Muscarinic response in rat lacrimal glands

Intracellular Ca2+ oscillation frequency and amplitude modulation mediate epithelial apical and basolateral membranes crosstalk

Since the early seminal studies on epithelial solute transport, it has been understood that there must be crosstalk among different members of the transport machinery to coordinate their activity and, thus, generate localized electrochemical gradients that force solute flow in the required direction that would otherwise be thermodynamically unfavorable. However, mechanisms underlying intracellular crosstalk remain unclear. We present evidence that crosstalk between apical and basolateral membrane transporters is mediated by intracellular Ca2+ signaling in insect renal epithelia. Ion flux across the basolateral membrane is encoded in the intracellular Ca2+ oscillation frequency and amplitude modulation and that information is used by the apical membrane to adjust ion flux accordingly. Moreover, imposing experimentally generated intracellular Ca2+ oscillation modulation causes cells to predictably adjust their ion transport properties. Our results suggest that intracellular Ca2+ oscillation frequency and amplitude modulation encode information on transmembrane ion flux that is required for crosstalk.

Randomized, crossover clinical efficacy trial in humans and mice on tear secretion promotion and lacrimal gland protection by molecular hydrogen

The incidence of dry eye disease is increasing worldwide because of the aging population and increasing use of information technology. Dry eye disease manifests as tear-layer instability and inflammation caused by osmotic hypersensitization in tear fluids; however, to our knowledge, no agent that treats both pathologies simultaneously is available. Molecular hydrogen (H₂) is known to be effective against various diseases; therefore, we aimed to elucidate the effects of H₂ on tear dynamics and the treatment of dry eye disease. We revealed that administering a persistent H₂-generating supplement increased the human exhaled H₂ concentration (p < 0.01) and improved tear stability (p < 0.01) and dry eye symptoms (p < 0.05) significantly. Furthermore, H₂ significantly increased tear secretion in healthy mice (p < 0.05) and significantly suppressed tear reduction in a murine dry eye model (p = 0.007). H₂ significantly and safely improved tear stability and dry eye symptoms in a small exploratory group of 10 human subjects, a subset of whom reported dry eye symptoms prior to treatment. Furthermore, it increased tear secretion rapidly in normal mice. Therefore, H₂ may be a safe and effective new treatment for dry eye disease and thus larger trials are warranted.

Modulation of Ca2+ oscillation and melatonin secretion by BKCa channel activity in rat pinealocytes

The pineal glands regulate circadian rhythm through the synthesis and secretion of melatonin. The stimulation of nicotinic acetylcholine receptor due to parasympathetic nerve activity causes an increase in intracellular Ca(2+) concentration and eventually downregulates melatonin production. Our previous report shows that rat pinealocytes have spontaneous and nicotine-induced Ca(2+) oscillations that are evoked by membrane depolarization followed by Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs). These Ca(2+) oscillations are supposed to contribute to the inhibitory mechanism of melatonin secretion. Here we examined the involvement of large-conductance Ca(2+)-activated K(+) (BKCa) channel conductance on the regulation of Ca(2+) oscillation and melatonin production in rat pinealocytes. Spontaneous Ca(2+) oscillations were markedly enhanced by BKCa channel blockers (1 μM paxilline or 100 nM iberiotoxin). Nicotine (100 μM)-induced Ca(2+) oscillations were also augmented by paxilline. In contrast, spontaneous Ca(2+) oscillations were abolished by BKCa channel opener [3 μM 12,14-dichlorodehydroabietic acid (diCl-DHAA)]. Under whole cell voltage-clamp configurations, depolarization-elicited outward currents were significantly activated by diCl-DHAA and blocked by paxilline. Expression analyses revealed that the α and β3 subunits of BKCa channel were highly expressed in rat pinealocytes. Importantly, the activity of BKCa channels modulated melatonin secretion from whole pineal gland of the rat. Taken together, BKCa channel activation attenuates these Ca(2+) oscillations due to depolarization-synchronized Ca(2+) influx through VDCCs and results in a recovery of reduced melatonin secretion during parasympathetic nerve activity. BKCa channels may play a physiological role for melatonin production via a negative-feedback mechanism.

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.

Differences in stimulus induced calcium increases in lacrimal gland acinar cells from normal and NZB/NZW F1 female mice

PURPOSE

To compare the changes in intracellular free calcium in response to both cholinergic and adrenergic agonists in cells isolated from exorbital lacrimal glands of NZB/NZW F1 female mice (NZB/W) and normal Swiss Webster mice (SW).

