Phosphorylation of purified bovine cardiac sarcolemma and potassium-stimulated calcium uptake

Signaling complexes of voltage-gated sodium and calcium channels

Membrane depolarization and intracellular Ca(2+) transients generated by activation of voltage-gated Na+ and Ca(2+) channels are local signals, which initiate physiological processes such as action potential conduction, synaptic transmission, and excitation-contraction coupling. Targeting of effector proteins and regulatory proteins to ion channels is an important mechanism to ensure speed, specificity, and precise regulation of signaling events in response to local stimuli. This article reviews experimental results showing that Na+ and Ca(2+) channels form local signaling complexes, in which effector proteins, anchoring proteins, and regulatory proteins interact directly with ion channels. The intracellular domains of these channels serve as signaling platforms, mediating their participation in intracellular signaling processes. These protein-protein interactions are important for regulation of cellular plasticity through modulation of Na+ channel function in brain neurons, for short-term synaptic plasticity through modulation of presynaptic Ca(V)2 channels, and for the fight-or-flight response through regulation of postsynaptic Ca(V)1 channels in skeletal and cardiac muscle. These localized signaling complexes are essential for normal function and regulation of electrical excitability, synaptic transmission, and excitation-contraction coupling.

Phospholamban: protein structure, mechanism of action, and role in cardiac function

A comprehensive discussion is presented of advances in understanding the structure and function of phospholamban (PLB), the principal regulator of the Ca2+-ATPase of cardiac sarcoplasmic reticulum. Extensive historical studies are reviewed to provide perspective on recent developments. Phospholamban gene structure, expression, and regulation are presented in addition to in vitro and in vivo studies of PLB protein structure and activity. Applications of breakthrough experimental technologies in identifying PLB structure-function relationships and in defining its interaction with the Ca2+-ATPase are also highlighted. The current leading viewpoint of PLB's mechanism of action emerges from a critical examination of alternative hypotheses and the most recent experimental evidence. The potential physiological relevance of PLB function in human heart failure is also covered. The interest in PLB across diverse biochemical disciplines portends its continued intense scrutiny and its potential exploitation as a therapeutic target.

Apparent heterogeneity of cardiac A1 adenosine receptors as revealed by radioligand binding experiments on N-ethylmaleimide-treated membranes

While G protein-coupled receptors are often studied by analyzing antagonist radioligand: "cold" agonist inhibition curves using an independent site model, it is now clear that KL and KH values determined in these analyses are not reliable estimates of the affinities of the agonists for "free" and G protein-coupled forms of the receptor. Thus, such experiments cannot be used to contrast the characteristics of a given type of receptor in different tissues, i.e., to probe for the existence of receptor subtypes. Since treatment with N-ethylmaleimide treatment blocks receptor: Gi/Go protein interactions, such analyses on N-ethylmaleimide-pretreated membranes should allow direct assessment of the affinities of competing ligands for the free receptor or for multiple receptor subtypes. As A1 adenosine receptors couple to Gi, and perhaps to Go, we have performed A1 adenosine receptor radioligand "competition" studies first on control, then on N-ethylmaleimide-pretreated bovine cardiac and cerebral cortical membranes. Results of experiments with the antagonist radioligand [3H]xanthine amine congener appeared to be confounded by ligand binding to A2 adenosine receptors present in the cardiac membrane preparations. Further experiments utilized the A1-specific radioligand [3H]1,3-dipropyl-8-cyclopentylxanthine. These experiments confirmed once more that the KL values determined by computer analysis of "competition" curves performed on control membranes are not reliable estimates of the affinities of the competing ligand for free receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of the 23,000-Da phosphoproteins in cardiac sarcolemma and sarcoplasmic reticulum

The calmodulin- and cAMP-dependent protein kinase-mediated phosphorylations of isolated sarcolemma and sarcoplasmic reticulum vesicles have been compared. Similarities in the calmodulin-mediated phosphorylation of the sarcolemma and sarcoplasmic reticulum 23,000-Da phosphoproteins included their Mg2+, Na+, Ca2+, and calmodulin sensitivities, as well as the size of their dissociated subunits. In contrast, a number of differences between these phosphoproteins were indicated in their sensitivity to detergents (Triton X-100 and sodium dodecyl sulfate) and calmodulin antagonists (R24571 and trifluoperazine). Furthermore, in contrast to the sarcoplasmic reticulum phosphoprotein, the sarcolemma phosphoprotein could not be affinity labeled with 125I-calmodulin. While these results indicate the probable chemical similarity of the sarcolemma and sarcoplasmic reticulum 23,000-Da phosphoproteins, they also indicate there are differences in the lipid/phosphoprotein interactions in these two membranes.

