Single cell assay of exocytosis from adrenal chromaffin cells using "perforated patch recording"

Explicit Theoretical Analysis of How the Rate of Exocytosis Depends on Local Control by Ca2+ Channels

Hormones and neurotransmitters are released from cells by calcium-regulated exocytosis, and local coupling between Ca²⁺ channels (CaVs) and secretory granules is a key factor determining the exocytosis rate. Here, we devise a methodology based on Markov chain models that allows us to obtain analytic results for the expected rate. First, we analyze the property of the secretory complex obtained by coupling a single granule with one CaV. Then, we extend our results to a more general case where the granule is coupled with n CaVs. We investigate how the exocytosis rate is affected by varying the location of granules and CaVs. Moreover, we assume that the single granule can form complexes with inactivating or non-inactivating CaVs. We find that increasing the number of CaVs coupled with the granule determines a much higher rise of the exocytosis rate that, in case of inactivating CaVs, is more pronounced when the granule is close to CaVs, while, surprisingly, in case of non-inactivating CaVs, the highest relative increase in rate is obtained when the granule is far from the CaVs. Finally, we exploit the devised model to investigate the relation between exocytosis and calcium influx. We find that the quantities are typically linearly related, as observed experimentally. For the case of inactivating CaVs, our simulations show a change of the linear relation due to near-complete inactivation of CaVs.

Calcium entry through slow-inactivating L-type calcium channels preferentially triggers endocytosis rather than exocytosis in bovine chromaffin cells

Calcium (Ca(2+))-dependent endocytosis has been linked to preferential Ca(2+) entry through the L-type (α(1D), Ca(V)1.3) of voltage-dependent Ca(2+) channels (VDCCs). Considering that the Ca(2+)-dependent exocytotic release of neurotransmitters is mostly triggered by Ca(2+) entry through N-(α(1B), Ca(V)2.2) or PQ-VDCCs (α(1A), Ca(V)2.1) and that exocytosis and endocytosis are coupled, the supposition that the different channel subtypes are specialized to control different cell functions is attractive. Here we have explored this hypothesis in primary cultures of bovine adrenal chromaffin cells where PQ channels account for 50% of Ca(2+) current (I(Ca)), 30% for N channels, and 20% for L channels. We used patch-clamp and fluorescence techniques to measure the exo-endocytotic responses triggered by long depolarizing stimuli, in 1, 2, or 10 mM concentrations of extracellular Ca(2+) ([Ca(2+)](e)). Exo-endocytotic responses were little affected by ω-conotoxin GVIA (N channel blocker), whereas ω-agatoxin IVA (PQ channel blocker) caused 80% blockade of exocytosis as well as endocytosis. In contrast, nifedipine (L channel blocker) only caused 20% inhibition of exocytosis but as much as 90% inhibition of endocytosis. Conversely, FPL67146 (an activator of L VDCCs) notably augmented endocytosis. Photoreleased caged Ca(2+) caused substantially smaller endocytotic responses compared with those produced by K(+) depolarization. Using fluorescence antibodies, no colocalization between L, N, or PQ channels with clathrin was found; a 20-30% colocalization was found between dynamin and all three channel antibodies. This is incompatible with the view that L channels are coupled to the endocytotic machine. Data rather support a mechanism implying the different inactivation rates of L (slow-inactivating) and N/PQ channels (fast-inactivating). Thus a slow but more sustained Ca(2+) entry through L channels could be a requirement to trigger endocytosis efficiently, at least in bovine chromaffin cells.

