Modulation of the unitary exocytic event amplitude by cAMP in rat melanotrophs

Local electrostatic interactions determine the diameter of fusion pores

In regulated exocytosis vesicular and plasma membranes merge to form a fusion pore in response to stimulation. The nonselective cation HCN channels are involved in the regulation of unitary exocytotic events by at least 2 mechanisms. They can affect SNARE-dependent exocytotic activity indirectly, via the modulation of free intracellular calcium; and/or directly, by altering local cation concentration, which affects fusion pore geometry likely via electrostatic interactions. By monitoring membrane capacitance, we investigated how extracellular cation concentration affects fusion pore diameter in pituitary cells and astrocytes. At low extracellular divalent cation levels predominantly transient fusion events with widely open fusion pores were detected. However, fusion events with predominately narrow fusion pores were present at elevated levels of extracellular trivalent cations. These results show that electrostatic interactions likely help determine the stability of discrete fusion pore states by affecting fusion pore membrane composition.

Effects of isoquinolonesulfonamides on action potential secretion coupling in pituitary cells

BACKGROUND

Pituitary lactotrophs fire action potentials spontaneously and the associated voltage-gated calcium influx is sufficient to maintain high and steady prolactin release. Several intracellular proteins can mediate the action of calcium influx on prolactin secretion, including calmodulin-dependent protein kinases. Here, we studied effects of isoquinolonesulfonamides KN-62 and KN-93, calmodulin-dependent protein kinase inhibitors, and KN-92, an inactive analog, on spontaneous electrical activity, voltage-gated calcium influx, cyclic nucleotide production, and basal prolactin release.

METHODS

The effects of these compounds on electrical activity and calcium signaling was measured in single lactotrophs and cyclic nucleotide production and prolactin release were determined in static culture and perifusion experiments of anterior pituitary cells from postpubertal female rats.

RESULTS

KN-62 and KN-93 blocked basal prolactin release in a dose- and time-dependent manner, suggesting that calmodulin-dependent protein kinase could mediate the coupling of electrical activity and secretion. However, a similar effect on basal prolactin release was observed on application of KN-92, which does not inhibit this kinase. KN-93 also inhibited cAMP and cGMP production, but inhibition of prolactin release was independent of the status of cyclic nucleotide production. Single cell measurements revealed abolition of spontaneous and depolarization-induced electrical activity and calcium transients in KN-92/93-treated cells, with a time course comparable to that observed in secretory studies.

CONCLUSIONS

The results suggest that caution should be used when interpreting data from studies using isoquinolonesulfonamides to evaluate the role of calmodulin-dependent protein kinases in excitable endocrine cells, because inactive compounds exhibit comparable effects on action potential secretion coupling to those of active compounds.

cAMP signalling in the normal and tumorigenic pituitary gland

cAMP signalling plays a key role in the normal physiology of the pituitary gland, regulating cellular growth and proliferation, hormone production and release. Deregulation of the cAMP signalling pathway has been reported to be a common occurrence in pituitary tumorigenesis. Several mechanisms have been implicated including somatic mutations, gene-gene interactions and gene-environmental interactions. Somatic mutations in G-proteins and protein kinases directly alter cAMP signalling, while malfunctioning of other signalling pathways such as the Raf/MAPK/ERK, PI3K/Akt/mTOR and Wnt pathways which normally interact with the cAMP pathway may mediate indirect effects on cAMP and varying downstream effectors. The aryl hydrocarbon receptor signalling pathway has been implicated in pituitary tumorigenesis and we review its role in general and specifically in relation to cAMP de-regulation.

cAMP-mediated stabilization of fusion pores in cultured rat pituitary lactotrophs

