Purified dihydropyridine-binding site from skeletal muscle t-tubules is a functional calcium channel

Subunit identification and reconstitution of the N-type Ca2+ channel complex purified from brain

Calcium channels play an important role in regulating various neuronal processes, including synaptic transmission and cellular plasticity. The N-type calcium channels, which are sensitive to omega-conotoxin, are involved in the control of transmitter release from neurons. A functional N-type calcium channel complex was purified from rabbit brain. The channel consists of a 230-kilodalton subunit (alpha 1B) that is tightly associated with a 160-kilodalton subunit (alpha 2 delta), a 57-kilodalton subunit (beta 3), and a 95-kilodalton glycoprotein subunit. The complex formed a functional calcium channel with the same pharmacological properties and conductance as those of the native omega-conotoxin-sensitive calcium channel in neurons.

Stimulation of the slow calcium current in bullfrog skeletal muscle fibers by cAMP and cGMP

The effects of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) on slow calcium currents (ICa) were investigated using the Vaseline-gap voltage-clamp technique in bullfrog skeletal muscle cut fibers. Both cAMP and cGMP induced a pronounced increase in the amplitude of ICa when applied to the cut ends of fibers. Both cyclic nucleotides also decreased time to peak current at all membrane potentials. The current-voltage relationship was shifted toward more negative potentials by cAMP as well as cGMP. The potentiating effects of cAMP and cGMP on ICa were additive. 8-Bromo analogues of both nucleotides had similar effects on ICa. The beta-adrenergic agonist isoproterenol, applied extracellularly, also produced an increase in the amplitude of ICa and produced a leftward shift in the current-voltage relationship. These results suggest that both cAMP and cGMP modulate calcium slow channels in bullfrog skeletal muscle fibers, causing stimulation of the ICa. The effect of cyclic nucleotides on ICa in bullfrog skeletal muscle contrasts with that in mammalian cardiac muscle, in which the same nucleotides produce opposite effects on the slow ICa, i.e., in cardiac muscle cAMP stimulates, and cGMP inhibits, the slow ICa.

Regulation of cardiac L-type Ca channels in planar lipid bilayers by G proteins and protein phosphorylation

We have studied the effects of activated G proteins (Gs alpha and Gi1 alpha), adenosine 3',5'-cyclic monophosphate-dependent protein kinase (PKA), and okadaic acid on L-type Ca channels incorporated from porcine ventricular sarcolemma into planar lipid bilayers. Channel activity evoked by membrane depolarizations diminished to extremely low levels within 2 min of incorporation (channel "rundown"). When Gs alpha [activated with guanosine 5'-O-(3-thiotriphosphate)] was present in the intracellular chamber, the initial level of channel activity was increased and rundown was delayed, so that channel activity was sustained for longer times after incorporation. The effect was specific for activated Gs alpha; activated Gi1 alpha, heat-denatured, activated Gs alpha, and unactivated Gs alpha did not augment channel activity. Activated Gi1 alpha inhibited the stimulation of Ca channel activity by Gs alpha. Treatment of the sarcolemmal membranes with PKA and Mg-ATP also increased the initial channel open probability and delayed their rundown. Addition of intracellular Gs alpha to PKA-treated channels increased the initial level of activity above that seen with PKA or Gs alpha alone, suggesting different nonocclusive pathways for the channel stimulation. This was also supported by the observation that activated Gi1 alpha had no effect on PKA-treated channels. Okadaic acid (100 nM) increased the level of Ca channel activity, suggesting that dephosphorylation by endogenous phosphatases participated in the downregulation of the channels in cell-free membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfection of the CD20 cell surface molecule into ectopic cell types generates a Ca2+ conductance found constitutively in B lymphocytes

CD20 is a plasma membrane phosphoprotein expressed exclusively by B lymphocytes. mAb binding to CD20 alters cell cycle progression and differentiation, indicating that CD20 plays an essential role in B lymphocyte function. Whole-cell patch clamp and fluorescence microscopy measurements of plasma membrane ionic conductance and cytosolic-free Ca2+ activity, respectively, were used to directly examine CD20 function. Transfection of human T and mouse pre-B lymphoblastoid cell lines with CD20 cDNA and subsequent stable expression of CD20 specifically increased transmembrane Ca2+ conductance. Transfection of CD20 cDNA and subsequent expression of CD20 in nonlymphoid cells (human K562 erythroleukemia cells and mouse NIH-3T3 fibroblasts) also induced the expression of an identical transmembrane Ca2+ conductance. The binding of a CD20-specific mAb to CD20+ lymphoblastoid cells also enhanced the transmembrane Ca2+ conductance. The mAb-enhanced Ca2+ currents had the same conductance characteristics as the CD20-associated Ca2+ currents in CD20 cDNA-transfected cells. C20 is structurally similar to several ion channels; each CD20 monomer possesses four membrane spanning domains, and both the amino and carboxy termini reside within the cytoplasm. Biochemical cross-linking of cell-surface molecules with subsequent immunoprecipitation analysis of CD20 suggests that CD20 may be present as a multimeric oligomer within the membrane, as occurs with several known membrane channels. Taken together, these findings indicate that CD20 directly regulates transmembrane Ca2+ conductance in B lymphocytes, and suggest that multimeric complexes of CD20 may form Ca2+ conductive ion channels in the plasma membrane of B lymphoid cells.

