Biomedical subjects

A E Martynyuk

Publications and source records attributed to A E Martynyuk.

At least 19 recordsLinked to original sources

PubMed2005

Impaired glutamatergic synaptic transmission in the PKU brain.

This paper reviews recent results of our investigation of the mechanisms whereby hyperphenylalaninemia may cause brain dysfunction in classical phenylketonuria (PKU). Acute applications of L-Phe in rat and mouse hippocampal and cerebrocortical cultured neurons, at a range of concentrations found in PKU brain, significantly and reversibly depressed glutamatergic synaptic transmission by a combination of pre- and postsynaptic actions: (1) competition for the glycine-binding site of the N-methyl-D-aspartate (NMDA) receptors; (2) attenuation of neurotransmitter release; (3) competition for the glutamate-binding site of (RS)-amino-3-hydroxy-5-methyl-4-isoxazolepropioinic acid and kainate (AMPA/kainate) receptors. Unlike L-Phe, its non-tyrosine metabolites, phenylacetic acid, phenylpyruvic acid, and phenyllactic acid, did not produce antiglutamatergic effects. L-Phe did not affect inhibitory gamma-aminobutyric (GABA)-ergic transmission. Consistent with this specific pattern of effects caused by L-Phe in neuronal cultures, the expression of NMDA receptor NR2A and AMPA receptor Glu1 and Glu2/3 subunits in brain of hyperphenylalaninemic PKU mice (Pah(enu2) strain) was significantly increased, whereas expression of the NMDA receptor NR2B subunit was decreased. There was no change in GABA alpha1 subunit expression. Considering the important role of glutamatergic synaptic transmission in normal brain development and function, these L-Phe-induced changes in glutamatergic synaptic transmission in PKU brain may be a critical element of the neurological symptoms of PKU.

Animals↗

PubMed2005

Differential modulation of glutamatergic transmission by 3,5-dibromo-L-phenylalanine.

An increasing body of evidence supports the hypothesis that diminished function of N-methyl-D-aspartate (NMDA) receptors and the associated increase in glutamate release and overstimulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors are critical elements of the pathophysiology of schizophrenia. Here, we describe a halogenated derivative of the aromatic amino acid L-phenylalanine that 1) activates NMDA receptors, 2) depresses presynaptic glutamate release, and 3) blocks AMPA/kainate receptors. The experiments were conducted in rat cerebrocortical cultured neurons by using the patch-clamp technique. 3,5-Dibromo-L-phenylalanine (3,5-DBr-L-Phe) augmented NMDA miniature excitatory postsynaptic currents (mEPSCs) and activated the steady-state current, effects that were eliminated by NMDA receptor antagonists DL-2-amino-5-phosphonopentanoic acid and MK-801 (dizocilpine maleate; 5H-dibenzo[a,d]cyclohepten-5,10-imine). 3,5-DBr-L-Phe was a partial agonist at the glutamate-binding site of NMDA receptors with an EC50 of 331.6 +/- 78.6 microM and with an efficacy of 30.5 +/- 4.7% compared with NMDA. 3,5-DBr-L-Phe depressed both amplitude and frequency of AMPA/kainate mEPSCs. The IC50 of 3,5-DBr-L-Phe to inhibit AMPA/kainate mEPSC frequency was 29.4 +/- 4.3 microM. 3,5-DBr-L-Phe significantly decreased paired pulse depression of AMPA/kainate EPSCs and attenuated current activated by AMPA with higher efficacy at lower concentration of AMPA. 3,5-DBr-L-Phe neither affected GABA miniature inhibitory postsynaptic currents nor elicited action potentials. By enhancing NMDA receptor function, reducing glutamate release and blocking AMPA/kainate receptors 3,5-DBr-L-Phe represents a new type of polyvalent modulator of glutamatergic synaptic transmission with potential therapeutic applications.

Animals↗

PubMed2005

Long-term changes in glutamatergic synaptic transmission in phenylketonuria.

