Calcium and calmodulin stimulate phospholipase A2 and fusion of H,K-ATPase-containing membrane vesicles isolated from pig gastric mucosa

Topographical distribution of cells in the rat submandibular gland duct system with special reference to dark cells and tuft cells

The duct system of the rat submandibular gland consists of the intercalated duct, the granular convoluted tubule, the striated duct, the excretory duct, the main excretory duct, and the salivary bladder. The duct system contains special cell types, such as dark cells and tuft cells, in addition to principal cells. However, little is known about cell distribution in the duct system. The purpose of the present study was to examine cell distribution and to perform a morphometric analysis of the duct system. Transmission and scanning electron microscopy were used to examine the duct system of the rat submandibular gland. Six regions in the duct system, the striated duct, the interlobular excretory duct, the 5-mm proximal excretory duct from the hilus, the main excretory duct at the hilus, the 10-mm distal main excretory duct from the hilus, and the salivary bladder, were investigated. Morphometric and statistical analyses of the data were then performed. The epithelium of the duct system consisted of a heterogeneous cell population. Dark cells and tuft cells were present throughout the duct system. The principal, dark, and tuft cells were distinguished by their different microvilli by using a scanning electron microscope. The frequency of these cells in the total epithelial cell population was as follows: The percentage of principal cells in the six regions of the duct system varied from 87.5% to 94.4%, that of dark cells varied from 4.1% to 7.2%, and that of tuft cells varied from 1.8% to 7.2%. The number of principal and tuft cells was significantly different between the striated duct and the main excretory duct at the hilus (P < 0.01). However, no significant difference in number of dark cells throughout the duct system was observed (P > 0.05). The abundance of the principal, dark, and tuft cells in the duct system of the rat submandibular gland was determined. Few tuft cells were distributed in the striated duct, and most were found at the hilus. Dark cells were distributed equally throughout the duct system.

Interactions between Ca2+- and cAMP-dependent stimulatory pathways in parietal cells

Isolated rat parietal cells were used to investigate the role of intracellular Ca2+ in the action of cAMP-dependent secretagogues and cross talk between cAMP- and Ca2+ -dependent stimulatory pathways. Aminopyrine accumulation (an index of acid produced and trapped by the parietal cells), cytosolic free Ca2+, morphological transformation and cell viability were used to investigate parietal cell function and stimulation. The increase of cytosolic free Ca2+ promoted by gastrin, or carbachol, was abolished by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 microM). Also, the morphological transformations induced by dibutyryladenosine 3':5'-cyclic monophosphate (DBcAMP), gastrin, and Sp-adenosine-cyclic-3', 5'-monophosphothioate (Sp-cAMPS) were completely abolished by BAPTA (10 microM). In aminopyrine accumulation the action of 1 mM DBcAMP was dose-dependently reduced by BAPTA. The Ca2+ ionophore A23187 alone, in the range of 1 pM to 1 microM, had no effect but it dose-dependently potentiated the action of 1 mM DBcAMP in aminopyrine accumulation. The inhibitory actions of BAPTA on DBcAMP- and histamine-stimulated aminopyrine accumulation were dose-dependently reversed by A23187. Histamine-stimulated protein kinase activity and viability parameters as cellular lactate dehydrogenase (LDH) and trypan blue exclusion were not changed by BAPTA. These results indicated that in isolated parietal cells: (1) the action of cAMP-dependent secretagogues in aminopyrine accumulation and morphological transformation are dependent on cytosolic free Ca2+; (2) Ca2+ -induced morphological transformation is essential for aminopyrine accumulation; (3) a threshold level of one second messenger is required for stimulation of aminopyrine accumulation by the other second messenger.

