Calcium and exocytosis

Amine secretion from electropermeabilized bovine chromaffin cells and human platelets requires Ca2+ and MgATP. There appears to be little correlation between the pH or potential of the interior of the amine storage granules of the chromaffin cells and the Ca2+ sensitivity or extent of secretion. The Ca2+ sensitivities of secretion for both preparations are increased by activators of protein kinase C. In the platelet, thrombin also increases the Ca2+ sensitivity. The thrombin-induced response is further enhanced by micromolar levels of GTP. The non-hydrolysable analogue GTP gamma S also potentiates the Ca2+-dependent secretory response, but this effect is additive to that seen by thrombin rather than synergistic, as is the case with GTP. GTP gamma S inhibits catecholamine secretion from bovine chromaffin cells. In both preparations the effects of GTP gamma S are inhibited by 10 microM GTP, even though GTP concentrations up to 1 mM are without effect when added alone. These results are consistent with there being two sites of action for the guanine nucleotides, one at the level of the agonist receptor and activated by GTP or one of its breakdown products, and the other one activated by GTP gamma S--possibly at the level of protein kinase C itself.

Secretion from bovine chromaffin cells acutely expressing exogenous proteins using a recombinant Semliki Forest virus containing an EGFP reporter

Acute expression of recombinant proteins throughout a population of postmitotic bovine chromaffin cells was achieved using the Semliki Forest virus expression system (P. Liljestrom and H. Garoff (1991) Biotechnology 9:1356-1361). The virus was modified to express a green fluorescent protein, which faithfully reported the expression of the recombinant proteins. Two types of reporting virus were constructed: the first included a second subgenomic element, and the second an internal ribosome entry site. Both were used to express the recombinant proteins beta-galactosidase, 5HT3 receptor, or tetanus toxin light chain. Beta-galactosidase was used to quantify the rate of expression of recombinant protein in chromaffin cells, the 5HT3 receptor to trigger secretion, and the toxin to block secretion. The experiments clearly show that infection and expression of recombinant proteins throughout a population of chromaffin cells do not, per se, affect the rate and extent of triggered exocytosis, endocytosis, or membrane recycling pathways. The catecholamine content of the cell is unaltered, and the secretory mechanism can be accessed within a few hours after infection. This noncytopathic method of acutely expressing specific proteins at physiological levels in chromaffin cells offers a powerful new tool for dissecting the roles of many proteins implicated in exo- and endocytosis.

A novel use of differential equations to fit exponential functions to experimental data

A procedure for fitting multi-exponential functions to experimental data is described. It is fast, requires no initial parameter estimates and is particularly suited to sums of several closely spaced exponentials. The method comprises the application of three well tried numerical techniques: (i) the signal is smoothed by representing it as an abbreviated Legendre series; (ii) the coefficients of a certain kind of differential equation are determined such that it's solution is the closest fit to the smoothed signal; and (iii) the amplitudes of the exponential components are determined, given the calculated values of the exponential rate constants. The method is computationally efficient, since determination of amplitudes and exponents involves the use of linear techniques, and therefore does not require multiple iterations, and the smoothed signal is contained in a handful of coefficients rather than as a lengthy time series. The severe ill-conditioning that is unavoidable in this problem is contained within the well-understood procedures of inverting a matrix and determining the roots of a polynomial. This method is particularly appropriate for analysis of data that may be modelled by a scheme of linked first-order reactions, describing for example the stochastic behaviour of ion channels, a chemical reaction, or the uptake and distribution of a drug within body compartments.

Triggered exocytosis and endocytosis have different requirements for calcium and nucleotides in permeabilized bovine chromaffin cells

The intracellular requirements for membrane recapture in permeabilized chromaffin cells were compared to the requirements for exocytosis from the same cells. In permeabilized bovine chromaffin cells, calcium-driven exocytosis also triggers, with a short delay, uptake of extracellular horseradish peroxidase (HRP). This internalized HRP remains compartmentalized within the cell and migrates to a low density band on a Percoll gradient which is distinct from the heavier chromaffin granules. The amount of horseradish peroxidase internalized is similar in intact and leaky cells and is approximately equivalent to the volumes secreted. Endocytosis in both preparations is blocked by botulinum toxin, operates in a collapsed membrane potential, and is inhibited by low temperature. In permeabilized cells, exocytosis and coupled endocytosis are activated by the same concentrations of Ca2+ and MgATP. Although secretion requires Ca2+ and MgATP, once exocytosis has occurred the subsequent endocytosis can proceed in the virtual absence of Ca2+ or MgATP, and is largely unaffected by a variety of nucleotide triphosphates (including nonhydrolyzable analogues), and cyclic nucleotides. These data suggest that endocytosis can proceed, once exocytosis has been triggered, under conditions that are quite different from those necessary to support exocytosis, and that the specific requirements for Ca2+ and MgATP in secretion are for the exocytotic limb of the secretory cycle rather than for the associated endocytotic pathway.

