Release of sialic acid alters the stability of the membrane potential in cardiac muscle

FTIR evidence for alcohol binding and dehydration in phospholipid and ganglioside micelles

We theorize that intoxicants and modern anesthetics bind at the membrane-water interface and displace (dehydrate) bound water molecules by breaking the hydrogen bonds. We tested this hypothesis by examining the effect of butanol on the binding of water to the polar regions of lipids in reversed micelles. Understanding the mechanisms of intoxication requires studies in physiologically relevant systems such as systems containing sialoglycoconjugates, especially gangliosides, which concentrate in the synapses of neural tissue. Therefore, we compared butanol effects on phospholipid with effects on ganglioside. Hydrogen-bond breaking activity of 1-butanol was studied in reversed micelles made of dipalmitoylphosphotidylcholine (DPPC), ganglioside (GM1 and GT1b) or the lipid mixture in a D2O-CCl4 medium. Fourier transform infrared spectroscopy (FTIR) data indicated that 1-butanol binds to DPPC and to gangliosides. Adding GM1 to the DPPC micelles introduces a new binding site for the alcohol. GT1b binds more butanol than GM1, because of more binding sites provided by extra sialic acid moieties. Spectral red shifts indicate that both water and butanol bind to the C = O group of sialic acid. Butanol partially releases the surface-bound water by disrupting hydrogen bonds, as indicated by an appearance of a sharp new free OD stretching band of the released D2O molecules. However, control studies with lipid-free systems in CCl4 revealed that a free OD peak could occur from a deuterium exchange reaction between D2O and 1-butanol(ol-h).(ABSTRACT TRUNCATED AT 250 WORDS)

Sialic acid removal modulates the myocardial and vascular activity of calcium channel ligands

Selective removal of sialic acid from isolated guinea pig left atrial strips and rabbit thoracic aortic ring segments was performed by neuraminidase prepared from Clostridium perfringens and was controlled electron microscopically. Preincubation of these organs (2 units/mL; 2 hr) resulted in enzyme mediated hydrolysis of total tissue sialic acid; 55.2% for atria and 60.9% for aorta. Contractile force of atria and arterial diameter of thoracic aorta were measured isometrically and isotonically by means of a force displacement transducer. Pretreatment of both organs with neuraminidase (2 units/mL; 2 hr) in a carbogen saturated organ bath caused a moderate left-hand shift of the cumulative concentration response curves for the dihydropyridine type calcium antagonist nisoldipine, the phenylalkylamine derivative gallopamil and the benzothiazepine diltiazem. EC50 values were significantly lower (P less than 0.05), particularly in the atrial muscle, when compared to untreated preparations. There was no effect of neuraminidase on the negative inotropic and vasodilator potency of the calcium channel modulator fendiline. Conversely, neuraminidase induced a right-hand shift in the concentration response curves shown by the pure calcium agonist (-)-S-Bay K 8644 leading to significantly higher EC50 values in both organs. Similarly, the contractile potency of calcium chloride (atria) and potassium chloride (aorta) was attenuated upon neuraminidase treatment. From the results obtained it is concluded that sialic acid removal may modulate the action of calcium channel ligands through an inhibitory effect on transmembrane calcium fluxes and/or by decreasing the external calcium availability. Whether the present results suggest a functional role for sialic acid in the regulation of calcium channels warrants further investigation.

Effects of neuropeptides, ruthenium red and neuraminidase on chemoreflexes mediated by afferents in the dog epicardium

