Control of secretion in anterior pituitary cells--linking ion channels, messengers and exocytosis

Nestin affects fusion pore dynamics in mouse astrocytes

AIM

Astrocytes play a homeostatic role in the central nervous system and influence numerous aspects of neurophysiology via intracellular trafficking of vesicles. Intermediate filaments (IFs), also known as nanofilaments, regulate a number of cellular processes including organelle trafficking and adult hippocampal neurogenesis. We have recently demonstrated that the IF protein nestin, a marker of neural stem cells and immature and reactive astrocytes, is also expressed in some astrocytes in the unchallenged hippocampus and regulates neurogenesis through Notch signalling from astrocytes to neural stem cells, possibly via altered trafficking of vesicles containing the Notch ligand Jagged-1.

METHODS

We thus investigated whether nestin affects vesicle dynamics in astrocytes by examining single vesicle interactions with the plasmalemma and vesicle trafficking with high-resolution cell-attached membrane capacitance measurements and confocal microscopy. We used cell cultures of astrocytes from nestin-deficient (Nes^-/- ) and wild-type (wt) mice, and fluorescent dextran and Fluo-2 to examine vesicle mobility and intracellular Ca²⁺ concentration respectively.

RESULTS

Nes^-/- astrocytes exhibited altered sizes of vesicles undergoing full fission and transient fusion, altered vesicle fusion pore geometry and kinetics, decreased spontaneous vesicle mobility and altered ATP-evoked mobility. Purinergic stimulation evoked Ca²⁺ signalling that was slightly attenuated in Nes^-/- astrocytes, which exhibited more oscillatory Ca²⁺ responses than wt astrocytes.

CONCLUSION

These results demonstrate at the single vesicle level that nestin regulates vesicle interactions with the plasmalemma and vesicle trafficking, indicating its potential role in astrocyte vesicle-based communication.

Astrogliopathology in the infectious insults of the brain

Astroglia, a heterogeneous type of neuroglia, play key homeostatic functions in the central nervous system (CNS) and represent an important defence system. Impaired homeostatic capacity of astrocytes manifests in diseases and this is mirrored in various astrocyte-based pathological features including reactive astrogliosis, astrodegeneration with astroglial atrophy and pathological remodelling of astrocytes. All of these manifestations are most prominently associated with infectious insults, mediated by bacteria, protozoa and viruses. Here we focus onto neurotropic viruses such as tick-borne encephalitis (TBEV) and Zika virus (ZIKV), both belonging to Flaviviridae and both causing severe neurological impairments. We argue that astrocytes provide a route through which neurotropic infectious agents attack the CNS, since they are anatomically associated with the blood-brain barrier and exhibit aerobic glycolysis, a metabolic specialisation of highly morphologically dynamic cells, which may provide a suitable metabolic milieu for proliferation of infectious agents, including viral bodies.

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.

Ångstrom-size exocytotic fusion pore: Implications for pituitary hormone secretion

In the past, vesicle content release was thought to occur immediately and completely after triggering of exocytosis. However, vesicles may merge with the plasma membrane to form an Ångstrom diameter fusion pore that prevents the exit of secretions from the vesicle lumen. The advantage of such a narrow pore is to minimize the delay between the trigger and the release. Instead of stimulating a sequence of processes, leading to vesicle merger with the plasma membrane and a formation of a fusion pore, the stimulus only widens the pre-established fusion pore. The fusion pore may be stable and may exhibit repetitive opening of the vesicle lumen to the cell exterior accompanied by a content discharge. Such release of vesicle content is partial (subquantal), and depends on fusion pore open time, diameter and the diffusibility of the cargo. Such transient mode of fusion pore opening was not confirmed until the development of the membrane capacitance patch-clamp technique, which enables high-resolution measurement of changes in membrane surface area. It allows millisecond dwell-time measurements of fusion pores with subnanometer diameters. Currently, the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) proteins are considered to be key entities in end-stage exocytosis, and the SNARE complex assembly/disassembly may regulate the fusion pore. Moreover, lipids or other membrane constituents with anisotropic (non-axisymmetric) geometry may also favour the establishment of stable narrow fusion pores, if positioned in the neck of the fusion pore.

