Intracellular [Ca2+] and inositol phosphates in avian nasal gland cells

Increasing Ca2+ accumulation in salt glands under salt stress increases stronger selective secretion of Na+ in Plumbago auriculata tetraploids

Under salt stress, recretohalophyte Plumbago auriculata tetraploids enhance salt tolerance by increasing selective secretion of Na+ compared with that in diploids, although the mechanism is unclear. Using non-invasive micro-test technology, the effect of salt gland Ca2+ content on Na+ and K+ secretion were investigated in diploid and tetraploid P. auriculata under salt stress. Salt gland Ca2+ content and secretion rates of Na+ and K+ were higher in tetraploids than in diploids under salt stress. Addition of exogenous Ca2+ increased the Ca2+ content of the salt gland in diploids and is accompanied by an increase in the rate of Na+ and K+ secretion. With addition of a Ca2+ channel inhibitor, diploid salt glands retained large amounts of Ca2+, leading to higher Ca2+ content and Na+ secretion rate than those of tetraploids. Inhibiting H2O2 generation and H+-ATPase activity altered Na+ and K+ secretion rates in diploids and tetraploids under salt stress, indicating involvement in regulating Na+ and K+ secretion. Our results indicate that the increased Na+ secretion rate of salt gland in tetraploids under salt stress was associated with elevated Ca2+ content in salt gland.

ATP Release from Human Airway Epithelial Cells Exposed to Staphylococcus aureus Alpha-Toxin

Airway epithelial cells reduce cytosolic ATP content in response to treatment with S. aureus alpha-toxin (hemolysin A, Hla). This study was undertaken to investigate whether this is due to attenuated ATP generation or to release of ATP from the cytosol and extracellular ATP degradation by ecto-enzymes. Exposure of cells to rHla did result in mitochondrial calcium uptake and a moderate decline in mitochondrial membrane potential, indicating that ATP regeneration may have been attenuated. In addition, ATP may have left the cells through transmembrane pores formed by the toxin or through endogenous release channels (e.g., pannexins) activated by cellular stress imposed on the cells by toxin exposure. Exposure of cells to an alpha-toxin mutant (H35L), which attaches to the host cell membrane but does not form transmembrane pores, did not induce ATP release from the cells. The Hla-mediated ATP-release was completely blocked by IB201, a cyclodextrin-inhibitor of the alpha-toxin pore, but was not at all affected by inhibitors of pannexin channels. These results indicate that, while exposure of cells to rHla may somewhat reduce ATP production and cellular ATP content, a portion of the remaining ATP is released to the extracellular space and degraded by ecto-enzymes. The release of ATP from the cells may occur directly through the transmembrane pores formed by alpha-toxin.

Staphylococcus aureus α-toxin-mediated cation entry depolarizes membrane potential and activates p38 MAP kinase in airway epithelial cells

Membrane potential (Vm)-, Na(+)-, or Ca(2+)-sensitive fluorescent dyes were used to analyze changes in Vm or intracellular ion concentrations in airway epithelial cells treated with Staphylococcus aureus α-toxin (Hla), a major virulence factor of pathogenic strains of these bacteria. Gramicidin, a channel-forming peptide causing membrane permeability to monovalent cations, a mutated form of Hla, rHla-H35L, which forms oligomers in the plasma membranes of eukaryotic cells but fails to form functional transmembrane pores, or the cyclodextrin-derivative IB201, a blocker of the Hla pore, were used to investigate the permeability of the pore. Na(+) as well as Ca(2+) ions were able to pass the Hla pore and accumulated in the cytosol. The pore-mediated influx of calcium ions was blocked by IB201. Treatment of cells with recombinant Hla resulted in plasma membrane depolarization as well as in increases in the phosphorylation levels of paxillin (signaling pathway mediating disruption of the actin cytoskeleton) and p38 MAP kinase (signaling pathway resulting in defensive actions). p38 MAP kinase phosphorylation, but not paxillin phosphorylation, was elicited by treatment of cells with gramicidin. Although treatment of cells with rHla-H35L resulted in the formation of membrane-associated heptamers, none of these cellular effects were observed in our experiments. This indicates that formation of functional Hla-transmembrane pores is required to induce the cell physiological changes mediated by α-toxin. Specifically, the changes in ion equilibria and plasma membrane potential are important activators of p38 MAP kinase, a signal transduction module involved in host cell defense.

S. aureus haemolysin A-induced IL-8 and IL-6 release from human airway epithelial cells is mediated by activation of p38- and Erk-MAP kinases and additional, cell type-specific signalling mechanisms

Soluble virulence-associated factors of Staphylococcus aureus like haemolysin A (Hla) induce secretion of chemo/cytokines from airway epithelial cells. To elucidate the potential roles of specific signalling pathways in this response, we treated 16HBE14o-, S9 or A549 cells with recombinant Hla (rHla). In a dose-dependent manner, rHla induced secretion of IL-8 in all three cell types, but IL-6 release only in 16HBE14o- and S9 cells. rHla-mediated secretion of IL-8 and IL-6 was suppressed by pre-incubation of cells with inhibitors of Erk type or p38 MAP kinases, indicating that activation of these signalling pathways is essential for IL-8 release in all three cell types and for IL-6 release in 16HBE14o- and S9 cells. The rHla-mediated phosphorylation and activation of p38 MAP kinase seem to depend on elevations in [Ca(2+)]i, an early response in rHla-treated cells. Inhibitors of calmodulin or calcium/calmodulin-dependent kinase II attenuated rHla-mediated release of IL-8 in 16HBE14o- and A549 cells and of IL-6 in 16HBE14o- cells. This indicates that rHla may mediate simultaneous activation of calmodulin-dependent processes as additional prerequisites for chemo/cytokine secretion.However, the inhibitors of calmodulin-dependent signalling did not affect rHla-induced p38 MAP kinase phosphorylation, indicating that this pathway works in parallel with p38 MAP kinase.

