Calcium and the exportable protein in rat parotid gland. Parallel subcellular distribution and concomitant secretion

Protein Production and Secretion in Exocrine Cells

Acinar cells of exocrine glands are highly specialized for producing, storing, and discharging secretory proteins for use on surfaces that represent interfaces between the organism and the surrounding environment. These functions are achieved through the secretory pathway that includes a series of functionally distinct intracellular compartments — the endoplasmic reticulum, subcompartments of the Golgi complex, and the secretion granule in which exportable macromolecules are stored at high concentrations. Most secretion occurs by granule exocytosis in response to external hormonal or neural stimuli. Although these processes have been traced in a variety of morphological and biochemical studies, very Utile is known about the mechanisms involved in facilitating and maintaining secretory storage, orchestrating discharge at the apical cell surface, and in ensuring conservation and re-internalization of the granule membrane. Recent studies initiated on cell fractions obtained from the rat parotid gland have provided significant insight into the protein storage conditions that prevail in the granule interior and the components of the granule membrane that are likely to be involved in general secretory function such as exocytosis.

Differential aggregation properties of secretory proteins that are stored in exocrine secretory granules of the pancreas and parotid glands

Low-pH- and calcium-induced aggregation of regulated secretory proteins has been proposed to play a role in their retention and storage in secretory granules. However, this has not been tested for secretory proteins that are stored in the exocrine parotid secretory granules. Parotid granule matrix proteins were analyzed for aggregation in the presence or absence of calcium and in the pH range of 5.5 to 7.5. Amylase did not aggregate under these conditions, although <10% of parotid secretory protein (PSP) aggregated below pH 6.0. To test aggregation directly in isolated granules, rat parotid secretory granules were permeabilized with 0.1% saponin in the presence or absence of calcium and in the pH range of 5.0 to 8.4. In contrast to the low-pH-dependent retention of amylase in exocrine pancreatic granules, amylase was quantitatively released and most PSP was released from parotid granules under all conditions. Both proteins were completely released upon granule membrane solubilization. Thus neither amylase nor PSP show low-pH- or calcium-induced aggregation under physiological conditions in the exocrine parotid secretory granules.

Response of the parotid gland of the brushtail possum, Trichosurus vulpecula, to adrenergic stimulation

Adrenergic stimulation of parotid secretion was investigated in anaesthetised brushtail possums to ascertain fluid secretion rates and salivary composition. Because neither alpha- nor beta-adrenergic stimulation evoked saliva output, infusion of the adrenergic agonists was superimposed on a pre-existing bethanechol-stimulated flow. Isoprenaline infusion (2.4 nmol min-1) increased salivary amylase activity, [protein]; [HCO3]; [PO4] and [Ca], and amylase/Ca and protein/Ca ratios; reduced [Cl]; [K] and osmolality; but did not alter H+ activity; [urea]; [Na]; [Mg]; amylase/protein or saliva/plasma urea ratios. These data are consistent with isoprenaline stimulating acinar secretion of protein, Ca and PO4 but not the ion transport necessary for primary fluid formation at the endpieces and modifying transport of monovalent ions in the excurrent ducts. Consequently, the possum parotid has beta-adrenergic receptors in both the endpieces and excurrent ducts. Phenylephrine infusions at 2.4 and 24 nmol min-1 were without effect whereas phenylephrine at 240 nmol min-1 caused changes in salivary composition which paralleled those for isoprenaline administration but were generally of lesser magnitude. Thus, the possum parotid has few or no alpha-adrenergic receptors and the salivary response elicited was the result of cross-reaction of phenylephrine with beta-adrenergic receptors.

Intracellular transport and secretion of salivary proteins

Intracellular transport and secretion of salivary proteins are major activities of salivary acinar cells. While the major intracellular pathway followed by salivary proteins following their synthesis has been described previously, there is only limited understanding of how this process is regulated at the molecular level. Studies of salivary proteins, especially proline-rich proteins, expressed in an endocrine cell line have begun to provide insight regarding intermolecular interactions during transport and the role played by structural signals during intracellular sorting. Analysis of the secretion of newly synthesized salivary proteins in parotid tissue has shown that there are multiple pathways of discharge from acinar cells. While granule exocytosis is the major pathway, at least two other pathways that export salivary proteins have been found to originate from maturing secretion granules. These pathways may contribute to other acinar cell functions, including secretion of proteins in the absence of acute stimulation and support of the secretory process for fluid and electrolytes.