Comparative proteomic analysis of pituitary glands from Huoyan geese between pre-laying and laying periods using an iTRAQ-based approach

In this study, we performed a comprehensive evaluation of the proteomic profile of the pituitary gland of the Huoyan goose during the laying period compared to the pre-laying period using an iTRAQ-based approach. Protein samples were prepared from pituitary gland tissues of nine pre-laying period and nine laying period geese. Then the protein samples from three randomly selected geese within each period were pooled in equal amounts to generate one biological sample pool. We identified 684 differentially expressed proteins, including 418 up-regulated and 266 down-regulated proteins. GO annotation and KEGG pathway analyses of these proteins were conducted. Some of these proteins were found to be associated with hormone and neurotransmitter secretion and transport, neuropeptide signalling and GnRH signalling pathways, among others. Subsequently, the modification of the abundance of three proteins (prolactin, chromogranin-A and ITPR3) was verified using western blotting. Our results will provide a new source for mining genes and gene products related to the egg-laying performance of Huoyan geese, and may provide important information for the conservation and utilization of local goose breeds.

Non-silent story on synonymous sites in voltage-gated ion channel genes

Synonymous mutations are usually referred to as “silent”, but increasing evidence shows that they are not neutral in a wide range of organisms. We looked into the relationship between synonymous codon usage bias and residue importance of voltage-gated ion channel proteins in mice, rats, and humans. We tested whether translationally optimal codons are associated with transmembrane or channel-forming regions, i.e., the sites that are particularly likely to be involved in the closing and opening of an ion channel. Our hypothesis is that translationally optimal codons are preferred at the sites within transmembrane domains or channel-forming regions in voltage-gated ion channel genes to avoid mistranslation-induced protein misfolding or loss-of-function. Using the Mantel-Haenszel procedure, which applies to categorical data, we found that translationally optimal codons are more likely to be used at transmembrane residues and the residues involved in channel-forming. We also found that the conservation level at synonymous sites in the transmembrane region is significantly higher than that in the non-transmembrane region. This study provides evidence that synonymous sites in voltage-gated ion channel genes are not neutral. Silent mutations at channel-related sites may lead to dysfunction of the ion channel.

Cell Cycle-Dependent Localization of Voltage-Dependent Calcium Channels and the Mitotic Apparatus in a Neuroendocrine Cell Line(AtT-20)

Changes in intracellular calcium are necessary for the successful progression of mitosis in many cells. Both elevation and reduction in intracellular calcium can disrupt mitosis by mechanisms that remain ill defined. In this study we explore the role of transmembrane voltage-gated calcium channels (CaV channels) as regulators of mitosis in the mouse corticotroph cell line (AtT-20). We report that the nifedipine-sensitive isoform CaV1.2 is localized to the "poleward side" of kinetechores during metaphase and at the midbody during cytokinesis. A second nifedipine-sensitive isoform, CaV1.3, is present at the mid-spindle zone in telophase, but is also seen at the midbody. Nifedipine reduces the rate of cell proliferation, and, utilizing time-lapse microscopy, we show that this is due to a block at the prometaphase stage of the cell cycle. Using Fluo-4 we detect calcium fluxes at sites corresponding to the mid-spindle zone and the midbody region. Another calcium dye, Fura PE3/AM, causes an inhibition of mitosis prior to anaphase that we attribute to a chelation of intracellular calcium. Our results demonstrate a novel, isoform-specific localization of CaV1 channels during cell division and suggest a possible role for these channels in the calcium-dependent events underlying mitotic progression in pituitary corticotrophs.

Alpha 1-adrenergic modulation of metabotropic glutamate receptor-induced calcium oscillations and glutamate release in astrocytes

Astrocytic responses to activation of metabotropic glutamate receptors group I (mGluRs I) and alpha(1)-adrenoreceptors in cultured cells have been assessed using spectral analyzes and calcium imaging. Concentration-dependent changes were observed after stimulation with the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG). These responses changed from a regular low frequency signal with sharp peaks at 1 microm to a pronounced stage of irregularity at 10 microm. After stimulation with 100 microm the signal was again homogenous in shape and regularity but occurred at a higher frequency. In contrast, the spectral properties after stimulation with the alpha(1)-adrenoreceptor agonist phenylephrine, exhibited considerable variation for all investigated concentrations. DHPG-induced increases in [Ca(2+)](i) were also associated with astroglial glutamate release, whereas no release was observed after noradrenergic stimulation. Both DHPG-mediated calcium signaling and glutamate release were inhibited by preincubation with 10 or 100 microm phenylephrine. Collectively, the present investigation provides new information about the spatial-temporal characteristics of astroglial intracellular calcium responses and demonstrates distinct differences between noradrenergic and glutamatergic receptors regarding intracellular calcium signaling and coupling to glutamate release. The noradrenergic modulation of DHPG-induced responses indicates that intracellular astroglial processes can be regulated in a bi-directional feedback loop between closely connected astrocytes and neurons in the central nervous system.

