alpha-adrenergic stimulation of cytosolic Ca2+ oscillations and exocytosis in identified rat corticotrophs

Gonadotrope plasticity at cellular, population and structural levels: A comparison between fishes and mammals

Often referred to as "the master gland", the pituitary is a key organ controlling growth, maturation, and homeostasis in vertebrates. The anterior pituitary, which contains several hormone-producing cell types, is highly plastic and thereby able to adjust the production of the hormones governing these key physiological processes according to the changing needs over the life of the animal. Hypothalamic neuroendocrine control and feedback from peripheral tissues modulate pituitary cell activity, adjusting levels of hormone production and release according to different functional or environmental requirements. However, in some physiological processes (e.g. growth, puberty, or metamorphosis), changes in cell activity may be not sufficient to meet the needs and a general reorganization of cell composition and pituitary structure may occur. Focusing on gonadotropes, this review examines plasticity at the cellular level, which allows precise and rapid control of hormone production and secretion, as well as plasticity at the population and structural levels, which allows more substantial changes in hormone production. Further, we compare current knowledge of the anterior pituitary plasticity in fishes and mammals in order to assess what has been conserved or not throughout evolution, and highlight important remaining questions.

Activation of alpha-1 adrenergic receptors increases cytosolic calcium in neurones of the paraventricular nucleus of the hypothalamus

Norepinephrine (NE) activates adrenergic receptors (ARs) in the hypothalamic paraventricular nucleus (PVN) to increase excitatory currents, depolarise neurones and, ultimately, augment neuro-sympathetic and endocrine output. Such cellular events are known to potentiate intracellular calcium ([Ca²⁺ ]_i ); however, the role of NE with respect to modulating [Ca²⁺ ]_i in PVN neurones and the mechanisms by which this may occur remain unclear. We evaluated the effects of NE on [Ca²⁺ ]_i of acutely isolated PVN neurones using Fura-2 imaging. NE induced a slow increase in [Ca²⁺ ]_i compared to artificial cerebrospinal fluid vehicle. NE-induced Ca²⁺ elevations were mimicked by the α₁ -AR agonist phenylephrine (PE) but not by α₂ -AR agonist clonidine (CLON). NE and PE but not CLON also increased the overall number of neurones that increase [Ca²⁺ ]_i (ie, responders). Elimination of extracellular Ca²⁺ or intracellular endoplasmic reticulum Ca²⁺ stores abolished the increase in [Ca²⁺ ]_i and reduced responders. Blockade of voltage-dependent Ca²⁺ channels abolished the α₁ -AR induced increase in [Ca²⁺ ]_i and number of responders, as did inhibition of phospholipase C inhibitor, protein kinase C and inositol triphosphate receptors. Spontaneous phasic Ca²⁺ events, however, were not altered by NE, PE or CLON. Repeated K⁺ -induced membrane depolarisation produced repetitive [Ca²⁺ ]_i elevations. NE and PE increased baseline Ca²⁺ , whereas NE decreased the peak amplitude. CLON also decreased peak amplitude but did not affect baseline [Ca²⁺ ]_i . Taken together, these data suggest receptor-specific influence of α₁ and α₂ receptors on the various modes of calcium entry in PVN neurones. They further suggest Ca²⁺ increase via α₁ -ARs is co-dependent on extracellular Ca²⁺ influx and intracellular Ca²⁺ release, possibly via a phospholipase C inhibitor-mediated signalling cascade.

Strong stimulation triggers full fusion exocytosis and very slow endocytosis of the small dense core granules in carotid glomus cells

Chemosensory glomus cells of the carotid bodies release transmitters, including ATP and dopamine mainly via the exocytosis of small dense core granules (SDCGs, vesicular diameter of ∼100 nm). Using carbon-fiber amperometry, we showed previously that with a modest uniform elevation in cytosolic Ca²⁺ concentration ([Ca²⁺]_i of ∼0.5 µM), SDCGs of rat glomus cells predominantly underwent a "kiss-and-run" mode of exocytosis. Here, we examined whether a larger [Ca²⁺]_i rise influenced the mode of exocytosis. Activation of voltage-gated Ca²⁺ channels by a train of voltage-clamped depolarizations which elevated [Ca²⁺]_i to ∼1.6 μM increased the cell membrane capacitance by ∼2.5%. At 30 s after such a stimulus, only 5% of the added membrane was retrieved. Flash photolysis of caged-Ca²⁺ (which elevated [Ca²⁺]_i to ∼16 μM) increased cell membrane capacitance by ∼13%, and only ∼30% of the added membrane was retrieved at 30 s after the UV flash. When exocytosis and endocytosis were monitored using the two-photon excitation and extracellular polar tracer (TEP) imaging of FM1-43 fluorescence in conjunction with photolysis of caged Ca²⁺, almost uniform exocytosis was detected over the cell's entire surface and it was followed by slow endocytosis. Immunocytochemistry showed that the cytoplasmic densities of dynamin I, II and clathrin (key proteins that mediate endocytosis) in glomus cells were less than half of those in adrenal chromaffin cells, suggesting that a lower expression of endocytotic machinery may underlie the slow endocytosis in glomus cells. An analysis of the relative change in the signals from two fluorescent dyes that simultaneously monitored the addition of vesicular volume and plasma membrane surface area, suggested that with an intense stimulus, SDCGs of glomus cells underwent full fusion without any significant "compound" exocytosis. Therefore, during a severe hypoxic challenge, glomus granules undergo full fusion for a more complete release of transmitters.