The cellular mechanisms that underlie impaired brain function during phenylketonuria (PKU), the most common biochemical cause of mental retardation in humans, remain unclear. Acute application of L-Phe at concentrations observed in the PKU brain depresses glutamatergic synaptic transmission but does not affect GABA receptor activity in cultured neurons. If these depressant effects of L-Phe take place in the PKU brain, then chronic impairment of the glutamate system, which may contribute to impaired brain function, could be detected as changes in postsynaptic glutamate receptors. This hypothesis was tested by using a combination of liquid chromatography-mass spectrometry, patch-clamp, radioligand binding and western blot approaches in forebrain tissue from heterozygous and homozygous (PKU) Pah(enu2) mice. Brain concentrations of L-Phe were nearly six-fold greater in PKU mice (863.12 +/- 17.96 micromol/kg) than in their heterozygous counterparts (149.32 +/- 10.23 micromol/kg). This concentration is significantly higher than the K(B) of 573 microM for L-Phe to compete for N-methyl-D-aspartate (NMDA) receptors. Receptor binding experiments with [3H]MK-801 showed significant up-regulation of NMDA receptor density in PKU mice. Consistent with the depressant effects of L-Phe, expression of NMDA receptor NR2A and (RS)-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor Glu1 and Glu2/3 subunits was significantly increased, whereas expression of the NR2B subunit was decreased. There was no change in GABA alpha1 subunit expression. Given the role of the glutamatergic system in brain development and function, these changes may, at least in part, explain the brain disorders associated with PKU.

Animals↗

PubMed2003

L-phenylalanine selectively depresses currents at glutamatergic excitatory synapses.

To explore the hypothesis that L-phenylalanine (L-Phe) depresses glutamatergic synaptic transmission and thus contributes to brain dysfunction in phenylketonuria (PKU), the effects of L-Phe on spontaneous and miniature excitatory postsynaptic currents (s/mEPSCs) in rat and mouse hippocampal and cerebrocortical cultured neurons were studied using the patch-clamp technique. L-Phe depressed the amplitude and frequency of both N-methyl-D-aspartate (NMDA) and non-NMDA components of glutamate receptor (GluR) s/mEPSCs. The IC(50) of L-Phe to inhibit non-NMDAR mEPSC frequency was 0.98 +/- 0.13 mM, a brain concentration seen in classical PKU. In contrast, D-Phe had a significantly smaller effect, whereas L-leucine, an amino acid that competes with L-Phe for brain transporter, had no effect on mEPSCs. Unlike GluR s/mEPSCs, GABA receptor mIPSCs were not attenuated by L-Phe. A high extracellular concentration of glycine prevented the attenuation by L-Phe of NMDAR current, activated by exogenous agonist, and of NMDAR s/mEPSC amplitude, but not of NMDAR s/mEPSC frequency. On the other hand, L-Phe significantly depressed non-NMDAR current activated by low but not high concentrations of exogenous agonists. Glycine-independent attenuation of NMDAR s/mEPSC frequency suggests decreased presynaptic glutamate release caused by L-Phe, whereas decreased amplitudes of NMDAR and non-NMDAR s/mEPSCs are consistent with competition of L-Phe for the glycine- and glutamate-binding sites of NMDARs and non-NMDARs, respectively. The finding that GluR activity is significantly depressed at conditions characteristic of classical PKU indicates a potentially important contribution of impaired GluR function to PKU-related mental retardation and provides important insights into the potential physiological consequences of impaired GluR function.

Animals↗

PubMed2002

Specific inhibition of N-methyl-D-aspartate receptor function in rat hippocampal neurons by L-phenylalanine at concentrations observed during phenylketonuria.

