Distribution of secretory granules from adrenal medulla and neurohypophysis on continuous isoosmotic density gradients formed with a new ioxaglic derivative AG-6227

Characterization of newly formed and aged granules in the neurohypophysis

Neurosecretory granules from the rat and bovine neurohypophysis were isolated and some of their biochemical and biophysical properties studied. Neurosecretory granules (NSG) from rat neurohypophysis were labeled, in vivo, with [35S]cysteine and isolated on isoosmotic gradients. Whereas 1 day after labeling most of the radioactivity was found in the lower part of the gradient, 35 days later the isotope was also located in the lighter NSG-containing fraction. Different analytical procedures showed that the lighter fraction, both in bovine and rat NSG, contain more subpopulations of neurophysin-like material than the heavier fraction. The first material to be released during stimulation of secretion, in vivo or in vitro, is mobilized from the heavy NSG. Isolation of rat NSG, at different times during and after dehydration of the animals, reveals that the newly synthesized material is found in the heavy NSG-containing fraction. Furthermore, the results indicate that the newly synthesized NSG are more resistant to lysis than the lighter granules. The results are discussed in relation to the maturation and degradation processes of the granule content and to the functional state of the NSG.

Chromaffin granule membrane-F-actin interactions and spectrin-like protein of subcellular organelles: a possible relationship

The membrane of chromaffin granule, the secretory vesicle of adrenal medullary cells storing catecholamines, enkephalins, and many other components, interacts with F-actin. Using low shear falling ball viscometry to estimate actin binding to membranes, we demonstrated that mitochondrial and plasma membranes from chromaffin cells also provoked large increases in viscosity of F-actin solutions. Mitochondrial membranes also had the capacity to cause complete gelation of F-actin. In addition, vasopressin-containing granules from neurohypophysial tissue were shown to bind F-actin and to increase the viscosity of F-actin solutions. Using an antibody directed against human erythrocyte spectrin, it was found that a spectrin-like protein was associated with secretory granule membrane, mitochondrial membrane, and plasma membrane. The chromaffin granule membrane-associated spectrin-like protein faces the cytoplasmic side, is composed of two subunits (240 kD and 235kD ), the alpha-subunit (240 kD, pHi5 .5) being recognized by the antibody. Nonionic detergents such as Triton X-100 or Nonidet P40 failed to release fully active spectrin-like protein. In contrast, Kyro EOB , a different nonionic detergent, was found to release spectrin-like protein while keeping intact F-actin binding capacity, at least below 0.5% Kyro EOB concentration. Chromaffin cells in culture were stained with antispectrin antibody, showing the presence of spectrin-like protein in the cell periphery close to the cell membrane but also in the cytoplasm. We conclude that in living cells the interaction of F-actin with chromaffin granule membrane spectrin observed in vitro is important in controlling the potential function of secretory vesicles.

Method for quantitating the molecular content of a subcellular organelle: hormone and neurophysin content of newly formed and aged neurosecretory granules

A method is described for the quantitative determination of the content of subcellular organelles such as secretory granules. Purified subcellular fractions of the organelle are prepared and aliquots are assayed for hormones, for example. To determine the number of organelles per fraction, known numbers of latex particles of a size similar to the organelle are added to other aliquots of the subcellular fractions. Latex particles and organelles are then pelleted together by centrifugation. The ratio between latex particles and organelles can be determined by morphometric analysis of ultrathin sections taken through the full thickness of the pellet. The number of organelles and hence their content of the substance assayed can then be calculated. We have applied this technique to posterior pituitary neurosecretory granules, the content of which has already been estimated by a different method. Newly formed neurosecretory granules from oxen and rats were found to have a content of approximately equal to 85,000 molecules of hormone and neurophysin. Aged neurosecretory granules from the same neural lobes appeared to contain less hormone and neurophysin, but this was shown to be the result of loss of material from the granules during isolation in media of 360 mosM. Such loss could be prevented by isolation in hypertonic (660 mosM) media.

Internal pH of isolated newly formed and aged neurohypophysial granules

The pH gradient (delta pH) across the membrane of bovine neurohypophysial granules was estimated by using the [14C]methylamine partition technique. The granule membrane showed high permeability to sugars such as sucrose or sorbitol and to cations. Granules suspended in sucrose medium showed an acidic internal delta pH that decreased with external pH from 1.0 pH unit at pH 7.5 to 0 at pH 4.5. High ionic strength of the external medium destroyed the delta pH, indicating that it originated from a Donnan equilibrium. In a medium constituted to simulate cytoplasm, the delta pH was approximately 0.5 pH unit. Aged neurosecretory granules were more acidic inside than were newly formed granules. The results are discussed in relationship to the nature and the duration of maturation and degradation process of the granule matrix.