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M T Nelson

Publications and source records attributed to M T Nelson.

119 records · Page 7Linked to original sources
PubMed1986

Interactions of divalent cations with single calcium channels from rat brain synaptosomes.

Voltage-dependent calcium channels from a rat brain membrane preparation ("synaptosomes") were incorporated into planar lipid bilayers. The effects of calcium, barium, strontium, manganese, and cadmium ions on the amplitudes and kinetics of single channel currents were examined. The order of single channel conductances was gBa greater than gSr greater than gMn, which was the inverse of the order of the mean channel open times: TMn greater than TCa = TSr greater than TBa. In contrast, the identity of the charge carrier had little or no effect on the mean closed times of the channel. Manganese, in the absence of other permeant ions, can pass through single channels (gMn = 4 pS). However, when added to a solution that contained another type of permeant divalent cation, manganese reduced the single channel current in a voltage-dependent manner. Cadmium, a potent blocker of macroscopic "ensemble" calcium currents in many preparations, reduced the current through an open channel in a manner consistent with Cd ions both not being measurably permeant and interacting with a single site. The permeant ions competed with cadmium for this site with the following order: Mn greater than Sr = Ca greater than Ba. These results are consistent with the existence of no less than one divalent cation binding site in the channel that regulates ion permeation.

Animals↗
PubMed1984

Sodium pump stoicheiometry determined by simultaneous measurements of sodium efflux and membrane current in barnacle.

Ouabain-sensitive Na efflux, membrane potential and membrane current were measured in single, perfused muscle cells taken from the giant barnacle, Balanus nubilus. This preparation permits control of the intracellular and extracellular solution composition as well as control of the membrane potential while measuring ion fluxes across the plasma membrane. The addition of ouabain (10(-4) M) to the extracellular solution produces a rapid depolarization of membrane potential (1-4 mV) and a simultaneous and proportional reduction of Na efflux (10-40 pmol/s). Ouabain-induced changes in membrane potential or Na efflux do not depend on the presence of extracellular Na. Under voltage control, the application of ouabain (10(-4) M) produces a rapid monotonic fall in outward current (1-3 microA) and a simultaneous fall in Na efflux (10-40 pmol/s). The fraction of ouabain-dependent Na efflux that appears as outward current is constant in any given preparation as the Na-pump turnover rate varies. Over a limited range, changes in membrane potential do not affect ouabain-sensitive Na efflux. The ouabain-sensitive Na efflux and membrane current are not altered by the presence of 50 mM-internal tetraethylammonium (TEA) ions. We conclude that the Na pump is electrogenic in barnacle muscle and that 49 +/- 10% of the extruded Na+ leaves the intracellular compartment as uncompensated charge. We find that the transport stoicheiometry of Na:K, calculated from the ouabain-dependent changes in membrane current and Na efflux, is between 3:2 and slightly more than 2:1.

Animals↗
PubMed1983

Effects of extracellular sodium on calcium efflux and membrane current in single muscle cells from the barnacle.

The actions of extracellular sodium (Nao) on membrane potential, membrane current, membrane conductance and Ca efflux were examined in single muscle cells from the giant barnacle, Balanus nubilus. The intracellular compartment was perfused to facilitate the control of intracellular constituents including calcium ions (Ca2i+). Changing Nao has no large effect on Ca efflux when free intracellular calcium activity, [Ca2+]i, is low (about 0.1 microM). However, increasing [Ca2+]i leads to the development of Nao-dependent Ca efflux as well as to an augmentation in Nao-independent Ca efflux. Reducing Nao (using Li+ as a substitute cation) leads to a depolarization of the membrane when [Ca2+]i is low (about 0.1 microM). Increasing [Ca2+]i causes the membrane to depolarize. With [Ca2+]i at about 10.0 microM, reduction of Nao produces a hyperpolarization of the membrane. Significant Nao-dependent inward current is seen when [Ca2+]i is high. This current is large with respect to the Nao-dependent changes in Ca efflux (about 1 microA per p-mole/sec). The Ca2i+-activated, Nao-dependent changes in Ca efflux and membrane current are not sensitive to La3o+. However, Lao3+ does inhibit a fraction of the Cai2+-activated changes in membrane current and Ca efflux which are not dependent on Nao. Over a limited range of membrane potential Ca efflux is not voltage-dependent. Possible relationships between the Nao-dependent changes in Ca efflux and Nao-dependent changes in membrane potential or current are discussed. We find that these changes cannot be readily interpreted in terms of a single transport mechanism.

Animals↗
PubMed1983

Single K+-channel current measurements from brain synaptosomes in lipid bilayers.

Ion channels from a rat brain preparation enriched in presynaptic nerve terminals (synaptosomes) were incorporated into planar lipid bilayers. Experiments examined macroscopic (channel-ensemble) currents as well as single-channel currents. Four single-channel conductances (ranging from 10 to 40 pS) were usually observed, each with distinct kinetic properties. All the observed channels selected for K+ over Cl-. These K+ channels may contribute to the resting K+ conductance of brain nerve terminals. Furthermore, this report demonstrates that the properties of ion channels from mammalian brain can be studied in planar lipid bilayers and suggests that this system can be readily extended to many additional investigations on the electrical properties of brain membranes.

Animals↗
PubMed1982

GABA efflux from synaptosomes: effects of membrane potential, and external GABA and cations.

