Discharge and restitution of secretory material in the rat parotid gland in response to isoproterenol

Localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) α, β, γ in the three major salivary glands in situ of mice and their response to β-adrenoceptor stimulation

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K), which is composed of three isozymes (α, β and γ), catalyzes the production of phosphatidylinositol bisphosphate (PIP2). This phospholipid functions in membrane trafficking, as an anchor for actin cytoskeletons and as a regulator of intramembranous channels/transporters. It is also a precursor of such second messengers as diacylglycerol, inositol triphosphate and phosphatidylinositol (3,4,5)-triphosphate. In the present study, the expression and localization of endogenous PIP5Ks were examined in the three major salivary glands of young adult mice in situ. In western blotting of normal control glands, immunoreactive bands for individual PIP5Ks were detectable, with the highest density in the parotid gland and the weakest density in the submandibular gland. In immuno-light microscopy under non-stimulated condition, weak immunoreactivity for PIP5Kα was confined to the apical plasmalemma in parotid, but not sublingual or submandibular, acinar cells. Immunoreactivity for PIP5Kβ was weak to moderate and confined to ductal cells but not acinar cells, whereas that for PIP5Kγ was selectively and intensely detected in myoepithelial cells but not acinar cells, and it was weak in ductal cells in the three glands. In western blot of the parotid gland stimulated by isoproterenol, a β-adrenoceptor agonist, no changes were seen in the intensity of immunoreactive bands for any of the PIP5Ks. In contrast, in immuno-light microscopy, the apical immunoreactivity for PIP5Kα in parotid acinar cells was transiently and distinctly increased after the stimulation. The increased immunoreactivity was ultrastructurally localized on most apical microvilli and along contiguous plasma membrane, where membranous invaginations of various shapes and small vesicles were frequently found. It was thus suggested that PIP5Kα is involved in post-exocytotic membrane dynamics via microvillous membranes. The present finding further suggests that each of the three isoforms of PIP5K functions through its product PIP2 discretely in different cells of the glands to regulate saliva secretion.

Stereological study of acinar growth in the rat parotid gland induced by isoproterenol

The growth of the rat parotid gland induced by daily treatment with isoproterenol (IPR) for 2 weeks was investigated by stereological methods applied to light microscopy. After 7 days of treatment, the glandular mass presented a 286% growth, with the first 3 days being the period of greatest growth. Total acinar volume exhibited a 363% increase during the period from 0 to 7 days, while acinar-cell volume presented a 468% growth from 0 to 5 days of treatment. On the other hand, total acinar-cell number did not increase during the study period. Thus, under the conditions used, IPR-stimulated gland growth was essentially hypertrophic. However, a significant increase in the number of bipolar and multipolar mitoses was also observed, especially on the third and fifth days of treatment. As no increase in acinar-cell number occurred during growth, the presence of these mitoses suggests that cell death occurred during gland growth. On this basis, bipolar mitoses may occur to replace cells that probably degenerated during treatment, whereas multipolar mitoses may lead to the occurrence of polyploidy.

A review of electron probe X-ray microanalysis studies of salivary gland cells

Electron probe X-ray microanalysis (EPXMA) has now been successfully applied to several salivary gland preparations. This paper briefly reviews the principles underlying this technique and the specific sample preparation procedures which permit accurate measurement of elemental concentrations in the various intracellular spaces. Findings from salivary gland studies indicate that cytoplasmic and nuclear spaces of nonstimulated acinar cells have high concentrations of K and P, and low concentrations of Mg, Ca, and S; and that mature secretory granules have high concentrations of Ca and S, and relatively low concentrations of K and P. No consistent differences have been found between the elemental concentrations of mucous and serous secretory granules. In vivo and in vitro EPXMA studies of the elemental changes associated with secretory granule maturation indicate there are at least two stages in this process: an early stage during which granule S concentration increases in parallel with mass density as condensing vacuoles mature into secretory granules, and a late stage during which granule mass density and protein content increase with no further elemental concentration changes. Findings from other in vivo and in vitro studies indicate that secretory granule membranes are permeable to Na, K, and Cl ions because the granular concentrations of these elements are altered by electrochemical gradients. Recent EPXMA results indicate that cells stimulated with parasympathomimetic agonists have decreased K and Cl concentrations, and increased Na concentrations. Furthermore, the magnitude of these changes are quantitatively consistent with changes measured using radio-isotope equilibration and other techniques. In contrast, cells stimulated with the beta-adrenergic agonist, isoproterenol, have increased concentrations of Na and Cl, but unchanged K concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Microprobe analysis of maturation-related elemental changes in rat parotid secretory granules

