Search PubMed⌕ Search

Biomedical subjects

J Meldolesi

Publications and source records attributed to J Meldolesi.

At least 217 records · Page 12Linked to original sources
PubMed1976

Dynamic changes of the luminal plasmalemma in stimulated parotid acinar cells. A freeze-fracture study.

In the acinar cells of the rat parotid gland the two membranes participating in exocytosis, i.e., the luminal plasmalemma and the secretory granule membrane, are clearly distinguishable in freeze-fracture because of their different densities in particles. In order to obtain point-specific information about the fusion-fission of these two membranes that occurs during the secretory cycle, glands were studied at various times (5 min to 6 h) after stimulation with isoproterenol. We observed that, in the course of the release of secretion products and shortly afterwards, the enlarged luminal plasmalemma exhibits a mosaic organization consisting of an alternation of membrane patches of high (original plasmalemma) and low (fused granule membrane) particle density. The transition between these two patterns is usually sharp. Later, concomitant with the reformation of acinar canaliculi, the low particle density membrane is found at the cell surface but only bounding vacuolar infoldings, and then it finally disappears. These results suggest that (a) fusion of these membranes does not result in a random intermixing of the molecular components of the participating membranes, which retain their structural identity; and (b) the enlarged luminal plasmalemma reverts to its original size by a progressive, specific removal of the regions of low particle density from the cell surface.

Animals↗
PubMed1976

Liver protection by silymarin: in vitro effect on dissociated rat hepatocytes.

The liver-protective effects of silymarin have been investigated in isolated hepatocytes suspended in vitro in sucrose solutions of graded hypotonicity. Cells preincubated with the drug at the concentration of 10(-4) M were found to be entirely resistant or relatively little affected when exposed to hypotonic solutions which have a lethal effect on a considerable proportion of the control liver cells, as shown by the determination in the cell preparations (1) of the proportion trypan blue-positive (broken) cells, (2) of the protein synthesizing capacity. The protection was reduced but not abolished upon removal of the drug and cell washing. A smaller effect was observed with a drug concentration of 10(-5) M. At 10(-6) M the effect was no longer detectable. Chlorpromazine, a drug which belongs to the group of the so called "unspecific membrane stabilizers", was without effect in this test.

Animals↗
PubMed1975

Molecular organization of rat prolactin granules. I. In vitro stability of intact and "membraneless" granules.

Studies carried out on a number of secretory cell systems suggest that the specific cytoplasmic granules in which the secretion products are stored before their release are complex organelles which can possess a distinct molecular organization. For instance, it has been reported that in some granules the segregated secretion products are organized into crystalline structures (1-3) or large intermolecular aggregates (4-8). It is likely that all phenomena of this type are favorable to the economy of the cell, in the sense that they reduce the energy required for storage of the secretion products. The prolactin (LTH) granules of the rat pituitary possess a number of morphological features which strongly suggest that the molecules(s) of their content might be arranged in a relatively stable structure. Thus, these granules are remarkably polymorphic in shape, and their membrane is usually separated from their content by a clear space. Furthermore, identifiable LTH granules devoid of their membrane are often seen in the pericapillary space, suggesting that upon discharge by exocytosis they are dissolved only slowly (9). However, no studies specifically concerned with the mechanisms of LTH storage have been reported so far. In order to obtain some information on this question, we have studied the behavior of isolated granule fractions incubated in vitro under a variety of carefully controlled experimental conditions.

Animals↗
PubMed1975

Calcium and pancreatic secretion. I. Subcellular distribution of calcium and magnesium in the exocrine pancreas of the guinea pig.

The distribution of calcium and magnesium has been studied in the acinar cells of the pancreas of the guinea pig. Most of the magnesium was found to be associated with the rough microsomes (probably bound to the ribosomes) and with the postmicrosomal supernate. In contrast, calcium was distributed among all the particulate fractions, primarily the mitochondria, microsomes (especially smooth surfaced), zymogen granules, and the plasmalemma, and was low in the postmicrosomal supernate. Most of the calcium recovered in the particulate fractions was found to be membrane bound. The highest concentrations were found in the membranes of the zymogen granules and in the plasmalemma. By means of control experiments using -45Ca as the tracer, it was established that a considerable redistribution of calcium occurs during homogenization and cell fractionation. At least some of the resulting artifacts were estimated quantitatively and the data were corrected accordingly. The biochemical results were confirmed with the cytochemical antimonate technique carried out on the tissue as well as on isolated fractions. The role of calcium associated with the zymogen granules and with their limiting membranes is discussed in relation to the architecture of the granule and to the functionality of the pancreatic juice.

Animals↗
PubMed1975

Calcium and pancreatic secretion-dynamics of subcellur calcium pools in resting and stimulated acinar cells.