Molecular components of large conductance calcium-activated potassium (BK) channels in mouse pituitary corticotropes

Large-conductance calcium- and voltage- activated potassium (BK) channels play a fundamental role in the signaling pathways regulating mouse anterior pituitary corticotrope function. Here we describe the cloning and functional characterization of the components of mouse corticotrope BK channels. RT-PCR cloning and splice variant analysis of mouse AtT20 D16:16 corticotropes revealed robust expression of mslo transcripts encoding pore-forming alpha-subunits containing the mouse homolog of the 59-amino acid STREX-1 exon at splice site 2. RT-PCR and functional analysis, using the triterpenoid glycoside, DHS-1, revealed that native corticotrope BK channels are not functionally coupled to beta-subunits in vivo. Functional expression of the STREX-1 containing alpha-subunit in HEK 293 cells resulted in BK channels with calcium sensitivity, single-channel conductance, and inhibition by protein kinase A identical to that of native mouse corticotrope BK channels. This report represents the first corticotrope ion channel to be characterized at the molecular level and demonstrates that mouse corticotrope BK channels are composed of alpha-subunits expressing the mouse STREX-1 exon.

Glucocorticoid block of protein kinase C signalling in mouse pituitary corticotroph AtT20 D16:16 cells

1. The regulation of large conductance calcium- and voltage-activated potassium (BK) currents by activation of the protein kinase C (PKC) and glucocorticoid signalling pathways was investigated in AtT20 D16:16 clonal mouse anterior pituitary corticotroph cells. 2. Maximal activation of PKC using the phorbol esters, 4beta-phorbol 12-myristate, 13-acetate (PMA), phorbol 12, 13 dibutyrate (PDBu) and 12-deoxyphorbol 13-phenylacetate (dPPA) elicited a rapid, and sustained, inhibition of the outward steady-state voltage- and calcium- dependent potassium current predominantly carried through BK channels. 3. The effect of PMA was blocked by the PKC inhibitors bisindolylmaleimide I (BIS; 100 nM) and chelerythrine chloride (CHE; 25 microM) and was not mimicked by the inactive phorbol ester analogue 4alpha-PMA. 4. PMA had no significant effect on the 1 mM tetraethylammonium (TEA)-insensitive outward current or pharmacologically isolated, high voltage-activated calcium current. 5. PMA had no significant effect on steady-state outward current in cells pre-treated for 2 h with 1 microM of the glucocorticoid agonist dexamethasone. Dexamethasone had no significant effect on steady-state outward current amplitude or sensitivity to 1 mM TEA and did not block PMA-induced translocation of the phorbol ester-sensitive PKC isoforms, PKCalpha and PKCepsilon, to membrane fractions. 6. Taken together these data suggest that in AtT20 D16:16 corticotroph cells BK channels are important targets for PKC action and that glucocorticoids inhibit PKC signalling downstream of PKC activation.

Long-term effects of calcium availability on prolactin and protein synthesis in human decidual cells

Ever since decidual cells were recognized as the source of decidual prolactin (dPRL), very few reports have dealt with the role of calcium (Ca2+) on dPRL synthesis and release. In a recent work, we described the presence of T-type Ca2+ channels in these cells, giving Ca(2+)-dependent action potentials. However, we failed to demonstrate any action of decidual cell Ca2+ modulation on acute dPRL release, but observed only long-term effects. We have now investigated these effects on decidual protein and dPRL synthesis after 24 h treatments. When Ca2+ channel blockers or EGTA (2 mM) were added to the culture medium, dPRL release and [3H] leucine incorporation into proteins decreased. Increasing external Ca2+ up to 2 mM instead of 0.8 mM or changing the external K+ concentration (30 mM instead of 5.6) had no consequence on dPRL release, whereas 2 mM of Ca2+ enhanced total protein synthesis. No toxicity was noted with these treatments. Finally a possible effect of Ca2+ modulation on dPRL synthesis was studied using [35S] methionine. The specific activity of [35S] methionine on dPRL was similar in control and treated cells (EGTA, 2 mM Ca2+, cobalt). These results support the idea that Ca2+ controls dPRL synthesis in decidual cells, acting only on general protein synthesis processes.

