Modulation of single Ca2+-dependent K+-channel activity by protein phosphorylation

Metabolic control of the K+ channel of human red cells

The effects of cAMP, ATP and GTP on the Ca2(+)-dependent K+ channel of fresh (1-2 days) or cold-stored (28-36 days) human red cells were studied using atomic absorption flame photometry of Ca2(+)-EGTA loaded ghosts which had been resealed to monovalent cations in dextran solution. When high-K+ ghosts were incubated in an isotonic Na+ medium, the rate constant of Ca2(+)-dependent K+ efflux was reduced by a half on increasing the theophylline concentration to 40 mM. This effect was observed in ghosts from both fresh and stored cells, but only if they were previously loaded with ATP. The inhibition was more marked when Mg2+ was added together with ATP, and it was abolished by raising free Ca2+ to the micromolar level. Like theophylline, isobutyl methylxanthine (10 mM) also affected K+ efflux. cAMP (0.2-0.5 mM), added both internally and externally (as free salt, dibutyryl or bromide derivatives), had no significant effect on K+ loss when the ghost free-Ca2+ level was below 1 microM, but it was slightly inhibitory at higher concentrations. The combined presence of cAMP (0.2 mM) plus either theophylline (10 mM), or isobutyl methylxanthine (0.5 mM), was more effective than cAMP alone. This inhibition showed a strict requirement for ATP plus Mg2+ and it was not overcome by raising internal Ca2+. Ghosts from stored cells seemed more sensitive than those from fresh cells, to the combined action of cAMP and methylxanthines. Loading ATP into ghosts from fresh or stored cells markedly decreased K+ loss. Although this effect was observed in the absence of added Mg2+ (0.5 mM EDTA present), it was potentiated upon adding 2 mM Mg2+. The K+ efflux from ATP-loaded ghosts was not altered by dithio-bis-nitrobenzoic acid (10 mM) or acridine orange (100 microM), while it was increased two- to fourfold by incubating with MgF2 (10 mM), or MgF2 (10 mM) + theophylline (40 mM), respectively. By contrast, a marked efflux reduction was obtained by incorporating 0.5 mM GTP into ATP-containing ghosts. The degree of phosphorylation obtained by incubating membranes with (gamma-32P)ATP under various conditions affecting K+ channel activity, was in direct correspondence to their effect on K+ efflux. The results suggest that the K+ channel of red cells is under complex metabolic control, via cAMP-mediated and nonmediated mechanisms, some which require ATP and presumably, involve phosphorylation of the channel proteins.

Regulation of maxi-K+ channels on pancreatic duct cells by cyclic AMP-dependent phosphorylation

Using the patch-clamp technique we have identified a Ca2(+)-sensitive, voltage-dependent, maxi-K+ channel on the basolateral surface of rat pancreatic duct cells. The channel had a conductance of approximately 200 pS in excised patches bathed in symmetrical 150 mM K+, and was blocked by 1 mM Ba2+. Channel open-state probability (Po) on unstimulated cells was very low, but was markedly increased by exposing the cells to secretin, dibutyryl cyclic AMP, forskolin or isobutylmethylxanthine. Stimulation also shifted the Po/voltage relationship towards hyperpolarizing potentials, but channel conductance was unchanged. If patches were excised from stimulated cells into the inside-out configuration, Po remained high, and was not markedly reduced by lowering bath (cytoplasmic) Ca2+ concentration from 2 mM to 0.1 microM. However, activated channels were still blocked by 1 mM Ba2+. Channel Po was also increased by exposing the cytoplasmic face of excised patches to the purified catalytic subunit of cyclic AMP-dependent protein kinase. We conclude that cyclic AMP-dependent phosphorylation can activate maxi-K+ channels on pancreatic duct cells via a stable modification of the channel protein itself, or a closely associated regulatory subunit, and that phosphorylation alters the responsiveness of the channels to Ca2+. Physiologically, these K+ channels may contribute to the basolateral K+ conductance of the duct cell and, by providing a pathway for current flow across the basolateral membrane, play an important role in pancreatic bicarbonate secretion.

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

Patch cramming: monitoring intracellular messengers in intact cells with membrane patches containing detector ion channels

This paper introduces "patch cramming," a new procedure that utilizes an ion channel gated directly by an intracellular messenger molecule as a probe for detecting changes in the concentration of that molecule in an intact cell. A patch pipette containing the channel in a membrane patch is inserted into a recipient cell where the channel locally "senses" the intracellular messenger. In this study patches containing Ca2(+)-dependent K+ channels were inserted into Helix neurons, where they were activated by Ca2+ influx during trains of action potentials. Channels gated directly by other messengers, including cyclic nucleotides and IP3, have also been identified. Hence, by using detector channels with appropriate specificity, it may be possible to detect local intracellular fluctuations of these molecules.

Phosphorylation modulates potassium conductance and gating current of perfused giant axons of squid

The presence of internal Mg-ATP produced a number of changes in the K conductance of perfused giant axons of squid. For holding potentials between -40 and -50 mV, steady-state K conductance increased for depolarizations to potentials more positive than approximately -15 mV and decreased for smaller depolarizations. The voltage dependencies of both steady-state activation and inactivation also appears shifted toward more positive potentials. Gating kinetics were affected by internal ATP, with the activation time constant slowed and the characteristic delay in K conductance markedly enhanced. The rate of deactivation also was hastened during perfusion with ATP. Internal ATP affected potassium channel gating currents in similar ways. The voltage dependence of gating charge movement was shifted toward more positive potentials and the time constants of ON and OFF gating current also were slowed and hastened, respectively, in the presence of ATP. These effects of ATP on the K conductance occurred when no exogenous protein kinases were added to the internal solution and persisted even after removing ATP from the internal perfusate. Perfusion with a solution containing exogenous alkaline phosphatase reversed the effects of ATP. These results provide further evidence that the effects of ATP on the K conductance are a consequence of a phosphorylation reaction mediated by a kinase present and active in perfused axons. Phosphorylation appears to alter the K conductance of squid giant axons via a minimum of two mechanisms. First, the voltage dependence of gating parameters are shifted toward positive potentials. Second, there is an increase in the number of functional closed states and/or a decrease in the rates of transition between these states of the K channels.

