The Role of Nicotinic Receptors on Ca2+ Signaling in Bovine Chromaffin Cells

Chromaffin cells have been used as a physiological model to understand neurosecretion in mammals for many years. Nicotinic receptors located in the cells' membrane are stimulated by acetylcholine, and they participate in the exocytosis of chromaffin granules, releasing catecholamines in response to stress. In this work, we discuss how the participation of nicotinic receptors and the localization of active zones in the borders of the cytoskeleton can generate local calcium signals leading to secretion. We use a computational model of a cytoskeleton cage to simulate Ca2+ levels in response to voltage and acetylcholine pulses. We find that nicotinic receptors are able to enhance the differences between local and average calcium values, as well as the heterogeneous distributions around the active zones, producing a non-linear, highly localized Ca2+ entry that, although consisting of a few ions, is able to improve secretion responses in chromaffin cells. Our findings emphasize the intricate interplay among nicotinic receptors, the cytoskeleton, and active zones within chromaffin cells as an example of Ca2+-dependent neurosecretion in mammals.

Sphingomyelin derivatives increase the frequency of microvesicle and granule fusion in chromaffin cells

Sphingomyelin derivatives like sphingosine have been shown to enhance secretion in a variety of systems, including neuroendocrine and neuronal cells. By studying the mechanisms underlying this effect, we demonstrate here that sphingomyelin rafts co-localize strongly with synaptosomal-associated protein of 25Kda (SNAP-25) clusters in cultured bovine chromaffin cells and that they appear to be linked in a dynamic manner. In functional terms, when cultured rat chromaffin cells are treated with sphingomyelinase (SMase), producing sphingomyelin derivatives, the secretion elicited by repetitive depolarizations is enhanced. This increase was independent of cell size and it was significant 15min after initiating stimulation. Interestingly, by evaluating the membrane capacitance we found that the events in control untreated cells corresponded to two populations of microvesicles and granules, and the fusion of both these populations is clearly enhanced after treatment with SMase. Furthermore, SMase does not increase the size of chromaffin granules. Together, these results strongly suggest that SNARE-mediated exocytosis is enhanced by the generation of SMase derivatives, reflecting an increase in the frequency of fusion of both microvesicles and chromaffin granules rather than an increase in the size of these vesicles.

A theoretical study of factors influencing calcium-secretion coupling in a presynaptic active zone model

A theoretical analysis of some of the relevant factors influencing the calcium time course and the strength and timing of release probabilities of vesicles evoked by an action potential in a calyx-type active zone is presented in this paper. In particular, our study focus on the comparison of cooperative vs non-cooperative calcium binding by the release site and the effect of the number of Ca(2+) binding sites on the calcium sensitivity for release. Regarding the comparison of cooperative and non-cooperative kinetic schemes, our simulations show that quite different results are obtained when considering one or another: a reduction in the release probability of more than a 50% is obtained when considering the cooperative kinetic scheme. Also, a delay in the average time for release appears when using this model for the calcium sensor. Our study also shows that a non-cooperative kinetic binding scheme gives rise to a well defined average calcium level for release assuming that the same kinetic constants are considered for all the sites. Our results also suggest that the central value of the calcium sensitivity for release depends on the number of binding sites N and the dissociation constant KD with a scaling law depending on NKD.

P2X7 receptors trigger ATP exocytosis and modify secretory vesicle dynamics in neuroblastoma cells

Previously, we reported that purinergic ionotropic P2X7 receptors negatively regulate neurite formation in Neuro-2a (N2a) mouse neuroblastoma cells through a Ca²⁺/calmodulin-dependent kinase II-related mechanism. In the present study we used this cell line to investigate a parallel though faster P2X7 receptor-mediated signaling pathway, namely Ca²⁺-regulated exocytosis. Selective activation of P2X7 receptors evoked exocytosis as assayed by high resolution membrane capacitance measurements. Using dual-wavelength total internal reflection microscopy, we have observed both the increase in near-membrane Ca²⁺ concentration and the exocytosis of fluorescently labeled vesicles in response to P2X7 receptor stimulation. Moreover, activation of P2X7 receptors also affects vesicle motion in the vertical and horizontal directions, thus, involving this receptor type in the control of early steps (docking and priming) of the secretory pathway. Immunocytochemical and RT-PCR experiments evidenced that N2a cells express the three neuronal SNAREs as well as vesicular nucleotide and monoamine (VMAT-1 and VMAT-2) transporters. Biochemical measurements indicated that ionomycin induced a significant release of ATP from N2a cells. Finally, P2X7 receptor stimulation and ionomycin increased the incidence of small transient inward currents, reminiscent of postsynaptic quantal events observed at synapses. Small transient inward currents were dependent on extracellular Ca²⁺ and were abolished by Brilliant Blue G, suggesting they were mediated by P2X7 receptors. Altogether, these results suggest the existence of a positive feedback mechanism mediated by P2X7 receptor-stimulated exocytotic release of ATP that would act on P2X7 receptors on the same or neighbor cells to further stimulate its own release and negatively control N2a cell differentiation.

