Spontaneous and secretagogue-induced changes in cytosolic free Ca concentration measured by microfluorimetry with fura-2 on single bovine adrenal chromaffin cells

Modulation of intracellular Ca2+ levels in chromaffin cells by nanoelectropulses

Exposing chromaffin cells to a single 5 ns, 5 MV/m pulse causes Ca(2+) influx and a rapid, transient rise in intracellular calcium concentration ([Ca(2+)](i)). A comparison of responses at room temperature versus 37°C revealed no effect of temperature on the magnitude of the increase in [Ca(2+)](i). The Ca(2+) transient, however, was shortened in duration almost twofold at 37°C, indicating that the rate of recovery was temperature-sensitive. Temperature also affected the interval required for a second pulse to elicit another maximal rise in [Ca(2+)](i), which was shorter at the higher temperature. In addition, a second pulse applied 5s after the first pulse was sufficient to cause cells at room temperature to become refractory to subsequent stimulation. At 37°C, cells became refractory after 5 pulses regardless of whether pulse delivery was at low (1 and 10 Hz) or high (1 kHz) rates. When refractory, cells showed no signs of swelling or uptake of the impermeant dye YO-PRO-1. These results demonstrate that temperature plays a role in determining how chromaffin cells respond to and become refractory to nanoelectropulses. They also indicate that despite the ultra-short duration of the pulses, pronounced effects on cell excitability result from the application of only very few pulses.

Generation of functionally competent single bovine adrenal chromaffin cells from cell aggregates using the neutral protease dispase

A simple and efficient procedure has been developed to enzymatically dissociate aggregates of bovine adrenal chromaffin cells in suspension culture into viable, responsive single cells. For dissociation, the neutral protease dispase is added directly to the culture medium for a minimum of 3 h, followed by incubation of the cells in Hank's calcium-magnesium-free balanced salt solution at 37 degrees C with intermittent trituration to facilitate dispersion. This procedure generates a population of phase-bright single cells that are round in morphology, take up the dye neutral red, exclude the dye trypan blue and readily attach to tissue culture dishes coated with collagen, fibronectin or polylysine, thereby permitting applications that require plated-down conditions. When transferred to culture medium, the cells begin to reaggregate. By altering the length of time the cells are incubated in culture medium prior to attachment, the degree of reaggregation can be controlled to obtain plate-down profiles that consist of both isolated cells and cells in aggregates of varying sizes. Returning dissociated cells to suspension culture results in the reformation of large cell aggregates. Several measures of chromaffin cell function were indistinguishable for dissociated cells placed either in monolayer culture or suspension culture versus non-dissociated cells, implying that the dissociation procedure does not alter cellular responses or cause cellular damage.

Intracellular calcium activity in isolated bovine adrenal chromaffin cells in the presence and absence of 60 Hz magnetic fields

This study examined whether 60 Hz magnetic field (MF) exposure alters intracellular calcium levels ([Ca(2+)](i)) in isolated bovine adrenal chromaffin cells, a classic model of neural responses. [Ca(2+)](i) was monitored by fluorescence video imaging of cells loaded with the calcium indicator fluo-4 during exposures to magnetic flux densities of 0.01, 0.1, 1.0, 1.4, or 2.0 mT. MFs generated by Helmholtz coils constructed from bifilar wire allowed both 60 Hz field and sham exposures. Following a 5 min monitoring period to establish baseline patterns, cells were subjected for 10 min to a 60 Hz MF, sham field or no field. Reference calcium responses and assessment of cell excitability were obtained by the sequential addition of the nicotinic cholinergic receptor agonist dimethylphenylpiperazinium (DMPP) and a depolarizing concentration of KCl. Throughout an 8 day culture period, cells exhibited spontaneous fluctuations in [Ca(2+)](i). Comparisons of the number of cells exhibiting transients, the number and types of calcium transients, as well as the time during monitoring when transients occurred showed no significant differences between MF exposed cells and either sham exposed or nonexposed cells. With respect to the percentage of cells responding to DMPP, differences between 1 and 2 mT exposed cells and both nonexposed and sham exposed cells reached statistical significance during the first day in culture. No statistically significant differences were observed for responses to KCl. In summary, our data indicate that [Ca(2+)](i) in chromaffin cells is unaffected by the specific 60 Hz MF intensities used in this study. On the other hand, plasma membrane nicotinic receptors may be affected in a manner that is important for ligand-receptor interactions.