Growth factor expression during rat development: a comparison of TGF-beta 3, TGF-alpha, bFGF, PDGF and PDGF-R

At least part of the mechanism underlying fetal development appears to be the production of a number of growth factors considered important in the process of tumour formation. Using immunocytochemistry, we have investigated the temporal and spatial pattern of expression of some of the important growth factors, by the fetus. We describe here the cellular localization of transforming growth factor beta 3 (TGF-beta 3), platelet derived growth factor (PDGF) and its receptor (PDGF-R), TGF-alpha and basic fibroblast growth factor (bFGF) in the fetal rat from day 13 to 21 of gestation. Using antisera raised against an N-terminal portion of TGF-beta 3, immunoreactivity peaked around day 16 and was seen predominantly within epithelial cells. However, using antisera raised against the C-terminal of this molecule immunoreactivity was seen exclusively within the extracellular matrix underlying adjacent epithelia, and was maintained up until day 21 of gestation. Strong expression of TGF-alpha was seen in cells of most organs throughout the gestation period studied. Immunoreactivity for bFGF, PDGF and PDGF-R peaked around day 18 in both epithelial and mesenchymal cells of all major organ systems and then declined by day 21. These data suggest distinct roles for each factor during embryogenesis and tumorigenesis.

Markov modelling of ensemble current relaxations: bovine adrenal nicotinic receptor currents analysed

1. A general approach to the analysis of ensemble currents of ligand-gated channels is presented, with a variety of examples that include single and multiple agonist binding steps, desensitization and several blocking pathways. 2. The use of matrix methods to describe model reaction schemes leads to a simplification if the reaction scheme is irreversible: the product of the exponential relaxation rate constants is exactly equal to the product of the forward reaction steps. 3. This method of analysis applied to the bovine adrenal nicotinic receptor suggests that in the range of acetylcholine concentrations from 1 microM to 2 mM, a model with a single kinetically relevant agonist binding step is appropriate. 4. Complex models, to explain the presence of two desensitizing components in currents recorded from whole cells, may be discounted in favour of two distinct receptor types. 5. A simple model of open channel block is discounted, and desensitization of the blocked state proposed.

A study of the bovine adrenal chromaffin nicotinic receptor using patch clamp and concentration-jump techniques

1. Voltage clamp records have been obtained from bovine adrenal chromaffin cells in the outside-out and whole-cell configurations, in response to step changes of acetylcholine (ACh) concentration. The concentrations used ranged from 50 nM to 20 mM. 2. At high acetylcholine concentrations, the activation and desensitization kinetics of the nicotinic receptor, as observed in outside-out patches, may be described by a model incorporating a single, fast agonist binding step, and relatively slow isomerization to the open state. The affinity of the closed receptor for ACh is 310 microM, the channel opening rate constant is 460 s-1, and the closing rate constant is 29 s-1. 3. Single channel events, observed when nanomolar ACh concentrations are applied to whole cells, have two distinct channel lifetimes: 0.6 ms and 11-15 ms. The variation of the frequencies of the events with ACh concentration, suggests that the short lifetimes are openings of a singly liganded receptor and the longer lifetimes are openings of a doubly liganded receptor. 4. Only a single exponential associated with receptor desensitization is seen with outside-out patches, but two are seen with whole cells. It is postulated that there are two nicotinic receptor types present on adrenal chromaffin cells. 5. The rate of desensitization (9 s-1 and 26 s-1, whole cells; 24 s-1, patches), is fast enough to be significant in determining the open channel lifetime. 6. A sudden increase in current (rebound) is observed when a high concentration of ACh is abruptly removed from outside-out patches. This is evidence for a blocked state. The affinity of the blocking site for ACh is 1400 microM (outside-out patches). 7. The total number of activatable nicotinic channels per whole cell is estimated to be 2600.

Parathyroid hormone-related peptide can regulate the growth of human lung cancer cells, and may form part of an autocrine TGF-alpha loop

Parathyroid hormone-related peptide (PTHrP) and transforming growth factor-alpha (TGF-alpha) were found to stimulate proliferation of human lung cancer cells (BEN-57). TGF-alpha stimulated PTHrP secretion from these cells. The polyclonal antisera raised against PTHrP significantly inhibited the growth of BEN-57 cells, and also the proliferation induced by TGF-alpha. Treatment of cells for up to 10 days with either a PTHrP receptor antagonist (PTHrP(7-34)) or PTHrP antiserum significantly inhibited the subsequent growth of these cells. We suggest that PTHrP may be a component of a complex autocrine loop involving TGF-alpha.

Parathyroid hormone-related peptide: expression in fetal and neonatal development

Hypercalcaemia frequently complicates the clinical management of cancer. Many factors have been implicated in the pathogenesis of this humoral hypercalcaemia of malignancy, the most recent candidate being parathyroid hormone-related peptide (PTHrP). Until now, this peptide has been detected only in some normal and transformed adult tissues. In recent years, it has become apparent that tumours are capable of expressing and secreting factors previously elaborated only during fetal life. Many of these factors act to stimulate the growth of both tumour and fetal cells in an autocrine manner. The data presented here demonstrate that PTHrP is expressed in the human and rat fetus throughout gestation. Immunocytochemistry reveals a gestationally related, changing pattern of expression which is paralleled by changes in mRNA transcription. These data support the hypothesis that PTHrP may function as a fetal growth factor.