Hippocampal N-methyl-D-aspartate receptors (NMDARs) are thought to be involved in the regulation of memory formation and learning. Investigation of NMDAR function during experimental conditions known to be associated with impaired cognition in vivo may provide new insights into the role of NMDARs in learning and memory. Specifically, the mechanism whereby high concentrations of L-phenylalanine (L-Phe) during phenylketonuria (>1.2 mM) cause mental retardation remains unknown. Therefore, the effects of L-Phe on NMDA-activated currents (I(NMDA)) were studied in cultured hippocampal neurons from newborn rats using the patch-clamp technique. L-Phe specifically and reversibly attenuated I(NMDA) in a concentration-dependent manner (IC(50) = 1.71 +/- 0.24 mM). In contrast, L-tyrosine (L-Tyr), an amino acid synthesized from L-Phe in normal subjects, did not significantly change I(NMDA). Although the L-Phe-I(NMDA) concentration-response relationship was independent of the concentration of NMDA, it was shifted rightward by increasing the concentration of glycine. Consistent with an effect of L-Phe on the NMDAR glycine-binding site, L-Phe (1 mM) did not attenuate I(NMDA) in the presence of D-alanine (10 microM). Furthermore, L-Phe significantly attenuated neither glutamate-activated current in the presence of MK-801, nor current activated by AMPA. The finding that L-Phe inhibits specifically NMDAR current in hippocampal neurons by competing for the glycine-binding site suggests a role for impaired NMDAR function in the development of mental retardation during phenylketonuria and accordingly an important role for NMDARs in memory formation and learning.

Animals↗

PubMed2001

Structure-activity relationships and electrophysiological effects of short-acting amiodarone homologs in guinea pig isolated heart.

Antiarrhythmic agents with amiodarone-like electrophysiological actions, but with a more favorable pharmacokinetic profile than amiodarone would be extremely useful for the treatment of many tachyarrhythmias. We designed a series of amiodarone homologs with an alkyl ester group at position 2 of the benzofurane moiety. It was hypothesized that the electrophysiological and pharmacokinetic properties of these compounds are closely related to the size and branching of the ester group. The magnitude and time course of electrophysiological effects caused by methyl (ATI-2001), ethyl (ATI-2010), isopropyl (ATI-2064), sec-butyl (ATI-2042), and neopentyl (ATI-2054) homologs, and their common metabolite (ATI-2000) were investigated in guinea pig isolated heart. In paced hearts (atrial cycle length = 300 ms), each homolog (1 microM) was infused for 90 min followed by a 90-min washout. The stimulus-to-atrium (St-A), atrium-to-His bundle (AH), His bundle-to-ventricle (HV), QRS, and QT intervals, and ventricular monophasic action potential duration at 90% repolarization (MAPD(90)) were measured every 10 min. ATI-2001 and ATI-2064 significantly lengthened the St-A, HV, and QRS intervals, whereas ATI-2042 and ATI-2054 prolonged only the St-A interval. All compounds except the metabolite prolonged the AH interval. The relative rank order for the homologs to lengthen ventricular repolarization (MAPD(90)) was ATI-2042 > or = 2001 = 2010 = 2064 > 2054 > or = 2000. The metabolite was electrophysiologically inactive. Thus, modification of the benzofurane moiety ester group size and branching markedly altered the magnitude and time course of the electrophysiological effects caused by the ATI compounds. The different structure-activity relationships among the amiodarone homologs may have important consequences for further development of amiodarone-like antiarrhythmic agents.

Action Potentials↗

PubMed2000

Antagonism of the positive dromotropic effect of isoproterenol by adenosine: role of nitric oxide, cGMP-dependent cAMP-phosphodiesterase and protein kinase G.

We hypothesized that nitric oxide (NO) plays an important role in mediating the anti-adrenergic effect of adenosine on atrioventricular (AV) nodal conduction. In guinea-pig hearts instrumented for measurement of AV nodal conduction time (atrium-to-His bundle, A-H, interval), the NO synthase (NOS) inhibitor, l-NMMA (100 microm), reversibly inhibited 80% (P=0.009, n=6) of adenosine's anti-adrenergic action on the positive dromotropic effect of isoproterenol (0.01 microm). In parallel studies carried out in rabbit AV nodal myocytes, intracellular mechanisms whereby NO mediates the inhibitory effect of adenosine on isoproterenol-induced A-H interval shortening were studied. Adenosine (3 microm) inhibited isoproterenol-stimulated (0.1 microm) I(Ca,L)(beta -I(Ca,L)) by 46+/-6% (P<0.001, n=17). Consistent with isolated heart data, the NOS inhibitors, l -NMMA (100 microm) and L-NNA (500 microm) attenuated the effect of adenosine on beta -I(Ca,L)by 69+/-8% (P<0.001, n=16) and 69+/-7% (P<0.001, n=10), respectively. An inhibitor of NO-stimulated guanylyl cyclase LY83538 (40 microm) reduced the inhibitory effect of adenosine on beta -I(Ca,L)by 97+/-6% (P=0.004, n=15). Similarly, the non-specific inhibitor of cAMP-phosphodiesterases IBMX (50 microm) decreased the anti-adrenergic effect of adenosine by 60% (P=0.02, n=6), whereas the extracellular application of the non-hydrolyzeable cAMP analog 8-Br-cAMP (500 microm) prevented this action of adenosine. Activation of cGMP-dependent protein kinase (PKG) by CPT-cGMP (300 microm) diminished beta -I(Ca,L), but to a significantly smaller degree (16+/-4%, P=0.025, n=12) than that caused by adenosine. NO mediates the anti-adrenergic effect of adenosine on AV nodal conduction by a mechanism predominately involving activation of cGMP-dependent cAMP-phosphodiesterase and to a lesser extent activation of PKG.