Evidence for a Ca2+ pool associated with secretory granules in rat submandibular acinar cells

Intracellular Ca2+ stores in rat submandibular acinar cells were characterized using the Ca(2+)-sensitive fluorescent indicator fura 2 and the radiotracer 45Ca2+. Acetylcholine induced a rapid Ca2+ release from a store sensitive to inositol 1,4,5-trisphosphate (IP3) and to thapsigargin (TG). After this store was presumably depleted, ionomycin caused a further increase in cytosolic free Ca2+ concentration ([Ca2+]i), suggesting the presence of an IP3-insensitive Ca2+ release from a store that is more extensive and heterogeneous than the IP3-sensitive one and includes a small mitochondrial component. After both of these stores had been discharged, exposure to monensin caused an additional release of Ca2+ from a third store. This store appears to be associated with secretory granules, since Ca2+ release was significantly reduced when degranulation was induced by isoprenaline. This third store appears to be insensitive to IP3, discharges Ca2+ when the pH gradient across the limiting membrane is collapsed with monensin and only in the presence of both ionomycin and monensin. Ca2+ release from this store is not by Na+/Ca2+ exchange, since simply altering [Na+]i did not cause significant Ca2+ release. In permeabilized cells, IP3 and TG released approx. 35% of 45Ca2+, and ionomycin released an additional 57%, whereas monensin only caused a small additional release, suggesting that only IP3- and ionomycin-sensitive stores are loaded with 45Ca2+ under these conditions. The absence of significant isotope uptake into the ionomycin+monensin-sensitive store may result from a low rate of tracer accumulation or from the lack of Ca2+ pumps in the store. The pattern of response was similar in the presence and absence of mitochondrial inhibitors, indicating that the store is not located in mitochondria. In summary, these results suggest that a substantial IP3-insensitive Ca2+ store is present in secretory granules in rat submandibular acinar cells.

Localization of salivary proteins in granules of human parotid and submandibular acinar cells

Electron microscopic immunocytochemistry was used to investigate the distribution of four secretory proteins within the granules of human parotid and submandibular gland acinar cells. The results demonstrate that the distribution of the individual proteins differs significantly. Amylase is concentrated within the electron-dense regions of the granules, whereas agglutinin is present mainly in the regions of lower density. In contrast, the proline-rich proteins and histatins are distributed throughout the content of the granules and do not appear to be concentrated in either the high- or low-density regions. The results also suggest that different mechanisms, such as self-aggregation or ionic complexes with metal ions and charged macromolecules, may be involved in the packaging of individual proteins within the same granule. Further, they also show that among the salivary glands of various species, the intragranular distribution of similar proteins (e.g., amylase and proline-rich proteins) may differ.

The effects of clonidine and three 2-imidazoline derivatives on the secretion of protein and some electrolytes by rat submandibular and parotid glands

1. Three imidazoline analogues of clonidine were potent secretagogues for the parotid and submandibular glands at relatively high doses. 2. Salivation in response to clonidine was completely abolished by prazosin, phentolamine, phenoxybenzamine and dihydroergotamine. 3. The gamma-type of proteins was secreted in response to three of the analogues, whereas with p-aminoclonidine the alpha-type of proteins was secreted by the submandibular gland. 4. Albumin was specifically secreted by the submandibular gland in response to clonidine but not to isoproterenol or phenylephrine.

Effects of chronic amitriptyline administration on saliva from the parotid and submandibular glands of the rat

The effect of prolonged treatment with amitriptyline on the secretory activity of rat salivary glands evoked by parasympathetic nerve stimulation and isoprenaline administration has been studied. Low doses of amitriptyline (10 mg/kg per day for 2 or 4 weeks), did not significantly affect salivary flow evoked by either parasympathetic nerve or isoprenaline stimulation. Higher doses of amitriptyline (50 mg/kg/day for 2 or 4 weeks) however, markedly decreased parasympathetic-evoked salivary secretion (flow and volume) from both parotid and submandibular glands, while isoprenaline-evoked secretions were unaffected. Sodium, potassium, and calcium concentrations of nerve-elicited or isoprenaline-evoked saliva were not significantly altered by amitriptyline treatment. Protein concentration and amylase activity of nerve-elicited parotid saliva were, however, greatly increased by chronic amitriptyline administration. Possible mechanisms for drug-induced increase in nerve-elicited salivary protein concentration include changes in cholinergic receptor binding, release of neuropeptides and variations in phosphatidylinositol turnover, which need further study.