Electron probe X-ray microanalysis was use to quantitate the elemental and mass changes that take place during the secretory granule maturation process. A single injection of isoproterenol stimulated the depletion of secretory granules from rat parotid acinar cells. Granules at different stages of maturation were analyzed as they reaccumulated within the cells over time. Dry mass measurements revealed that secretory material becomes concentrated about twofold within maturing granules. Nearly all of the increase in mass concentration could be attributed to a reduction in water space. Data are presented that indicate that Na, K, Cl, and water all efflux from secretory granules during maturation. In contrast, granule S content is positively correlated with maturation. Hence, significant changes in granule elemental and water contents occur during the maturation process.

Maturation-related changes in mass and elemental contents of secretory granules as measured by electron-microprobe

The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P less than 0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures.

Effects of secretory stimulation on the Golgi apparatus and GERL of rat tracheal gland cells

The morphological and cytochemical changes of the Golgi apparatus and GERL of rat tracheal serous cells were studied after in vivo secretory stimulation with pilocarpine. Discharge of PAS-positive secretory granules had started within 30 min and complete discharge was finished within 2 h after stimulation. Reaccumulation of secretory granules became evident at 6 h and the volume of granules was similar to that of unstimulated control cells at 8 h. At 6 h after pilocarpine administration, acid phosphatase (AcPase) activity was markedly increased in GERL and immature secretory granules as compared to uninjected controls and this phenomenon continued 12 h after stimulation. GERL appeared to increase in extent and numerous continuities with immature granules were observed. Thiamine pyrophosphatase (TPPase) activity, normally present in trans Golgi saccules was not altered during the secretory cycle. At 2 h after stimulation, tubular structures of GERL, which also react to AcPase, were seldom observed. At 6 h, these structures were more numerous than those of control cells. These results demonstrate that, in rat tracheal serous cells, pilocarpine induces alteration of only GERL, and reconstitution followed by resynthesis of secretory granules is performed without participation of the Golgi stack.

In vivo effects of tunicamycin on the secretory processes of rat parotid glands

The morphological and functional effects of tunicamycin were studied in rat parotid glands at the stage of the reformation of secretory granules following secretory stimulation by isoproterenol. Tunicamycin inhibited the incorporation of (3H)-mannose into the acid-insoluble fraction but had no effect on total protein synthesis as determined by the incorporation of (14C)-leucine. Thus the administration of tunicamycin in vivo inhibits the synthesis of mannose-rich glycoproteins in a manner similar to that in an in vitro system. The ultrastructure of the acinar cell showed little change following treatment with this drug, except that the number of reaccumulated secretory granules was greater than in the control. Amylase secretion stimulated by isoproterenol was inhibited in tunicamycin-treated cells, but did not decrease following treatment with N6,2'-O-dibutyryladenosine 3'-5'-cyclic monophosphate, a secretory stimulator bypassing the beta-receptor. A radio-receptor assay using (3H)-dihydroalprenolol and direct localization using the fluorescent beta-adrenergic blocker 9-amino-acridin-propranolol showed a marked reduction in the binding activity of beta-receptor following treatment with tunicamycin. Thus the inhibition of N-linked glycosylation appears to produce profound effects on the beta-adrenergic receptor-adenylate cyclase complex of acinar cells, although the steps of the transport and the exocytotic discharge of secretory materials are not affected.