Regulated exocytosis mediates the release of hormones and transmitters. The last step of this process is represented by the merger between the vesicle and the plasma membranes, and the formation of a fusion pore. Once formed, the initially stable and narrow fusion pore may reversibly widen (transient exocytosis) or fully open (full-fusion exocytosis). Exocytosis is typically triggered by an elevation in cytosolic calcium activity. However, other second messengers, such as cAMP, have been reported to modulate secretion. The way in which cAMP influences the transitions between different fusion pore states remains unclear. Here, hormone release studies show that prolactin release from isolated rat lactotrophs stimulated by forskolin, an activator of adenylyl cyclases, and by membrane-permeable cAMP analog (dbcAMP), exhibit a biphasic concentration dependency. Although at lower concentrations (2-10 μm forskolin and 2.5-5 mm dbcAMP) these agents stimulate prolactin release, an inhibition is measured at higher concentrations (50 μm forskolin and 10-15 mm dbcAMP). By using high-resolution capacitance (Cm) measurements, we recorded discrete increases in Cm, which represent elementary exocytic events. An elevation of cAMP leaves the frequency of full-fusion events unchanged while increasing the frequency of transient events. These exhibited a wider fusion pore as measured by increased fusion pore conductance and a prolonged fusion pore dwell time. The probability of observing rhythmic reopening of transient fusion pores was elevated by dbcAMP. In conclusion, cAMP-mediated stabilization of wide fusion pores prevents vesicles from proceeding to the full-fusion stage of exocytosis, which hinders vesicle content discharge at high cAMP concentrations.

High-resolution membrane capacitance measurements for the study of exocytosis and endocytosis

In order to understand exocytosis and endocytosis, it is necessary to study these processes directly. An elegant way to do this is by measuring plasma membrane capacitance (C(m)), a parameter proportional to cell surface area, the fluctuations of which are due to fusion and fission of secretory and other vesicles. Here we describe protocols that enable high-resolution C(m) measurements in macroscopic and microscopic modes. Macroscopic mode, performed in whole-cell configuration, is used for measuring bulk C(m) changes in the entire membrane area, and it enables the introduction of exocytosis stimulators or inhibitors into the cytosol through the patch pipette. Microscopic mode, performed in cell-attached configuration, enables measurements of C(m) with attofarad resolution and allows characterization of fusion pore properties. Although we usually apply these protocols to primary pituitary cells and astrocytes, they can be adapted and used for other cell types. After initial hardware setup and culture preparation, several C(m) measurements can be performed daily.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

cAMP increases the sensitivity of exocytosis to Ca²+ primarily through protein kinase A in mouse pancreatic beta cells

Cyclic AMP regulates the late step of Ca²+-dependent exocytosis in many secretory cells through two major mechanisms: a protein kinase A-dependent and a cAMP-GEF/Epac-dependent pathway. We designed a protocol to characterize the role of these two cAMP-dependent pathways on the Ca²+ sensitivity and kinetics of regulated exocytosis in mouse pancreatic beta cells, using a whole-cell patch-clamp based capacitance measurements. A train of depolarizing pulses or slow photo-release of caged Ca²+ were stimuli for the exocytotic activity. In controls, due to exocytosis after slow photo-release, the C(m) change had typically two phases. We observed that the Ca²+-dependency of the rate of the first C(m) change follows saturation kinetics with high cooperativity and half-maximal rate at 2.9±0.2 μM. The intracellular depletion of cAMP did not change amp1, while rate1 and amp2 were strongly reduced. This manipulation pushed the Ca²+-dependency of the exocytotic burst to significantly lower [Ca²+](i). To address the question of which of the cAMP-dependent mechanisms regulates the observed shifts in Ca²+ dependency we included regulators of PKA and Epac2 activity in the pipette solution. PKA activation with 100 μM 6-Phe-cAMP or inhibition with 500 μM Rp-cAMPs in beta cells significantly shifted the EC(50) in the opposite directions. Specific activation of Epac2 did not change Ca²+ sensitivity. Our findings suggest that cAMP modulates Ca²+-dependent exocytosis in mouse beta cells mainly through a PKA-dependent mechanism by sensitizing the insulin releasing machinery to [Ca²+](i); Epac2 may contribute to enhance the rates of secretory vesicle fusion.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Multiple roles of Gi/o protein-coupled receptors in control of action potential secretion coupling in pituitary lactotrophs