The use of intracellular dialysis to study signal transduction coupling in the squid giant axon

The squid giant axon has proved a useful model in the study of ionic channel gating, intracellular homeostasis and receptor-mediated signal transduction leading to generation of intracellular second messengers. In the latter category, previous studies on activation of adenylate or guanylate cyclase have used intact and intracellularly perfused axons to investigate the effects of extra- and intracellular agents on the transduction processes. However, the perfusion of the axon interior washes out many factors which may be important in the processes under study. We introduce here the use of porous cellulose dialysis tubing as a means to circumvent these problems. We find that this dialysis technique is a simple procedure to set-up, and the serotonin/G-protein/adenylate cyclase system can readily be studied in the dialysed axon. This approach should allow investigation under conditions which retain asymmetric transmembrane conditions.

The effect of botulinum toxin type D on the triggered and constitutive exocytosis/endocytosis cycles in cultures of bovine adrenal medullary cells

The extracellular fluid phase marker, horseradish peroxidase, enters chromaffin cells when triggered to secrete catecholamine. This triggered uptake, like secretion, is abolished in cells pre-incubated with botulinum toxin. Endocytosis of horseradish peroxidase into unstimulated cells is unaffected by botulinum toxin but is inhibited when the temperature is reduced. Once internalised by the unstimulated cells, horseradish peroxidase is released back into the extracellular fluid, the rate of release being temperature sensitive but unaffected by carbamylcholine or botulinum toxin. These results suggest that triggered exocytosis is a necessary event to precede triggered endocytosis, and that botulinum toxin may affect only the triggered exocytosis/endocytosis cycle and not the constitutive cycle.

Membrane recapture and early triggered secretion from the newly formed endocytotic compartment in bovine chromaffin cells

1. Recycling of secretory vesicles in cultured bovine adrenal medullary cells was investigated. 2. Extracellular horseradish peroxidase (HRP), a fluid phase marker, was taken up into cultured adrenal medullary cells following carbamylcholine-induced secretion of catecholamine. 3. The endocytosed HRP remained compartmentalized within the cell, migrating to a low density band on a Percoll density gradient. The endocytotic compartment was distinct from the major pool of catecholamine-containing chromaffin granules, which were found at much higher densities on the Percoll gradient. 4. The chromaffin granule membrane marker dopamine beta-hydroxylase was associated with the endocytosed HRP compartment as well as with the heavier chromaffin granules. 5. A subsequent challenge of the cells with carbamylcholine triggered the release of up to forty per cent of the endocytosed HRP. 6. The time course for secretion of the fluid phase marker was similar to that for catecholamine secretion. 7. Triggered release of HRP was dependent on extracellular calcium. The dependence on the extracellular calcium concentration was similar to that of catecholamine release. 8. Release of HRP could be triggered from electropermeabilized cells by raising the intracellular Ca2+ into the micromolar range. The intracellular Ca2+ dependence of triggered HRP release was similar to that for catecholamine release. 9. HRP could be secreted as early as 5 min, and as late as 2 h after endocytosis. 10. These data provide evidence that endocytotic vesicles can rapidly re-enter the secretory cycle. Endocytosed vesicles may therefore not have to recycle via the trans-Golgi reticulum to form high-density chromaffin granules in order to re-enter the regulated secretory pathway.

Parathyroid hormone-related peptide in normal human fetal development

Parathyroid hormone-related peptide (PTHrP) has been detected in fetal serum and amniotic fluid. Using a combination of immunocytochemistry and molecular biology we have detected the peptide and its mRNA in a variety of fetal tissues throughout gestation. Tissue-specific mRNA isoforms were observed, the pattern of hybridization of which changed throughout gestation. In addition, the intensity and pattern of immunocytochemical localization of the peptide was found to vary over the time-period studied (8-30 weeks). PTHrP is expressed by a variety of tumours associated with the syndrome of humoral hypercalcaemia of malignancy and probably accounts for the hypercalcaemia by virtue of its limited amino acid homology with parathyroid hormone. These data demonstrate for the first time that PTHrP, a tumour-related peptide, is expressed during normal human fetal development, and suggest the possibility that it may function to regulate fetal calcium balance and growth in utero.

Parathyroid hormone related peptide can function as an autocrine growth factor in human renal cell carcinoma

Parathyroid hormone related peptide (PTHrP) has been implicated in the cause of the hypercalcemia associated with a number of malignant tumours. The data presented here suggests that PTHrP (in addition to its known role of mediating hypercalcemia) may be involved in the autocrine regulation of growth of some tumours. Polyclonal PTHrP antiserum almost totally inhibited the growth of a human renal cell carcinoma cell line, known to secrete PTHrP, in vitro and growth was significantly inhibited by the competitive PTH antagonist PTH (3-34)NH2.

Parathyroid hormone-related peptide in the human fetal uro-genital tract

Using a polyclonal antiserum raised against the first 34 amino acids of human parathyroid hormone-related peptide (PTHrP), we have localized PTHrP throughout the uro-genital tract of the human fetus aged between 8 and 40 weeks. Staining was present in the developing mesonephros, metanephros, gonads and in both the adrenal cortex and medulla. In particular, the developing mesonephric and metanephric renal tubules were intensely positive. Using Northern hybridization analysis we have detected a complex pattern of PTHrP mRNA transcripts ranging in size from 1.4 to 4.5 kb in early second trimester human fetal kidney. The presence of PTHrP in the mesonephros and metanephros provides evidence for a role for PTHrP in the regulation of fetal calcium metabolism. However, its presence in the gonad and adrenal gland invites the possibility of a wider role for PTHrP.