Streptococcus pneumoniae infection of host epithelial cells via polymeric immunoglobulin receptor transiently induces calcium release from intracellular stores

The pneumococcal surface protein C (PspC) is a major adhesin of Streptococcus pneumoniae (pneumococci) that interacts in a human-specific manner with the ectodomain of the human polymeric immunoglobulin receptor (pIgR) produced by respiratory epithelial cells. This interaction promotes bacterial colonization and bacterial internalization by initiating host signal transduction cascades. Here, we examined alterations of intracellular calcium ([Ca(2+)](i)) levels in epithelial cells during host cell infections with pneumococci via the PspC-hpIgR mechanism. The release of [Ca(2+)](i) from intracellular stores in host cells was significantly increased by wild-type pneumococci but not by PspC-deficient pneumococci. The increase in [Ca(2+)](i) was dependent on phospholipase C as pretreatment of cells with a phospholipase C-specific inhibitor U73122 abolished the increase in [Ca(2+)](i). In addition, we demonstrated the effect of [Ca(2+)](i) on pneumococcal internalization by epithelial cells. Uptake of pneumococci was significantly increased after pretreatment of epithelial cells with the cell-permeable calcium chelator 1,2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid-tetraacetoxymethyl ester or use of EGTA as an extracellular Ca(2+)-chelating agent. In contrast, thapsigargin, an inhibitor of endoplasmic reticulum Ca(2+)ATPase, which increases [Ca(2+)](i) in a sustained fashion, significantly reduced pIgR-mediated pneumococcal invasion. Importantly, pneumococcal adherence to pIgR-expressing cells was not altered in the presence of inhibitors as demonstrated by immunofluorescence microscopy. In conclusion, these results demonstrate that pneumococcal infections induce mobilization of [Ca(2+)](i) from intracellular stores. This may constitute a defense response of host cells as the experimental reduction of intracellular calcium levels facilitates pneumococcal internalization by pIgR-expressing cells, whereas elevated calcium levels diminished bacterial internalization by host epithelial cells.

Effects of Staphylococcus aureus-hemolysin A on calcium signalling in immortalized human airway epithelial cells

Part of the innate defence of bronchial epithelia against bacterial colonization is secretion of salt and water which generally depends on coordinated actions of receptor-mediated cAMP- and calcium signalling. The hypothesis that Staphylococcus aureus-virulence factors interfere with endogenous signals in host cells was tested by measuring agonist-mediated changes in [Ca(2+)](i) in S9 cells upon pre-incubation with bacterial secretory products. S9 cells responded to mAChR-activation with calcium release from intracellular stores and capacitative calcium influx. Treatment of cells with culture supernatants of S. aureus (COL) or with recombinant alpha-hemolysin (Hla) resulted in time- and concentration-dependent changes in [Ca(2+)](i). High concentrations of Hla (2000 ng/ml) resulted in elevations in [Ca(2+)](i) elicited by accelerated calcium influx. A general Hla-mediated permeabilization of S9 cell membranes to small molecules, however, did not occur. Lower concentrations of Hla (200 ng/ml) induced a reduction in [Ca(2+)](i)-levels during the sustained plateau phase of receptor-mediated calcium signalling which was abolished by pre-incubation of cells with carboxyeosin, an inhibitor of the plasma membrane calcium-ATPase. This indicates that low concentrations of Hla change calcium signalling by accelerating pump-driven extrusion of Ca(2+) ions. In vivo, such a mechanism may result in attenuation of calcium-mediated cellular defence functions and facilitation of bacterial adherence to the bronchial epithelium.

Morphological substrate of autonomic failure and neurohormonal dysfunction in multiple system atrophy: impact on determining phenotype spectrum

Autonomic failure is a prominent clinical feature of patients with multiple system atrophy (MSA). Neurohormonal dysfunction is also a frequent accompaniment in patients with MSA. The determination of the pathological involvement of the autonomic neurons, which are responsible for circadian rhythms and responses to stress, provides new insight into autonomic failure and neurohormonal dysfunction in MSA. The disruptions of circadian rhythms and responses to stress may underlie the impairment of homeostatic integration responsible for cardiovascular and respiratory failures. These notions lead to the hypothesis that a pathological involvement of autonomic neurons is a significant factor of the poor prognosis of MSA. Beyond this perspective, endeavors to find the morphological phenotype that represents a predominant loss of autonomic neurons may elucidate the full spectrum of pathological involvements in MSA.

Cardiovascular effects of noradrenaline microinjection in the bed nucleus of the stria terminalis of the rat brain

The bed nucleus of the stria terminalis (BST) is a limbic structure involved in regulating the hypothalamic-pituitary-adrenal axis as well as in central cardiovascular control. We report here on cardiovascular effects caused by microinjection of noradrenaline (NA) in the BST of the rat brain and the peripheral mechanisms involved in their mediation. Injection of NA (3, 7, 10, 15, 30, or 45 nmol in 100 nl) in the BST of unanesthetized rats caused long-lasting dose-related pressor and bradycardiac responses. No responses were observed when the dose of 10 nmol NA was microinjected into surrounding structures, such as the anterior commissure, the stria terminalis, the fornix, and the internal capsule, indicating a predominant action at the BST. Additionally, microinjection of 50 nmol tyramine, an indirectly acting sympathomimetic amine, caused similar pressor response, indicating local NA release in the BST. Responses to NA microinjection in the BST were markedly reduced in urethane-anesthetized rats, favoring the idea of a central action without significant leakage to the peripheral circulation. The pressor response was potentiated by i.v. pretreatment with the ganglion blocker pentolinium and blocked by i.v. pretreatment with the selective V(1)-vasopressin antagonist dTyr(CH(2))(5)(Me)AVP, suggesting its mediation by vasopressin release into circulation. The bradycardiac response to NA microinjected into the BST was also abolished by pretreatment with the vasopressin antagonist, indicating its reflex origin. In conclusion, results indicate that microinjection of NA into the BST evokes pressor responses, which are mediated by acute vasopressin release.