Control of anterior pituitary cell excitability by calcium-activated potassium channels

In anterior pituitary endocrine cells, large (BK), small (SK) and intermediate (IK) conductance calcium activated potassium channels are key determinants in shaping cellular excitability in a cell type- and context-specific manner. Indeed, these channels are targeted by multiple signaling pathways that stimulate or inhibit cellular excitability. BK channels can, paradoxically, both promote electrical bursting as well as terminate bursting and spiking dependent upon intrinsic BK channel properties and proximity to voltage gated calcium channels in somatotrophs, lactotrophs and corticotrophs. In contrast, SK channels are predominantly activated by calcium released from intracellular IP3-sensitive calcium stores and mediate membrane hyperpolarization in cells including gonadotrophs and corticotrophs. IK channels are predominantly expressed in corticotrophs where they limit membrane excitability. A major challenge for the future is to determine the cell-type specific molecular composition of calcium-activated potassium channels and how they control anterior pituitary hormone secretion as well as other calcium-dependent processes.

Granule matrix property and rapid “kiss-and-run” exocytosis contribute to the different kinetics of catecholamine release from carotid glomus and adrenal chromaffin cells at matched quantal size

Catecholamine-containing small dense core granules (SDCGs, vesicular diameter of ∼100 nm) are prominent in carotid glomus (chemosensory) cells and some neurons, but the release kinetics from individual SDCGs has not been studied in detail. In this study, we compared the amperometric signals from glomus cells with those from adrenal chromaffin cells, which also secrete catecholamine but via large dense core granules (LDCGs, vesicular diameter of ∼200–250 nm). When exocytosis was triggered by whole-cell dialysis (which raised the concentration of intracellular Ca2+ ([Ca2+]i) to ∼0.5 µmol/L), the proportion of the type of signal that represents a flickering fusion pore was 9-fold higher for glomus cells. Yet, at the same range of quantal size (Q, the total amount of catecholamine that can be released from a granule), the kinetics of every phase of the amperometric spike signals from glomus cells was faster. Our data indicate that the last phenomenon involved at least 2 mechanisms: (i) the granule matrix of glomus cells can supply a higher concentration of free catecholamine during exocytosis; (ii) a modest elevation of [Ca2+]i triggers a form of rapid “kiss-and-run” exocytosis, which is very prevalent among glomus SDCGs and leads to incomplete release of their catecholamine content (and underestimation of their Q value).

Ca2+ signaling and exocytosis in pituitary corticotropes

The secretion of adrenocorticotrophin (ACTH) from corticotropes is a key component in the endocrine response to stress. The resting potential of corticotropes is set by the basal activities of TWIK-related K(+) (TREK)-1 channel. Corticotrophin-releasing hormone (CRH), the major ACTH secretagogue, closes the background TREK-1 channels via the cAMP-dependent pathway, resulting in depolarization and a sustained rise in cytosolic [Ca(2+)] ([Ca(2+)](i)). By contrast, arginine vasopressin and norepinephrine evoke Ca(2+) release from the inositol trisphosphate (IP(3))-sensitive store, resulting in the activation of small conductance Ca(2+)-activated K(+) channels and hyperpolarization. Following [Ca(2+)](i) rise, cytosolic Ca(2+) is taken into the mitochondria via the uniporter. Mitochondrial inhibition slows the decay of the Ca(2+) signal and enhances the depolarization-triggered exocytotic response. Both voltage-gated Ca(2+) channel activation and intracellular Ca(2+) release generate spatial Ca(2+) gradients near the exocytic sites such that the local [Ca(2+)] is ~3-fold higher than the average [Ca(2+)](i). The stimulation of mitochondrial metabolism during the agonist-induced Ca(2+) signal and the robust endocytosis following stimulated exocytosis enable corticotropes to maintain sustained secretion during the diurnal ACTH surge. Arachidonic acid (AA) which is generated during CRH stimulation activates TREK-1 channels and causes hyperpolarization. Thus, corticotropes may regulate ACTH release via an autocrine feedback mechanism.

Arachidonic acid mobilizes Ca2+ from the endoplasmic reticulum and an acidic store in rat pancreatic β cells

In rat pancreatic β cells, arachidonic acid (AA) triggered intracellular Ca(2+) release. This effect could be mimicked by eicosatetraynoic acid, indicating that AA metabolism is not required. The AA-mediated Ca(2+) signal was not affected by inhibition of ryanodine receptors or emptying of ryanodine-sensitive store but was reduced by ∼70% following the disruption of acidic stores (treatment with bafilomycin A1 or glycyl-phenylalanyl-β-naphthylamide (GPN)). The action of AA did not involve TRPM2 channels or NAADP receptors because intracellular dialysis of adenosine diphosphoribose (ADPR; an activator of TRPM2 channels) or NAADP did not affect the AA response. In contrast, stimulation of IP(3) receptors via intracellular dialysis of adenophostin A, or exogenous application of ATP largely abolished the AA-mediated Ca(2+) signal. Intracellular dialysis of heparin abolished the ATP-mediated Ca(2+) signal but not the AA response, suggesting that the action of AA did not involve the IP(3)-binding site. Treatment with the SERCA pump inhibitor, thapsigargin, reduced the amplitude of the AA-mediated Ca(2+) signal by ∼70%. Overall, our finding suggests that AA mobilizes Ca(2+) from the endoplasmic reticulum as well as an acidic store and both stores could be depleted by IP(3) receptor agonist. The possibility of secretory granules as targets of AA is discussed.

