Characterization of protein kinases from bovine parotid glands. The effect of tolbutamide and its derivative on these partially purified enzymes

Detection and characterization of a rat parotid gland protein kinase that catalyzes phosphorylation of matured destrin at Ser-2

Destrin, an actin-binding protein, is partly phosphorylated at Ser-2 (numbering on the matured form) in the resting rat parotid gland, and beta-adrenergic or cholinergic stimulation of this gland induces its dephosphorylation. In this study, we searched for the protein kinase involved in phosphorylation of destrin. We developed an assay method for the kinase, using an antibody specific to destrin phosphorylated at Ser-2, and detected the kinase in the rat parotid homogenate. This enzyme was predominantly (93%) present in the soluble fraction, and the enzyme in this fraction was characterized. It had an optimum pH at 6.8 and required 3-5 mM Mg2+ for its maximum activity. Ca2+ (1 mM) had no effect whereas Mn2+ (5 mM) inhibited the enzyme activity by 75%. The apparent Km values for destrin and ATP were 92 microg/ml and 170 microM, respectively. GTP was an inefficient phosphate donor, and cAMP had no effect. Heat-denatured destrin was poorly phosphorylated. Two-dimensional PAGE analysis of destrin phosphorylated with the soluble fraction indicated that it was exclusively phosphorylated at Ser-2. None of the protein kinase inhibitors tested here was specific to this enzyme. At 1 mM, ML-7, Y-27632, KN-93, HA-1077, H-7, and H-8 inhibited the activity by 88, 61, 58, 49, 46, and 42%, respectively. Staurosporine (2 microM) and H-89 (50 microM) inhibited the activity by 48 and 33%, respectively. Heparin (30 microg/ml) had no effect. These results suggest that the rat parotid gland contains a novel, constitutively active, soluble protein kinase catalyzing specific phosphorylation of destrin at Ser-2.

Tolbutamide blocks Ca(2+)- and voltage-dependent K+ currents of hippocampal Ca1 neurons

In current-clamp recordings with KMeSO4 electrodes (either whole-cell or intracellular), though tolbutamide (0.5-1 mM) did not change the resting potential, it increased both input resistance (by 12 +/- 3.8%) and spontaneous firing, and spikes were evoked by smaller depolarizing pulses. Tolbutamide reduced in a dose-dependent manner both components of post-burst afterhyperpolarizations: IC50 was 0.15 mM for medium afterhyperpolarizations and 0.33 mM for slow afterhyperpolarizations. In whole-cell recordings under voltage-clamp, 0.5-1 mM tolbutamide depressed slow outward currents by 65 +/- 5.3%. The tolbutamide-sensitive current was Ca(2+)-dependent-tolbutamide being ineffective in Mn2+, low Ca(2+)-containing medium-though tolbutamide did not significantly depress high voltage-activated Ca2+ currents. Tolbutamide reduced C-type outward currents by 45 +/- 5.9% and M-type current inward relaxations by 41 +/- 12.9%, as well as Q-type current inward relaxations by 22 +/- 5.7%. Glyburide (10 microM) did not depress afterhyperpolarizations or outward currents, even in recordings with electrodes containing 1 mM guanosine diphosphate. We conclude that the most prominent effects of 0.5-1 mM tolbutamide on CA1 neurons are caused by suppression of Ca(2+)-and voltage-dependent outward currents, including IAHP, IC and IM.

The hypoglycemic sulphonylurea tolbutamide increases N-methyl-D-aspartate- but not kainate-activated currents in hippocampal neurons in culture

The effects of the hypoglycemic sulphonylurea tolbutamide, a marker of K(+)-ATP channels, on the N-methyl-D-aspartate- (NMDA) and kainate-activated currents were studied in rat hippocampal neurons in culture, using the patch-clamp technique in a whole-cell configuration. Tolbutamide (500 microM) reversibly increased the peak amplitude and the steady state level of NMDA- but not kainate-evoked currents. This effect was not glycine dependent as it was observed at low and saturated concentrations of glycine. The affinity of the NMDA receptor-channel complex for glycine did not change in the presence of tolbutamide. The action of tolbutamide on the NMDA-activated current was not mediated by K(+)-ATP channels since CsCl was added intracellularly at concentrations which completely blocked all K+ channels. Possible mechanisms explaining the effect of tolbutamide via the modulation of intracellular messengers are discussed.