Downregulation of aquaporins 1 and 5 in nasal gland by osmotic stress in ducklings, Anas platyrhynchos: implications for the production of hypertonic fluid

Using primers against highly conserved regions of mammalian and bird aquaporins in RT-PCR experiments, we amplified products derived from duck (Anas platyrhynchos) nasal gland RNA that were identified as homologues of mammalian and chicken aquaporin 1 and aquaporin 5 cDNAs by sequencing. Using digoxigenin-labelled probes derived from these PCR products in northern blot analyses of mRNA isolated from nasal glands of untreated (naïve) or osmotically stressed ducklings (replacement of drinking water with a 1% NaCl solution), we observed a decrease in aquaporin 1 (AQP1) and aquaporin 5 (AQP5) mRNA abundance (by approximately 40%) during saline adaptation in the animals. Western blot analysis of AQP1 and AQP5 expression in the glands revealed that protein abundance decreased in a similar fashion. Immunohistochemical analysis of AQP1 distribution in cryosections of nasal gland indicated that AQP1 is mainly expressed in endothelial cells of the capillaries, but definitely not in the secretory or ductal cells of the gland. AQP5 distribution in the gland, however, seems to be different, since staining was exclusively observed in apical and basolateral plasma membranes of individual epithelial cells of the primary and central ducts, which collect fluid from the secretory tubules. The observations are consistent with the hypothesis that strongly hyperosmotic fluid is produced by the secretory cells at very low (unstimulated gland) or high (activated gland) rates. In the unstimulated gland, secretions may be diluted by aquaporin-mediated transcellular water flux while passing through the ductal system flushing the glandular ducts, thereby potentially preventing ascending infections. In the activated gland, however, downregulation of aquaporins in capillaries and duct cells may prevent dilution of the initially secreted fluid, enabling the animals to excrete large volumes of a highly concentrated salt solution.

Noradrenergic regulation of galanin expression in the supraoptic nucleus in the rat hypothalamus. An ex vivo study

Galanin is coexpressed with vasopressin and oxytocin in magnocellular neurons of the rat neuroendocrine hypothalamus. Various physiological stimuli, such as osmotic stimulation or lactation, that affect vasopressin and oxytocin expression and release also modulate galanin expression. Magnocellular neurons are highly innervated by noradrenergic inputs from the brainstem. The noradrenergic system plays a critical excitatory role in the activation of vasopressin-expressing and oxytocin-expressing neurons. Here, we have evaluated the possible regulation of Gal expression by noradrenaline in the magnocellular neurons of supraoptic nucleus in an ex vivo acute model of rat hypothalamic slices. The slices containing the supraoptic nucleus were incubated with 10(-4) M noradrenaline for 1 or 4 hr. The levels of galanin and galanin mRNA were estimated by semiquantitative immunohistochemistry and in situ hybridization, respectively. Our results show that the amount of galanin-immunopositive material in the cell bodies of the magnocellular neurons increased significantly after incubation with noradrenaline compared with control slices at the same time point and that this effect was more pronounced after 4 hr than after 1 hr. In situ hybridization showed that radiolabeling of the supraoptic nucleus with a radioactive galanin probe increased slightly after 1 hr of incubation and increased considerably after 4 hr of incubation with noradrenaline. Our study shows that galanin may be a target in the regulation of the hypothalamic magnocellular-neurohypophysial system by noradrenaline.

Coping with excess salt: adaptive functions of extrarenal osmoregulatory organs in vertebrates

In all organisms, changing environmental conditions require appropriate regulatory measures to physiologically adjust to the altered situation. Uptake of excess salt in non-mammalian vertebrates having limited or no access to freshwater is balanced by extrarenal salt excretion through specialized structures called 'salt glands'. Nasal salt glands of marine birds are usually fully developed in very early stages of their lives since individuals of these species are exposed to salt soon after hatching. In individuals of other bird species, salt uptake may occur infrequently. In these animals, glands are usually quiescent and glandular cells are kept in a fairly undifferentiated state. This is the situation in 'naive' ducklings, Anas platyrhynchos, which have never been exposed to excess salt. When these animals become initially osmotically stressed, the nasal glands start to secrete a moderately hypertonic sodium chloride solution but secretory performance is meager. Within 48 h after the initial stimulus, however, the number of cells per gland is elevated by a factor of 2-3, the secretory cells differentiate and acquire full secretory capacity. During this differentiation process, extensive surface specializations are formed. The number of mitochondria is increased and metabolic enzymes and transporters are upregulated. These adaptive growth and differentiation processes result in a much higher efficiency of salt excretion in acclimated ducklings compared with naive animals. Receptors and signal transduction pathways in salt gland cells controling the adaptive processes seem to be the same as those controling salt secretion, namely muscarinic acetylcholine receptors and receptors for vasoactive intestinal peptide. This review focusses on signal transduction pathways activated by muscarinic receptors which seem to fine-tune salt secretion in salt-adapted ducklings and may control adaptive growth and differentiation processes in the nasal gland of naive animals.

Attenuation of cell cycle regulator p27(Kip1) expression in vertebrate epithelial cells mediated by extracellular signals in vivo and in vitro

Cell cycle arrest in potentially dividing cells is often mediated by inhibitors of G1/S-phase cyclin-dependent kinases. The cyclin E/CDK2-inhibitor p27(Kip1) has been implicated in this context in epithelial cells. We cloned and sequenced p27(Kip1) of ducklings (Anas platyrhynchos) and used an in vitro assay system to study the mechanism of p27(Kip1) downregulation in the nasal gland which precedes an increase in proliferation rate upon initial exposure of the animals to osmotic stress. Western blot studies revealed that p27(Kip1) is downregulated during 24 h of osmotic stress in ducklings with the steepest decline in protein levels between 5 and 8 h. As indicated by the results of Northern blot and semi-quantitative PCR studies, protein downregulation is not accompanied by similar changes in mRNA levels indicating that Kip1 is regulated mainly at the translational (synthesis) or posttranslational level (degradation). Using recombinant duck Kip1 protein expressed in E. coli, we showed that Kip1 is subject to polyubiquitinylation by cytosolic enzymes from nasal gland cells indicating that loss of Kip1 may be regulated, at least in part, by acceleration of protein degradation. In cultured nasal gland tissue, attenuation of Kip1 expression could be induced by activation of the muscarinic acetylcholine receptor indicating that mAChR-receptor signalling may play a role in the re-entry of quiescent gland cells into the cell cycle.

