Ultrastructural changes during stimulation of amphibian oxyntic cells viewed by scanning and transmission electron microscopy

Changes in the cell membranes of the bullfrog gastric mucosa with Acid secretion

The effective area, resistance, and configuration of the apical and basolateral cell membranes of the bullfrog gastric mucosa were studied as a function of acid secretion rate, by alternating-current impedance methods. The drop in transepithelial resistance with acid secretion is attributed to the great increase in apical membrane area (hence conductance) associated with tubulovesicles. There is no evidence of a change in basolateral membrane resistance or of apical membrane premeability per unit area.

Expression of H(+),K(+)-ATPase and glycopattern analysis in the gastric glands of Rana esculenta

A multidisciplinary study involving lectin histochemistry, IHC, immuno-lectin blotting, and immunogold was carried out to determine the distribution of sugar residues in the glycoproteins of Rana esculenta oxynticopeptic cells. We considered animals in two experimental conditions, fasting and fed. It is known that, in mammals, the tubulovesicular membranes are rich in proteins with several functions. The proton pump H(+),K(+)-ATPase, a heterodimeric complex with a catalytic alpha-subunit and a heavily glycosylated beta-subunit, responsible for acid secretion, is the most abundant. No data have been published regarding the localization and the structures of H(+),K(+)-ATPase in amphibians. In the water frog, the luminal membrane and tubulovesicular system of oxynticopeptic cells, which differ in morphology according to their functional stage, reacted with the primary gold-conjugated antibody against the H(+),K(+)-ATPase alpha-subunit. By lectin histochemistry and immunoblotting, in the oxynticopeptic cells of R. esculenta we detected the presence of N-linked glycans having fucosylated (poly)lactosamine chains, which could correspond to the oligosaccharide chains of the beta subunit. The latter are somewhat different from those described in mammals, and this is probably because of an adaptation to the different microenvironmental conditions in which the oxynticopeptic cells find themselves, in terms of their different habits and phylogeny.

Ultra-high-resolution scanning electron microscopy of the continuity of cytoplasmic and luminal membranes in frog oxyntic cells

BACKGROUND

Despite numerous previous studies, the presumed continuity of the luminal and tubulovesicular membranes in frog oxyntic (oxyntico-peptic) cells remains to be convincingly demonstrated. This study was undertaken to clarify this question by ultra-high-resolution scanning electron microscopy of specially prepared frog stomach specimens before and during histamine stimulation.

METHODS

Fasted Japanese meadow frogs stimulated with histamine were used. Oxyntic cell cytoplasmic matrix was removed by the aldehyde-osmium-DMSO-osmium maceration procedure and impregnated with osmium-hydrazine. Specimens were examined without metal coating.

RESULTS

In the resting oxyntic cell, the luminal membrane had closely packed surface folds forming a rather flat surface with a few short microvilli. In the cytoplasm, flattened 200-500 nm vesicles were interconnected by numerous slender 20-60 nm tubules forming the tubulovesicular network. Occasional slender tubular branches were found in continuity with the luminal membrane. After histamine stimulation, the number and length of microvilli and surface folds increased, whereas the tubulovesicular membrane system decreased. Sites of clear continuity between the luminal and tubulovesicular membranes were not abundant but were clearly demonstrated in histamine-stimulated oxyntic cells. The small size of the tubules connecting the tubulovesicular system to the plasma membrane renders this observation by transmission electron microscopy very difficult. In these specimens, the clear continuity of the tubulovesicular network to the luminal plasma membrane became more evident.

CONCLUSIONS

This study confirms previous findings of increased luminal membrane and depletion of the tubulovesicle system. The demonstration of continuity between these two compartments in our SEM preparations supports the hypothesis of direct transfer of tubulovesicular membrane to oxyntic cell luminal secretory membrane.

Activation of apical chloride channels in the gastric oxyntic cell

Oxyntic cells that retain distinct morphological polarity between apical and basolateral membranes were isolated from the gastric mucosa of the amphibian Necturus. Patch-clamp techniques were applied to these cells to identify apical membrane ion channels associated with hydrochloric acid secretion. A single class of voltage-dependent, inwardly rectifying chloride channels was observed in the apical membranes of both resting and stimulated (acid-secreting) oxyntic cells. Stimulation of the cells with dibutyryladenosine 3',5'-monophosphate and isobutylmethylxanthine increased channel open probability and simultaneously increased apical membrane surface area. This chloride channel is probably responsible for electrogenic chloride secretion by the gastric mucosa and may also participate in the fluid- and enzyme-secretory functions of the oxyntic cell, analogous to the chloride channels found in the apical membranes of other exocrine cells.