Phosphorylation of the porcine skeletal and cardiac muscle sarcoplasmic reticulum ryanodine receptor

Porcine skeletal and cardiac muscle sarcoplasmic reticulum (SR) vesicle fractions enriched in the ryanodine receptor were phosphorylated in the presence of [gamma-32P]MgATP and either exogenous cAMP-dependent protein kinase (cAMP-PK), or Ca2+ plus calmodulin. Phosphorylation of the cardiac muscle ryanodine receptor in the presence of either cAMP-PK or calmodulin (6.4 and 10.6 pmol Pi/mg SR respectively) was approximately equal to or twice the [3H]ryanodine binding activity of this preparation (5.2 pmol/mg). Furthermore, cardiac muscle ryanodine receptor Pi incorporation catalyzed by cAMP-PK and calmodulin was approximately additive. In skeletal muscle SR, however, the level of cAMP-PK or calmodulin catalyzed phosphorylation of the intact ryanodine receptor (0.2 or 2.9 pmol Pi/mg SR, respectively) was much less than the [3H]ryanodine binding activity of this fraction (11.6 pmol/mg). Furthermore, Pi incorporation into the intact skeletal muscle ryanodine receptor was 3-8-fold less than that incorporated into a component of slightly lower M(r). Although this latter component comigrated with an immunoreactive fragment of the ryanodine receptor on polyacrylamide gels, it did not appear to be derived from the ryanodine receptor. We conclude that the significant phosphorylation of the cardiac muscle SR ryanodine receptor indicates a likely physiological role for protein kinase-mediated regulation of this Ca(2+)-channel. In contrast, the minimal phosphorylation of the skeletal muscle SR ryanodine receptor indicates that such a role of protein kinases is unlikely in this tissue.

Solubilized proteins from carrot (Daucus carota L.) membranes bind calcium channel blockers and form calcium-permeable ion channels

Calcium channels have been suggested to play a major role in the initiation of a large number of signal transduction processes in higher plant cells. However, molecular components of higher plant Ca2+ channels remain unidentified to date. Calcium channel blockers of the phenylalkylamine family and bepridil specifically inhibit Ca2+ influx into carrot (Daucus carota L.) cells. By using a phenylalkylamine azido derivative, a 75-kDa carrot membrane protein has been previously identified. Here we have partially purified this Ca2+ channel blocker-binding protein by lectin-affinity and ion-exchange chromatographies. The protein fraction containing the 75-kDa binding protein was incorporated into giant liposomes. Single-channel patch-clamp studies on these proteoliposomes showed the presence of Ca2+-permeable channel currents. These Ca2+-permeable channels were not stable. Recordings after durations of 2-10 min showed the appearance of nonselective ion channels with a permeability to calcium and chloride ions. These nonselective Ca2+-permeable ion channels, in contrast, were stable and were recorded for extended durations. The addition of the Ca2+ channel-blocker bepridil (10 M) led to the inhibition of these nonselective Ca2+-permeable channels by reducing the probability of channel opening. These results suggest that the 75-kDa Ca2+ channel blocker-binding protein from carrot cells plays a role in channel sensitivity to Ca2+ channel inhibitors and may constitute one of the components of Ca2+ channels in higher plants.

Adenylate cyclase induces intracellular calcium increase in single human epidermal keratinocytes measured by fluorescence microscopy using Fura 2-AM

Intracellular calcium ([Ca2+]i) is an important second messenger of extracellular signals to induce various cellular responses. Extracellular and intracellular Ca2+ are considered to be important for cellular differentiation and proliferation of epidermal keratinocytes. Several mechanisms which increase [Ca2+]i have been demonstrated in various tissues, but in epidermal keratinocytes these mechanisms are poorly understood. In epidermal keratinocytes the adenylate cyclase-cyclic AMP response is thought to regulate cell proliferation and differentiation. However, the series of reactions which follow the cyclic AMP response remain unknown. Beta-adrenergic agonists increase [Ca2+]i in cultured epidermal keratinocytes, and we have therefore studied whether stimulation of keratinocyte adenylate cyclase could induce [Ca2+]i increase, by using fluorescence microscopy with Fura 2-AM. Adenosine and histamine, which are known to be keratinocyte adenylate cyclase receptor agonists, induced transient [Ca2+]i increase, as did epinephrine. In addition, forskolin, a direct adenylate cyclase activator, and dibutyryl-cyclic AMP also induced an increase in [Ca2+]i. In a calcium-free medium epinephrine, adenosine, histamine and dibutyryl-cyclic AMP induced an increase in [Ca2+]i. These results suggest that cyclic AMP in human epidermal keratinocytes regulates [Ca2+]i, which is released from intracellular stores.

