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Query: UNIPROT:P06889 (Mol)
 630,302 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Calcium uptake was measured using 47Ca2+ in the isolated and arterially perfused interventricular septum of the rabbit. Experiments were undertaken to determine whether calcium uptake on reoxygenation is linked to recovery of mechanical function and whether calcium uptake is through the sodium-calcium exchange mechanism. During substrate-free hypoxia for 45 min total tissue calcium remained unchanged but immediately upon reoxygenation there was a substantial net gain of calcium. Recovery of mechanical function upon reoxygenation was inversely related to the increase in tissue calcium. Activation of sodium-calcium exchange by perfusion with a low-sodium, zero-potassium, sucrose solution also increased tissue calcium and the relation to mechanical recovery was similar to that observed on reoxygenation. The sodium-calcium exchange mechanism was not affected by hypoxia and could be demonstrated during perfusion with a substrate-free hypoxic solution. Lithium (100 mM) substitution for sucrose prevented the calcium influx induced by a low-sodium and zero-potassium perfusate under normoxic conditions. Lithium substitution early during hypoxia or on reoxygenation did not affect the increase in myocardial calcium on reoxygenation. Amiloride (10(-4)M), presumed to inhibit sodium-hydrogen exchange during hypoxia, had no effect upon reoxygenation induced calcium uptake. It is concluded that the increase in calcium uptake that occurs on reoxygenation after a period of substrate-free hypoxia is related to mechanical recovery. Sodium-calcium exchange may contribute to calcium uptake on reoxygenation in this experimental model but is not the major mechanism.
J Mol Cell Cardiol 1990 Oct
PMID:Calcium exchange in rabbit myocardium during and after hypoxia: role of sodium-calcium exchange. 209 31

The modification of cardiac sodium channels by DPI 201-106, its S-enantiomeric form (S)-DPI, and its R-enantiomeric form (R)-DPI was investigated with whole-cell voltage-clamp recording in single cultured ventricular myocytes obtained from late-fetal rats. From a holding potential of -100 mV, depolarizing pulses to -30 mV of 50-msec duration were applied at 0.2 Hz. Extracellular [Na] was reduced to 70 mM; temperature was 20 degrees. Drugs were administered directly on the cell by a double-barrelled microsuperfusion system. Sodium current inactivation was progressively slowed when the concentration of DPI 201-106 was increased from 0.3 to 3 microM. At 10 microM DPI 201-106, this effect was followed by a blocking effect on peak inward sodium current (INa), and at 30 microM inward sodium current was fully blocked within 2 min. The slowing of inactivation was produced by (S)-DPI (maximally effective at 3 microM), whereas (R)-DPI had little effect on inactivation at 3 microM. Conversely, (R)-DPI reduced INa at 10 microM, whereas (S)-DPI did not reduce INa at 3 microM. The effects of both (S)-DPI and (R)-DPI were partially reversed by washout. (R)-DPI retained its blocking activity on INa when the interval between depolarizing pulses was prolonged to 90 sec. In order to test whether the different sodium channel modifications produced by (S)-DPI and (R)-DPI were mutually exclusive, the INa-reducing activity of (R)-DPI was measured in the absence of (S)-DPI and after equilibration with a maximally effective (S)-DPI concentration. In the absence of (S)-DPI, 3 microM (R)-DPI reduced INa by 35% and in the presence of 3 microM (S)-DPI, by 51%. Thus, modification by (S)-DPI of sodium channels did not prevent their block by (R)-DPI. The INa-reducing activity of (R)-DPI was even significantly augmented by (S)-DPI after a 1-sec depolarization to -30 mV. During such prolonged pulses, (R)-DPI accelerated the monoexponential decay of the (S)-DPI-induced slow phase of sodium current inactivation. The results are consistent with an irreversible binding reaction between (R)-DPI and (S)-DPI-modified open sodium channels (association rate constant, 4.7 x 10(5) M-1sec-1). We conclude that (R)-DPI reduces INa by interacting both with resting sodium channels and with (S)-DPI-modified open sodium channels. The corresponding receptor site is stereoselective and distinct from and allosterically coupled to the (s)-DPI receptor that mediates slowing of inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)
Mol Pharmacol 1990 Jan
PMID:Interaction between DPI 201-106 enantiomers at the cardiac sodium channel. 215 6

