Changes in parietal cell membranous structures that accompany the onset of acid secretion were studied with electron microscopy using isolated gastric glands from rabbit. A stereological analysis was performed to quantitate the morphological changes occurring within 5 min following histamine stimulation. These changes were compared to the changes resulting from osmotic expansion of parietal cell components following addition of 1 mM aminopyrine (AP) to glands incubated in medium containing 108 mM K + (high-K+). Morphometric analyses, together with measurements of glandular water content, indicated that parietal cells swell in high-K + medium. Addition of 1 mM AP to glands incubated in high-K + medium resulted in massive distention of the secretory canaliculus but no difference was observed in the amount of tubulovesicular membrane or the relative size of these cytoplasmic structures. In the histamine-treated glands the parietal cells displayed a rapid loss of tubulovesicular membrane and a reciprocal increase in canalicular membrane. These morphological changes were complete long before a maximum level of acid formation was achieved. Taken together, these results indicate that; (i) the morphological change accompanying stimulation does not require acid formation per se; (ii) the site of acid secretion is the intracellular canaliculus and not the tubulovesicles; (iii) there is no preexisting actual or potential continuity between the tubulovesicular space and the canalicular space; and (iv) the AP-induced expansion of the canaliculus in high-K + medium, while yielding some valuable information, is not an appropriate model for studying the normal stimulus-induced morphological transition, despite a superficial similarity of appearance.Key words gastric glands . acid secretion -morphometry 9 parietal cells . histamine stimulation . aminopyrine accumulation 0022-2631/82/0067-0113 $02.40 9 1982 Springer-Verlag N e w Y o r k Inc.

Gastric glands were isolated from rabbit gastric mucosa and incubated in the presence of secretin, cholecystokinin octapeptide (CCK-8), and in hyperosmolar medium. These agents all elicited a compound exocytotic response from the chief cells observed by electron microscopy. Responses to secretagogues yielded significantly higher values than controls at 2.5 min after exposure, and were essentially complete within 30 min. The response to hyperosmolarity was more gradual and continued over a 60 min period. Glands incubated in hyperosmolar medium were still capable of responding to secretagogues, and the combination was additive. The results indicate that both secretagogues, which initiate specific receptor mediated events, and hyperosmolarity, which causes a more generalized change in the cells, result in pepsinogen-granule release by compound exocytosis.

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Gastric glands were isolated from rabbit stomach and pepsinogen secretion was measured after stimulation with isoproterenol, forskolin, 8-bromo cyclic adenosine monophosphate (8-bromo cAMP), cholecystokinin octapeptide (CCK-OP), carbachol, and hyperosmolar medium. The responses to these stimuli in medium containing 143 mM Na+ and 5.4 mM K+ (normal medium) were compared with responses to the same stimuli in media containing either 0 Na+ and 5.4 mM K+, or 143 mM Na+ and O K+. In addition, the effects of ouabain and furosemide on secretion elicited by these stimuli were determined. Medium containing 0 Na+ inhibited all stimuli. Medium containing 0 K+ inhibited the action of 8-bromo cAMP and stimuli postulated to be mediated by cAMP. Ouabain inhibited the same stimuli as O K+ medium, and, in addition, inhibited the response to hyperosmolar medium. However, ouabain enhanced the response to CCK-OP. Furosemide inhibited the response to hyperosmolar medium but had no effect on the action of any secretagogue employed. Intraglandular [Na+] increased and [K+] decreased after exposure to K+-free medium or ouabain. cAMP content of the glands was assayed after stimulation with both isoproterenol and hyperosmolar medium. Isoproterenol and hyperosmolar medium significantly increased cAMP levels. The results are discussed in relation to possible involvement of ion transport or intracellular ion concentration in the secretory process.