Calcium-dependent and calcium-independent exocytosis

A large body of evidence supports the concept that calcium (Ca2+) plays a pivotal role in the control of exocytosis. However, recent experiments suggest that a rise in intracellular Ca2+ does not necessarily trigger secretion, and also that secretion can occur independently of cytosolic free calcium levels. This article briefly summarizes the early evidence that has formulated the role of Ca2+ in secretion, and then examines some of the recent evidence suggesting a Ca2+-independent mechanism of exocytosis.

A method for making solution changes in the sub-millisecond range at the tip of a patch pipette

A method is described here for making multiple fast external solution changes at the tip of a patch pipette. The time for the change, 0.2 ms, has been established by measuring changes in liquid junction potential at the tip of an open patch pipette. This technique of producing an abrupt change in solution allows agonist/receptor reactions to be studied under non-equilibrium conditions. We have applied this technique to the nicotinic receptors in outside-out patches from skeletal muscle cell line C2 (Jaffe and Saxel 1979) and from bovine adrenal chromaffin cells. The application of step changes in acetylcholine concentration produces current traces with a characteristic shape, which may be compared with the predictions of established models for the activation and desensitisation of the nicotinic receptor. The results of making single steps and also short pulses in acetylcholine concentration are demonstrated. The direct comparison of two different cholinergic agonists is demonstrated.

Ionic requirements of the endothelin response in aorta and portal vein

The vasoconstrictor responses of isolated rat portal vein and aorta to synthetically prepared endothelin are investigated. Both preparations respond to 10(-9) M levels of the peptide although the aortic response is more sustained than that of the portal vein. Endothelin-evoked contractions, unlike those evoked by scorpion alpha-toxins (which are homologous to endothelin) or by veratridine, are insensitive to tetrodotoxin or to the removal of sodium ions from the tissue-bathing medium. Contractile responses to endothelin may still be observed in high-potassium depolarizing medium and are not dependent on the presence of extracellular chloride; however, the responses are dependent on the presence of extracellular calcium and are blocked by nitrendipine, nifedipine, or nickel. Endothelin-evoked uptake of 45Ca into aortic tissue is also independent of extracellular sodium or potassium and is blocked by nifedipine. These data strongly suggest that endothelin acts at a site closely coupled to the calcium channel and that depolarization by sodium influx through voltage-dependent channels is not involved in endothelin-induced vasoconstriction.

Tissue selectivity of endothelin

The effects of endothelin, a potent endogenous vasoconstrictor peptide, were examined in a range of vascular and non-vascular tissues. At concentrations that cause vasoconstriction in portal vein and aorta, the peptide strongly contracted rat uterus, trachea and vas deferens, but not guinea pig ileum. Nifedipine, a dihydropyridine calcium anatgonist, partially inhibited these contractions. Endothelin had no inotropic or chronotropic effect on the isolated rat heart. The peptide did not modulate secretion at the neuromuscular junction, from adrenal medullary cells or neutrophils, nor affect secretion or aggregation of platelets. The tissue responsiveness to endothelin was not the same as the tissue distribution of dihydropyridine receptors. This supports the idea that endothelin interacts with a specific receptor distinct from dihydropyridine sensitive calcium channels. The contractile effect of endothelin on non vascular smooth muscle suggests that the concept of endothelium dependent modulation of vascular smooth muscle tone may be extended to include epithelium dependent modulation of non vascular tissues.

Activation of sodium channels is not essential for endothelin induced vasoconstriction

The role of sodium and calcium ions in the vasoconstrictor response of isolated rat aorta and protal vein to synthetically prepared endothelin is investigated. Contractile responses to endothelin, unlike those induced by the sodium channel activator veratridine, are unaffected by tetrodotoxin or by the removal of sodium chloride from the solution bathing the tissue. The responses are the same whether sodium chloride is replaced iso-osmotically with either sucrose or potassium chloride. The endothelin responses in all media are entirely dependent on the presence of extracellular calcium, and can be blocked by 1 microM nitrendipine. These findings offer no support to the idea that voltage activated sodium channels are the primary site of action of endothelin as suggested by sequence homologies to scorpion alpha-toxins, but are entirely consistent with the possibility that the site of action of endothelin is closely coupled to the calcium channel (Yanagisawa et al, 1988).

Botulinum toxin-induced ADP-ribosylation and inhibition of exocytosis are unrelated events

The hypothesis that inhibition of secretion by botulinum neurotoxin type D occurs by an intracellular process involving ADP-ribosylation has been directly tested by measuring both the extent of inhibition of secretion and of ADP-ribosylation in the same cells. Although the inhibitory effect of unpurified toxin closely parallels intracellular ribosylation, the two events are clearly unrelated, as using purified D and C3 toxins together with their antibodies, each of these events can be either stimulated or inhibited independently of each other.