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.

Calcium uptake in rat parotid gland secretory granules

45Ca2+ uptake in isolated rat parotid secretory granules was examined in the presence of oxalate. Uptake of calcium was dependent on time, with the maximum occurring at 15 min. The uptake of calcium was dependent on adenosine-5'-triphosphate (ATP), and substitution of ATP with beta, gamma-methylene-ATP did not stimulate calcium uptake. Enzyme marker analysis indicated that mitochondria accounted for no greater than 3.0 +/- 0.2% of the observed ATP-dependent calcium uptake. Calcium uptake was blocked by the ATPase inhibitors tributyltin, IC50 = 12.2 +/- 0.6 nmol/L and 4-acetamido-4'-isothiocyano-2,2'-stilbene disulphonic acid (SITS), IC50 = 3.0 +/- 0.3 mumol/L. These results indicate that in the parotid secretory granule there is a calcium uptake mechanism that is dependent on the hydrolysis of ATP and is suppressed by two inhibitors of granule ATPase.

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.

Separation of Ca2(+)-ATPase from Mg2(+)-ATPase in plasma membrane-rich fraction of bovine parotid gland

Ca2(+)-ATPase, which does not require Mg2+ for its activation, was separated from Mg2(+)-ATPase by papain treatment of a membrane-rich fraction of bovine parotid gland. The enzyme was partially purified 48-fold by subsequent chromatography on DEAE-cellulose, gel filtration on HPLC, and ion-exchange HPLC. The enzyme showed a molecular weight of 100,000, as estimated by gel filtration on HPLC. The Ca2(+)-ATPase was activated by Ca2+ but not by Mg2+, and this enzyme did not require Mg2+ for its activation by Ca2+. In fact, Mg2+ was inhibitory. p-Nitrophenyl phosphate was not hydrolyzed in the presence of Ca2+ or Mg2+, and this enzyme had no activities of other phosphatases tested. These results suggest that the Ca2(+)-ATPase is a separate enzyme from Mg2(+)-ATPase, Ca2(+)-stimulated Mg2(+)-dependent ATPase, and alkaline phosphatase, all of which are well known to be present in other tissues.

Does beta-adrenoceptor activation stimulate Ca2+ mobilization and inositol trisphosphate formation in parotid acinar cells?

The effects of the beta-adrenoceptor agonist, isoprenaline, on Ca2+ mobilization and inositol phosphate formation in parotid acinar cells were examined. Isoprenaline (2 microM) failed to increase cytosolic [Ca2+] in acinar cells, as measured by Fura-2 fluorescence, even in the presence of a phosphodiesterase inhibitor. Likewise, neither the 8-bromo nor the dibutyryl derivatives of cAMP (both at 2 mM concentration) increased [Ca2+]i. However, in confirmation of results previously published, a higher concentration of isoprenaline (200 microM) increased cytosolic [Ca2+]i of rat parotid acinar cells, from 104 +/- 4 nM to 151 +/- 18 nM. The increase in [Ca2+]i in response to isoprenaline, while transient in the absence of extracellular Ca2+, was sustained in Ca2(+)-containing medium. This isoprenaline-stimulated Ca2+ signal was more potently antagonized by phentolamine than by propranolol, suggesting that the higher concentration of isoprenaline activated alpha-adrenoceptors. Furthermore, the Ca2+ signal generated in response to the alpha-adrenoceptor agonist, phenylephrine, also was blocked by the same concentrations of propranolol necessary to block the effects of isoprenaline, suggesting that propranolol may block alpha-adrenoceptors under certain experimental conditions. The high concentration of (-)isoprenaline (200 microM) also increased inositol (1,4,5) trisphosphate and inositol (1,3,4) trisphosphate formation 45% within 30 s. Analogous to the increase in intracellular Ca2+, the formation of inositol phosphates stimulated by isoprenaline was more potently antagonized by the alpha-adrenoceptor antagonist, phentolamine, than by the beta-adrenoceptor antagonist, propranolol, again suggesting that isoprenaline interacts with alpha-adrenoceptors on parotid cells. Thus, the effects of isoprenaline on [Ca2+]i do not appear to be mediated by cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Age changes in secretory function of male and female rat parotid glands in response to methoxamine and pilocarpine