Limited intercellular spread of spontaneous Ca2+ signals via gap junctions between mouse chromaffin cells in situ

Using a confocal laser-scanning microscope, we measured the changes in the cytosolic Ca(2+) concentration ([Ca]i) of chromaffin cells on adrenal slice preparations of mouse. The spontaneous fluctuations of [Ca]i were often observed in situ, as reported in isolated rat and bovine cells. Intriguingly, the spontaneous [Ca]i changes in one cell were often transmitted to one or two adjacent cells, and the synchronized [Ca]i changes were often observed in two adjacent cells, both of which failed to respond to ATP. The synchronized [Ca]i changes between two cells were much less frequently observed in rat than in mouse adrenals. The spontaneous [Ca]i changes were also synchronized between closely touching mouse chromaffin cells in culture. These results suggest that the spread of Ca(2+) signaling is mediated by low-conductance intercellular coupling.

Excitatory effect of Cd2+ on cat adrenal chromaffin cells

To understand the mechanism(s) underlying the Cd2+- and Co2+-induced increases in the cytosolic free Ca2+ concentration ([Ca]i) in cat adrenal chromaffin cells, we used nystatin-perforated patch recording method and fura-2 microfluorometry. Under the current-clamp conditions, the external application of 5x10(-7) M Cd2+ slowly depolarized the cells resulting in the bursting of action potentials. Under the voltage-clamp conditions, Cd2+ evoked a slow inward current accompanied by a decrease of K+ conductance at a holding potential of -40 mV, and Co2+ mimicked Cd2+ action. In some cells (16%), Cd2+ evoked an additional rapid transient outward current associated with an increased K+ conductance and a successive slow inward current. The Cd2+-induced inward current was activated in a concentration-dependent manner with a half-maximum concentration of 9.3x10(-8) M. The Cd2+- and Co2+-induced [Ca]i increases measured with fura-2 microfluorometry were maximal at 10(-6) and 10(-5) M, respectively, and the higher concentrations of both cations caused the smaller responses. Additional transient increase in [Ca]i was often evoked upon the removal of relatively higher concentrations of these metals. It was concluded that the Cd2+-induced membrane depolarization due to the decrease in K+ conductances evoked the bursting firings resulting in the increase in [Ca]i, and consequently might stimulate the catecholamine secretion.

Cd2+ and Co2+ at micromolar concentrations mobilize intracellular Ca2+ via the generation of inositol 1,4,5-triphosphate in bovine chromaffin cells

To understand the mechanisms underlying the Cd2+- and Co2+-induced intracellular Ca2+ mobilization, we measured the levels of inositol phosphates using bovine chromaffin cells. Studies using HPLC indicated that Cd2+, Co2+ and methacholine significantly increased the generation of 1,4,5-IP3. The results suggest that Cd2+ and Co2+ mobilize Ca2+ from IP3-sensitive Ca2+ stores, possibly through the presumptive Cd2+ receptor.

Ca2+ signalling in rat chromaffin cells: interplay between Ca2+ release from intracellular stores and membrane potential

Cytosolic Ca2+ oscillations are physiologically important in a range of excitable and non-excitable cells. The combined techniques of whole-cell patch clamp and photometric measurement of cytosolic Ca2+ has enabled us to identify the components of Ca2+ spiking in rat chromaffin cells. We show that Ca2+ oscillations continue at a fixed membrane potential and that infusion of the InsP3 receptor antagonist, heparin, substantially blocked the cytosolic Ca2+ spikes. However, even in the presence of heparin we observed spikes of membrane potential depolarization due to the repetitive activation of a transient inward cation current. We conclude that Ca2+ oscillations are dependent on Ca2+ release from heparin sensitive Ca2+ stores and possibly on Ca2+ entry associated with the repetitive activation of a transient cation current. The depolarizing action of the cation current would, in turn, recruit voltage-sensitive Ca2+ channels and further Ca2+ entry would augment the cytosolic Ca2+ spikes. Our results demonstrate that Ca2+ oscillations in rat chromaffin cells are due to a complex interplay of Ca2+ entry and Ca2+ release.