METHODS

We have loaded cells with Fura-2 and measured total intracellular calcium using ratiomicrofluorometric methods. Isolated cells were also patch-clamped using perforated patch.

RESULTS

In all cells, both carbachol (CCh) and phenylephrine (PE) increased intracellular calcium. The calcium increase to both CCh and PE was less in cells from NZB/W animals than in cells from SW animals. In cells from young animals, the baseline calcium levels were the same, but in cells from older (6 months) NZB/W animals, the baseline levels of calcium were 50% higher than those seen in SW animals.

CONCLUSION

These data suggest that cells from the NZB/W mice have different calcium dynamics, which could contribute to their compromised fluid secretion.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Calcium signalling in secretory cells

Stimulation of secretory cells with muscarinic agonists leads to an increase in the intracellular Ca (2+)concentration ([Ca (2+)]( i)), which activates protein secretion through exocytosis and causes closure of gap junctions between adjacent cells. In addition, the increase in [Ca (2+)](i) activates three different kinds of ion channels: large K(+) channels, Cl(-) channels and non-specific cation channels. The opening of those channels leads to an increase of [Na(+ )] and a decrease of [Cl(-)] and [K(+) ] in the cell. The two components that contribute to the increase in [Ca (2+)]( i) are calcium release from intracellular stores, localised in the endoplasmic reticulum and calcium influx through the plasma membrane. Several models for the regulation of [Ca (2+)](i) have been proposed, including a recently suggested model whereby a distinct pathway involving arachidonic acid is added to the well-established capacitative model. Different hypotheses concerning coupling between the intra-cellular calcium stores and membrane channels co-exist. In addition to a historical overview, recent developments and future challenges are discussed in this review.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Nonselective cation channels in exocrine gland cells

The nonselective cation channel has been described in a wide variety of nonexcitable cells. However even in such closely related tissues as the pancreatic acinar cell and the lacrimal acinar cell, which both possess a superficially similar channel, recent work has shown fundamental differences in channel regulation (Sasaki and Gallacher, 1992; Thorn and Petersen, 1992). These differences are a reflection of a diverse function of the nonselective channel in different tissues.

Diffusion into the patch-clamp recording pipette of a factor necessary for muscarinic current response

Ca-dependent K and Cl currents were measured in isolated cells from rat lacrimal glands using the tight-seal whole-cell recording method. Upon application of acetylcholine (ACh), both K and Cl-selective currents were activated. The size of the ACh-activated currents declined after a few minutes of whole-cell recording. The rundown curve was composed of an initial stable period followed by a rather rapid decline. Both the length of the initial plateau and the speed of the falling phase were dependent on cell size and recording pipette resistance. The results suggest that the rundown was due to washout of an unknown cytosolic substance. Another manifestation of washout was an increase in the delay of the response. Plots of the inverse of the delay as a function of time in whole-cell recording showed again an initial plateau and a falling phase, but the stable period lasted less than in amplitude plots. Analysis of the washout time course suggested that the cytosolic substance has a diffusion coefficient of 5.4 x 10(-6) cm2/s, corresponding to a molecular weight of 350. Washed-out cells were insensitive to GTP-gamma-S, but responded normally to an internal application of inositol-trisphosphate (InsP3), introduced through the pipette. Thus, the step of the response which is sensitive to washout is closely related to the production of InsP3. Addition of various exogenous water soluble substances failed to halt washout. Among the inactive substances were GTP (or a combination of Mg and GTP) and small water soluble precursors of InsP3. The results imply that the production of InsP3 by muscarinic agonists in exocrine glands requires the presence of a small molecular weight, water soluble substance. It is suggested that this substance is an unknown co-factor of phospholipase C or of Gp, the GTP binding protein governing the production of InsP3.