Saliva secreted in response to methoxamine and pilocarpine was collected from the cannulated ducts of both parotid glands of male and female rats at weekly age intervals from three to 10 weeks, and at 3.5, 8, and 15 months of age. It was analyzed for the concentrations of protein, potassium, calcium, inorganic phosphate, and for amylase activity. The type of protein were determined electrophoretically, and an amino acid analysis of the total protein was also carried out. The wet weights of the glands increased substantially up to eight weeks of age, then reached almost plateau values, and finally tended to decrease at 15 months of age in both sexes. The salivary volumes secreted in response to methoxamine and pilocarpine were positively correlated with the parotid gland weights in both sexes. The concentrations of protein, potassium, and inorganic phosphate were inversely related to the salivary flow rates only at relatively low rates of flow. The amylase activity was positively correlated with the concentration of protein, independent of the nature of the stimulus, age, and sex. With methoxamine as a stimulus, the amylase activity was positively correlated with the concentration of calcium, independent of age and sex. The types of protein and amino acid concentrations were independent of the nature of the stimulus, age, and sex up to 15 months of age. However, in parotid saliva of several rats at 8 and 15 months of age, unusual proteins were observed electrophoretically, independent of the nature of the stimulus and sex.

Similarities and differences among neuroendocrine, exocrine, and endocytic vesicles

Secretory and endocytic vesicles have analogous functions as cyclic carriers between specific cellular compartments. The centrifugally functioning secretory system operates from the Golgi complex, whereas the centripetally functioning endocytic system operates from the cell surface. Further, within polarized epithelial cells the export traffic can be directed to a distinct plasmalemmal domain which distinguishes exocrine from endocrine secretion and import traffic can be directed transcellularly. These shuttle operations involve a special class of lipid-rich, protein-poor membranes that appear to use an inwardly directed H+-translocase activity to varying extents for pH-dependent sorting and for accumulation and concentration of transported molecules. Comparative analyses of purified membrane preparations from exocrine and endocrine sources identify compositional overlap between different types of shuttle membrane. However, the structural elements that specify a particular origin or destination for a given carrier or determine function in storage and stimulus-dependent shuttling remain unknown.

Developmental changes in the volumes, protein, and some electrolyte concentrations of male and female rat submandibular saliva secreted in response to methoxamine and pilocarpine

Saliva secreted in response to methoxamine and pilocarpine was collected from the cannulated ducts of the submandibular glands of male and female rats at weekly intervals from two to 10 weeks of age. It was analyzed for volume and for concentrations of protein, potassium, calcium, and inorganic phosphate. Following the collection of saliva, the submandibular glands were removed and weighted. The wet weights of the glands increased substantially up to seven weeks of age and then reached almost plateau values in both sexes. The salivary volumes secreted in response to both agents in both sexes were positively correlated with the gland weights, except that after five to six weeks of age there was no correlation between gland weight and methoxamine-stimulated salivary volume. The concentrations of protein, potassium, and inorganic phosphate were inversely related to the flow rates only at relatively low rates of flow. The concentration of calcium was positively correlated with the protein concentration and was independent of the nature of the stimulus and of sex differences during postnatal development.

Protein production and secretion in exocrine cells

Acinar cells of exocrine glands are highly specialized for producing, storing, and discharging secretory proteins for use on surfaces that represent interfaces between the organism and the surrounding environment. These functions are achieved through the secretory pathway that includes a series of functionally distinct intracellular compartments--the endoplasmic reticulum, subcompartments of the Golgi complex, and the secretion granule in which exportable macromolecules are stored at high concentrations. Most secretion occurs by granule exocytosis in response to external hormonal or neural stimuli. Although these processes have been traced in a variety of morphological and biochemical studies, very little is known about the mechanisms involved in facilitating and maintaining secretory storage, orchestrating discharge at the apical cell surface, and in ensuring conservation and re-internalization of the granule membrane. Recent studies initiated on cell fractions obtained from the rat parotid gland have provided significant insight into the protein storage conditions that prevail in the granule interior and the components of the granule membrane that are likely to be involved in general secretory function such as exocytosis.