Cross-talk of phosphoinositide- and cyclic nucleotide-dependent signaling pathways in differentiating avian nasal gland cells

In many bird species, the nasal glands secrete excess salt ingested with drinking water or food. In ducks ( Anas platyrhynchos), osmotic stress results in adaptive cell proliferation and differentiation in the gland. Using 'naive' nasal gland cells isolated from animals that had never ingested excess salt or 'differentiated' cells from animals fed with a 1% NaCl solution for 48 h, we investigated the allocation of metabolic energy to salt excretory processes and to other cellular activities. Activation of muscarinic acetylcholine receptors (carbachol) or beta-adrenergic receptors (isoproterenol) in nasal gland cells resulted in a transient peak in metabolic rate followed by an elevated plateau level that was maintained throughout the activation period. Activation of cells using vasoactive intestinal peptide, however, had only marginal effects on metabolic rate. In differentiated cells, sequential stimulation with carbachol and isoproterenol resulted in additive changes in metabolic rate during the plateau phase. Naive cells, however, developed supra-additive plateau levels in metabolic rates indicating cross-talk of both signaling pathways. Using bumetanide, TEA or barium ions to block different components of the ion transport machinery necessary for salt secretion, the relative proportion of energy needed for processes related to ion transport or other cellular processes was determined. While differentiated cells in the activated state allocated virtually all metabolic energy to processes related to salt secretion, naive cells reserved a significant amount of energy for other processes, possibly sustaining cellular signaling and regulating biosynthetic mechanisms related to adaptive growth and differentiation.

Vasopressin response to osmotic and hemodynamic stress: neurotransmitter involvement

Osmotic and hemodynamic stress are the two primary regulators of vasopressin (VP) release from the posterior pituitary. The pathways providing information about plasma osmolality and blood pressure or blood volume are distinct and utilize different chemical neurotransmitters. Osmotic regulation of VP release is dependent upon afferents from the lamina terminalis region. Glutamate is an important transmitter in this system and angiotensinergic afferents from this region to the VP neurons modulate responses to osmotic challenges. Hemodynamic information is transmitted to the VP neurons via multisynaptic pathways from the brainstem with the A1 catecholamine neurons of the ventrolateral medulla providing the final link for information about decreases in blood pressure and volume. Several neurotransmitters and neuropeptides are expressed in the A1 neurons including norepinephrine (NE), ATP, neuropeptide Y, and substance P. The impact of co-release of these agents on VP release is reviewed and the potential physiological significance is discussed.

Pain-autonomic interactions: a selective review

The nociceptive and the autonomic systems interact at the level of the periphery, spinal cord, brainstem, and forebrain. Spinal and visceral afferents provide converging information to spinothalamic neurons in the dorsal horn and to neurons of the nucleus tractus solitarius and parabrachial nuclei. These structures project to areas involved in reflex, homeostatic, and behavioral control of autonomic outflow, endocrine function, and nociception. These include monoaminergic cell groups of the medulla and pons, periaqueductal gray, hypothalamus, amygdala, insular cortex, and anterior cingulate gyrus. These interactions should be taken into account to understand the complex pathophysiology of chronic pain disorders.

Ca(2+) and p38 MAP kinase regulate mAChR-mediated c-Fos expression in avian exocrine cells

Muscarinic acetylcholine receptors (mAChRs) in exocrine tissue from the avian nasal salt gland are coupled to phospholipase C and generate inositol phosphate and Ca(2+) signals upon activation. An early effect of receptor activation in the secretory cells is a transient accumulation of c-Fos protein. In cooperation with constitutively expressed Jun, Fos presumably serves as a transcription factor altering gene expression during cell growth and differentiation processes in the gland associated with adaptation to osmotic stress in animals. Nothing is known, however, about the mAChR-dependent signaling pathways leading to Fos expression in these cells. By incubation of isolated nasal gland tissue in short-term culture with activators or inhibitors of signaling pathways and quantitative Western blot analysis of Fos abundance, we have now identified the sustained elevation of the intracellular Ca(2+) concentration and the activation of the p38 mitogen-activated protein (MAP) kinase as intermediate signaling elements for the regulation of c-Fos by muscarinic receptor activation. It is suggested that p38 MAP kinase, rather than exclusively mediating stress responses, is involved in the regulation of cellular growth and differentiation controlled by G protein-coupled receptors.

Vertebrate salt glands: short- and long-term regulation of function

Excess salt loads in most non-mammalian vertebrates are dealt with by a variety of extra-renal salt-secreting structures collectively described as salt glands. The best studied of these are the supra-orbital nasal salt glands of birds. Two distinct types of response to osmoregulatory disturbances are shown by this structure: a progressive adaptive response on initial exposure to a salt load that results in the induction and enhancement of the secretory performance or capabilities of the gland; and the rapid activation of existing osmoregulatory mechanisms in the adapted gland in response to immediate osmoregulatory imbalance. Not only is the time-frame of these two types of response very different, but the responses usually involve fundamentally different processes: e.g., the growth and differentiation of osmoregulatory structures and their components in the former case, compared with the rapid activation of ion channels, pumps etc. in the latter. Despite marked differences in the nature and time-frame of these responses, they both are apparently triggered by neuronally released acetylcholine, which acts at muscarinic receptors on the secretory cells to induce an inositol phosphate-dependent increase in cytosolic-free calcium concentrations ([Ca2+]i). Therefore, the question arises as to how the cells produce the appropriate distinct response using a single common signal (i.e., an increase in [Ca2+]i). Examination of the features of this signaling pathway in the two conditions described, reveals that they each are uniquely tuned to generate a response with the characteristics appropriate for the cells' requirements. This tuning of the signal involves often rather subtle changes in the overall signaling pathway that are part of the adaptive differentiation process.

In vivo and in vitro induction of c-fos in avian exocrine salt gland cells

Osmotic stress in ducklings (Anas platyrhynchos) results in salt secretion and adaptive cell proliferation and differentiation in the nasal glands. We investigated whether osmotic stress in vivo or muscarinic ACh receptor activation in vitro changed the expression levels of the cellular protooncogene products Fos and Jun, which may play a role in the initiation of the adaptive processes. Using Fos- and Jun-specific polyclonal antisera in Western blot experiments, we demonstrated that Jun is constitutively expressed in nasal gland tissue, whereas Fos is not detectable in tissue from unstressed (naive) animals. Under conditions of osmotic stress imposed by replacing the drinking water of the animals with a 1% NaCl solution, Jun protein remains constant in nasal gland tissue, whereas Fos protein is transiently upregulated. Treatment of cultured nasal gland tissue with muscarinic agonists results in a transcriptionally regulated expression of Fos in an atropine-sensitive manner. Immunohistochemical experiments show that Fos accumulation occurs in the nuclei of the secretory cells. These results indicate that the activation of the c-fos gene induced by muscarinic ACh receptor-mediated signaling pathways may play an important role in the initiation of adaptive growth and differentiation processes in nasal glands of osmotically stressed ducklings.