Changes in the polypeptide composition of mouse parotid glands after stimulation of secretion and DNA synthesis by isoproterenol

The polypeptide composition of mouse parotid glands has been analysed by unidimensional SDS-polyacrylamide slab gel electrophoresis and Coomassie blue staining after isoproterenol stimulation of secretion and DNA synthesis. Two polypeptides (polypeptides A and B) are lost within 2 h and their restoration in the glands occurs according to a chronology which is identical to that of the alpha-amylase activity. On the other hand, five clearly defined new bands appear consistently during the late prereplicative period of isoproterenol-stimulated mouse parotid acinar cells (polypeptides C, D, E, F and G). These new polypeptides are induced by doses of isoproterenol which provoke secretion and DNA synthesis, but not by doses which provoke only secretion. Although no function has been assigned to any of the above-described polypeptides, a relation between polypeptides A and B and secretion and between polypeptides C, D, E, F and G and the proliferative response is suggested.

Ultrastructure of the secretory response of male mouse submandibular gland granular tubules

The organization and fine structure of granular convoluted tubule cells (GCT) from male mouse submandibular glands have been examined in controls and in animals injected with adrenergic and cholinergic secretagogues. Control submandibular glands exhibited a single population of GCT cells with numerous homogeneous granules filling the apical two-thirds of the cytoplasm. A zone of transition cells, exhibiting characteristics of both GCT and striated duct cells, was found between the agranular intercalated duct and GCT segments. These transition cells possessed apical granules of variable size as well as prominent basal striations. Dramatic changes in the morphology of GCT cells followed administration of the alpha-adrenergic agent, phenylephrine. The extensive degranulation involved formation of "secretory pools" of fused granules and release of secretory material into the lumen. The appearance of numerous smooth vesicles near luminal membranes suggested extensive membrane retrieval. Intracellular membrane-limited aggregates of membrane fragments suggested that much of the retrieved membrane was destined for degradation. Rough endoplasmic reticulum was highly dilated but there was no indication of increased size or activity of the Golgi complex. Ultrastructural evidence indicated that the secretory responses to isoproterenol, a beta-adrenergic agent, and to pilocarpine, a cholinergic agent, were much more modest, but it is clear that some secretory response to these agents does occur. The other cell types of the duct and tubule system did not exhibit comparable morphological changes in response to the agents used.

Alteration of tight junctional permeability in the rat parotid gland after isoproterenol stimulation

The permeability of junctional complexes to ultrastructural tracers of different molecular weight and the freeze-fracture appearance of junctional structure were investigated in the resting and stimulated rat parotid gland. Tracers were administered retrogradely via the main excretory duct, and allowed to flow by gravity (16 mmHg) into the gland for 15-60 min. Secretion was induced in some animals by intraperitoneal injection of isoproterenol. In resting glands, the tracers microperoxidase , cytochrome c, myoglobin, tyrosinase (subunits), and hemoglobin were restricted to the luminal space of the acini and ducts. In glands stimulated 1-4 h before tracer administration, reaction product for microperoxidase , cytochrome c, myoglobin, and tyrosinase was found in the intercellular and interstitial spaces, whereas hemoglobin was usually retained in the lumina. In contrast, horseradish peroxidase and lactoperoxidase appeared to penetrate the tight junctions and reaction product was localized in the extracellular spaces in both resting and stimulated glands. Diffuse cytoplasmic staining for horseradish peroxidase and lactoperoxidase was frequently observed in acinar and duct cells. The distribution of horseradish peroxidase was similar in both Sprague-Dawley and Wistar-Furth rats, and at concentrations of 0.1-10 mg/ml in the tracer solution. Freeze-fracture replicas of stimulated acinar cells revealed an increased irregularity of the tight junction meshwork, but no obvious gaps or discontinuities were observed. These findings indicate that (a) tight junctions in the resting rat parotid gland are impermeable to tracers of molecular weight greater than or equal to 1,900; (b) stimulation with isoproterenol results in a transient increase in junctional permeability allowing passage of tracers of molecular weight less than or equal to 34,500; (c) junctional permeability cannot be directly correlated with junctional structure; and (d) the behavior of horseradish peroxidase and lactoperoxidase in the rat parotid gland is inconsistent with their molecular weights. Cell membrane damage due to the enzymatic activity or binding of these two tracers may account for the observed distribution.