Structural analysis of muscle development: transverse tubules, sarcoplasmic reticulum, and the triad

Increased interest in the mechanism of excitation-contraction (E-C) coupling over the last few years has been accompanied by numerous investigations into the development of the underlying cellular structures. Areas of particular interest include: (1) the compartmentalization and specialization of an external and an internal membrane system, the T-tubules, and the sarcoplasmic reticulum, respectively; (2) interactions between the membrane proteins of both systems upon the formation of a junction, the triad; and (3) membrane-cytoskeletal interactions leading to the orderly arrangement of the triads with respect to the myofibrils. Structural studies using newly available specific molecular probes and a variety of in vivo and in vitro model systems have provided new insights into the cellular and molecular mechanisms involved in the development of the E-C coupling apparatus in skeletal muscle.

Ion channels

Insulin secretion by the pancreatic Beta cell is dependent upon transmembrane ion fluxes gated by the ATP-regulated potassium channel and the voltage regulated, L-type calcium channel. This work group examined major recent advances in the structure and modulation of ion channels and how those advances may pertain to the physiology of insulin secretion and the pharmacological treatment of Type 2 (non-insulin-dependent) diabetes mellitus. Structural studies have revealed that voltage gated ion channels are related, complex, and comprised of multiple components: sodium channels consist of three distinct subunits. L-type calcium channels, crucial to the insulin secretory response are structurally related to the sodium channel but contain additional subunits. Potassium channels are less closely related and appear to function as homotetramers. Modulation of ion channel activity is similarly complex: site specific phosphorylation by multiple protein kinases under the control of several intracellular second messenger systems may increase or decrease conductance. Subunit composition and relatively stable changes in the modal state of ion channels also appear to be critical to ion channel gating properties. Functional studies of the Beta-cell ATP-regulated potassium channel suggest two distinct nucleotide binding sites which link this channel to the metabolic state of the Beta cell. The multiple paths of ion channel modulation provide multiple targets for therapeutic intervention. Where detailed characterisation of ion channel structure has been achieved, those targets are being used for specific drug design. Such complete characterisation has not yet been achieved for Beta-cell ion channels and this presents a major goal for diabetes research.

Cyclic AMP-dependent phosphorylation and regulation of the cardiac dihydropyridine-sensitive Ca channel

A polyclonal antibody, CR2, prepared using the C-terminal peptide of the alpha 1 subunit of the rabbit cardiac DHP-sensitive Ca channel, specifically immunoprecipitated the [3H]PN200-110-labeled Ca channel solubilized from cardiac microsomes. The antibody recognized 250 and 200-kDa cardiac microsomal proteins as determined by immunoblotting, and cAMP-dependent protein kinase phosphorylated the 250-kDa, but not the 200-kDa protein in vitro. CHO cells, transfected with the cardiac alpha 1 subunit cDNA carried by an expression vector, synthesized a 250-kDa protein which was recognized by CR2. Adding db-cAMP or forskolin to the transformed CHO cells induced phosphorylation of the 250-kDa protein and stimulated the DHP-sensitive Ba current under patch-clamp conditions. These results suggested that the cardiac DHP-sensitive Ca channel was regulated by cAMP-dependent phosphorylation of the alpha 1 subunit.

Dihydropyridine-sensitive skeletal muscle Ca channels in polarized planar bilayers. 3. Effects of phosphorylation by protein kinase C

The effects of protein kinase C (PKC) were studied on dihydropyridine (DHP)-sensitive Ca channels from rabbit skeletal muscle T tubule membranes. To determine which channel subunits become phosphorylated under the conditions used for electrophysiological studies, we first performed biochemical studies of phosphorylation. T tubular membranes were fused with vesicles of the lipid mixture used in the planar bilayers, and phosphorylation was assessed using the same concentrations of PKC, adenosine 5'-triphosphate, and buffers as were used in the electrophysiological experiments. The alpha 1 subunit of the DHP receptors was phosphorylated by PKC to an extent of 1 mol phosphate/mol protein. The beta subunit was also phosphorylated but to a significantly lesser extent. The DHP-sensitive Ca channel activity was studied after fusing T tubule membranes with planar bilayers (Ma, J., C. Mundiña-Weilenmann, M. M. Hosey, and E. Ríos. 1991. Biophys. J. 60:890-901). The bilayers were held at -80 mV and activated by depolarizing voltage clamp pulses. The observed Ca channels exhibited two open states (tau o1 = 5 ms and tau o2 = 25 ms). On addition of purified PKC to the intracellular side, the proportion of the longer open state increased threefold. The average open probability during a 2-s, maximally activating pulse (Pmax) increased from 10 to 15%. The voltage dependence of activation was not changed by PKC; the Boltzmann parameters were V1 = -20.5 mV and K = 10.5 mV, which were not significantly different from the reference channels. The deactivation (closing) time constant was increased from 7 to 12 ms after PKC. The inactivation time constant during the pulse was slightly increased(from 1.2 to 1.6 s), and the channel availability at the holding potential was decreased from 76 to 71%. Taken together, the results revealed that PKC increased Pmax largely through a shift in the voltage independent open-close equilibrium of the fully activated channels.This is in contrast with the effect of phosphorylation by PKA (Mundir'a-Weilenmann, C., J. Ma, E. Rios, and M. M. Hosey. 1991. Biophys.J. 60:902-909), which also increases Pmax but mostly by increasing the availability of channels and slowing inactivation during the pulse.