The effects of bradykinin on K+ currents in NG108-15 cells treated with U73122, a phospholipase C inhibitor, or neomycin

1. Bradykinin has multiple effects on differentiated NG108-15 neuroblastoma x glioma cells: it increases Ins(1,4,5)P3 production and intracellular Ca2+ concentration [Ca2+]i evokes a Ca2+ activated K+ current (IK(Ca)) and inhibits M current (IM). We studied the effect of the aminosteroid U73122 and the antibiotic neomycin, both putative blockers of phospholipase C (PLC), on these four bradykinin effects. 2. Preincubation with 1 or 5 microM U73122 for 15 min partly suppressed Ins(1,4,5)P3 generation and the increase in [Ca2+]i induced by 1 microM bradykinin. U73122 10 microM caused total and irreversible inhibition. The inactive analogue U73343 was without effect. 3. Resting levels of Ins(1,4,5)P3 were not affected. However, resting [Ca2+]i was increased by 10 microM U73122, but not by U73343. Individual cells responded to 10 microM U73122 with a small increase in [Ca2+]i, followed in some cells by a large further rise. 4. Pretreatment of whole-cell clamped cells with 1 microM U73122 for 30 min reduced the bradykinin-induced IK(Ca) to a fifth of its normal size. To suppress it totally, a 7-12 min pretreatment with 5 microM U73122 was required. Again, U73343 was without effect. 5. U73122 and U73343 at concentrations of 5-10 microM irreversibly decreased the holding current (Ih) which at a holding potential of -30 or -20 mV mainly flows through open M channels. The decrease was often preceded by a transient increase. 6. M current (IM) measured with 1 s pulses, was also decreased by 5-10 microM U73122 and U73343, but short applications of U73122 could cause a small increase. The bradykinin-induced inhibition of IM was not affected by U73122. 7. Preincubation with 1 or 3 mM neomycin for 15 min did not affect Ins(1,4,5)P3 generation and the increase in [Ca2+]i induced by bradykinin. Pretreatment with 3 mM neomycin for about 20 min diminished the bradykinin-induced IK(Ca) to a fifth of its normal size. 8. The four main conclusions drawn from the results are: (a) U73122 suppresses bradykinin-induced PLC activation and IK(Ca), but not IM inhibition. (b) This indicates that the transient outward current IK(Ca), but not the decrease of IM in response to bradykinin, is mediated by PLC. (c) U73122 itself inhibits IM and mobilizes Ca2+ from intracellular stores. (d) Externally applied neomycin is not an effective inhibitor of PLC-mediated signalling pathways in NG108-15 cells.

Lysophosphatidic acid depletes intracellular calcium stores different from those mediating capacitative calcium entry in C6 rat glioma cells

Lysophosphatidic acid (LPA) functions as an extracellular lipid mediator stimulating phospholipase C and affecting the structure of the cytoskeleton in several cell types. In rat glioma C6 cells, LPA mobilizes calcium from intracellular calcium stores and reverts morphological changes induced by elevated cytosolic cAMP-concentrations. Here we show that LPA-stimulation of C6 cells loaded with the calcium-sensitive fluorescent dye indo-1 results in calcium release from a subset of intracellular calcium stores that are not sensitive to the tumor promoter thapsigargin and do not overlap with calcium stores depleted during purinergic receptor stimulation with ATP. Furthermore, depletion of LPA-sensitive calcium stores does not induce capacitative calcium entry from the extracellular space into the cytosol to the same extent as ATP. These results indicate that inositol phosphate signaling induced by LPA or ATP may differ in kinetics or in spatial organisation within the cell. This may represent a possible explanation for the previous observation that only LPA, but not other calcium-mobilizing agonists, reverts cAMP-induced changes in the cytoskeletal organization in C6 cells.

Ca2+ entry modulates oscillation frequency by triggering Ca2+ release

As in many cells, the frequency of agonist-induced cytosolic Ca2+ concentration ([Ca2+]1) oscillations in exocrine avian nasal gland cells is dependent on the rate of Ca2+ entry. Experiments reveal that the initiation of each oscillatory spike is independent of the relative fullness of the stores and, furthermore, the oscillating pool is normally fully refilled by the end of each [Ca2+]1 spike. Therefore, contrary to current models, the interspike interval (which essentially sets the frequency) does not reflect the time taken to recharge the oscillating stores. Instead, the data show that it is the previously demonstrated role that Ca2+ entry plays in triggering the repetitive release of Ca2+ from the oscillating stores, rather than the recharging of those stores, that provides the basis for the observed effects of Ca2+ entry rate on oscillation frequency.

Activation by ATP of a P2U 'nucleotide' receptor in an exocrine cell

1. We employed the perforated patch whole-cell technique to investigate the effects of ATP and other related nucleotides on membrane conductances in avian exocrine salt gland cells. 2. ATP (10 microM-1 mM) evoked an increase in maxi-K+ and Cl- conductances with a reversal potential of -35 mV. At lower concentrations of ATP (< or = 100 microM) responses were generally oscillatory with a sustained response observed at higher concentrations (> or = 200 microM). 3. Both oscillatory and sustained responses were abolished by the removal of bath Ca2+. In cells preincubated in extracellular saline containing reduced Ca2+, the application of ATP resulted in a transient increase in current. 4. As increasing concentrations of ATP (and related nucleotides) evoked a graded sequence of events with little run-down we were able to establish a rank order of potency in single cells. The order of potency of ATP analogues and agonists of the various P2-receptor subtypes was UTP > ATP = 2-methylthio-ATP > ADP. Adenosine (1 microM-1 mM), AMP (1 microM-1 mM), alpha,beta-methylene-ATP (1 microM-1 mM) and beta,gamma-methylene-ATP (1 microM-1 mM) were without effect. 5. In conclusion, although unable to preclude a role for a P2Y-receptor, our results suggest that ATP binds to a P2U-receptor increasing [Ca2+]i and subsequently activating Ca(2+)-sensitive K+ and Cl- currents.

