Engineered peptides corresponding to segments of the H3 domain of syntaxin inhibit insulin release both in intact and permeabilized mouse pancreatic beta cells

Syntaxin is one of the proteins involved in the exocytotic event through sequential binding to specific proteins, including SNAP25 and synaptobrevin. In a previous work in digitonin-permeabilized beta cells, we characterized the functional role of two segments: synA and synB of the H3 domain of syntaxin. As a continuation of these experiments in the present study we have initially outlined a zone of 17 residues as the very effective uncoupling element of the synA segment. Further functional studies have been accomplished in intact pancreatic beta cells with a specific myristoylated (myr) 13-mer peptide comprised in this active zone. These experiments showed a concentration-dependent inhibition of glucose-induced insulin release (IC50 = 4 microM) of this engineered peptide that was specific since a myristoylated random peptide with the same composition was ineffective. A second myristoylated 13-mer peptide comprised into the synB segment was shown to be even more potent promoting a selective inhibition of insulin release. These data show for the first time, that nutrient-induced secretory process can be specifically uncoupled in intact beta cells demonstrating at the same time that syntaxin plays a central role in this mechanism.

Inhibition of insulin release by synthetic peptides shows that the H3 region at the C-terminal domain of syntaxin-1 is crucial for Ca(2+)- but not for guanosine 5'-[gamma-thio]triphosphate-induced secretion

Recently, we have described the presence and possible role of syntaxin in pancreatic beta-cells by using monoclonal antibodies [F. Martin, F. Moya, L. M. Gutierrez, J.A. Reig, B. Soria (1995) Diabetologia 38, 860-863]. In order to characterize further the importance of specific domains of this protein, the functional role of a particular region of the syntaxin-1 molecule has now been investigated by using two synthetic peptides, SynA and SynB, corresponding to two portions of the H3 region at the C-terminal domain of the protein, residues 229-251 and 197-219 respectively. Functional experiments carried out in permeabilized pancreatic beta-cells demonstrate that these peptides inhibit Ca(2+)-dependent insulin release in a dose-dependent manner. This effect is specific because peptides of the same composition but random sequence do not show the same effect. In contrast with this inhibitory effect on Ca(2+)-induced secretion, both peptides increase basal release. However, under the same conditions, SynA and SynB do not affect guanosine 5'-[gamma-thio]triphosphate-induced insulin release. These results demonstrate that specific portions of the H3 region of syntaxin-1 are involved in critical protein-protein interactions specifically during Ca(2+)-induced insulin secretion.

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.

Secretagogue-induced [Ca2+]i changes in single rat pancreatic islets and correlation with simultaneously measured insulin release

The effects of secretagogues (glucose, tolbutamide and phorbol esters) on simultaneously measured intracellular free calcium concentration ([Ca2+]i) and insulin release were studied in rat pancreatic islets of Langerhans. Stimulatory concentrations (11mM) of glucose caused a transient [Ca2+]i. In contrast with mouse islets, rat islets scarcely showed glucose-induced [Ca2+]i oscillations. Digital image analysis showed that [Ca2+]i changes occurred synchronously across the whole islets. As expected simultaneously measured insulin release was biphasic with a clear second phase. This clearly indicated that in rat islets there is a lack of correlation between [Ca2+]i and insulin release. This was further explored using agents which separately promoted the first (tolbutamide, 200 microns and second (phorbol-12-myristate-13-acetate; PMA; 5nM) phases of insulin release. Tolbutamide induced a transient increase in [Ca2+]i. These results suggest that in rat islets the first phase of insulin release is calcium dependent, whereas the second phase is related to the activation of protein kinase C (PKC). However, the glucose-induced second phase of insulin release did not coincide with an increase in membrane-associated PKC activity. Other messengers may contribute to this late phase of insulin release.

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.