Potassium channel openers and vascular smooth muscle relaxation

Potassium channel openers comprise a diverse group of chemical agents which open plasma-lemmal K-channels. They show selectivity for smooth muscle, although K-channels in cardiac and skeletal muscle, neurones and the pancreatic beta-cell are also affected at relatively high concentrations. In addition, at least one endogenous K-channel opener of vascular origin--endothelium-derived hyperpolarizing factor--exists and in man plays a role in modulating blood vessel tone. The type of K-channel involved in the actions of both exogenous and endogenous K-channel openers is still uncertain, although a prime candidate in smooth muscle seems similar to the [ATPi]-modulated K-channel in the pancreatic beta-cell. This review focuses attention on the action of these agents in vascular smooth muscle and on the possible clinical exploitation of their powerful vasorelaxant properties.

The influence of substance P and its fragments on endogenous phosphorylation of synaptosomal membrane protein (synapsin) from cerebral cortex of rat brain

1. The effects of substance P and its fragments and analogue of a C-terminal fragment on cyclic AMP-dependent phosphorylation of synapsin I in synaptosomal membranes (SM) from cerebral cortex were investigated. 2. SP(I-II) and SP(1-4) at 10(-3) M caused a marked stimulation of synapsin I phosphorylation. 3. A C-terminal fragment of SP (SP6-11) had no effect on phosphorylation of synapsin 1. 4. Analogue of C-terminal fragment [(Tyr8)SP6-11] at 10(-3) M distinctly inhibits phosphorylation of synapsin I. 5. These data suggest that SPI-II and its C- and N-terminal fragments have a modulator function against the phosphorylation of some rat brain proteins.

The activation of calcium and calcium-activated potassium channels in mammalian colonic smooth muscle by substance P

1. The regulation of Ca2(+)-activated K+ channels by the agonist substance P in freshly dissociated smooth muscle cells from the rabbit longitudinal colonic muscle was characterized using the patch clamp technique. 2. In the cell-attached recording mode, when pipette and bath solutions contained equal [K+] (126 mM), the Ca2(+)-activated K+ channels showed a linear current-voltage relationship (between -50 mV and 50 mV) with a slope conductance of 210 +/- 35 pS (n = 12). Reversal potential measurements indicated that the channel was highly selective for K+ over Na+ (PK/PNa = 110). 3. Channels were activated by depolarizing membrane voltages and cytosolic Ca2+, and in inside-out patches channel activation depended sigmoidally on voltage and [Ca2+]. The potential for half-activation at a cytosolic [Ca2+] of 5 x 10(-6) M was 0 mV. A tenfold increase in cytosolic Ca2+ resulted in a 60 mV shift of the sigmoidal voltage activation curve to more negative potentials. 4. Threshold concentrations of substance P (10(-12) M), which did not result in cell contraction, caused a prolonged activation of K+ channels. The K+ channels were observed to open in clusters: simultaneous opening of multiple channels was interrupted by complete, prolonged channel closure. 5. Lowering bath [Ca2+] to submicromolar concentrations abolished the effect of substance P. The activation of K+ channels by substance P (10(-12) M) was also inhibited by the dihydropyridine nifedipine (10(-6) M), a blocker of L-type Ca2+ channels. 6. In the whole-cell recording mode, with the pipette solution containing 126 mM-KCl, 0.77 mM-EGTA and 1 mM-ATP, depolarization from a holding potential of -70 mV elicited outward currents which increased to steady-state values. These were K+ currents as they were blocked by TEA (tetraethylammonium, 30 mM) and Ba2+ (1 mM) and were abolished when pipette K+ was replaced by Cs+. 7. The depolarization-activated outward current was not affected by lowering extracellular [Ca2+] or by the Ca2+ channel antagonists Cd2+ (200 microM), nifedipine (10(-6)-10(-5) M) or verapamil (10(-6) M). The current was greatly reduced when the EGTA concentration in the pipette solution was increased from 0.77 to 10 mM. 8. When the pipette solution contained CsCl, membrane depolarization activated inward currents. The peak inward current was identified as current through L-type Ca2+ channels based on its voltage- and time-dependent kinetics, and its modulation by dihydropyridines.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2(+)-activated K+ current involvement in neuronal function revealed by in situ single-channel analysis in Helix neurones

1. The properties of single calcium-activated potassium channels (or C-channels) were studied in cell-attached patches using the patch-clamp technique. Experiments were performed on identified Ca2(+)-dependent U cells in juvenile specimens (1-2 months old) of Helix aspersa. 2. The criteria used to identify C-channels were based on comparison between macroscopic C-currents and currents reconstructed from unitary recordings. Both currents had a slow activation rate at large positive potentials which turned into fast activation after large Ca2+ entries. Both currents were blocked by intracellularly injected EGTA. 3. The unitary conductance in normal (5 mM) or reduced (0.5 mM) [K+]o ranged from 24 to 65 pS (mean +/- S.D., 48 +/- 13; n = 64). With 85-110 mM [K+]o, which is approximately equal to the internal [K+], the conductance was 64 pS and the reversal potential was approximately 0 mV. 4. C-channels in U cells were distributed in clusters of three to ten channels (mean 5.05 channels in seventy-five patches). Calcium channels were present in patches containing clustered C-channels. C-channels within clusters behaved independently. 5. With patch electrode containing 8 mM-calcium, C-channels opened transiently upon patch depolarization. Reopenings in quiescent depolarized patches were induced by whole-cell spikes triggered by current pulses applied to an intracellular electrode. Apparent inactivation of C-channels in depolarized patches was in fact due to a decrease in [Ca2+]i resulting from inactivation of Ca2+ channels. 6. Calcium-free saline solutions in the patch electrodes prevented C-channels from opening upon patch depolarization. Entry of calcium through the surrounding membrane induced delayed openings in the patch. Peak opening probability Po occurred 330 +/- 30 ms after a brief Ca2+ entry with a lag period of 50-80 ms. With patch electrodes filled with Ca2(+)-containing saline solutions and under conditions which maximized C-channel opening, peak Po was reached in 20-50 ms. The same value was observed for the whole-cell C-current. 7. The peak Po at a given patch potential and in response to a whole-cell spike was not altered by a previous long-lasting patch depolarization, or by producing several successive Ca2+ entries. Thus, C-channels did not appear to be inactivated by depolarization or increase in [Ca2+]i. 8. C-channels were found to be relatively highly voltage dependent, with an e-fold increase in Po per 14.9 mV increase in potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Blockade of a mitochondrial cationic channel by an addressing peptide: an electrophysiological study