Lysophosphatidic acid induces inositol phosphate and calcium signals in exocrine cells from the avian nasal salt gland

We tested lysophosphatidic acid (LPA) known to induce inositol phosphate generation and calcium signals as well as rearrangements of the cytoskeleton and mitogenic responses in fibroblasts, for its ability to activate phospholipase C in an exocrine cell system, the salt-secreting cells from the avian nasal salt gland. LPA (> 10 nmol/l) caused the generation of inositol phosphates from membrane-bound phosphatidylinositides. The resulting calcium signals resembled those generated upon activation of muscarinic receptors, the physiological stimulus triggering salt secretion in these cells. However, close examination of the LPA-mediated calcium signals revealed that the initial calcium spike induced by high concentrations of LPA (> 10 mumol/l) may contain a component that is not dependent upon generation of inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3) and may result from calcium influx from the extracellular medium induced by LPA in a direct manner. Low concentrations of LPA (< 10 mumol/l), however, induce inositol phosphate generation, Ins(1,4,5)P3-mediated release of calcium from intracellular pools and calcium entry. These effects seem to be mediated by a specific plasma membrane receptor and a G protein transducing the signal to phospholipase C in a pertussis-toxin-insensitive manner. Signaling pathways of the muscarinic receptor and the putative LPA-receptor seem to merge at the G-protein level as indicated by the fact that carbachol and LPA trigger hydrolysis of the same pool of phosphatidylinositol (4,5)-bisphosphate (PIP2) and mobilize calcium from the same intracellular stores.

Receptor-mediated calcium signals in astroglia: multiple receptors, common stores and all-or-nothing responses

Calcium signals following activation of P2Y purinergic, alpha 1 adrenergic, and muscarinic cholinergic receptors were examined in individual astroglial cells. ATP, phenylephrine and carbachol, each increased intracellular calcium levels ([Ca2+]i) to similar amplitudes in the presence or absence of extracellular Ca2+. The dose-response relationship showed that less than an order of magnitude increase in ligand concentration led to maximal increase in [Ca2+]i from basal levels. Simultaneous application of multiple ligands did not produce additive effects on [Ca2+]i. These data suggested that different ligands released Ca2+ from common stores and that each of the ligands could cause maximal release. Application of a second ligand immediately after the first ligand produced an additional Ca2+ rise, suggesting that the Ca2+ stores were rapidly refilled and that receptor desensitization rather than Ca2+ depletion accounted for the rapid decline of the Ca2+ peak. Caged IP3 produced Ca2+ signals similar to those produced by ligands. For a given cell, both caged IP3 and ligands sometimes produced only one level of partial Ca2+ increases, suggesting the presence of a pool of high IP3-sensitive stores. Together, our results indicate that neuroligands tend to generate an all-or-nothing Ca2+ release from IP3 sensitive stores. The interactions between different receptor systems most likely occur at the level of IP3 accumulation.

Ca2+ influx drives agonist-activated [Ca2+]i oscillations in an exocrine cell

In current models describing agonist-induced oscillations in [Ca2+]i, Ca2+ entry is generally assumed to have a simple sustaining role, replenishing Ca2+ lost from the cell and recharging intracellular Ca2+ stores. In cells from the avian nasal gland, a model exocrine cell, we show that inhibition of Ca2+ entry by La3+, SK&F 96365, or by membrane depolarization, rapidly blocks [Ca2+]i oscillations but does so without detectable depletion of agonist-sensitive Ca2+ stores. As the rate of Mn2+ quenching during [Ca2+]i oscillations is constant, Ca2+ entry is not directly contributing to the [Ca2+]i changes and, instead, appears to be involved in inducing the repetitive release of Ca2+ from internal stores. Together, these data contradict current models in that (i) at the low agonist concentrations where [Ca2+]i oscillations are seen, generated levels of Ins(1,4,5)P3 are themselves inadequate to result in a regenerative [Ca2+]i signal, and (ii) Ca2+ entry is necessary to actually drive the intrinsic oscillatory mechanism.

Temporal relationships between Ca2+ store mobilization and Ca2+ entry in an exocrine cell

Consideration of the principal current models for agonist-induced activation of Ca2+ entry in electrically non-excitable cells suggests that it may be possible to distinguish between them on the basis of predicted differences in the temporal relationship(s) between intracellular Ca2+ release and the activation of Ca2+ entry. Measurements of changes in [Ca2+]i and Mn2+ quench in individual exocrine cells from the avian nasal gland indicate that, whereas Ins(1,4,5)P3-induced release of intracellular Ca2+ occurs within 3-5 s, the increase in Mn2+ quench is delayed by some 20-30 s. Mn2+ quench rate is similarly increased by thapsigargin, and is blocked by SK&F 96365, indicating that the increased Mn2+ quench observed genuinely reflects agonist-enhanced activity of the divalent cation entry pathway normally traversed by Ca2+. Additional experiments indicate that the observed delay is not due to inhibition of this pathway by elevated [Ca2+]i. Furthermore, the delay cannot be explained by the time required for Ins(1,3,4,5)P4 generation, which is essentially maximal within 10 s of agonist addition. It is concluded that the observed delay in the activation of the Ca2+ entry pathway is best explained by 'capacitative' models where increased entry requires the generation, and transmission to the plasma membrane, of an unknown messenger as a direct result of the depletion of intracellular Ca2+ stores.

Muscarinic-receptor activation stimulates oscillations in K+ and Cl- currents which are acutely dependent on extracellular Ca2+ in avian salt gland cells

By utilizing the perforated-patch variant of the whole-cell patch-clamp recording technique, in order to maintain the integrity of the normal cellular buffering systems, we demonstrate that carbachol (CCh) stimulates simultaneous oscillations in a Ca(2+)- and voltage-activated K+ current and a linear Ca(2+)-activated Cl- current in an exocrine avian salt gland cell preparation. Similar conductance changes, although sustained rather than oscillatory, are stimulated by the Ca2+ ionophore A23187. The outward K+ current can be inhibited by tetraethylammonium chloride (TEA) whereas the Cl- current is inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino) (NPPB) and N-phenylanthranilic acid (DPC). The oscillations in current stimulated by CCh are acutely dependent on extracellular Ca2+ and are not affected by the application of low doses of caffeine. In addition, the application of caffeine at all doses fails to mimic the current transients stimulated by CCh. As both caffeine and A23187 are unable to stimulate oscillations under the perforated-patch conditions we suggest that in avian salt gland cells the primary oscillatory mechanism probably involves a one-pool mechanism of Ca2+ release which is intimately related to the activation of a Ca2+ influx pathway.