The involvement of dihydropyridine-sensitive calcium channels in phorbol ester-induced luteinizing hormone and growth hormone release

We examined the role of voltage-activated, L-type, Ca2+ channels in phorbol ester-induced luteinizing hormone (LH) and growth hormone (GH) release from rat anterior pituitary tissue. The L-type Ca2+ channel inhibitor, nimodipine (NMD), inhibited phorbol 12,13-dibutyrate (PDBu)-induced GH release but had no significant effect on LH release. The L-type Ca2+ channel activator BAY K 8644 had no effect on PDBu-induced GH release but potentiated PDBu-induced LH release. In contrast, 60 mM K(+)-induced LH and GH release were inhibited by NMD, whereas BAY K 8644 had no effect. When PDBu and either K+ or BAY K 8644 were used together, they acted synergistically to evoke levels of LH release greater than addition of release caused by each secretagogue alone. However, the release of GH was additive with PDBu and either K+, BAY K 8644. The protein kinase C (PKC) inhibitor staurosporine inhibited both PDBu-induced LH release and GH release. A structurally different PKC inhibitor, H7, significantly inhibited PDBu-induced LH release but had no effect on PDBu-induced GH release. Both staurosporine and H7 inhibited LH release induced by PDBu and BAY K 8644 together. In contrast, although staurosporine inhibited GH release induced by PDBu and BAY K 8644, H7 significantly potentiated this response. A difference in the action of these two inhibitors was also apparent on K(+)-induced hormone release where staurosporine partially blocked K(+)-induced LH and GH release but H7 had no effect on the release of either hormone. Data obtained in 45Ca2+ influx experiments further suggested that a staurosporine-sensitive, but H7-resistant, PKC-like kinase may tonically maintain L-channels in a voltage-sensitive state, as down-regulation of PKC in dispersed anterior pituitary cells by long term PDBu treatment caused a significant reduction in K(+)-induced 45Ca2+ influx. We conclude that phorbol ester-induced GH release, but not LH release, is a result of L-type Ca2+ channel activation which may occur by means of alterations in the channel itself to increase its responsiveness to a given depolarisation.

Activation of calcium entry by cyclopiazonic acid in thyroid FRTL-5 cells

The aim of the present study was to investigate whether the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA) could empty intracellular Ca2+ stores and activate Ca2+ influx in thyroid FRTL-5 cells. Addition of CPA to Fura-2 loaded cells rapidly increased intracellular free Ca2+ ([Ca2+]i) which then stabilized at a new elevated steady state level. The initial increase was mainly dependent on the release of sequestered Ca2+, but was decreased in Ca(2+)-free buffer and in depolarized cells. The plateau phase was totally dependent on extracellular Ca2+. Addition of Ca2+ to cells exposed to CPA in Ca(2+)-free buffer rapidly increased [Ca2+]i. This influx was decreased in depolarized cells and inhibited by SKF 96365. Addition of CPA to cells prior to stimulating the cells with ATP totally abolished the ATP-induced increase in [Ca2+]i. In Ca(2+)-free buffer, addition of ATP prior to CPA decreased the response in [Ca2+]i evoked by CPA. The results show that emptying intracellular Ca2+ stores with CPA rapidly activates influx of Ca2+ in FRTL-5 cells. Furthermore, ATP and CPA appear to release Ca2+, at least in part, from the same intracellular Ca2+ store in these cells.