Cytoskeletal control of vesicle transport and exocytosis in chromaffin cells

Chromaffin cell exocytosis is a fascinating interplay between secretory vesicles and cellular components. One of these components is the cytoskeleton and its associated regulatory proteins. Transport of chromaffin secretory granules from their site of biosynthesis towards the active site of exocytosis requires both F-actin fine remodelling as well as microtubule trails. At least two molecular motors, myosins II and V, seem to play a crucial role in the control of F-actin dynamics and vectorial vesicle displacement respectively. Vesicle movement experiences spatial restrictions as they approach the cell cortical region, where the F-actin meshwork constitutes a barrier-limiting vesicle access to the plasmalemma. During secretion, cortical F-actin is locally disrupted providing access of vesicles to release sites on the plasmalemma. Removal of the stimulus restores cortical F-actin. Two pathways (Ca2+-scinderin and PKC-MARCKS) control F-actin changes during the secretory cycle . Furthermore, GTPases such as RhoA, that controls F-actin network integrity, and Cdc42 signalling which induces the formation of local actin filaments at active sites, provide additional evidence on the importance of F-actin as a key element in vesicle transport and in the exocytotic machinery of chromaffin cells.

Vesicle movements are governed by the size and dynamics of F-actin cytoskeletal structures in bovine chromaffin cells

Dense vesicles can be observed in live bovine chromaffin cells using fluorescent reflection confocal microscopy. These vesicles display a similar distribution, cytoplasmic density and average size as the chromaffin granules visualized by electron microscopy. In addition, the acidic vesicles labeled with Lysotracker Red comprised a subpopulation of the vesicles that are visualized by reflection fluorescence. A combination of fluorescence reflection and transmitted light images permitted the movements of vesicles in relation to the cortical cytoskeleton to be studied. The movement of vesicles located on the outside of this structure was restricted, with an apparent diffusion coefficient of 1.0+/-0.4 x 10(-4) microm(2)/s. In contrast, vesicles located in the interior moved much more freely and escaped from the visual confocal plane. Lysotracker labeling was more appropriate to study the movement of the faster moving vesicles, whose diffusion coefficient was five times higher. Using this type of labeling we confirmed the restriction on cortical movement and showed a clear relationship between vesicle mobility and the kinetics of cytoskeletal movement on both sides of the cortical cytoskeleton. This relationship was further emphasized by studying cytoskeletal organization and kinetics. Indeed, an estimate of the size of the cytoskeletal polygonal cages present in the cortical region and in the cell interior agreed well with the calculation of the theoretical radius of the cages imprisoning vesicle movement. Therefore, these data suggest that the structure and kinetics of the cytoskeleton governs vesicle movements in different regions of chromaffin cells.

Comparative analysis of cognitive impairment among Mexicans and Spanish-speaking immigrant's elders

OBJECTIVE

This study aims to compare the prevalence of cognitive impairment among older Mexicans and Spanish-speaking U.S. immigrants.

METHODS

The relationship of cognitive impairment with social and demographic variables was explored using data from five different population based survey studies.

RESULTS

Prevalence rates increased with age, with low educational level, didn't show a clear association with gender and decreased in married subjects. Estimates of cognitive and functional impairment combined are greater in the immigrant samples than those for Mexican residents. Primary memory measures are the least discriminative for cognitive impairment, whereas orientation, attention, and secondary verbal memory measures discriminate better.

DISCUSSION

Mexicans and Spanish-speaking USA immigrants have similar prevalence estimates of cognitive impairment. However, prevalence of functional impairment differs between Mexicans and immigrants because of measurement issues and cultural factors.