A voltage-dependent cationic channel of large conductance is observed in phospholipid bilayers formed at the tip of microelectrodes from proteoliposomes derived from mitochondrial membranes. This channel was blocked by a 13-residue peptide with the sequence of the amino terminal extremity of the nuclear-coded subunit IV of cytochrome c oxidase. The blockade was reversible, voltage- and dose-dependent. The peptide did not affect the activity of a Torpedo chloride channel observed under the same conditions. From experiments with phospholipid monolayers, it is unlikely that the peptide inserts into bilayers under the experimental conditions used. The blockade was observed from both sides of the membrane, being characterized by more frequent transitions to the lower conductance states, and a maximum effect was observed around 0 mV. Channels, the gating mechanism of which had been eliminated by exposure to trypsin, were also blocked by the peptide. For trypsinized channels, the duration of the closure decreased and the blockade saturated at potentials below -30 mV. These observations are consistent with a translocation of the peptide through the channel. Dynorphin B, which has the same length and charge as the peptide, had some blocking activity. Introduction of negative charges in the peptide by succinylation suppressed the activity.

Role of calcium in mediating action of carbachol in T84 cells

To examine the role of calcium in mediating carbachol's action in secretory epithelia, we simultaneously measured intracellular free [Ca] [( Ca]i) and transepithelial chloride transport in T84 cells grown on collagen-coated filters. [Ca]i was measured with fura-2 and fluorescence microscopy and expressed as a relative value [( Ca]'i) normalized to control. Chloride transport was measured as the short-circuit current (Isc) with a voltage clamp. Monolayers were pretreated with cyclic AMP to augment the response of Isc to carbachol, a procedure that did not qualitatively change the response of the monolayer to carbachol. The carbachol-induced changes in Isc appeared to be dependent on the increase in [Ca]i. First, carbachol caused both Isc and [Ca]'i to increase in parallel. Isc increased from 32 +/- 5 to 70 +/- 9 microA and then declined to 57 +/- 16 microA while [Ca]'i increased from 72 +/- 14 to 156 +/- 22 nM and then declined to 133 +/- 45 nM. Second, the carbachol-induced increases in Isc and [Ca]'i were correlated. The greater the hormone-stimulated rise in [Ca]'i, the higher the increase in Isc. Third, carbachol's stimulation of Isc was blunted by preventing the calcium spike with the cellular calcium buffer 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA). Although the carbachol-induced increase in [Ca]'i appeared necessary for the increase in Isc, it was not clear if carbachol's action was solely the result of an increase in [Ca]'i. Increasing [Ca]'i with ionomycin, although causing Isc and [Ca]'i to increase in parallel, failed to increase Isc to the levels observed with carbachol. These experiments suggest that although the carbachol-induced increase in Isc is dependent on the increase in [Ca]i, the hormone may activate a second process that increases the sensitivity of the calcium-activated transport process to changes in [Ca]i.

The physiological role of calcium-dependent channels

Calcium (Ca2+)-dependent channels may be classified in three broad categories, which are, respectively, selective for potassium ions, for chloride ions, and for monovalent cations. The usual action of Ca2+ is to increase the probability of opening of the channels, but examples of the reverse, Ca2+-induced inhibition of ion channels, have recently been found. Ca2+-dependent channels help to shape the action potentials of excitable cells as well as the synaptic currents of muscular and neuronal preparations. They are involved in several aspects of electrolyte transport including regulation of osmolarity in animal cells and of turgor in plant cells, electrolyte secretion in exocrine glands, fluid absorption and secretion in epithelial tissues.

Regulation of Ca2+-dependent K+-channel activity in tracheal myocytes by phosphorylation

Isoprenaline is a beta-adrenergic agonist of clinical importance as a remedy for asthma. In airway smooth muscle its relaxant action is accompanied by hyperpolarization of the membrane and elevation of the level of intracellular cyclic AMP. Hyperpolarization and relaxation are also induced by drugs such as forskolin, theophylline and dibutyryl cAMP, indicating that cAMP-dependent phosphorylation is involved in producing the electrical response. Cyclic AMP-dependent protein kinase (protein kinase A) has been reported to activate Ca2+-dependent K+ channels in cultured aortic smooth muscle cells and snail neurons. The membrane of tracheal smooth-muscle cells is characterized by a dense distribution of Ca2+-dependent K+-channels. We have now examined the effect of isoprenaline and protein kinase A on Ca2+-dependent K+-channels in isolated smooth muscle cells of rabbit trachea, using the patch-clamp technique. Our results show that the open-state probability of Ca2+-dependent K+-channel of tracheal myocytes is reversibly increased by either extracellular application of isoprenaline or intracellar application of protein kinase A. We also show that this effect is significantly enhanced and prolonged in the presence of a potent protein phosphatase inhibitor, okadaic acid.

Differential modulation of Ca2+-activated K+ channels in ovine pituitary gonadotrophs by GnRH, Ca2+ and cyclic AMP

Patch-clamp techniques were employed to examine the effects of cAMP in relation to gonadotrophin-releasing hormone (GnRH) action on Ca2+-activated K+ channels in pituitary gonadotrophs derived from ovine pars tuberalis. GnRH applied extracellularly increased channel openings in cell-attached patches similar to calcium ionophore (A23187), while raising intracellular cAMP concentration with dibutyryl cAMP or forskolin decreased the number of functional channels (Nf) and the open state probability (Po). Both cAMP and the catalytic subunit of cAMP-dependent protein kinase produced similar results when applied to the cytoplasmic membrane face of inside-out patches, and the effect of cAMP was abolished by the protein kinase inhibitor. Our results suggest that decreased permeability through these channels modulated by cAMP through a phosphorylation-dependent route can modulate luteinizing hormone release.