Influence of dietary fat and feeding period on phosphoinositide metabolism in rat colonocytes

The objective of the present study was to examine the effect of dietary fat content on phosphoinositide (PI) metabolism, fatty acid composition in colonocytes, and colonic luminal content of bile acids (BA) and free fatty acids (FFA) in rats. Male Sprague-Dawley rats weighing approximately 166 g were fed to semipurified diet containing 3% or 21.5% beef fat and 2% corn oil. The nonfat ingredients were adjusted to correct for differences in food consumption of these diets. Animals were fed these diets ad libitum for one or four weeks. The isolated colonocytes had a viability of 88.9% in all groups. PI metabolism was examined in the absence (basal) or presence of agonists, 2 mM deoxycholic acid or 10 microM A23187. Dietary fat concentration had no effect on PI metabolism, but the length of feeding had a significant effect on basal and stimulated PI metabolism. Colonocytes of animals fed the diets for four weeks were less sensitive to stimulation of PI cycle by agonists than those of animals fed for one week. Colonocyte fatty acid composition was influenced by dietary fat and feeding period. Only the relative percentage of 20:3(n-6) was significantly lower in rats fed the high-fat diet for one week; 18:0 was lower and 18:3(n-6) was higher in colonocytes of animals fed the diets for one week than in those fed for four weeks. Several colonic fatty acids, namely, 16:0, 20:3(n-6), and 22:5(n-6), also exhibited diet-by-feeding period interaction. Intracolonic luminal contents from rats fed the high-fat diet contained elevated concentrations of BA and FFA (44% and 62%, respectively). It was concluded that despite the effects of dietary fat concentration on increased colonic BA and FFA and on altered membrane fatty acid composition, dietary fat had no effect on PI metabolism in colonocytes under the conditions in the present experiment. A difference in components between the purified diet and the commercial rat chow and/or an aging effect of the rats may alter the PI cycle of colonocytes.

Concerning stimulation by injected fluoroaluminate of the sodium efflux in barnacle muscle fibers

Single barnacle muscle fibers from Balanus nubilus were used primarily to examine the validity of two ideas: first, that the injection of KF stimulates the ouabain-insensitive Na+ efflux, and that this action is potentiated by adding AlCl3 (Al) in a low concentration to the solution of KF prior to injection. And second, that the injection of a KF-AlCl3 solution into ouabain-poisoned, K(+/-)-depolarized fibers elicits a stimulatory response resembling that obtained by injecting GTP. The results of this study are as follows: injection of 0.5 M KF into unpoisoned fibers causes a sustained rise in the resting Na+ efflux. However, injection of a 0.5 M KF, 10(-3) M AlCl3 solution leads to a reduced rather than an augmented response. Whereas injection of 0.5 M KF into ouabain-poisoned fibers elicits a marked stimulatory response, the injection of 0.5 M KF, 10(-3) M AlCl3 reduces the remaining Na+ efflux. Injection of KF-AlCl3 in equimolar concentrations, e.g., 0.25 M, elicits a response that is significantly larger than that obtained by injecting 0.25 M KF. A dose-response curve indicates that a 0.2 M solution of fluoroaluminate probably represents an optimal concentration. Injection of 0.3 M KF following peak stimulation by injecting 0.3 M AlCl3 completely reverses this response to Al. In sharp contrast, injection of a 0.3 M KF, 0.3 M AlCl3 mixture following peak stimulation by injecting 0.3 M AlCl3 is ineffective. Injection of KF into ouabain-poisoned, K+ depolarized fibers does not always cause sustained stimulation of the remaining Na+ efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of inositol(1,4,5)trisphosphate-sensitive calcium store content during continuous receptor activation and its effects on calcium entry

Changes in intracellular Ca2+ concentration ([Ca2+]i) following the activation of muscarinic receptors with carbachol were studied in cells from the exocrine avian nasal gland that had been maintained in culture for 40-48 h. In these cells, the carbachol-induced sustained increase in [Ca2+]i could be further increased by the subsequent addition of thapsigargin. This increase was due to an additional release of intracellular Ca2+ and a corresponding further enhancement of Ca2+ entry. However, thapsigargin-sensitive and Ins(1,4,5)P3-sensitive stores appeared to be coincident and the initial carbachol stimulus was sufficient to completely empty these stores. It was concluded that the subsequent effect of thapsigargin was due to a partial refilling of the Ins(1,4,5)P3-sensitive stores despite the continued presence of agonist, an effect that was not the result of any decline in levels of cellular Ins(1,4,5)P3 or changes in the generation of Ins(1,3,4,5)P4, which were sustained throughout. Possible explanations for this refilling response include compartmentalization of intracellular Ins(1,4,5)P3, or a desensitization of the Ins(1,4,5)P3 receptor/Ca(2+)-release channel. Alternatively, the data are also compatible with a recently proposed kinetic separation of Ca2+ uptake and release sites. An important implication of this particular interpretation of our findings would be an apparent dependence of Ca2+ entry specifically on the status of the Ca(2+)-uptake component of the agonist-sensitive store, rather than the Ca(2+)-release component.

Calcium-sensitivity of inositol 1,4,5-trisphosphate metabolism in exocrine cells from the avian salt gland

