

In the isolated, superfused mouse lacrimal gland, intracellular Na+ activities (aNai) of the acinar cells were directly measured with double-barreled Na+-selective microelectrodes. In the nonstimulated condition aNai was 6.5 +/- 0.5 mM and membrane potential (Vm) was -38.9 +/- 0.4 mV. Addition of 1 mM ouabain or superfusion with a K+-free solution slightly depolarized the membrane and caused a gradual increase in aNai. Stimulation with acetylcholine (ACh, 1 microM) caused a membrane hyperpolarization by about 20 mV and an increase in aNai by about 9 mM in 5 min. The presence of amiloride (0.1 mM) reduced the ACh-induced increase in aNai by approximately 50%, without affecting Vm and input resistance in both nonstimulated and ACh-stimulated conditions. Acid loading the acinar cells by an addition/withdrawal of 20 mM NH4Cl or by replacement of Tris+-buffer saline solution with HCO3-/CO2-buffered solution increased aNai by a few mM. Superfusion with a Cl(-)-free NO3- solution or 1 mM furosemide or 0.5 mM bumetanide-containing solution had little effect on the resting aNai levels, however, it reduced the ACh-induced increase in aNai by about 30%. Elimination of metabolite anions (glutamate, fumarate and pyruvate) from the superfusate reduced both the resting aNai and the ACh-induced increase in aNai. The present results suggest the presence of multiple Na+ entry mechanisms activated by ACh, namely, Na+/H+ exchange, Na-K-Cl cotransport and organic substrate-coupled Na+ transport mechanisms.

Growth of exocrine acinar cells on a reconstituted basement membrane gel

Methods have been developed for culturing a dividing population of morphologically differentiated rat parotid, lacrimal, and pancreatic acinar cells in vitro. Isolated acinar cells were plated onto tissue culture dishes coated with a three-dimensional, reconstituted basement membrane gel. After attachment in Ham's nutrient mixture F12, the cells were cultured at 35 degrees C in F12 supplemented with 10% heat inactivated rat serum, epidermal growth factor, dexamethasone, insulin, transferrin, selenium, putrescine, reduced glutathione, ascorbate, penicillin, streptomycin, and the appropriate secretagogue. Under these conditions, the cells attached rapidly and DNA synthesis was initiated within 2 to 3 d. Although the cells flattened on the substratum, they continued to maintain their differentiated morphology. The cells contained secretory granules, and the secretory enzymes peroxidase and amylase could be detected. The use of a reconstituted basement membrane gel proved critical for the attachment and growth of exocrine acinar cells.

ATP keeps exocytosis sites in a primed state but is not required for membrane fusion: an analysis with Paramecium cells in vivo and in vitro

We have tried to specify a widespread hypothesis on the requirement of ATP for exocytosis (membrane fusion). With Paramecium tetraurelia cells, synchronously (approximately 1 s) exocytosing trichocysts, ATP pools have been measured in different strains, including wild type cells, "non-discharge" (nd), "trichless" (tl), and other mutations. The occurrence of a considerable and rapid ATP consumption also in nd and tl mutations as well as its time course (with a maximum 3-5 s after exocytosis) in exocytosis-competent strains does not match the actual extent of exocytosis performance. However, from in vivo as well as from in vitro experiments, we came to the conclusion that ATP might be required to keep the system in a primed state and its removal might facilitate membrane fusion. (For the study of exocytosis in vitro we have developed a new system, consisting of isolated cortices). In vivo as well as in vitro exocytosis is inhibited by increased levels of ATP or by a nonhydrolyzable ATP analogue. In vitro exocytosis is facilitated in ATP-free media. In vivo-microinjected ATP retards exocytosis in response to chemical triggers, whereas microinjected apyrase triggers exocytosis without exogenous trigger. Experiments with this system also largely exclude any overlaps with other processes that normally accompany exocytosis. Our data also explain why it was frequently assumed that ATP would be required for exocytosis. We conclude that membrane fusion during exocytosis does not require the presence of ATP; the occurrence of membrane fusion might involve the elimination of ATP from primed fusogenic sites; most of the ATP consumption measured in the course of exocytosis may be due to other effects, probably to recovery phenomena.