3',5'-Cyclic-AMP Phosphodiesterases↗

PubMed2000

Midazolam selectively potentiates the A(2A) - but not A1- receptor--mediated effects of adenosine: role of nucleoside transport inhibition and clinical implications.

BACKGROUND: Inhibition of adenosine metabolism offers a unique approach to harness the cardioprotective properties of adenosine in a site- and event-specific manner. Benzodiazepines inhibit adenosine metabolism by blocking nucleoside transporter. Therefore, the authors studied the binding affinities of structurally different benzodiazepines to nucleoside transporter and benzodiazepine-induced potentiation of A1-adenosine (negative dromotropy) and A2A-adenosine (coronary vasodilation) receptor-mediated effects. METHODS: In membranes from porcine striatum and guinea pig ventricle, competition binding assays to displace [3H]nitrobenzylmercaptopurine riboside ([3H]NBMPR) from nucleoside transporter were performed using alprazolam, chlorodiazepoxide, diazepam, flurazepam, and midazolam. The augmentation by the most potent benzodiazepine of A1- and A2A-adenosine receptor-mediated responses, elicited by exogenous administration of adenosine or brief periods of global hypoxia, was subsequently studied in guinea pig Langendorff-perfused hearts. RESULTS: All benzodiazepines completely displaced [3H]NBMPR in a concentration-dependent manner with Hill coefficients not significantly different from unity in both striatal and ventricular membranes. Midazolam was the most potent inhibitor of nucleoside transporter (ventricle:pKi = 5.22+/-0.41, Ki = 6 microM). In isolated hearts, midazolam (5, 10, 20 microM) significantly augmented coronary flow in a concentration-dependent manner in the presence of adenosine (30 nM), an effect reversed by ZM 241385, a selective A2A-receptor antagonist. In contrast, midazolam did not increase the effect of adenosine (30 nM) on atrioventricular conduction. Similarly, midazolam potentiated A2A- but not A1-receptor-mediated effects of endogenous adenosine released during hypoxia. CONCLUSIONS: Structurally distinct benzodiazepines inhibit nucleoside transporter to different degrees. Midazolam selectively augments A2A- but not A1-receptor-mediated effects of adenosine by inhibiting nucleoside transporter.

Adenosine↗

PubMed1999

Hyperkalemia enhances the effect of adenosine on IK,ADO in rabbit isolated AV nodal myocytes and on AV nodal conduction in guinea pig isolated heart.