G(i/o) protein-coupled receptors, signaling through G protein-dependent and protein-independent pathways, have prominent effects on secretion by modulating calcium signaling and regulating the size of the releasable secretory pool, the rates of exocytosis and endocytosis, and de novo synthesis. Pituitary cells fire action potentials spontaneously, and the associated calcium influx is sufficient to maintain prolactin (PRL) release but not gonadotropin release at high and steady levels for many hours. Such secretion, termed intrinsic, spontaneous, or basal, reflects fusion of secretory vesicles triggered by the cell type-specific pattern of action potentials. In lactotrophs, activation of endothelin ET(A) and dopamine D(2) receptors causes inhibition of spontaneous electrical activity and basal adenylyl cyclase activity accompanied with inhibition of basal PRL release. Agonist-induced inhibition of cAMP production and firing of action potentials is abolished in cells with blocked pertussis toxin (PTX)-sensitive G(i/o) signaling pathway. However, agonist-induced inhibition of PRL release is only partially relieved in such treated cells, indicating that both receptors also inhibit exocytosis downstream of cAMP/calcium signaling. The PTX-insensitive step in agonist-induced inhibition of PRL release is not affected by inhibition of phosphoinositide 3-kinase and glycogen synthase kinase-3 but is partially rescued by downregulation of the G(z)alpha expression. Thus, ET(A) and D(2) receptors inhibit basal PRL release not only by blocking electrical activity but also by desensitizing calcium-secretion coupling.

Phosphatidylinositol-4,5-bisphosphate-dependent facilitation of the ATP-dependent secretory activity in mouse pituitary cells

Phosphatidylinositol-4, 5-bisphosphate [PI(4,5)P(2)] has been implicated in the priming of large dense-core vesicles in many secretory cells; however, its role in the Ca(2+)-dependent secretory activity in pituitary cells remains elusive. We assessed the effect of elevated intracellular PI(4,5)P(2) on the kinetics of Ca(2+)-dependent exocytosis, using a whole-cell patch-clamp technique in wild-type mouse melanotrophs from fresh pituitary tissue slices. We found that 1 micromol/L PI(4,5)P(2) significantly increased Ca(2+)-dependent exocytosis of vesicles that need to go through ATP-dependent reactions; however, the exocytosis of release-ready vesicles (ATP-independent release) and voltage-activated Ca(2+) currents remained unaffected. We suggest that PI(4,5)P(2) increases the size of the readily releasable vesicle pool by regulating the effectiveness of vesicular mobilization and fusion in an ATP-dependent manner.

Molecular, pharmacological and functional properties of GABA(A) receptors in anterior pituitary cells

Anterior pituitary cells express gamma-aminobutyric acid (GABA)-A receptor-channels, but their structure, distribution within the secretory cell types, and nature of action have not been clarified. Here we addressed these questions using cultured anterior pituitary cells from postpubertal female rats and immortalized alphaT3-1 and GH(3) cells. Our results show that mRNAs for all GABA(A) receptor subunits are expressed in pituitary cells and that alpha1/beta1 subunit proteins are present in all secretory cells. In voltage-clamped gramicidin-perforated cells, GABA induced dose-dependent increases in current amplitude that were inhibited by bicuculline and picrotoxin and facilitated by diazepam and zolpidem in a concentration-dependent manner. In intact cells, GABA and the GABA(A) receptor agonist muscimol caused a rapid and transient increase in intracellular calcium, whereas the GABA(B) receptor agonist baclofen was ineffective, suggesting that chloride-mediated depolarization activates voltage-gated calcium channels. Consistent with this finding, RT-PCR analysis indicated high expression of NKCC1, but not KCC2 cation/chloride transporter mRNAs in pituitary cells. Furthermore, the GABA(A) channel reversal potential for chloride ions was positive to the baseline membrane potential in most cells and the activation of ion channels by GABA resulted in depolarization of cells and modulation of spontaneous electrical activity. These results indicate that secretory pituitary cells express functional GABA(A) receptor-channels that are depolarizing.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

cAMP increases Ca2+-dependent exocytosis through both PKA and Epac2 in mouse melanotrophs from pituitary tissue slices

