Preparation of solutions with free calcium concentration in the nanomolar range using 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid

Oriented endocytic recycling of α5β1 in motile neutrophils

During cell migration, integrin attachments to the substratum provide the means to generate the traction and force necessary to achieve locomotion. Once the cell has moved over these attachments, however, it is equally important that integrins detach from the substratum. The fate of integrins after detachment may include release from the cell, lateral diffusion across the cell surface, or endocytosis and redelivery to the cell surface. Polymorphonuclear neutrophils (PMNs) become stuck on the extracellular matrix proteins fibronectin and vitronectin when their intracellular free calcium concentration ([Ca++]i) is buffered. Taking advantage of this feature of PMN migration, we investigated the fate of integrins to differentiate among various models of migration. We demonstrate that 5β1, one of the fibronectin-binding integrins, is responsible for immobilization of [Ca++]i-buffered PMNs on fibronectin. We find that 5 and β1 are in endocytic vesicles in PMNs and that 5 colocalizes with a marker for an endocytic recycling compartment. When [Ca++]i is buffered, 5 and β1 become concentrated in clusters in the rear of the adherent cells, suggesting that [Ca++]i transients are required for 5β1 detachment from the substratum. Inhibition of 5β1 detachment by buffering [Ca++]i results in the depletion of 5 from both endocytic vesicles and the recycling compartment, providing compelling evidence that integrins are normally recycled by way of endocytosis and intracellular trafficking during cell migration. This model is further refined by our demonstration that the endocytic recycling compartment reorients to retain its localization just behind the leading lamella as PMNs migrate, indicating that membrane recycling during neutrophil migration has directionality.

Oriented endocytic recycling of α5β1 in motile neutrophils

During cell migration, integrin attachments to the substratum provide the means to generate the traction and force necessary to achieve locomotion. Once the cell has moved over these attachments, however, it is equally important that integrins detach from the substratum. The fate of integrins after detachment may include release from the cell, lateral diffusion across the cell surface, or endocytosis and redelivery to the cell surface. Polymorphonuclear neutrophils (PMNs) become stuck on the extracellular matrix proteins fibronectin and vitronectin when their intracellular free calcium concentration ([Ca++]i) is buffered. Taking advantage of this feature of PMN migration, we investigated the fate of integrins to differentiate among various models of migration. We demonstrate that 5β1, one of the fibronectin-binding integrins, is responsible for immobilization of [Ca++]i-buffered PMNs on fibronectin. We find that 5 and β1 are in endocytic vesicles in PMNs and that 5 colocalizes with a marker for an endocytic recycling compartment. When [Ca++]i is buffered, 5 and β1 become concentrated in clusters in the rear of the adherent cells, suggesting that [Ca++]i transients are required for 5β1 detachment from the substratum. Inhibition of 5β1 detachment by buffering [Ca++]i results in the depletion of 5 from both endocytic vesicles and the recycling compartment, providing compelling evidence that integrins are normally recycled by way of endocytosis and intracellular trafficking during cell migration. This model is further refined by our demonstration that the endocytic recycling compartment reorients to retain its localization just behind the leading lamella as PMNs migrate, indicating that membrane recycling during neutrophil migration has directionality.

Effects of cytoplasmic and luminal pH on Ca(2+) release channels from rabbit skeletal muscle

Ryanodine receptor (RyR)-Ca(2+) release channels from rabbit skeletal muscle were incorporated into lipid bilayers. The effects of cytoplasmic and luminal pH were studied separately over the pH range 5-8, using half-unit intervals. RyR activity (at constant luminal pH of 7.5) was inhibited at acidic cytoplasmic pH, with a half-inhibitory pH (pH(I)) approximately 6.5, irrespective of bilayer potential and of whether the RyRs were activated by cytoplasmic Ca(2+) (50 microM), ATP (2 or 5 mM), or both. Inhibition occurred within approximately 1 s and could be fully reversed within approximately 1 s after brief inhibition or within approximately 30-60 s after longer exposure to acidic cytosolic pH. There was no evidence of any hysteresis in the cytoplasmic pH effect. Ryanodine-modified channels were less sensitive to pH inhibition, with pH(I) at approximately 5.5, but the inhibition was similarly reversible. Steady-state open and closed dwell times of RyRs during cytoplasmic pH inhibition suggest a mechanism where the binding of one proton inhibits the channel and the binding of two to three additional protons promotes further inhibited states. RyR activity was unaffected by luminal pH in the pH range 7.5 to 6.0. At lower luminal pH (5-5.5) most RyRs were completely inhibited, and raising the pH again produced partial to full recovery in only approximately 50% of cases, with the extent of recovery not detectably different between pH 7.5 and pH 9. The results indicate that isolated skeletal muscle RyRs are not inhibited as strongly by low cytoplasmic and luminal pH, as suggested by previous single-channel studies.

The effect of static magnetic fields on the rate of calcium/calmodulin-dependent phosphorylation of myosin light chain

This study reports an attempt to confirm a published and well-defined biological effect of magnetic fields. The biological model investigated was the phosphorylation of myosin light chain in a cell free system. The rate of phosphorylation has been reported to be affected in an approximately linear manner by static magnetic field strengths in the range 0-200 microT. We performed three series of experiments, two to test the general hypothesis and a third that was a direct replication of published work. We found no effect of static magnetic field strength on the rate of phosphorylation. Hence, we were unable to confirm that weak static magnetic fields affect the binding of calcium to calmodulin. In view of the difficulty we and other authors have had making independent verifications of claimed biological effects of magnetic fields, we would urge caution in the interpretation of published data until they have been independently confirmed. There are still few well defined biological effects of low level magnetic fields that have been successfully transferred to an independent laboratory.

Biochemical analysis of a dimerization domain mutation in RetGC-1 associated with dominant cone-rod dystrophy

Mutations in the photoreceptor membrane guanylyl cyclase RetGC-1 have been linked to autosomal dominant cone-rod dystrophy. Three mutations were identified that alter strictly conserved residues within the RetGC-1 dimerization domain, a region predicted to form an amphipathic alpha-helical coil. Here we report on a biochemical characterization of one of the mutations, a substitution of cysteine for arginine at residue 838. We generated this mutation in vitro and measured its catalytic activity and sensitivity to guanylyl cyclase activating protein 1 (GCAP-1) and GCAP-2. The R838C substitution has several effects. It reduces the overall catalytic ability of RetGC-1 and dramatically reduces stimulation by GCAP-2, although GCAP-2 still appears to interact with the protein. The R838C substitution also increases the apparent affinity of RetGC-1 for GCAP-1 and alters the Ca(2+) sensitivity of the GCAP-1 response, allowing the mutant to be stimulated by GCAP-1 at higher Ca(2+) concentrations than wild type. The diminished response to GCAP-2, which we propose is not likely the cause of cone-rod degeneration in these patients, is interesting mechanistically because it separates the ability to bind a specific GCAP from the ability to be stimulated by it, and it also discriminates between the mechanisms of activation of GCAP-1 vs. GCAP-2. We suggest that the gain-of-function effects of R838C on RetGC-1 stimulated by GCAP-1, which are dominant in vitro and may cause an abnormal increase in cGMP synthesis in dark-adapted photoreceptors, may be the cause of the cone-rod degeneration.