Functional interactions between nicotinic and P2X receptors in celiac ganglia neurons

Here we characterized the cross-inhibitory interactions between nicotinic and P2X receptors of celiac neurons from the guinea pig by recording whole-cell currents induced by 1mM ACh (I(ACh)), 1mM ATP (I(ATP)) and by the simultaneous application of both agonists (I(ACh)(+ATP)). I(ACh) and I(ATP) were inhibited by hexamethonium (nicotinic channel blocker) and PPADS (P2X receptor antagonist), respectively. The amplitude of I(ACh)(+ATP) was equal to the current induced by the most effective agonist, indicating a current occlusion. Various observations indicate that I(ACh)(+ATP) is carried out through both nicotinic (nACh) and P2X channels: i) I(ACh)(+ATP) desensitisation kinetics were in between that of I(ACh) and I(ATP); ii) application of ATP+ACh, decreased I(ACh) and I(ATP), whereas no cross-desensitisation was observed between nACh and P2X receptors; iii) ATP did not affect I(ACh) in the presence of PPADS or after P2X receptor desensitisation; and iv) ACh did not affect I(ATP) when nACh channels were blocked with hexamethonium or after nACh receptor desensitisation. Current occlusion is not mediated by activation of metabotropic receptors as it is: i) voltage dependent (was not observed at + 5 mV); ii) present at low temperature (10 degrees C) and after inhibition of protein kinase activity (with staurosporine); and iii) absent at 30 microM ATP and 30 microM ACh (concentrations that should activate metabotropic receptors). In conclusion, current occlusion described here is similar to the previously reported myenteric neurons. This occlusion is likely the result of allosteric interactions between these receptors.

Dielectrophoretic field-flow fractionation system for detection of aquatic toxicants

Dielectrophoretic field-flow fractionation (dFFF) was applied as a contact-free way to sense changes in the plasma membrane capacitances and conductivities of cultured human HL-60 cells in response to toxicant exposure. A micropatterned electrode imposed electric forces on cells in suspension in a parabolic flow profile as they moved through a thin chamber. Relative changes in the dFFF peak elution time, reflecting changes in cell membrane area and ion permeability, were measured as indices of response during the first 150 min of exposure to eight toxicants having different single or mixed modes of action (acrylonitrile, actinomycin D, carbon tetrachloride, endosulfan, N-nitroso- N-methylurea (NMU), paraquat dichloride, puromycin, and styrene oxide). The dFFF method was compared with the cell viability assay for all toxicants and with the mitochondrial potentiometric dye assay or DNA alkaline comet assay according to the mode of action of the specific agents. Except for low doses of nucleic acid-targeting agents (actinomycin D and NMU), the dFFF method detected all toxicants more sensitively than other assays, in some cases up to 10 (5) times more sensitively than the viability approach. The results suggest the dFFF method merits additional study for possible applicability in toxicology.

Cross-inhibitory interactions between GABAA and P2X channels in myenteric neurones

Inhibitory interactions between GABA(A)[induced by gamma-aminobutyric acid (GABA)] and P2X [activated by adenosine 5'-triphosphate (ATP)] receptors of myenteric neurones from the guinea pig small intestine were characterized using whole-cell recordings. Currents induced by GABA (I(GABA)) or ATP (I(ATP)) were inhibited by picrotoxin or pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, respectively. Currents induced by GABA + ATP (I(GABA+ATP)) were only as large as the current induced by the most effective transmitter, revealing current occlusion. This occlusion requires maximal activation of at least one of these receptors. Sequential applications of neurotransmitters, and kinetic and pharmacological properties of I(GABA+ATP) indicate that they are carried through both GABA(A) and P2X channels. ATP did not affect I(GABA) in neurones: (i) in which P2X channels were not present; (ii) after inhibiting P2X channels with Ca2+ (iii) in the presence of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, a P2X receptor antagonist; (iv) after P2X receptor desensitization or (v) at I(ATP) reversal potential. Similarly, GABA did not affect P2X-mediated currents in neurones: (i) in which GABA(A) channels were not present; (ii) in the presence of picrotoxin, a GABA(A) channel blocker; (iii) after GABA(A) receptor desensitization or (iv) at the I(GABA) reversal potential. Current occlusion occurred as fast as current activation and it was still present in the absence of Ca2+, at 11 degrees C, after adding to the pipette solution a cocktail of protein kinase inhibitors (staurosporine + genistein + K-252a), after substituting the GTP in the pipette with GDP-beta-S and after treating the cells with N-ethylmaleimide. Taken together, all of these results are consistent with a model of cross-inhibition between GABA(A) and P2X.