Ca2+-activated K channels of canine colonic myocytes

K channels in enzymatically dispersed circular smooth muscle cells from the canine proximal colon were studied with the patch-clamp technique. The most prominent channel in cell-attached and excised, inside-out patches was a K channel, which had slope conductances of approximately 100 pS at a holding potential of 0 mV in a physiological K+ gradient and approximately 200 pS in symmetrical 140 mM K+ solutions. The relative permeabilities of the channel for monovalent cations were 1.0 K+:0.5 Rb+: less than 0.07 Li+:less than 0.07 Na+. The channels were activated by potential and intracellular Ca2+. At Ca2+ concentrations less than 10(-7) M, channel openings were rare except at very positive potentials. At Ca2+ concentrations between 10(-7) and 10(-6) M the probability of channel opening increased steeply, and the voltage for channel activation shifted to a negative potential range, which cells experience during electrical slow wave events in situ. The effect of Ca2+ on the open-state probability of single channels was mainly due to a decrease in mean close time. Channels were blocked by 1 mM tetraethylammonium applied to the outside of the patch but up to 10 mM tetraethylammonium applied to the inside of the patch, and 4-aminopyridine applied to either side did not block the channel. The data suggest that this channel mediates a current important in the termination of electrical slow waves, which are the primary excitable event in colonic circular muscles.

Modification by pyrethroids and DDT of phosphorylation activities of rat brain sodium channel

The effects of pyrethroids and DDT on the alpha-subunit protein of the rat brain sodium channel were studied by using both native and exogenously added cAMP-dependent protein kinases. For this purpose, the sodium channel was partially purified, using the method of Hartshorne and Catterall [J Biol Chem 259: 1667-1675, 1984], and 32P-phosphorylated using [gamma-32P]ATP and exogenously added catalytic subunit of cAMP-dependent protein kinase. By comparing the phosphorylation patterns of the isolated sodium channel to those of the partially purified or unpurified (i.e. intact synaptosomes) preparations, it was concluded that the alpha-subunit of the voltage-sensitive sodium channel protein is the only phosphorylatable protein present at the 260 kD molecular weight range on the sodium dodecyl sulfate-polyacrylamide gel electrophoretogram. Phosphorylation of the alpha-subunit was induced by depolarization, and this process was inhibited by 10(-6) to 10(-10) M 1R-deltamethrin, but not by 1S-deltamethrin, the latter being an inactive enantiomer of the former. DDT produced a similar effect, but only at a higher concentration range. By using lysed synaptosomal membranes, it was possible to study the direct effects of these compounds on the alpha-subunit, which were similar to those produced by depolarization of intact synaptosomes.

Neurotransmitter inhibition of neuronal calcium currents by changes in channel voltage dependence

The voltage-dependent calcium current of many neurons is depressed by transmitters such as noradrenaline, GABA, and kappa-opiate agonists. This modulation probably constitutes a major mechanism of presynaptic inhibition. Although recent work has implicated GTP-binding proteins in the mechanism of current inhibition, it is still unknown how the activation of those proteins alters the operation of the channels. In their initial description of the phenomenon, Dunlap and Fischbach proposed that noradrenaline acts by somehow reducing the number of functions calcium channels in the cell. By contrast with this hypothesis, I have found that inhibition of Ca2+ current is primarily due to a transmitter-induced change in the voltage-dependence with which channels are opened. Transmitters profoundly alter the voltage-dependence of channel activation, but there is little or no change in the number of functional channels activated by very large depolarizations. There is also little effect on the voltage-dependence of inactivation.

Activation of K+ channels in renal medullary vesicles by cAMP-dependent protein kinase

ADH, acting through cAMP, increases the potassium conductance of apical membranes of mouse medullary thick ascending limbs of Henle. The present studies tested whether exposure of renal medullary apical membranes in vitro to the catalytic subunit of cAMP-dependent protein kinase resulted in an increase in potassium conductance. Apical membrane vesicles prepared from rabbit outer renal medulla demonstrated bumetanide- and chloride-sensitive 22Na+ uptake and barium-sensitive, voltage-dependent 86Rb+ influx. When vesicles were loaded with purified catalytic subunit of cAMP-dependent protein kinase (150 mU/ml), 1 mM ATP, and 50 mM KCl, the barium-sensitive 86Rb+ influx increased from 361 +/- 138 to 528 +/- 120 pM/mg prot.30 sec (P less than 0.01). This increase was inhibited completely when heat-stable protein kinase inhibitor (1 microgram/ml) was also present in the vesicle solutions. The stimulation of 86Rb+ uptake by protein kinase required ATP rather than ADP. It also required opening of the vesicles by hypotonic shock, presumably to allow the kinase free access to the cytoplasmic face of the membranes. We conclude that cAMP-dependent protein kinase-mediated phosphorylation of apical membranes from the renal medulla increases the potassium conductance of these membranes. This mechanism may account for the ADH-mediated increase in potassium conductance in the mouse mTALH.

Delayed activation of large-conductance Ca2+-activated K channels in hippocampal neurons of the rat

We applied a fast concentration jump system to produce step changes in Ca2+ concentration [( Ca2+]i) on the cytoplasmic side of the inside-out membrane patch, excised from isolated rat hippocampal pyramidal neurons, and examined the time course of the activation phase of the large-conductance K channel (the BK channel; approximately 266 pS) after a step rise in [Ca2+]i. Diffusion of Ca2+ from the electrode tip to the cytoplasmic surface of the patch was estimated to be almost completed in 10 ms. After a step increase in [Ca2+]i from 0.04 to 3.2-1,000 microM, the activation of the K channel started after a clear latency of 280-18 ms and proceeded along a sigmoidal function. This was in sharp contrast with the rapid deactivation that began without delay and that was completed within 50 ms. The latency in activation was not accounted for by the binding of Ca2+ to EGTA in unstirred layers in the patch, since this binding was reported to be slow, taking up to seconds at physiological pH. Calmodulin (1 microM) did not affect the delay, the activation rate, or the steady-state current level. The calmodulin inhibitors W-7 and W-5 caused flickering of the single-channel current. These results indicate a delayed activation of the BK channel after a step rise in [Ca2+]i, suggesting that the BK current does not contribute to the repolarization of the action potential. Calmodulin is probably not involved in the activation process of the channel.