A zinc-dependent Cl- current in neuronal somata

Extracellular Zn2+ modulates current passage through voltage- and neurotransmitter-gated ion channels, at concentrations less than, or near, those produced by release at certain synapses. Electrophysiological effects of cytoplasmic Zn2+ are less well understood, and effects have been observed at concentrations that are orders of magnitude greater than those found in resting and stimulated neurons. To examine whether and how neurons are affected by lower levels of cytoplasmic Zn2+, we tested the effect of Zn2+-selective chelators, Zn2+-preferring ionophores, and exogenous Zn2+ on neuronal somata during whole-cell patch-clamp recordings. We report here that cytoplasmic zinc facilitates the downward regulation of a background Cl- conductance by an endogenous protein kinase C (PKC) in fish retinal ganglion cell somata and that this regulation is maintained if nanomolar levels of free Zn2+ are available. This regulation has not been described previously in any tissue, as other Cl- currents have been described as reduced by PKC alone, reduced by Zn2+ alone, or reduced by both independently. Moreover, control of cation currents by a zinc-dependent PKC has not been reported previously. The regulation we have observed thus provides the first electrophysiological measurements consistent with biochemical measurements of zinc-dependent PKC activity in other systems. These results suggest that contributions of background Cl- conductances to electrical properties of neurons are susceptible to modulation.

Effects of ivermectin and midecamycin on ryanodine receptors and the Ca2+-ATPase in sarcoplasmic reticulum of rabbit and rat skeletal muscle

1. Ryanodine receptor (RyR) Ca2+ channels in the sarcoplasmic reticulum (SR) of skeletal muscle are regulated by the 12 kDa FK506- (or rapamycin-) binding protein (FKBP12). Rapamycin can also activate RyR channels with FKBP12 removed, suggesting that compounds with macrocyclic lactone ring structures can directly activate RyRs. Here we tested this hypothesis using two other macrocyclic lactone compounds, ivermectin and midecamycin. 2. Rabbit skeletal RyRs were examined in lipid bilayers. Ivermectin (cis, 0.66-40 microM) activated six of eight native, four of four control-incubated and eleven of eleven FKBP12-'stripped' RyR channels. Midecamycin (cis, 10-30 microM) activated three of four single native channels, six of eight control-incubated channels and six of seven FKBP12-stripped channels. Activity declined when either drug was washed out. 3. Neither ivermectin nor midecamycin removed FKBP12 from RyRs. Western blots of terminal cisternae (TC), incubated for 15 min at 37 C with 40 microM ivermectin or midecamycin, showed normal amounts of FKBP12. In contrast, no FKBP12 was detected after incubation with 40 microM rapamycin. 4. Ivermectin reduced Ca2+ uptake by the SR Ca2+-Mg2+-ATPase. Ca2+ uptake by TC fell to approximately 40% in the presence of ivermectin (10 microM), both with and without 10 microM Ruthenium Red. Ca2+ uptake by longitudinal SR also fell to approximately 40% with 10 microM ivermectin. Midecamycin (10 microM) reduced Ca2+ uptake by TC vesicles to approximately 76% without Ruthenium Red and to approximately 90 % with Ruthenium Red. 5. The rate of rise of extravesicular [Ca2+] increased approximately 2-fold when 10 microM ivermectin was added to TC vesicles that had been partially loaded with Ca2+ and then Ca2+ uptake blocked by 200 nM thapsigargin. Ivermectin also potentiated caffeine-induced Ca2+ release to approximately 140% of control. These increases in Ca2+ release were not seen with midecamycin. 6. Ivermectin, but not midecamycin, reversibly reduced Ca2+ loading in four of six skinned rat extensor digitorum longus (EDL) fibres to approximately 90%, and reversibly increased submaximal caffeine-induced contraction in five of eight fibres by approximately 110% of control. Neither ivermectin nor midecamycin altered twitch or tetanic tension in intact EDL muscle fibres within 20 min of drug addition. 7. The results confirm the hypothesis that compounds with a macrocyclic lactone ring structure can directly activate RyRs. Unexpectedly, ivermectin also reduced Ca2+ uptake into the SR. These effects of ivermectin on SR Ca2+ handling may explain some effects of the macrolide drugs on mammals.

Accumulation of bisphosphonates in human artery and their effects on human and rat arterial function in vitro

Clodronate, etidronate and pamidronate are bisphosphonates introduced in the treatment of hypercalcaemia and osteoporosis. Interestingly, they also inhibit development of experimental atherosclerosis and affect smooth muscle tone of isolated rat tail artery. We have studied in vitro whether these hydrophilic compounds 1) accumulate in the wall of the human artery, 2) influence human arterial tone, and 3) interfere with the vascular action of L-type Ca2+ antagonists. Human internal mammary artery rings were incubated with 14C-labelled bisphosphonates. After a 2-hr incubation, the ratios of artery-to-incubate concentrations with 4 and 40 mumol/l of clodronate were, respectively, 3.0 +/- 0.5 (mean +/- S.E.M.) and 1.3 +/- 0.2, with 4 and 40 mumol/l of etidronate 7.4 +/- 0.9, and 3.2 +/- 0.4, and with 0.4 and 4 mumol/l of pamidronate 4.7 +/- 0.7 and 3.9 +/- 0.8. Both tested bisphosphonates, clodronate and pamidronate, reduced the arterial contractile force induced by alpha-adrenergic stimulation with noradrenaline and membrane depolarization with high concentration of KCl. Clodronate also decreased the arterial contraction induced by cumulative addition of Ca2+ with KCl as the agonist, and had an additive inhibitory effect on this response with the L-type Ca2(+)-channel blocker nifedipine. The results demonstrate that 1) bisphosphonates accumulate markedly in human artery, 2) clodronate and pamidronate reduce human arterial contactile force to alpha-adrenergic and depolarizing stimuli, and 3) as shown with clodronate, bisphosphonates may exert an additive inhibitory effect on human arterial contractions with an L-type Ca2(+)-channel blocker.