Molecular mechanisms of pituitary endocrine cell calcium handling

Endocrine pituitary cells express numerous voltage-gated Na(+), Ca(2+), K(+), and Cl(-) channels and several ligand-gated channels, and they fire action potentials spontaneously. Depending on the cell type, this electrical activity can generate localized or global Ca(2+) signals, the latter reaching the threshold for stimulus-secretion coupling. These cells also express numerous G-protein-coupled receptors, which can stimulate or silence electrical activity and Ca(2+) influx through voltage-gated Ca(2+) channels and hormone release. Receptors positively coupled to the adenylyl cyclase signaling pathway stimulate electrical activity with cAMP, which activates hyperpolarization-activated cyclic nucleotide-regulated channels directly, or by cAMP-dependent kinase-mediated phosphorylation of K(+), Na(+), Ca(2+), and/or non-selective cation-conducting channels. Receptors that are negatively coupled to adenylyl cyclase signaling pathways inhibit spontaneous electrical activity and accompanied Ca(2+) transients predominantly through the activation of inwardly rectifying K(+) channels and the inhibition of voltage-gated Ca(2+) channels. The Ca(2+)-mobilizing receptors activate inositol trisphosphate-gated Ca(2+) channels in the endoplasmic reticulum, leading to Ca(2+) release in an oscillatory or non-oscillatory manner, depending on the cell type. This Ca(2+) release causes a cell type-specific modulation of electrical activity and intracellular Ca(2+) handling.

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.

Arachidonic acid stimulates extracellular Ca(2+) entry in rat pancreatic beta cells via activation of the noncapacitative arachidonate-regulated Ca(2+) (ARC) channels

Arachidonic acid (AA) is generated in the pancreatic islets during glucose stimulation. We investigated whether AA activated extracellular Ca(2+) entry in rat pancreatic beta cells via a pathway that was independent of the activation of voltage-gated Ca(2+) channels. The AA triggered [Ca(2+)](i) rise did not involve activation of GPR40 receptors or AA metabolism. When cells were voltage clamped at -70mV, the AA-mediated intracellular Ca(2+) release was accompanied by extracellular Ca(2+) entry. AA accelerated the rate of Mn(2+) quench of indo-1 fluorescence (near the Ca(2+)-independent wavelength of indo-1), reflecting the activation of a Ca(2+)-permeable pathway. The AA-mediated acceleration of Mn(2+) quench was inhibited by La(3+) but not by 2-APB (a blocker of capacitative Ca(2+) entry), suggesting the involvement of arachidonate-regulated Ca(2+) (ARC) channels. Consistent with this, intracellular application of the charged membrane-impermeant analog of AA, arachidonyl-coenzyme A (ACoA) triggered extracellular Ca(2+) entry, as well as the activation of a La(3+)-sensitive small inward current (1.7pA/pF) at -70mV. Our results indicate that the activation of ARC channels by intracellular AA triggers extracellular Ca(2+) entry. This action may contribute to the effects of AA on Ca(2+) signals and insulin secretion in rat beta cells.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates the oxygen sensing type I (glomus) cells of rat carotid bodies via reduction of a background TASK-like K+current

Pituitary adenylate cyclase-activating polypeptide (PACAP)-deficient mice are prone to sudden neonatal death and have reduced respiratory response to hypoxia. Here we found that PACAP-38 elevated cytosolic [Ca(2+)] ([Ca(2+)](i)) in the oxygen sensing type I cells but not the glial-like type II (sustentacular) cells of the rat carotid body. This action of PACAP could not be mimicked by vasoactive intestinal peptide but was abolished by PACAP 6-38, implicating the involvement of PAC(1) receptors. H89, a protein kinase A (PKA) inhibitor attenuated the PACAP response. Simultaneous measurement of membrane potential and [Ca(2+)](i) showed that the PACAP-mediated [Ca(2+)](i) rise was accompanied by depolarization and action potential firing. Ni(2+), a blocker of voltage-gated Ca(2+) channels (VGCC) or the removal of extracellular Ca(2+) reversibly inhibited the PACAP-mediated [Ca(2+)](i) rise. In the presence of tetraethylammonium (TEA) and 4-aminopyridine (4-AP), PACAP reduced a background K(+) current. Anandamide, a blocker of TWIK-related acid-sensitive K(+) (TASK)-like K(+) channel, occluded the inhibitory action of PACAP on K(+) current. We conclude that PACAP, acting via the PAC(1) receptors coupled PKA pathway inhibits a TASK-like K(+) current and causes depolarization and VGCC activation. This stimulatory action of PACAP in carotid type I cells can partly account for the role of PACAP in respiratory disorders.

