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D Njus

Publications and source records attributed to D Njus.

At least 37 records · Page 2Linked to original sources
PubMed1985

Electron transfer across posterior pituitary neurosecretory vesicle membranes.

Secretory vesicles from the neurohypophysis have a transmembrane electron carrier very similar to that found in adrenal medullary chromaffin granules. Two different tests show that ascorbic acid contained in the vesicles will reduce an external electron acceptor. First, reduction of cytochrome c or ferricyanide in the medium by a neurosecretory vesicle suspension can be followed spectrophotometrically. Second, the membrane potential (inside positive) generated by electron transfer can be monitored using the membrane potential-sensitive optical probe Oxonol VI. As in chromaffin granules, this electron transfer is probably mediated by cytochrome b561. It may function to regenerate internal ascorbic acid and to provide reducing equivalents needed by the intravesicular amidating enzyme.

Animals↗
PubMed1984

Kinetics of tyramine transport and permeation across chromaffin-vesicle membranes.

Tyramine permeates chromaffin-granule membranes via a reserpine-insensitive mechanism. The rate is unsaturable and increases with pH, indicating permeation of the unprotonated form of the amine. Reserpine-insensitive dopamine uptake is at least 10 times slower, consistent with dopamine's lesser lipophilicity. Dopamine is transported into chromaffin-granule membrane vesicles via a saturable, reserpine-sensitive, proton-linked mechanism. Tyramine inhibits dopamine transport with a Ki of 5-10 microM. Tyramine is not accumulated nearly as well as dopamine because inward transport is opposed by outward permeation. Nevertheless, the velocity of reserpine-sensitive tyramine transport can be deduced from the steady-state level of tyramine accumulation and the rate of permeation. Vmax for tyramine transport is about one-third of the value for dopamine transport. Therefore, two aromatic hydroxyls are not needed for monoamine transport but are required for efficient accumulation and storage.

Adrenal Medulla↗
PubMed1983

A spin-label study of plasma membranes of adrenal chromaffin cells.

Chromaffin-cell membranes were labeled with two nitroxide spin labels, one probing the interior of the membrane and one probing the interfacial region. Both spin labels indicate that the membrane undergoes a phase transition at about 26 degrees C. An Arrhenius plot of acetylcholinesterase activity exhibits a discontinuity at 26 degrees C, consistent with the existence of a phase transition at that temperature. Acetylcholine, which stimulates chromaffin cells to secrete catecholamines, and hexamethonium, a cholinergic blocker, do not affect the rotational correlation times of the spin labels. These results argue that cholinergic stimulation does not affect the fluidity of the chromaffin-cell membrane.

Acetylcholinesterase↗
PubMed1983

Electron transfer across the chromaffin granule membrane.

Membrane vesicles (ghosts) containing ascorbic acid were prepared from bovine chromaffin granules. When ferricyanide or ferricytochrome c were added to the external medium, a membrane potential (interior positive) developed across the ghost membrane. This membrane potential could not be elicited from ascorbate-free ghosts or by ferrocyanide added instead of ferricyanide. These results indicate that the chromaffin-granule membrane has a transmembrane electron carrier with a midpoint potential between that of ascorbate (+85 mV) and that of cytochrome c (+255 mV). The most likely candidate is cytochrome b-561 (+140 mV).

Adrenal Medulla↗
PubMed1983

The chromaffin vesicle and the energetics of storage organelles.

In the chromaffin vesicle, energy for amine transport is provided by a proton-translocating adenosine triphosphatase. The ATPase pumps protons into the vesicle; a pair of protons is then exchanged for each catecholamine taken up. In terms of transport, the adrenal chromaffin vesicle is an important model for non-mitochondrial organelles of all types. These organelles include adrenergic synaptic vesicles, other secretory and neurotransmitter storage vesicles, and lysosomes and other intracellular organelles as well. The membranes of these organelles all possess an ATPase that pumps H+ ions into the vesicles. This proton pump presumably drives the transport of ions and molecules into the organelle and powers other energy-requiring functions of the membrane.