BACKGROUND: The atrioventricular (AV) node is insensitive to changes in extracellular potassium concentration, [K+]o, because of the absence of the inward rectifier potassium current (IK1). However, we propose that in the presence of adenosine, elevated [K+]o should increase the adenosine-activated inward rectifier potassium current (IK,ADO) in AV nodal myocytes and hence augment the negative dromotropic effect of the nucleoside. METHODS AND RESULTS: The effects of normal (4.8 mmol/L) and high (8.0 mmol/L) [K+]o on adenosine-induced changes in resting membrane potential (Vm), IK,ADO, and membrane resistance (Rm) in rabbit isolated AV nodal myocytes and in AV nodal conduction delay (atrium-to-His bundle, AH, interval) in guinea pig isolated hearts were determined with the use of whole-cell patch-clamp and His bundle electrogram techniques, respectively. High [K+]o alone did not significantly affect membrane current, Rm, or Vm in AV nodal myocytes. However, high [K+]o in the presence of adenosine (3 micromol/L) markedly increased Im (-0. 249+/-0.038 to -0.571+/-0.111 nA, P<0.05) at -100 mV and reduced Rm (151+/-21 to 77+/-8 MOmega, P<0.02). Adenosine still hyperpolarized Vm from -48+/-2 to -65+/-1 mV (P<0.001). High [K+]o alone did not significantly affect the AH interval in isolated hearts. However, high [K+]o markedly lengthened the AH interval prolongation caused by adenosine (4 micromol/L, 7.9+/-0.8 vs 22.1+/-3.0 ms, P<0.001). The potentiating effect of high [K+]o on adenosine-induced delay in AV nodal conduction was abolished by BaCl2 (100 micromol/L). CONCLUSIONS: By increasing IK,ADO and decreasing Rm of AV nodal myocytes, elevated [K+]o, augments the depressant effect of adenosine on AV nodal conduction.

Adenosine↗

PubMed1999

Ionic mechanisms mediating the differential effects of methohexital and thiopental on action potential duration in guinea pig and rabbit isolated ventricular myocytes.

BACKGROUND: Commonly used barbiturate anesthetics may significantly influence cardiac electrophysiologic characteristics. The authors evaluated thiopental (a thiobarbiturate) and methohexital (an oxybarbiturate), two compounds with similar physicochemical properties but different structures, to determine whether they have distinct effects on the major ionic currents that determine action potential duration (APD) in ventricular myocytes. METHODS: The effects of thiopental and methohexital (50 microM) on APD at 50% (APD50) and 90% (APD90) repolarization were studied in guinea pig and rabbit single ventricular myocytes using the patch-clamp technique in a whole-cell configuration. The ionic mechanisms underlying the APD changes were evaluated by measuring the anesthetics' effects on the L-type calcium inward current, the inward rectifier potassium current, and the delayed rectifier potassium current in guinea pig cells and on the transient outward potassium current in rabbit cells. RESULTS: Thiopental and methohexital caused opposite effects on APD. Whereas thiopental prolonged APD50 and APD90 in guinea pig and rabbit ventricular myocytes, methohexital shortened them. Thiopental markedly depressed both the inward and outward components of the inward rectifier potassium current, whereas methohexital caused minimal inhibition of the inward component and no change in the outward component. The delayed rectifier potassium current was inhibited by thiopental but significantly potentiated by methohexital. Neither thiopental nor methohexital significantly affected the transient outward potassium current or the L-type calcium inward current. CONCLUSIONS: Despite their similar lipid solubilities, molecular weights, and pKa values, thiopental increased and methohexital decreased the APD in ventricular myocytes by predominantly inhibiting the inward rectifier potassium current and the delayed rectifier potassium current and by increasing the delayed rectifier potassium current, respectively. These characteristics suggest distinct structure-specific actions of barbiturates on the function of myocardial ionic channels.

Action Potentials↗

PubMed1997

Role of nitric oxide, cyclic GMP and superoxide in inhibition by adenosine of calcium current in rabbit atrioventricular nodal cells.