The generation of inositol phosphates upon muscarinic-receptor activation was studied in [3H]inositol-loaded exocrine cells from the nasal salt glands of the duck Anas platyrhynchos, and the metabolism of different inositol phosphates in vitro was studied in tissue homogenates, with particular reference to the possible interaction of changes in intracellular [Ca2+] ([Ca2+]i) with the metabolic processes. In intact cells, there was a rapid (within 15 s) generation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, followed by an accumulation of their breakdown products, Ins(1,3,4)P3 and inositol bis- and monophosphates. Ca(2+)-sensitivity of the Ins(1,4,5)P3 3-kinase was demonstrated in tissue homogenates, with the rate of phosphorylation increasing 2-fold at free Ca2+ concentrations greater than 1 microM. However, addition of calmodulin or the presence of the calmodulin inhibitor W-7 (up to 100 microM) had no effect. 3-Kinase activity increased proportionally with the initial Ins(1,4,5)P3 concentration up to 1 microM, but a 10-fold higher substrate concentration produced only a doubling in the phosphorylation rate. Ins(1,3,4,5)P4 was dephosphorylated to Ins(1,3,4)P3, which accumulated in the homogenate assays as well as in intact cells. Depending on its concentration, Ins(1,3,4)P3 was phosphorylated [in part to Ins(1,3,4,6)P4] or dephosphorylated. To investigate the Ca(2+)-sensitivity of the 3-kinase in intact cells, excess quin2 was used to buffer the receptor-mediated transient changes in [Ca2+]i in [3H]inositol-loaded cells. These experiments revealed that increasing [Ca2+]i from less than 100 to approx. 400 nM (i.e. within the physiological range) has no effect on the partitioning of Ins(1,4,5)P3 metabolism (phosphorylation versus dephosphorylation) and on the accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. This indicates that activation of the 3-kinase by physiologically relevant Ca2+ concentrations may not play a major role in the generation of Ins(1,3,4,5)P4 signals upon receptor activation in these cells. The latter are mainly achieved by the receptor-mediated increase in Ins(1,4,5)P3 in the cell and its phosphorylation by the 3-kinase in a substrate-concentration-dependent manner.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Chloride secretagogues stimulate inositol phosphate formation in shark rectal gland tubules cultured in suspension

Neuroendocrine activation of transepithelial chloride secretion by shark rectal gland cells is associated with increases in cellular cAMP, cGMP, and free calcium concentrations. We report here on the effects of several chloride secretagogues on inositol phosphate formation in cultured rectal gland tubules. Vasoactive intestinal peptide (VIP), atriopeptin (AP), and ionomycin increase the total inositol phosphate levels of cultured tubules, as measured by ion exchange chromatography. Forskolin, a potent chloride secretagogue, has no effect on inositol phosphate formation. The uptake of 3H-myo-inositol into phospholipids is very slow, preventing the detection of increased levels of inositol trisphosphate. However, significant increases in inositol monophosphate (IP1) and inositol biphosphate (IP2) were measured. The time course of VIP- and AP-stimulated IP1 and IP2 formation is similar to the effects of these agents on the short-circuit current responses of rectal gland monolayer cultures. In addition, aluminum fluoride, an artificial activator of guanine nucleotide-binding proteins, stimulates IP1 and IP2 formation. We conclude that rectal gland cells contain VIP and AP receptors coupled to the activation of phospholipase C. Coupling may be mediated by G-proteins. Receptor-stimulated increases in inositol phospholipid metabolism is one mechanism leading to increased intracellular free calcium concentrations, an important regulatory event in the activation of transepithelial chloride secretion by shark rectal gland epithelial cells.

Fluoroaluminate activation of different components of the calcium signal in an exocrine cell

In isolated cells from the avian supra-orbital nasal gland, used as a model for exocrine ion secretion, addition of NaF (2-15 mM) produced a slow Al3(+)-enhanced increase in intracellular Ca2+ concn. ([Ca2+]i), resulting in a more than 2-fold sustained elevation in [Ca2+]i. Simultaneously, cellular Ins(1,4,5)P3 contents became markedly elevated, suggesting an AlF4- activation of a phospholipase C-specific G-protein. Subsequent addition of the muscarinic agonist carbachol failed to produce any further sustained increase in [Ca2+]i, indicating that the AlF4(-)-induced increase in [Ca2+]i involves a Ca2(+)-entry pathway identical with that activated by carbachol. In low-Ca2+ media (extracellular [Ca2+] = 0.04 mM) no such increase in [Ca2+]i, either sustained or transient, is seen, although cellular Ins(1,4,5)P3 levels were markedly elevated. Despite the failure to observe any change in [Ca2+]i in the low-Ca2+ medium, estimation of the size of the agonist-sensitive Ca2+ stores (determined as the magnitude of the transient change in [Ca2+]i induced by carbachol) revealed that these are progressively emptied by the action of AlF4-. However, the onset of this emptying showed an initial lag period of at least 2 min (with 5 mM-NaF plus 10 microM-AlCl3). In marked contrast, determinations of the magnitude of the Ca2(+)-entry pathway under identical conditions showed that this was significantly activated after as little as 1 min of AlF4- treatment. This suggests that, under these conditions, activation of Ca2+ entry in these cells preceded the release of Ca2+ from agonist-sensitive stores, contradicting current models in which the receptor-enhanced entry of extracellular Ca2+ is entirely dependent on, and subsequent to, the prior release of Ca2+ from the intracellular stores.

Receptor-activated calcium entry in exocrine cells does not occur via agonist-sensitive intracellular pools

Currently, most models describing receptor-activated Ca2+ entry in exocrine cells invoke a pathway for the entry of extracellular Ca2+ directly linking the agonist-sensitive intracellular Ca2+ pools with the plasma membrane. In the avian nasal gland, a model exocrine ion-secreting tissue, we have found that Ca2+ entry during refilling of the intracellular pools following termination of receptor activation (by atropine) occurs via the cytoplasm and not directly into the empty pools. Under appropriate conditions this can be demonstrated as a transient increase in [Ca2+]i (intracellular Ca2+ concn.) seen on restoration of normal extracellular Ca2+ concentrations after atropine to stimulated cells whose intracellular stores have been prevented from refilling by incubation in a low-extracellular-Ca2+ medium. The magnitude of these [Ca2+]i transients decays with time, but with a time course markedly slower than for the corresponding decrease in intracellular Ins(1,4,5)P3. Further experiments have revealed that Ca2+ entry into the cytoplasm during the initial stimulation phase is also direct and not via the intracellular pools. Thus the initial rates of increase in [Ca2+]i during stimulation are always faster in conditions where both Ca2+ entry and Ca2+ release occur (i.e. they are additive). These differences could not be explained by any effects of extracellular Ca2+ on the initial increases in intracellular Ins(1,4,5)P3 after addition of carbachol. These data are therefore inconsistent with the current models in which the rate of Ca2+ entry through the agonist-sensitive pools cannot exceed the rate of Ca2+ release. It appears therefore that Ca2+ entry and Ca2+ release must occur via separate pathways operating in parallel, and not in series as previously predicted.