1. Experiments were performed on anaesthetized, open-chest dogs to determine reflex effects on blood pressure and heart rate produced by stimulation of neural afferents of the left ventricular epicardium by local application of capsaicin, bradykinin, nicotine and the neuropeptides substance P (SP), neurokinin A (NKA), neurokinin B (NKB) and calcitonin gene-related peptide (CGRP). 2. Studies also included assessing whether reflexogenic actions of capsaicin, bradykinin and nicotine are influenced by epicardial treatment with either neuropeptides, Ruthenium Red or neuraminidase. 3. Epicardial application of either capsaicin (0.1-10 micrograms) or bradykinin (0.1-1 micrograms), consistently resulted in dose-related increases in blood pressure and heart rate, whereas reflex bradycardia and hypotensive effects were initiated by the application of nicotine (30-50 micrograms). 4. SP, NKA, NKB and CGRP caused marked hypotensive effects and tachycardia when injected intravenously (1 microgram), but failed to produce any cardiovascular response when applied to the epicardium of the left ventricle (0.1-1 microgram). Treatment of the heart surface with these neuropeptides (0.05-0.5 micrograms min-1) was also without any effect on the magnitude of reflex responses evoked by epicardial application of either capsaicin, bradykinin or nicotine. 5. Superfusion of the ventricular epicardium with Ruthenium Red (10-30 microM), a cationic dye known to have sialic acid as a molecular target, antagonized the reflexogenic effects of capsaicin but not those of bradykinin or nicotine. The reflex effects of capsaicin, but not those of bradykinin, were also sensitive to inhibition by epicardial treatment with neuraminidase, an enzyme which cleaves sialic acid residues from glycosides and sialoglycoproteins. 6. We conclude that neuropeptides which may be released from the peripheral endings of some cardiac sensory neurons neither directly activate nor sensitize spinal sympathetic and vagal afferents in the dog heart to the reflexogenic action of bradykinin, nicotine or capsaicin. 7. We further suggest that activation of the cardiac sympathetic chemoreflex by capsaicin involves its interaction with calcium-binding sialic acid moieties present on the surface of axons and/or terminals of chemosensitive sympathetic afferents distributed in the dog ventricular epicardium.

Lectin binding of the interphotoreceptor matrix during retinal development in normal and RCS rats

The retinas of both normal and Royal College of Surgeons (RCS) rats with inherited retinal dystrophy have been examined using lectin histochemistry to determine the developmental and degenerative changes of the glycoconjugates in the interphotoreceptor matrix (IPM) between postnatal day (P) 10 and P25, when the adult lectin binding patterns are seen in normal rats. Wheat germ agglutinin (WGA; recognizing sialic acid and/or N-acetyl-D-glucosamine) bound to the apical surface of the retinal pigment epithelium (RPE) sparsely at P10 and prominently at P12 in both strains. In both strains at P14, WGA also stained the basal outer segment zone at the inner segment-outer segment junction. Between P14 and P16 in both strains, there was a dramatic increase in the binding of the interstitial region, the space alongside the outer segments and between the apical and basal outer segment zones. The binding pattern of WGA in normal rats remained basically unchanged from P16 to P25, although the intensity of binding was increased somewhat. Ricinus communis agglutinin-1 (RCA-1; specific for galactosyl residues) bound to the outer segment zone prominently and diffusely with increasing intensity with age at P10, P12 and P14 in both strains. At P16 and older, the intense binding of the interstitial zone was dramatically reduced and the RCA-1 bound primarily to the inner and outer segment junctional region, with weak binding to the apical surface of the RPE in both strains. At P25, the binding of the inner and outer segment junctional region was even more restricted, limited to punctate sites in this zone in normal rats and almost missing in RCS rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Sialic acid and the surface charge associated with hyperpolarization-activated, inward rectifying channels

The whole-cell configuration of the patch-clamp technique was used with cultured pacemaker cells from the rabbit sinoatrial node to test the hypothesis that sialic acid residues (NANA) constitute much of the negative surface charge associated with hyperpolarization-activated, inward rectifying channels. Activation-voltage relationships (between -70 and -140 mV) were determined for hyperpolarization-activated (inward rectifying) current (i(f)). Addition of 10 mM Ca2+ shifted the half-activation potential (V 1/2) from -89.5 +/- 0.9 mV to -77.9 +/- 2.6 mV (P less than 0.01), confirming the presence of negative fixed charges on the myocytes after 3 to 5 days in culture. Addition of 20 mM dimethonium, an organic divalent cation that "screens" but does not bind to negative surface charge, shifted V 1/2 from -86.8 +/- 1.4 mV to -75.0 +/- 1.7 mV (P less than 0.001) without affecting the amplitude of the current. In contrast, 10 mM Ca2+ reduced the amplitude of i(f) significantly. Incubation of cells with a highly purified preparation of neuraminidase (0.1-2.0 U/ml, 1 hr, 37 degrees C), an enzyme that selectively removes NANA from glycoproteins and glycolipids, failed to alter V 1/2 or the amplitude of i(f) significantly. Pretreatment of cells with neuraminidase (1.0 U/ml, 1 hr, 37 degrees C) failed to alter the positive shift of V 1/2 produced by dimethonium. The results suggest that NANA does not constitute the negative surface charge associated with hyperpolarization-activated, inward rectifying channels.