Stimulus-secretion coupling: a perspective highlighting the contributions of Peter Baker

Many investigators are using numerous preparations for contributing to our present understanding of stimulus-secretion coupling, by which we mean stimulus-dependent exocytosis, sometimes known as the regulated pathway. However, a few model systems have been particularly illuminating and several of these were exploited by Peter Baker and his close associates: namely, the motor nerve terminal, the adrenal chromaffin cell, the sea urchin egg and the blood platelet. In fact, Peter's first real contribution in this area came from his seminal studies on calcium transport in his favourite preparation, the squid giant axon, where he investigated Ca2+/Na+ exchange, Ca2+ distribution and voltage-gated Ca2+ entry. More direct investigations into stimulus-secretion coupling came from work on neurone transmitter release in collaboration with Andrew Crawford, and on catecholamine secretion from the adrenal medulla in collaboration (with TJR). His most important generic contribution to this field was in the development (with DEK), of the electropermeabilized cell, which allows control of the low molecular weight components of the cytosol while leaving the exocytotic apparatus and process intact. In the initial experiments on the cells it was finally proved that Ca2+-dependent secretion of catecholamines is indeed from the granules and not from the cytosol. The quantification of the Ca2+ requirement of secretory exocytosis was an important step, as was the investigation of many factors purported to be important in the coupling mechanism or in the exocytotic process itself. Work with the human platelet, using this technique, has proved to be especially valuable in unravelling the complex interactions between different second messengers and has been neatly complemented by work in intact cells containing Ca2+-indicator fluorescent dyes. Peter was also intrigued by post-secretory events both in the early seventies, and at the end of his career when he embarked on analysis of the membrane retrieval process and the associated uptake of extracellular medium.

Evidence implicating protein kinase C in exocytosis from electropermeabilized bovine chromaffin cells

The calcium sensitivity of exocytosis from electro-permeabilized chromaffin cells is increased by activators of protein kinase C, such as TPA and certain phorbol esters, diacylglycerols, and mezerein. A range of putative inhibitors of protein kinase C block both the phorbol ester-sensitive component of secretion and also the underlying insensitive component. These inhibitors are also shown to inhibit medulla protein kinase C activity in vitro. The extent of secretion is reduced when electro-permeabilized cells are exposed to Ca2+ levels much in excess of 50 microM. The onset of inhibition is faster than the relatively slow rate of Ca-dependent exocytosis and is insensitive to inhibitors of proteolysis. Adrenal medulla protein kinase C activity is also irreversibly inhibited by high Ca2+ concentrations. Both the secretory response and the protein kinase C activity in vitro have similar nucleotide and cation specificities. Although these data do not definitely establish an involvement of protein kinase C in exocytosis, none argue against it.

Secretion of 5-hydroxytryptamine from electropermeabilised human platelets. Effects of GTP and cyclic 3',5'-AMP

Enhancement by thrombin of Ca2+-dependent 5HT secretion in the absence of added GTP decreases as the time between electropermeabilisation and addition of thrombin is increased. No decrease occurs if thrombin is added with GTP. Observation of apparent GTP-independent receptor/phospholipase C coupling may result from the presence of bound GTP in the preparation. Enhancement by GTP of Ca2+-dependent 5HT secretion occurs with a significant lag indicating an agonist-independent effect. Cyclic 3'5'-AMP inhibits enhancement by GTP of Ca2+-dependent 5HT secretion while having no effect on enhancement induced by GTP gamma S. Hence cyclic AMP may impair receptor/phospholipase C coupling by enhancing Np GTPase activity.

Calcium and diacylglycerol control of secretion

Measurements of intracellular Ca2+ in adrenal medullary cells suggest that a transient rise in Ca2+ leads to a transient secretory response, the rise in Ca2+ being brought about by an influx through voltage-sensitive Ca channels which subsequently inactivate. The level of Ca2+ observed is much smaller than the Ca2+ needed to trigger secretion when introduced directly into the cell. The discrepancy is removed by the presence of diacylglycerol, which increases the sensitivity of the secretory process to Ca2+. The site of action of Ca2+ and diacylglycerol is probably protein kinase C, and the different secretory responses to increases of Ca2+ and diacylglycerol can be modelled in terms of a preferential order of binding of these two substrates to the enzyme. ATP is needed for secretion: one role is possibly to confer stability to the secretory apparatus; another may involve phosphorylation of some key protein. The kinetics of secretion suggest that if Ca2+ regulates phosphorylation or dephosphorylation, then it is the rate of change of phosphorylation that controls secretion rather than the extent of phosphorylation or dephosphorylation. Guanine nucleotide-binding proteins may play a role not only at the level of signal transduction coupling, but also at or near the site of exocytosis, and the mechanism by which some Botulinum toxins inhibit secretion may be associated with these proteins.

Botulinum toxin types A, B and D inhibit catecholamine secretion from bovine adrenal medullary cells

Evoked catecholamine secretion from cultured bovine adrenal medullary cells is inhibited by commercially available botulinum toxins - types A, B and D (10(4)-10(6) MLD/ml of culture medium). Basal secretion is also inhibited. The catecholamine content of such toxin-treated cells is larger than that of control cells and may in part be a result of the inhibition of basal release. The onset of action of botulinum toxin types A and D can be neutralised by their respective antisera. Concentrations of botulinum toxins A, B or D that inhibit secretion leave unaffected the 45Ca2+ influxes normally associated with secretion. These data provide further evidence to support the idea [(1985) Nature 317, 719-721] that botulinum toxins block secretion by acting downstream of the Ca2+ transient at or near the site of exocytosis.