On the mechanism of beta-adrenergic regulation of the Ca channel in the guinea-pig heart

Dose-response relations for the increase in the amplitude of Ca current (ICa) on external application of isoprenaline (ISP) and internally applied cyclic AMP (cAMP) or catalytic subunit of cAMP-dependent protein kinase (C subunit) were established in single ventricular cells of the guinea pig. An intracellular dialysis technique was used. The threshold concentration was for ISP 10(-9) M, for cAMP 3 microM (pipette concentration to which 10(-5) M 3-isobutyl-1-methylxanthine was added) and for C subunit around 0.4 microM (pipette concentration). The concentrations for the half-maximal effect were 3.7 X 10(-8) M (ISP), 5.0 microM (cAMP) and 0.95 microM (C subunit) and for the maximum effect 10(-6) M (ISP), 15-20 microM (cAMP) and 3-4 microM (C subunit). For all three agents the maximum increase in the Ca current density was similar (a factor of 3-4), suggesting that they converge on the same site of the Ca channel. Accordingly, the effects of cAMP and C subunit on ICa were non-additive to those of ISP. From these data the relationship both between concentrations of ISP and cAMP and between those of cAMP and active C subunit in terms of their effects on ICa could be estimated and were compared with those obtained in broken cell preparations. A competitive inhibitor of phosphorylation, 5'-adenylyl-imidodiphosphate (5 mM), greatly reduced the effects of ISP and C subunit on ICa. Cell dialysis with 3 mM adenosine-5'-(gamma-thio)-triphosphate, which produces a dephosphorylation-resistant phosphorylation, markedly potentiated the effects of ISP and cAMP on ICa.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the binding sites for nimodipine and (-)-desmethoxyverapamil in bovine cardiac sarcolemma

The bovine cardiac sarcolemmal binding sites for the dihydropyridine nimodipine and the phenylalkylamine (-)-desmethoxyverapamil were studied. The density of the nimodipine and (-)-desmethoxyverapamil binding sites increased 8.3-fold and 3.4-fold with the sarcolemma. The binding sites for both compounds were destroyed by trypsin. Nimodipine bound in the presence of 1 mM free calcium to a high-affinity and a low-affinity site with apparent Kd values of 0.35 +/- 0.09 nM (n = 9) and 33 +/- 6.0 nM (n = 9) and with apparent densities of 0.3 +/- 0.05 pmol/mg (n = 9) and 8.2 +/- 1.0 pmol/mg (n = 9). The binding to the high-affinity site was abolished by 1 mM EGTA. The binding sites were specific for dihydropyridines. The (-)-isomers of several phenylalkylamines inhibited nimodipine binding by an apparent allosteric mechanism. (-)-Desmethoxyverapamil bound in the presence of 5 mM EGTA to a high-affinity and a low-affinity site with apparent Kd values of 1.4 +/- 0.3 nM (n = 6) and 171 +/- 26 nM (n = 6) and with apparent densities of 0.16 +/- 0.02 pmol/mg (n = 6) and 13.6 +/- 2.7 pmol/mg (n = 6). The binding to both sites was inhibited by calcium with a half-maximal concentration of 4.3 mM. The binding sites were specific for the other phenylalkylamines and had a higher affinity for the (-)-isomers than for the (+)-isomers. Nimodipine inhibited the binding of (-)-desmethoxyverapamil by an apparent allosteric mechanism. d-cis-Diltiazem inhibited non-competitively the binding of (-)-[3H]desmethoxyverapamil with a Ki of 3.7 microM. Diltiazem up to concentrations of 10 microM did not affect the amount of nimodipine bound at equilibrium at 20 degrees C. However, but in agreement with this result, diltiazem decreased threefold at 20 degrees C the dissociation and association rates for the high-affinity nimodipine receptor. These rates were only marginally affected at 4 degrees C and 37 degrees C. d-cis-Diltiazem reversed in a competitive manner the inhibition of nimodipine binding elicited by the addition of (-)-desmethoxyverapamil with a Ka value of 1.6 microM. The amount of nimodipine bound was inhibited by 50% by the adenosine uptake inhibitors nitrobenzylthioinosine and hexobendine with apparent median inhibitory concentrations of 1 nM and 3 nM, respectively. Nitrobenzylthioinosine completely abolished binding of nimodipine to the low-affinity site, but did not affect binding to the high-affinity site.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of depolarizing concentrations of potassium on calcium uptake and metabolism in rat liver