ATP inhibition and rectification of a Ca2+-activated anion channel in sarcoplasmic reticulum of skeletal muscle

We describe ATP-dependent inhibition of the 75-105-pS (in 250 mM Cl-) anion channel (SCl) from the sarcoplasmic reticulum (SR) of rabbit skeletal muscle. In addition to activation by Ca2+ and voltage, inhibition by ATP provides a further mechanism for regulating SCl channel activity in vivo. Inhibition by the nonhydrolyzable ATP analog 5'-adenylylimidodiphosphate (AMP-PNP) ruled out a phosphorylation mechanism. Cytoplasmic ATP (approximately 1 mM) inhibited only when Cl- flowed from cytoplasm to lumen, regardless of membrane voltage. Flux in the opposite direction was not inhibited by 9 mM ATP. Thus ATP causes true, current rectification in SCl channels. Inhibition by cytoplasmic ATP was also voltage dependent, having a K(I) of 0.4-1 mM at -40 mV (Hill coefficient approximately 2), which increased at more negative potentials. Luminal ATP inhibited with a K(I) of approximately 2 mM at +40 mV, and showed no block at negative voltages. Hidden Markov model analysis revealed that ATP inhibition 1) reduced mean open times without altering the maximum channel amplitude, 2) was mediated by a novel, single, voltage-independent closed state (approximately 1 ms), and 3) was much less potent on lower conductance substates than the higher conductance states. Therefore, the SCl channel is unlikely to pass Cl- from cytoplasm to SR lumen in vivo, and balance electrogenic Ca2+ uptake as previously suggested. Possible roles for the SCl channel in the transport of other anions are discussed.

Ca2+- and voltage-dependent inactivation of Ca2+ channels in nerve terminals of the neurohypophysis

Ca2+ channel inactivation was investigated in neurohypophysial nerve terminals by using patch-clamp techniques. The contribution of intracellular Ca2+ to inactivation was evaluated by replacing Ca2+ with Ba2+ or by including BAPTA in the internal recording solution. Ca2+ channel inactivation during depolarizing pulses was primarily voltage-dependent. A contribution of intracellular Ca2+ was revealed by comparing steady-state inactivation of Ca2+ channels with Ca2+ current and with intracellular [Ca2+]. However, this contribution was small compared to that of voltage. In contrast to voltage-gated Ca2+ channels in other preparations, in the neurohypophysis Ba2+ substitution or intracellular BAPTA increased the speed of inactivation while reducing the steady-state level of inactivation. Ca2+ channel recovery from inactivation was studied by using a paired-pulse protocol. The rate of Ca2+ channel recovery from inactivation at negative potentials was increased dramatically by Ba2+ substitution or intracellular BAPTA, indicating that intracellular Ca2+ inhibits recovery. Stimulation with trains of brief pulses designed to mimic physiological bursts of electrical activity showed that Ca2+ channel inactivation was much greater with 20 Hz trains than with 14 Hz trains. Inactivation induced by 20 Hz trains was reduced by intracellular BAPTA, suggesting an important role for Ca2+-dependent inactivation during physiologically relevant forms of electrical activity. Inhibitors of calmodulin and calcineurin had no effect on Ca2+ channel inactivation, arguing against a mechanism of inactivation involving these Ca2+-dependent proteins. The inactivation behavior described here, in which voltage effects on Ca2+ channel inactivation predominate at positive potentials and Ca2+ effects predominate at negative potentials, may be relevant to the regulation of neuropeptide release.

Measurement of intracellular pH and pCa with a confocal microscope

In the late 1980s, the field of biological confocal microscopy exploded. So did traffic on the Internet. Considering the ongoing interest in the role of intracellular pH and pCa in all aspects of cell physiology, it is not surprising that the most frequently asked question on the Internet's confocal forum has been: 'How do I measure pH/pCa with a confocal microscope?' This article was inspired by these Internet discussions and attempts to answer this question by presenting the rationale for using (or not using) a confocal approach to measure intracellular ion concentration, assessing the feasibility of performing this task with currently prevailing hardware, assembling the currently available 'know-how' and telling 'how'.

ATPase activity of myosin in hair bundles of the bullfrog's sacculus

Mechanoelectrical transduction by a hair cell displays adaptation, which is thought to occur as myosin-based molecular motors within the mechanically sensitive hair bundle adjust the tension transmitted to transduction channels. To assess the enzymatic capabilities of the myosin isozymes in hair bundles, we examined the actin-dependent ATPase activity of bundles isolated from the bullfrog's sacculus. Separation of 32P-labeled inorganic phosphate from unreacted [gamma-32P]ATP by thin-layer chromatography enabled us to measure the liberation of as little as 0.1 fmol phosphate. To distinguish the Mg(2+)-ATPase activity of myosin isozymes from that of other hair-bundle enzymes, we inhibited the interaction of hair-bundle myosin with actin and determined the reduction in ATPase activity. N-ethylmaleimide (NEM) decreased neither physiologically measured adaptation nor the nucleotide-hydrolytic activity of a 120-kDa protein thought to be myosin 1 beta. The NEM-insensitive, actin-activated ATPase activity of myosin increased from 1.0 fmol x s-1 in 1 mM EGTA to 2.3 fmol x s-1 in 10 microM Ca2+. This activity was largely inhibited by calmidazolium, but was unaffected by the addition of exogenous calmodulin. These results, which indicate that hair bundles contain enzymatically active, Ca(2+)-sensitive myosin molecules, are consistent with the role of Ca2+ in adaptation and with the hypothesis that myosin forms the hair cell's adaptation motor.

Inactivation of EF-hands makes GCAP-2 (p24) a constitutive activator of photoreceptor guanylyl cyclase by preventing a Ca2+-induced "activator-to-inhibitor" transition

Guanylyl cyclase activator proteins GCAP-1 and GCAP-2 (Dizhoor et al. , 1995, Gorczyca et al., 1995) are members of a recently identified subclass of EF-hand type Ca2+-binding proteins that respond to Ca2+ differently than any other known members of the EF-hand superfamily. GCAPs acquire an activating conformation only in their Ca2+-free form. Free Ca2+ concentrations corresponding to levels in dark-adapted vertebrate photoreceptors inhibit the ability of GCAPs to activate photoreceptor guanylyl cyclases (RetGCs). We studied the effects of mutations that block binding of Ca2+ to the EF-hands of GCAP-2. Unlike other EF-hand proteins, which fail to activate their target when their EF-hands are inactivated by mutations, GCAP-2 with any single EF-hand inactivated remains active and is 3-6 times less sensitive to the inhibitory effect of Ca2+. Inactivation of any two or all three EF-hands produces active forms of GCAP-2 that are insensitive to inhibition by physiological intracellular concentrations of Ca2+. Unexpectedly we also found that activation of RetGCs by a Ca2+-insensitive mutant is inhibited by Ca2+-loaded wild type GCAP-2. We propose the following. 1) GCAP-2 can exist in two extreme functional forms: an apo form that activates RetGCs and a Ca2+-loaded form that blocks activation of RetGCs. 2) All three EF-hands of GCAP-2 contribute to the inhibitory effect of Ca2+. 3) Inactivation of two or three EF-hands is sufficient to shift the "activator-inhibitor" transition outside the physiological range of intracellular free Ca2+.