Effects of guanine nucleotides on the properties of 5-hydroxytryptamine secretion from electropermeabilised human platelets

Guanosine 5'-[gamma-thio]triphosphate and guanosine 5'-[beta,gamma-imido]triphosphate enhance Ca2+-dependent 5-hydroxytryptamine secretion from electropermeabilised human platelets. GTP has little such effect except when the platelets are permeabilised, and incubated with this nucleotide, at 2 degrees C and pH 7.4. The lag phase observed in the time course of 5-hydroxytryptamine secretion induced by addition of guanosine 5'-[gamma-thio]triphosphate is markedly longer than that characterising secretion induced by Ca2+ alone, by thrombin +/- GTP or by guanosine 5'-[gamma-thio]triphosphate in the presence of thrombin. GTP causes competitive inhibition of the enhancement of the Ca2+-dependent secretory response induced by guanosine 5'-[gamma-thio]triphosphate when both nucleotides are added simultaneously. The extent of inhibition is decreased if guanosine 5'-[gamma-thio]triphosphate is added prior to GTP. GTP markedly enhances the effect of thrombin on Ca2+-dependent 5-hydroxytryptamine secretion by increasing the maximal extent of the response and decreasing the thrombin concentration required to give half-maximal response. A similar effect is observed on addition of guanosine 5'-[gamma-thio]triphosphate in the presence of thrombin at short incubation times. On more prolonged incubation the effects of thrombin and guanosine 5'-[gamma-thio]triphosphate are additive. Guanosine 5'-[beta-thio]diphosphate completely inhibits the response induced by guanosine 5'-[gamma-thio]triphosphate or guanosine 5'-[beta,gamma-imido]triphosphate but has little effect on the response induced by Ca2+ when added alone or in the presence of thrombin. Partial inhibition is observed for the response induced by thrombin + GTP. Cyclic-AMP effectively inhibits the response induced by thrombin + GTP but has little effect on that induced by guanosine 5'-[gamma-thio]triphosphate or guanosine 5'-[beta,gamma]imidotriphosphate. The results provide further support for the proposal [Haslam, R.J. & Davidson, M.M.L. (1984) FEBS Lett. 170, 90-95], that receptor--phospholipase-C coupling in platelets is mediated in part by a guanine-nucleotide-binding (Np) protein but that a coupling mechanism may also exist which is independent of such a protein. The properties of guanine-nucleotide-dependent coupling resemble those previously described for receptor--adenylate-cyclase coupling.

Observations on the muscarinic activation of catecholamine secretion in the chicken adrenal

Cells were isolated by collagenase digestion of chicken adrenal glands. Catecholamine secretion could be stimulated by acetylcholine, carbamylcholine, potassium or veratridine. Methacholine, muscarine and oxotremorine were also effective secretagogues whereas nicotine was not. Secretion evoked by acetylcholine was blocked by low concentrations of atropine but was relatively insensitive to hexamethonium. Atropine-sensitive secretion required both external sodium and calcium, was unaffected by tetrodotoxin, blocked by methoxy verapamil and nifedipine, and potentiated by BAY-K-8644. These data suggest that muscarinic activation of these cells facilitates tetrodotoxin insensitive depolarization, thereby opening conventional voltage-sensitive calcium channels. The mechanism by which calcium activates catecholamine secretion was investigated in cells that had been made permeable by exposure to brief intense electric fields. Catecholamine release required Mg-adenosine 5' triphosphate, was half-maximally activated by 1 microM Ca2+ and could be inhibited by high concentrations of Mg2+. At low Ca2+ concentrations, release was potentiated by 12-O-tetradecanoylphorbol 13-acetate, dioctanoylglycerol, guanosine 5'-O-(3-thiotriphosphate) and 5'-guanylylimidodiphosphate, all of which increased the apparent affinity of exocytosis for Ca2+.

Inhibition of exocytosis in bovine adrenal medullary cells by botulinum toxin type D

Botulinum toxins are known to block transmitter release at peripheral cholinergic synapses, producing muscular weakness and paralysis. The toxins may also block adrenergic transmission, although this effect is less well understood. The mechanisms by which toxins act are unclear. They are proteins of relative molecular mass approximately 150,000 and are structurally similar to tetanus toxin. It is generally accepted that a rise in intracellular calcium concentration is sufficient to trigger secretion by exocytosis, but it is not known whether the toxins block secretion by preventing this Ca transient or whether they act downstream from Ca entry by interfering with the process of exocytosis itself. We have attempted to resolve these questions in the case of the adrenergic system by studying the effects of botulinum toxins (types A, B, D and E) on the secretory response of isolated bovine adrenal medullary cells maintained in culture. The cells were either challenged with various secretagogues or rendered leaky and challenged directly with Ca buffers. We report here that botulinum toxin type D inhibits secretion in a time- and dose-dependent manner, the results being entirely consistent with the idea that the toxin acts at or near the site of exocytosis rather than at the sites controlling the rise in free Ca.

Guanine nucleotides and Ca-dependent exocytosis. Studies on two adrenal cell preparations

Exposure of 'leaky' adrenal medullary cells to GTP-y-S inhibits Ca-dependent exocytosis in bovine cells, but stimulates exocytosis in chicken cells. The inhibitory action on bovine cells persists in the presence of TPA suggesting that in this tissue an inhibitory GTP-binding protein may modulate the action of protein kinase C on exocytosis.