The low-affinity dihydropyridine receptor and Na+/Ca2+ exchanger are associated in adrenal medullary mitochondria

The effect of Ca2+ channel-acting drugs on bovine adrenal mitochondria Ca2+ movements was investigated. Mitochondrial Ca2+ uptake is performed by an energy-driven Ca2+ uniporter with a Km of 20.9 +/- 3.2 microM and Vmax of 148.1 +/- 7.2 nmol 45Ca2+ min-1 mg-1. Ca2+ release is performed through an Na+/Ca2+ antiporter with a Km for Na+ of 4.2 +/- 0.5 mM, a Vmax of 7.5 +/- 0.4 nmol 45Ca2+ min-1 mg-1, and a Hill coefficient of 1.4 +/- 0.2 Ca2+ efflux through the mitochondrial Na+/Ca2+ exchanger was inhibited by several dihydropyridines (nitrendipine, felodipine, nimodipine, (+)isradipine) and by the benzothiazepine diltiazem with similar potencies. In contrast, neither CGP 28392, Bay-K-8644, amlodipine, nor verapamil had any effect on Ca2+ efflux. Nitrendipine at 20 microM modified neither the Km nor the Hill coefficient for Na+, whereas the Vmax was reduced to 2.9 nmol 45Ca2+ min-1 mg-1, thus demonstrating noncompetitive modulation of the Na+/Ca2+ exchanger. None of the Ca2+ channel-acting drugs assayed at 100 microM affected Ca2+ influx through the uniporter. Ca2+ channel blockers inhibited the Na+/Ca2+ antiporter and displaced the specific binding of [3H]nitrendipine to intact mitochondria with Ki values similar to the IC50s obtained for the inhibition of the Ca2+ efflux. Ca2+ channel-acting drugs that did not inhibit the Na+/Ca2+ exchanger (amlodipine, CGP 28392, Bay-K-9644, and verapamil, at concentrations of 100 microM or higher) had no effect on [3H]nitrendipine binding. These results suggest that the adrenomedullary mitochondrial dihydropyridine receptor is associated with the Na+/Ca2+ exchanger.

The protein phosphatase inhibitor calyculin-A affects catecholamine secretion and granular distribution in cultured adrenomedullary chromaffin cells

Calyculin-A, a potent inhibitor of types 1 and 2A protein phosphatases, increases basal catecholamine secretion in cultured chromaffin cells with a maximum effect observed at 100 nM. This effect was increased by forskolin and the calmodulin antagonist W7, but was modified neither by phorbol esters nor the protein kinase inhibitor, H7. The effect of the toxin, calyculin-A, on basal secretion was completely prevented by the protein kinase inhibitor K252a. In digitonin-permeabilized cells calyculin-A induced an increase in basal release, but, in contrast, it partially reduced calcium-induced secretion. Analysis of total proteins revealed that calyculin-A treatment of the cells increased the level of phosphorylation of different protein bands. Examination of the Triton X-100-insoluble fraction revealed a clear increase in the phosphorylation level of various proteins, including vimentin. Calyculin-A provoked a rapid morphological change in chromaffin cells in the same range of concentration (50-300 nM). Cells became rounder and were partially detached from the substratum forming clusters, this effect was also blocked by K252a. Transmission electron microscopy of calyculin-A-treated cells showed an increase in the proportion of chromaffin granules located closer to the membrane. These results suggest that calyculin-A induces changes both in the catecholamine secretory response and in the cytoskeletal elements of chromaffin cells by protein phosphorylation.

Role of syntaxin in mouse pancreatic beta cells

The role of syntaxin 1, a protein involved in the docking of synaptic vesicles at presynaptic active zones, has been investigated in pancreatic islet cells. Using two different monoclonal antibodies we have shown that syntaxin 1 is present in the pancreatic islet cell microsomal fraction. Furthermore, functional experiments demonstrate that anti-syntaxin antibodies inhibit CA(2+)-dependent insulin secretion in permeabilized islet cells. These data indicate that syntaxin 1 is present in the pancreatic beta cell and it is likely to play a functional role in the exocytosis of secretory granules.