Adenosine stimulates depolarization and rise in cytoplasmic [Ca2+] in type I cells of rat carotid bodies

During hypoxia, the level of adenosine in the carotid bodies increases as a result of ATP catabolism and adenosine efflux via adenosine transporters. Using Ca2+imaging, we found that adenosine, acting via A2Areceptors, triggered a rise in cytoplasmic [Ca2+] ([Ca2+]i) in type I (glomus) cells of rat carotid bodies. The adenosine response could be mimicked by forskolin (but not its inactive analog), and could be abolished by the PKA inhibitor H89. Simultaneous measurements of membrane potential (perforated patch recording) and [Ca2+]ishowed that the adenosine-mediated [Ca2+]irise was accompanied by depolarization. Ni2+, a voltage-gated Ca2+channel (VGCC) blocker, abolished the adenosine-mediated [Ca2+]irise. Although adenosine was reported to inhibit a 4-aminopyridine (4-AP)-sensitive K+current, 4-AP failed to trigger any [Ca2+]irise, or to attenuate the adenosine response. In contrast, anandamide, an inhibitor of the TWIK-related acid-sensitive K+-1 (TASK-1) channels, triggered depolarization and [Ca2+]irise. The adenosine response was attenuated by anandamide but not by tetraethylammonium. Our results suggest that adenosine, acting via the adenylate cyclase and PKA pathways, inhibits the TASK-1 K+channels. This leads to depolarization and activation of Ca2+entry via VGCC. This excitatory action of adenosine on type I cells may contribute to the chemosensitivity of the carotid body during hypoxia.

Dominant role of sarcoendoplasmic reticulum Ca2+-ATPase pump in Ca2+ homeostasis and exocytosis in rat pancreatic beta-cells

The exocytosis of insulin-containing granules from pancreatic beta-cells is tightly regulated by changes in cytosolic Ca2+ concentration ([Ca2+]i). We investigated the role of the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pump, Na+/Ca2+ exchanger, and plasma membrane Ca2+-ATPase pump in the Ca2+ dynamics of single rat pancreatic beta-cells. When the membrane potential was voltage clamped at -70 mV (in 3 mm glucose at approximately 22 or 35 C), SERCA pump inhibition dramatically slowed (approximately 4-fold) cytosolic Ca2+ clearance and caused a sustained rise in basal [Ca2+]i via the activation of capacitative Ca2+ entry. SERCA pump inhibition increased ( approximately 1.8-fold) the amplitude of the depolarization-triggered Ca2+ transient at approximately 22 C. Inhibition of the Na+/Ca2+ exchanger or plasma membrane Ca2+-ATPase pump had only minor effects on Ca2+ dynamics. Simultaneous measurement of [Ca2+]i and exocytosis (with capacitance measurement) revealed that SERCA pump inhibition increased the magnitude of depolarization-triggered exocytosis. This enhancement in exocytosis was not due to the slowing of the cytosolic Ca2+ clearance but was closely correlated to the increase in the peak of the depolarization-triggered Ca2+ transient. When compared at similar [Ca2+]i with controls, the rise in basal [Ca2+]i during SERCA pump inhibition did not cause any enhancement in the magnitude of the ensuing depolarization-triggered exocytosis. Therefore, we conclude that in rat pancreatic beta-cells, the rapid uptake of Ca2+ by SERCA pump limits the peak amplitude of depolarization-triggered [Ca2+]i rise and thus controls the amount of insulin secretion.

Dominant role of mitochondria in calcium homeostasis of single rat pituitary corticotropes

The rise in cytosolic free Ca2+ concentration ([Ca2+]i) is the major trigger for secretion of ACTH from pituitary corticotropes. To better understand the shaping of the Ca2+ signal in corticotropes, we investigated the mechanisms regulating the depolarization-triggered Ca2+ signal using patch-clamp techniques and indo-1 fluorometry. The rate of cytosolic Ca2+ clearance was unaffected by inhibitors of Na+/Ca2+ exchanger or plasma membrane Ca2+-ATPase (PMCA), slightly slowed by sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor, but dramatically slowed by mitochondrial uncouplers or inhibitor of mitochondrial uniporter. Measurements with rhod-2 revealed that depolarization-triggered increase in mitochondrial Ca2+ concentration. Thus, mitochondria have a dominant role in cytosolic Ca2+ clearance. Using the Mn2+ quench technique, we found the presence of a continuous basal Ca2+ influx in corticotropes. This basal Ca2+ influx was balanced by the combined actions of mitochondrial uniporter and PMCA and SERCA pumps. Inhibition of the mitochondrial uniporter or PMCA or SERCA pumps elevated basal [Ca2+]i. Using membrane capacitance measurement, we found that the change in the shape of the depolarization-triggered Ca2+ signal after mitochondrial inhibition was associated with enhancement of the exocytotic response. Thus, mitochondria have a dominant role in the regulation of Ca2+ signal and exocytosis in corticotropes.

Maintenance of quantal size and immediately releasable granules in rat chromaffin cells by glucocorticoid

Glucocorticoid is reported to regulate catecholamine synthesis and storage. However, it is not clear whether the actual amount of catecholamine released from individual granules (quantal size, Q) in mature chromaffin cells is affected by glucocorticoid. Using carbon fiber amperometry, we found that dexamethasone did not affect mean cellular Q or the proportional release from different populations of granules in rat chromaffin cells cultured for 1 day in a serum-free defined medium. After two extra days of culture in the defined medium, there was a rundown in mean cellular Q, and it was associated with a shift in the proportional release from the different granule populations. This phenomenon could not be rescued by serum supplementation but could be prevented by dexamethasone via an action that was independent of changes in voltage-gated Ca(2+) channel (VGCC) density. Using simultaneous measurements of membrane capacitance and cytosolic Ca(2+) concentration, we found that for cells cultured in defined medium dexamethasone enhanced the exocytotic response triggered by a brief depolarization (50 ms) without affecting the VGCC density or the fast exocytotic response triggered via flash photolysis of caged Ca(2+). Thus glucocorticoid may regulate the number of immediately releasable granules that are in close proximity to a subset of VGCC. Because chromaffin cells in vivo are exposed to high concentrations of glucocorticoid, our findings suggest that the paracrine actions of glucocorticoid maintain the mean catecholamine content in chromaffin cell granules as well as the colocalization of releasable granules with VGCCs.

