Calcium and sulphur in neurosecretory granules and calcium in mitochondria as determined by electron microscope x-ray microanalysis

Calcium loading of secretory granules in stimulated neurohypophysial nerve endings

The total calcium content of secretory granules, Cag, was evaluated in isolated neurohypophysial nerve endings. The Cag in the resting state, as measured by X-ray microanalysis, is relatively high with an average of 7.4 +/- 0.6 mmol/kg wet weight. Following a depolarizing potassium challenge, a subpopulation of granules with even higher Cag could be detected, dispersed over a wider range of concentrations (up to 70 mmol/kg wet weight). After subsequent rinsing in physiological saline, Cag decreased to control values. This could have resulted from Ca2+ extrusion, or from preferential secretion of calcium-enriched granules. Our data can be interpreted in favor of the second explanation since no decrease in Cag was observed when secretion was blocked by a hyperosmotic saline. The effect of hyperosmotic conditions on isolated nerve endings was further studied by monitoring free cytoplasmic Ca2+ with the calcium-sensitive dye Fura-2 and by conventional electron microscopy. It was demonstrated that hyperosmotic treatment alone did not increase basal cytosolic Ca2+ concentrations but did significantly reduce the potassium-induced cytosolic rise in Ca2+. Electron microscopy of nerve endings in hyperosmotic conditions showed numerous exocytotic figures at various stages. The observed changes in Cag are in accord with a published hypothesis which proposes that intragranular calcium is a significant variable in regulated secretion.

Calcium/sodium exchange in purified secretory vesicles from bovine neurohypophyses

Purified secretory vesicles isolated from bovine neurohypophyses take up Na+ under the same circumstances where an efflux of Ca2+ takes place, suggesting a Na+/Ca2+ exchange. Potassium cannot substitute for Na+ in this process. Also, a Ca2+/Ca2+ exchange can occur. Inhibiting the latter process by Mg2+ allowed to estimate an apparent KM of 0.7 microM free Ca2+ and a maximal uptake of 1.5 nmol X mg protein-1 X min-1 Ca2+ in exchange for Na+. The vesicles did not contain plasma membrane marker (Na+/K+ ATPase) as shown by distribution analyses on the density gradients on which they were purified. Similarly, distribution studies also showed that no other ATPase activity could be detected in the purified vesicle fraction. It is concluded that a Na+/Ca2+ exchange is operating across the secretory vesicle membrane and that it is not directly dependent on ATP hydrolysis.

Ca2+ uptake to purified secretory vesicles from bovine neurohypophyses

Purified secretory vesicles isolated from bovine neurohypophyses were found to take up Ca2+ when incubated at 30 degrees C in media containing 10(-7) to 10(-4) M free Ca2+. At 10(-4) free Ca2+ 19 nmol/mg protein were taken up within 30 min. The initial uptake at this Ca2+ concentration was about 2 nmol/mg protein per min. The uptake of Ca2+ to secretory vesicles was not affected by ATP, oligomycin, ruthenium red, trifluoperazine, Mg2+ or K+, but was inhibited by Na+ and Sr2+. From these characteristics it can be concluded that the uptake system does not utilize directly ATP (as the Ca2+-ATPases known to be present in the cell membrane and the endoplasmic reticulum) and is different from the mitochondrial Ca2+ uptake system driven by respiration and/or ATP hydrolysis. However, Ca2+-Na+ exchange may well operate: In experiments using different concentrations of Na+ we found half-maximal inhibition of Ca2+ uptake with 33.3 mM Na+. An analysis of the data in a Hill plot indicated that at least 2 Na+ would be exchanged for 1 Ca2+. Also, it was found that Ca2+ previously taken up could be released again by external Na+ but not by K+.

Ultrastructural dynamics of exocytosis in the ovulation-neurohormone producing caudo-dorsal cells of the freshwater snail Lymnaea stagnalis (L.)