Cognition, Functional Status, Education, and the Diagnosis of Dementia and Mild Cognitive Impairment in Spanish-Speaking Elderly

A group of 314 Spanish-speaking elders were classified in 55 participants with mild to moderate dementia, 74 participants with mild cognitive impairment (MCI), and 185 control participants, according to clinical evaluation derived. Sensitivity, specificity, and detection characteristics of frequently cognitive and functional tests were calculated in comparison with the clinical evaluation: Minimental State Examination, Brief Neuropsychological Test Battery, Short Blessed test, Pfeffer Functional Activities Questionnaire, and Blessed Dementia Scale. Influence of education on sensitivity and specificity values varied along the tests. For all the cognitive and functional measures, a great number of MCI participants who fulfilled Mayo's (Mayo's Clinical School) clinical criteria (Petersen et al., 1999) were misclassified as controls and a few were misclassified as demented. Level of education plays a very important role in both cognitive and functional assessment. The cognitive tests that are commonly used to screen demented patients may fail to detect MCI particularly in high-functioning individuals as well as those who are well educated.

Overweight in elderly Mexican women of a marginal community

Obesity is a main risk factor in the development of diseases, which are causes of death in elderly population. Socio-economic characteristics had an impact in the prevalence of obesity. The purpose of this study was to estimate the prevalence of obesity in elderly Mexican women living in a marginal community and compare their results with those of women of urban and rural areas in Mexico. The study has a cross sectional design. Standardised nutritionist carried out the measurements. The international techniques recommended were used to measure weight and height. To estimate the proportion of overweight and obesity, the body mass index (BMI) was used following the WHO categories. A total of 249 elderly women were examined. The mean age was 71.6 (sd 7.8). No difference in age was found in the three communities visited (p>0.05). The mean BMI in the urban women was 26.7 (sd 4.6), in the rural 24.5 (sd 3.9), and in the marginal group 28.6 (sd 5.5) (p<0.0001). The proportion of overweight and obese women was in the urban group 60.7%, in the rural 36.2%, and in the marginal women 76.5% (p<0.001). The results of this study indicated the presence of a high prevalence of overweight and obesity in elderly women of urban and particularly in the marginal areas. Marginality seams to be a risk marker of obesity in elderly women in Mexico. Intervention programs aimed to decrease the prevalence of obesity and improve nutritional condition in this group are urgent.

Co-localization of vesicles and P/Q Ca2+-channels explains the preferential distribution of exocytotic active zones in neurites emitted by bovine chromaffin cells

We have taken advantage of the differences between the preferential localization of secretion in the terminals of neurite-emitting bovine chromaffin cells in contrast with the random distribution secretion in spherical cells to study the possible molecular factors determining such localization by using immunofluorescence and confocal microscopy techniques. By analyzing the distribution of dopamine beta-hydroxylase present in the membrane of chromaffin granules, we found that vesicles migrate and accumulate in dense packages in the terminals of neurite processes. Neither members of the fusion core complex such as SNAP-25, nor nicotinic receptors are preferentially located in the terminals as would be expected from elements defining sites of release, thereby suggesting the presence of additional factors. Interestingly, we observed a preferential distribution of the P/Q subtype of Ca2+ channels in these neurite terminals and co-localization with vesicles present in these structures, in sharp contrast with the overall distribution of the L subtype channels. Using the same immunofluorescence techniques we were unable to detect N-type calcium channels. In addition, omega-agatoxin IVA was able to block 70% of the exocytotic release occurring into the neurites, whereas L-type blockers had a weak effect. Taken together our results strongly indicate that the co-localization of vesicles and clusters of P/Q Ca2+ channels may explain the precise localization of exocytotic sites in the terminals of neurite-emitting chromaffin cells, whereas the distribution of secretory sites in round cells may arise from the random presence of these factors as indicated by their partial co-localization.