Correlation of real-time catecholamine release and cytosolic Ca2+ at single bovine chromaffin cells

Previous investigations of the role of Ca2+ in stimulus-secretion coupling have been undertaken in populations of adrenal chromaffin cells. In the present study, the simultaneous detection of intracellular Ca2+, with the fluorescent probe fura-2, and catecholamine release, using a carbon-fiber microelectrode, are examined at single chromaffin cells in culture. Results from classic depolarizing stimuli, high potassium (30-140 mM) and 1,1-dimethyl-4-phenylpiperazinium (3-50 microM), show a dependence of peak cytosolic Ca2+ concentration and catecholamine release on secretagogue concentration. Catecholamine release induced by transient high K+ stimulation increases logarithmically with K+ concentration. Continuous exposure to veratridine (50 microM) induces oscillations in intracellular Ca2+ and at higher concentrations (100 microM) concomitant fluctuation of cytosolic Ca2+ and catecholamine secretion. Mobilization of both caffeine- and inositol trisphosphate-sensitive intracellular Ca2+ stores is found to elicit secretion with or without extracellular Ca2+. Caffeine-sensitive intracellular Ca2+ stores can be depleted, refilled, and cause exocytosis in medium without Ca2+. Single cell measurement of exocytosis and the increase in cytosolic Ca2+ induced by bradykinin-activated intracellular stores reveal cell to cell variability in exocytotic responses which is masked in populations of cells. Taken together, these results show that exocytosis of catecholamines can be induced by an increase in cytosolic Ca2+ either as a result of transmembrane entry or by release of internal stores.

Cd2+ and Co2+ at micromolar concentrations stimulate catecholamine secretion by increasing the cytosolic free Ca2+ concentration in cat adrenal chromaffin cells

Cd2+ and Co2+ at micromolar concentrations increased the cytosolic free Ca2+ concentration, which was measured by fura-2 microfluorometry, in cat adrenal chromaffin cells. Simultaneously, these divalent cations stimulated catecholamine secretion from the perfused adrenal. The present findings suggest that these cations increase the Ca2+ influx by depolarizing the cell membrane and consequently stimulate catecholamine secretion.

Single-cell fura-2 microfluorometry reveals different purinoceptor subtypes coupled to Ca2+ influx and intracellular Ca2+ release in bovine adrenal chromaffin and endothelial cells

ATP and adenosine(5')tetraphospho(5')adenosine (Ap4A), released from adrenal chromaffin cells, are potent stimulators of endothelial cell function. Using single-cell fura-2 fluorescence recording techniques to measure free cytosolic Ca2+ concentration ([Ca2+]i), we have investigated the role of purinoceptor subtypes in the activation of cocultured chromaffin and endothelial cells. ATP evoked concentration-dependent [Ca2+]i rises (EC50 = 3.8 microM) in a subpopulation of chromaffin cells. Both ATP-sensitive and -insensitive cells were potently activated by nicotine, bradykinin and muscarine. Reducing extracellular free Ca2+ concentration to around 100 nM suppressed the [Ca2+]i transient evoked by ATP but not the [Ca2+]i response to bradykinin. ATP-sensitive chromaffin cells were also potently stimulated by 2-methylthioadenosine triphosphate (2MeSATP; EC50 = 12.5 microM) and UTP, but did not respond to either adenosine 5'-[beta-thio]diphosphate (ADP[beta S]), a P2Y receptor agonist, adenosine 5'-[alpha,beta-methylene]triphosphate (pp-[CH2]pA), a P2X agonist or AMP. Adrenal endothelial cells displayed concentration-dependent [Ca2+]i responses when stimulated with ATP (EC50 = 0.86 microM), UTP (EC50 = 1.6 microM) and 2MeSATP (EC50 = 0.38 microM). 2MeSATP behaved as a partial agonist. Ap4A and ADP[beta S] also raised the [Ca2+]i in endothelial cells, whereas AMP and pp[CH2]pA were ineffective. Lowering extracellular free Ca2+ to around 100 nM did not affect the peak ATP-evoked [Ca2+]i rise in these cells. It is concluded that different purinoceptor subtypes are heterogeneously distributed among the major cell types of the adrenal medulla. An intracellular Ca(2+)-releasing P2U-type purinoceptor is specifically localized to adrenal endothelial cells, while a subpopulation of chromaffin cells expresses a non-P2X, non-P2Y subtype exclusively coupled to Ca2+ influx.