The ultrastructural dynamics of exocytosis in the ovulation-stimulating neurosecretory Caudo-Dorsal Cells (CDC) of the freshwater snail L. stagnalis were studied after incubation of cerebral ganglia in Ringer's solutions with different concentrations of K+ and Ca2+. Detection of exocytosis was facilitated by the use of the tannic acid-glutaraldehyde fixation method (TAGO-method). In control Ringer (low K+) the frequency of exocytosis was rather low. Exocytosis mainly occurred as "terminal" exocytosis (TE); "intracellular" (ICE) and, particularly, "multiple" exocytosis (ME) took place infrequently. Incubation in high K+-containing Ringer strongly increased exocytotic activity. Compared to the controls the total frequency of exocytosis was 50 X as high, whereas TE, ICE and ME occurred 6 X, 47 X, and more than 300 X as frequently, respectively. In high K+/Ca2+-free Ringer the total frequency of exocytosis was only 2 X as high as in control Ringer. It is concluded that TE, ICE, and ME are normal, Ca2+-dependent exocytotic phenomena. The significance of their dynamics in response to K+-stimulation is discussed. The extremely high frequency of exocytosis, as well as the presence of "unaltered granule contents in transit", is explained by assuming that an exocytotic event in the CDC lasts rather long, viz. some minutes. The results may reflect the physiological mechanism by which the CDC release their ovulation hormone. The possible involvement of "clear" and "large" electron lucent vesicles in membrane reuptake after exocytosis is considered.

Energy-dispersive X-ray analysis of the mitochondria of sideroblastic anaemia

Energy-dispersive X-ray analysis has been performed on marrow sideroblasts obtained from 10 patients with sideroblastic anaemias or erythroleukaemia (six primary refractory sideroblastic anaemia, two pyridoxine-responsive, one secondary sideroblastic anaemia, two erythroleukaemia). Irrespective of the nature of the disorder associated with the presence of sideroblasts, X-ray analysis of siderotic mitochondria consistently revealed the presence of iron and phosphorus with the average Fe/P intensity ratio measuring 1.4-1.5. Other elements variably detected within siderotic mitochondria included calcium, lead, potassium and zinc. Variation in the presence of these latter elements was detected not only between different patients, but also within different samples taken at different times from a single patient and even among different cells of the same sample. Despite the detection of lead in siderotic mitochondria of a significant number of patients (five out of seven), there was no clinical evidence of lead toxicity. The elemental composition of the intramitochondrial deposits in sideroblasts was distinct from that of ferritin or haemosiderin and probably consists of ferric phosphate, possibly, ferric orthophosphate (FePO4).

Effect of measured calcium chloride injections on the membrane potential and internal pH of snail neurones

1. Ion-sensitive micro-electrodes were used to measure changes in intracellular pH (pHi) and internal chloride which resulted from the pressure injection of calcium chloride into identified Helix aspersa neurones. The internal chloride measurement allowed the quantity of calcium chloride injected to be estimated. 2. Application of the metabolic inhibitor carbonyl cyanide m-chlorophenyl hydrazone (CCmP) to a calcium-loaded cell caused an increase in the membrane potential comparable to the effect of injecting calcium itself. The effect was not observed in normal cells. 3. When injected in the presence of CCmp, calcium caused a much larger and longer-lasting effect on the membrane potential than that observed in untreated cells. 4. The injection of ruthenium red can increase and/or prolong the hyperpolarization caused by a given quantity of injected calcium. The pHi changes following calcium injection were biphasic and slower than normal. 5. In seven experiments, both hydrogen chloride and calcium chloride were injected into the same cell. The relative changes in pHi corresponded to the production of one hydrogen ion for each calcium ion injected. 6. The relationship between the quantity of calcium injected and the size of the induced hyperpolarization suggested that at least three calcium ions acting cooperatively are required to activate a potassium 'channel'. 7. Injection of barium chloride hyperpolarized the membrane after a delay of about 1 min. After several injections of barium the cell lost this response while retaining its normal response to calcium injection. Injection of barium also caused a slowly developing (biphasic) fall in pHi. 8. We conclude that injected calcium is normally rapidly taken up by mitochondria in exchange for hydrogen ions. If this uptake process is blocked by ruthenium red or CCmP, the calcium is taken up by a second, slower, process which also releases hydrogen ions. When pre-loaded, but not otherwise, the mitochondria will release calcium ions on treatment with CCmP. Injection of barium does not directly affect the membrane conductance, but causes the release of calcium from intracellular binding sites.