Changes in elemental concentrations of rat parotid acinar cells following pilocarpine stimulation

1. Quantitative electron microprobe analysis was used to measure elemental dry weight concentrations in cytoplasm, secretory granules and nuclei of resting and pilocarpine-stimulated rat parotid gland acinar cells. 2. Secretory granules in resting cells had lower concentrations of Na, Mg, P, Cl and K, and higher concentrations of S and Ca than cytoplasm or nuclei. Nuclei in resting cells had lower S and higher K concentrations than cytoplasm. 3. Three major pilocarpine-related changes were found: (i) cytoplasmic dry weight concentrations of Na and Ca increased and the concentration of K decreased, (ii) the nuclear concentration of Na increased while that of K decreased and (iii) the concentrations of Na and Cl increased in secretory granules. 4. These results indicate that the nuclear, and cytoplasmic compartments have different mechanisms for regulating their elemental concentrations relative to the secretory granules. 5. The present results are largely consistent with X-ray microanalysis results from the pilocarpine-stimulated dog submandibular gland.

Forskolin, a tool for rat parotid secretion studies: 45Ca efflux is not related to cAMP

In the present work, we investigated, by use of forskolin, whether adenosine 3',5'-cyclic monophosphate (cAMP) level and Ca movements were modulated sequentially or in parallel by the activation of the beta-adrenergic receptor in the rat parotid gland. Forskolin-induced [3H]protein secretion was dependent on external Ca, whereas isoproterenol-induced secretion was not. This effect was not due to a requirement of adenylate cyclase for Ca, since the cAMP level increase induced by forskolin was not Ca dependent. Furthermore isoproterenol induced 45Ca efflux, whereas forskolin did not. 45Ca efflux was correlated neither to cAMP nor to secretion, since when there was a massive augmentation of cAMP there was no change in 45Ca efflux, and forskolin, which induced much secretion, was unable to induce Ca efflux. Carbachol potentiated the secretion induced by forskolin in the absence of Ca, whereas it did not potentiate the isoproterenol-induced response. From these results we suggest that beta-adrenergic receptor activation would lead to two parallel events, cAMP accumulation and Ca movements, which together would lead to maximal secretion.

Changes in cytosolic free calcium concentration in isolated rat parotid cells by cholinergic and beta-adrenergic agonists

The alteration in the concentration of cytosolic free calcium ([Ca2+]i) in isolated rat parotid cells caused by autonomic agents was directly measured using the Ca-sensitive fluorescent probe, quin2. [Ca2+]i of unstimulated cells was estimated to be 162.7 +/- 3.2 nM in normal medium. Carbachol (CCh) and isoproterenol (ISP) caused a rapid rise in [Ca2+]i in a dose-dependent manner. Maximum increases in [Ca2+]i induced by CCh and ISP were approximately 100% and 25% of resting level, respectively. In Ca-free medium, CCh produced a small, rapid rise in [Ca2+]i, followed by a slow decay and a return to resting level within 3-4 min, while all doses of ISP tested failed to change [Ca2+]i. These results suggest that CCh mobilizes Ca2+ from both extracellular and intracellular pools and then results in a rise in [Ca2+]i, whereas ISP may slightly mobilize only the extracellular Ca pool.

Isolation and partial characterization of two populations of secretory granules from rat parotid glands

A method is described for the isolation of two populations of secretory granules from rat parotid glands utilizing differences in their sedimentation characteristics. The granule preparations were analyzed for homogeneity by electron microscopy and chemical analyses. The soluble contents of both types of granules were obtained by hypotonic lysis, and the proteins compared by SDS-PAGE and ion exchange-gel filtration chromatography. Both populations of secretory granules appear to have the same protein composition as that of the parotid saliva. The secretory granules with the smaller apparent buoyant density became labelled with radioactive leucine earlier than the heavier granules when a pulse of this amino acid was supplied to a gland slice system. The lighter granules appear to represent an earlier stage in maturation.

