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N A Thorn

Publications and source records attributed to N A Thorn.

11 recordsLinked to original sources
PubMed1979

ATP-induced release of vasopressin from isolated bovine neurohypophyseal secretory granules. Dependency on chloride and effects of analogues of ATP.

Secretory granules isolated from bovine neurohypophyses released vasopressin in the presence of a buffered medium containing ATP, Mg2+ and KCl. Substitution of K+ in the medium with Na+ or choline did not affect the release. Substitution of Cl- with either sucrose, sulphate or acetate strongly reduced the release. Analogues of ATP, substituted at the beta-gamma anhydride bond with methylene or imido groups caused a smaller release which was not related to a very small breakdown of analogues that occurred. It is suggested that at least part of the ATP induced release is due to a physicochemical action.

Adenosine Triphosphate↗
PubMed1978

ATP-induced release of vasopressin associated with phosphorylation of isolated bovine neurohypophyseal secretory granule membranes.

Secretory granules isolated from ox neurohypophyses released their content of vasopressin in the presence of ATP and Mg2+. A half maximal ATP concentration of 0.25 mM was found. Ca2+ was not necessary for the effect. High concentrations of ADP, AMP and ITP were shown to mimic the effect of ATP. Utilizing this effect of ATP combined with iodonitrotetrazolium treatment to make mitochondria heavier, a method is described to obtain granule "ghosts" in a purified form. They were shown to be phosphorylated when granules were incubated with [gamma-32P] ATP.

Adenosine Triphosphate↗
PubMed1977

Fusion of neurohypophyseal membranes in vitro.

Freeze cleaving electron microscopy has shown that fusion of isolated secretory vesicles from bovine neurohypophyses was induced by Ca2+ in micromolar concentrations. Mg2+ and Sr2+ were ineffective. Mg2+ inhibited Ca2+-induced fusion. In suspensions containing secretory vesicles as well as sheets of cell membrane, release of vasopressin parallel to intervesicular fusion and fusion of secretory vesicles with sheets of cell membrane was observed after exposure to Ca2+. Mg2+ and Sr2+ were ineffective in replacing Ca2+ as trigger for fusion or vasopressin release. Intervesicular fusion and exocytotic profiles were observed when isolated neurohypophyses or neurosecretosomes were exposed to cold.

Animals↗
PubMed1977

Isolation and purification of calcium-binding proteins from bovine neurohypophyses.

An acidic calcium-binding protein was isolated from the soluble fraction of the homogenate of ox neurohypophyses. The protein has a molecular weight of 35 000 and a subunit weight of 15 000. The purification procedure involved ammonium sulphate fractionation, DEAE-cellulose chromatography and gel filtration on Sephadex G-100 and Sephadex G-50. Conventional and sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated it to be a protein distinct from the S-100 protein and the soluble hormone-binding proteins (neurophysins) abundant in the neurohypophysis. This appears to be the only Ca2+-binding protein in the soluble part of the homogenate, with an apparent Kdiss for Ca2+ of 1.1 X 10(-5) M (at 22 degrees C) and a binding capacity of 2 mol of calcium per mol of protein. Two different Ca2+-binding proteins of molecular weights 16 500 and 68 000, respectively, were identified in the sodium-deoxycholate-soluble proteins from an ox neurohypophysial microsome fraction. One of them (the former) has been isolated in high purity by DEAE-cellulose chromatography and gel filtration on Sephadex G-200. This protein binds 4 mol of calcium per mol of protein with an apparent Kdiss of 1.0 X 10(-5) M (at 22 degrees C). The sodium-deoxycholate-insoluble proteins from the microsomal fraction also have Ca2+-binding components. The soluble Ca2+-binding protein has properties similar to and may be identical to Ca2+-binding proteins which have been isolated from bovine brain and have been demonstrated to be modulators of brain cyclic nucleotide phosphodiesterase and of actinomyosin ATPase. It also resembles Ca2+-binding proteins isolated from bovine adrenals and the electroplax from electrophorus electricus.

Animals↗
PubMed1976

Calcium and stimulus secretion coupling in the neurohypophysis. V. The effects of the Ca2+ ionophores A23187 and X537 A on vasopressin release and 45Ca2+ efflux; interactions with sodium and a verapamil analogue (D600).