ATP inhibits the hypoxia response in type I cells of rat carotid bodies

Summary During hypoxia, ATP was released from type I (glomus) cells in the carotid bodies. We studied the action of ATP on the intracellular Ca(2+) concentration ([Ca(2+)](i)) of type I cells dissociated from rat carotid bodies using a Ca(2+) imaging technique. ATP did not affect the resting [Ca(2+)](i) but strongly suppressed the hypoxia-induced [Ca(2+)](i) elevations in type I cells. The order of purinoreceptor agonist potency in inhibiting the hypoxia response was 2-methylthioATP > ATP > ADP >> alpha, beta-methylene ATP > UTP, implicating the involvement of P2Y(1) receptors. Simultaneous measurements of membrane potential and [Ca(2+)](i) show that ATP inhibited the hypoxia-induced Ca(2+) signal by reversing the hypoxia-triggered depolarization. However, ATP did not oppose the hypoxia-mediated inhibition of the oxygen-sensitive TASK-like K(+) background current. Neither the inhibition of the large-conductance Ca(2+)-activated K(+) (maxi-K) channels nor the removal of extracellular Na(+) could affect the inhibitory action of ATP. Under normoxic condition, ATP caused hyperpolarization and increase in cell input resistance. These results suggest that the inhibitory action of ATP is mediated via the closure of background conductance(s) other than the TASK-like K(+), maxi-K or Na(+) channels. In summary, ATP exerts strong negative feedback regulation on hypoxia signaling in rat carotid type I cells.

Simultaneous monitoring of Zn2+ secretion and intracellular Ca2+ from islets and islet cells by fluorescence microscopy

A method for simultaneously imaging Zn2+ secretion and intracellular Ca2+ at β-cell clusters and single islets of Langerhans was developed. Cells were loaded with the Ca2+ indicator Fura Red, incubated in buffer containing the Zn2+ indicator FluoZin-3, and imaged via laser scanning fluorescence confocal microscopy. FluoZin-3 and Fura Red are excited at 488 nm and emit at 515 and 665 nm, respectively. Zn2+, which is co-released with insulin, reacts with extracellular FluoZin-3 to form a fluorescent product. Stimulation of cell clusters with glucose evoked increases and oscillations in intracellular Ca2+ and Zn2+ secretion that were correlated with each other and were synchronized among cells. In single islets, spatially resolved dynamics of secretion including detection of first phase, second phase, and synchronized oscillations around the islet were observed. Fura Red did not yield detectable Ca2+ signals at islets. For islet measurements, cells were loaded with Fura-2 and incubated in FluoZin-3 while sequentially illuminating the islets with 340, 380, and 470 nm light and acquiring epi-fluorescence images with a charge-coupled device (CCD) camera. This allowed Ca2+ and secretion to be observed with approximately 2 s temporal resolution. This method should be useful for studying Ca2+ secretion coupling and any application requiring rapid assays of secretion.

Alpha-1 adrenergic input to solitary nucleus neurones: calcium oscillations, excitation and gastric reflex control

The nucleus of the solitary tract (NST) processes substantial visceral afferent input and sends divergent projections to a wide array of CNS targets. The NST is essential to the maintenance of behavioural and autonomic homeostasis and is the source, as well as the recipient, of considerable noradrenergic (NE) projections. The significance of NE projections from the NST to other CNS regions has long been appreciated, but the nature of NE action on NST neurones themselves, especially on the alpha-1 receptor subtype, is controversial. We used a combination of methodologies to establish, systematically, the effects and cellular basis of action of the alpha-1 agonist, phenylephrine (PHE), to control NST neurones responsible for vago-vagal reflex regulation of the stomach. Immunocytochemical and retrograde tracing studies verified that the area postrema, A2, A5, ventrolateral medulla and locus coeruleus regions are sources of catecholaminergic input to the NST. In vivo electrophysiological recordings showed that PHE activates physiologically identified, second-order gastric sensory NST neurones. In vivo microinjection of PHE onto NST neurones caused a significant reduction in gastric tone. Finally, in vitro calcium imaging studies revealed that PHE caused dramatic cytosolic calcium oscillations in NST neurones. These oscillations are probably the result of an interplay between agonist-induced and inositol 1,4,5-trisphosphate (IP(3))-mediated intracellular calcium release and Ca(2+)-ATPase control of intracellular calcium storage pumps. The oscillations persisted even in perfusions of zero calcium-EGTA Krebs solution suggesting that the calcium oscillation is mediated principally by intracellular calcium release-reuptake mechanisms. Cyclical activation of the NST may function to increase the responsiveness of these neurones to incoming afferent input (i.e., elevate the "gain"). An increase in gain of afferent input may cause an amplification of the response part of the reflex and help explain the powerful effects that alpha-1 agonists have in suppressing gastric motility and producing anorexia.

Adrenergic regulation of the intracellular [Ca2+] and voltage-operated Ca2+ channel currents in the rat prostate neuroendocrine cells

BACKGROUND

The prostate gland contains numerous neuroendocrine cells (PNECs) innervated by adrenergic neurons. PNECs are believed to influence the growth and physiological function of the prostate gland via paracrine release of hormones.