Postnatal changes in calcium and amylase of rat salivary glands, including calcium changes with senescence

Postnatal changes occur in glandular Ca concentration of rat parotid and submandibular glands. At 4 days of age, Ca concentration was low in both glands (only one-third to one-half that of adults) and increased gradually with age. The pattern of change was generally similar for male and female rats, but in submandibular gland, adult levels of 9-10 m-equiv./kg were reached by weaning, whereas for parotid gland, a gradual increase in Ca concentration occurred with adult levels of 9-10 m-equiv./kg reached by 7 weeks of age. The pattern of change was the same whether Ca concentration was expressed per kg wet or dry weight albeit water content changed with age. The changes in Ca concentration of parotid paralleled the age-associated increases in amylase activity of parotid gland. Amylase activity of submandibular gland was much less than that of parotid and similarly low at all ages examined, and did not parallel the age-associated increases in Ca concentration. The regulatory role of the sympathetic innervation on glandular Ca concentration was examined by effecting surgical denervation of parotid and submandibular glands at 8 days of age, and then determining Ca concentration of the denervated glands at 32 days. A three-fold increase in Ca concentration, similar to that following acute sympathectomy in adults, occurred in submandibular gland but no change was seen in parotid. An unexpectedly high concentration of Ca was also found in submandibular (but not parotid) gland of old rats.

The effects of alpha-methylnoradrenaline on protein and electrolyte secretion by rat submandibular and parotid glands

alpha-Methylnoradrenaline (alpha-mNA) is a potent secretagogue for the parotid and submandibular glands of rats. With regard to the parotid glands, alpha-mNA activates mainly beta-adrenoceptors. In the submandibular glands, alpha-mNA activates alpha-adrenoceptors at higher doses whereas at relatively lower doses it activates beta-adrenoceptors. alpha-mNA may not stimulate the specific alpha 2-adrenoceptors of the salivary glands of rats.

Surface architecture of the mucosal epithelium of the cat trachea: II. Structure and dynamics of the membranous portion

Bronchoscopic examination of anesthetized cats revealed that the trachea is capable of considerable change in caliber via lateral expansion of the membranous trachea. The morphological basis for this expansile capacity was determined by correlated light microscopy and scanning and transmission electron microscopy. The organization of the membranous trachea differs from that of the cartilaginous trachea. The mucosa is arranged in a series of longitudinal folds that open like an accordion when stretched laterally. These fold are not random, but appear to depend on rows of nonciliated, microvillus-rich cells that form flexure lines in the surface epithelium. The mitochondria in such cell have a condensed configuration, indicating a high level of oxidative metabolism and suggesting that they may participate in transport processes that modify the luminal contents. Goblet cells, which are relatively sparse in the membranous trachea, have mitochondria in which the prominence of matrix granules and degree of mucus storage are inversely related. Mitochondrial morphology allows goblet cells that have discharged their mucin content to be readily distinguished from the microvillus-rich cells, even when their luminal surfaces lie outside the thickness of a section.

Vitamin D-mediated decrease of Ca2+-pump activity in the rat parotid gland

In vitro activities of Ca2+-ATPase and 45Ca2+ uptake in microsomes, which were prepared from vitamin D-deficient rat parotid glands, were decreased in parallel by the oral administration of vitamin D3 as compared with those of control animals (r = 0.83). In vivo 45Ca2+ uptake in the parotid glands of vitamin D-deficient rats was also decreased by the oral administration of vitamin D3.

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.

Vitamin D receptors in isolated rat parotid gland acinar cells

Rat parotid gland was examined for the presence of 1 alpha, 25-dihydroxycholecalciferol receptors using sucrose density gradient ultracentrifugation techniques. [3H] DHCC bound specifically and with high affinity to a 3.2 S protein present in nuclear and cytosolic fractions of isolated parotid acinar cells. Values for the equilibrium dissociation constant and for the receptor concentration were determined to be approx. 0.1 nM, and 12 fmol/mg protein, respectively. In competitive inhibition experiments, the 3.2 S protein displayed 100-fold lower affinity for 25-hydroxycholecalciferol than for DHCC, and did not bind estradiol or methylprednisolone. These results suggest that rat parotid gland acinar cells contain classical DHCC receptors. A similar approach failed to provide evidence of DHCC receptors in isolated pancreas acinar cells, lacrimal gland or submandibular gland. It has been previously reported that vitamin D is essential for normal exocrine secretion from the rat parotid gland (Tenenhouse, A. and Afari, G. (1978) Biochim. Biophys. Acta 538, 631-634). The present findings suggest that this effect is the result of a direct action of DHCC on the parotid gland acinar cell. The absence of DHCC receptors in other exocrine cells suggests that tissue sensitivity to DHCC is not a general property of exocrine systems.