1 Pulse-chase experiments were carried out on pancreatic tissue lobules incubated in vitro, with 45Ca as the tracer, in order to shed some light on the functional significance of the calcium pools associated with the various cell organelles of the acinar cell, especially in relation to stimulus-secretion coupling. 2 The kinetics of tracer uptake and release which were observed in the intact lobules suggest the existence of a number of intracellular pools, whose rate of exchange is slower than that across teh plasmalemma. 3 The various subcellular fractions accumulate the tracer in different amounts: some (rough microsomes and postmicrosomal supernatant) showed little radioactivity and some (smooth microsomes and zymogen granule membranes) were heavily labelled; mitochondria and zymogen granules showed intermediate values. 4 The fractions are heterogeneous also in relation to the time course of uptake and release of the tracer: in rough and smooth microsomes and, especially, in the postmicrosomal supernatant both rates were fast; zymogen granules and zymogen granule membranes showed slow rates of uptake and little release during chase; intermediate rates were found in mitochondria. 5 In agreement with previous findings we observed that in 45Ca preloaded lobules, stimulation of secretion (brought about by the secretagogue polypeptide caerulein) results in an increase of the tracer release which seems to be due primarily to the rise of the intracellular concentration of free Ca2+ and to the consequent increase of the transmembrane Ca2+ efflux. Among the cell fractions isolated from stimulated lobules only the mitochondria exhibited a significantly lower 45Ca level relative to the unstimulated controls. 6 It is concluded that, of the organelle-bound calcium pools, that associated with the mitochondria might be involved in the regulation of the calcium-dependent functions, including stimulus-secretion coupling; the calcium associated with the zymogen granule content probably has a role in the architecture of the organelle and in the functionality of the pancreatic juice, while the calcium bound to the membrane of the granules might be concerned with the regulation of its permeability properties.

Animals↗
PubMed1975

Secretion of calcium in pancreatic juice.

1. The orgin of the calcium secreted by the pancreas has been investigated in vivo in the guinea-pig by a study carried out in parallel (a) in the juice secreted in response to the injection of either secretin or caerulein and (b) in the pancreatic tissue and in cell fractions isolated thereform. 2. In agreement with previous findings we observed that the concentration of calcium is low in the secretin-stimulated and high in the caerulein-stimulated juice. In the latter calcium and protein are proportional (cal0 n-mole:mg). 3. After I.V. injection of 45Ca the radioactivity decreases rapidly and quasi-exponentially in the blood plasma. A roughly parallel time course is found in the secretin-stimulated juice: the evolution of the juice: plasma radioactivity ratio resembles that observed with the extraceullar space marker [3H]D-sorbitol. In contrast, the time course of 45Ca in plasma and caerulein-stimulated juice are not proportional: the high levels characteristic of this juice are reached several minutes after the injection and maintained thereafter. This increase is followed ca. 50 min later by the appearance of the newly synthesized [3H]L-leucine-labelled proteins. 4. The pancreatic tissue is rich in calcium which is localized primarily in zymogen granules (Ca.36 n-mole:mg protein) and mitochondria; the soluble cytoplasm is low in calcium. 5. The injected 45Ca accumulates in zymogen granules faster than [3H]L-leucine-labelled proteins. The 45Ca:protein ratio of these organelles is considerably lower than that of the caerulein-stimulated juice. 6. It is concluded (a) that calcium is secreted in to the pancreatic juice in two fractions, one (possibly released by simple diffusion) associated with the electrolyte component, the other with protein of the juice, (b) that zymogen granules are the major, but not the only source of the latter fraction, and (c) that the zymogen granule-associated calcium joins the exportable proteins some time after their synthesis, possibly in the Golgi complex and/or in the condensing vacuoles.

Animals↗
PubMed1974

Dynamics of cytoplasmic membranes in guinea pig pancreatic acinar cells. I. Synthesis and turnover of membrane proteins.

The rate of synthesis and the turnover of cytoplasmic membrane proteins were determined in the acinar cells of guinea pig pancreas with the aim of investigating the mechanisms by which the intracellular transport of secretion products occurs. These cells are highly specialized toward protein secretion. By means of in vitro pulse-chase experiments and in vivo double-labeling experiments, using radioactive L-leucine as the tracer, it was found that the turnover of secretory proteins is much faster than that of all membranes involved in their transport (rough and smooth microsome and zymogen granule membranes). Sodium dodecyl sulfate-polyacrylamide disk gel electrophoresis of membrane proteins revealed that in each of these membranes there is a marked heterogeneity of turnover; generally the high molecular weight polypeptides have a shorter half-life than the low molecular weight polypeptides. These data indicate that the membranes participating in the intracellular transport of secretory proteins are not synthesized concomitantly with the latter. Rather, they are probably reutilized in several successive secretory cycles. The possible relevance of these findings to other secretory systems is discussed.

Animals↗