Slices from ox neurohypophyses were incubated in a calcium-free medium with the ionophores A23187 or X537A. X537A (5 X 10(-5) mol/l) caused a marked release of vasopressin, neurophysin and protein to the medium. A23187 (2 X 10(-5) mol/l) did not cause any release by itself, but when Ca2+ was added to the medium in the presence of the ionophore, an increase in the release of vasopressin, neuorphysin and protein occurred. Release of lactate dehydrogenase and peptidase were not affected by the ionophores. The secretion caused by A23187 was abolished by D600 (a verapamil analogue) (2 X 10(-5) mol/l) whereas the effect of X537A was unchanged. The effects of X537A were strongly inhibited by removal of sodium from the medium. Re-addition of sodium to the medium caused a marked release. Gramicidin (10(-6) or 5 X 10(-5) mol/l) had no effect on secretion. Efflux of 45Ca2+ from pre-loaded slices was drastically reduced in a sodium-free medium. X537A caused an increase in the efflux rate of 45Ca2+ both in medium with a normal concentration of sodium and when slices had been incubated in a sodium-free medium. A23187 and X537A both released 45Ca2+ from a neurohypophyseal mitochondrial fraction. When sodium in a concentration of 20 mmol/l was added to this fraction, the Ca2+ accumulation was inhibited. This effect was reduced by inorganic phosphate up to a concentration of 2 mmol/l.

Animals↗
PubMed1975

Adenosine triphosphate dependent calcium uptake by subcellular fractions from bovine neurohypophyses.

Bovine neurohypophyses were fractionated by differential and density gradient ultracentrifugation and the Ca-2+ uptake and ATPase activities in the microsomal, mitochondrial and secretory granule fractions were studied. The microsomal and mitochondrial fractions accumulated Ca-2+ in the presence of ATP. The accumulation by the latter per mg protein was at least twice as large as by the former. This Ca2+ accumulation was accompanied by liberation of inorganic phosphate (Pi). In the presence of sodium azide (2 mM) Ca-2+ uptake and Pi liberation were inhibited in the mitochondrial, but not in the microsomal fraction. Further studies of the microsomal fractions revealed that the ATP-dependent Ca-2+ uptake and Pi liberation activities were temperature and pH-dependent and required Mg-2+. Both activities were stimulated by very low concentrations of Ca-2+ (1-10 muM) and were inhibited by EGTA (2 mM). N-ethylmaleimide (2 mM) inhibited both the Ca-2+ uptake and ATPase activities of the microsomal fraction. These results suggest the presence of a membrane ATPase that is stimulated by both Ca-2+ and Mg-2+. It is suggested that the observed Ca-2+ uptake activities are involved in maintaining a low axoplasmic free Ca-2+ concentration, thus playing an important role in the release mechanism of vasopressin by the neurosecretory terminals.

Adenosine Triphosphatases↗
PubMed1975

Effects of calcium and sodium on vasopressin release in vitro induced by a prolonged potassium stimulation.

Groups of isolated hemilobes of rat neurohypophyses were stimulated for 90 min with a 56 mM K+ concentration in the surrounding medium. The rate of release of vasopressin into the medium showed an early maximum followed by a long phase of decline which lasted for most of the stimulation period. When 3 min collection periods were employed, higher secretion rates were obtained than when using 10 min periods. This may be due to a higher rate of removal of vasopressin from the tissue and a lower vasopressin inactivation when 3 min periods are used. When Na+ was replaced by sucrose in the incubation medium for the whole stimulation period, much higher secretion rates than previously reported were obtained (13% of the total hormone content in the first 30 min period and 21% for the total 90 min period). When Na+ was removed from the incubation medium late in the stimulation period, a new high secretion rate was induced, provided that extracellular calcium was present. When Ca-2+ was removed from the medium 3 min after start of the stimulation with 56 mM K+, the secretion rate fell very rapidly. Reintroduction of a normal Ca-2+ concentration caused a new marked increase in secretion rate. These results further stress the importance of extra-cellular calcium for release of vasopressin. In addition they show that a considerable fraction of the vasopressin present in the neurohypophysis can be mobilized during a continued strong stimulation if sodium is omitted from the medium.

Animals↗