The purification and partial characterization of phospholipase A2, a secretory protein of rabbit parotid gland

Phospholipase A2 (EC 3.1.1.4), a soluble enzyme present in secretion granule lysates and in the discharged secretion of rabbit parotid gland, has been purified by gel filtration. The enzyme preparations obtained from both lysates and secretion have been found to have identical amino acid compositions, amino terminal residues (Asx), isoelectric points (10.2) and electrophoretic behavior in polyacrylamide gels. The reduced and alkylated protein yields a single band upon electrophoresis in the presence of sodium dodecyl sulfate; the mobility corresponds to an apparent molecular weight of 16000. The enzyme is capable of action on phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine and phosphatidylinositol. Activity has been examined using as substrate sonicated vesicles consisting of PC and PE. Rates of hydrolysis have been determined by densitometry of lysophosphatides resolved and charred on thin-layer chromatograms. This approach has been used to follow enzyme purification, to indicate the preferential hydrolysis (approx. 2-fold) of PE vs. PC and to demonstrate that enzyme purified from granule lysates and discharged secretion shows the same heat stability and activity profile as a function of pH. A highly specific Ca2+ requirement for activity also has been identified for substances organized as phospholipid bilayers; the apparent inactivity of this enzyme within a Ca2+-containing storage organelle, the secretion granule, presents an interesting problem for future investigation.

parallel secretion of secretory proteins and calcium by the rat parotid gland

1. The secretion of amylase, deoxyribonuclease, ribonuclease, protein and Ca2+ by the rat parotid gland in vitro was studied. 2. Isoproterenol and carbamoylcholine elicited a parallel discharge of amylase, deoxyribonuclease, ribonuclease and protein over a 40 min time period. 3. The composition of the secretion was independent of the secretogogue used for stimulation. When gland slices from the same animal were stimulated with isoproterenol, adrenaline, phenylephrine or carbamoylcholine, secretory enzymes and protein were secreted in constant proportions. 4. 45Ca injected intraperitoneally 16 h before stimulation with either isoproterenol or carbamoylcholine was released in parallel with amylase and protein. 5. The relative proportions of amylase, ribonuclease, deoxyribonuclease, protein and Ca present in isolated parotid gland secretory granules was identical to that of isoproterenol stimulated gland secretion. 6. It is concluded that the secretory proteins and Ca2+ are discharged in constant proportions by the rat parotid gland regardless of the mode of stimulation or the rate of secretion. The similarity in the composition of gland secretion and granule contents also suggests that enzymes and Ca2+ are released by exocytosis and not by diffusion across the apical plasma membrane.

Differential effects of cobalt on the initiation of fast axonal transport

Effects of Co2+ on the fast axonal transport of individual proteins were examined in vitro in bullfrog spinal/sciatic nerves. 35S-methionine-labeled proteins, fast-transported in control and Co2+-treated preparations were separated via two-dimensional gel electrophoresis. While the overall amount of protein transported was reduced, no qualitative differences could be seen when gel fluorographic patterns were compared. Quantitative analyses of the 48 most abundantly transported species revealed two significantly different populations (p less than 0.01) differentially sensitive to Co2+ and distinguishable to a large extent by molecular weight. Those proteins less sensitive to Co2+ ranged from approximately 20,000 to 35,000 daltons while those more sensitive to Co2+ were greater than approximately 35,000 daltons. The finding that all proteins are affected by Co2+ supports the proposal that fast-transported proteins are subject to a common Co2+-sensitive, Ca2+-requiring step. The observed differential effects are consistent with more than one Ca2+-dependent step occurring during the initiation phase of fast transport.