Microheterogeneity of rat parotid gland proteins after chronic treatment with isoproterenol

Two ultrasensitive methods were employed to determine the effect of repeated in vivo stimulation with isoproterenol on the protein composition of parotid acinar cells. Silver staining showed electrophoretic banding pattern changes in a wide range of cellular proteins during and after multiple injections of the beta-adrenergic agonist. Photo-affinity labeling with [32P]-N3-cyclic AMP showed the disappearance and eventual return (after discontinuation of the injections) of the cellular cyclic AMP-binding proteins as measurable cellular components. These findings indicate that major translation-associated changes may occur in target cell protein composition both during and after hormonal stimulation.

Isoproterenol-induced changes in glutathione metabolism in the rat parotid gland

Rat parotid glands were removed at timed intervals after intraperitoneal injection of isoproterenol (IPR). Reduced/oxidized glutathione ratios, glutathione peroxidase, glutathione reductase and amylase activities were determined. Parotid gland amylase content fell to less than 5 per cent of control values within 1 h after IPR, then slowly returned to control levels by 24 h. The ratio of reduced/oxidized glutathione was decreased at 2 h after IPR injection, returned to near control values at 6 h, then increased above control levels at 9, 12 and 24 h. Glutathione peroxidase activity was increased only at 12 h after injection of IPR, whereas glutathione reductase activity initially decreased, then reached peak values at 12 h. These results indicate that alterations in metabolic activity during the secretory cycle are accompanied by changes in the reduced/oxidized glutathione ratio and the activity of glutathione-metabolizing enzymes. Cellular events contributing to these alterations in glutathione metabolism may include exocytosis, endocytosis and membrane recycling, increased amino-acid transport and resynthesis of secretory proteins.

Inhomogeneities in glycoprotein cytochemistry of secretory granules in rat-parotid acinar cells after selective beta 1-adrenoceptor stimulation

The distribution of periodate-positive glycoproteins was studied in parotid acinar cells of the rat after stimulation with the specific beta 1-adrenoceptor agonist prenalterol. After two weeks of daily injections, the majority of secretory granules were larger, less electron dense, and often exhibited a bipartite or pleomorphic PA-CrA-silver staining pattern. A great variation in the staining pattern of the secretory granules was seen among different cells but, within any individual cell, most granules displayed a similar pattern. The staining reaction suggests a mucoid transformation of the granules, or a reorganization of the glycoproteins within the granules. The variation among cells in the staining pattern of granules may suggest that there are different populations of acinar cells or of secretory granules.

Freeze-fracture and lead ion tracer evidence for a paracellular fluid secretory pathway in rat parotid glands

The morphology and permeability of tight junctions of the three major epithelial constituents of rat parotid gland-acinus, intercalated duct, and striated duct-have been examined ultrastructurally. Acinar and intercalated duct junctions (including those surrounding intercellular canaliculi) averaged two to three sealing strands, whereas striated duct junctions had five to eight sealing strands. When the permeability of the junctional complex was probed by means of a recently devised lead ion tracer technique, acinar junctions were found to be very permeable, intercalated duct junctions were somewhat permeable, and striated duct junctions were essentially impermeable to the tracer. Thus, by both morphological and tracer-permeability criteria, acinar tight junctions appear to be "leaky." These data provide strong evidence that, in rat parotid glands, a potential paracellular secretory pathway exists in the acinar region for the transepithelial passage of fluid.