An endogenous membrane-bound protein inhibits the phosphorylation of the L-type calcium channel by the cAMP-dependent protein kinase

An endogenous protein inhibits the PKA-phosphorylation of the DHP-binding calcium channel complex in vitro. The inhibitory activity could be reduced by a treatment with detergents or dithiothreitol. Further purification separates the inhibitory activity from the dihydropyridine-binding calcium channel complex. Both activities are localized in the plasma membrane indicating that this protein kinase-inhibitor could interfere with the phosphorylation of the calcium channel by the cAMP-dependent protein kinase. The inhibitory activity may therefore take part in the regulation of the calcium channel current.

A two-motif isoform of the major calcium channel subunit in skeletal muscle

Evidence is presented that two isoforms of the voltage-dependent, dihydropyridine-sensitive calcium channel alpha 1 subunit are present in newborn and adult skeletal muscle and that expression of these isoforms is developmentally regulated. A voltage-dependent calcium channel alpha 1 cDNA from newborn muscle was cloned and found to be identical to that published from the adult, except that it was 2 kb shorter owing to an internal deletion. Nucleotide sequences, Northern blots, reverse-transcriptase PCR experiments, and sequencing of the PCR product confirmed that a segment corresponding to the inner two repeats of the structural prototype four homologous motifs is missing from the immature isoform. Immunological studies using antisera raised against synthetic peptides that correspond to sequences in the two isoforms show that the abbreviated transcript is predominant in newborn muscle, whereas the four-repeat isoform is the major species in the adult.

Inactivation of the Ba2+ current in dissociated Helix neurons: voltage dependence and the role of phosphorylation

The rate of inactivation of the voltage-dependent Ba2+ current in dissociated neurons from the snail Helix aspersa was found to be modulated by phosphorylation. Conditions were chosen such that the most likely mechanism of inactivation of the Ba2+ current was a voltage-dependent/calcium-independent inactivation process. If adenosine-triphosphate (ATP) was not included in the patch electrode filling solution, or if alkaline phosphatase was added, the Ba2+ current rapidly ran down and the rate of inactivation greatly increased with time. Dialysis with either ATP gamma S or the phosphatase inhibitor okadaic acid (OA) either enhanced the amplitude or greatly reduced the rate of run-down of the Ba2+ current (depending upon the presence of ATP), as well as reducing the rate of inactivation. However, dialysis with either the catalytic subunit of the cyclic-adenosine-mono-phosphate-dependent protein kinase (cAMP-PK), a synthetic peptide inhibitor of this enzyme, or staurosporine (a potent inhibitor of protein kinase C), did not have any significant effect on the amplitude or kinetics of the Ba2+ current. Surprisingly, dialysis with a peptide inhibitor (CKIP) of the Ca2+/calmodulin-dependent protein kinase II (Ca(2+)-CaM-PK) significantly reduced the rate of inactivation of this current. These results suggest that phosphorylation may exert its effect by modulating the gating properties of the Ca2+ channels.

Dihydropyridine binding of the calcium channel complex from skeletal muscle is modulated by subunit interaction

The dihydropyridine-binding subunit alpha 1 of the calcium channel complex from rabbit skeletal muscle can be partially depleted from the alpha 2 delta beta-complex using wheat germ agglutinin-affinity chromatography. This depletion of the alpha 1 from the other subunits leads to a loss of dihydropyridine-binding, which can be fully reconstituted by repletion of the alpha 1 with the other subunits. Reassembly of these subunits results in an increase in the Kd and Bmax of the dihydropyridine-binding indicating that the non-dihydropyridine-binding subunits influence dihydropyridine-binding. The affinity of the alpha 1 subunit for the other subunits was determined to be approximately 35 nM. Since the free alpha 1 subunit will not bind to the beta subunit alone, there is evidence, given the selective partitioning of the beta subunit to the lectin-bound subunit pool, that either beta binds with higher affinity to the alpha 2 delta-complex than to the free alpha 1 subunit or that the bound alpha 1 creates or modulates beta-binding. This indicates a functional high affinity interaction between the dihydropyridine-binding alpha 1 subunit and the alpha 2 delta beta-complex.