A common spectrum of polypeptides occurs in secretion granule membranes of different exocrine glands

A highly purified membrane preparation from rat parotid secretion granules has been used as a comparative probe to examine the extent of compositional overlap in granule membranes of three other exocrine secretory tissues--pancreatic, lacrimal, and submandibular--from several standpoints. First, indirect immunofluorescent studies using a polyclonal polyspecific anti-parotid granule membrane antiserum has indicated a selective staining of granule membrane profiles in all acinar cells of all tissues. Second, highly purified granule membrane subfractions have been isolated from each exocrine tissue; comparative two-dimensional (isoelectric focusing; SDS) PAGE of radioiodinated granule membranes has identified 10-15 polypeptides of identical pI and apparent molecular mass. These species are likely to be integral membrane components since they are not extracted by either saponin-sodium sulfate or sodium carbonate (pH 11.5) treatments, and they do not have counterparts in the granule content. Finally, the identity among selected parotid and pancreatic radioiodinated granule membrane polypeptides has been documented using two-dimensional peptide mapping of chymotryptic and tryptic digests. These findings clearly indicate that exocrine secretory granules, irrespective of the nature of stored secretion, comprise a type of vesicular carrier with a common (and probably refined) membrane composition. Conceivably, the polypeptides identified carry out general functions related to exocrine secretion.

Relationship between lacrimal gland, isolated cells (lacrimocytes) and tears: biochemical and histological studies in the rabbit eye

The rabbit defense system has a number of specific features: no lactoferrin and lysozyme are detectable and peroxidase activity is only demonstrated in the cubic epithelial cells of the ducts. Experiments carried out with radioactive amino acid, demonstrate the absence of secreted proteins with molecular weights corresponding to those of albumin and transferrin, indicating that these proteins are not synthesized by the lacrimal gland tissue. Rabbit tear pattern presents a set of acidic proteins secreted by the lacrimal gland tissue, with small molecular weight and acidic pI's.

Calcium-dependent chloride currents in isolated cells from rat lacrimal glands

Isolated cells from rat lacrimal glands were studied with the tight-seal whole-cell recording technique. Cells were dialysed with K-free solutions containing a high concentration of Ca2+ buffer in order to record Ca-dependent Cl- currents at a Ca2+ level fixed between 0.1 and 10 microM. HEDTA was preferable to EGTA as a Ca2+ buffer, even under conditions of equivalent equilibrium buffering power. After replacement of all internal K+ with Na+, the cells displayed a small conductance component which could be abolished by removal of external K+ or by external application of 2 mM-tetraethylammonium. It is suggested that this conductance is due to Ca-dependent K+ channels. The main part of the cell current was Cl selective. The Cl- conductance was negligible at 0.5 microM-Ca2+, and fully activated at 2 microM-Ca2+. The dose-response curve relating Cl- currents to the internal Ca2+ concentration, [Ca]i, was steeper than predicted by a simple binding isotherm reaction. Relaxations observed in response to voltage jumps could, in most cases, be fitted with single exponentials. At [Ca]i 0.5 microM, the curve relating the relaxation time constant, tau, to the membrane potential, displayed a maximum near +20 mV and 250 ms. At hyperpolarized potentials, tau varied by an e-fold factor in 130 mV. At [Ca]i 1 microM, tau decreased from 100 ms at -120 mV to 60 ms at +60 mV. Relaxation analysis gave an estimate of the variation of the channel open state probability, Po, with potential. At [Ca]i 0.5 microM, Po varied by an e-fold factor in 50-70 mV at hyperpolarized potentials, and saturated above +60 mV. At [Ca]i 1 microM, Po varied e-fold in 100 to 110 mV at hyperpolarized potentials, and saturated near +20 mV. External Cl- was substituted with various anions. From reversal potential measurements, the following permeability sequence was obtained: I- greater than NO3- greater than Br- greater than Cl- greater than F- greater than isethionate, methanesulphonate greater than glutamate. The corresponding normalized permeability coefficients were 2.7, 2.4, 1.6, 1, 0.2, 0.1, 0.1, 0.05. Replacement of external Cl- with Br-, isethionate, methanesulphonate or glutamate did not alter current kinetics as obtained during or after a depolarizing voltage jump.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein secretion in lacrimal gland: alpha 1-beta-adrenergic synergism