High affinity GTPase in membranes from NG108-15 cells was differentially affected by opioid competitive antagonists; one type of antagonist [( N,N'-diallyl-Tyr1-Aib2,3]Leu-enkephalin) reduced the basal rate of GTP hydrolysis, whereas a second type (MR 2266) produced no changes. The inhibitory effect of the "active" antagonist was stereospecifically reversed by the "inactive" antagonist, indicating that it was receptor mediated. This suggests that part of basal GTPase activity in this system results from a spontaneous interaction between opioid receptors and GTP-binding proteins (G proteins) and that some antagonists exhibit negative intrinsic activity by hindering such an interaction. The inhibitory effect of the antagonist was minimal in the presence of Na+ and maximal when Na+ was replaced by K+ in the reaction. When the ratio [Na+]/[K+] was progressively increased at constant [Cl-], total GTPase activity (i.e., net difference between activity stimulated by agonist and that inhibited by antagonist) did not change, but the activity measured in the absence of ligand was selectively decreased. Thus, Na+ does not alter the total proportion of G proteins that can be activated by ligand-occupied receptors and instead regulates the interaction between receptor and G protein in the absence of ligand. Upon examination of several opioid agonist and antagonists, we found an inverse relation between the intrinsic activity (either negative or positive) of each ligand and the sensitivity to Na+ of the GTPase elicited upon occupation of the receptor by that ligand. Sodium-mediated and ligand-mediated regulations of GTPase had identical requirements for Mg2+ [( Mg2+]free greater than 10 microM), and were both abolished with a similar potency by pertussis toxin. There was no effect of Na+ on the basal rate of GTP hydrolysis of Gi/Go purified from bovine brain. However, addition of these proteins to membranes prepared from cells that had been previously exposed to pertussis toxin partially restored both receptor- and sodium-mediated regulations of GTPase in parallel and in a concentration-dependent fashion. We conclude that sodium ions play a key role in the mechanism underlying the spontaneous interaction between "empty" receptors and G proteins in intact membranes.
Mol Pharmacol 1990 Mar
PMID:Spontaneous association between opioid receptors and GTP-binding regulatory proteins in native membranes: specific regulation by antagonists and sodium ions. 215 52

Using chromatography on DE-52 and acetylcholine-Affi-Gel columns, nicotinic acetylcholine receptor was purified to approximately 10,000 fold from Lubrol extract of rat brain with a recovery of 15%. The purified preparation contained no cholinesterase activity. alpha-Bungarotoxin did not inhibit [3H]acetylcholine binding to the purified preparation. Sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed 4 major protein bands with apparent molecular weights of 53,000, 67,000, 80,000 and 108,000. When nicotinic acetylcholine receptor was eluted with either carbachol or nicotine from the affinity column, these major bands were found on SDS-PAGE gels. Immunoblot analysis showed that the Mr 80,000 protein was an acetylcholine-binding subunit and that the Mr 48,000 protein, a minor band on SDS-PAGE gel, was a structural subunit.
Brain Res Mol Brain Res 1990 Apr
PMID:Affinity purification of nicotinic acetylcholine receptor from rat brain. 215 81

Sodium (Na+) channels are members of a multigene family and are responsible for generation and propagation of the action potential in excitable cells. We have assembled, in a transcription-competent vector, a full-length cDNA clone encoding the rat brain type III Na+ channel. Xenopus oocytes microinjected with in vitro synthesized mRNA expressed functional rat brain Na+ channels from such 'cloned' RNA transcripts. We found that type III Na+ currents in whole cell microelectrode voltage clamp and in cell-attached patch recordings decayed much more slowly than any other reported Na+ current. In addition, we saw typical and additive effects of alpha- and beta-scorpion toxins, suggesting that the Na+ channel alpha-subunit itself contains functional and distinct toxin binding sites.
Brain Res Mol Brain Res 1990 Feb
PMID:Toxin and kinetic profile of rat brain type III sodium channels expressed in Xenopus oocytes. 216 38

