Formation of an IPN (interpenetrating polymer network) of organic polymer and silica gel in the form of polymer hybrids was accomplished by utilizing the Diels-Alder reaction between maleimide and furan. Maleimide and furan groups were introduced in the side chain of poly(2-methyl-2-oxazoline), respectively. Polymer hybrids were prepared by acid-catalyzed sol-gel reaction of tetramethoxysilane (TMOS) in the presence of these polymers. The progress of the Diels-Alder reaction between maleimide and furan was confirmed by UV and FT-IR spectroscopy. The solvent resistance of the polymer hybrids was improved by the formation of the IPN structure. Retro-Diels-Alder reaction takes place at an elevated temperature, and these reactions can be cycled.

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The gastric mucosa is covered with a viscoelastic and lubricant mucous layer. The chief determinants of the physiological role of the mucous layer are mucins, which are high molecular weight glycoproteins. Gastric mucins are produced in and secreted from specialized differentiated mucous cells located in the epithelium lining the gastric pits, and play an important role in the protection of gastric mucosa from acid-peptic injury. The production and release of mucins are regulated by neurotransmitters, hormones and biologically active peptides. Mucins are synthesized in the Golgi apparatus, and stored in intracellular granules which are transported to the luminal surface of the cell (Forstner & Forstner, 1994). These mucin granules finally discharge their contents through holes in the plasma membrane. This process is generally known as exocytosis. The first event in exocytosis is the fusion of a granule with the plasma membrane at the fusion pore, and is mediated by the exocytosis-related proteins, which are activated by intracellular Ca¥, protein kinase A (PKA) and protein kinase C (PKC) (Forstner & Forstner, 1994). Intracellular Ca¥, in particular, is widely accepted as playing a key role in exocytosis in epithelial cells, endocrine cells and nerve endings. In the gastric mucosa, elevation of intracellular Ca¥ concentration, [Ca¥]é, also increases mucin release (Seidler & Sewing, 1989), and cholinergic stimulation is well known to increase [Ca¥]é. The present experiments were designed to investigate the effects of acetylcholine (ACh) on the frequency of exocytotic events.

Effects of cAMP accumulation on ATP-dependent priming and Ca(2+)-dependent fusion in Ca(2+)-regulated exocytosis were examined in antral mucous cells of guinea pigs by using video-enhanced contrast microscopy. The Ca(2+)-regulated exocytosis activated by 1 microM ACh consisted of two phases, an initial transient phase followed by a sustained phase, which were potentiated by cAMP accumulation. Depletion of ATP by 100 microM dinitrophenol (uncoupler of oxidative phosphorylation) or anoxia induced the sustained phase without the initial transient phase in Ca(2+)-regulated exocytosis. However, accumulation of cAMP before depletion of ATP induced and potentiated an initial transient phase followed by a sustained phase in Ca(2+)-regulated exocytosis. This suggests that the initial transient phase of Ca(2+)-regulated exocytosis is induced by fusion of all primed granules maintained by ATP and that accumulation of cAMP accelerates ATP-dependent priming of the exocytotic cycle. Moreover, ACh and Ca(2+) dose-response studies showed that accumulation of cAMP shifted the dose-response curves to the low concentration side, suggesting that it increases Ca(2+) sensitivity in the fusion of the exocytotic cycle. In conclusion, cAMP accumulation increases the number of primed granules and Ca(2+) sensitivity of the fusion, which potentiates Ca(2+)-regulated exocytosis in antral mucous cells.

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