Exposure of perfused livers of fed rats to 60 mM K+ induces rapid responses in the Ca2+-sensitive metabolic events, glycogenolysis, cytoplasmic and mitochondrial NADH/NAD ratios and octanoate oxidation. All increase within 45 s of K+ addition. Metabolic responses were not observed following K+ addition to livers perfused in the absence of added Ca2+. Movements of Ca2+ into the liver were suggested from experiments in which 45Ca2+ uptake was measured. The Ca2+ antagonists verapamil, diltiazem and Ni2+ essentially abolished changes to tissue metabolism and Ca2+ fluxes induced by K+ addition. K+-induced changes were consistent with Ca2+ channel activation.

Involvement of superoxide radicals on adrenochrome formation stimulated by arachidonic acid in bovine heart sarcolemmal vesicles

Highly purified sarcolemmal membranes prepared from bovine heart muscle produced superoxide radicals, especially when incubated with NADPH or NADH, as revealed by the oxidation of adrenaline to adrenochrome. The reaction was inhibited by superoxide dismutase or by heat denaturation of the sarcolemmal vesicles. Less evident was the inhibitory effect shown by catalase, while mannitol, deferoxamine or dicumarol were uneffective. The formation of adrenochrome was an oxygen-dependent reaction with a Km for adrenaline of 8-10 microM. Moreover, the reaction was inhibited by preincubating the sarcolemmal membranes with propranolol, while the alpha-antagonist phentolamine was without effect. Adrenaline oxidation was unaffected by the presence of exogenous linolenic acid or methylarachidonic acid, while arachidonic acid, with a Km for this reaction of 175 microM, showed a marked stimulatory effect. This activation was suppressed by superoxide dismutase, catalase and NaCN, while mannitol was without effect. Moreover, the reaction was blocked by the cyclooxygenase inhibitor indomethacin, differently from the lipooxygenase inhibitor nordihydroguaiaretic acid. Also, the incubation of the sarcolemmal vesicles with phospholipase A2 and calcium produced a stimulation of adrenochrome formation which was partially suppressed by albumin. In the experiments using arachidonic acid or phospholipase A2, the addition of indomethacin blocked the adrenaline oxidation. These results indicate that arachidonic acid accentuated the heart sarcolemmal adrenochrome formation presumably by participating in the cyclooxygenase reaction.

Phosphorylation of ventricular sarcolemmal membranes does not alter binding properties of nitrendipine

Isoproterenol increased contractility in isolated cat papillary muscles 2-fold with an EC50 of 6.3 X 10(-8) M. Nifedipine (3 X 10(-7) M) reduced contractility in control muscles by 43%; however, inotropic state was restored by isoproterenol with a comparable EC50 of 5 X 10(-8) M. To test the hypothesis that this effect might result from cAMP-dependent phosphorylation of a Ca2+ channel-associated protein, [3H]nitrendipine binding was used to probe the high-affinity 1,4-dihydropyridine site in either phosphorylated or dephosphorylated sarcolemmal vesicles. Kd and Bmax values for binding to phosphorylated sarcolemmal vesicles (0.14 +/- 0.027 nM and 479 +/- 62 fmol/mg protein, respectively) were not significantly different from control values P greater than 0.4). Similarly, dephosphorylation of sarcolemmal vesicles did not alter binding parameters. These data demonstrate that phosphorylation of sarcolemmal vesicles neither alters the binding affinity for [3H]nitrendipine nor promotes an interconversion of dihydropyridine-binding sites from high to low affinity or vice versa. While phosphorylation may regulate the slow Ca2+ channel, this is not reflected as changes in [3H]nitrendipine-binding parameters determined in vitro. Furthermore, the cyclic AMP-dependent phosphorylation state of sarcolemmal proteins does not appear to account for wide variations (more than 100-fold) between Kd values from binding studies and IC50 values determined in pharmacological investigations.