A characterization of the properties of voltage-gated sodium channels expressed in the human cerebellar medulloblastoma cell line TE671 is presented. Membrane currents were recorded under voltage clamp conditions using the patch clamp technique in both the whole-cell and the excised-patch configurations. Macroscopic sodium currents display a typical transient time course with a sigmoidal rise to a peak followed by an exponential decay. The rates of early activation and subsequent inactivation accelerate and approach a maximum in response to test potentials, V, of greater depolarization. The magnitude of peak sodium current increased from negligible values below V = -50 mV and reached a maximum at V = -3.6 mV +/- 2.7 mV (mean +/- S.E.M., n = 12). Sodium currents reversed at V = + 70 mV, near the predicted Nernst equilibrium potential for a Na+ selective channel. The peak sodium conductance, gpeak increased with depolarizing voltages to a maximum at V = approximately 0 mV, exhibiting half-activation voltage at V approximately equal to -36.8 mV and an e-fold change in gpeak/9.5 mV. The Hodgkin-Huxley inactivation parameter h infinity indicates that at V = -73.6 mV half of the sodium currents were inactivated. Single channel current recordings demonstrated the occurrence of discrete events: the latency for first opening was shorter as the depolarizing pulse became more positive. The single-channel current amplitude was ohmic with a slope conductance, gamma = 17.13 pS +/- 0.66 pS. Sodium channel currents were reversibly blocked by tetrodotoxin (TTX).(ABSTRACT TRUNCATED AT 250 WORDS)
Brain Res Mol Brain Res 1990 Feb
PMID:Voltage-gated sodium channels expressed in the human cerebellar medulloblastoma cell line TE671. 216 39

Saturable high and low affinity binding sites for [3H]saxitoxin were identified in myometrial membranes of pregnant rats, with dissociation constants of 0.53 and 27 nM, respectively. The maximal binding capacity of the low affinity binding sites was about 10 times higher than that of the high affinity binding sites. The dissociation constants obtained from association and dissociation kinetics of [3H]saxitoxin were similar to those obtained from equilibrium binding. Saxitoxin and tetrodotoxin specifically displaced [3H]saxitoxin binding at both types of sites. Isradipine (1-10 microM) and amiloride (50-100 microM) were without effect on the binding of [3H]saxitoxin. At high concentrations (10-100 microM), veratridine induced a partial inhibition of [3H]saxitoxin binding. In dispersed myometrial cells, [3H]saxitoxin binding revealed the presence of both high and low affinity binding sites, with KD values similar to those obtained in myometrial membranes. Sodium currents were studied in both freshly dispersed and cultured myometrial cells in the presence of veratridine (100 microM), using the whole-cell patch-clamp technique. Steady state inactivation curves indicated that sodium channels were available at negative membrane potentials (between -110 and -40 mV). Isradipine (1-10 microM) and amiloride (50-100 microM) were without effect on the sodium current. Applications of saxitoxin or tetrodotoxin inhibited the amplitude of the sodium current in a concentration-dependent manner. The concentrations of saxitoxin and tetrodotoxin producing half-maximal inhibition were 1.4 and 8.8 nM, respectively. Although the IC50 values for saxitoxin and tetrodotoxin found from electrophysiological experiments are not identical to the equilibrium dissociation constants for the high and low affinity sites found from binding experiments, these results suggested that binding of the neurotoxins to the high affinity sites may be involved in their inhibitory effects on sodium channels. Furthermore, low affinity binding sites may be associated with a non-functional subtype of sodium channels in myometrial cells.
Mol Pharmacol 1990 Nov
PMID:Identification and properties of voltage-sensitive sodium channels in smooth muscle cells from pregnant rat myometrium. 217 74