Anti-syntaxin antibodies inhibit calcium-dependent catecholamine secretion from permeabilized chromaffin cells

Adrenomedullary chromaffin cells release catecholamines in response to the intracellular calcium rise upon stimulation by different secretagogues. The presence of syntaxin 1, a protein presumably involved in docking of synaptic vesicles to presynaptic membranes, has been investigated in chromaffin cells. The study using two different monoclonal antibodies shows that syntaxin 1 is present in the chromaffin cell membrane fraction. Functional experiments demonstrate that anti-syntaxin antibodies inhibit calcium-dependent secretion in permeabilized cells. These results suggest that syntaxin 1 is an important component of the secretory machinery in chromaffin cells.

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.

Partial characterization of neuropathy target esterase and related phenyl valerate esterases from bovine adrenal medulla

The mechanism by which organophosphorus-induced delayed polyneuropathy is induced relates to the specific inhibition and subsequent modification ("aging") of a protein known as neuropathy target esterase (NTE), operatively defined as paraoxon-resistant and mipafox-sensitive phenyl valerate (PV) esterase activity. This protein has fundamentally been investigated in hen brain, the latter being the habitually employed OPIDP study model. In the present article, a partial characterization is made of the NTE and other related PV esterases in the bovine adrenal medulla and brain; NTE sensitivity to the neurotoxic organophosphorus compound mipafox is investigated, and its subcellular distribution is studied. The NTE activity of the adrenal medulla was found to be the highest of those among the tissues studied to date (5000 +/- 1400 mU/g tissue; +/- SD, n = 12). This activity represented 93% of the PV esterase activity resistant to 40 microM paraoxon in the particulate fraction of the adrenal medulla and approximately 50% of total PV esterase activity. In the bovine brain, these proportions were 72 and 26%, respectively, i.e., similar to those described in hen brain. The mipafox inhibition curve of PV esterase activity resistant to 40 microM paraoxon in the particulate fraction of the adrenal medulla suggests that NTE activity fundamentally comprises a mipafox-sensitive component with an I50 of 6.39 microM at 30 minutes, which is similar to the value reported in hen brain. NTE activity in the bovine adrenal medulla is almost exclusively limited to the particulate fraction, the microsomal fraction, plasma membrane, and chromaffin granule-enriched fractions being the highest in terms of specific activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Ruthenium red inhibits selectively chromaffin cell calcium channels

The effect of Ruthenium red (RR) on ionic currents and catecholamine secretion was studied in chromaffin cells. This polycation inhibited 59 mM potassium-stimulated 45Ca2+ uptake in a concentration-dependent manner (IC50 = 5 +/- 0.2 microM). This effect was more evident at extracellular calcium concentrations over 1 mM and was not abolished by neuraminidase pretreatment. RR also inhibited potassium-stimulated catecholamine secretion (IC50 = 6 +/- 0.9 microM). These results were corroborated by patch-clamp in whole-cell recordings. RR inhibited chromaffin cell calcium currents (IC50 = 7 microM) without affecting significantly either sodium or potassium currents. Radioligand binding studies in adrenomedullary plasma membranes showed that RR inhibited [125I]omega-conotoxin GVIA binding but it had no effect on specific binding of [3H]nitrendipine. The effect of the RR on calcium currents was additive with the inhibitory effect observed with 10 microM nitrendipine. The residual dihydropyridine-resistant calcium current was inhibited with a potency similar to that determined under control conditions in the absence of nitrendipine. These results demonstrate that RR selectively inhibits calcium channels; however, this polycation was not selective for a particular calcium channel subtype.

Solubilization, characterization and photoaffinity labeling of the mitochondrial dihydropyridine receptor from bovine adrenal medulla

1. The mitochondrial dihydropyridine receptor was solubilized with Chaps at a detergent/protein ratio of 2.5, during 45 min at 4 degrees C. 2. From the rate constants of association (8.10 +/- 0.25 x 10(4) M-1 min-1) and dissociation (0.022 +/- 0.001 min-1) a Kd of 275 nM was calculated, while from saturation experiments a Kd of 270 +/- 30 nM and a density of receptors of 106 +/- 9 pmol/mg protein was obtained. 4. The solubilized receptors are heat-resistant, sensitive to the trypsin and to the reduction of disulfide bonds. 5. In native membranes, a polypeptide of 50 kDa was specifically photolabelled with [3H]Azidopine.