Cyclic AMP regulates Ca(2+)-dependent exocytosis through a classical protein kinase A (PKA)-dependent and an alternative cAMP-guanine nucleotide exchange factor (GEF)/Epac-dependent pathway in many secretory cells. Although increased cAMP is believed to double secretory output in isolated pituitary cells, the direct target(s) for cAMP action and a detailed and high-time resolved analysis of the effect of intracellular cAMP levels on the secretory activity in melanotrophs are still lacking. We investigated the effect of 200 microM cAMP on the kinetics of secretory vesicle depletion in mouse melanotrophs from fresh pituitary tissue slices. The whole-cell patch-clamp technique was used to depolarize melanotrophs and increase the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Exogenous cAMP elicited an about twofold increase in cumulative membrane capacitance change and approximately 34% increase of high-voltage activated Ca(2+) channel amplitude. cAMP-dependent mechanisms did not affect [Ca(2+)](i), since the application of forskolin failed to change [Ca(2+)](i) in melanotrophs, a phenomenon readily observed in anterior lobe. Depolarization-induced secretion resulted in two distinct kinetic components: a linear and a threshold component, both stimulated by cAMP. The linear component (ATP-independent) probably represented the exocytosis of the release-ready vesicles, whereas the threshold component was assigned to the exocytosis of secretory vesicles that required ATP-dependent reaction(s) and > 800 nM [Ca(2+)](i). The linear component was modulated by 8-pCPT-2Me-cAMP (Epac agonist), while either H-89 (PKA inhibitor) or Rp-cAMPS (the competitive antagonist of cAMP binding to PKA) completely prevented the action of cAMP on the threshold component. In line with this, 6-Phe-cAMP, (PKA agonist), increased the threshold component. From our study, we suggest that the stimulation of cAMP production by application of oestrogen, as found in pregnant mice, increases the efficacy of the hormonal output through both PKA and cAMP-GEFII/Epac2-dependent mechanisms.

Distinct potentiation of L-type currents and secretion by cAMP in rat chromaffin cells

We have investigated the potentiating action of cAMP on L-currents of rat chromaffin cells and the corresponding increase of Ca(2+)-evoked secretory responses with the aim of separating the action of cAMP on Ca(2+) entry through L-channels and the downstream effects of cAMP/protein kinase A (PKA) on exocytosis. In omega-toxin-treated rat chromaffin cells, exposure to the permeable cAMP analog 8-(4-chlorophenylthio)-adenosine 3',5'-monophosphate (pCPT-cAMP; 1 mM, 30 min) caused a moderate increase of Ca(2+) charge carried through L-channels (19% in 10 mM Ca(2+) at +10 mV) and a drastic potentiation of secretion ( approximately 100%), measured as membrane capacitance increments (deltaC). The apparent Ca(2+) dependency of exocytosis increased with pCPT-cAMP and was accompanied by 83% enhancement of the readily releasable pool of vesicles with no significant change of the probability of release, as evaluated with paired-pulse stimulation protocols. pCPT-cAMP effects could be mimicked by stimulation of beta(1)-adrenoreceptors and reversed by the PKA inhibitor H89, suggesting strict PKA dependence. For short pulses to +10 mV (100 ms), potentiation of exocytosis by pCPT-cAMP was proportional to the quantity of charge entering the cell and occurred independently of whether L, N, or P/Q channels were blocked, suggesting that cAMP acts as a constant amplification factor for secretion regardless of the channel type carrying Ca(2+). Analysis of statistical variations among depolarization-induced capacitance increments indicates that pCPT-cAMP acts downstream of Ca(2+) entry by almost doubling the mean size of unitary exocytic events, most likely as a consequence of an increased granule-to-granule rather than a granule-to-membrane fusion.