"Caged calcium" in Aplysia pacemaker neurons. Characterization of calcium-activated potassium and nonspecific cation currents

We have studied calcium-activated potassium current, IK(Ca), and calcium-activated nonspecific cation current, INS(Ca), in Aplysia bursting pacemaker neurons, using photolysis of a calcium chelator (nitr-5 or nitr-7) to release "caged calcium" intracellularly. A computer model of nitr photolysis, multiple buffer equilibration, and active calcium extrusion was developed to predict volume-average and front-surface calcium concentration transients. Changes in arsenazo III absorbance were used to measure calcium concentration changes caused by nitr photolysis in microcuvettes. Our model predicted the calcium increments caused by successive flashes, and their dependence on calcium loading, nitr concentration, and light intensity. Flashes also triggered the predicted calcium concentration jumps in neurons filled with nitr-arsenazo III mixtures. In physiological experiments, calcium-activated currents were recorded under voltage clamp in response to flashes of different intensity. Both IK(Ca) and INS(Ca) depended linearly without saturation upon calcium concentration jumps of 0.1-20 microM. Peak membrane currents in neurons exposed to repeated flashes first increased and then declined much like the arsenazo III absorbance changes in vitro, which also indicates a first-order calcium activation. Each flash-evoked current rose rapidly to a peak and decayed to half in 3-12 s. Our model mimicked this behavior when it included diffusion of calcium and nitr perpendicular to the surface of the neuron facing the flashlamp. Na/Ca exchange extruding about 1 pmol of calcium per square centimeter per second per micromolar free calcium appeared to speed the decline of calcium-activated membrane currents. Over a range of different membrane potentials, IK(Ca) and INS(Ca) decayed at similar rates, indicating similar calcium stoichiometries independent of voltage. IK(Ca), but not INS(Ca), relaxes exponentially to a different level when the voltage is suddenly changed. We have estimated voltage-dependent rate constants for a one-step first-order reaction scheme of the activation of IK(Ca) by calcium. After a depolarizing pulse, INS(Ca) decays at a rate that is well predicted by a model of diffusion of calcium away from the inner membrane surface after it has entered the cell, with active extrusion by surface pumps and uptake into organelles. IK(Ca) decays somewhat faster than INS(Ca) after a depolarization, because of its voltage-dependent relaxation combined with the decay of submembrane calcium. The interplay of these two currents accounts for the calcium-dependent outward-inward tail current sequence after a depolarization, and the corresponding afterpotentials after a burst

Modulation of K channels in dialyzed squid axons. ATP-mediated phosphorylation

In squid axons, internally applied ATP potentiates the magnitude of the potassium conductance and slows down its activation kinetics. This effect was characterized using internally dialyzed axons under voltage-clamp conditions. Both amplitude potentiation and kinetic slow-down effects are very selective towards ATP, other nucleotides like GTP and ITP are ineffective in millimolar concentrations. The current potentiation Km for ATP is near 10 microM with no further effects for concentrations greater than 100 microM. ATP effect is most likely produced via a phosphorylative reaction because Mg ion is an obligatory requirement and nonhydrolyzable ATP analogues are without effect. In the presence of ATP, the K current presents more delay, resembling a Cole-Moore effect due to local hyperpolarization of the channel. ATP effect induces a 10-20 mV shift in both activation and inactivation parameters towards more depolarized potentials. As a consequence of this shift, conductance-voltage curves with and without ATP cross at approximately -40 mV. This result is consistent with the hyperpolarization observed with ATP depletion, which is reversed by ATP addition. At potentials around the resting value, addition of ATP removes almost completely K current slow inactivation. It is suggested that a change in the amount of the slow inactivation is responsible for the differences in current amplitude with and without ATP, possibly as a consequence of the additional negative charge carried by the phosphate group. However, a modification of the local potential is not enough to explain completely the differences under the two conditions.

Voltage-dependent opening of single calcium-activated potassium channels in Helix neurons

Single Ca-activated K channels (C channels) were identified in cell-attached patches in Helix neurons. The respective effects of membrane potential and calcium on the opening probability Po were separated by using Ca-free filled patch electrodes. C channels were found to be highly voltage-dependent, with an e-fold increase in Po per 15 mV increase in potential. The location of the Po(V) curve on the voltage axis was dependent on the intracellular Ca concentration. These data indicate that C channels provide the cell with a powerful repolarizing mechanism.

Enhancement of kainate-gated currents in retinal horizontal cells by cyclic AMP-dependent protein kinase

Dopamine, acting via cyclic adenosine 3':5'-monophosphate (cAMP), has been shown to enhance a kainate-gated ionic conductance in white perch retinal horizontal cells in vitro. To determine whether this effect involves stimulation of a protein kinase, kainate-gated currents were observed in cultured horizontal cells that were dialyzed with the catalytic subunit of cAMP-dependent protein kinase. Intracellular application of catalytic subunit or cAMP, but not heat-inactivated catalytic subunit, caused significant enhancement of the kainate-evoked currents. These results suggest that kainate-gated channels in horizontal cells may be modified by a phosphorylation event.

Augmentation by external Mg ions of beta-adrenoceptor-mediated actions in the longitudinal muscle of rat uterus

1. The longitudinal muscle isolated from the uterus of oestrogen-treated rats was not spontaneously active in Locke solution, and electrical stimulation evoked phasic contraction. Isoprenaline (3 x 10(-11) - 10(-8) M) and dibutyryl cyclic AMP (db cyclic AMP, 0.1-0.8 mM) depressed the phasic contraction; the depression was enhanced in the presence of 0.6 mM Mg. 2. The contracture generated by 40 mM K was partially relaxed by isoprenaline (10(-11) - 10(-8) M) and db cyclic AMP (0.1-0.8 mM). Mg (0.6 mM) enhanced the isoprenaline-induced relaxation, but not that induced by db cyclic AMP. 3. The membrane potential of the muscle was -61 mV, and electrical stimulation induced an action potential which consisted of spike and plateau components. Application of isoprenaline and db cyclic AMP mainly reduced the duration of the plateau potential. The effect was potentiated by 0.6 mM Mg. 4. The membrane was hyperpolarized, accompanied by a decrease in membrane resistance, when 10(-8) M isoprenaline or 0.8 mM db cyclic AMP was applied. The effects of isoprenaline were prominently augmented in the presence of 1.2 mM Mg, while those of db cyclic AMP were slightly potentiated. 5. Forskolin (0.1 microM) or papaverine (10 microM) inhibited the phasic contraction and the K-contracture. The effect on the phasic contraction was potentiated by 0.6 mM Mg, while that on the K-contracture was not affected. 6. Forskolin shortened the action potential at 0.3 microM, and hyperpolarized the membrane with a decrease in membrane resistance at 3.0 microM. The membrane effects were augmented by 0.6 and 1.2 mM Mg, respectively. 7. It was hypothesized that external Mg ions could affect at least two processes involved in actions at beta-adrenoceptors on rat myometrium; receptor-agonist interaction and cyclic AMP-mediated inhibition of membrane excitability.