Tolbutamide suppresses anoxic outward current of hippocampal neurons

In hippocampal slices, 2-3 min of hypoxia often evokes a hyperpolarisation or outward current. In the presence of tetrodotoxin and kynurenic acid (to minimize indirect effects of the drugs), we applied two sulphonylureas to detect a possible involvement of ATP-sensitive K (KATP) channels. In all 9 cells tested, tolbutamide (TOLB, 0.1-1 mM) greatly reduced both the hypoxic current (by 81.3 +/- 9.4%) and the conductance increase (by 77.2 +/- 10.2%). By contrast, glibenclamide (GLIB, 10-30 microM) tested on 5 cells, had no comparable effects. We therefore conclude that if KATP channels play a role in the hypoxic response, they are likely to be of the low affinity type found in neocortical and hypothalamic neurons.

Dissociation between cellular K+ loss, reduction in repolarization time, and tissue ATP levels during myocardial hypoxia and ischemia

The mechanisms underlying the marked increase in [K+]o in response to ischemia are not fully understood. Accordingly, the present study was performed to assess the contribution of ATP-regulated K+ channels by using simultaneous measurements of cellular K+ efflux, [K+]o, transmembrane action potentials, and tissue ATP, ADP, phosphocreatine, and creatine content in a unique isolated, blood-perfused papillary muscle preparation during hypoxia compared with ischemia. During 15 minutes of hypoxic perfusion (PO2, 6.1 +/- 0.9 mm Hg) with normal [K+]o of 4.1 +/- 0.1 mM, action potential duration (APD) was not altered even though tissue ATP levels decreased markedly from 33.5 +/- 1.8 to 14.7 +/- 2.0 nmol.mg protein-1 (p < 0.01). Net cellular K+ efflux, based on measured differences of [K+] between the venous effluent and the perfusate, was 13.23 +/- 0.79 mumol.g wet wt-1 during hypoxia. In contrast, after 15 minutes of zero-flow ischemia, APD at 80% of repolarization (APD80) decreased by 47% from 171 +/- 5 to 92 +/- 5 msec (p < 0.01), but integrated net cellular K+ efflux over 15 minutes of ischemia was 8.4-fold less (1.57 +/- 0.13 mumol.g wet wt-1) than during hypoxia. Tissue ATP levels, however, decreased by only 35.2% to 21.7 +/- 2.1 nmol.mg protein-1, which was significantly less than that induced by 15 minutes of hypoxia. Perfusion with hypoxic blood containing high [K+]o of 10.3 +/- 0.3 mM resulted in APD shortening similar to that observed during ischemia. Cellular K+ loss, however, was inhibited markedly by high [K+]o perfusion (only 4.51 +/- 0.28 mumol.g wet wt-1). Pretreatment with glibenclamide (5 microM), a drug that has been reported to inhibit ATP-regulated K+ channels and accelerate glycolysis in normoxic tissue, partially inhibited cellular K+ efflux during hypoxic perfusion with normal [K+]o (7.35 +/- 0.71 versus 13.23 +/- 0.79 mumol.g wet wt-1, p < 0.01) but had no significant influence on repolarization time or tissue ATP levels. Although glibenclamide partially prevented action potential shortening induced by hypoxic perfusion in the presence of elevated [K+]o, the proportion of cellular K+ efflux reduced by glibenclamide was less (23%) than that observed with glibenclamide in hypoxic perfusion with normal [K+]o (44%).(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine responses in Helix neurons

1. Using internal perfusion and concentration-clamp procedures applied to Helix neurons, the effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine Cl-dependent responses were determined. 2. Intracellular cAMP (10(-4) M) depressed those acetylcholine responses which were blocked by ouabain but had no effect on ouabain-insensitive acetylcholine responses. In the presence of elevated intracellular cAMP, ouabain had no further depressant effect on these acetylcholine responses. Both elevated cAMP and ouabain reduced the acetylcholine response without altering the current-voltage curves. 3. An increase in intracellular Ca2+ concentration depressed the amplitude of current induced by application of acetylcholine in neurons with ouabain-sensitive responses and shifted the dose-response relationship to the right. However, elevated Ca2+ did not reduce the maximal response induced by acetylcholine, nor did it prevent the reduction of that response by ouabain. 4. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent stimulator of protein kinase C activity, caused depression of both the ouabain-sensitive and the ouabain-insensitive acetylcholine responses. The inhibitory effect of TPA was markedly enhanced after addition of ATP to the intracellular medium and was greatly reduced by cooling to 5 degrees C. The blocking effect of ouabain, however, reexamined in the presence of TPA. 5. These observations are consistent with the hypothesis that the depression of acetylcholine induced Cl--responses in Helix neurons is a result of an increase in intracellular cAMP concentration but is unrelated to activation of protein kinase C or increases in intracellular Ca2+.