A cytochemical study of the effect of cholinergic and beta-adrenergic stimulation on calcium fluxes of rat parotid gland

Parotid gland tissue from carbachol-treated and isoproterenol-treated rats was studied cytochemically by pyroantimonate precipitation and x-ray microanalysis in an effort to identify any intracellular calcium reservoirs that might be available for use in a stimulus-secretion coupling mechanism, and to determine any differences that might exist in terms of calcium utilization due to the cholinergic or beta-adrenergic receptor mechanisms. Stimulation by either secretagogue results in a reduction of mitochondrial and plasma membrane calcium, but the reduction in mitochondrial calcium deposits of carbachol-treated animals is only one-half that of the beta-adrenergic-treated animals. This could possibly suggest a greater dependency on mitochondrial calcium for beta-adrenergic stimulated animals.

Membrane dynamics in the parotid acinar cell during regranulation: a stereological study following isoprenaline-induced secretion

Recently weaned male rabbits were injected either with 150 micrograms/kg isoprenaline in saline containing 0.01 M ascorbic acid or simply with the drug vehicle. Groups of drug-injected animals were killed at various time after injection. Parotid gland tissue samples from all animals were fixed, embedded and thin sectioned, and micrographs were prepared at standard magnification. Estimations of membrane areas of each membrane type in parotid acinar cells were made. It was found that in animals killed 2 hours after induced secretion apical area was larger than in controls. In animals killed at successively later times the apical area was progressively less. No elevation of any internal smooth membrane areas was ascertained at any sampling time, though the areas of rough endoplasmic reticulum in 2-12 hour samples were larger. It is suggested that excess apical membrane, though probably removed by interiorization, is afterwards disassembled in side the cell to create fresh macromolecular building units (protein molecules), perhaps after passing through the Golgi apparatus. This cryptic pool of building units can provide about 900 micrometers2 of secretion granule membrane per cell, the supply apparently being exhausted in the first eight hours after degranulation, whilst granule numbers are being increased. Thereafter, apparently, limited granule fusion occurs, so that ultimately the cellular complement of secretion granule membrane comes to enclose a greater volume of secretory product, though the average granule number per cell is small.

Secretion granule formation in the rabbit parotid gland after isoprenaline-induced secretion: stereological reconstructions of granule populations

The development of the secretion granule population of rabbit parotid glands after isoprenaline (IPR)-induced degranulation has been analyzed at the E.M. level using stereological techniques. Young male New Zealand White rabbits were sacrificed 2, 4, 8, 12 and 16 hours after IPR administration and the granule population compared with those of starved controls. In control glands a third of cell volume was occupied by stored secretory material, and it was estimated that, on average, cells contained 386 granules. The granule population as a whole had a mean diameter of 0.94 micrometers, with a unimodal positively skewed size distribution. Two hours after IPR treatment overall granule volume density was only 15% of that of control glands, but there was evidence that the process of restitution had already begun. At 8 hours about a fifth of acinar cell volume was occupied by electron-dense granules with an estimated mean diameter of only 0.58 micrometers, and the population as a whole was more strongly skewed than in the controls. In the later stages of restitution (12 and 16 hours), the volume of stored secretory material continued to rise, mean granule diameter increased, the size-frequency distribution became less skewed and the estimated number of granules per cell fell to 277 by 16 hours, suggesting that some granule fusion occurs during development. The analyses are discussed in relation to the techniques employed, and the results are equated with other independent evidence of the mode of granule genesis.

Localization of 5'-nucleotidase activity in the parotid acinar cells of a rat treated with isoproterenol