A family of calcium-dependent potassium channels from rat brain

By incorporating rat brain plasma membrane vesicles into planar lipid bilayers, we have found and characterized four types of Ca2(+)-activated K+ channels. The unitary conductances of these channels are 242 +/- 14 pS, 236 +/- 16 pS, 135 +/- 10 pS, and 76 +/- 6 pS in symmetrical 150 mM KCI buffers. These channels share a number of properties. They are all activated by depolarizing voltages, activated by micromolar concentrations of internal Ca2+ with a Hill coefficient for Ca2+ activation of between 2 and 3, noninactivating under our assay conditions, blocked by low millimolar concentrations of TEA from the outside, apamin-insensitive, and very selective for K+ over Na+ and Cl-. Three of the four channels are also blocked by nanomolar concentrations of charybdotoxin. One of the high conductance Ca2(+)-activated K+ channels is novel in that it is not blocked by charybdotoxin and exhibits gating kinetics highlighted by long closed times and long open times. This family of closely related Ca2(+)-activated K+ channels may share structural domains underlying particular functions.

Inositol 1,4,5-trisphosphate activates a channel from smooth muscle sarcoplasmic reticulum

Inositol 1,4,5-trisphosphate (InsP3) can initiate calcium release into the cytoplasm in a variety of cells. From experiments using permeabilized cells, membrane vesicles, and patch-clamp techniques, it has been suggested that InsP3 acts by directly opening calcium channels. Here, we show that InsP3 induced openings of channels in planar lipid bilayers into which vesicles made from aortic muscle sarcoplasmic reticulum (SR) were incorporated. Activation of channels by InsP3 was not observed when vesicles made from SR of cardiac or skeletal muscle were incorporated into planar lipid bilayers. The present study demonstrates for the first time unique properties of an InsP3-gated calcium channel in sarcoplasmic reticulum vesicles from vascular smooth muscle. This InsP3-activated channel from aortic SR differs strikingly from the calcium-gated calcium channel of striated muscle SR in single-channel conductance and pharmacology.

Single-channel recordings from the apical membrane of the toad urinary bladder epithelial cell

The patch-clamp technique for the recording of single-channel currents was used to investigate the activity of ion channels in the intact epithelium of the toad urinary bladder. High resistance seals were obtained from the apical membrane of tightly stretched tissue. Single-channel recordings revealed the activity of a variety of ion channels that could be classified in 4 groups according to their mean ion conductances, ranging from 5 to 59 pS. In particular, we observed highly selective, amiloride-sensitive Na channels with a mean conductance of 4.8 pS, channels with a similar conductance that were not Na-selective and channels with mean conductance values of 17-58 pS that were mostly seen after stimulation of the tissue with vasopressin or cAMP. When inside-out patches from the apical membrane were exposed to 110 mM fluoride, large conductances (86-490 pS) appeared.

Cyclic AMP-dependent protein phosphorylation is involved in activation of the potassium current associated with endogenous cellular calcium in Euhadra neurons

Intracellular injection of the catalytic subunit of cAMP-dependent protein kinase stimulated the outward potassium current associated with endogenous cellular Ca2+ which were abolished by either caffeine or cAMP-dependent protein kinase inhibitor. The principal action of this kinase in current activation may be to release calcium from the intracellular reservoir.

Calcium and voltage dependence of single Ca2+-activated K+ channels from cultured hippocampal neurons of rat

Calcium and voltage dependence of the Ca2+-activated K+ channel, K(Ca), was studied at the single-channel level in cultured hippocampal neurons from rat. The K(Ca) channel has approx. 220 pS conductance in symmetrical 150 mM K+, and is gated both by voltage and by Ca2+ ions. For a fixed Ca2+ concentration at the inner membrane surface, [Ca]i, channel open probability, Po, increases e-fold for 14 mV positive change in membrane potential. At a fixed membrane potential (0 mV), channel activity is first observed at [Ca]i = 10(-6) M, and increases with Ca2+ concentration approximating an absorption isotherm with power 1.4. The [Ca]i required to half activate (Po = 0.5) the channel is 4.10(-6) M. When compared to other preparations, the K(Ca) channel from hippocampal neurons reported here shows the lowest Ca2+ sensitivity and the highest voltage sensitivity. These findings are interpreted in evolutionary terms.

Cyclic AMP-dependent mechanism regulates acetylcholine receptor function on bovine adrenal chromaffin cells and discriminates between new and old receptors

Bovine adrenal chromaffin cells have nicotinic acetylcholine receptors (AChRs) that mediate release of catecholamines from the cells in response to synaptic input, and resemble neuronal AChRs in pharmacology and antigenic profile. Results presented here show that a cAMP-dependent process enhances the function of adrenal chromaffin AChRs as a population in the plasma membrane. This was demonstrated by showing that cAMP analogues cause specific increases both in the level of nicotine-induced catecholamine release from the cells and in the level of the nicotine-induced conductance change occurring in the cells. Neither de novo synthesis of receptors nor transport of preexisting intracellular receptors to the plasma membrane is necessary for the enhancement. The responsiveness of AChRs to regulation by the cAMP-dependent process appears to depend on the length of time the receptors have been on the cell surface. AChRs newly inserted into the plasma membrane generate a greater nicotinic response than do older AChRs and, unlike older AChRs, their response to agonist is not enhanced after treatment of the cells with cAMP analogues. The findings indicate that the AChRs and/or associated components undergo a maturation in the plasma membrane that alters their function and their regulation by secondary messenger systems.

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.