Acetylcholine-induced closure of gap junction channels in rat lacrimal glands is probably mediated by protein kinase C

In rat lacrimal gland cells, application of acetylcholine (ACh) opens Ca-dependent channels and closes gap junction channels. We have shown previously that the increase in intracellular calcium concentration induced by ACh, is not required for the closure of gap junctions. We have examined the effects of activators of protein kinase C on gap junction conductance measured with the double patch-clamp technique. This conductance was markedly reduced by incubating the cell pairs for a few minutes with 100 nM phorbol dibutyrate (PdBu). Two membrane permeant analogues of diacylglycerol, OAG (1-oleoyl-2-acetyl-glycerol) and DOG (1,2-dioleoyl-glycerol) also induced a closure of gap junction channels. This effect was observed in the concentration range 10-100 microM when the diacylglycerol was added on intact cells, and at 75 microM when it was applied on dialysed cells. The cell uncoupling was not mediated by phosphatidate, a degradation product of OAG, nor by a phospholipase A2-induced increase in arachidonate concentration. The OAG-induced closure of gap junction channels reversed spontaneously upon prolonged exposure (more than 90 min at 37 degrees C) to 25 microM OAG. After a prolonged OAG treatment, the ability of ACh to uncouple the cells was markedly reduced. ACh induced uncoupling was modulated to some extent by intracellular Ca and had an absolute requirement for Mg. These results indicate that ACh-induced closure of gap junction channels may be mediated by PKC.

Small-conductance chloride channels activated by calcium on cultured endocrine cells from mammalian pars intermedia

Porcine intermediate lobe (IL) endocrine cells maintained in primary culture have been studied using patch-clamp derived configurations to record unitary activity on outside-out vesicles. Solutions were devised so as to record Cl current in isolation and to fix cytoplasmic Ca concentration [Ca]i between 0.1 microM and 3 microM. Between [Ca]i 0.5 and 1 microM, the chloride permeability was restricted to single events with a small amplitude, that varied linearly with the membrane potential. Mean slope conductance of this chloride channel was 2.5 pS. Single channel analysis yielded two mean open time values of 10 and 55 ms at -80 mV. Relaxations of chloride currents on outside-out patches was examined at different [Ca]i. Relaxation was negligible at 0.15 microM [Ca]i, whereas at higher [Ca]i, the current exhibited relaxation in response to voltage jumps the kinetic of which could be fitted by two exponentials. At 0.5 microM [Ca]i, the fast relaxation time constant was shown to be voltage insensitive with a value of about 10 ms. The slow relaxation time constant had a mean value of 75 ms at -60 mV and increased with membrane depolarization with a twofold change over 120 mV. Another voltage effect was to favour the slow opening mode at the more depolarized potentials: the ratio of fast to slow relaxations being 5:1 at -60 mV as compared to 1:1 at +80 mV). Finally the estimated probability of opening (po) linearly increased with voltage. po displayed a bell-shaped dependence on [Ca]i, so that full activation of the channels was not achieved.

Regulation of calcium influx by second messengers in rat mast cells

Biphasic increases in the free intracellular calcium concentration, consisting of a large initial transient followed by a sustained elevation, are frequently observed in non-excitable cells following stimulation. In rat peritoneal mast cells a cAMP- and Ca-activated chloride current can interact with IP3-dependent calcium influx to provide the sustained elevation of intracellular Ca concentration following transient IP3-induced release of calcium from intracellular stores. This novel combination of second messenger systems provides a flexible means to modulate calcium-dependent processes such as exocytosis.

Vasoactive intestinal peptide activates Ca2(+)-dependent K+ channels through a cAMP pathway in mouse lacrimal cells

The action of vasoactive intestinal peptide (VIP) on Ca2(+)-dependent K+ currents, in dissociated mouse lacrimal cells, was investigated using patch clamp techniques. In whole cell recordings, VIP (10-100 pM) increased the magnitude of the Ca2(+)-dependent K+ current. In single channel recordings, VIP increased the fraction of time the large charybdotoxin-sensitive Ca2(+)-activated K+ channel spent in the open state. The activity of this channel was also increased by adding forskolin or 8-bromo cAMP to the bath. Additionally, application of either cAMP or catalytic subunit of cAMP-dependent protein kinase directly to the cytoplasmic surface of excised inside out patches reversibly lengthened the time Ca2(+)-activated K+ channels spent in the open state. These data suggest that VIP stimulates Ca2(+)-activated K+ channels by a cAMP-dependent pathway in mouse lacrimal acinar cells.

Calcium and secretagogues-induced conductances in rat exocrine pancreas

The electrical properties of single acinar cells isolated from rat pancreas were studied with the whole-cell tight-seal recording method. Under resting conditions, the relative permeabilities of Cl and K were PCl/PK approximately equal to 3. At 1 microM internal calcium, a Ca and voltage-dependent Cl conductance was activated. At 10 microM internal calcium, the major conductance was selective for cations. It was not voltage-dependent. Acetylcholine and cholecystokinin induced an increase of internal Ca which in turn activated either only a Cl conductance or both Cl and cationic conductances. The secretagogue-induced conductance was increased to a variable extent by depolarisation. The absence of K channels activated by internal calcium indicates that, in pancreatic acinar cells, the mechanism of fluid secretion differs from that observed in other exocrine glands.