The secretion of protein and of some electrolytes in response to alpha- and beta- adrenergic agonists by rat parotid and submandibular salivary glands enlarged by chronic treatment with isoproterenol

In comparison with control rats. chronic administration of isoproterenol (IPR) caused enlargement of the parotid and submandibular glands and the synthesis of apparently new proteins, probably by acinar cells. The parotid and submandibular saliva secreted in response to methoxamine or IPR differed markedly in protein and electrolyte content. In addition, with each stimulus, the salivas secreted by control and experimental glands differed in flow rate, protein concentration, and in the concentrations of certain of the electrolytes.

Effects on rat parotid amylase and Ca of alpha- and beta-adrenergic sympathetic stimulation

The roles of alpha- and beta-adrenergic receptors in the regulation of the secretion of amylase and calcium from rat parotid gland were studied by using direct electrical stimulation of the sympathetic innervation to the gland in the presence of selective adrenergic blocking agents. When phenoxybenzamine was administered intraperitoneally 25 min prior to nerve stimulation, the highest [Ca] (12-14 meq/liter) and amylase activity (1,000 mg/microliter) in the evoked saliva were observed. On the other hand, stimulation of the nerve in the presence of propranolol evoked a saliva that contained the lowest [Ca] (5-6 meq/liter) and amylase activity (170 mg/microliter). Furthermore, salivary flow (4.27 +/- 0.42 microliter/min X g) induced by sympathetic nerve stimulation in the presence of phenoxybenzamine was higher than that induced by sympathetic nerve stimulation in the presence of propranolol (2.55 +/- 0.39 microliter/min X g). Therefore, it was concluded that beta-adrenergic receptors play the major role in the regulation of salivary flow and the secretion of amylase and calcium, whereas alpha-adrenergic receptors play a minor role in the regulation of these parameters.

A micropuncture study of the handling of calcium by the rat parotid

The handling of Ca by the rat parotid gland was investigated using micropuncture, microperfusion, and microanalytical techniques. Concentrations of Ca were measured in salivary fluid from intercalated, lobular and main ducts, and net transductal fluxes of Ca and water were calculated during stimulation of secretion with acetylcholine, pilocarpine, and DL-isoproterenol.

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

Sections of neurosecretory cells fixed in glutaraldehyde and osmium tetroxide were studied by means of an EMMA-4 analytical microscope. Secretory granules in neurosecretory cells of the corpus cardiacum and of the brain, both in the desert locust Schistocerca and in the blowfly Calliphora, as well as neurosecretory granules in posterior pituitaries of the frog Rana and of the albino rat all contain a high concentration of calcium. A distinct sulphur peak was also a constant feature. In neurosecretory cells of the corpus cardiacum of Schistocerca the chromatin contained a high concentration of calcium. The mitochondria also contained much calcium, but part of this disappeared during preparation except when fixative and wash contained calcium chloride. By block staining with uranyl acetate most calcium is displaced from the mitochondria, whereas most of the calcium remains in the neurosecretory granules. Since the calcium peaks in spectra from neurosecretory granules appear of similar size, regardless of variations in the preparative procedure, this calcium must be firmly bound. The possible role of the calcium bound to the neurosecretory substance is discussed. The presence of sulphur in insect neurosecretory granules indicated the presence of a protein besides the hormone, i.e., an insect neurophysin.

Stimulation of renin secretion and calcium efflux from the isolated perfused cat kidney by noradrenaline after prolonged calcium deprivation

1. The effects of noradrenaline on the secretion of renin and on the efflux of Ca have been investigated in the isolated cat kidney perfused with Locke solution containing the alpha-adrenergic blocking agent, phenoxybenzamine, to block increases in renal vascular resistance. 2. Noradrenaline elicited a marked increase in renin secretion in the absence of any discernible alterations in renal arterial pressure, and prolonged perfusion with Ca-free Locke solution failed to depress noradrenaline-evoked renin secretion. 3. Noradrenaline caused an increase in the efflux of 45Ca from prelabelled kidneys perfused with Ca-free solution. Increasing the noradrenaline concentration produced graded increases in 45Ca efflux and renin release, and the peak rise in efflux preceded or coincided with peak renin secretion. 4. DL-Propranolol inhibited the increase in 45Ca efflux and renin release resulting from noradrenaline stimulation. 5. Electrical stimulation of the renal nerve enhanced 45Ca efflux and renin release from prelabelled kidneys perfused with noraml Locke solution. 6. These findings provide further support for the view that the process of catecholamine-induced renin secretion involves mobilization of Ca from a cellular site.