The membrane guanylyl cyclase, retinal guanylyl cyclase-1, is activated through its intracellular domain

Retinal guanylyl cyclase-1 (RetGC-1) is a membrane guanylyl cyclase found in photoreceptor outer segments. It consists of an apparent extracellular domain (ECD) linked by a single transmembrane segment to an intracellular domain (ICD). Guanylyl cyclase activating protein-2 (GCAP-2) is a Ca2+-binding protein that activates RetGC-1 in a Ca2+-sensitive manner. To establish whether GCAP-2 stimulates RetGC-1 through the ECD or ICD, we made deletion mutants lacking either the ECD or both the ECD and transmembrane domains (TMD) of RetGC-1. Recombinant wild type RetGC-1 and both deletion mutants were expressed in HEK 293 cells, and their sensitivities to GCAP-2, Ca2+, and ATP were compared. Our data demonstrate that both deletion mutants are regulated similarly to wild type RetGC-1 with indistinguishable EC50 values for Ca2+ and similar K1/2 values for activation by GCAP-2. This shows that GCAP-2 functions through the ICD of RetGC-1 and that removal of the ECD and TMD do not significantly alter regulation by these factors. Our data also show that ATP potentiates stimulation of guanylyl cyclase activity by GCAP-2 and that neither the ECD nor the TMD of RetGC-1 participate in its regulation by ATP.

Cloning, sequencing, and expression of a 24-kDa Ca(2+)-binding protein activating photoreceptor guanylyl cyclase

Two vertebrate photoreceptor-specific membrane guanylyl cyclases, RetGC-1 and RetGC-2, are activated by a soluble 24-kDa retinal protein, p24, in a Ca(2+)-sensitive manner (Dizhoor, A.M., Lowe, D.G., Olshevskaya, E.V., Laura, R.P., and Hurley, J.B. (1994) Neuron 12, 1345-1352; Lowe, D.G., Dizhoor, A.M., Liu, K., Gu, O., Laura, R., Lu, L., and Hurley, J.B. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 5535-5539). The primary structure of bovine p24 has been derived from peptide sequencing and from its cDNA. p24 is a new EF-hand-type Ca(2+)-binding protein, related but not identical to another guanylyl cyclase-activating protein, GCAP (Palczewski, K., Subbaraya, I., Gorczyca, W.A., Helekar, B.S., Ruiz, C.C., Ohguro, H. Huang, J., Zhao, X., Crabb, J.W., Johnson, R.S., Walsh, K.A., Gray-Keller, M.P., Detwiler, P.B., and Baehr, W. (1994) Neuron 13, 395-404) and other members of the recovering family of Ca(2+)-binding proteins. Antibodies against a truncated fusion protein and against a p24-specific synthetic peptide specifically recognize retinal p24 on immunoblot. Both antibodies inhibit activation of photoreceptor membrane guanylyl cyclase by purified p24. p24 is found only in retina, and it copurifies with outer segment membranes. Immunocytochemical analysis shows that it is present in rod photoreceptor cells. An immobilized antibody column was used to purify p24 from a heat-treated retinal extract. Purified p24 appears on SDS-polyacrylamide gel electrophoresis as a homogeneous protein not contaminated with GCAP, and it activates photoreceptor guanylyl cyclase in vitro at submicromolar concentrations. Ca2+ inhibits this activation with an EC50 near 200 nM and a Hill coefficient of 1.7. Recombinant p24 expressed in 293 cells effectively stimulates photoreceptor guanylyl cyclase. These findings demonstrate that p24, like GCAP, imparts Ca2+ sensitivity to photoreceptor membrane guanylyl cyclase. We propose that p24 be referred to as GCAP-2 and that GCAP be referred to as GCAP-1.

Different spatial patterns of [Ca2+] increase caused by N- and L-type Ca2+ channel activation in frog olfactory bulb neurones

1. The intracellular calcium concentration ([Ca2+]i) in cultured olfactory bulb neurones of Xenopus laevis tadpoles was imaged using the calcium indicator dyes fluo-3 and Fura Red as well as a laser scanning microscope. 2. Upon extracellular application of brief pulses of a solution with high potassium concentration (high [K+]o), an increase in [Ca2+]i occurred in all neurones observed. During the first 2 days in culture this increase was highest. At later stages (more than 2 days in culture) the increase in [Ca2+]i was non-homogeneous and highest in the dendritic processes. 3. Nifedipine (10 microM) reduced the high [K+]o-induced increase in [Ca2+]i. The reduction was greatest in somata and proximal dendrites. 4. With nifedipine in the bath, the high [K+]o-induced increase of [Ca2+]i was further reduced by the application of omega-conotoxin GVIA (1 microM). The omega-conotoxin-sensitive Ca2+ influx occurred predominantly on dendritic processes. 5. Noradrenaline (NA), as well as the alpha 2-adrenergic receptor agonist clonidine, reduced the high [K+]o-induced increase of [Ca2+]i. This reduction occurred mainly on dendritic processes. 6. Our results suggest a highly non-homogeneous spatial distribution of voltage-gated Ca2+ channels in cultured olfactory bulb neurones. L-type channels were found mainly on somata and their density seemed to decrease on the dendrites with increasing distance from the soma. In contrast, nifedipine-insensitive N-type channels were mainly observed on dendrites and were blocked by omega-conotoxin. NA, as well as clonidine, markedly blocked Ca2+ influx through dendritic N-type Ca2+ channels.

Cloning and expression of a second photoreceptor-specific membrane retina guanylyl cyclase (RetGC), RetGC-2

One of the membrane guanylyl cyclases (GCs), RetGC, is expressed predominantly in photoreceptors. No extracellular ligand has been described for RetGC, but it is sensitive to activation by a soluble 24-kDa protein (p24) and is inhibited by Ca2+. This enzyme is, therefore, thought to play a role in resynthesizing cGMP for photoreceptor recovery or adaptation. By screening a human retinal cDNA library at low stringency with the cytoplasmic domains from four cyclases, we cloned cDNAs encoding a membrane CG that is most closely related to RetGC. We have named this GC RetGC-2, and now term the initially described RetGC RetGC-1. By in situ hybridization, mRNA encoding RetGC-2 is found only in the outer nuclear layer and inner segments of photoreceptor cells. By using synthetic peptide antiserum specific for each RetGC subtype, RetGC-2 can be distinguished from RetGC-1 as a slightly smaller protein in immunoblots of bovine rod outer segments. Membrane GC activity of recombinant RetGC-2 expressed in human embryonic kidney 293 cells is stimulated by the activator p24 and is inhibited by Ca2+ with an EC50 value of 50-100 nM. Our data reveal a previously unappreciated diversity of photoreceptor GCs.

Calcium transients in growth cones and axons of cultured Helisoma neurons in response to conditioning factors

Accumulating evidence indicates that cytosolic calcium levels regulate growth cone motility and neurite extension. The purpose of this study was to determine if intracellular calcium levels also influence the initiation of neurite extension induced by growth-promoting factors. An in vitro preparation of axotomized neurons that can be maintained in the absence of growth-promoting factors was utilized. The distal axons of cultured Helisoma neurons plated into defined medium do not extend neurites until they are exposed to Helisoma brain-conditioned medium. This provided the opportunity to study the intracellular changes associated with neurite extension. Cytosolic calcium levels were monitored with the calcium-sensitive dye fura 2 at the distal axon. In control medium calcium levels in the distal axon were constant. However, transient elevations in cytosolic calcium in the axonal growth cone occurred after addition of conditioned medium and coincident with the initiation of neurite extension. Application of calcium channel blockers showed that the transients resulted from calcium influx across the neuronal membrane. The transients, however, were not required for neurite extension, although they did influence the rate and extent of neurite outgrowth. Simultaneous extracellular patch recordings demonstrated that the calcium transients were correlated temporally with an increase in rhythmic spontaneous electrical activity of cells, suggesting that conditioned medium influences ionic membrane properties of these neurons.