MATERIALS AND METHODS

Using fura-2 fluorescence measurement and patch-clamp techniques, we investigated the effects of adrenergic stimulation on cytosolic concentration of Ca2+ ([Ca2+]c) and high voltage-activated Ca2+ channel currents (HVA-I(Ca)) of the putative rat prostate neuroendocrine cells (RPNECs) freshly isolated by an enzymic digestion.

RESULTS

Noradrenaline (NA, 1 microM) induced a sharp, transient increase of [Ca2+]c measured by the fura-2 fluorescence. Pharmacological studies showed that alpha1-adrenoceptors (alpha1-ARs) coupled with PLC/IP3 signaling pathway induce the release of stored Ca2+, which subsequently recruits store-operated Ca2+ entry pathways. In the whole-cell voltage clamp experiment, NA decreased the amplitude of HVA-I(Ca) by 40%, which was mimicked by an alpha2-AR agonist (UK14304) but not by an alpha1-AR agonist (phenyleprine). After selective blockade of N-type Ca2+ channels by omega-conotoxin GVIA, the addition of NA showed no further inhibition on the remaining L-type Ca2+ channel currents. The adrenergic inhibition of HVA-I(Ca) was partially prevented by the pretreatment with pertussis toxin (PTX) (5 microg/ml, 4 hr, 37 degrees C).

CONCLUSIONS

RPNECs express both alpha1- and alpha2-ARs, signaling the release of stored Ca2+ and the inhibition of N-type Ca2+ channels, respectively.

ATP triggers intracellular Ca2+ release in type II cells of the rat carotid body

Using a Ca2+-imaging technique, we studied the action of ATP on the intracellular Ca2+ concentration ([Ca2+]i) of fura-2-loaded mixtures of type I and type II cells dissociated from rat carotid bodies. ATP (100 micro M) triggered a transient rise in [Ca2+]i in the spindle-shaped type II (sustentacular) cells, but not the ovoid type I (glomus) cells. When challenged with ionomycin (1 micro M), no amperometry signal could be detected from the ATP-responsive type II cells, suggesting that these cells lacked catecholamine-containing granules. In contrast, KCl depolarization triggered robust quantal catecholamine release from type I cells that were not responsive to ATP. In type II cells voltage clamped at -70 mV, the ATP-induced [Ca2+]i rise was not accompanied by any current change, suggesting that P2X receptors are not involved. The ATP-induced Ca2+ signal could be observed in the presence of Ni2+ (a blocker of voltage-gated Ca2+ channels) or in the absence of extracellular Ca2+, indicating that Ca2+ release from intracellular stores was the dominant mechanism. The order of purinoreceptor agonist potency in triggering the [Ca2+]i rise was UTP > ATP > 2-methylthioATP >> alpha,beta-methyleneATP, implicating the involvement of P2Y2 receptors. In carotid body sections, immunofluorescence revealed localization of P2Y2 receptors on spindle-shaped type II cells that partially enveloped ovoid type I cells. Since ATP is released from type I cells during hypoxia, we suggest that the ATP-induced Ca2+ signal in type II cells can mediate paracrine interactions within the carotid bodies.

Differential regulation of transmitter release by presynaptic and glial Ca2+ internal stores at the neuromuscular synapse

The differential regulation of synaptic transmission by internal Ca(2+) stores of presynaptic terminals and perisynaptic Schwann cells (PSCs) was studied at the frog neuromuscular junction. Thapsigargin (tg), an inhibitor of Ca(2+)-ATPase pumps of internal stores, caused a transient Ca(2+) elevation in PSCs, whereas it had no effect on Ca(2+) stores of presynaptic terminals at rest. Tg prolonged presynaptic Ca(2+) responses evoked by single action potentials with no detectable increase in the resting Ca(2+) level in nerve terminals. However, Ca(2+) accumulation was observed during high frequency stimulation. Tg induced a rapid rise in endplate potential (EPP) amplitude, accompanied by a delayed and transient increase. The effects appeared presynaptic, as suggested by the lack of effects of tg on the amplitude and time course of miniature EPPs (MEPPs). However, MEPP frequency was increased when preparations were stimulated tonically (0.2 Hz). The delayed and transient increase in EPP amplitude was occluded by injections of the Ca(2+) chelator BAPTA into PSCs before tg application, whereas a rise in intracellular Ca(2+) in PSCs induced by inositol 1,4,5-triphosphate (IP(3)) injections potentiated transmitter release. Furthermore, increased Ca(2+) buffering capacity after BAPTA injection in PSCs resulted in a more pronounced synaptic depression induced by high frequency stimulation of the motor nerve (10 Hz/80 sec). It is concluded that presynaptic Ca(2+) stores act as a Ca(2+) clearance mechanism to limit the duration of transmitter release, whereas Ca(2+) release from glial stores initiates Ca(2+)-dependent potentiation of synaptic transmission.