Temperature and PMA affect different phases of exocytosis in bovine chromaffin cells

Amperometry was used to study secretory kinetics of single bovine chromaffin cells stimulated by transient depolarizations at different temperatures. The initial rate of release was moderately enhanced when the temperature was raised from 18 to 22 and 37 degrees C. Secretion increased drastically at a later period, 5-10 s after the initiation of stimulus. Interestingly, incubation of the cells with phorbol 12-myristate 13-acetate (PMA) clearly enhanced fast secretory components. In addition, the rate of secretion of the slower component recruited by prolonged depolarizations (t > 30 s) was unaffected at the range of temperatures normally used in secretory experiments (22-37 degrees C). A 'counting events' analysis of secretion, which avoids the influence of event charge changes, showed specific increases in a population of vesicles fusing between 7 and 12 s over the same range of temperatures, and a marked increase in vesicles fusing during the initial phase (1-5 s), of PMA-treated cell secretion. An analysis of temperature influence on transient components released by high sucrose, the secretion elicited by cell permeabilization with digitonin, and studies of the individual characteristics of amperometric events, allow us to conclude that an increase in the size of a secondary-released vesicle population is the main factor contributing to temperature-dependent enhancement of secretion, in clear contrast to the enhancement of fast releasable pools caused by phorbol esters.

The F-actin cytoskeleton modulates slow secretory components rather than readily releasable vesicle pools in bovine chromaffin cells

Adrenal chromaffin cells were used to test the role of the peripheral cytoskeleton of F-actin in controlling different vesicle pools. Phorbol 12-myristate 13-acetate and calyculin A, two substances affecting phosphorylation-dephosphorylation cycles, produced different degrees of F-actin reorganization, inducing the partial and the almost total disassembly of this structure, respectively, as visualized using rhodamine-phalloidin staining. Consequently, electron microscopy studies revealed the higher efficiency of calyculin-A over phorbol 12-myristate 13-acetate in promoting vesicle access to the plasmalemma boundary. Surprisingly, only the phorbol ester enhanced fast kinetics and the population of rapidly releasable vesicle pools as studied by single-cell amperometry, whereas both agents, as well as the F-actin severing compound, Latrunculin A, promoted an increase in the population of vesicles recruited in response to prolonged or repetitive stimulations. Taken together, our data support the notion that the F-actin peripheral barrier controls primary granule recruitment from reserve vesicle pools, whereas the phorbol ester effect on the rapidly releasable pools might be related to the alteration of late secretory stage through protein kinase C-dependent phosphorylation of an unidentified target.

A single amino acid near the C terminus of the synaptosomeassociated protein of 25 kDa (SNAP-25) is essential for exocytosis in chromaffin cells

Amperometry in chromaffin cells expressing green fluorescent protein (GFP) fused to synaptosome-associated protein of 25 kDa (SNAP-25) have been used to test the involvement of single amino acids in exocytotic function, overcoming some of the limitations of studies based on Botulinum neurotoxin cleavage, as this occurs at defined sites of the protein. Constructs containing either the whole SNAP-25 polypeptide or several deleted forms lacking its C-terminal domain were heavily overexpressed in transfected cells. All GFP-fusions were located in both the cytoplasm and the plasma membrane. Although a construct containing complete SNAP-25 sustained normal secretion, removal of four or more amino acids of its C terminus greatly altered the overall rate and extent of exocytosis. Further mutational analysis proved that Leu203, the fourth residue from the C terminus, is critical for secretion. Kinetics of single granule fusions from cells expressing truncated forms showed slow onset and decay times when compared with control cells expressing full SNAP-25. Thus, these data provide direct evidence for the involvement of a specific residue of SNAP-25 in exocytosis and show that overexpression of GFP-SNAP contructs combined with single vesicle fusion measurements constitutes a powerful approach to dissect the structural elements playing a role in individual steps of the exocytotic cascade.

Dual effects of botulinum neurotoxin A on the secretory stages of chromaffin cells