Salivary cystatin SA-III, a potential precursor of the acquired enamel pellicle, is phosphorylated at both its amino- and carboxyl-terminal regions

Cystatin SA-III was purified from human submandibular/sublingual glandular secretions by adsorption to hydroxyapatite, gel filtration chromatography, and reversed-phase HPLC. The amino acid sequence of its amino-terminus was deduced by sequential Edman degradation and found to be identical to the first 10 residues of cystatin HSP-12. The purified protein was digested with endoproteinase Asp-N and the digestion products were subjected to fast atom bombardment mass spectroscopy. m/z values corresponding to 12 peptides were aligned to the sequence of cystatin S preceded by the eight-residue amino-terminal peptide detected in HSP-12. This process resulted in the assignment of peptides corresponding with 118 out of the 121 amino acid residues predicted from the nucleotide sequence for cystatin SA-III. In order to align several peptides, it was necessary to substitute four residues of phosphoserine for four residues of serine. Fast atom bombardment mass spectrometry and additional Edman degradation procedures localized the phosphate moieties to Ser-3, Ser-99, Ser-112, and Ser-116. This is the first report of the structure of cystatin SA-III deduced by amino acid sequencing techniques and indicates the sites of phosphoserine within the molecule. Based on these assignments, cystatin SA-III is unique among salivary proteins in that it possesses phosphate groups at its amino-terminus as well as its carboxyl-terminus.

The effect of oxytocin on potential-dependent calcium current in snail neurons, Helix pomatia

1. The effect of external application of oxytocin on inward calcium current in dialyzed snail neurons has been investigated under clamp conditions. 2. External application of oxytocin in a dose-dependent manner (Kd 0.9 microM) inhibits inward calcium current in dialyzed neurons of the snail, Helix pomatia. 3. Inhibition of calcium current developed with the time constant of about 2 min. The degree of restoration of calcium current after oxytocin washout depends on duration of oxytocin action. 4. It has been suggested that inhibition of calcium current by oxytocin occurs in two stages, the initial one is more fast and reversible and the second one--more slow and irreversible. The participation of soluble second messengers in the inhibitory effect of oxytocin on calcium current is discussed.

Inhibition by oxytocin of voltage-activated calcium influx in cultured PC12 pheochromocytoma cells

1. Voltage-activated dihydropyridine-sensitive Ca2+ influx was measured in PC12 pheochromocytoma cells using 45Ca. 2. It has been found that oxytocin inhibits voltage-activated dihydropyridine-sensitive Ca2+ influx with ED50 about 0.30 x 10(-6) M. 3. Tolbutamide (1.3 x 10(-3) M) has no visible effect on both Ca2+ influx itself and on the inhibitory oxytocin effect. 4. External application of Li+ (10 mM) causes a slight shift of ED-curve to lower oxytocin concentrations. 5. It is suggested that the hydrolysis of phosphoinositides may play a role in oxytocin action on Ca2+ influx in PC12 cells.

Stimulation of synaptosomal dopamine synthesis by corticotropin-releasing factor in rat striatum: role of Ca2+-dependent mechanisms

Corticotropin-releasing factor (CRF) stimulated synaptosomal dopamine (DA) synthesis in rat striatal homogenates. The stimulatory effect of CRF was antagonized by alpha-helical CRF 9-41 and was dependent on the extracellular Ca2+ concentration, being absent after removal of Ca2+ from the incubation medium and maximal at about 1.2 mM extracellular Ca2+. The stimulation of DA synthesis by forskolin (FSK) was less sensitive to a change of extracellular Ca2+ concentration. 3-Isobutyl-1-methylxanthine potentiated the FSK effect but did not affect the response to CRF. CRF stimulation was additive to that produced by 40 mM KCl and was little affected by the calmodulin inhibitor, W-7, which markedly suppressed the veratridine-induced stimulation of DA synthesis. Polymyxin B antagonized the CRF stimulation while it had a weaker effect on FSK-stimulated DA synthesis. Tolbutamide reduced the FSK stimulation more effectively than the CRF response. CRF elicited a non-significant increase of cyclic AMP formation by striatal membranes. These results indicate that CRF stimulates striatal DA synthesis by a Ca2+-dependent mechanism possibly involving the activation of the protein kinase C pathway.