Cell-specific purinergic receptors coupled to Ca2+ entry and Ca2+ release from internal stores in adrenal chromaffin cells. Differential sensitivity to UTP and suramin

We have assessed the relative contribution of Ca2+ entry and Ca2+ release from internal stores to the [Ca2+]i transients evoked by purinergic receptor activation in bovine adrenal chromaffin cells. The [Ca2+]i was recorded from single cells using ratiometric fura-2 microfluorometry. Two discrete groups of ATP-sensitive cells could be distinguished on the basis of their relative capacity to respond to ATP in the virtual absence of extracellular Ca2+. One group of cells (group I) failed to respond to ATP in the absence of Ca2+, was completely insensitive to UTP, and displayed suramin-blockable [Ca2+]i transients when challenged with ATP in the presence of external Ca2+. ATP activated a prominent and rapidly inactivating Mn2+ influx pathway in group I cells, as assessed by monitoring Mn2+ quenching of fura-2 fluorescence. In contrast, a second group of ATP-sensitive cells (group II) exhibited pronounced [Ca2+]i rises when challenged with ATP and UTP in the absence of Ca2+ and was completely insensitive to suramin. ATP and UTP activated a delayed and less prominent Mn2+ influx pathway in group II cells. Contrary to the nicotinic receptor agonist DMPP, which evoked a preferential release of epinephrine, ATP evoked a preferential release of norepinephrine, and UTP had no effect on secretion. Suramin nearly suppressed ATP-evoked norepinephrine release. We conclude that chromaffin cells contain two distinct and cell-specific purinoceptor subtypes. Although some cells express a P2U-type purinoceptor coupled to Ca2+ release from internal stores and to the associated slow Ca2+ refilling mechanism, other cells express a suramin-sensitive and UTP-insensitive purinoceptor exclusively coupled to Ca2+ influx, probably an ATP-gated channel. It is suggested that the ATP-gated channel is preferentially localized to norepinephrine-secreting chromaffin cells and supports specifically hormone output from these cells. Thus, the biochemical pathways involved in the exocytotic release of the two major stress-related hormones appear to be regulated by distinct signaling systems.

Calcium entry in rabbit corneal epithelial cells: evidence for a nonvoltage dependent pathway

We performed experiments to elucidate the calcium influx pathways in freshly dispersed rabbit corneal epithelial cells. Three possible pathways were considered: voltage-gated Ca++ channels, Na+/Ca++ exchange, and nonvoltage-dependent Ca(++)-permeable channels. Whole cell inward currents carrying either Ca++ or Ba++ were not detected using voltage clamp techniques. We also used imaging technology and the Ca(++)-sensitive ratiometric dye fura 2 to measure changes in intracellular Ca++ concentration ([Ca]i). Bath perfusion with NaCl Ringer's solution containing the calcium channel agonist Bay-K-8644 (1 microM), or Ni++ (40 microM), a blocker of many voltage-dependent calcium channels, did not affect [Ca++]i. Membrane depolarization with a KCl Ringer's bath solution resulted in a decrease in [Ca++]i. These results are inconsistent with the presence of voltage gated Ca++ channels. Nonvoltage gated Ca++ entry, on the other hand, would be reduced by membrane depolarization and enhanced by membrane hyperpolarization. Agents which hyperpolarize via stimulation of K+ current, such as flufenamic acid, resulted in an increase in ratio intensity. The cells were found to be permeable to Mn++ and bath perfusion with 5 mM Ni++ decreased [Ca++]i suggesting that the Ca++ conductance was blocked. These results are most consistent with a nonvoltage gated Ca++ influx pathway. Finally, replacing extracellular Na+ with Li+ resulted in an increase in [Ca++]i if the cells were first Na(+)-loaded using the Na+ ionophore monensin and ouabain, a Na(+)-K(+)-ATPase inhibitor. These results suggest that Na+/Ca++ exchange may also regulate [Ca++]i in this cell type.

Photophysics of the fluorescent Ca2+ indicator Fura-2

The photophysics of the complex forming reaction of Ca2+ and Fura-2 are investigated using steady-state and time-resolved fluorescence measurements. The fluorescence decay traces were analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution with BAPTA as Ca2+ buffer: k01 = 1.2 x 10(9)s-1, k21 = 1.0 x 10(11) M-1 s-1, k02 = 5.5 x 10(8) s-1, k12 = 2.2 x 10(7) s-1, and with EGTA as Ca2+ buffer: k01 = 1.4 x 10(9) s-1, k21 = 5.0 x 10(10) M-1 s-1, k02 = 5.5 x 10(8) s-1, k12 = 3.2 x 10(7) s-1. k01 and k02 denote the respective deactivation rate constants of the Ca2+ free and bound forms of Fura-2 in the excited state. k21 represents the second-order rate constant of binding of Ca2+ and Fura-2 in the excited state, whereas k12 is the first-order rate constant of dissociation of the excited Ca2+:Fura-2 complex. The ionic strength of the solution was shown not to influence the recovered values of the rate constants. From the estimated values of k12 and k21, the dissociation constant K*d in the excited state was calculated. It was found that in EGTA Ca2+ buffer pK*d (3.2) is smaller than pKd (6.9) and that there is negligible interference of the excited-state reaction with the determination of Kd and [Ca2+] from fluorimetric titration curves. Hence, Fura-2 can be safely used as an Ca2+ indicator. From the obtained fluorescence decay parameters and the steady-state excitation spectra, the species-associated excitation spectra of the Ca2+ free and bound forms of Fura-2 were calculated at intermediate Ca2+ concentrations.

The role of conditioning factors in the formation of growth cones and neurites from the axon stump after axotomy

Knowledge of the cellular events that underlie initiation of outgrowth is crucial to understanding regulation of development and regeneration of the nervous system. This study utilized a culture preparation in which growth cone formation could be studied independent of cellular responses to the presence of conditioning factors. Identified neurons were removed from the buccal ganglion of the mollusc, Helisoma trivolvis, and plated into defined culture medium. A large growth cone formed at the end of the attached axon stump. Although this axonal growth cone exhibited filopodial and lamellipodial activity, it did not advance across the substrate, suggesting that growth cone formation and motility were independent of the presence of conditioning factors. Axonal growth cones of identified neurons B19 and B5 exhibited differences in their morphological and behavioral properties. In response to the addition of conditioning factors, several new neurites extended from the periphery of the axonal growth cone. Extension of outgrowth from the axonal growth cone was accompanied by a redistribution of cytoskeletal elements in the axonal growth cone. Cytoskeletal staining revealed a loss of the peripheral actin filament network and microtubules were found to extend into the peripheral lamellipodium of the axonal growth cone, an area normally devoid of microtubule staining. Thus, these experiments indicate that growth cone formation is an intrinsic property of the distal axon stump and that neurite extension from this structure involves reorganization of the neuronal cytoskeleton.