Truncation of the C-terminal domain of the synaptosomal associated protein of 25 kDa (SNAP-25) by botulinum neurotoxin A (BoNT A) has been shown to block neuroendocrine cell secretion. It is unclear, however, if toxin mechanism involved the affection of a single or more events of the exocytotic cascade. BoNT A induced changes in both the degree of inhibition and the kinetics of catecholamine secretion from populations of cultured bovine chromaffin cells. Ca2+-dependent secretion from digitonin-permeabilized cells showed partial inhibition associated with the alteration of a slow secretory phase at different toxin concentrations. In contrast, in intact cells stimulated by depolarization, cell treatment with low concentrations (1 nM) of the toxin affected the late phase of secretion, whereas 100 nM BoNT A-poisoned cells showed an alteration even of fast components. The high degree of inhibition associated with fast secretory component alteration was dependent on Ca2+ entry through the Ca2+ channels, as it was absent from cells permeated with the A23187 Ca2+ ionophore. Vesicle pools implicated in the effect of BoNT A on the secretory response from single cells were identified using amperometry. These studies supported the macroscopic view by showing that secretion from BoNT A-treated permeabilized cells presented specific inhibition of late vesicle fusions. Intact cells showed alterations in the late vesicle pool (t1/2 = 39 s) recruited during prolonged or repetitive KCI depolarizations using 1 nM BoNT A-treated cells as well as in an intermediate kinetic pool (t1/2 = 18 s) at higher toxin concentrations (100 nM). The faster resolved component (t1/2 = 8 s) or the membrane fusion event itself were not affected. Our results demonstrate that removal of the last nine C-terminal amino acids of SNAP-25 by BoNT A has a specific effect on two different and distal secretory stages in chromaffin cells.

The 26-mer peptide released from SNAP-25 cleavage by botulinum neurotoxin E inhibits vesicle docking

Botulinum neurotoxin E (BoNT E) cleaves SNAP-25 at the C-terminal domain releasing a 26-mer peptide. This peptide product may act as an excitation-secretion uncoupling peptide (ESUP) to inhibit vesicle fusion and thus contribute to the efficacy of BoNT E in disabling neurosecretion. We have addressed this question using a synthetic 26-mer peptide which mimics the amino acid sequence of the naturally released peptide, and is hereafter denoted as ESUP E. This synthetic peptide is a potent inhibitor of Ca2+-evoked exocytosis in permeabilized chromaffin cells and reduces neurotransmitter release from identified cholinergic synapses in in vitro buccal ganglia of Aplysia californica. In chromaffin cells, both ESUP E and BoNT E abrogate the slow component of secretion without affecting the fast, Ca2+-mediated fusion event. Analysis of immunoprecipitates of the synaptic ternary complex involving SNAP-25, VAMP and syntaxin demonstrates that ESUP E interferes with the assembly of the docking complex. Thus, the efficacy of BoNTs as inhibitors of neurosecretion may arise from the synergistic action of cleaving the substrate and releasing peptide products that disable the fusion process by blocking specific steps of the exocytotic cascade.

Preferential localization of exocytotic active zones in the terminals of neurite-emitting chromaffin cells

Amperometry using 2.5 microm radius carbon fiber electrodes was employed to study exocytotic catecholamine release from individual cultured bovine chromaffin cells. The secretory responses to either direct depolarization or nicotinic receptor stimulation were focal in nature in both round and neurite-emitting cells. In contrast to the random distribution of active sites found in round cells, bipolar and tripolar chromaffin cells had responsive zones preferentially located at neurite terminals as indicated by the lower probability of finding "silent" electrode positions and an increased nicotinic-receptor responsiveness when compared with the cell body. In agreement with these data we have observed a preferential deposition of dopamine-beta-hydroxylase into the neurite terminal plasmalemma after stimulation of intact cells. These observations might be of interest since the differences in the distribution of secretory "spots" between round and neurite-emitting chromaffin cells could be used to study the molecular factors determining active site localization.

The role of nicotinic receptors and calcium channels in mipafox induced inhibition of catecholamine release in bovine chromaffin cells

Depolarization induced catecholamine release from chromaffin cells was decreased 28% by N,N'-diisopropyl diamido-phosphorofluoridate (mipafox), an organophosphorus compound (OP) causing neurotoxic effects, while secretion stimulated by nicotinic agonist was inhibited 65%. The reversibility of this effect and the fact that calcium-dependent secretion from digitonin-permeabilized cells was unaffected by mipafox suggest that this compound affects the ionic currents implicated in catecholamine release. Patch-clamp experiments showed that the activity of voltage-dependent calcium channels (VDCC) was inhibited 35% by mipafox being this effect reversible whereas only minor effects were detected on Na(+) and K(+) currents. Finally, we studied the effect of mipafox on nicotinic ionic currents in chromaffin cells. In this case, the OP was able to cause reversible inhibition reaching maximal effects of 50-60%. In conclusion, nicotinic receptors and VDCC should be considered as potential targets in order to understand the neurotoxicity of these chemicals.