Cytochemical localization of 5'-nucleotidase (AMPase) has been investigated in the parotid acinar cells of rats at various stages of exocytic secretion induced by an administration of isoproterenol (IPR). In the resting stage, the acinar cells show AMPase activity located on the baso-lateral and luminal plasmalemma, and in the earliest secretory stage the luminal plasma membranes are devoid of the enzymatic activity. However, these particular regions exhibit AMPase activity during the advanced stages of secretion, and the AMPase positive membranes become absorbed into the cytoplasm by endocytic activity. The absorbed membrane components then seem to be degraded by the action of lysosomes. The intracellular fate of the endocytic vacuoles has been examined by the aid of ferritin particles introduced retrogradely through ductal lumina. Ferritin containing vacuoles are distributed in the cytoplasm, and these droplets change into secondary lysosomes. No tracer particles are recognized in the internal space of the Golgi lamella and its associated vesicles. The results suggested that in the exocytic secretion of parotid acinar cells, AMPase originating from plasma membrane intermingles with the membranes derived from secretion granules, and is translocated into cytoplasm by an endocytic mechanism. The internalized membrane components are, at least partly, degraded by lysosome action.

Morphology and cytochemistry of acinar secretory granules in normal and isoproterenol-treated rat submandibular glands

Several fixation procedures have been utilized in a fine-structural study of rat submandibular glands with the aim of correlating the morphology of granule substructure with its composition. Procedures included fixation with aldehyde in a variety of buffers, with or without fixation additives such as tannic acid or calcium chloride. Both immersion and perfusion fixation studies were performed. Osmium tetroxide postfixed tissue was compared with non-postfixed tissue. Thin sections were stained for carbohydrate-containing constituents by either periodic acid-thiocarbohydrazide-silver proteinate or tannic acid-ferric chloride sequences. The morphology and cytochemistry of acinar secretory granules were highly dependent upon the fixation procedure utilized. In postfixed tissue, fine filaments and vesicles were the major granule constituents. Filaments often aggregated into fibrils in fixatives containing phosphate buffer or calcium, whereas vesicles were prominent with fixatives containing collidine buffer. Tannic acid, as a fixation additive, imparted enhanced density to the peripheral rim of aggregated filaments. If post fixation was eliminated, an amorphous meshlike material was the major granule component. This stained readily with methods for carbohydrate whereas the granules of post-osmicated tissue did not. Following chronic isoproterenol treatment the degree of filament aggregation in postfixed tissue was augmented, and this corresponded to an increase in amount of material stainable for carbohydrate in non-post-osmicated tissue.

Hypophysial portal vascular infusion of TRH in the rat: an ultrasturctural and radioimmunoassay study

The hypophysial portal vessels and anterior pituitary gland of adult male Wistar rats were exposed surgically. A hypophysial portal vessel was cannulated and infused for one minute with saline or thyrotrophin (TRH). Anterior pituitary glands were collected at 1,5,15,30 or 60 minutes after cessation of infusion, for light and electron microscopic examination. Before and immediately after cannulation of a portal vessel, a 1-ml sample of blood was collected at 1,5,15,30, or 60 minutes, from the femoral vein for radioimmunoassay (RIA) of growth hormone. Thyrotrophs from anterior pituitary glands of rats infused with TRH displayed emiocytic activity at all time-periods studied. Rough endoplasmic reticular (RER) cisternae were dilated at 15 minutes following infusion and remained dilated at 30 and 60 minutes. TRH was observed to stimulate emiocytic activity in most pituitary cell-types. Extensive dilations of RER cisternae were also observed in mammotrophs and gonadotrophs, but were not observed in somatotrophs or adrenocorticotrophs. The demonstration that thyrotrophs, mammotrophs, somatotrophs, and gonadotrophs respond to TRH suggests that some common features may be shared by these cells. Preliminary analysis of the RIA data show that TRH was potent in elevating radioimmunoassayable growth hormone levels. Significant increases (p less than 0.02) in plasma GH levels were present at the earlier time periods studied (1,5, and 15 minutes) following the infusion of TRH, but no at 30 or 60 minutes. These findings provide additional support for the non-specific action of TRH upon hte various adenohypophysial cell types, and demonstrate that TRH stimulates these cells by a direct action on the adenohypophysis.