Endocytosis in identified rat corticotrophs

1. We used the patch-clamp technique, in conjunction with membrane capacitance measurement, fluorescence measurement of intracellular calcium concentration ([Ca(2+)](i)), and flash photolysis of caged Ca(2+) to study exo- and endocytosis in identified rat corticotrophs. 2. Exocytosis stimulated by depolarization pulses was typically followed by a 'slow' endocytosis that retrieved the membrane with a time constant of approximately 6 s. The efficiency (the endocytosis/exocytosis amplitude ratio) of 'slow' endocytosis was approximately 1.2 at [Ca(2+)](i) < 3 microM and increased to approximately 1.6 at [Ca(2+)](i) > 3 microM. 3. Whole-cell dialysis through a patch pipette did not affect the kinetics and the efficiency of 'slow' endocytosis, but the amplitude of exocytosis was reduced. 4. 'Slow' endocytosis did not require sustained [Ca(2+)](i) elevation and its kinetics was only weakly [Ca(2+)](i) dependent. Our results suggest that 'slow' endocytosis involves a Ca(2+) sensor with a high Ca(2+) affinity (approximately 500 nM). 5. At high [Ca(2+)](i) (> 10 microM), the 'slow' endocytosis was frequently preceded by a 'fast' endocytosis that comprised multiple steps of rapid decrease in membrane capacitance. 6. Neither calmodulin nor calcineurin appeared to be the Ca(2+) sensor for endocytosis because the two forms of endocytosis were not affected by the calmodulin inhibitor calmidazolium (500 microM) or the calcineurin inhibitors cyclosporin A (1 microM) and calcineurin autoinhibitory peptide (1 mg ml(-1)). Ba(2+), a poor activator of calmodulin, could support both forms of endocytosis but slowed the kinetics of 'slow' endocytosis approximately 2-fold. 7. Non-hydrolysable analogues of GTP (GDP-beta-S) and ATP (ATP-gamma-S) also failed to inhibit either form of endocytosis, indicating that neither GTP nor ATP was essential for endocytosis. 8. We suggest that the high Ca(2+) affinity of 'slow' endocytosis may be important for maintaining continuous cycles of exocytosis-endocytosis during sustained adrenocorticotropin secretion in corticotrophs.

Differential regulation of transmitter release by presynaptic and glial Ca2+ internal stores at the neuromuscular synapse

The differential regulation of synaptic transmission by internal Ca(2+) stores of presynaptic terminals and perisynaptic Schwann cells (PSCs) was studied at the frog neuromuscular junction. Thapsigargin (tg), an inhibitor of Ca(2+)-ATPase pumps of internal stores, caused a transient Ca(2+) elevation in PSCs, whereas it had no effect on Ca(2+) stores of presynaptic terminals at rest. Tg prolonged presynaptic Ca(2+) responses evoked by single action potentials with no detectable increase in the resting Ca(2+) level in nerve terminals. However, Ca(2+) accumulation was observed during high frequency stimulation. Tg induced a rapid rise in endplate potential (EPP) amplitude, accompanied by a delayed and transient increase. The effects appeared presynaptic, as suggested by the lack of effects of tg on the amplitude and time course of miniature EPPs (MEPPs). However, MEPP frequency was increased when preparations were stimulated tonically (0.2 Hz). The delayed and transient increase in EPP amplitude was occluded by injections of the Ca(2+) chelator BAPTA into PSCs before tg application, whereas a rise in intracellular Ca(2+) in PSCs induced by inositol 1,4,5-triphosphate (IP(3)) injections potentiated transmitter release. Furthermore, increased Ca(2+) buffering capacity after BAPTA injection in PSCs resulted in a more pronounced synaptic depression induced by high frequency stimulation of the motor nerve (10 Hz/80 sec). It is concluded that presynaptic Ca(2+) stores act as a Ca(2+) clearance mechanism to limit the duration of transmitter release, whereas Ca(2+) release from glial stores initiates Ca(2+)-dependent potentiation of synaptic transmission.

Voltage-gated Ca2+ channels and intracellular Ca2+ release regulate exocytosis in identified rat corticotrophs

1. The patch clamp technique was used in conjunction with a fluorescent Ca2+ indicator (indo-1, or indo-1FF) to measure simultaneously cytosolic Ca2+ concentration ([Ca2+]i) and exocytosis (changes in membrane capacitance) in single, identified rat corticotrophs. 2. Exocytosis could be stimulated by extracellular Ca2+ entry (via voltage-gated Ca2+ channels). A train of depolarizations could exhaust the pool of readily releasable granules and the pool replenished with a time constant of 42 s (at 22-25 C). 3. Recordings from cells with 0.5 mM intracellular cAMP showed that the amplitude of the depolarization-triggered exocytosis, the Ca2+ sensitivity of exocytosis, as well as the rate of replenishment of the readily releasable pool, were similar to the controls. 4. Exocytosis could also be stimulated by intracellular Ca2+ release from the inositol 1,4, 5-trisphosphate (IP3)-sensitive store (via flash photolysis of caged IP3). At comparable [Ca2+]i, extracellular Ca2+ entry and intracellular Ca2+ release had similar efficacy in triggering exocytosis. 5. The rate of exocytosis triggered via depolarization or intracellular Ca2+ release was much faster than that triggered via uniform elevation of [Ca2+]i (Ca2+ dialysis or flash photolysis of caged Ca2+). 6. The above findings suggest that both intracellular Ca2+ release and voltage-gated extracellular Ca2+ entry generate a spatial Ca2+ gradient, such that the local [Ca2+] near the exocytic sites was approximately 3-fold higher than the mean cytosolic [Ca2+]. However, neither cAMP nor the spatial Ca2+ gradient generated during depolarization could account for the high efficacy of corticotropin-releasing hormone (CRH) in stimulating adrenocorticotropic hormone (ACTH) secretion from corticotrophs.