A highly Ca2+-sensitive pool of granules is regulated by glucose and protein kinases in insulin-secreting INS-1 cells

We have used membrane capacitance measurements and carbon-fiber amperometry to assay exocytosis triggered by photorelease of caged Ca²⁺ to directly measure the Ca²⁺ sensitivity of exocytosis from the INS-1 insulin-secreting cell line. We find heterogeneity of the Ca²⁺ sensitivity of release in that a small proportion of granules makes up a highly Ca²⁺-sensitive pool (HCSP), whereas the bulk of granules have a lower sensitivity to Ca²⁺. A substantial HCSP remains after brief membrane depolarization, suggesting that the majority of granules with high sensitivity to Ca²⁺ are not located close to Ca²⁺ channels. The HCSP is enhanced in size by glucose, cAMP, and a phorbol ester, whereas the Ca²⁺-sensitive rate constant of exocytosis from the HCSP is unaffected by cAMP and phorbol ester. The effects of cAMP and phorbol ester on the HCSP are mediated by PKA and PKC, respectively, because they can be blocked with specific protein kinase inhibitors. The size of the HCSP can be enhanced by glucose even in the presence of high concentrations of phorbol ester or cAMP, suggesting that glucose can increase granule pool sizes independently of activation of PKA or PKC. The effects of PKA and PKC on the size of the HCSP are not additive, suggesting they converge on a common mechanism. Carbon-fiber amperometry was used to assay quantal exocytosis of serotonin (5-HT) from insulin-containing granules following preincubation of INS-1 cells with 5-HT and a precursor. The amount or kinetics of release of 5-HT from each granule is not significantly different between granules with higher or lower sensitivity to Ca²⁺, suggesting that granules in these two pools do not differ in morphology or fusion kinetics. We conclude that glucose and second messengers can modulate insulin release triggered by a high-affinity Ca²⁺ sensor that is poised to respond to modest, global elevations of [Ca²⁺]_i.

Physiological stimuli evoke two forms of endocytosis in bovine chromaffin cells

1. Exocytosis and endocytosis were measured following single, or trains of, simulated action potentials (sAP) in bovine adrenal chromaffin cells. Catecholamine secretion was measured by oxidative amperometry and cell membrane turnover was measured by voltage clamp cell capacitance measurements. 2. The sAPs evoked inward Na(+) and Ca(2+) currents that were statistically identical to those evoked by native action potential waveforms. On average, a single secretory granule underwent fusion following sAP stimulation. An equivalent amount of membrane was then quickly internalised (tau = 560 ms). 3. Stimulation with sAP trains revealed a biphasic relationship between cell firing rate and endocytic activity. At basal stimulus frequencies (single to 0.5 Hz) cells exhibited a robust membrane internalisation that then diminished as firing increased to intermediate levels (1.9 and 6 Hz). However at the higher stimulation rates (10 and 16 Hz) endocytic activity rebounded and was again able to effectively maintain cell surface near pre-stimulus levels. 4. Treatment with cyclosporin A and FK506, inhibitors of the phosphatase calcineurin, left endocytosis characteristics unaltered at the lower basal stimulus levels, but blocked the resurgence in endocytosis seen in control cells at higher sAP frequencies. 5. Based on these findings we propose that, under physiological electrical stimulation, chromaffin cells internalise membrane via two distinct pathways that are separable. One is prevalent at basal stimulus frequencies, is lessened with increased firing, and is insensitive to cyclosporin A and FK506. A second endocytic form is activated by increased firing frequencies, and is selectively blocked by cyclosporin A and FK506.

Imaging constitutive exocytosis with total internal reflection fluorescence microscopy

Total internal reflection fluorescence microscopy has been applied to image the final stage of constitutive exocytosis, which is the fusion of single post-Golgi carriers with the plasma membrane. The use of a membrane protein tagged with green fluorescent protein allowed the kinetics of fusion to be followed with a time resolution of 30 frames/s. Quantitative analysis allowed carriers undergoing fusion to be easily distinguished from carriers moving perpendicularly to the plasma membrane. The flattening of the carriers into the plasma membrane is seen as a simultaneous rise in the total, peak, and width of the fluorescence intensity. The duration of this flattening process depends on the size of the carriers, distinguishing small spherical from large tubular carriers. The spread of the membrane protein into the plasma membrane upon fusion is diffusive. Mapping many fusion sites of a single cell reveals that there are no preferred sites for constitutive exocytosis in this system.