OBJECTIVE: To study the intracellular pathways which mediate the inhibitory actions of adenosine on isoprenaline-stimulated calcium current (ICa) in atrioventricular (AV) nodal myocytes. METHODS: The whole-cell patch-clamp technique was used to record ICa from rabbit AV nodal cells, isolated by enzymatic and mechanical dispersion. RESULTS: Isoprenaline, 0.1 microM, increased peak ICa from 0.58 +/- to 1.23 +/- 0.1 nA, and this increase was reversibly inhibited by adenosine, 10 microM (83 +/- 6%), which we have previously shown to be mediated by nitric oxide (NO) production. A membrane-permeable analogue of cyclic GMP, 8-Br-cGMP (300 microM), an inhibitor of cGMP-stimulated phosphodiesterase, prevented the effect of adenosine on ICa-Methylene blue (10 microM), an inhibitor of NO-sensitive guanylyl cyclase and a generator of superoxide (.02-), did not prevent, but increased, the inhibiting action of adenosine (49.5 +/- 6.6%, P < 0.01). Methylene blue (50 microM) caused a reduction of ICa, with further inhibition when combined with adenosine. A .O(2-)-generating system, xanthine oxidase (0.02 U/ml) and purine (2.3 mM), also increased the inhibitory action of adenosine on ICa. Inhibition of ICa by adenosine in the presence of xanthine oxidase was not prevented by 8-Br-cGMP (300 microM) and was not influenced by pre-incubation of cells with a NO synthase inhibitor, L-NAME (0.5 mM). CONCLUSIONS: The inhibitory effect of adenosine on ICa in rabbit AV nodal myocytes can be mediated by two mechanisms--stimulation of cGMP-stimulated phosphodiesterase by NO-induced cGMP, and a mechanism which involves interaction with .O2- production.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1997

Ionic basis of the differential effects of intravenous anesthetics on erythromycin-induced prolongation of ventricular repolarization in the guinea pig heart.

BACKGROUND: Dysrhythmias and death occur in patients with acquired long QT syndrome (LQTS). Little information exists regarding interactions between anesthetics and drugs that prolong ventricular repolarization. Therefore the effects of three commonly used intravenous anesthetics on ventricular repolarization were investigated in the setting of drug-induced, long QT syndrome. METHODS: The effects of increasing concentrations (0, 10, 25, and 50 microM) of propofol, ketamine, and thiopental on ventricular repolarization were evaluated by measuring the monophasic action potential duration at 90% repolarization (MAPD90) in guinea pig Langendorff-perfused hearts in the absence or presence of erythromycin (100 microM). If an anesthetic enhanced erythromycin-induced prolongation of MAPD90, its effects on the delayed rectifier (I[K]) and inward rectifier (I[Kl]) potassium currents were measured using the whole-cell patch-clamp technique. RESULTS: At clinically relevant concentrations, only thiopental significantly modulated erythromycin's effect on MAPD90. Thiopental at 10, 25, and 50 microM prolonged MAPD90 from a control of 163 +/- 6 ms by 18 +/- 4, 30 +/- 3, and 31 +/- 4 ms, respectively. In a separate group, erythromycin prolonged MAPD90 from 155 +/- 2 ms to 171 +/- 2 ms (n = 21, P < 0.001). In the presence of erythromycin, thiopental at 10, 25, and 50 microM caused significantly greater prolongation from a control of 171 +/- 2 ms by 39 +/- 2, 58 +/- 3, and 72 +/- 6 ms, respectively. Whole-cell patch-clamp experiments indicated that thiopental inhibited I(K) and I(Kl). CONCLUSIONS: Intravenous anesthetics caused markedly different effects on ventricular repolarization. Thiopental, unlike propofol and ketamine, potentiated the effects of erythromycin on ventricular repolarization by inhibiting I(K) and I(Kl).

Action Potentials↗

PubMed1997

Ionic mechanisms of the effect of adenosine on single rabbit atrioventricular node myocytes.

The ionic mechanisms underlying the negative dromotropic effect of adenosine were studied in calcium-tolerant myocytes isolated from the region of the rabbit atrioventricular (AV) node. Action potentials and membrane currents were recorded by using the whole cell patch clamp technique. Adenosine (1 to 50 microM) abolished the spontaneous activity of AV node myocytes with hyperpolarization of the membrane potential. Voltage clamp experiments showed that adenosine induced an inwardly rectifying, time-independent potassium current. These effects were antagonized by 8-cyclopentyl-1,3-dipropylxanthine and produced by ribose 5-phosphate isomerase A, indicating that they were mediated by the A1 adenosine receptor. Adenosine also had a small direct inhibitory action on the inward calcium current (ICa) but had a more marked indirect action following stimulation of the calcium current by isoprenaline. The isoprenaline-induced increase in ICa was abolished in the presence of adenosine 10 microM. In cells pretreated with the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME), the isoprenaline-induced increase in ICa was not reduced by the addition of adenosine. Coincubation of the cells with L-NAME plus L-arginine (the endogenous substrate of nitric oxide synthase) restored the adenosine-induced attenuation of ICa. A membrane permeable analogue of cGMP, 8Br cGMP, an inhibitor of cGMP-stimulated phosphodiesterase, prevented the antiadrenergic effect of adenosine. These results suggest that adenosine activates guanylyl cyclase following the production of nitric oxide, and the subsequent stimulation of phosphodiesterase enhances the breakdown of isoprenaline-elevated cAMP leading to a reduction in the stimulated ICa. In conclusion, the important ionic mechanisms of the actions of adenosine on AV nodal cells are a direct effect, with activation of a potassium conductance and an indirect antiadrenergic effect on ICa, which is mediated by nitric oxide production and phosphodiesterase stimulation.

