Modulation of gastric mucosal calcium channel activity by mucus glycoprotein

1. The effect of gastric mucus glycoprotein on the activity of calcium channel isolated from gastric epithelial cell membrane was investigated. The 45Ca2+ uptake into the vesicle-reconstituted channels, while only moderately (14%) affected by the intact mucus glycoprotein, was found significantly inhibited (59%) by the acidic glycoprotein fraction. This effect was associated with the sialic acid and sulfate ester groups of the glycoprotein, as their removal caused a loss in the inhibition. 2. The channel complex in the presence of epidermal growth factor (EGF) and ATP responded by an increase in protein tyrosine phosphorylation of 55 and 170 kDa proteins, and the vesicles containing the phosphorylated channels showed a 50% increase in 45Ca2+ uptake. The phosphorylation and the calcium uptake were susceptible to inhibition by a specific tyrosine kinase inhibitor, genistein. 3. The channel protein phosphorylation was inhibited by the acidic mucus glycoprotein, which also interfered with the binding of EGF to the channel protein. The inhibitory effect was dependent upon the presence of sulfate ester and sialic acid groups, as evidenced by the loss of the glycoprotein inhibitory capacity following their removal. 4. The results suggest that the acidic gastric mucus glycoproteins, by modulating the EGF-controlled calcium channel phosphorylation, play a major role in gastric mucosal calcium homeostasis.