Arterial contractions induced by cumulative addition of calcium in hypertensive and normotensive rats: influence of endothelium

Responses to cumulative addition of Ca2+ (0.2-2.5 mM) after precontraction with potassium chloride (KCl) and noradrenaline in Ca(2+)-free medium were studied in isolated mesenteric arterial rings from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). The Ca2+ contractions in 125 mM KCl-stimulated endothelium-denuded rings in the presence of atenolol (10 microM) and phentolamine (10 microM) were less marked in SHR than WKY, although the contractions to high concentrations of KCl in normal organ bath Ca2+ (1.6 mM) were similar in these strains. The difference in Ca2+ contractions between SHR and WKY during KCl stimulation was also present after 10-min pretreatment with 1 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) in Ca(2+)-free medium. However, when noradrenaline (1 microM) was used as the agonist the Ca2+ contractions of endothelium-denuded rings in the two strains were comparable, while exposure to EGTA reduced these responses more effectively in SHR than WKY. Nifedipine (0.5 nM and 10 nM in KCl- and noradrenaline-stimulated rings, respectively) more efficiently inhibited the Ca2+ contractions in hypertensive than in normotensive rats. The presence of intact vascular endothelium attenuated the contractions to Ca2+ addition comparably (during KCl stimulation) or even more (during noradrenaline) in SHR when compared with WKY. NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM) counteracted this attenuation correspondingly in WKY and SHR, and L-arginine (1 mM) restored it in both strains, whereas indomethacin (10 mM) was without effect on the response. However, mesenteric arterial relaxations induced by the endothelium-dependent agonists acetylcholine and ADP in noradrenaline-precontracted (1 microM) rings were clearly impaired in SHR, and also L-NAME (0.1 mM) reduced the responses to acetylcholine more efficiently in SHR. In contrast, the relaxations to acetylcholine and ADP in KCl-precontracted (60 mM) rings in the absence and presence of L-NAME were comparable between the two strains. In conclusion, attenuated contractile response to cumulative Ca2+ addition during stimulation with KCl clearly differentiated arterial smooth muscle of hypertensive and normotensive rats, suggesting altered function of cell membrane in SHR. The more pronounced effect of nifedipine on the response indicates abnormal function of voltage-dependent Ca2+ channels, and higher diminishing effect of EGTA on the contraction during noradrenaline suggests exaggerated action of the chelator on membrane-bound Ca2+ in SHR. Interestingly, the depressant effect of intact endothelium on the Ca2+ contraction response, mediated largely via nitric oxide, was not attenuated in SHR.(ABSTRACT TRUNCATED AT 400 WORDS)

Intramolecular and intermolecular enzymatic modulation of ion channels in excised membrane patches

A calcium-activated potassium channel in posterior pituitary nerve terminals was modulated by phosphorylation and dephosphorylation. Nearly every patch of membrane containing this channel also contained both membrane bound protein phosphatase and membrane-bound protein kinase. By examining the statistical and kinetic nature of phosphorylation and dephosphorylation in excised patches, it was possible to evaluate two contrasting models for these enzymatic reactions. One of these models treated catalysis as an intermolecular process in which the enzyme and substrate are separate molecular species that diffuse and encounter one another during collisions. The second model treated catalysis as an intramolecular process in which the enzyme and substrate reside within a stable macromolecular complex. The study began with a Poisson analysis of the distribution of channel number in patches, and of the number of protein phosphatase-free and protein kinase-free patches. Subsequent kinetic analysis of dephosphorylation yielded an estimate of the mean number of protein phosphatase molecules per patch that was similar to the value obtained from Poisson analysis. Because these two estimates were independent predictions based on the intermolecular model, their agreement supported this model. Analysis of channel number in protein phosphatase-free patches and of the rarity of patches showing partial but incomplete rundown provided additional support for the intermolecular model over the intramolecular model. Furthermore, dephosphorylation exhibited monotonic kinetics with a rate well below the diffusion limit. Thus, several different lines of analysis support the intermolecular model for dephosphorylation, in which the protein phosphatase must encounter its substrate to effect catalysis. In contrast to the monotonic kinetics of dephosphorylation, the phosphorylation reaction exhibited sigmoidal kinetics, with a rate that depended on membrane potential. Voltage dependence is an unlikely property for a kinetic step involving encounters resulting from diffusion. Furthermore, the velocity of the phosphorylation reaction exceeded the diffusion limit, and this observation is inconsistent with the intermolecular model. Thus, both intermolecular and intramolecular enzymatic mechanisms operate in the modulation of the calcium-activated potassium channel of the posterior pituitary. These studies provide a functional characterization of the interactions between enzyme and substrate in intact patches of cell membrane.

Localization of calcium entry through calcium channels in olfactory receptor neurones using a laser scanning microscope and the calcium indicator dyes Fluo-3 and Fura-Red

The intracellular calcium concentration [Ca2+]i in olfactory receptor neurones of Xenopus laevis was imaged with high spatial and temporal resolution. A new method using a mixture of the calcium indicator dyes Fluo-3 and Fura-Red was employed. The fluorescence patterns in two wavelength bands were measured on the emission side of a confocal laser scanning microscope, and the ratio R of the fluorescence intensities was taken as an estimate of [Ca2+]i. When the neurones were depolarized by elevating the extracellular potassium concentration [K+]o they showed one of three types of responses: a fast increase in [Ca2+]i, a slow increase in [Ca2+]i, or no change in [Ca2+]i. The fast increase in [Ca2+]i took place in the soma compartment. For at least 4 s after the onset of depolarization the calcium distribution in the dendrite remained essentially unchanged. To study the fast increase with high time resolution, line scan images were taken. The neurones were depolarized for brief periods applying a solution containing high [K+] onto the soma from an application pipette. The fast increase in [Ca2+]i began with a delay of about 200 ms and went from the resting concentration to about 110 nM above resting concentration. Following the depolarization, recovery from elevated [Ca2+]i to resting levels had a time constant of about 15 s. The slow response seemed to depend on the removal of [Na+] from the bath rather than on the elevated [K+] in the bath. The response was also observed with Cd2+, Ni2+, and Co2+ (1.5 mM each) in the bath.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-cell fura-2 microfluorometry reveals different purinoceptor subtypes coupled to Ca2+ influx and intracellular Ca2+ release in bovine adrenal chromaffin and endothelial cells