Effect of Ca2+ and cAMP on capacitance-measured hormone secretion in human GH-secreting adenoma cells

Membrane capacitance (Cm) was measured as an index of exocytosis in human growth hormone-secreting adenoma cells using the perforated whole cell, patch-clamp technique; the effects of membrane depolarization, growth hormone-releasing hormone, and 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) were examined. Cm was increased by membrane depolarization to potentials beyond the threshold necessary to open voltage-gated Ca2+ channels. These voltage-dependent changes in Cm varied as a function of both depolarization amplitude and duration and were blocked in the presence of the Ca2+ channel antagonist nitrendipine (10(-6) M). When membrane potential was clamped at the holding potential (-78 mV), voltage-gated Ca2+ channels were closed, and neither application of growth hormone-releasing hormone nor 8-BrcAMP affected Cm. However, when these agents were applied to depolarized cells, where the voltage-gated Ca2+ channels were open, the increases in Cm were augmented. From these data, it was concluded that elevation of intracellular cAMP, per se, did not stimulate exocytosis. Rather, Ca2+ influx through voltage-gated channels was a prerequisite for cAMP-induced exocytosis.

Relation of exocytosis and Ca2+-activated K+ current during Ca2+ release from intracellular stores in individual rat chromaffin cells

Measurement of the change in cell membrane capacitance (Cm) along with the change in IK(Ca) was used to investigate the effects of bradykinin and caffeine on the secretory process in rat adrenal chromaffin cells. In a Ca2+-free external solution, bradykinin (100 nM) caused a transient increase in Cm with a concurrent change in IK(Ca). Extracellular application of neomycin as an inhibitor of phospholipase C activity reversibly inhibited the bradykinin-activated event, implying an IP3-mediated increase of submembrane-free Ca2+. The increases in Cm and IK(Ca) caused by bradykinin were transient even with the sustained application of bradykinin. Caffeine also caused exocytosis in the Ca2+-free solution, and this was irreversibly blocked by ryanodine (1 microM) in a use-dependent manner. Caffeine-sensitive intracellular Ca2+ stores were also depleted in several seconds and recovered by an influx of external Ca2+. The sequential application of bradykinin and caffeine showed that these are likely to activate Ca2+ release from the same or distinct but rapidly equilibrating intracellular Ca2+ stores. The single cell assay of exocytosis and the increase in IK(Ca) revealed cell-to-cell variability in bradykinin- and caffeine-induced exocytotic response. Our results suggest that Ca2+ release from intracellular stores potentially increases submembrane Ca2+ concentration and modulates simultaneously two submembrane Ca2+-dependent processes, exocytosis and IK(Ca), in rat adrenal chromaffin cells.

Barium triggers rapid endocytosis in calf adrenal chromaffin cells

1. Changes in cell capacitance were monitored in whole-cell patch-clamp recordings from calf adrenal chromaffin cells using a software-based phase-tracking technique. Rapid endocytosis and exocytosis were observed in extracellular solutions containing either Ca2+ or Ba2+. 2. There was no significant difference in the magnitude or the time course of rapid endocytosis of cells stimulated in Ca2+ as compared to Ba2+. When cells were pretreated with caffeine and thapsigargin in order to deplete intracellular Ca2+ stores, rapid endocytosis in Ba2+ was not affected. This indicates that Ba2+ itself is capable of supporting rapid endocytosis. 3. The application of the calmodulin inhibitor calmidazolium via the intracellular pipette solution did not inhibit rapid endocytosis. Although our findings are inconsistent with an immediate requirement for calmodulin in rapid endocytosis, they do not rule out an involvement on a longer time scale. 4. While rapid endocytosis was not affected by the substitution of Ca2+ with Ba2+, the maximum rate of exocytosis was higher in cells stimulated in Ca2+ than in Ba2+. Since Ba2+ currents were much larger than Ca2+ currents during depolarizations to +10 mV (the test potential used in these experiments), Ba2+ appears to be less efficient at promoting exocytosis than Ca2+.