Changes in rat parotid salivary proteins induced by chronic isoproterenol administration

Changes in the rat parotid gland and its secretion, brought about by chronic isoproterenol administration, were studied. In addition to the expected enlargement, morphological and biochemical analyses of the glands showed evidence of changes in the secretory components. Chromatographic and electrophoretic experiments revealed both qualitative and quantitative changes in the secretory proteins.

Molecular basis of a control mechanism of DNA synthesis in mammalian cells

Personal observations made on the model of isoproterenol-stimulated DNA synthesis have pointed out the following: 1) cell hypertrophy precedes constantly the onset of DNA synthesis; 2) the length of the G1 phase is mass-dependent; 3) accumulation of ribosomes is needed for cell progress through G1; 4) ribosomal protein synthesis is involved in cell growth activation. These results together with a consideration of the pertinent literature allow us to formulate a hypothesis on the control of cell division in mammalian cells. DNA synthesis might be the terminal event in a chain of metabolic processes whereby a cell adjusts itself to increased functional demands (Increased Functional Demand Hypothesis). The main points of this model are the following: the interaction of the extracellular effector on the target cells first activates the pre-existent protein-synthesizing apparatus of the cell, which in turn brings about the activation of the "translation-transcription connecting mechanism" whereby the cells adjusts itself to an increased need for protein synthesis. Such a mechanism is characterized by cytoplasmic signals arising from the protein-synthesizing apparatus of the cell which reach the nucleus and call forth a messenger RNA for ribosomal proteins. The latter, once synthesized, protect the nascent ribosomal RNA from nuclease attack, resulting in an accumulation of ribosomes in the cytoplasm. Once the ribosomes have reached a "critical amount", the cell is triggered to enter DNA synthesis. As a link between the enhanced ribosomal RNA synthesis and DNA synthesis a reduction in the capacity of the ribonucleotide pool as source of DNA precursors has been suggested.

Correlative morphological and biochemical study of the effects of isoprenaline on the organelle and membrane content of the rabbit parotid gland

A quantitative study has been made on the enzymic, chemical and ultrastructural changes that occur in the parotid glands of rabbits as a result of Isoprenaline-induced secretion. Emphasis has been placed on correlating changes in organelle and membrane content which are evident 2 hr after Isoprenaline administration and which have been measured stereologically with the levels of appropriate enzymic or chemical markers, taking into account the contribution made by both the acinar and duct tissue. Lower protein, alpha-amylase and beta-glycerophosphatase levels correlated with reductions in zymogen granule and lysosome volume with whilst plasmalemmal and Golgi membrane areas and their marker enzyme concentrations remained unchanged. However, declines in alkaline phosphatase and succinate dehydrogenase activity (illustrated histochemically), and p-nitrophenyl phosphatase activity at pH 4-5 in the presence of tartrate occurred without any detectable decrease in membrane area. Conversely, an increase in rough endoplasmic reticulum area was measured stereologically but no increases in chemical markers were detected. The extent of correlation of the data is discussed in the context of the mechanism of secretion and the action of Isoprenaline.

Secretory dynamics of the rat submandibular gland. An ultrastructural and cytochemical study of the isoproterenol-induced secretory cycle

The presence of skull fracture has been associated with a higher risk of intracranial sequelae than if a fracture were not present. This is true for the total population of head-injury patients. However, reanalysis of the patient selection criteria data from two large published series on skull imaging in head trauma revealed that this increased risk factor for intracranial sequelae did not apply to a specific subset of minor-head-trauma patients. The patients in this subset were characterized by the presence of one or more of five “low-yield” criteria: (1) asymptomatic (no complaints), (2) headaches, (3) dizziness, (4) scalp hematoma, and (5) scalp laceration. All other criteria were absent. Results of the reanalysis showed (from a total population of 3031 head-trauma patients) a subset of 1184 patients characterized by these five criteria. In these 1184 minor-head-trauma patients there were 19 fractures, all linear, with none depressed or basilar. There were no intracranial sequelae. This change in the concept of fracture as a risk factor for intracranial sequelae has major implications in the future development of strategies for selecting patients for not having skull films or head computed tomograms.