Acute and long-term biphasic volume alterations in rat type-II somatotrophs during GH secretory stimulation

The size and shape of growth hormone (GH)-producing rat (type II)-somatotrophs was studied during secretory stimulation by either human GH-releasing hormone (hGHRH(1-29)) or 50 nM extracellular potassium. A new type of perfusion chamber for light microscopy allowed the interpretation of early changes in cell morphology. The exposure to 10 nM hGHRH caused a significant transient decrease in cell volume to 94.4 +/- 2.1% within 2 s. The cell volume recovered to 99.2 +/- 1.3% at 30 s. A second, more gradual volume decrease then followed (60 s), which stabilized at about 92.5 (8 min) and was still present after 20 min of continuous hGHRH exposure. Potassium, 50 mM, gave an immediate and persistent cell volume increase of about 5%. The fluctuation in rat somatotroph volume after hGHRH exposure correlates to a previously observed biphasic GH-secretory pattern involving an initial burst secretion and a second slow phase secretion. The transient early volume decrease may reflect ion fluxes across the membrane and/or the response of the cytoskeleton to calcium mobilization during the GH-secretory onset. The second, persistent, volume decrease closely correlates to the calculated volume of lost GH vesicles.

Evidence for receptor-mediated calcium entry and refilling of intracellular calcium stores in FRTL-5 rat thyroid cells

The aim of the present study was to investigate the relationship between agonist-induced changes in intracellular free Ca2+ ([Ca2+]i) and the refilling of intracellular Ca2+ stores in Fura 2-loaded thyroid FRTL-5 cells. Stimulating the cells with ATP induced a dose-dependent increase in ([Ca2+]i). The ATP-induced increase in [Ca2+]i was dependent on both release of sequestered intracellular Ca2+ as well as influx of extracellular Ca2+. Addition of Ni2+ prior to ATP blunted the component of the ATP-induced increase in [Ca2+]i dependent on influx of Ca2+. In cells stimulated with ATP in a Ca(2+)-free buffer, readdition of Ca2+ induced a rapid increase in [Ca2+]i; this increase was inhibited by Ni2+. In addition, the ATP-induced influx of 45Ca2+ was blocked by Ni2+. Stimulating the cells with noradrenaline (NA) also induced release of sequestered Ca2+ and an influx of extracellular Ca2+. When cells were stimulated first with NA, a subsequent addition of ATP induced a blunted increase in [Ca2+]i. If the action of NA was terminated by addition of prazosin, and ATP was then added, the increase in [Ca2+]i was restored to control levels. Addition of Ni2+ prior to prazosin inhibited the restoration of the ATP response. In the presence of extracellular Mn2+, ATP stimulated quenching of Fura 2 fluorescence. The quenching was probably due to influx of Mn2+, as it was blocked by Ni2+. The results thus suggested that stimulating release of sequestered Ca2+ in FRTL-5 cells was followed by influx of extracellular Ca2+ and rapid refilling of intracellular Ca2+ stores.

Depolarization of the membrane potential decreases the ATP-induced influx of extracellular Ca2+ and the refilling of intracellular Ca2+ stores in rat thyroid FRTL-5 cells

The aim of the present study was to investigate the effect of membrane depolarization on ATP-induced changes in intracellular Ca2+ ([Ca2+]i) and the refilling of intracellular Ca2+ stores in thyroid follicular FRTL-5 cells. Depolarizing the cells with 50 mM K+, an amount sufficient to almost totally depolarize the cells as determined by bisoxonal, significantly reduced the ATP-induced uptake of 45Ca2+. This effect was not dependent on an enhanced efflux of Ca2+, as no difference in the ATP-induced efflux of 45Ca2+ was obtained between control cells and depolarized cells. The ATP-induced transient increase in [Ca2+]i in Fura-2 loaded cells was not altered by depolarization, whereas the ATP-induced plateau in [Ca2+]i was decreased compared with control cells. Furthermore, in cells stimulated with ATP in a Ca(2+)-free buffer, readdition of Ca2+ after the termination of the ATP response induced a decreased response in [Ca2+]i in depolarized cells. Refilling of intracellular Ca2+ stores was investigated by first stimulating the cells with noradrenaline (NA). The effect of NA was then terminated with prazosin, and the cells restimulated with ATP. In cells depolarized with high K+, the response to ATP was decreased compared with that seen in control cells. The results thus suggest that both the ATP-induced influx of extracellular Ca2+ and the refilling of intracellular Ca2+ stores is decreased in depolarized FRTL-5 cells.