Distinct effect of actin cytoskeleton disassembly on exo- and endocytic events in a membrane patch of rat melanotrophs

We used the cell-attached mode of patch-clamp technique to measure discrete attofarad steps in membrane capacitance (C(m)), reporting area changes in the plasma membrane due to unitary exocytic and endocytic events. To investigate the role of the actin cytoskeleton in elementary exocytic and endocytic events, neuroendocrine rat melanotrophs were treated with Clostridium spiroforme toxin (CST), which specifically depolymerises F-actin. The average amplitude of exocytic events was not significantly different in control and in CST-treated cells. However, the amplitude of endocytic events was significantly smaller in CST-treated cells as compared to controls. The frequency of exocytic events increased by 2-fold in CST-treated cells relative to controls. In control cells the average frequency of exocytic events (upsilon;(exo)) was lower than the frequency of endocytic events (upsilon;(endo)) with a ratio upsilon;(exo)/upsilon;(endo) < 1. In the toxin treated cells, the predominant process was exocytosis with a ratio (upsilon;(exo)/upsilon;(endo) > 1). To study the coupling between the two processes, the slopes of regression lines relating upsilon;(exo) and upsilon;(endo) in a given patch of membrane were studied. The slopes of regression lines were similar, whereas the line intercepts with the y-axis were significantly different. The increased frequency of unitary exocytic events in CST-treated cells is consistent with the view, that the actin cytoskeleton acts as a barrier for exocytosis. While the disassembly of the actin cytoskeleton diminishes the size of unitary endocytic events, suggesting an important role of the actin cytoskeleton in determining the size of endocytic vesicles, the coupling between exocytosis and endocytosis in a given patch of membrane was independent of the state of the actin cytoskeleton.

Dopamine modulates exocytosis independent of Ca(2+) entry in melanotropic cells

Dopamine is a known inhibitor of pituitary melanotropic cells. It reduces Ca(2+) influx by hyperpolarizing the cell membrane and by modulating high- and low-voltage-activated (HVA and LVA) Ca(2+) channels. As a result, dopamine reduces the hormonal output of the cell. However, it is unknown how dopamine affects each of the four different HVA Ca(2+) channel types individually. Moreover, it is unknown whether dopamine interacts with exocytosis independent of Ca(2+) channels. Here we show that dopamine differentially modulates the HVA Ca(2+) channels and that it affects the stimulus-secretion coupling through a direct effect on the exocytotic machinery. Sustained L- and P-type Ba(2+) currents are reduced in amplitude and inactivating N- and Q-type currents acquire different activation and inactivation kinetics in the presence of dopamine. The Q-type current shows slow activation, which is a hallmark for direct G-protein modulation. We used membrane capacitance measurements to monitor exocytosis. Surprisingly, we find that the amount of exocytosis per step depolarization is not diminished by dopamine despite the reduction in Ca(2+) current. To test whether dopamine affects the release machinery downstream of Ca(2+) entry, we stimulated exocytosis by dialyzing cells with buffered high-Ca(2+) solutions. Dopamine increased the amount and the rate of exocytosis. In the first 90 s, the rate of secretion was increased two- to threefold, but it was normalized again at 180 s, suggesting that predominantly vesicles that fuse early in the exocytotic phase are modulated by dopamine. Thus while Ca(2+) channels are inhibited by dopamine, the exocytotic machinery downstream of Ca(2+) influx is sensitized. As a result, release is more effectively stimulated by Ca(2+) influx during dopamine inhibition.