Neuraminidase selectively enhances transient Ca2+ current in cardiac myocytes

Sialic acid, an anionic sugar moiety found peripherally on membrane glycoconjugates, is specifically hydrolyzed from the cell surface by neuraminidase. Because neuraminidase has previously been demonstrated to augment myocardial cell calcium content, the effects of neuraminidase on Ca channel function were studied on voltage-clamped guinea pig ventricular myocytes. In 25-50% of cells, neuraminidase treatment (0.12 U/ml for 20 min) enhanced current through the transient (T) Ca channel by 304 +/- 35% without significantly altering the magnitude of the long-lasting (L) Ca channel current. Exposure to neuraminidase did not affect the voltage dependence of activation or inactivation, nor did it affect the selective inhibition of the T-channel current by amiloride or the L-channel current by nifedipine. After neuraminidase treatment, the T-channel current inactivated more rapidly (time constant decreasing from 8.9 +/- 0.9 to 7.7 +/- 0.6 ms), whereas there was no change in the rate of inactivation of the L-channel current. Neuraminidase treatment removed approximately 20% of the total cellular sialic acid. These results indicate that neuraminidase treatment selectively modulates the function of the T Ca channel in ventricular myocytes, possibly through removal of sarcolemmal sialic acid, suggesting that glycosylation of membrane macromolecules may influence membrane function.

Sialic acid is selectively involved in the interaction of agonists with M2 muscarinic acetylcholine receptors

Neuraminidase and slight acid hydrolysis were used to investigate the role of sialic acid residues in the binding of muscarinic agonists and antagonists to membranes from tissues rich in M1 and M2 receptors. Membranes were pretreated with neuraminidase at pH 5 and the binding parameters were determined from competitive experiments with (3H)-quinuclidinylbenzylate. The removal of sialic acid residues reduced the affinity of muscarinic agonists for cerebellum, heart and lung membranes (M2), in contrast to striatum (M1). The affinity of antagonists was not affected. Thus, sialic acid is selectively involved in the interaction of agonists with M2 muscarinic receptors.

On the action of ruthenium red and neuraminidase at the frog neuromuscular junction

1. The actions of the polyvalent cationic dye Ruthenium Red and the enzyme neuraminidase were studied at the frog neuromuscular junction. 2. Both Ruthenium Red (0.5-40 microM) and neuraminidase (1, 2 and 4 u.) reduced the miniature end-plate potential (m.e.p.p.) amplitude irreversibly with little change in frequency. This effect could be attributed to the depressant action of these agents on the amplitude of extracellularly recorded miniature end-plate currents (m.e.p.c.s) coupled with their shortening effect on the duration of these m.e.p.c.s. For example, 10 microM-Ruthenium Red reduced the time constant of decay (tau D) of extracellular m.e.p.c.s to 35% of the control value. 3. Ruthenium Red (5, 10 and 15 microM) produced a pronounced reduction in the post-synaptic end-plate sensitivity to microiontophoretically applied acetylcholine. In some cases this effect was fully reversible on wash-out of the dye. 4. Amplitude histograms of m.e.p.c.s recorded under voltage clamp showed a similar shift of the modal value to lower levels in the presence of Ruthenium Red. Over the voltage range -70 to -110 mV the action of Ruthenium Red on the tau D was not voltage dependent. 5. The actions of Ruthenium Red on tau D were not affected by anaesthetic agents, such as the short-chain alcohols which increased tau D, or ketamine which decreased tau D. 6. Some possible modes of action of Ruthenium Red and neuraminidase are briefly discussed.

The role of sialic acid in the nerve terminal for the release of transmitter

The role of sialic acid in the frequency of miniature endplate potentials (MEPPs) was examined using neuraminidase and gangliosides in the mouse diaphragm. Neuraminidase increased and decreased MEPP frequency in normal K+ and high K+ solution, respectively. The effects were dependent on the presence of Ca2+ in extracellular medium. Neuraminidase liberated sialic acid from and lowered Ca2+-binding capacity of synaptosomal membrane. Gangliosides treatment of the tissue partially restored the effects of neuraminidase on the frequency of MEPP and Ca2+-binding capacity. It is possible that sialic acid in the nerve endings provides a functional storage site which supply intracellular Ca2+ to cause a transmitter release.