Parallel processing and selection of the responses to serotonin during reinnervation of an identified leech neuron

In an attempt to define the mechanism of synaptic specificity, we have been studying pairs of identified leech neurons isolated in tissue culture. The cultured neurons reform specific synapses when paired with appropriate partners in the absence of other cell types. In recent studies, we have examined in detail the reformation of a serotoninergic synapse between the Retzius cell and one of its targets, the pressure sensitive (P) cell. The P cell in vivo and its soma in vitro have two types of responses to serotonin (5-HT). From voltage clamp analysis of cultured P cells, we demonstrated the parallel activation of chloride (gCls) and monovalent cation (gCations) channels coupled to distinct receptor subtypes and gated by separate second messengers. Only gCls was activated by 5-HT released from the presynaptic Retzius cell both in vivo and in vitro. This demonstrates the remarkable specificity of the reformation of this synapse in culture since only the correct 5-HT receptor subtype is activated. An 80% reduction of gCations was observed in P cells that had failed to be innervated by Retzius cells in culture, suggesting that gCations may be lost prior to synapse formation. Retzius cells depleted of 5-HT also reduced gCations in the paired P cells and incubating single P cells in 5-HT did not reduce gCations. In addition, aldehyde-fixed Retzius cells were able to selectively reduce gCations when paired with P cells. We conclude that the loss of gCations was due to contact between the neurons. The early clearing of counter-effective receptor subtypes may be a prelude to synapse formation.

Unusual biochemistry of changes in neuron membrane permeability evoked by cAMP

Influence of different metabolic poisons on cAMP-evoked neuron membrane permeability is investigated. Drugs preventing cAMP binding with R subunits of protein kinase decrease the cAMP-evoked current, but the inhibitor of the C subunit. H8, has no effect. The cAMP-dependent current is increased by uncouplers and decreased by inhibitors of glycolysis and oxidative phosphorylation. The mechanism of cAMP action on neuron permeability is discussed.

Phosphorylation of salivary proteins by salivary gland protein kinase

Human saliva contains a number of phosphorylated acidic proline-rich proteins (APRP). Monkey parotid saliva contains a similar protein with the same phosphorylated sequences as the human proteins. A crude protein kinase was prepared from Macaca fascicularis parotid glands which phosphorylated human APRP. The enzyme was activated by Mg2+, it had a pH optimum between pH 7.0 and 7.5, the Km for ATP was 78 mumol/L, and for APRP it was 85 mumol/L. Phosphorylation of APRP was independent of cAMP and calmodulin. Phosphate was incorporated as phosphoserine, and the kinase phosphorylated the same residues in dephosphorylated APRP which are phosphorylated in the secreted protein. In addition, the enzyme preparation also phosphorylated dephosphorylated and native APRP in a region which is not phosphorylated in the secreted protein. There was no difference in the rate of phosphorylation of APRPs and their tryptic peptides. The kinase also phosphorylated other dephosphorylated salivary phosphoproteins. An enzyme was demonstrated in the human salivary gland which gave the same pattern of phosphorylation of APRP as did the simian kinase. More than one kinase may be necessary for the observed phosphorylation.

Comparison of cyclic AMP-dependent protein kinases from salivary glands of four species

Cyclic AMP-dependent protein kinase activity, subcellular distribution, and isozyme profile were compared in rabbit, rat, guinea pig, and mouse in both parotid and submandibular glands. Glands were homogenized under hypotonic conditions and the following fractions isolated: 600 g pellet, 27,000 g pellet, and 27,000 g supernatant. The specific activity of the enzyme was similar in the eight glands and was highest in the 27,000 g supernatant. The average activity in the 27,000 g supernatant was approximately 75% of the total gland activity, although there was considerable variability between tissues and species. After being washed with isotonic buffer, this percentage was increased to an average of 84%. When isozyme patterns of the kinase were examined, the rabbit parotid was unique in that it contained a high percentage of isozyme I as isolated on DEAE cellulose columns.

