1. The mechanism of interaction of the polyacrylates, carbopols with the mucus barrier in vivo has been investigated in vitro.

2. Carbopol caused a dramatic increase in the viscosity of porcine gastric mucin solutions that was up to 19-fold greater than that of the sum of the individual polymers.

3. The mucin-carbopol interaction was stable after an initial 30 min period for up to 36 h at 25°C or 37°C. It was reduced by increasing the temperature from 20°C to 45°C, was unaffected by pH and ionic strength, but was enhanced by Ca2+.

4. The magnitude of the interaction between mucin and carbopol depended on the polymeric structure of the mucin and the molecular size and level of cross-linking of the carbopol.

5. The interactions were reversible and increased with increasing carbopol and mucin concentration. The dramatic increase in viscosity can be explained in terms of space filling by the mucin molecules leading to predominantly carbopol-carbopol interactions.

6. Carbopol 934P inhibits pepsin hydrolysis and therefore has potential as a mucosal protective agent in vivo.

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