[Functional guanine nucleotide-binding proteins in receptor-mediated modulation of voltage-dependent ion channels]

G-proteins act as transducers between cell surface receptors activated by extracellular signals and enzymatic effectors which control the concentrations of cytosolic signal molecules such as cAMP, cGMP, inositol phosphates and calcium. The receptor/G-protein-induced changes of the intracellular concentration of such signal molecules correlates with activity changes of various voltage-dependent ion channels. In some instances, cytosolic signal molecules appear to interact directly with ion channels, thereby causing an alteration of ion channel activity. In other instances, signal molecules affect the function of ion channels by activating protein kinases which, in turn, phosphorylate either proteins constituting extracellular signal- and voltage-dependent ion channels or non-identified membranous regulatory components. Recent findings suggest a third, membrane-confined mechanism which does not involve cytosolic signal molecules but a close control of voltage-dependent ion channels by G-proteins. Ion channels that are modulated by extracellular signals according to this newly discovered principle include those for calcium and potassium in neuronal, cardiac and endocrine cells. G-proteins involved in the hormonal stimulation of potassium and calcium channels belong to the family of Gi-type G-proteins which are functionally uncoupled from activating receptors by pertussis toxin. In addition, the cholera toxin-sensitive G-protein, Gs, may directly stimulate cardiac calcium channels. Hormonal inhibition of calcium channels is possibly mediated by Go which, like G-proteins of the Gi-family, is functionally impaired by pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple types of voltage-dependent Ca2+-activated K+ channels of large conductance in rat brain synaptosomal membranes

K+-selective ion channels from a mammalian brain synaptosomal membrane preparation were inserted into planar phospholipid bilayers on the tips of patch-clamp pipettes, and single-channel currents were measured. Multiple distinct classes of K+ channels were observed. We have characterized and described the properties of several types of voltage-dependent, Ca2+-activated K+ channels of large single-channel conductance (greater than 50 pS in symmetrical KCl solutions). One class of channels (Type I) has a 200-250-pS single-channel conductance. It is activated by internal calcium concentrations greater than 10(-7) M, and its probability of opening is increased by membrane depolarization. This channel is blocked by 1-3 mM internal concentrations of tetraethylammonium (TEA). These channels are similar to the BK channel described in a variety of tissues. A second novel group of voltage-dependent, Ca2+-activated K+ channels was also studied. These channels were more sensitive to internal calcium, but less sensitive to voltage than the large (Type I) channel. These channels were minimally affected by internal TEA concentrations of 10 mM, but were blocked by a 50 mM concentration. In this class of channels we found a wide range of relatively large unitary channel conductances (65-140 pS). Within this group we have characterized two types (75-80 pS and 120-125 pS) that also differ in gating kinetics. The various types of voltage-dependent, Ca2+-activated K+ channels described here were blocked by charybdotoxin added to the external side of the channel. The activity of these channels was increased by exposure to nanomolar concentrations of the catalytic subunit of cAMP-dependent protein kinase. These results indicate that voltage-dependent, charybdotoxin-sensitive Ca2+-activated K+ channels comprise a class of related, but distinguishable channel types. Although the Ca2+-activated (Type I and II) K+ channels can be distinguished by their single-channel properties, both could contribute to the voltage-dependent Ca2+-activated macroscopic K+ current (IC) that has been observed in several neuronal somata preparations, as well as in other cells. Some of the properties reported here may serve to distinguish which type contributes in each case. A third class of smaller (40-50 pS) channels was also studied. These channels were independent of calcium over the concentration range examined (10(-7)-10(-3) M), and were also independent of voltage over the range of pipette potentials of -60 to +60 mV. Type III channels were unaffected by internal TEA concentrations <50 mM. Our results also indicate that the study of K+ channels in lipid bilayers may allow the identification and characterization of novel K+ channels from brain regions otherwise inaccessible to conventional recording techniques.

Vasoactive intestinal peptide activates Ca2(+)-dependent K+ channels through a cAMP pathway in mouse lacrimal cells

The action of vasoactive intestinal peptide (VIP) on Ca2(+)-dependent K+ currents, in dissociated mouse lacrimal cells, was investigated using patch clamp techniques. In whole cell recordings, VIP (10-100 pM) increased the magnitude of the Ca2(+)-dependent K+ current. In single channel recordings, VIP increased the fraction of time the large charybdotoxin-sensitive Ca2(+)-activated K+ channel spent in the open state. The activity of this channel was also increased by adding forskolin or 8-bromo cAMP to the bath. Additionally, application of either cAMP or catalytic subunit of cAMP-dependent protein kinase directly to the cytoplasmic surface of excised inside out patches reversibly lengthened the time Ca2(+)-activated K+ channels spent in the open state. These data suggest that VIP stimulates Ca2(+)-activated K+ channels by a cAMP-dependent pathway in mouse lacrimal acinar cells.

Cyclic AMP-stimulated sodium current in identified feeding neurons of Lymnaea stagnalis

Iontophoretic injection of cAMP elicits a slow, transient inward current in identified buccal feeding motoneurons and in the giant cerebral interneuron of the snail, Lymnaea stagnalis. The current is voltage independent, and is abolished in the absence of extracellular Na+. Application of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) causes a marked increase in both amplitude and duration of cAMP-stimulated inward current. The amplitude of the current is reduced following prolonged application of depolarizing pulses to the cell. However, generation of high-frequency bursts of action potentials lasting up to 20 s has no significant effect on the amplitude of the cAMP-induced current measured subsequently. Bath application of the cAMP analogue 8-chlorophenylthio-cAMP or of IBMX leads to enhanced bursting activity in buccal motoneurons. It is suggested that cAMP sensitivity in feeding motoneurons provides a mechanism for adjusting the cells' responsiveness to rhythmic synaptic inputs during the generation of feeding motor output.

Role of Ca2+-activated K+ channel in regulation of NaCl reabsorption in thick ascending limb of Henle's loop

1. Reabsorption of NaCl in the thick ascending limb of Henle's loop involves the integrated function of the Na+,K+,Cl- -cotransport system and a Ca2+-activated K+ channel in the luminal membrane with the Na+,K+-pump and a net Cl- conductance in the basolateral membrane. 2. Assay of K+ channel activity after reconstitution into phospholipid vesicles shows that the K+ channel is stimulated by Ca2+ in physiological concentrations and that its activity is regulated by calmodulin and phosphorylation from cAMP dependent protein kinase. 3. For purification luminal plasma membrane vesicles are isolated and solubilized in CHAPS. K+ channel protein is isolated by affinity chromatography on calmodulin columns. The purified protein has high Ca2+-activated K+ channel activity after reconstitution into vesicles. 4. The purified K+ channel consists of two proteins of 51 and 36 kDa. Phosphorylation from cAMP dependent protein kinase stimulates K+ channel activity and labels the 51 kDa band. The 36 kDa band is rapidly cleaved by trypsin and may be involved in Ca2+ stimulation. 5. Opening of the K+ channel by Ca2+ in physiological concentrations and regulation by calmodulin and phosphorylation by protein kinase may mediate kinetic and hormonal regulation of NaCl transport across the tubule cells in TAL.