Surface charge of mammalian neurones as revealed by microelectrophoresis

The surface charge of isolated rat dorsal root ganglion neurones was studied by microelectrophoresis technique. The increase of Ca concentration caused greater reduction of the electrophoretic mobility compared to that produced by an equivalent amount of divalent organic cations, dimethonium or hexamethonium. No charge reversal for Ca concentrations up to 80 mM was observed. These data fit the suggestion that two anion groups of the outer membrane surface can bind one Ca ion with apparent binding constant of about 50 M-1. In solutions of low pH the electrophoretic mobility of cells decreased corresponding to titration of acidic groups with apparent pK = 4.2. Trypsin treatment in mild conditions markedly reduced the surface charge; however, neuraminidase and hyaluronidase did not change it. N-bromosuccinimide (a specific reagent for carboxylic groups of proteins) decreased the electrophoretic mobility about 60%. However, no increase of the surface charge after the action of specific reagents for amino groups (2,4,6-trinitrobenzene-sulfonic acid and maleic anhydride) was observed. It was shown that the surface charge depends also on the intracellular metabolism. If 1 mM dibutyryl cAMP or theophilline was added to the culture medium (thus, raising the concentration of cAMP inside the cell) the surface charge increased. This effect developed slowly and reached its maximum on the third day of incubation. Treatment of cells by 5 mM tolbutamide (an inhibitor of some protein kinases) did not change cell mobility. Addition of 5 mM N-ethylmaleimide (an inhibitor of adenylate cyclase) to the culture medium produced some decrease of the surface charge.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular protein kinase and calcium inward currents in perfused neurones of the snail Helix pomatia

Changes in the amplitude of the calcium inward current caused by intracellular administration of tolbutamide (an inhibitor of the cyclic AMP-dependent protein kinase activity) or catalytic subunits of cAMP-dependent protein kinases from rabbit myocardium were studied on internally perfused nerve cells of the snail, Helix pomatia. Intracellular administration of 7 mM tolbutamide caused a rapid decline of the amplitude of the calcium current that had been stabilized by theophylline; the effect was practically completely reversible. In contrast, addition to the perfusing solution of exogenous catalytic subunits of cyclic AMP-dependent protein kinases (about 0.7 microM of protein) together with 2 mM adenosine 5'-triphosphate and 3 mM MgCl2, led to stabilization of the calcium conductance of the cell membrane or restored it if it had declined during the perfusion with basic solution. The effect depended largely on the presence of adenosine 5'-triphosphate. Its time course was very slow (dozens of minutes) due probably to slow diffusion of the protein inside the cell. Heat-inactivated catalytic subunits did not produce such a stabilizing or restoring action on the calcium conductance. The results substantiate the suggestion that the normal functioning of calcium channels depends on phosphorylation catalyzed by cyclic AMP-dependent protein kinases.

The effect of intracellular cAMP injections on stationary membrane conductance and voltage- and time-dependent ionic currents in identified snail neurons

3'5'-cAMP injected iontophoretically into identified neurons of the snail, Helix pomatia, induced depolarization of the membrane. Clamping the membrane potential revealed the appearance of a simultaneous inward transmembrane current ('cAMP-current') followed by a weaker outward current. External application of theophylline increased the amplitude of cAMP-current. Imidazole had an opposite effect on this current. Tolbutamide and lowering of temperature largely reduced its rate of rise and, correspondingly, its amplitude. Simultaneous removal of Na+ ions from external solution and addition of Cd2+ ions resulted in complete disappearance of the inward cAMP-current. Analysis of current-voltage characteristics of the cAMP-current at varying external concentrations of Na+, K+, Cl and Ca2+-ions has shown that the cAMP-current is due to an increase of membrane conductance to Na+, K+ and Ca2+ ions; a late component of the cAMP-current is associated with an increase of potassium conductance of the neuronal membrane induced, probably by Ca2+, influx. Besides the induction of stationary currents, cAMP injection also decreased the voltage- and time-dependent calcium currents reducing the maximum calcium conductance. After the end of injection, calcium currents restored their initial value in 1-2 min. An analogous decrease of the calcium current could be evoked by prolonged external application of theophylline. Possible mechanisms of intracellular effects of cAMP on electrical characteristics of the neuronal membrane are discussed.

Effect of tolbutamide on the protein secretion of the rat salivary glands

Protein and amylase content as well as the distribution of isoamylases were determined in the parotid and the submandibular glands of the rat following treatment with tolbutamide for 3-21 days. Significant changes were only observed in the submandibular glands in which, compared with the controls, the amylase activity decreased distinctly after 3 days but had increased 7-fold after 21 days of tolbutamide. These findings could be explained by secretion of epinephrine from the adrenal medulla supposed to take place independently of the effect of insulin and by the possible inhibition of a protein kinase involved in the process of protein secretion from the submandibular gland.

Role of neurotransmitter release and cyclic AMP-dependent membrane phosphorylation in low voltage myocardial automaticity

Low voltage myocardial automaticity (LVA) was investigated by pharmacological modulations of the presynaptic and postsynaptic processes. The sensitivity of LVA both to inhibitor and stimulator of neurotransmitter release suggests its involvement in LVA genesis. Moreover, LVA is blocked by the inhibition of the cyclic AMP system, supporting the participation of the c-AMP-dependent membrane phosphorylation in calcium-mediated cardiac electrogenesis.