Adenosine↗

PubMed1996

Nitric oxide mediates the anti-adrenergic effect of adenosine on calcium current in isolated rabbit atrioventricular nodal cells.

The aim of this study was to determine if adenosine exerts an anti-adrenergic effect on rabbit isolated atrioventricular (AV) nodal cells and, if so, the dependence of this effect on nitric oxide (NO) production. Inward Ca current, ICa, was measured in AV nodal cells, enzymatically isolated from rabbit hearts. Isoprenaline (0.1 microM) increased ICa from 676 +/-59 to 1102 +/-86 pA (n =25). This isoprenaline-induced increase in ICa(178 +/-15% of control) was abolished in the presence of 10 microM adenosine (ICa100 +/-2% of control, n =9, P <0.05). This effect of adenosine was completely blocked by the A1 receptor antagonist CPDPX (8-cyclopentyl l, 3-dipropylxanthine, 0.1 microM). In cells pre-treated with the NO synthase inhibitor, L-nitro-arginine methyl ester (L-NAME, 1 mM) the isoprenaline-induced increase in ICa(208 +/-39% of control, n=7) was not reduced by the addition of 10 microM adenosine (195 +/-32% of control). Co-incubation of cells in L-NAME with L-arginine (1 mM, the endogenous substrate of NO synthase) restored the adenosine-induced attenuation of ICa. In these cells, isoprenaline increased ICa (157 +/-7% of control, n =6), and, following addition of adenosine (10 microM) ICa was reduced to 107 +/-8% (P <0.05). The NO-releasing agent SIN-1 (3-morpholino-sydnonimine, 100 microM), inhibited ICa augmented by isoprenaline (n=5). It is concluded that adenosine exerts an anti-adrenergic effect on the AV node via A1 receptors to attenuate a catecholamine-stimulated increase in ICa and that this action involves the intracellular production of NO.

Adenosine↗

PubMed1995

Adenosine increases potassium conductance in isolated rabbit atrioventricular nodal myocytes.

OBJECTIVE: To study the actions of adenosine on the electrophysiology of spontaneously active, rod-shaped cells enzymatically isolated from rabbit atrioventricular (AV) node. METHODS: Calcium-tolerant myocytes were isolated from the region of the AV node by enzymatic and mechanical dispersion. They were rod- or spindle-shaped, with spontaneous activity at 35-37 degrees C, and had higher membrane resistances (776 +/- 283 M omega, n = 13), compared to atrial cells (41 +/- 18.2 M omega, n = 7; P < 0.001). Membrane potential, spontaneous action potentials and transmembrane ionic currents were studied using the whole-cell patch-clamp technique, in current-clamp and voltage-clamp mode. RESULTS: Adenosine (0.1-50 microM) slowed or abolished the spontaneous activity, with hyperpolarisation of the membrane potential. Voltage-clamp experiments showed that adenosine induced an inwardly rectifying time-independent current. The adenosine-induced current was shown to be carried by potassium ions by the effect of increasing external potassium, which altered the reversal potential in accordance with the calculated potassium equilibrium potential. The A1 adenosine receptor antagonist, CPDPX (8-cyclopentyl-1,3-dypropylxanthine), reversed the effects of adenosine and an A1 receptor agonist, R-PIA [R(-)N(6)-(2-phenylisopropyl)adenosine] had effects similar to adenosine. Adenosine also caused a small decrease in inward calcium current (ICa) in some AV nodal cells. CONCLUSIONS: These results indicate that adenosine acts at A1 adenosine receptors to suppress spontaneous activity, hyperpolarise membrane potential and induce a time-independent potassium current in AV nodal cells. These actions, combined with reduction in inward calcium current in some cells, may underlie the negative chronotropic and dromotropic actions of adenosine on rabbit AV nodal cells.