Neuropeptide inhibition of voltage-gated calcium channels mediated by mobilization of intracellular calcium

Many neurotransmitters and hormones regulate secretion from endocrine cells and neurons by modulating voltage-gated Ca2+ channels. One proposed mechanism of neurotransmitter inhibition involves protein kinase C, activated by diacylglycerol, a product of phosphatidyl-inositol inositol hydrolysis. Here we show that thyrotropin-releasing hormone (TRH), a neuropeptide that modulates hormone secretion from pituitary tumor cells, inhibits Ca2+ channels via the other limb of the phosphatidylinositol signaling system: TRH causes inositol trisphosphate-triggered Ca2+ release from intracellular organelles, thus causing Ca2(+)-dependent inactivation of Ca2+ channels. Elevation of intracellular Ca2+ concentration is coincident with the onset of TRH-induced inhibition and is necessary and sufficient for its occurrence. The inhibition is blocked by introducing Ca2+ buffers into cells and mimicked by a variety of agents that mobilize Ca2+. Treatments that suppress protein kinase C have no effect on the inhibition. Hence inactivation of Ca2+ channels occurs not only as a result of Ca2+ influx through plasma membrane channels, but also via neurotransmitter-induced Ca2+ mobilization. This phenomenon may be common but overlooked because of the routine use of Ca2+ buffers in patch-clamp electrodes.

Cytotoxicity of equinatoxin II from the sea anemone Actinia equina involves ion channel formation and an increase in intracellular calcium activity

Equinatoxin II is a 20-kDa basic protein isolated from the sea anemone Actinia equina. The aim of our work was to investigate the primary molecular basis for the cytotoxic effects of equinatoxin II in two model systems: single bovine lactotrophs and planar lipid bilayers. Previous work has shown that equinatoxin II produces rapid changes in cell morphology, which are dependent on external calcium. It has also been reported that addition of equinatoxin II increases membrane electrical conductance, which suggests that the cytotoxic action of equinatoxin II involves an increase in the permeability of membranes to Ca2+. Extensive changes in cytosolic Ca2+ activity are thought to invoke irreversible changes in cell physiology and morphology. In this paper, we show that morphological changes brought about by equinatoxin II in bovine lactotrophs are associated with a rapid rise in cytosolic Ca2+ activity, monitored with a fura-2 video imaging apparatus. Moreover, incorporation of equinatoxin II into planar lipid bilayers produces Ca2+ permeable ion channels. This suggests that the mode of equinatoxin II cytotoxicity involves the formation of cation (Ca2+) permeable channels in cell membranes.

Hypoxic suppression of K+ currents in type I carotid body cells: selective effect on the Ca2(+)-activated K+ current

Whole-cell K+ currents were recorded in isolated type I carotid body cells using the patch-clamp technique. Hypoxia (pO2 25 torr) reversibly suppressed K+ currents in a voltage-dependent manner: maximal effects were seen at low, positive test potentials, where the Ca2(+)-activated component of K+ currents was greatest. Enhancing this component with 5 microM BAY K 8644 exaggerated the effects of hypoxia, and when the component was inhibited (100 microM Cd2+ or 5 microM nifedipine) hypoxic effects were abolished. As hypoxia does not affect Ca2+ currents directly, these data indicate the suppressive effect of hypoxia is selective for the Ca2(+)-activated component of K+ currents in type I cells.

Whole-cell K+ currents in type II pneumocytes freshly isolated from rat lung: pharmacological evidence for two subpopulations of cells

The patch clamp technique was used to record whole cell K+ currents in type II pneumocytes freshly isolated from adult rats. Depolarizing voltage steps evoked outward K+ currents which were distinguished into low and high threshold types, only one type being apparent in any one cell. Low-threshold (LT) currents were activated at test potentials of -40 mV to -20 mV and were reduced in amplitude by 20 mM tetraethylammonium (TEA). High-threshold (HT) currents were activated only at test potentials positive to -20 mV and current noise was always greater than for LT currents. HT currents were also significantly more sensitive than were LT currents to block by TEA. Quinine (1 mM) blocked LT currents reversibly at all activating test potentials. HT currents were also reversibly blocked by 1 mM quinine, but in a voltage-dependent manner, the degree of block increasing with increasing test potential. 4-Aminopyridine (2 mM) further distinguished the two current types: it was virtually without effect on HT currents but caused large reductions in LT current amplitudes, apparently by acting on the open channels underlying this current. These data clearly distinguish type II pneumocytes into two subpopulations and suggest that they may play separate roles in the functioning of the intact alveolar epithelium.