Phosphorylation modulates the voltage dependence of channels reconstituted from the major intrinsic protein of lens fiber membranes

Major intrinsic polypeptide (MIP), a 28-kDa protein isolated from lens fiber cell membranes, forms large, nonselective channels when reconstituted into lipid bilayers. MIP channels are regulated by voltage, such that these channels close when the potential across the membrane is greater than 30 mV. We have investigated the modulation of the voltage-dependent closure of MIP channels by phosphorylation. In this report, we describe the isolation of two isomers of MIP from lens fiber cell membranes. These isomers differ by a single phosphate at a protein kinase A phosphorylation site. The phosphorylated isomer produces channels that close in response to applied voltages when reconstituted into bilayers. The nonphosphorylated isomer produces voltage-independent channels. Direct phosphorylation with protein kinase A converts voltage-independent channels to voltage-dependent channels in situ. Analyses of macroscopic and single-channel currents suggest that phosphorylation increases the voltage-dependent closure of MIP channels by increasing closed channel lifetimes and the rate of channel closure following the application of voltage.

Calcium channels and their involvement in cardiovascular disease

The widespread distribution of L-type Ca2+ channels in the cardiovascular system makes that system a natural 'target' for drugs which inhibit L-type Ca2+ channel activity. Now that tissue-dependent differences in the chemical composition of the calcium antagonist binding sites have been recognized it may be possible to develop drugs with enhanced tissue selectivity. The search for new compounds should not be restricted to improvements in tissue selectivity, however. Some of the ancillary properties of the L-type Ca2+ channel inhibitors--including their ability to protect against lipid peroxidation--should not be lost because these ancillary properties may contribute significantly to their usefulness as therapeutic agents.

Calmodulin antagonists and protein phosphatase inhibitor okadaic acid fasten the 'run-up' of high-voltage activated calcium current in rat hippocampal neurones

Voltage-activated Ca2+ channel currents were recorded from cultured rat hippocampal neurones using the whole-cell clamp technique with Ba2+ as a charge carrier. After breaking into the cell the amplitude of low-voltage activated Ca2+ channel current increased to a new steady value within 1 min whereas several minutes were required for a full development of the high-voltage activated current (IHVA). Pretreatment of cells with calmodulin antagonists (trifluoperazine or W-13) or protein phosphatase inhibitor, okadaic acid, fastened the development of IHVA. Trifluoperazine (6-40 microM) also increased IHVA when applied after breaking into the cell in standard external solution. Incubation of cells in the presence of permeable precursor of Ca2+ chelator, BAPTA, was without effect. The effects of all inhibitors studied allow to suggest that IHVA in intact cells is largely masked due to activity of calmodulin-activated protein phosphatase.

Characterization of the two size forms of the alpha 1 subunit of skeletal muscle L-type calcium channels

The molecular properties of two size forms of the alpha 1 subunit of purified skeletal muscle calcium channels were analyzed. The minor, full-length, form, alpha 1(212), was found to have an apparent molecular mass of 214 kDa by Ferguson plot analysis, while the major, truncated, form, now designated alpha 1(190), had an apparent molecular mass of 193 kDa. Antibody mapping of the C-terminal region of alpha 1(190) with 10 anti-peptide antibodies placed the C terminus between residues 1685 and 1699. Three consensus sites for cAMP-dependent protein phosphorylation are present in the C-terminal region of alpha 1(212) but not in alpha 1(190), and they may be important for the regulation of the ion conductance activity of the calcium channel.

Dihydropyridine receptor alpha subunits in normal and dysgenic muscle in vitro: expression of alpha 1 is required for proper targeting and distribution of alpha 2

We have studied the subcellular distribution of the alpha 1 and alpha 2 subunits of the skeletal muscle dihydropyridine (DHP) receptor with immunofluorescence labeling of normal and dysgenic (mdg) muscle in culture. In normal myotubes both alpha subunits were localized in clusters associated with the T-tubule membranes of longitudinally as well as transversely oriented T-tubules. The DHP receptor-rich domains may represent the sites where triad junctions with the sarcoplasmic reticulum are being formed. In cultures from dysgenic muscle the alpha 1 subunit was undetectable and the distribution patterns of the alpha 2 subunit were abnormal. The alpha subunit did not form clusters nor was it discretely localized in the T-tubule system. Instead, alpha 2 was found diffusely distributed in parts of the T-system, in structures in the perinuclear region and in the plasma membrane. These results suggest that an interaction between the two alpha subunits is required for the normal distribution of the alpha 2 subunit in the T-tubule membranes. Spontaneous fusion of normal non-muscle cells with dysgenic myotubes resulted in a regional expression of the alpha 1 polypeptide near the foreign nuclei, thus defining the nuclear domain of a T-tubule membrane protein in multi-nucleated muscle cells. Furthermore, the normal intracellular distribution of the alpha 2 polypeptide was restored in domains containing a foreign "rescue" nucleus; this supports the idea that direct interactions between the DHP receptor alpha 1 and alpha 2 subunits are involved in the organization of the junctional T-tubule membranes.