Presynaptic GABAergic nerve terminals accumulate gamma-aminobutyric acid (GABA) by a sodium-dependent carrier mechanism in which two Na+ are co-transported with one GABA. We have examined the influence of external GABA and cations on GABA efflux from 3H-GABA loaded rat brain synaptosomes, to determine whether or not the carriers can also mediate GABA efflux. We observed that, in Ca-free media (to minimize Ca-dependent, evoked release), external GABA promotes GABA efflux when the medium contains Na+, but inhibits GABA efflux in the absence of Na+. The efflux of GABA into Ca-free media is stimulated by depolarization (either with veratridine or increased external K+). These data, and published data on the internal Na+ dependence of GABA efflux into Ca-free media, indicate that exiting GABA is cotransported with Na+. The stimulatory effect of depolarization is consistent with efflux of Na+ along with the uncharged GABA. The (carrier-mediated) efflux is also stimulated when the carriers cycle inward with Na+ + GABA. The inhibitory effect of GABA in Na+-free media implies that GABA can bind to unloaded carriers and that the carriers loaded only with GABA cycle very slowly, if at all. Our data, and data from the literature, can be fitted to a simple model with sequential binding of solutes: external GABA binds to the carrier first, and only the free or fully-loaded (with 2Na+ + 1GABA) carriers can cycle. Other binding sequences and random binding, do not fit the experimental observations.

Animals↗
PubMed1981

Effects of ATP and vanadate on calcium efflux from barnacle muscle fibres.

Calcium ions carry the inward current during depolarization of barnacle muscle fibres and are involved in the contraction process. Intracellular ionized calcium ([Ca2+]i) in barnacle muscle, as in other cells, is kept at a very low concentration, against a large electrochemical gradient. This large gradient is maintained by Ca2+ extrusion mechanisms. When [Ca2+]i is below the contraction threshold, Ca2+ efflux from giant barnacle muscle fibres is, largely, both ATP dependent and external Na+ (Na+0) dependent (see also refs 5,6). When [Ca2+]i is raised to the level expected during muscle contraction (2-5 muM), most of the Ca2+ efflux from perfused fibres is Na0 dependent; as in squid axons, this Na+0-dependent Ca2+ efflux is ATP independent. Orthovanadate is an inhibitor of (Na+ + K+) ATPase and the red cell Ca2+-ATpase. We report here that vanadate inhibits ATP-promoted, Na+0-dependent Ca2+ efflux from barnacle muscle fibres perfused with low [Ca2+]i (0.2-0.5 microM), but has little effect on the Na+0-dependent, ATP-independent Ca2+ efflux from fibres with a high [Ca]i (2-5 microM). Nevertheless, ATP depletion or vanadate treatment of high [Ca2+]i fibres causes an approximately 50-fold increase of Ca2+ efflux into Ca2+-containing lithium seawater. These results demonstrate that both vanadate and ATP affect Ca2+ extrusion, including the Na+0-dependent Ca2+ efflux (Na-Ca exchange), in barnacle muscle.

Adenosine Triphosphate↗
PubMed1980

Properties of sodium pumps in internally perfused barnacle muscle fibers.

To study the properties of the Na extrusion mechanism, giant muscle fibers from barnacle (Balanus nubilus) were internally perfused with solutions containing tracer 22Na. In fibers perfused with solutions containing adenosine 5'-triphosphate (ATP) and 30 mM Na, the Na efflux into 10 mM K seawater was approximately 25-30 pmol/cm2.s; 70% of this efflux was blocked by 50-100 microM ouabain, and approximately 30% was blocked by removal of external K. The ouabain-sensitive and K-dependent Na effluxes were abolished by depletion of internal ATP and were sigmoid-shaped functions of the internal Na concentration ([Na]i), with half-maxima at [Na]i approximately or equal to 20 mM. These sigmoid functions fit the Hill equation with Hill coefficients of approximately 3.5. Ouabain depolarized ATP-fueled fibers by 1.5-2 mV ([Na]i greater than or equal to 30 mM) but had very little effect on the membrane potential of ATP-depleted fibers; ATP depletion itself caused a 2-2.5-mV depolarization. When fueled fibers were treated with 3,4-diaminopyridine or Ba2+ (to reduce the K conductance and increase membrane resistance), application of ouabain produced a 4-5 mV depolarization. These results indicate that an electrogenic, ATP-dependent Na-K exchange pump is functional in internally perfused fibers; the internal perfusion technique provides a convenient method for performing transport studies that require good intracellular solute control.

Adenosine Triphosphate↗
PubMedDate not supplied

Voltage-dependent calcium channels from brain incorporated into planar lipid bilayers.

Many important physiological processes, including neurotransmitter release and muscle contraction, are regulated by the concentration of Ca2+ ions in the cell. Levels of cytoplasmic Ca2+ can be elevated by the entry of Ca2+ ions through voltage-dependent channels which are selective for Ca2+, Ba2+ and Sr2+ ions. We have measured currents through single, voltage-dependent calcium channels from rat brain that have been incorporated into planar lipid bilayers. Channel gating was voltage-dependent: membrane depolarization increased the channel open times and decreased the closed times. The channels were selective for divalent cations over monovalent ions. The well-known calcium channel blockers, lanthanum and cadmium, produced a concentration-dependent reduction of the apparent single-channel conductance. Contrary to expectations, the nature of the divalent cation carrying current through the channel affected not only the single-channel conductance, but also the channel open times, with mean open times being shortest for barium.

Animals↗