Pharmacological control of the pattern of activity in leech Retzius neurones

1. Changes in the activity pattern of leech Retzius (R) cells were investigated using intracellular recording. 2. The presence of an after-hyperpolarisation (AHP) is closely related to activity pattern; regular firing being associated with an AHP, bursting with its absence. 3. Increasing external calcium (Cao), cyclic AMP levels or activity of kinase A enhanced the AHP. 4. Bursting was induced by low Cao, EGTA, barium, cobalt or injection of phorbol ester. 5. Reduction of Cao to zero caused long paroxysmal depolarising shifts of potential which could be reversed to bursting by cobalt, IBMX or injection of kinase A catalytic subunit. 6. The possible roles of a calcium-activated potassium channel and protein phosphorylation in regulating the activity of the cell are discussed.

Calcium dependence of voltage sensitivity in adenosine 3',5'-cyclic phosphate-stimulated sodium current in Pleurobranchaea

1. Ionophoretic injection of cyclic AMP into a voltage-clamped molluscan neurone caused a transient slow inward current (Isi) whose amplitude was enhanced by depolarization. Na+-replaced salines abolished the current, placing it with cyclic AMP-stimulated Na+ currents of other gastropod species. 2. Isi amplitude was suppressed by extracellular Ca2+. The amplitude increased up to 4-fold at holding potentials of -50 mV in nominally Ca2+-free saline. Ion substitutions showed that Ca2+ suppressed Isi more effectively than Mg2+, Co2+, Cd2+, Mn2+, Ba2+ or Sr2+. 3. Voltage sensitivity of Isi was abolished by low-Ca2+ salines, by the Ca2+ current blocker Co2+ and by substitution of Ba2+ or Sr2+ as Ca2+ channel current carriers. In such salines Isi showed no appreciable change in amplitude at holding potentials between -70 and -25 mV. 4. Intracellular injection of the Ca2+ chelator EGTA both augmented the amplitude of the current and its duration. EGTA injection failed to suppress the Ca2+-dependent voltage sensitivity of Isi. Intracellular injection of concentrated 3-N-(morpholino) propanesulphonic acid (MOPS) pH buffer to inhibit secondary, Ca2+-dependent intracellular acidification also failed to suppress the voltage sensitivity, as did injections of a mixed EGTA and MOPS solution. 5. While the data indicate a requirement for extracellular Ca2+ in conferring voltage sensitivity, they do not support a role for an intracellular action. An extracellular binding site for Ca2+ could mediate the voltage sensitivity, either by local depolarization-dependent changes in extracellular Ca2+ concentration or through direct voltage-sensitive block of the Isi channel.

Involvement of a 65 kDa phosphoprotein in the regulation of membrane fusion during exocytosis in Paramecium cells

Antisera were raised against a phosphoprotein of 65 kDa (PP65) from Paramecium cells (shown before to be selectively dephosphorylated during synchronous exocytosis) and specified by immunoblotting. By immunofluorescence PP65 has been localized within the cortex, beneath the cell membrane. This corresponds to data obtained by cell fractionation, applying SDS-PAGE autoradiography to cortices prepared from 32P-prelabeled cells. Antisera against PP65 inhibit exocytosis in vivo (microinjection). Applying anti-PP65 antisera in vitro to cortices we could demonstrate inhibition not only of exocytosis, but also of PP65 dephosphorylation. We conclude that PP65 is involved in the regulation of membrane fusion during exocytosis.

Effects of pCai and pHi on cell-to-cell coupling

Internal longitudinal resistance (ri), a determinant of cardiac conduction, is affected by changes in intracellular calcium and protons. However, the role and mechanism by which H+ and Ca2+ may modulate ri is uncertain. Cable analysis was performed in cardiac Purkinje fibers to measure ri during various interventions. In some experiments, intracellular pH (pHi) was recorded simultaneously to study the pHi-ri relation. Both intracellular Ca2+ and H+ independently modified ri. However, internal resistance of cardiac fibers was insensitive to pHi changes compared to other tissues. A latent period preceded the pHi-related changes in ri and the amount of change depended upon methodology. The results suggest that direct action of protons or ri may be subordinate to other regulatory processes. Ionic regulation of internal longitudinal resistance may occur by more than one mechanism: i) direct cationic binding to sites on junctional membrane proteins; and ii) H+- or Ca2+-dependent phosphorylation of junctional proteins.

Retinal horizontal cell gap junctional conductance is modulated by dopamine through a cyclic AMP-dependent protein kinase

The action of many neuromodulators is mediated by intracellular second messengers such as cyclic AMP. In the retina, exogenously applied dopamine alters the conductance of gap junctions between cultured horizontal cells and this effect is mediated by cyclic AMP. However, it is not known how cyclic AMP modulates horizontal cell gap junction function. Here I report that cyclic AMP works by way of a cyclic AMP-dependent protein kinase. Cyclic AMP-dependent protein kinase injected into coupled horizontal cells from white bass (Roccus chrysops) rapidly and reversibly uncoupled the cells, mimicking the actions of dopamine. The threshold for the effect was between 0.06 and 0.03 microM. Injection of Walsh inhibitor of protein kinase [Walsh, D. A., Ashby, C. D., Gonzalez, C., Calkins, D., Fischer, E. H. & Krebs, E. G. (1971) J. Biol. Chem. 246, 1977-1985] blocked the effect of dopamine. Thus, the action of dopamine is to raise intracellular levels of cyclic AMP, which then activates a cyclic AMP-dependent protein kinase. Although not tested, it is likely that the cyclic AMP-dependent protein kinase phosphorylates a protein, possibly a gap junction protein, to alter conductance.

Axonal transport of neuroreceptors: possible involvement in long-term memory

Evidence is presented that neuroreceptors move bidirectionally along axons of neurons through fast axoplasmic transport mechanisms. The retrograde transport of receptor-bound signal molecules from nerve terminals to the perikaryon represents a corridor of information between synapses and the cell body of neurons. It is speculated that this process could be involved in long-term memory.