Purification and characterization of guinea pig gastric phospholipase A2 of the pancreatic type

Guinea pig gastric mucosa and juice contained exceptionally high phospholipase-A2 activity, whereas the activity in the pancreas was only minimal. Phospholipases A2 were purified to homogeneity from these three tissues. Structural evidence, including the sequence of the NH2-terminal 41 residues, the amino-acid composition and the molecular mass (13902 +/- 3 Da) determined accurately by mass spectrometry, showed that the gastric mucosa enzyme belongs to the pancreatic type. An unique feature of the sequence is the substitution of Phe for the hitherto invariant Tyr28 in the calcium-binding loop of pancreatic phospholipases A2. The affinity of the guinea pig enzyme for Ca2+ in the presence of substrate was, however, identical to that of the rat enzyme with Tyr28, suggesting the interaction of a phenolic hydroxyl group of the Tyr with its neighboring residues is not significantly linked to the binding of Ca2+. The NH2-terminal sequences and immunochemical properties of the enzymes purified from the gastric juice and pancreas were identical to those of the gastric mucosa enzyme. The distribution of cells immunoreactive with anti-(gastric PLA2) immunoglobulin in the stomach was quite similar to that of the chief cells. Unlike in pancreas of other animals, the prophospholipase A2 was not detectable in gastric mucosa or juice homogenates treated with diisopropyl fluorophosphate or in column effluents during purification under acidic conditions. An appreciable prophospholipase-A2-activating activity was not detectable in gastric mucosa extracts at low pH relevant to gastric juice, using rat prophospholipase A2 as substrate. This opposes the activation of secreted proenzyme in the gastric juice.

Production of monoclonal antibodies against gastric parietal cell antigens

Two mice DBA/1 were each immunized with a single injection of one million enriched parietal cells in the hind foot pads. Monoclonal antibodies to be used as research tools in studies on regulatory mechanisms in gastric parietal cells were obtained after fusion of mouse myeloma cells (SP2) with cells from the popliteal lymph nodes of the mice. Twelve hybridomas produced antibodies reactive with structures only present in parietal cells as assessed by immunohistochemistry of oxyntic mucosa sections. Three hybridomas were subcloned and the antibodies produced by them, designated as PC4, PC8, and PC117, were characterized. In an enzyme-linked immunosorbent assay, all antibodies reacted with H,K-ATPase-containing vesicles. The antibody PC8 recognized a 94 kDa protein after immunoblotting of H,K-ATPase-containing vesicles and all antibodies precipitated a 94 kDa protein from [125I]H,K-ATPase-containing vesicles. The antibodies PC4 and PC117 recognized extracellular structures with a polarized distribution in viable, purified parietal cells. The results suggest that the structure recognized by all three antibodies is the alpha-subunit of the H,K-ATPase. The antibodies produced by another hybridoma, PC43, recognized a structure present in parietal and surface epithelial cells of the oxyntic mucosa. In an enzyme-linked immunosorbent assay, they reacted with a high-activity carbonic anhydrase which had been affinity-purified from pig oxyntic mucosa and they recognized a 30 kDa protein after immunoblotting. Thus, monoclonal antibodies against both intracellular and extracellular parietal cell structures were obtained after immunization with a small number of parietal cells.

Occurrence of phospholipase A2 and lysophospholipase in a gastric H,K-ATPase-containing membrane fraction, and the formation of lysophosphatidylcholine in stimulated pig parietal cells

A membrane fraction containing H,K-ATPase (EC 3.6.1.36) was prepared from pig gastric mucosa and found to contain phospholipase A2 (EC 3.1.1.4) and lysophospholipase (EC 3.1.1.5) activities. Washing the membranes decreased their protein content by 25%. Recovery profiles of H,K-ATPase, phospholipase A2 and lysophospholipase were similar for membranes washed either with water or with 0.15 or 1.5 M KCl. Nearly identical distribution profiles were obtained for the three enzyme activities after centrifugation of washed vesicle membranes on a linear sucrose gradient. The phospholipase A2 activity was stimulated by calcium and increased further in the presence of calmodulin. The amount of cellular radioactively labelled lysophosphatidylcholine was doubled upon cholinergic stimulation of isolated parietal cells prelabelled with [3H]glycerol or 32Pi. The liberated lyso[32P]phosphatidylcholine had its acyl chain in the sn-1 position, which implies an activation of a phospholipase A2. These findings indicate that secretagogues which increase the cytosolic Ca2+ concentration, i.e. acetylcholine, histamine and gastrin, may activate a phospholipase A2 in the parietal cell.