We have shown the existence of a homogeneous population of specific binding sites for [3H]prazosin in membranes from rat lacrimal glands. The value of the equilibrium dissociation constant was 0.186 +/- 0.07 nM, and the density of specific binding sites was 20.4 +/- 1 fmol/mg protein. Taking into account the potency sequences for adrenergic agonists and antagonists for competition with these [3H]prazosin binding sites, we identified them as alpha 1-adrenergic receptors. Moreover, we have demonstrated that the stimulation of protein discharge evoked by epinephrine could be partly attributed to the occupation of this alpha-adrenergic receptor subtype. However, the inhibition pattern of the epinephrine effect by a beta-adrenergic antagonist, 1-propranolol, and the characteristics of the secretory response observed when selectively stimulating the alpha 1- and beta-adrenergic receptors, either separately or simultaneously, suggest that 1) a simultaneous activation of both receptors is necessary to produce a maximal secretory response to catecholamines; and 2) a synergism may exist between these two routes of stimulation, leading to an amount of protein discharge higher than that expected in the case of additive effects.

Membrane fluidity and adenylate cyclase activity in the lacrimal glands of rats fed diets containing trans fatty acids

The effects of feeding a diet containing trans fatty acids on the fatty acid composition of plasma membrane phospholipids, fluorescence polarization of diphenylhexatriene (DPH) and adenylate cyclase activity in the exorbital lacrimal glands of rats were studied. Three groups of male, weanling rats were fed semipurified diets containing 20% corn oil (CO), 20% partially hydrogenated soybean oil (PHSBO) (a source of trans fatty acids) and 19% PHSBO + 1% CO. Plasma membranes of the lacrimal glands from rats fed 20% PHSBO showed higher adenylate cyclase activity and lower fluidity as shown by a lower double bond index of the fatty acids of their phospholipids and higher fluorescence polarization of DPH. When 1% CO was included with the diet containing PHSBO, the adenylate cyclase activity and membrane fluidity tended to be normal. The results suggest that feeding of a diet containing trans fatty acids in the absence of sufficient linoleic acid (18:2) can result in a decrease in membrane fluidity and an increase in adenylate cyclase activity in the lacrimal glands.

Identification and localization of synaptophysin, an integral membrane glycoprotein of Mr 38,000 characteristic of presynaptic vesicles

A polypeptide of Mr 38,000 has been identified as a specific component of the membrane of presynaptic vesicles, using the monoclonal antibody SY38. This protein, which is acidic (isoelectric at approximately pH 4.8) and glycosylated, appears to be an integral membrane protein, as suggested by its solubilization with the nonionic detergent Triton X-100 and the finding that the epitope recognized by antibody SY38 is located on the cytoplasmic surface of those vesicles. It is found in presynaptic vesicles of neurons of the brain, spinal cord, and retina as well as at neuromuscular junctions. It is also found in the adrenal medulla. Its occurrence in diverse vertebrate species indicates its stability during evolution. This protein, for which we propose the name synaptophysin*, provides a molecular marker for the presynaptic vesicle membrane and may be involved in synaptic vesicle formation and exocytosis.

Stimulus-secretion coupling in exocrine glands: the role of inositol-1,4,5-trisphosphate, calcium and cAMP

Enzyme, electrolyte and fluid secretion from exocrine glands is stimulated by neurotransmitters and peptide hormones. Whereas for some of these secretagogues calcium is an important intracellular messenger, for others it is cyclic AMP. Regulation of steady state free Ca2+ concentration at rest and at stimulation have been studied in isolated permeabilized acinar cells from pancreas, parotid and lacrimal glands by measuring the free Ca2+ concentration of the surrounding incubation medium with a Ca2+-specific macroelectrode. Ca2+ transport mechanisms have been further characterized in subcellular membrane fractions by measuring 45Ca2+ uptake into membrane vesicles from rough endoplasmic reticulum (RER) and plasma membranes (PM). The data show that the intracellular messenger for secretagogue-induced Ca2+ release from RER is inositol-1,4,5-trisphosphate (IP3) which is produced during stimulation by phospholipase C mediated hydrolysis of phosphatidylinositol-bisphosphate. At rest both Ca2+ uptake into RER and Ca2+ extrusion from the cell is promoted by (Ca2+ + Mg2+)-ATPases with different characteristics in both types of membranes and by a coupled Na+/Ca2+ countertransport in the PM which keep cytosolic free Ca2+ concentration at a low level of approximately 2 - 4 X 10(-7) mol/l. During stimulation the Ca2+ permeability of endoplasmic reticulum membrane increases via IP3 and that of the PM by a yet unknown "receptor-operated" mechanism. These events lead to increase in cytosolic free Ca2+ concentration that is a trigger for enzyme, electrolyte and fluid secretion.