Multiple forms of endocytosis in bovine adrenal chromaffin cells

We studied endocytosis in chromaffin cells with both perforated patch and whole cell configurations of the patch clamp technique using cell capacitance measurements in combination with amperometric catecholamine detection. We found that chromaffin cells exhibit two relatively rapid, kinetically distinct forms of stimulus-coupled endocytosis. A more prevalent "compensatory" retrieval occurs reproducibly after stimulation, recovering an approximately equivalent amount of membrane as added through the immediately preceding exocytosis. Membrane is retrieved through compensatory endocytosis at an initial rate of approximately 6 fF/s. Compensatory endocytotic activity vanishes within a few minutes in the whole cell configuration. A second form of triggered membrane retrieval, termed "excess" retrieval, occurs only above a certain stimulus threshold and proceeds at a faster initial rate of approximately 248 fF/s. It typically undershoots the capacitance value preceding the stimulus, and its magnitude has no clear relationship to the amount of membrane added through the immediately preceding exocytotic event. Excess endocytotic activity persists in the whole cell configuration. Thus, two kinetically distinct forms of endocytosis coexist in intact cells during perforated patch recording. Both are fast enough to retrieve membrane after exocytosis within a few seconds. We argue that the slower one, termed compensatory endocytosis, exhibits properties that make it the most likely mechanism for membrane recycling during normal secretory activity.

Protein kinase C enhances exocytosis from chromaffin cells by increasing the size of the readily releasable pool of secretory granules

We have used membrane capacitance measurements to assay Ca2+-triggered exocytosis in single bovine adrenal chromatin cells. Brief application of phorbol ester (PMA) enhances depolarization-evoked exocytosis severalfold while actually decreasing the Ca2+ current. Ca2+ metabolism is unchanged. Three different protocols were used to show that PMA increases the size of the readily releasable pool of secretory granules. PMA treatment leads to a large increase in amplitude, but little change in the time course of the exocytic burst that results from rapid elevation of [Ca2+]i upon photolysis of DMI-Nitrophen. Thus, PKC appears to affect a late step in secretion but not the Ca2+ sensitivity of the final step.

Calcium- and barium-dependent exocytosis from the rat insulinoma cell line RINm5F assayed using membrane capacitance measurements and serotonin release

Electrophysiological measurements of cell capacitance (Cm) and biochemical assays of [3H] serotonin ([3H]5-hydroxytryptamine or [3H]5-HT) release were combined to study the control of secretion in rat insulinoma RINm5F cells. Depolarizing pulses produced Cm changes (DeltaCm), indicative of exocytosis, with the same voltage and Ca2+ dependency as the inward Ca2+ currents (ICa). Ba2+ was able to substitute for Ca2+ in stimulating exocytosis, but not endocytosis. However, both the relative potency and kinetics of Ca2+-versus Ba2+-triggered exocytosis differed significantly. 5-HT synthesis and uptake were demonstrated in RINm5F cells. This allowed the use of [3H]5-HT to study hormone release from cell populations. [3H]5-HT was released in a depolarization-, Ca2+- and time-dependent manner. Ba2+ also substituted for Ca2+ in depolarization-induced [3H]5-HT release. Thapsigargin, used to deplete Ca2+ stores, had no effects on Ca2+-triggered Cm increases, but Ca2+-triggered [3H]5-HT release was abolished. Ba2+-triggered [3H]5-HT release, however, was only slightly affected by Ca2+ store depletion. Ba2+ was found to act directly as a secretagogue of [3H]5-HT in intact cells, but not in Cm measurements of voltage-clamped cells, suggesting that cell depolarization is a prerequisite for this action.