Inhibition of nicotinic receptor-mediated responses in bovine chromaffin cells by diltiazem

1. The effects of diltiazem on various functional parameters were studied in bovine cultured adrenal chromaffin cells stimulated with the nicotinic receptor agonist dimethylphenylpiperazinium (DMPP) or with depolarizing Krebs-HEPES solutions containing high K+ concentrations. 2. The release of [3H]-noradrenaline induced by DMPP (100 microM for 5 min) was gradually and fully inhibited by increasing concentrations of diltiazem (IC50 = 1.3 microM). In contrast, the highest concentration of diltiazem used (10 microM) inhibited the response to high K+ (59 mM for 5 min) by only 25%. 3. 45Ca2+ uptake into cells stimulated with DMPP (100 microM for 1 min) was also blocked by diltiazem in a concentration-dependent manner (IC50 = 0.4 microM). Again, diltiazem blocked the K(+)-evoked 45Ca2+ uptake (70 mM K+ for 1 min) only by 20%. In contrast, the N-P-Q-type Ca2+ channel blocker omega-conotoxin MVIIC depressed the K+ signal by 70%. In the presence of this toxin, diltiazem exhibited an additional small inhibitory effect, indicating that the compound was acting on L-type Ca2+ channels. 4. Whole-cell Ba2+ currents through Ca2+ channels in voltage-clamped chromaffin cells were inhibited by 3-10 microM diltiazem by 20-25%. The inhibition was readily reversed upon washout of the drug. 5. The whole-cell currents elicited by 100 microM DMPP (IDMPP) were inhibited in a concentration-dependent and reversible manner by diltiazem. Maximal effects were found at 10 microM, which reduced the peak IDMPP by 70%. The area of each curve represented by total current (QDMPP) was reduced more than the peak current. At 10 microM, the inhibition amounted to 80%; the IC50 for QDMPP inhibition was 0.73 microM, a figure close to the IC50 for 45Ca2+ uptake (0.4 microM) and [3H]-noradrenaline release (1.3 microM). The blocking effects of diltiazem developed very quickly and did not exhibit use-dependence; thus the drug blocked the channel in its closed state. The blocking effects of 1 microM diltiazem on IDMPP were similar at different holding potentials (inhibition by around 30% at -100, -80 or -50 mV). Diltiazem did not affect the current flow through voltage-dependent Na+ channels. 6. These data are compatible with the idea that diltiazem has little effect on Ca2+ entry through voltage-dependent Ca2+ channels in bovine chromaffin cells. Neither, does diltiazem affect INa. Rather, diltiazem acts directly on the neuronal nicotinic receptor ion channel and blocks ion fluxes, cell depolarization and the subsequent Ca2+ entry and catecholamine release. This novel effect of diltiazem might have clinical relevance since it might reduce the sympathoadrenal drive to the heart and blood vessels, thus contributing to the well established antihypertensive and cardioprotective effects of the drug.

A peptide that mimics the carboxy-terminal domain of SNAP-25 blocks Ca(2+)-dependent exocytosis in chromaffin cells

SNAP-25, a synaptosomal associated membrane protein of 25 kDa, participates in the presynaptic process of vesicle-plasma membrane fusion that results in neurotransmitter release at central nervous system synapses. SNAP-25 occurs in neuroendocrine cells and, in analogy to its role in neurons, has been implicated in catecholamine secretion, yet the nature of the underlying mechanism remains obscure. Here we use an anti-SNAP-25 monoclonal antibody to show that SNAP-25 is localized at the cytosolic surface of the plasma membrane of chromaffin cells. This antibody inhibited the Ca(2+)-evoked catecholamine release from digitonin-permeabilized chromaffin cells in a time- and dose-dependent manner. Remarkably, a 20-mer synthetic peptide representing the sequence of the C-terminal domain of SNAP-25 blocked Ca(2+)-dependent catecholamine release with an IC50 = 20 microM. The inhibitory activity of the peptide was sequence-specific as evidenced by the inertness of a control peptide with the same amino acid composition but random order. The C-terminal segment of SNAP-25, therefore, plays a key role in regulating Ca(2+)-dependent exocytosis, presumably mediated via interactions with other protein components of the fusion complex.

alpha-Bungarotoxin-sensitive nicotinic receptors on bovine chromaffin cells: molecular cloning, functional expression and alternative splicing of the alpha 7 subunit