Demonstration of Ri-type adenosine receptors in bovine myocardium by radioligand binding

Adenosine has been shown to have negative inotropic, chronotropic and dromotropic effects on the heart. The pharmacological profiles of these effects suggest that they are mediated via Ri (A1) adenosine receptors, but a direct demonstration of these receptors is still missing. In the present study we report direct labelling of these receptors with (-)N6-[125I]-p-hydroxyphenylisopropyladenosine [( 125I]HPIA)1. The radioligand bound in a saturable and reversible manner to a crude membrane preparation, the Bmax-value was 30.5 fmol/mg protein and the KD-value 1.1 nmol/l. A similar affinity of the ligand was obtained in kinetic and competition experiments. Competition experiments with a variety of adenosine analogues gave a pharmacological profile characteristic of Ri adenosine receptors with high affinities of N6-substituted derivatives and a marked stereospecificity for N6-phenylisopropyladenosine (PIA). Purification of the membrane preparation by density gradient centrifugation resulted in a 30-fold increase in the number of binding sites which was paralleled by a similar increase in the number of binding sites for [3H]ouabain. Guanine nucleotides decreased binding of [125I]HPIA in a dose-dependent manner, but the IC50-values were considerably higher than those reported in other tissues. Finally, binding of [125I]HPIA appeared to be entropy-driven which has been shown to be characteristic of agonist binding to Ri adenosine receptors. These results suggest the presence of Ri adenosine receptors in ventricular myocardium which may be responsible for the mediation of the effects of adenosine and its analogues.

The calmodulin-dependent phosphorylation of cardiac myosin

Cardiac myosin light chains are phosphorylated in vivo and in vitro. The enzyme myosin light-chain kinase, has been purified and found to be very specific for cardiac myosin light chains. Experiments with skinned cardiac fibers suggest that phosphorylation of myosin light chain-2-decreases ATP consumption, presumably by lowering the cross-bridge cycle. These results are discussed in this chapter.

A protein activator of the plasma membrane Ca++-ATPase of heart sarcolemma

A detergent extract of dog or beef heart sarcolemmal vesicles was prepared and found to have a stimulatory effect on the Ca++-ATPase of plasma membranes from human erythrocyte and cardiac sarcolemma. A procedure is described which enriches the activating fraction. The protein nature of the preparation is illustrated by its sensitivity to boiling and to the proteolytic enzyme(s) trypsin and chymotrypsin. SDS polyacrylamide gels indicate that the protein(s) involved have a molecular weight of 56 and 60 kDa. The sarcolemmal activator can stimulate the Ca++-ATPase activity of the isolated enzyme more than 100% in the presence of saturating amounts of calmodulin. The activation is calcium dependent, being greatest at approximately 10 microns Ca++, free, but does not change the Km for Ca++. A possible physiological role for the activator is discussed.

Ca2+-dependent K+ permeability of heart sarcolemmal vesicles. Modulation by cAMP-dependent protein kinase activity and by calmodulin

The Ca2+-dependent K+ permeability of heart sarcolemma vesicles was measured by following the transmembrane movement of the charge compensating tetraphenylborate anion. The increase in vesicles permeability induced by Ca2+ is lost when membrane proteins are dephosphorylated by an endogenous protein phosphatase and is restored by a phosphorylation process catalysed by a cAMP-dependent protein kinase. The calmodulin antagonist R 24571 lowers the Ca2+-dependent K+ permeability by decreasing the Ca2+ affinity of the K+ transporting system.

The effect of lipid intermediates on Ca2+ and Na+ permeability and (Na+ + K+)-ATPase of cardiac sarcolemma. A possible role in myocardial ischemia