Effect of various excitatory agonists on the secretion of 5-hydroxytryptamine from permeabilised human platelets induced by Ca2+ in the presence or absence of GTP

Addition of GTP markedly enhances the ability of thrombin to cause a leftward shift in the Ca2+ dose/response curve for 5-hydroxytryptamine secretion from permeabilised human platelets. Little effect is observed on addition of GTP in the absence of thrombin. Neither ADP nor adrenaline, in the presence or absence of GTP, causes such a shift, whereas 5-hydroxytryptamine does so to a small extent but only in the presence of GTP. The leftward shift in the Ca2+ dose/response curve induced by 12-O-tetradecanoyl-phorbol-13-acetate or 1-oleyl-2-acetylglycerol is not enhanced by addition of GTP. The thrombin concentration required for half-maximal enhancement of the response to Ca2+ is markedly reduced by addition of GTP. The results support the postulate that the effects of excitatory agonists in this system correlate with their ability to activate phospholipase C and provide further evidence for a role for GTP in signal transduction between the receptor and phospholipase C.

Calcium clamp of the intracellular environment

Quantitative analysis of the effects of calcium on cell function requires methods for altering intracellular free Ca in a precise and reproducible manner. Microinjection of Ca is very unreliable largely because of the powerful Ca-binding properties of cytoplasm. Much more satisfactory are microinjection of Ca-buffers - provided enough buffer is introduced - and various forms of intracellular dialysis and perfusion which permit full equilibration of the cell interior with a defined artificial intracellular environment.

The chromaffin granule proton pump and calcium-dependent exocytosis in bovine adrenal medullary cells

Calcium-dependent exocytosis in 'leaky' bovine adrenal medullary cells has a requirement for Mg-ATP. One possibility is that exocytosis depends in some way on the operation of the ATP-dependent proton pump that serves to maintain the core of the secretory vesicles both acid and at a positive potential with respect to the cytosol. This possibility has been tested in 'leaky' cells by monitoring exocytosis under conditions where the secretory vesicle pH and potential gradients are measured in situ. The results show rather clearly that exocytosis can persist, with unchanged Ca-activation kinetics, in the virtual absence both of a difference in pH between the cytosol and secretory vesicle core and also of a difference in potential across the vesicle membrane. The results do not, however, exclude a small modulating effect of vesicle pH or potential on exocytosis and shed no light on whether or not the plasma membrane potential, which is maintained close to zero in these experiments, influences exocytosis.

The relationship between intracellular second messengers and platelet secretion

Abbreviations used: PAF, platelet-activating factor; [Ca]i, cytosolic Ca2+ concentration; OAG, 1-oleyl-2-acetylglycerol; TPA, 12-O-tetradecanoylphorbol-13-acetate; [1,2-diocylglycerol]i, cytosolic 1,2-diacylglycerol concentration, BAPTA, bis(O-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid; IMBX, 3-isobutyl-1-methylxanthine.

Thrombin and activators of protein kinase C modulate secretory responses of permeabilised human platelets induced by Ca2+

Addition of thrombin enhances secretion of both [14C]serotonin and beta-N-acetylglucosaminidase induced by Ca2+ in human platelets rendered permeable by exposure to intense electric fields. Enhancement of beta-N-acetylglucosaminidase secretion by thrombin results from an increase in the maximal extent of the response with no significant change in the median effective concentration EC50 for Ca2+. In contrast, thrombin shifts the dose/response curve for Ca2+-induced [14C]serotonin secretion to the left and has little effect on the maximal extent of this response even when this extent is reduced by use of a non-saturating concentration of MgATP2-. The relationship between extent of response and [MgATP2-] is similar for secretion of [14C]serotonin and of beta-N-acetylglucosaminidase in the presence or absence of thrombin. Similar nucleotide specificities are also observed. Activators of protein kinase C have previously been shown to mimic quantitatively the effect of thrombin on [14C]serotonin secretion induced by Ca2+ [D. E. Knight & M. C. Scrutton (1984) Nature (Lond.) 309, 66-68]. Such activators have the same qualitative effect as thrombin on the properties of beta-N-acetylglucosaminidase secretion induced by Ca2+ but are less effective. The EC50 for thrombin observed for enhancement of [14C]serotonin and beta-N-acetylglucosaminidase secretion is in the same range as that obtained for intact platelets under comparable conditions [D. E. Knight, T. J. Hallam & M. C. Scrutton (1982) Nature (Lond.) 296, 256-257]. The EC50, and the specificity of response, observed for activators of protein kinase C in these systems are consistent with those reported previously for the purified enzyme. Addition of 1-10 microM Ca2+ to permeabilised platelets in the presence of [gamma-32P] ATP causes marked enhancement of 32P incorporation into polypeptides of molecular mass 20 kDA, 45 kDA and 66 kDA. No additional polypeptides become phosphorylated in this system when thrombin is added together with 10 microM Ca2+, but some increase is observed in the extent of phosphorylation of the 45-kDa polypeptide. Addition of 1-oleyl-2-acetylglycerol + 1 - 2 microM Ca2+ causes enhanced phosphorylation of the 45-kDa polypeptide and to a lesser extent of the 20-kDa polypeptide. The dose/response curves for Ca2+-dependent phosphorylation of the 45-kDa polypeptide in the presence and absence of 1-oleyl-2-acetylglycerol are similar to those observed for Ca2+-dependent [14C]serotonin secretion under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Tyrosine hydroxylase in "leaky" adrenal medullary cells: evidence for in situ phosphorylation by separate Ca2+ and cyclic AMP-dependent systems