Naphthalenesulfonamide derivatives ML9 and W7 inhibit catecholamine secretion in intact and permeabilized chromaffin cells

The role of protein phosphorylation in catecholamine secretion from bovine adrenomedullary chromaffin cells was studied using different protein kinase inhibitors. Naphthalenesulfonamide derivatives as ML9 and ML7, more specific for the myosin light chain kinase, and the calmodulin antagonist W7 inhibited catecholamine secretion 20 and 40% respectively in digitonin-permeabilized chromaffin cells. ML9 also decreased calcium evoked protein phosphorylation of different proteins including tyrosine hydroxylase in permeabilized cells. These naphthalenesulfonamide derivatives showed also an effect in intact cells, ML9 and W7 produced 50% inhibition in catecholamine secretion and 45Ca2+ uptake, however H8 had no effect. The partial [3H]nitrendipine binding displacement of these drugs to adrenomedullary membranes suggests that these sulfonamide derivatives could interact directly with L-type calcium channels in intact cells. The results obtained in permeabilized cells suggest a possible role of protein phosphorylation in the regulation of catecholamine secretion in chromaffin cells.

Muscarinic receptor subtypes in bovine adrenal medulla

Catecholamine secretion in the bovine adrenal medulla is evoked largely by nicotinic receptor activation. However, bovine adrenal medulla also contain muscarinic receptors that mediate several cell responses. To understand the physiological role of muscarinic receptors in the bovine adrenal medulla it is important to identify the pharmacological subtypes present in this tissue. For this, we analyzed the abilities of different selective muscarinic antagonists in displacing the binding of the non-selective antagonist [3H] quinuclidinyl benzylate to an enriched plasma membrane fraction prepared from bovine adrenal medulla. All the selective antagonists bind at least two bindings sites with different affinities. The binding profile of the sites with high proportion is similar to the M2 subtype and those present in low proportion have a M1 profile. However, some variation in the proportion of the sites for the different ligands suggest the presence of the third pharmacological subtype (M3). We conclude that the sites in high proportion (60-80%) correspond to M2 muscarinic subtypes, and the rest is constituted by M1 plus M3 subtypes. The presence of multiplicity of subtypes in the adrenal medulla membranes suggests a diversity of functions of muscarinic receptors in the adrenal gland.

Separate [3H]-nitrendipine binding sites in mitochondria and plasma membranes of bovine adrenal medulla

1. Two binding sites for the 1,4-dihydropyridine (DHP) derivative [3H]-nitrendipine have been found in the bovine adrenal medulla. The high-affinity site (Kd = 0.48 nM and Bmax = 128 fmol mg-1 protein) was specifically located in purified plasma membranes. The low-affinity site (Kd = 252 nM and Bmax = 169 pmol mg-1 protein) was located only in mitochondria. Chromaffin granule membranes lacked specific binding sites for [3H]-nitrendipine. 2. Kinetic analysis of the rates of association and dissociation of [3H]-nitrendipine, saturation isotherms and displacement experiments with unlabelled nitrendipine and PN200-110 revealed single, homogeneous populations of high- and low-affinity sites in plasma and mitochondrial membranes, respectively. 3. The high affinity site was sensitive to Ca2+ deprivation and heating; it was practically unaffected by changes in ionic strength of the medium and its optimal pH was slightly alkaline. This site exhibited a strong DHP stereoselectivity; diltiazem increased and verapamil decreased the affinity of [3H]-nitrendipine. 4. In contrast, binding of [3H]-nitrendipine to the low affinity site was more heat resistant and less affected by Ca2+ removal. Its optimal pH was slightly acid and the increase in ionic strength enhanced the number of available sites. The site had no DHP stereoselectivity. Verapamil decreased the dissociation constant of [3H]-nitrendipine acting in a non-competitive manner; diltiazem did not affect equilibrium binding parameters of [3H]-nitrendipine. 5. These results suggest that both biding sites reflect different receptor entities. The high-affinity binding site corresponds to the dihydropyridine receptor associated with the L-type calcium channel. The function of the mitochondrial, low-affinity binding site is, at present, unknown.