Caffeine- and muscarinic receptor agonist-sensitive Ca2+ stores in chick ciliary ganglion cells

To investigate the presence and the role of intracellular Ca2+ stores in chick ciliary ganglion cells, the concentration of cytosolic free Ca2+ ([Ca]in) was measured in acutely isolated neurons, using fura-2 microfluorometry. Caffeine caused a substantial increase in [Ca]in following or during high K+ depolarization; this response was inhibited by treatment of the cells with thapsigargin or with caffeine plus ryanodine. The peak value and the rate of the depolarization-induced [Ca]in increase were not much altered by either of these treatments, which deplete caffeine-sensitive Ca2+ stores. The muscarinic receptor agonists muscarine, oxotremorine M, and methacholine, caused substantial increases in [Ca]in, in a manner that was partially dependent on Ca2+. These agonists also caused a rise in [Ca]in during K+ depolarization, which rise was inhibited by treatment with thapsigargin or with caffeine plus ryanodine. The response to oxotremorine M during depolarization was strongly inhibited by 10 nM 4-DAMP, but was not inhibited by 1 microM pirenzepine or by 1 microM AF-DX 116. These results indicate that chick ciliary ganglion cells possess Ca2+ stores that are activated by both caffeine and a second messenger generated by the activation of the M3 muscarinic receptor subtype.

Difference in the effectiveness of Ca2+ to evoke catecholamine secretion between adrenaline- and noradrenaline-containing cells of bovine adrenal medulla

Differential adrenaline (Ad) and noradrenaline (NA) secretions evoked by secretagogues were investigated using digitonin-permeabilized adrenal chromaffin cells, cultured adrenal chromaffin cells, and perfused adrenal glands of the ox. In digitonin-permeabilized cells, Ca2+ (0.8-160 microM) caused a concentration-dependent increase in catecholamine secretion, which was characterized by a predominance of NA over Ad secretion. Acetylcholine (10-1,000 microM), high K+ (14-56 mM), and bradykinin (0.1-1,000 nM) all were confirmed to induce the release of more NA than Ad at all concentrations used. There was no apparent difference in the ratios of NA/Ad between Ca(2+)-induced catecholamine secretion from digitonin-permeabilized cells and those induced by secretagogues from cultured cells. Qualitatively the same result was obtained in the secretory responses to acetylcholine and high K+ in perfused adrenal glands. These results indicate that the effectiveness of Ca2+ for catecholamine secretion is higher in the secretory apparatus of NA cells than in that of Ad cells of the bovine adrenal medulla. This may be one of the reasons why the secretagogues cause a predominance of NA secretion over Ad secretion in the bovine adrenal medulla.

Calcium permeability of non-N-methyl-D-aspartate receptor channels in immature cerebellar Purkinje cells: studies using fura-2 microfluorometry

Using fura-2 microfluorometry, I investigated the mechanism by which non-N-methyl-D-aspartate (NMDA) receptor agonists increase the cytosolic free calcium concentration ([Ca]in) in single cerebellar Purkinje cells isolated from 3-10-day-old rats. Kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate dose-dependently increased the cytosolic free Na+ concentration, which was measured using sodium-binding benzofuran isophthalate microfluorometry, confirming the Na+ influx through ion channels linked to non-NMDA receptors. The [Ca2+] increases induced by relatively lower concentrations of agonists were entirely dependent on external Ca2+ and were reduced by removal of external Na+ or by addition of a Ca2+ channel blocker, D600. The results indicate that the non-NMDA agonist-induced [Ca]in increase was due mainly to Ca2+ influx through voltage-dependent Ca2+ channels, which were activated by a massive Na+ influx. On the other hand, higher concentrations of agonists dose-dependently increased [Ca]in under conditions in which activation of voltage-dependent Ca2+ channels were blocked by a combination of Na+ removal with D600. These [Ca]in increases were Ca2+ dependent and little affected by adding a competitive NMDA antagonist. Non-NMDA agonists also stimulated influxes of Mn2+ and Co2+, both of which can be monitored by quenching fura-2 fluorescence under the same conditions. These results suggest that ion channels linked to non-NMDA receptors on immature Purkinje cells are permeable to Ca2+, Mn2+, and Co2+.

Possible regulation of caffeine-induced intracellular Ca2+ mobilization by intracellular free Na+

To gain some understanding of the regulatory mechanism involved in caffeine-induced Ca2+ release in adrenal chromaffin cells, we took advantage of the paradoxical observation that removal of divalent cations potentiated the secretory response to caffeine. We measured the concentration of cytosolic free Ca2+ ([Ca]in) in isolated cat chromaffin cells, by fura-2 microfluorometry, to see whether there was any correlation between the secretory response and the rise in [Ca]in. The caffeine-induced [Ca]in rise and catecholamine secretion were increased by treatment of cells with a divalent cation-deficient solution. These potentiated responses were strongly inhibited either by pretreatment with ryanodine, by the reduction of the external Na+ concentration, or by the addition of Ca2+ channel blockers. Removal of divalent cations caused a large rise in the cytosolic free Na+ concentration ([Na]in), which was measured using SBFI microfluorometry. This rise in [Na]in was reduced either by adding Ca2+ channel blockers or by reducing the external Na+ concentration. These results show a good correlation between caffeine-induced Ca2+ release and [Na]in at the time of stimulation, suggesting that caffeine-induced Ca2+ release is regulated by [Na]in.