New ratiometric fluorescent calcium indicators with moderately attenuated binding affinities

Mono-halogenated derivatives of the calcium indicators fura-2 and indo-1 were synthesized and their spectroscopic properties evaluated. Halogenation ortho or para to the bridging oxygen in the BAPTA nucleus had a more pronounced weakening effect on binding affinity than in the meta position in the fura derivatives. Two new excitation ratioable fluorescent calcium indicators, benzothiaza-1 and 2, were also synthesized. Kd values of 400 nM to 5.3 microM [Ca2+] were observed in these families of new probes.

Stimulation of Ca(2+)-independent exocytosis in rat pituitary gonadotrophs by G-protein

We employed the whole-cell recording technique in conjunction with fluorometry to measure cytosolic Ca(2+) concentration ([Ca(2+)](i)) and exocytosis (capacitance measurement) in single, identified rat gonadotrophs. Direct activation of G-protein (via intracellular dialysis of non-hydrolysable analogues of GTP, but not of GDP) triggered a slow rise in capacitance even in the presence of a fast intracellular Ca(2+) chelator. The broad-spectrum kinase inhibitors H7 and staurosporine did not prevent this Ca(2+)-independent exocytosis, ruling out the involvement of the cAMP and PKC pathways. AlF(4)(-), a potent stimulator of heterotrimeric G-proteins, failed to stimulate any exocytosis when the intracellular Ca(2+) store was depleted, implicating the involvement of AlF(4)(-)-insensitive G-protein(s). Maximal stimulation of Ca(2+)-independent exocytosis by GTP analogues did not reduce the number of readily releasable granules that were available subsequently for Ca(2+)-dependent release. The last finding raises the possibility that the G-protein-stimulated Ca(2+)-independent exocytosis may regulate a pool of granules that is distinct from the Ca(2+)-dependent pool.

Luteinizing hormone (LH) drives diverse intracellular calcium second messenger signals in isolated porcine ovarian thecal cells: preferential recruitment of intracellular Ca2+ oscillatory cells by higher concentrations of LH

The present study examines Ca2+ second messenger signaling driven by LH in isolated porcine thecal cells. To this end, we implemented semiquantitative fluorescent (fura-2) videomicroscopic imaging of single thecal cells in vitro. Stimulation of 388 cells with LH (5 microg/ml) elicited an intracellular Ca2+ ([Ca2+]i) signal in 85+/-5.3% of individual thecal cells (n = 11 experiments). Among 337 LH-responsive cells, we identified four predominant temporal modes of [Ca2+]i signaling: 1) [Ca2+]i oscillations with periodicities of 0.5 to 4.5 min(-1) (63+/-4.5%), 2) a [Ca2+]i spike followed by a sustained plateau (17+/-2.6%), 3) a [Ca2+]i spike only (5.8+/-2.6%); and 4) a [Ca2+]i plateau only (3.8+/-1.5%). The prevalence, but not the amplitude or frequency, of LH-induced [Ca2+]i oscillations in thecal cells was dependent on the agonist concentration. Reduced availability of extracellular Ca2+ induced by treatment with EGTA or cobaltous chloride did not block the initiation, but reversibly abolished ongoing [Ca2+]i oscillations (72% of cells) or increased the mean [Ca2+]i interspike periodicity from 1.09+/-0.16 to 0.59+/-0.07 min(-1) (P < 0.05). Putative phospholipase C inhibition with U-73122 (10 microM) also abolished or frequency-damped LH-driven [Ca2+]i oscillations in 95+/-4.7% of cells. [Ca2+]i oscillations in thecal cells were not abrogated by overnight pretreatment with pertussis toxin. We conclude that 1) thecal cells (unlike earlier findings in granulosa cells) manifest a diverse array of [Ca2+]i signaling responses to LH at the single cell level; 2) LH can dose dependently recruit an increasing number of individually [Ca2+]i oscillating thecal cells; 3) extracellular Ca2+ is required for LH to sustain (but not initiate) frequent and high amplitude [Ca2+] oscillations in thecal cells; and 4) these signaling actions of LH are mediated via phospholipase C, but not a pertussis-toxin sensitive mechanism. Accordingly, the present data extend the apparent complexity of LH-induced [Ca2+]i second messenger signaling and identify at the single cell level LH's dose-responsive drive of [Ca2+]i oscillations in gonadal cells.

Regulation of exocytosis via release of Ca(2+) from intracellular stores

The release of Ca(2+) from intracellular stores is an important trigger for secretion in many cell types. Depending on the spatial relationship between the intracellular Ca(2+) stores and the site of exocytosis, the Ca(2+) signal can be very local or spread throughout the entire cell. Here, we review how the release of Ca(2+) from inositol trisphospate (IP(3))-sensitive stores contributes differently to the stimulus-secretion coupling in three types of secretory cells (acinar cells of the pancreas, gonadotrophs, and corticotrophs of the anterior pituitary gland). We propose that in both pancreatic acinar cells and pituitary gonadotrophs the IP(3)-sensitive stores may be in close proximity to the sites of exocytosis such that the concentration of Ca(2+) at these sites are transiently much higher than the average cytosolic Ca(2+) concentration. In contrast, the local Ca(2+) gradient is less prominent in pituitary corticotrophs. Finally, some recent technical developments that may contribute significantly to future investigations of local Ca(2+) signals are discussed.