We have studied a bretylium tosylate induced increase of the membrane potentials of partially depolarized rat, mouse and human lymphocytes, using the potential sensitive dye, bis [1,3, dibutylbarbituric acid-(5) trimethine oxonol]. The extent of this repolarization is dose-dependent and decreased in magnitude as the temp was reduced from 37 degrees C to room temp. The repolarizing effect is inhibited by K(+)-Na(+)-pump blockers or lack of extracellular Na+. Sodium ion channel blockers are effective in abolishing repolarization only if applied prior to, or simultaneously with, bretylium. Activation of Na+/H+ exchange is not involved in the mechanism of the phenomenon as the latter is completely eliminated in the presence of 10 microM amiloride (concn of the diuretics having no measurable inhibition on the action of the exchanger). These data suggest that bretylium opens ligand- and voltage-gated Na+ channels, and repolarization occurs due to higher activity of the K(+)-Na(+)-pump stimulated by the enhanced intracellular Na+ accumulation.
Mol Immunol 1990 Dec
PMID:Bretylium causes a K(+)-Na+ pump activation that is independent of Na+/H+ exchange in depolarized rat, mouse and human lymphocytes. 217 49

In order to investigate the existence of a chorionic gonadotropin (CG) in the rat, placental mRNA was prepared from either the foetal disc or the maternal site of implantation in pregnant rats and translated in a wheat-germ cell-free translation system in the presence of 35S-labeled methionine and cysteine. Sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the radioactive material immunoprecipitated using antiserum specific to native rat alpha-subunit allowed us to isolate in translation products from both sources (foetal disc and maternal site of implantation) a single polypeptide of 17 kDa having the electrophoretic properties of the rat pituitary alpha-precursor. This polypeptide was absent in media derived from translation of mRNA extracted from uterine horn of non-pregnant female rats, and was approximately 10 times more abundant when RNA was derived from the maternal part (about 0.25% of cpm in total proteins) rather than the foetal part (0.026%) of the placenta. Immunoprecipitation was prevented in the presence of an excess of rat (but not ovine) alpha-subunit. Antisera directed against denatured bovine alpha-subunit, which cross-react with the rat pituitary alpha-precursor, did not bind the placental peptide. These results suggest that this rat placental polypeptide and the rat alpha-subunit of pituitary glycoprotein hormones have important differences in their primary structure, but share discrete structurally and/or conformationally related regions in their polypeptide chains. The possibility that these partial homologies account for gonadotropin-like activity of a presumed rat CG remains to be ascertained.
Mol Cell Endocrinol 1987 Nov
PMID:Rat placental mRNA directs the synthesis of a polypeptide immunologically related to alpha-subunit of glycoprotein hormones. 244 7

This paper presents a view of the evolution and phylogenetic distribution of ionic channels of biological membranes. The view is based on the assumptions that ionic channels (1) appeared very early in the history of life, (2) have evolved from a common ancestor, and (3) have been subjected to evolutionary pressure to reach precision and high speed of signaling. We propose that Ca2+ was the intracellular messenger and modulator of the most primitive biological systems, which implies that the first channel to appear may have been a calcium channel. Then, very soon the entire group of potassium channels evolved from the calcium channel to improve the shape of signals and to restore initial conditions. Sodium channels probably appeared relatively late, diversifying from calcium channels in the early metazoan groups. Mainly because Na+ ions do not interfere with cellular metabolism (thus allowing the inward current--and, consequently, the speed of conduction--to be greatly increased), sodium channels probably proved advantageous in the generation of the action potential, and selection replaced calcium channels with sodium channels in this function. Finally, with the acquisition of multicellularity, channels responsible for synaptic transmission appeared. The case of the acetylcholine receptor channel is briefly discussed.
Mol Biol Evol 1989 Sep
PMID:Evolution of ionic channels of biological membranes. 247 61


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