Dependence of intracellular effects of GTP gamma S and inositoltrisphosphate on cell membrane potential and on external Ca ions

Single cells from rat lacrimal glands were studied with the tight-seal whole-cell recording method. Ca-dependent K and Cl currents were measured in response to dialysis with inositoltrisphosphate or GTP gamma S, two compounds known for elevating internal Ca2+ concentration. The activation of the Ca-dependent currents elicited by either compound was partially inhibited by sustained depolarization or by removal of external Ca2+. Conversely, hyperpolarization or removal of external Mg2+ led to augmentations of the Ca-dependent currents. These effects became apparent about 1 min after initiation of cell dialysis with inositoltrisphosphate- or GTP gamma S-containing solutions, and they further developed during the ensuing 10 min. Holding potential and external divalent cations did not affect the Ca-dependent currents elicited by dialysing the cells with strongly buffered solutions containing 0.5 microM free Ca2+. In Ca2+-free external saline, cell currents were independent of the holding potential. It is suggested that InsP3 augments intracellular Ca2+ levels not only by releasing Ca2+ from internal stores but also by slowly increasing the Ca permeability of the plasma membrane. The results indicate that Ca2+ entry through the plasma membrane increases with hyperpolarization. The similarity of the effects seen in InsP3- and GTP gamma S-dialysed cells gives support to the hypothesis that production of the former depends on a GTP-binding protein.

Spontaneous and GABA-evoked chloride channels on pituitary intermediate lobe cells and their internal Ca requirements

On porcine intermediate lobe (IL) endocrine cells, spontaneously opening chloride channels have been studied and compared to GABA-A activated chloride channels. Elementary currents were recorded mainly from outside-out patches excised from IL cells maintained in culture for 1-4 weeks. Spontaneous inward currents were observed in Cs-loaded cells after replacing Na in the extracellular medium by the impermeant ion choline. This activity, at an internal calcium concentration of 10(-8) M corresponded to a channel for chloride ions with a main conductance level of 26 pS, and substates around 11 pS. The sequence of permeabilities to halides was I greater than Br greater than Cl. These conductance characteristics were common to the GABA-operated channels which also showed a main conductance substate of 23-31 pS. The open time of the 26 pS level mostly encountered in spontaneous activity, was distributed along two modes: one, the most frequent, around 1 ms, and the other around 4 ms. This latter mode was the predominant one observed during GABA and isoguvacine applications but in addition a bursting activity of 19 ms duration was also seen. Specific GABA-A receptor antagonists (bicuculline and SR42641, 1 microM) blocked activity evoked by GABA (1-10 microM), but did not affect spontaneous events. These spontaneous Cl events were only observed in a restricted range of internal Ca concentrations, i.e. between 1 nM and 0.1 microM, and were practically abolished at Cai 1 microM. The GABA-induced activity of Cl channels was also Ca-sensitive, being reduced when Cai reached 1 microM.

Voltage-activation of high-conductance K+ channel in the insulin-secreting cell line RINm5F is dependent on local extracellular Ca2+ concentration

Patch-clamp single-channel current recording experiments have been carried out on intact insulin-secreting RINm5F cells. Voltage-activation of high-conductance K+ channels were studied by selectively depolarizing the electrically isolated patch membrane under conditions with normal Ca2+ concentration in the bath solution but with or without Ca2+ in the patch pipette solution. When Ca2+ was present in the pipette, 40 mV to 120 mV depolarizing pulses (100 ms) from the normal resting potential (-70 mV) regularly evoked tetraethylammonium-sensitive large outward single-channel currents and the average open state probability during the pulses varied from about 0.015 (40 mV pulses) to 0.1 (120 mV pulses). In the absence of Ca2+ in the pipette solution the same protocol resulted in fewer and shorter K+ channel openings and the open-state probability varied from about 0.0015 (40 mV pulses) to about 0.03 (120 mV pulses). It is concluded that Ca2+ entering voltage-gated channels raises [Ca2+]i locally and thereby markedly enhances the open-state probability of tetraethylammonium-sensitive voltage-gated high-conductance K+ channels.