Pineal transduction. Adrenergic----cyclic AMP-dependent phosphorylation of cytoplasmic 33-kDa protein (MEKA) which binds beta gamma-complex of transducin

Adrenergic regulation of phosphorylation of pineal proteins was studied. Norepinephrine treatment of intact pinealocytes incubated with 32Pi enhanced phosphorylation of a 33-kDa phosphoprotein (33PP). The effect of NE was rapid, sustained, and appeared to be mediated by a beta-adrenergic----cyclic AMP mechanism. Studies using broken cell preparations revealed that 33PP was phosphorylated by cyclic AMP-dependent protein kinase (PKA). It was also possible to demonstrate PKA-dependent phosphorylation of the 33-kDa protein in cytosol from rat retina and in cow and sheep pineal glands. Two-dimensional polyacrylamide gel electrophoresis revealed that 33PP is acidic (pI congruent to 4.5), appears to exist as two isoforms with slightly different charge, and has the same mobility as the retinal 33-kDa PKA substrate. Immunological analysis indicated 33PP in both tissues is a previously reported 33-kDa protein (MEKA); this protein is a PKA substrate which has been reported to form a cytoplasmic complex with the beta gamma complex of transducin. Consistent with this, it was possible to identify the beta-subunit in pineal cytoplasm and in the same congruent to 70-kDa gel permeation fraction which contained the 33-kDa protein identified as MEKA. Thus, it appears possible that MEKA is present in pineal cytoplasm in a 70-kDa complex with G beta gamma, as is the case in retina. The finding of MEKA in the pineal makes it the latest addition to a family of retinal/pineal proteins which are thought to have evolved from a common ancestral photochemical transduction system.

Phosphorylation of myosin light chain from adrenomedullary chromaffin cells in culture

The myosin-light-chain (MLC) phosphorylation accompanying catecholamine release in chromaffin cells was investigated with the objective of assessing the possible role of this contractile protein in catecholamine secretion. The electrophoretic characteristics of adrenomedullary MLC were determined by immunochemical techniques using two different specific antibodies. The identified 22 kDa phosphoprotein was mainly present in the cytosol, as demonstrated by ultracentrifugation and immunocytochemical analysis. A part of this protein was located on, or close to, the plasma membrane. Cell stimulation by secretagogues resulted in a Ca2(+)-dependent 32P incorporation into MLC, the time course of this process being related to catecholamine release. These findings were supported by a two-dimensional gel-electrophoretic analysis by which means this protein was resolved into two acidic forms. A role for Ca2(+)-calmodulin and Ca2(+)-phospholipid kinases in adrenomedullary MLC phosphorylation is reported. The results obtained suggest a regulatory role for such a protein in the underlying exocytotic event.

Separate binding and functional sites for omega-conotoxin and nitrendipine suggest two types of calcium channels in bovine chromaffin cells

Purified adrenomedullary plasma membranes contain two high-affinity binding sites for 125I-omega-conotoxin, with KD values of 7.4 and 364 pM and Bmax values of 237 and 1,222 fmol/mg of protein, respectively. Dissociation kinetics showed a biphasic component and a high stability of the toxin-receptor complex, with a t1/2 of 81.6 h for the slow dissociation component. Unlabeled omega-conotoxin inhibited the binding of the radioiodinated toxin, adjusting to a two-site model with Ki1 of 6.8 and Ki2 of 653 pM. Specific binding was not affected by Ca2+ channel blockers or activators, cholinoceptor antagonists, adrenoceptor blockers, Na+ channel activators, dopaminoceptor blockers, or Na+/H+ antiport blockers, but divalent cations (Ca2+, Sr2+, and Ba2+) inhibited the toxin binding in a concentration-dependent manner. The binding of the dihydropyridine [3H]nitrendipine defined a single specific binding site with a KD of 490 pM and a Bmax of 129 fmol/mg of protein. At 0.25 microM, omega-conotoxin was not able to block depolarization-evoked Ca2+ uptake into cultured bovine adrenal chromaffin cells depolarized with 59 mM K+ for 30 s, whereas under the same conditions, 1 microM nitrendipine inhibited uptake by approximately 60%. When cells were hyperpolarized with 1.2 mM K+ for 5 min and then Ca2+ uptake was subsequently measured during additions of 59 mM K+. Omega-conotoxin partially inhibited Ca2+ uptake in a concentration-dependent manner. These results suggest that two different types of Ca2+ channels might be present in chromaffin cells. However, the molecular identity of omega-conotoxin binding sites remains to be determined.