Differential regulation of granule-to-granule and granule-to-plasma membrane fusion during secretion from rat pituitary lactotrophs

We used fluorescence imaging of individual exocytic events together with electron microscopy to study the regulation of dense core granule-to-plasma membrane fusion and granule-to-granule fusion events that occur during secretion from rat pituitary lactotrophs. Stimulating secretion with elevated extracellular potassium, with the calcium ionophore ionomycin, or with thyrotropin releasing hormone or vasoactive intestinal polypeptide resulted in abundant exocytic structures. Approximately 67% of these structures consisted of multiple granules fused together sharing a single exocytic opening with the plasma membrane, i.e., compound exocytosis. For all of these stimulation conditions there appeared to be a finite number of plasma membrane fusion sites, approximately 11 sites around each cellular equator. However, a granule could fuse directly with another granule that had already fused with the plasma membrane even before all plasma membrane sites were occupied. Granule-to-plasma membrane and granule-to-granule fusion events were subject to different regulations. Forskolin, which can elevate cAMP, increased the number of granule-to-granule fusion events without altering the number of granule-to-plasma membrane fusion events. In contrast, the phorbol ester PMA, which activates protein kinase C increased both granule-to-granule and granule-to-plasma membrane fusion events. These results provide a cellular mechanism that can account for the previously demonstrated potentiation of secretion from lactotrophs by cAMP- and PKC-dependent pathways.

Regulation of exocytosis in neuroendocrine cells: spatial organization of channels and vesicles, stimulus-secretion coupling, calcium buffers and modulation

Neuroendocrine cells display a similar calcium dependence of release as synapses but a strongly different organization of channels and vesicles. Biophysical and biochemical properties of large dense core vesicle release in neuroendocrine cells suggest that vesicles and channels are dissociated by a distance of 100-300 nm. This distinctive organization relates to the sensitivity of the release process to mobile calcium buffers, the resulting relationship between calcium influx and release and the modulatory mechanisms regulating the efficiency of excitation-release coupling. At distances of 100-300 nm, calcium buffers determine the calcium concentration close to the vesicle. Notably, the concentration and diffusion rate of mobile buffers affect the efficacy of release, but local saturation of buffers, possibly enhanced by diffusion barriers, may limit their effects. Buffer conditions may result in a linear relationship between calcium influx and exocytosis, in spite of the third or fourth power relation between intracellular calcium concentration and release. Modulation of excitation-secretion coupling not only concerns the calcium channels, but also the secretory process. Transmitter regulation mediated by cAMP and PKA, as well as use-dependent regulation involving calcium, primarily stimulates filling of the releasable pool. In addition, direct effects of cAMP on the probability of release have been reported. One mechanism to achieve increased release probability is to decrease the distance between channels and vesicles. GTP may stimulate release independently from calcium. Thus, while in most cases primary inputs triggering these pathways await identification, it is evident that large dense core vesicle release is a highly controlled and flexible process.

Stimulation of exocytosis without a calcium signal

More than 30 years ago, Douglas (Douglas & Rubin, 1961; Douglas, 1968) proposed that intracellular Ca2+ controls stimulus-secretion coupling in endocrine cells, and Katz & Miledi (1967; Katz, 1969) proposed that intracellular Ca2+ ions control the rapid release of neurotransmitters from synapses. These related hypotheses have been amply confirmed in subsequent years and for students of excitable cells, they dominate our teaching and research. Calcium controls regulated exocytosis. On the other hand, many studies of epithelial and blood cell biology emphasize Ca2+-independent regulation of secretion of mucin, exocytotic delivery of transporters and degranulation. The evidence seems good. Are these contrasting conclusions somehow mistaken, or are the dominant factors controlling exocytosis actually different in different cell types? In this essay, we try to reconcile these ideas and consider classes of questions to ask and hypotheses to test in seeking a more integrated understanding of excitation-secretion coupling. Our review is conceptual and narrowly selective of a few examples rather than referring to a broader range of useful studies in the extensive literature. The examples are taken from mammals and are documented principally by citing other reviews and two of our own studies. The evidence shows that protein phosphorylation by kinases potentiates Ca2+-dependent exocytosis and often suffices to induce exocytosis by itself. Apparently, protein phosphorylation is the physiological trigger in a significant number of examples of regulated exocytosis. We conclude that although sharing many common properties, secretory processes in different cells are specialized and distinct from each other.