Signal transduction by cGMP in heart

Early studies in whole heart indicated that cGMP antagonized the positive inotropic effects of catecholamines and cAMP. However, the regulation of cGMP levels by a variety of agents was not always consistent with their effects on contractility. It is now clear that at least two major cell types in whole heart, cardiac myocytes and vascular smooth muscle cells, differ markedly in their mechanisms of cGMP regulation and response to cGMP. Furthermore, experiments on isolated cardiac myocytes indicate that the mechanism of cGMP action even in this single cell type can be multifaceted. Cyclic GMP inhibits the L-type calcium channel current (ICa), which is the major source of Ca++ entry into heart cells, and which plays a predominant role in the initiation and regulation of cardiac electrical and contractile activities. Patch-clamp measurements of ICa indicate that in isolated frog myocytes cGMP inhibits ICa by stimulation of cAMP phosphodiesterase (cGS-PDE), whereas in purified rat ventricular myocytes, cGMP predominantly inhibits ICa via a mechanism involving cGMP-dependent protein kinase (cGMP-PK). Under certain conditions, cGMP can also inhibit a cGMP-inhibited cAMP phosphodiesterase (cGI-PDE) and thereby produce a stimulatory effect on ICa. Biochemical characterization of the endogenous PDEs and cGMP-PK in purified cardiac myocytes provided further evidence in support of these mechanisms of cGMP action on ICa.

Enzymatic gating of voltage-activated calcium channels

The model of calcium-channel gating described above, although almost certainly too simple, suggests a direct role for protein kinases and phosphatases in determining the kinetics of calcium channel gating on a subsecond time scale. In addition, it provides a unique perspective for understanding studies of calcium channel gating under widely different metabolic and pharmacological conditions. Although many of these effects may be specific to the dihydropyridine-sensitive or L-type calcium channel, they give an indication of the range of possibilities for integrating calcium-channel activity with cellular biochemistry.

Dihydropyridine-sensitive skeletal muscle Ca channels in polarized planar bilayers. 2. Effects of phosphorylation by cAMP-dependent protein kinase

The effects of phosphorylation on the voltage-dependent properties of dihydropyridine-sensitive Ca channels of skeletal muscle were studied. Single channel currents were recorded upon incorporation of transverse tubule membranes into planar bilayers that were kept polarized at near physiological resting potential and subjected to depolarizing pulses under voltage clamp. Studies were conducted to analyze the properties of the channels at both the single channel and macroscopic level, using methods introduced in the preceding paper (Ma et al., 1991. Biophys. J. 60: 890-901.). Addition of the catalytic subunit of cAMP-dependent protein kinase to the cis (intracellular) side of the bilayers containing channels resulted in: (a) an increase in open channel probability at all voltages above -50 mV; (b) a leftward shift (by 7 mV) in the curve describing the voltage-dependence of activation; (c) an approximate twofold decrease in the rate of inactivation; and (d) an increase in the availability of the channel. These findings provide new insights at the single channel level into the mechanism of modulation of the dihydropyridine-sensitive Ca channels of skeletal muscle by signal transduction events that involve elevation in cAMP and activation of the cAMP-dependent protein kinase.

Dihydropyridine-sensitive skeletal muscle Ca channels in polarized planar bilayers. 1. Kinetics and voltage dependence of gating

Rabbit skeletal muscle transverse tubule (T) membranes were fused with planar bilayers. Ca channel activity was studied with a "cellular" approach, using solutions that were closer to physiological than in previous studies, including asymmetric extracellular divalent ions as current carriers. The bilayer was kept polarized at -80 mV and depolarizing pulses were applied under voltage clamp. Upon depolarization the channels opened in a steeply voltage-dependent manner, and closed rapidly at the end of the pulses. The activity was characterized at the single-channel level and on macroscopic ensemble averages of test-minus-control records, using as controls the null sweeps. The open channel events had one predominant current corresponding to a conductance of 9 pS (100 mM Ba2+). The open time histogram was fitted with two exponentials, with time constants of 5.8 and 30 ms (23 degrees C). Both types of events were virtually absent at -80 mV. The average open probability (fractional open time) increased sigmoidally from 0 to a saturation level of 0.08, following a Boltzmann function centered at -25 mV and with a steepness factor of 7 mV. Ensemble averages of test-minus-control currents showed a sigmoidal activation followed by inactivation during the pulse and deactivation (closing) after the pulse. The ON time course was well fitted with "m3h" kinetics, with tau m = 120 ms and tau h = 1.2 s. Deactivation was exponential with tau = 8 ms. This study demonstrates a technique for obtaining Ca channel events in lipid bilayers that are strictly voltage dependent and exhibit most of the features of the macroscopic ICa. The technique provides a useful approach for further characterization of channel properties, as exemplified in the accompanying paper, that describes the consequences on channel properties of phosphorylation by cAMP dependent protein kinase.