Adenosine↗

PubMed1991

Blocking effect of intraperitoneal injection of phenylalanine on high-threshold calcium currents in rat hippocampal neurones.

Calcium currents were recorded in cultured (5-7 days) hippocampal neurones isolated from one-day-old rats. The animals obtained intraperitoneal injections of L-phenylalanine which induces in the brain biochemical changes characteristic of phenylketonuria. It has been found that the amplitude of the low-threshold calcium current in L-phenylalanine-affected neurones was not appreciably changed compared with that in neurones from control (non-injected) animals. However, the amplitude of the high-threshold calcium current was essentially decreased. Its relative amplitude at Vt = +20 mV became 40 +/- 30% as contrasted to 416 +/- 130% in neurones from control animals (the amplitude of the calcium currents at Vt = -10 mV taken as 100%). The decrease remained during the whole time of culturing. Addition of L-tyrosine to the cultivation medium (50 microM) restored the high-voltage calcium current, its relative amplitude reaching 280 +/- 57%. The data are discussed in conjunction with the previously obtained results about antagonistic modulatory action of tyrosine and phenylalanine on the functioning of high-threshold calcium channels and possible mechanisms of brain dysfunction during phenylketonuria.

Animals↗

PubMed1991

Effects of intracellular administration of L-tyrosine and L-phenylalanine on voltage-operated calcium conductance in PC12 pheochromocytoma cells.

Changes induced by internal administration of L-tyrosine and L-phenylalanine in high-threshold calcium currents have been studied on perfused PC12 pheochromocytoma cells using whole-cell voltage-clamp technique. A method for rapid changes of perfusing solutions has been used. L-Tyrosine (20 microM) slowed down the decline ('wash-out') of ICa occurring during intracellular perfusion and in most cells induced its temporary recovery. alpha-Methyl-D,L-tyrosine (a tyrosine hydroxylase blocker) exerted a similar effect. On the other hand, L-phenylalanine (20 microM) in most cells speeded-up the decline of ICa. Replacement of ATP in the perfusing solution by an equivalent amount of ADP (2 mM) did not alter the effects of amino acids. The possible mechanisms of the described changes are discussed in connection with the known role of L-tyrosine in posttranslational modifications of microtubular proteins.

Adrenal Gland Neoplasms↗

PubMed1988

Potassium outward current dependent on extracellular calcium in snail neuronal membrane.

In experiments on isolated intracellularly dialysed neurons of the snail Helix pomatia a component of delayed inactivating potassium outward current depending on the presence of Ca2+ ions in the extracellular medium has been distinguished, which differs from the already known potassium current sensitive to intracellular calcium ions. This component decreases with a decrease in extracellular calcium (in the range of 10(-2) - 10(-5) M); it is not affected or even increased by intracellular introduction of ethyleneglycolbis(aminoethylether)tetra-acetate (10 mM) or fluoride ions (77 mM) and can be blocked by addition of 1.5 mM cobalt ions to the extracellular solution. Contrary to the slow rising potassium current dependent on intracellular calcium, this current has a fast rising phase (several milliseconds) and time-dependent inactivation. The inactivation depends on extracellular potassium ions: it slowed down when [K+]out is increased in the range of 1-10 mM. Extracellular application of calmodulin blockers calmidazolium (6.5 X 10(-7) M) and chlorpromazine (2.5 X 10(-6) M) selectively inhibits the potassium current dependent on intracellular calcium but does not affect that dependent on external calcium. Tetraethylammonium (10 mM) depresses the latter current on both intra- and extracellular application, the former being more effective. The existence of a special type of potassium channel sensitive to extracellular calcium ions is postulated.

Animals↗