The early changes of parietal cell structure in the course of secretory activity in the rat

The fine structure of the rat parietal cell was studied, both at rest and after stimulation by refeeding or insulin administration. Experiments on fixation procedures showed that whenever the fixative contained sucrose at a concentration higher than 0.2 M, the system of cytoplasmic membranes was clearly tubular in arrangement, whereas the omission of sucrose in the fixative usually resulted in a vesicular structure. The study with the high-voltage electron microscope of thick sections prepared by conventional techniques or by impregnation with zinc iodide-osmium (ZIO) revealed that the tubules are grouped into fascicles, and that these form a feltwork that is especially thick toward the cell apex. The development of the secretory canaliculus after stimulation appears to take place by an in situ remodeling of the cytoplasmic domain occupied by the tubular system. Cells examined after short periods of stimulation (5-15 min) showed images of the tubular system and of the canalicular structure which differed both from the nonstimulated and from the fully active (30-45 min of stimulation) cell. These features include the formation of wide cisternae and of pericanalicular cytoplasmic trabeculae or laminae, whose fine structure bears close resemblance to that of the intracanalicular processes in the same cells. These images can be ordered into a hypothetical sequence which is proposed as a model to explain the transformation of the tubular system and intervening cytoplasmic matrix into secretory canaliculus.

VIP inhibits histamine-induced ultrastructural changes related to acid secretion by parietal cells

We have studied the in vitro effect of VIP and histamine on ultrastructure of the parietal cells in isolated guinea pig fundic glands. The morphological changes induced by histamine in the parietal cells can be compared to those observed after histamine stimulation in vivo or in vitro on gastric mucosa preparations. In contrast, VIP incubation did not produce the ultrastructural changes related to gastric acid secretion, in resting parietal cells. Pretreatment of the glands by VIP resulted in a remarkable suppression of the histamine effect, since the parietal cells assumed an almost resting state. The data (1) indicate that the parietal cells in isolated gastric glands of the guinea pig retain in vitro the capacity to undergo the ultrastructural changes that are related to acid secretion in vivo after histamine or cAMP and (2) suggest that VIP is an inhibitor of histamine-induced gastric acid secretion in the guinea pig. It is proposed that VIP could act directly on the parietal cell via cAMP-phosphodiesterase activation, or indirectly via gastric somatostatin and/or prostaglandin secretions, inhibiting the H2 receptor-cAMP system of the parietal cell.

Use of AC impedance analysis to study membrane changes related to acid secretion in amphibian gastric mucosa

We have applied transepithelial AC impedance techniques to gastric mucosa to reconcile ultrastructural and electrophysiological findings about gastric acid secretion and the mucosal barrier. By fitting impedance data measured at different HCl secretion rates to equivalent circuit models, we extracted capacitances and resistances (as measures of membrane area and ionic conductance, respectively) for the apical and basolateral membranes. The impedance measurements were found to be incompatible with earlier equivalent circuit models that modeled membrane electrical properties as lumped circuits based on one or two cell types. A distributed circuit model was developed that assumed only one dominant electrical pathway (i.e., one cell type), but that incorporated electrical effects arising from long and narrow membrane-lined structures present in the epithelium (e.g., gastric crypts, tubulovesicles, lateral intercellular spaces). This morphologically based model was found to represent the measured data accurately, and to yield values for membrane capacitances consistent with morphometric measurements of membrane areas. The main physiological conclusions from this analysis were as follows: (a) The dominant transepithelial current pathway may reside in the oxyntic cells. (b) The transepithelial conductance increase associated with the onset of acid secretion is entirely due to increased conductance of the apical membrane. This is in turn due entirely to increased area of this membrane, resulting from incorporation of tubulovesicular membrane. (c) When membrane conductances are normalized to actual membrane area by use of membrane capacitances, it turns out that acid secretion is not associated with a change in specific ionic conductance (change in conductance per unit area) at either the apical or basolateral membrane. (d) The puzzlingly low value of transepithelial resistance (</=400 Omega-cm(2)) arises because there are hundreds or thousands of square centimeters of actual membrane area per square centimeter chamber area. Apical membrane resistance is 25 kOmega-cm(2) (actual membrane area), implying a tight barrier to back-diffusion of protons.