The effect of fatty acid and acylcarnitine on Ca2+ and Na+ transporting enzymes and carriers was studied in sealed cardiac sarcolemma vesicles of mixed polarity. Palmitoylcarnitine markedly reduced the Na+ gradient-induced Ca2+ uptake. Half-maximal reduction was obtained at 15 microM of the carnitine derivative. In a same concentration range palmitoylcarnitine caused a rapid release of accumulated Ca2+ when added to Ca2+-filled vesicles, which suggests that palmitoylcarnitine increases the permeability of the sarcolemma vesicles to Ca2+. A rapid release of Ca2+ was also observed if Ca2+ was taken up by action of the Ca2+ pump. The (Ca2+ + Mg2+)-ATPase, which most likely drives this active Ca2+ uptake, was 90% increased by 50 microM palmitoylcarnitine and evidence was presented that the acylcarnitine effect again was linked to an alteration of Ca2+ permeability of the vesicles. At the same concentration acylcarnitine was not able to unmask the latent protein kinase, so that probably the sarcolemma ATP permeability was not affected. Palmitoylcarnitine at 25 microM did not affect the ouabain-sensitive (Na+ + K+) -ATPase in native sarcolemma vesicles, however, it inhibited markedly if the enzyme was measured in SDS-treated vesicles. The effect of increased free fatty acid concentration on some of the sarcolemma transporting properties was tested by adding oleate-albumin complexes with different molar ratios to the sarcolemma vesicles. In contrast to molar ratios 1 and 5, the ratio of 7 was able to induce a rapid Ca2+ release and to inhibit (Na+ + K+)-ATPase in either native or SDS-treated vesicles markedly. 22Na release from 22Na-preloaded sarcolemma vesicles was shown to be stimulated by either palmitoylcarnitine (50 microM) or oleate-albumin complex (with a molar ratio of 7). The possible significance of the observed effects of lipid intermediates on ion permeability and (Na+ + K+)-ATPase activity in isolated sarcolemma vesicles for the derangement of cardiac cell function in ischemia is discussed.

Nitrendipine and isoproterenol induce phosphorylation of a 42,000 dalton protein that co-migrates with the affinity labeled calcium channel regulatory subunit

Slow inward calcium channels in canine cardiac membranes were affinity labeled with the calcium channel analogue, [3H]o-NCS [2,6 dimethyl-3,5-dicarbomethoxy-4-(2- isothiocyanatophenyl )-1, 4-dihydropyridine], in the presence and absence of cold o-NCS or nicardipine. A major specifically labeled peak was identified with Mr 42,000 on NaDodSO4 polyacrylamide gels. In parallel experiments the effects of the calcium channel antagonist, nitrendipine and a variety of other chemical mediators were tested for their ability to stimulate protein phosphorylation in cardiac membranes. These data demonstrate that both nitrendipine and isoproterenol induce the phosphorylation of a 42,000 dalton protein via a kinase endogenous to the cardiac membranes and that the effects of isoproterenol are attenuated by carbachol.

Formation of adrenochrome by bovine cardiac sarcolemma

A sarcolemma preparation from bovine heart was able to promote adrenaline oxidation especially when NADH and NADPH were added. The superoxide anion O(2) was demonstrated to be involved in the activation of adrenochrome production.

Biochemical properties of membranes isolated from calcium-depleted rabbit hearts

The purpose of this study was to define the biochemical properties of sarcolemma from the calcium-depleted rabbit heart. Calcium repletion after calcium-free perfusion results in irreversible damage to the heart (calcium paradox). No difference was found in specific activity of the Na+ -Ca++ antiporter in a crude preparation of sarcolemmal vesicles that was isolated from calcium-depleted hearts, compared with control perfused hearts. Likewise, the passive calcium efflux from sarcolemmal vesicles, preloaded with calcium via the Na+ -Ca++ antiporter, showed rates that were identical with control values. This indicates that the sarcolemma calcium permeability is not affected by calcium-free perfusion of the heart. Na+,K+ -ATPase activity in sarcolemma isolated from calcium-depleted hearts was reduced by 75% (P less than 0.005) compared with the control activity. Sarcolemmal phosphoproteins, whether produced by endogenous cyclic AMP- or calcium-calmodulin-dependent protein kinase, were not altered by calcium-free perfusion of the heart. The content of an important calcium-binding site in the myocardial cell, the sialic acid residues, was also estimated. Only a long period (60 minutes) of calcium-free perfusion resulted in a significant decrease (by 68%, P less than 0.025) of sialic acid content in the homogenate but not in the sarcolemma preparation. In hearts that were reperfused for 15 minutes with a normal calcium concentration (1.3 mM), sarcolemmal Na+,K+ -ATPase remained depressed and calcium permeability was still unchanged. It is possible that the sarcolemma isolation method selected a distinct part of the sarcolemma from the calcium-depleted and repleted heart that had no modified glycocalyx and permeability barriers to calcium ions, and that another part of the sarcolemma with altered properties was lost during the isolation procedure. Another possibility is that reconstitution processes during isolation affected membrane permeability properties. The results of the Na+,K+ -ATPase measurements provide evidence that the net calcium gain of the cells after calcium repletion may be associated, in part, with a loss in ability of the sarcolemma to remove calcium from the cytosol.