Gastrocnemius acetylcholinesterase activity in response to burn trauma in the mouse

In this study we examined the effect of increasing size of body surface area (BSA, 20, 30 and 50 per cent) burn on distally located gastrocnemius acetylcholinesterase (AChE) activity. Burn injury was applied to predefined areas of the dorsal and ventral skin surfaces of mice. AChE activity at 3 weeks postburn was measured by differential extraction and velocity sedimentation on sucrose gradients. Analysis of variance was used for all statistical evaluation. Within the sham-treated controls most of the AChE activity consisted entirely (92 per cent) of the globular forms (S1 and S2). Asymmetric (S3 and S4) and non-extractable (H5) forms were 2 and 5 per cent, respectively. The total globular, asymmetric and non-extractable forms were not significantly different in gastrocnemius among the burn groups. However, individual forms (6.5S, 10S) of the soluble globular group (S1) were decreased (P less than 0.05) by half for all burn groups. Some molecular forms (4S, 10S) of residual globular group (S2) were decreased (P less than 0.05) in the 20 per cent and 50 per cent burn groups. In the asymmetric group (S3) the molecular forms (4S, 10S, 12S and 16S) were increased (P less than 0.001) for the 20 and 30 per cent groups. Within and between extraction groups interrelationships of the molecular forms were apparent. These data provide evidence that in response to burn trauma AChE activity of the gastrocnemius showed a decrease in soluble globular and an increase in asymmetric forms with multimeric dependence.

The effect of beta 2-adrenoceptor stimulation and blockade of L-type calcium channels on in vivo Na+/H+ antiporter activity in rat skeletal muscle

We have studied the in vivo response of the Na+/H+ antiporter in skeletal muscle to beta 2-adrenoceptor stimulation with isoprenaline and the effect of blocking L-type calcium channels with nifedipine. Na+/H+ antiporter activity in skeletal muscle in vivo increased after beta 2-adrenoceptor stimulation with isoprenaline; nifedipine attenuated that effect. This suggests that opening of L-type calcium channels is necessary for full activation of the Na+/H+ antiporter in skeletal muscle. Bleeding also increased Na/H+ antiporter activity, which we believe could be explained by an increase in sympathetic nervous system activity as a result of hypotension. This may be one of the mechanisms by which animals under stress prepare their skeletal muscle for exercise as part of the 'fright and flight' reaction.

Glucocorticoids increase Ca2+ uptake and [3H]dihydropyridine binding in A7r5 vascular smooth muscle cells

Increased levels of corticosteroids result in the development of hypertension in vivo. To investigate whether corticosteroids modulate calcium handling in vascular smooth muscle cells, we studied 45Ca2+ uptake and binding of [methyl-3H]PN 200-110, a potent dihydropyridine Ca2+ antagonist, in A7r5 vascular smooth muscle cells. Forty-eight-hour treatment with 100 nM dexamethasone increased the unidirectional 45Ca2+ uptake during a 2-min period, and the 30-min 45Ca2+ uptake of dexamethasone-treated cells was 95% greater than that of nontreated cells. The lag time for the dexamethasone effect on Ca2+ uptake was approximately 8 h. The effect of dexamethasone was blocked by the glucocorticoid antagonist RU 38486, whereas it was not affected by the mineralocorticoid antagonist RU 26752. After cessation of the dexamethasone treatment, 45Ca2+ uptake returned to the control level by 24 h. The effect of dexamethasone was completely blocked by nifedipine in a dose-dependent manner. Scatchard plots of [methyl-3H]PN 200-110 binding revealed two binding sites (Kd; 0.02 and 1 nM), and dexamethasone increased the number of the higher affinity binding sites. These results indicate that glucocorticoids increase Ca2+ uptake possibly mediated by an increase in the number of dihydropyridine-sensitive Ca2+ channels.

Ca2+ channel agonists enhance thyrotropin-releasing hormone-induced inositol phosphates and prolactin secretion

The dihydropyridine Ca2+ channel activator BAY K 8644 (1 microM) stimulated basal prolactin secretion from perifused primary cultures of anterior pituitary cells and potentiated the stimulation of prolactin secretion by 1 microM thyrotropin-releasing hormone (TRH) 5-fold over 30 min. This potentiation was mimicked by other dihydropyridine agonists CGP 28392 and (+)-SDZ 202-791 and by (-)-BAY K 8644 (1 microM), but not by (+)-BAY K 8644. The Ca2+ channel antagonist nimodipine, at a concentration sufficient to block BAY K 8644-stimulated 45Ca2+ uptake in GH4C1 anterior pituitary tumor cells, decreased basal prolactin secretion and blocked the enhancement of basal and TRH-stimulated secretion by BAY K 8644. These results suggest that dihydropyridine agonists potentiate TRH-induced secretion through interaction with known stereospecific sites on Ca2+ channels. In GH4C1 cells, BAY K 8644 alone did not affect inositol polyphosphate accumulation, but potentiated TRH-stimulated accumulation of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Accumulation of the Ca(2+)-mobilizing isomer inositol 1,4,5-trisphosphate was not potentiated, suggesting that potentiation of TRH-stimulated hormone secretion by BAY K 8644 does not result from synergistic stimulation of phospholipase C, but may correlate with enhanced inositol trisphosphate-3-kinase activity.