ATP and adenosine(5')tetraphospho(5')adenosine (Ap4A), released from adrenal chromaffin cells, are potent stimulators of endothelial cell function. Using single-cell fura-2 fluorescence recording techniques to measure free cytosolic Ca2+ concentration ([Ca2+]i), we have investigated the role of purinoceptor subtypes in the activation of cocultured chromaffin and endothelial cells. ATP evoked concentration-dependent [Ca2+]i rises (EC50 = 3.8 microM) in a subpopulation of chromaffin cells. Both ATP-sensitive and -insensitive cells were potently activated by nicotine, bradykinin and muscarine. Reducing extracellular free Ca2+ concentration to around 100 nM suppressed the [Ca2+]i transient evoked by ATP but not the [Ca2+]i response to bradykinin. ATP-sensitive chromaffin cells were also potently stimulated by 2-methylthioadenosine triphosphate (2MeSATP; EC50 = 12.5 microM) and UTP, but did not respond to either adenosine 5'-[beta-thio]diphosphate (ADP[beta S]), a P2Y receptor agonist, adenosine 5'-[alpha,beta-methylene]triphosphate (pp-[CH2]pA), a P2X agonist or AMP. Adrenal endothelial cells displayed concentration-dependent [Ca2+]i responses when stimulated with ATP (EC50 = 0.86 microM), UTP (EC50 = 1.6 microM) and 2MeSATP (EC50 = 0.38 microM). 2MeSATP behaved as a partial agonist. Ap4A and ADP[beta S] also raised the [Ca2+]i in endothelial cells, whereas AMP and pp[CH2]pA were ineffective. Lowering extracellular free Ca2+ to around 100 nM did not affect the peak ATP-evoked [Ca2+]i rise in these cells. It is concluded that different purinoceptor subtypes are heterogeneously distributed among the major cell types of the adrenal medulla. An intracellular Ca(2+)-releasing P2U-type purinoceptor is specifically localized to adrenal endothelial cells, while a subpopulation of chromaffin cells expresses a non-P2X, non-P2Y subtype exclusively coupled to Ca2+ influx.

Phosphorylation and dephosphorylation modulate a Ca(2+)-activated K+ channel in rat peptidergic nerve terminals

1. Ca(2+)-activated K+ channels regulate the excitability of many nerve terminals. A Ca(2+)-activated K+ channel present in the membranes of rat posterior pituitary nerve terminals runs down following the formation of excised patches. This run-down process reflects enzymatic dephosphorylation. 2. Both Mg-ATP and the protein phosphatase inhibitor okadaic acid prevented run-down of channel activity in excised patches. The okadaic acid sensitivity suggests that run-down resulted from dephosphorylation by a type 1 protein phosphatase. 3. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) accelerated run-down by accelerating okadaic acid-sensitive dephosphorylation. GTP gamma S had no effect on the activity of the protein kinase in these patches. These results suggest a direct coupling between a G-protein and a protein phosphatase. 4. After run-down, channel activity could be restored by Mg-ATP; restoration depended on ATP hydrolysis, but did not require Ca2+ or a second messenger. Restoration of channel activity by ATP was blocked by staurosporine and 1-(5-isoquinolinylsulphonyl)-3-methylpiperizine, but not by more specific inhibitors of protein kinases. 5. Restoration of channel activity by phosphorylation was very sensitive to membrane potential; increasing the voltage by as little as 10 mV could dramatically enhance recovery. 6. Ca2+ and voltage acted synergistically to enhance phosphorylation; higher [Ca2+] permitted phosphorylation at more negative potentials. 7. During trains of high frequency stimulation under current clamp, action potentials were influenced by both the protein phosphatase and protein kinase, indicating that enzymatic modulation of channel gating occurs under physiological conditions. An important implication of these results is that voltage-dependent phosphorylation could play a role in use-dependent depression of secretion from nerve terminals.

A simultaneous evaluation method of purity and apparent stability constant of Ca-chelating agents and selectivity coefficient of Ca-selective electrodes

To determine the purity (q) of Ca-chelating agents (such as EGTA and BAPTA) and their apparent stability constants to Ca2+ (K') using Ca(2+)-selective electrodes precisely, we have developed a new method using the double-log optimization. Free Ca2+ concentration was plotted against the ratio of the concentrations of Ca(2+)-bound to Ca(2+)-free chelator on double logarithmic co-ordinates in which a linear relationship with a slope of -1 must hold for the metal-chelator reaction with a stoichiometry of 1 to 1. Not only the q and K' values but also the selectivity coefficient of the electrode could be simultaneously estimated on the double-log plot through an optimizing method. Error analyses using a Monte Carlo simulation showed that the double-log plot is statistically more reliable and robust than the Scatchard plot and that the optimizing method is more objective and reliable than previous methods involving extrapolation and truncation procedures.

Apparent stability constants and purity of Ca-chelating agents evaluated using Ca-selective electrodes by the double-log optimization method

Apparent calcium stability constants and the purity of Ca-chelating agents were evaluated using Ca(2+)-selective electrodes by the double-log optimization method [Oiki S. Yamamoto T. Okada Y. (1994) Cell Calcium, 15, 199-208]. The method was amended to allow evaluation of the free Ca2+ concentrations contaminating electrolyte solutions. The value thus estimated (3.7 microM) was not significantly different from the total contaminating Ca content measured by atomic absorption spectroscopy. The purity of EGTA of different commercial brands was found to be in the range from 95.5-98.0% and was almost stable over several years of storage. The impurity of EGTA was completely eliminated by baking at 150 degrees C for 3 h. The purity of BAPTA decreased from 85.8 to 77.2% after storing for 3 months at -20 degrees C. The impurity of BAPTA was also abolished by the same drying procedure. At physiological pH (7.30), the apparent stability constants (K's) of EGTA were determined to be 7.13 and 6.97 in KCl-based solutions of 0.10 M and 0.16 M ionic strength, respectively, at 25 degrees C in the absence of Mg. At pH 7.30 and 0.20 M ionic strength K' values of BAPTA were 6.50 at 22 degrees C and 6.69 at 37 degrees C. The K' value increased with decreasing ionic strength.

Amiloride-insensitive cation conductance in Xenopus laevis olfactory neurons: a combined patch clamp and calcium imaging analysis

We used digital calcium imaging with Fura-2 in conjunction with the tight-seal whole-cell patch clamp technique to describe a novel cation conductance in olfactory neurons of the clawed toad Xenopus laevis. Substitution of extracellular Ca2+ and Na+ was used as a tool to change [Ca2+]i. When [Ca2+]i was increased to about 450 nM, a conductance gcat activated that was permeable for cations. Upon gcat activation, an increase in [Ca2+]i occurred in the dendritic knob. Once activated, gcat showed no further dependence upon [Ca2+]i. Icat is shown to be different from the current activated by a mixture of the odorants citralva and amyl acetate. We conclude that there are two different cation conductances in the peripheral compartments of olfactory neurons in X. laevis.