The systems responsible for phosphorylating tyrosine hydroxylase, the rate-limiting enzyme of catecholamine biosynthesis, were investigated in situ in adrenal medullary cells made permeable to solutes of up to 1,000 dalton by exposure to brief intense electric fields. Two different phosphorylation systems were found. One is dependent on Ca2+, the other on cyclic AMP. The Ca2+-dependent system is half-maximally activated by 1-2 microM Ca2+ and 0.5 mM ATP, and follows a time course similar to that of secretion of catecholamines. Trifluoperazine (0.1 mM) does not inhibit significantly Ca2+-dependent phosphorylation of tyrosine hydroxylase in situ. The cyclic AMP-dependent system is half-maximally activated by addition of 0.5 microM cyclic AMP and about 0.3 mM ATP. Ca2+-dependent and cyclic AMP-dependent phosphorylations of tyrosine hydroxylase have roughly the same time course and are additive under conditions where one system is already saturated. Peptide maps of immunoprecipitated tyrosine hydroxylase, after in situ phosphorylation of the enzyme either in the presence of 10(-8)M Ca2+ plus 2 X 10(-5)M cyclic AMP or of 10(-5)M Ca2+, show a marked difference indicating that the enzyme contains several phosphorylation sites. At least one of these sites is phosphorylated only by the Ca2+-dependent system, whereas the other site(s) are phosphorylated by both the Ca2+- and cyclic AMP-dependent systems. The effect of in situ phosphorylation of tyrosine hydroxylase on its enzymatic activity was also investigated.

Ca2+ and cyclic nucleotide dependence of amylase release from isolated rat pancreatic acinar cells rendered permeable by intense electric fields

Enzyme digestion of rat pancreatic tissue yielded a preparation of isolated acinar cells, over 90% of which excluded trypan blue. These isolated cells responded to a variety of secretagogues, the responses being sensitive to the removal of extracellular calcium, increasing extracellular magnesium, and by trifluoperazine, an antagonist of Ca-dependent processes. When exposed to intense electric fields, isolated acinar cells became permeable to CaEGTA and MgATP, these markers gaining access to over 60% of the intracellular milieu within minutes. The accessibility to these markers seemed independent of the ionised Ca2+ level. Less than 0.5% of the cellular amylase was released when cells were rendered leaky in a medium containing about 10(-9) M Ca2+, but typically 4% was released when the Ca2+ level was subsequently raised to 10(-5)M levels, the EC50 for Ca2+ being 2 microM. This amount of amylase released was comparable to the amounts secreted from intact cells in response to a variety of agonists. The cytosolic marker lactate dehydrogenase was also released from leaky cells, but the extent was independent of Ca2+ concentration. No amylase was released at 10(-7)M Ca2+ when permeable cells were exposed to cyclic 3',5'-AMP or cyclic 3',5'-GMP. The calcium activation curve for amylase release seemed to be independent of cyclic nucleotides, but was markedly increased in both the extent of release and apparent affinity for Ca2+ in the presence of the phorbol ester 12-0-tetradecanoyl phorbol 13 acetate. These results suggest that when "functionally normal" isolated acinar cells are rendered permeable, Ca2+-but not cyclic nucleotides-acts as a second messenger for amylase secretion, and furthermore that protein kinase C may be involved in the secretory process.

Cyclic nucleotides control a system which regulates Ca2+ sensitivity of platelet secretion

Cellular responses to extracellular signals are mediated by changes in the intracellular concentrations of one or more second messengers. In platelets, inhibitory agonists increase intracellular cyclic-3',5'-AMP [( cyclic AMP]i (refs 2, 3] whereas excitatory agonists increase [Ca2+]i and/or [1,2-diacylglycerol]i (refs 4-9), and in some cases decrease [cyclic AMP]i (refs 10, 11). Both activation and inhibition of platelet responses have been attributed to an increase in [cyclic-3',5'-GMP]i (refs 8, 12). The activity of protein kinase C, which is associated with the platelet secretory response, is increased by both 1,2-diacylglycerol and Ca2+ (refs 4, 7, 8). The role of cyclic AMP may involve either inhibition of Ca2+ mobilization to the cytosol or stimulation of intracellular Ca2+ uptake, and in addition inhibition of 1,2-diacylglycerol formation. The relationship between cyclic-3',5'-GMP (cyclic GMP) and other second messengers in platelet activation has not been defined. Using platelets made permeable by exposure to an intense electric field, we demonstrate here modulation of the Ca2+ sensitivity of platelet secretion by thrombin, and by 12-O-tetradecanoylphorbol-13-acetate (TPA) and 1-oleyl-2- acetylglycerol ( OAG ), both potent activators of protein kinase C. The effect of thrombin is selectively modified by cyclic GMP and cyclic AMP. The response to OAG and TPA is also modulated by cyclic AMP but to a much lesser extent.

Primary linitis plastica carcinoma of urinary bladder

Primary linitis plastica type carcinoma of the urinary bladder in a sixty-one-year-old man is described. Only 2 other cases have been reported in the English medical literature.

The phorbol ester TPA increases the affinity of exocytosis for calcium in 'leaky' adrenal medullary cells

Exposure of 'leaky' bovine adrenal medullary cells to the phorbol ester TPA causes a shift in the calcium-activation curve to lower calcium concentrations without altering the levels of secretion at the extremes of the activation curve. These results are consistent with a role for protein kinase C in exocytosis.