Adenosine Triphosphate↗
PubMed1982

Multiple effects of reserpine on chromaffin-granule in membranes.

The tranquilizer reserpine has several effects on adrenal medullary chromaffin-granule membrane vesicles (ghosts). At low concentrations (0.20 +/0 0.12 nmol/mg of membrane protein), reserpine inhibits proton-linked epinephrine uptake but does not affect transmembrane pH and electrical potential gradients. Reserpine apparently binds to and blocks the catecholamine translocator. At intermediate concentrations (14.3 +/- 4.8 nmol/mg of membrane protein), reserpine abolishes the ATP-dependent enhancement of 8-anilinonaphthalene-1-sulfonate fluorescence without affecting the ATP-dependent membrane potential. At high concentrations (550 +/- 390 nmol/mg of membrane protein), reserpine stimulates the efflux of epinephrine from preloaded chromaffin-granule ghosts. Because it is highly hydrophobic, reserpine partitions into the membrane and probably exerts a nonspecific detergent-like action. At high concentrations (74 +/- 25 nmol/mg of lipid), reserpine also increases the permeability of phospholipid vesicles to epinephrine. The effectiveness of reserpine in inhibiting epinephrine transport correlates with the reserpine/membrane ratio but not with the molar concentration. This may account for the larger variation in reports of effective reserpine concentrations.

Adenosine Triphosphate↗
PubMed1982

Mechanisms of proton-linked monoamine transport in chromaffin granule ghosts.

In bovine chromaffin granules, an inwardly directed H+-translocating ATPase can create either a transmembrane pH gradient (inside acidic) or a transmembrane difference in electrical potential (inside positive). Both the pH gradient delta pH and the membrane potential delta psi can drive monoamine uptake into chromaffin granule membrane vesicles (ghosts). The monoamine concentration gradient is proportional to the square of the [H+] gradient and to exp(F delta psi/RT). This implies that monoamine uptake occurs via a proton antiport or exchange diffusion mechanism with a stoichiometry of two protons per cationic amine.

Adenosine Triphosphate↗
PubMed1981

Stoichiometry of H+-linked dopamine transport in chromaffin granule ghosts.

A proton-translocating adenosinetriphosphatase in adrenal medullary chromaffin granule ghosts can generate either a membrane potential (inside positive) or a pH gradient (inside acid). Dopamine uptake occurs in response to both the membrane potential and the pH gradient. The natural logarithm of the dopamine concentration gradient [In (Din/Dout)] is linearly related to the membrane potential with a slope of F/(RT). This dependence is not affected by the pH of the medium. In (Din/Dout) is linearly dependent on In ([H+]in/[H+]out) with a slope of 2. These results indicate that dopamine is taken up via an exchange diffusion or antiport mechanism. The stoichiometry of this exchange is two H+/dopamine cation and is independent of pH.

Adenosine Triphosphatases↗
PubMed1981

Amine transport in chromaffin granule ghosts. pH dependence implies cationic form is translocated.

Chromaffin granules have a translocator-mediated uptake system for the monoamines dopamine, serotonin, norepinephrine, and epinephrine. These substrates are predominantly cationic at physiological pH but they also exist in neutral, zwitterionic, and anionic forms. The cationic fraction is nearly pH-independent between pH 6.9 and pH 7.6. Over the same pH range, the neutral and zwitterionic fractions increase by a factor of 6.3 and the anionic fraction increases by a factor of 40. In chromaffin granule ghosts, the apparent Km values for dopamine and serotonin transport are independent of pH between 6.8 and 7.6. Consequently, the translocator probably binds the cationic form of the substrate. Vmax values for dopamine and serotonin uptake increase by a factor of 2 between pH 6.8 and pH 7.6.

Adrenal Medulla↗
PubMed1981

Precision of the Gonyaulax circadian clock.

Under constant conditions, the circadian bioluminescent glow rhythm in populations (10(5) cells) of Gonyaulax polyedra is accurate to within 2 min/day. On successive days following the transfer to constant conditions, however, the glow exhibits a progressively broader waveform, implying that individual clocks in the population are drifting out of synchrony. Analysis of the glow waveform suggests that the standard deviation in circadian period among individual clocks is about 18 min and that the period of a given clock varies by less than this from one day to the next.