Fast exocytosis and endocytosis triggered by depolarisation in single adrenal chromaffin cells before rapid Ca2+ current run-down

The kinetics of exocytosis and membrane retrieval (endocytosis) were examined in bovine chromaffin cells using membrane capacitance measurement during whole-cell recording. At early times after breakthrough to the whole-cell recording mode, depolarisation for 1 s resulted in a fast (600 vesicles per s) exocytotic response and efficient membrane retrieval with a time constant of 25 s. The ability to activate fast exocytosis and retrieval was lost during intracellular dialysis, with a time constant of 40 s. At later times, a slow exocytotic response could be elicited with no membrane retrieval following single depolarisations. The wash-out of the responses appeared to be due to a rapid loss of a portion of the Ca2+ current. Trains of depolarisation at late times after breakthrough could elicit a fast (time constant 4 s) retrieval. These data show that in addition to a previously studied slow Ca(2+)-independent retrieval mechanism, chromaffin cells also possess an efficient and rapid retrieval pathway coupled to exocytosis that can be activated following depolarisation. The fast endocytosis appears to have a higher threshold for activation than exocytosis, probably due to a higher Ca2+ requirement. Rapid membrane retrieval appears to occur via a clathrin-independent pathway in chromaffin cells.

Contribution of L- and N-type calcium currents to exocytosis in rat adrenal medullary chromaffin cells

The excitation-secretion coupling process requires Ca2+ influx through voltage-dependent Ca2+ channels (VDCC) but the contribution of L-type or N-type VDCC during the secretion from adrenal chromaffin cell is still on debate. In this study we explored the contribution of each VDCC to exocytosis in single rat adrenal chromaffin cells. Chromaffin cells were voltage-clamped clamped in the whole-cell recording mode. Ca2+ inward current (ICa) was elicited by depolarization from -70 mV to +10 mV and the change in cell membrane capacitance (Cm) was monitored as an indicator of the resultant exocytosis. The increase in Cm had positive correlation with the amount of ICa and replacing the internal Ca2+ buffer to high EGTA (5 mM) decreased the sensitivity of Cm increase to Ca2+ influx. After blockage of ICa with 100 microM Cd2+, there was no increase in Cm following membrane potential depolarization while INa was intact. To clarify the contribution of each type of VDCC to induce exocytosis during membrane potential depolarization, L- and N-type ICa were blocked selectively by Ca2+ channel antagonists. After blockage of L-type ICa with nicardipine (1 microM), ICa was blocked to 35 +/- 6.2% (mean +/- standard error) of control and the resultant change in Cm was reduced to 38 +/- 4.6% of control. Bay K-8644 (1 microM) enhanced ICa and the similar proportion of Cm was increased by this L-type VDCC agonist. On the other hand omega-conotoxin GVIA (1 microM), an N-type VDCC antagonist, blocked ICa to 60 +/- 4.3% of control and reduced the change in Cm to 58 +/- 3.9% of control.(ABSTRACT TRUNCATED AT 250 WORDS)

Two independently regulated secretory pathways in mast cells

We review here evidence that mast cells independently secrete large and small vesicles. Release of small vesicles is triggered by Ca2+. Combining capacitance measurements and flash photolysis of caged Ca2+, we demonstrate Ca-triggered exocytosis that is kinetically identical to the release of dense-core peptidergic vesicles from pituitary melanotrophs. It is suggested that the secretory pathway used for hormone secretion in neuroendocrine cells is also active in mast cells and eosinophils. The nature of the mediator thus secreted is unknown.

Single cell assay of exocytosis from pancreatic islet B cells

We have measured changes in membrane capacitance (delta Cm) in pancreatic B cells as a single cell assay of insulin secretion. Evidence that depolarization evoked delta Cm reflects exocytosis includes it's voltage, temperature and Ca2+ o dependence. Decreases in Cm, presumably reflecting endocytosis, occur on a variable time scale. Two features that make B cells unique among excitable cells are the large magnitude of delta Cm, when normalized to Ca2+ current, and the rapid "fatigue" of the response for even minimal stimulation, perhaps due to the depletion of a readily releasable pool of vesicles.