Chromaffin cells from the bovine adrenal medulla express alpha-bungarotoxin-sensitive acetylcholine receptors whose subunit composition is unknown. Northern blot analysis showed that the alpha 7 subunit, a main component of these alpha-bungarotoxin-sensitive acetylcholine receptors in avian and rat brain, is expressed in chromaffin cells. The cDNA of this bovine alpha 7 subunit was cloned by polymerase chain reaction amplification of adrenal medulla RNA for detailed characterization of structure and function. The protein-coding region revealed 92% amino acid sequence identity to rat alpha 7 and 89% to chicken alpha 7 subunits. The alpha-bungarotoxin affinity of alpha 7 homomers expressed in Xenopus oocytes was similar to that observed previously with native chromaffin alpha-bungarotoxin-sensitive acetylcholine receptors. Cross-linking and sucrose gradient experiments suggested that, like the muscular and neuronal acetylcholine receptors; the alpha 7 receptor has a pentameric structure. Upon activation with nicotinic agonists the alpha 7 receptor exhibited rapidly desensitizing cation currents that were blocked by nicotinic antagonists and showed inward rectification. The amplification of adrenal medulla RNA by reverse transcription-polymerase chain reaction methods revealed an alternatively spliced isoform of the bovine alpha 7 subunit, where the exon that codes for the M2 transmembrane segment was skipped during mRNA processing. Oocyte expression of this isoform does not yield functional channels. However, this alternative mRNA exhibits dose-dependent inhibition of alpha 7 homomer expression when coinjected with the undeleted isoform.

Calyculin A blocks bovine chromaffin cell calcium channels independently of phosphatase inhibition

The specific phosphatase inhibitor, Calyculin-A (CL-A), decreases high-K stimulated catecholamine secretion in bovine chromaffin cells. This effect can be split into two components: one needs long exposures to the drug to be elicited, and is sensitive to the protein kinase-inhibitor K252a; the other is observed after short incubations of CL-A, and is insensitive to K252a. Here we report that the latter component is due to an external block, by CL-A, of chromaffin cell calcium channels in a voltage-dependent, reversible and phosphorylation-independent manner.

Dihydropyridine-sensitive skeletal muscle Ca channels in polarized planar bilayers. 3. Effects of phosphorylation by protein kinase C

The effects of protein kinase C (PKC) were studied on dihydropyridine (DHP)-sensitive Ca channels from rabbit skeletal muscle T tubule membranes. To determine which channel subunits become phosphorylated under the conditions used for electrophysiological studies, we first performed biochemical studies of phosphorylation. T tubular membranes were fused with vesicles of the lipid mixture used in the planar bilayers, and phosphorylation was assessed using the same concentrations of PKC, adenosine 5'-triphosphate, and buffers as were used in the electrophysiological experiments. The alpha 1 subunit of the DHP receptors was phosphorylated by PKC to an extent of 1 mol phosphate/mol protein. The beta subunit was also phosphorylated but to a significantly lesser extent. The DHP-sensitive Ca channel activity was studied after fusing T tubule membranes with planar bilayers (Ma, J., C. Mundiña-Weilenmann, M. M. Hosey, and E. Ríos. 1991. Biophys. J. 60:890-901). The bilayers were held at -80 mV and activated by depolarizing voltage clamp pulses. The observed Ca channels exhibited two open states (tau o1 = 5 ms and tau o2 = 25 ms). On addition of purified PKC to the intracellular side, the proportion of the longer open state increased threefold. The average open probability during a 2-s, maximally activating pulse (Pmax) increased from 10 to 15%. The voltage dependence of activation was not changed by PKC; the Boltzmann parameters were V1 = -20.5 mV and K = 10.5 mV, which were not significantly different from the reference channels. The deactivation (closing) time constant was increased from 7 to 12 ms after PKC. The inactivation time constant during the pulse was slightly increased(from 1.2 to 1.6 s), and the channel availability at the holding potential was decreased from 76 to 71%. Taken together, the results revealed that PKC increased Pmax largely through a shift in the voltage independent open-close equilibrium of the fully activated channels.This is in contrast with the effect of phosphorylation by PKA (Mundir'a-Weilenmann, C., J. Ma, E. Rios, and M. M. Hosey. 1991. Biophys.J. 60:902-909), which also increases Pmax but mostly by increasing the availability of channels and slowing inactivation during the pulse.