The mechanical hypothesis of excitation-contraction (EC) coupling in skeletal muscle

The mechanism of transmission in skeletal muscle EC coupling is still an open question. There is some indirect evidence in favour of the mechanical coupling hypothesis, deriving mostly from consideration of the structure of the Ca2+ release channel protein. A new functional approach is proposed, that consists in comparing the properties of the complete system--EC coupling in a skeletal muscle fibre--with those of the EC coupling molecules in bilayers. In this approach, those properties of the whole system that are not traceable to its constitutive molecules, are ascribed to the physiological interaction, and are expected to yield new information on the nature of this interaction.

Phosphatidylserine vesicles increase Ca2+ uptake by rat brain synaptosomes

Phosphatidylserine (PS) vesicles incorporated into rat brain synaptosomes increased total Ca2+ uptake. Total Ca2+ uptake was resolved in three components: K+ depolarization-induced Ca2+ uptake, Na+/Ca2+ exchange, and passive Ca2+ entry, which were differently affected by PS depending on the amount of incorporated phospholipid. K+ depolarization-induced Ca2+ uptake was stimulated by 0.05-0.10 mumol PS/mg protein while 0.10-0.30 mumol PS/mg protein increased Na+/Ca2+ exchange activity and passive Ca2+ entry but not K+ depolarization-induced Ca2+ uptake. High amounts of incorporated PS also increased passive Rb+ uptake.

Beta-adrenergic stimulation induces intracellular Ca++ increase in human epidermal keratinocytes

Intracellular Ca++ ([Ca++]i) is one of the most important second messengers of extracellular signals that induce cellular responses. In epidermal keratinocytes, both extracellular and intracellular Ca++ are reported to be important to cell differentiation and proliferation. Several mechanisms that increase [Ca++]i have been elicited in various tissues; however, in epidermal keratinocytes they remain unknown. Thus, we investigated the [Ca++]i modulation in cultured human epidermal keratinocytes and the stimulation that increases the concentration. The [Ca++]i concentration of keratinocytes was increased immediately and transiently by epinephrine. Methoxamine hydrochloride and clonidine (alpha-1- and 2-adrenergic agonists) did not induce an increase in [Ca++]i. The beta-antagonist, propranolol, inhibited the [Ca++]i increase induced by epinephrine and salbutamol (a beta-2-agonist). These results reveal that the beta-adrenergic stimulation induces an immediate and transient [Ca++]i increase in human keratinocytes. Beta-adrenergic stimulation is known to induce adenylate cyclase activation, which results in cyclic AMP accumulation through stimulatory guanosine 5-triphosphate (GTP) binding proteins in the keratinocytes. Also, epinephrine is reported to inhibit cultured epidermal cell proliferation. The effect of epinephrine has been demonstrated by cyclic AMP accumulation; however, beta-adrenergic stimulation revealed a [Ca++]i increase in keratinocytes in our study. One of epinephrine's regulatory effects on epidermal cell proliferation is assumed to occur through the [Ca++]i increase as well.

Calcium channels from Cyprinus carpio skeletal muscle

The complete amino acid sequence of the L-type calcium channel alpha 1 subunit from the carp (Cyprinus carpio) white skeletal muscle was deduced by cDNA cloning and sequence analysis. The open reading frame encodes 1852 amino acids (Mr 210,060). A 155-amino acid COOH-terminal sequence (after the fourth internal repeat) is evolutionarily preserved (90% homology) and may represent an important functional domain of L-type calcium channels. The photolabeled, membrane-bound, and purified carp alpha 1 subunits have masses of 211 and 190 kDa. The purified channel could not be phosphorylated by cAMP-dependent protein kinase. Two glycoproteins (alpha 2 subunits) are associated with the alpha 1 subunit and change their apparent masses from 235 and 220 kDa to 159 kDa upon reduction of disulfide bonds. Nucleic acid hybridization with alpha 2 cDNA revealed an 8.0-kilobase transcript in carp skeletal muscle. Evidence for a copurification of subunits similar in size to mammalian beta or gamma subunits was not obtained.

Calcium channel ligand binding to intact, concanavalin A and cyclic AMP-treated cells of the immune system

To examine whether the cells of immune system express calcium channel-forming proteins, we studied the binding of calcium channel ligands, known to detect certain types of the above channels in excitable tissues, to murine splenic and human peripheral blood mononuclear cells. Specific (i.e., displaceable by excess cold ligand) binding of the 3H-labelled dihydropyridine drugs PN200-110 and nitrendipine, was not detected in these cells. Specific binding of a phenylalkylamine drug, [3H]verapamil, was detected, but cannot be attributed to the existence of certain specialized receptors, since multiple [3H]verapamil binding sites (about 10(6) per cell) appeared to be occupied. [3H]Verapamil binding to murine splenic mononuclear cells was inhibited following exposure to either the polyclonal T-cell activator, concanavalin A, or a cell-permeable analogue of the second messenger, cyclic AMP, suggesting that processes of lymphocyte activation and/or intracellular signalling may down-modulate at least some of calcium channel ligand binding sites.