Role of polyunsaturated fatty acids as signal transducers: amplification of signals from growth factor receptors by fatty acids in mammary epithelial cells

The growth, morphogenesis and differentiation of milk producing epithelial tissues in the developing mammary glands require interaction with extracellular matrices and stimulation by hormones, growth factors and essential fatty acids. In primary culture, the proliferation of mammary epithelial cells (MEC), induced by epidermal growth factor (EGF), is enhanced and sustained by linoleate and its eicosanoid metabolites. Since a combination of linoleic acid (18:2 omega 6) and prostaglandin E2 or cAMP has synergistic effect on EGF-stimulated growth, it is suggested that additional cAMP-dependent protein kinase A (PK-A) independent pathways may also contribute to the linoleate effect on EGF action. Possible involvement of Ca2+/phospholipid-dependent protein kinase C (PK-C) is explored. Both linoleate and arachidonate can activate Type-II and Type-III protein kinase-C in MEC and a PK-C inhibitor can block growth stimulation by EGF and fatty acids. Like 12-O-Tetradecanoly phorbol-13-acetate (TPA), a PK-C activator which also enhances EGF-stimulated growth of MEC, linoleate can phosphorylate a 40-42 KD protein. EGF itself can stimulate transient phosphorylation of the same protein in MEC cultures but when supplemented with linoleate, which does not influence the ligand binding affinity of EGF-receptors, the transient phosphorylation signal in 40-42 KD protein is sustained.(ABSTRACT TRUNCATED AT 250 WORDS)

Depolarization-induced changes in neurite elongation and intracellular Ca2+ in isolated Helisoma neurons

This study focuses on the effects of K+ depolarization on neurite elongation of identified Helisoma neurons isolated into culture. Application of K+ to the external medium caused a dose-dependent suppression of neurite elongation. Lower concentrations of K+ were associated with a slowing in the rate of neurite elongation, whereas higher concentrations produced neurite retraction. Surprisingly, the effects of K+ depolarization were transient, and neurite elongation rates recovered towards control levels within 90 min even though the neurons remained in high-K+ solution. Identified neurons differed in the magnitude of their response to K+ depolarization; neurite elongation of buccal neuron B4 was inhibited at 5 mM K+, but elongation in B5 and B19 was not affected until concentrations of 25 mM. Electrophysiologically, K+ application evoked a brief period (5-10 s) of action potential activity that was followed by a steady-state membrane depolarization lasting 2 h or more. The changes in neurite elongation induced by K+ depolarization occurred in isolated growth cones severed from their neurites and were blocked by application of calcium antagonists. Intracellular free Ca2+ levels in growth cones of B4 and B19 increased and then decreased during the 90-min depolarization, corresponding to the changes in elongation. B4 and B19 showed differences in the magnitude, time course, and spatial distribution of the Ca2+ change during depolarization, reflecting their different sensitivities to depolarization.

Protein kinase inhibitors prevent junction dissociation induced by low extracellular calcium in MDCK epithelial cells

When epithelial cell cultures are transferred from a medium with a normal extracellular calcium concentration (1-2 mM) to a medium with a low extracellular calcium concentration (LC, less than 50 microM free Ca2+) cell-cell contacts are disrupted, and the tight junction-dependent transepithelial resistance drops. In this study, I used MDCK epithelial cells to investigate the effects of LC on the localization of the tight junction protein cingulin, and the role of protein kinases in the events induced by LC. Immunofluorescence analysis showed that within 15 min of incubation of confluent monolayers in LC, cingulin labeling was dislocated from the cell periphery, as an array of granules forming a ring-like structure. At later times after calcium removal, cingulin labeling appeared mostly cytoplasmic, in a diffuse and granular pattern, and cells appeared rounded and smaller. These events were not influenced by lack of serum, or by preincubation with 10 mM sodium azide or 6 mg/ml of cycloheximide. However, the disruption of cell-cell contacts, the cell shape changes, and the redistribution of cingulin and other junctional proteins induced by LC were inhibited when cells were pretreated with the protein kinase inhibitor H-7 (greater than or equal to 30 microM). The inhibitors H-8 and, to a lesser degree, staurosporine were also effective, whereas HA-1004 and ML-7 showed essentially no activity, suggesting a specificity of action of different inhibitors. Measurement of the transepithelial resistance showed that the kinase inhibitors that could prevent junction disassembly could also reduce the drop in transepithelial resistance induced by LC. Dose-response curves demonstrated that H-7 is the most effective among the inhibitors, and the transepithelial resistance was 70% of control up to 1 h after calcium removal. These results suggest that low extracellular calcium modulates junctional integrity and cytoskeletal organization through an effector system involving protein kinases.

Bound and determined: a computer program for making buffers of defined ion concentrations

A computer program that allows the preparation of buffers containing known concentrations of metal-ligand complexes at defined pH values and temperatures is described. Ligands are defined as compounds that bind metals and may include AMP, ADP, ATP, GMP, GDP, GTP, EGTA, EDTA, BAPTA, phosphate, sulfate, chloride, monocarboxylic acids, dicarboxylic acids, organophosphates, and/or citric acid. Metals may include sodium, potassium, magnesium, calcium, and/or manganese. The program uses association constants corrected for temperature and ionic strength so that solutions between 0 and 40 degrees C and between pH values of 4 and 10 can be defined. The program can perform the following: (i) calculate the concentration of all metal-ligand complexes when total metal and total ligand concentrations are known, (ii) calculate the concentration of metal ion required to make a solution of known free metal ion concentration when total ligand concentrations are known, (iii) calculate the concentration of ligand required to make a solution of known free metal ion concentration when total metal concentrations are known, and (iv) calculate the total concentrations of metal and ligand required to make a buffer of known metal-ligand concentration. Options i-iii are useful for making buffers of defined free metal ion concentrations; option iv is useful for making buffers of defined metal-nucleotide concentrations.

ATP suppresses activity of Ca(2+)-activated K+ channels by Ca2+ chelation

Ca(2+)-activated maxi K+ channels were studied in inside-out patches from smooth muscle cells isolated from either porcine coronary arteries or guinea-pig urinary bladder. As described by Groschner et al. (Pfügers Arch 417:517, 1990), channel activity (NPo) was stimulated by 3 microM [Ca2+]c (1 mM Ca-EGTA adjusted to a calculated pCa of 5.5) and was suppressed by the addition of 1 mM Na2ATP. The following results suggest that suppression of NPo by Na2ATP is due to Ca2+ chelation and hence reduction of [Ca2+]c and reduced Ca2+ activation of the channel. The effect was absent when Mg ATP was used instead of Na2ATP. The effect was diminished by increasing the [EGTA] from 1 to 10 mM. The effect was absent when [Ca2+]c was buffered with 10 mM HDTA (apparent pKCa 5.58) instead of EGTA (pKCa 6.8). A Ca(2+)-sensitive electrode system indicated that 1 mM Na2ATP reduced [Ca2+]c in 1 mM Ca-EGTA from 3 microM to 1.4 microM. Na2ATP, Na2GTP, Li4AMP-PNP and NaADP reduced measured [Ca2+]c in parallel with their suppression of NPo. After the Na2ATP-induced reduction of [Ca2+]c was re-adjusted by adding either CaCl2 or MgCl2, the effect of Na2ATP on NPo disappeared. In vivo, intracellular [Mg2+] exceeds free [ATP4-], hence ATP modulation of maxi K+ channels due to Ca2+ chelation is without biological relevance.