Animals↗
PubMed1980

Electrogenic epinephrine transport in chromaffin granule ghosts.

An ATP-dependent proton pump drives epinephrine transport in chromaffin granule ghosts. When ghosts are suspended in a medium free of permeant anions, ATP addition leads to an increase in membrane potential (interior positive) and epinephrine uptake but not to a change in intravesicular pH. Since ATP does not affect the pH gradient, the energy for transport must be drawn from the membrane potential (delta psi), and epinephrine uptake must result in a net efflux of positive charge. This can be achieved by an antiport (exchange diffusion) mechanism in which each catecholamine cation is taken up in exchange for more than one H+. Measurements indicate that the stoichiometry is close to 2 H+/epinephrine cation, so the equilibrium epinephrine gradient is theoretically [E]in/[E]out = ([H+]in/[H+]out)2eFdelta psi/(RT). In deenergized ghosts, the epinephrine concentration gradient equals the [H+] gradient. This is consistent with a situation in which the H+ concentration gradient is in equilibrium with the membrane potential as described by the Nernst equation. Then, in the equation above, the membrane potential term (eFdelta psi/(RT)) will exactly cancel one power of the [H+] gradient, leaving [E]in/[E]out equal to [H+]in/[H+]out.

Adrenal Glands↗
PubMed1979

A potassium ion diffusion potential causes adrenaline uptake in chromaffin-granule 'ghosts'.

Membrane vesicles ('ghosts') formed from bovine chromaffin granules accumulate adrenaline in response to a diffusion potential produced by adding K+ in the presence of valinomycin. This uptake occurs as a short (2--5 min) burst because of the transient nature of the diffusion potential. The potential-driven uptake is optimal at pH approximately 7.2, is inhibited by reserpine, and has an initial rate comparable with that of ATP-driven uptake. These results show that ATP-dependent adrenaline uptake may occur at least partly in response to the membrane potential generated by an electrogenic proton-translocating adenosine triphosphatase found in chromaffin-granule membranes.

Animals↗
PubMed1978

Phosphorus-31 nuclear magnetic resonance studies of active proton translocation in chromaffin granules.

ATP hydrolysis and proton translocation in chromaffin granules were followed using 31P nuclear magnetic resonance. The intragranular pH affects the resonance frequency of the gamma-phosphate of granular ATP. By measuring frequency vs. pH in solutions which simulate the intragranular matrix, this may be calibrated to give quantitative pH measurements. The pH in the resting granule is 5.65 +/- 0.15. This drops by 0.4 to 0.5 pH unit when ATP is added externally and protons are actively pumped into the granules. Because of differences in the composition and pH of the internal and external solutions, the resonances of internal and external nucleotides and Pi can be distinguished. Consequently, ATP hydrolysis and changes in internal pH may be observed simultaneously and continuously in a single sample of chromaffin granules. From the measured buffering capacity of a reconstituted intragranular solution, pH changes were converted into an absolute number of protons translocated. The net proton flux (protons translocated/ATP hydrolyzed) was about 1.0 immediately after external ATP addition but fell toward zero as the pH gradient increased to a new steady state. These 31P NMR results agree with intragranular pH measurements determined from methylamine distribution and with H+/ATP stoichiometries calculated from pH changes observed in the external medium.

Adenosine Triphosphatases↗
PubMed1977

Active proton uptake by chromaffin granules: observation by amine distribution and phosphorus-31 nuclear magnetic resonance techniques.

The hydrogen ion activity within isolated chromaffin granules can be estimated from the distribution of the weak base methylamine and from phosphorus-31 nuclear magnetic resonance spectra of ATP contained in the granules. Following the addition of ATP to the external medium, the internal pH drops by 0.2 to 0.5 unit. This change occurs only in medium containing a permeant anion such as chloride and is abolished by an uncoupler of oxidative phosphorylation. These results indicate that the chromaffin granule membrane possess an electrogenic proton pump directed inward.

Adenosine Triphosphate↗