Changes in (Na+ + K+) ATPase activity and the composition of surface carbohydrates in erythrocyte membranes in alcoholics

(Na+ + K+)ATPase activity and sensitivity to the inhibitory effect of ethanol and noradrenaline in vitro as well as the concentrations of sialic acid, galactose, and hexosamine were determined in erythrocyte membranes in 20 healthy controls and in 20 alcoholic patients within 24 hr of withdrawal. Basal (Na+ + K+)ATPase activity, the sensitivity of the enzyme to inhibition by ethanol and noradrenaline added in vitro, and the concentrations of sialic acid and galactose were significantly reduced in the patients (p less than 0.0005). All of these abnormalities were significantly correlated to each other as well as to the estimated daily quantity of ethanol consumed. After enzymatic removal of terminal sialic acid and terminal and sialic acid-bound beta-galactose, the (Na+ + K+)ATPase differences between the patients and the controls were eliminated. The results showed that the previously reported reduced inhibition of (Na+ + K+)ATPase by ethanol in the presence of noradrenaline in brain membranes in chronically ethanol-fed animals is also found in erythrocyte membranes in alcoholic humans. Abnormal carbohydrate composition of glycolipids and/or glycoproteins of the membrane surface appeared to be related to the (Na+ + K+)ATPase changes, possibly due to interference either with K+ transport or surface electrostatics or, directly or indirectly, with the conformation of (Na+ + K+)ATPase.

Carbohydrate composition of erythrocyte membranes and glycosidase activities in serum in patients with myotonic dystrophy, limb-girdle dystrophy and congenital myotonia

A number of abnormalities in cell membrane function, including cells other than muscle cells, have been described in patients with inherited muscular diseases such as myotonic dystrophy and congenital myotonia. The basic molecular defects are, however, still unknown. The complex carbohydrates of membrane-bound glycoconjugates are of vital importance for the normal performance of the cell membrane. In this study the concentrations of the three major carbohydrates (sialic acid, galactose and hexosamines) of the erythrocyte membrane were therefore determined in patients with myotonic dystrophy, limb-girdle dystrophy and congenital myotonia. The activities of relevant glycosidases in serum were also assayed. In each of the three diseases pertinent changes of the carbohydrate pattern were found. In patients with myotonic dystrophy the sialic acid and in patients with limb-girdle dystrophy the hexosamine concentration was significantly reduced (P less than 0.0005). The sialic acid, galactose and hexosamine concentrations were all significantly increased in patients with congenital myotonia. No increase of the neuraminidase (sialidase) activity was found in sera from patients with myotonic dystrophy. In patients with limb-girdle dystrophy, the activities of serum hexosaminidases were normal. These results support the contention that certain inherited muscular diseases may represent generalized membrane disorders, and suggests that disturbances of membrane-bound glycoproteins and/or glycolipids might be of importance in the pathogenesis of some of these disorders.

Glycoprotein properties of the solubilized atrial muscarinic acetylcholine receptor

The muscarinic acetylcholine receptor from porcine atria exhibits sialoglycoprotein characteristics based on its sensitivity to neuraminidase digestion and its ability to interact specifically with lectin affinity resins when solubilized with a digitonin/cholate mixed detergent system. Differential lectin binding properties of the neuraminidase-treated and untreated receptor suggest that high-affinity binding to immobilized wheat germ agglutinin is accomplished through the presence of both terminal sialic acid and internal N-acetylglucosamine or its beta(1 leads to 4)-linked oligomers.

The sialic acid and galactose concentrations in erythrocyte membranes in patients with myotonic dystrophy, limb-girdle and facioscapulohumeral dystrophy

Sialic acid is an important constituent of membrane-bound glycoproteins and glycolipids. It occurs linked to galactose at the surface of the membrane and is involved in, e.g., cation exchange, receptor function, maintenance of membrane polarity and intercellular interactions. In myotonic dystrophy there is evidence of an as yet basically undefined plasma-membrane abnormality. Considering the importance of sialic acid in membrane function, sialic acid as well as galactose concentrations were measured in erythrocyte membranes from 17 patients with myotonic dystrophy and compared to 17 matched healthy controls. There was a highly significant (P less than 0.0005) reduction of the sialic acid concentration in the patients, while no significant difference in galactose concentration was found. In 16 patients with limb-girdle and facioscapulohumeral dystrophy, sialic acid and galactose concentrations did not differ from matched controls. The possible importance of a reduced concentration of membrane-bound sialic acid in myotonic dystrophy is discussed in relation to previously reported biochemical membrane abnormalities in this disease.