[3H]AHN 086 acylates peripheral benzodiazepine receptors in the rat pineal gland

AHN 086, an isothiocyanato derivative of Ro 5-4864 (4'-chlorodiazepam), inhibits radioligand binding to peripheral benzodiazepine receptors with characteristics of an irreversible (acylating) ligand. We now report that [3H]AHN 086 labels a approximately 30 kDa protein in the rat pineal gland determined by both SDS-polyacrylamide gel electrophoresis and gel filtration high-performance liquid chromatography of digitonin-solubilized membranes. Specific incorporation of [3H]AHN 086 into this protein was inhibited by preincubating membranes with excess AHN 086. Moreover, significant specific binding of [3H]AHN 086 was not observed in either bovine pineal gland (which does not possess high-affinity binding sites for Ro 5-4864) or ovalbumin. These findings suggest that the approximately 30 kDa protein labeled by [3H]AHN 086 in rat pineal gland is associated with peripheral benzodiazepine receptors in this tissue.

A two-dimensional electrophoresis study of phosphorylation and dephosphorylation of chromaffin cell proteins in response to a secretory stimulus

Phosphorylated proteins of bovine chromaffin cells, radioactively labeled with [32P]orthophosphate, have been analyzed by two-dimensional polyacrylamide gel electrophoresis and autoradiography. Complex two-dimensional electrophoretograms were studied with the aid of computer-assisted image analysis (CAIA). A database map of 32P-labeled proteins was constructed; approximately 500 polypeptides have been detected, numbered, and characterized according to the intensity of labeling, molecular weight, and isoelectric point. The database was constructed from cells kept in resting conditions or stimulated with 59 mM K+ in 2.5 mM Ca2+ or in 0 Ca2+ solution. These manipulations caused statistically significant changes in the degree of phosphorylation of 20 proteins; they were classified as Ca2+-dependent substrates for the phosphorylation or dephosphorylation processes. These changes were also shown in cells stimulated in the presence of the Ca2+ channel activator Bay K 8644. New proteins that show as much as a fivefold increase in their phosphorylation state during cell stimulation have been located with this methodology, as well as many others that had not previously been detected with conventional methods. These experiments provide the first CAIA database of chromaffin cell phosphoproteins; the map constructed with these data will allow the location of specific phosphoproteins and serve as a reference for future ongoing studies. The database will continue to grow to identify more proteins and to facilitate the comparison of complex patterns obtained in different laboratories for normal and transformed pheochromocytoma PC12 cells.

Activation of Neurospora crassa soluble adenylate cyclase by calmodulin

The soluble form of adenylate cyclase was extracted and purified from wild-type Neurospora crassa mycelia. Brain or N. crassa calmodulin significantly enhanced this enzyme activity in assay mixtures containing Mg2+-ATP as substrate. EGTA reverses this calmodulin activation.

Dihydropyridine BAY-K-8644 activates chromaffin cell calcium channels

Douglas and Rubin suggested that "the role of acetylcholine as a transmitter at the adrenal medulla is to cause some brief change in medullary cells which allows Ca ions to penetrate them and trigger the catecholamine ejection process". The Ca2+-channel blocking agents, verapamil, nifedipine and nitrendipine, have been used widely to investigate the properties of slow Ca2+ channels in a variety of tissues, including the adrenomedullary chromaffin cell. Recently, small modifications to the nifedipine molecule produced a derivative, BAY-K-8644 (methyl-1,4-dihydro-2, 6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5-carboxylate), that in contrast to the Ca2+-channel blocking agents, stimulated cardiac and vascular smooth muscle contractility. We have tested whether this compound behaves as a Ca2+-channel activator at the chromaffin cell membrane as shown by Schramm et al. in smooth muscle cells. The experiments described here strongly suggest that it does so.