Possible role of surface potential in the gating mechanism of Ca2+ channels in cat adrenal chromaffin cells: studies with fura-2 microfluorometry

The cytosolic free Ca2+ concentration ([Ca]in) in isolated cat chromaffin cells was measured by fura-2 microfluorometry. During 30 mM KCl depolarization or sucrose substitution for NaCl, a reduction in external Ca2+ concentration under optimal conditions paradoxically caused a rise in [Ca]in and, in separate experiments, in catecholamine secretion. The results support a previously suggested role of surface potentials in the gating mechanism of Ca2+ channels.

A muscarinic receptor agonist mobilizes Ca2+ from caffeine and inositol-1,4,5-trisphosphate-sensitive Ca2+ stores in cat adrenal chromaffin cells

To gain some understanding of the characteristics of intracellular Ca2+ stores of cat adrenal chromaffin cells, we investigated the effects of ryanodine, a blocker of Ca(2+)-induced Ca2+ release channels in muscle, on both cytosolic Ca2+ concentration and catecholamine secretion induced by caffeine or methacholine. The results suggest that Ca2+ stores consist of at least two compartments, one which is sensitive to both caffeine and inositol-1,4,5-trisphosphate (IP3), and the other which is sensitive to IP3 alone.

Ryanodine inhibits caffeine-evoked Ca2+ mobilization and catecholamine secretion from cultured bovine adrenal chromaffin cells

The effects of ryanodine, a selective inhibitor of the Ca(2+)-induced Ca2+ release mechanism, on caffeine-evoked changes in cytosolic Ca2+ concentration ([Ca2+]i) and catecholamine secretion were investigated using cultured bovine adrenal chromaffin cells. Caffeine (5-40 mM) caused a concentration-dependent transient rise in [Ca2+]i and catecholamine secretion in Ca2+/Mg(2+)-free medium containing 0.2 mM EGTA. Ryanodine (5 x 10(-5) M) alone had no effect on either [Ca2+]i or catecholamine secretion. Although the application of ryanodine plus caffeine caused the same increase in both [Ca2+]i and catecholamine secretion as those induced by caffeine alone, ryanodine (4 x 10(-7) - 5 x 10(-5) M) irreversibly prevented the increase in both [Ca2+]i and catecholamine secretion resulting from subsequent caffeine application over a range of concentrations. The secretory response to caffeine was markedly enhanced by replacement of Na+ with sucrose in Ca2+/Mg(2+)-free medium, and this enhanced response was also blocked by ryanodine. Caffeine was found to decrease the susceptibility of the secretory apparatus to Ca2+ in digitonin-permeabilized cells. These results indicate that caffeine mobilizes Ca2+ from intracellular stores, the function of which is irreversibly blocked by ryanodine, resulting in the increase in catecholamine secretion in the bovine adrenal chromaffin cell.

Internal Ca2+ mobilization by muscarinic stimulation increases secretion from adrenal chromaffin cells only in the presence of Ca2+ influx

The cytosolic free Ca2+ concentration ([Ca2+]in) in single cat and bovine adrenal chromaffin cells was measured to determine whether or not there was any correlation between the [Ca2+]in and the catecholamine (CA) secretion caused by muscarinic receptor stimulation. In cat chromaffin cells, methacholine (MCh), a muscarinic agonist, raised [Ca2+]in by activating both Ca2+ influx and intracellular Ca2+ mobilization with an accompanying CA secretion. In bovine cells, MCh elevated [Ca2+]in by mobilizing intracellular Ca2+ but did not cause CA secretion. The MCh-induced rise in [Ca2+]in in cat cells was much higher than that in bovine cells, but when Ca2+ influx was blocked, the rise was reduced, with a concomitant loss of secretion, to a level comparable to that in bovine cells. Intracellular Ca2+ mobilization due to muscarinic stimulation substantially increased secretion from depolarized bovine and cat cells, where a [Ca2+]in elevated above basal values was maintained by a continuous Ca2+ influx. These results show that Ca2+ released from internal stores is not effective in triggering secretion unless Ca2+ continues to enter across the plasma membrane, a conclusion suggesting that secretion depends on [Ca2+]in in a particular region of the cell.