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 investigat ed. The Ca-45(2+) uptake into the vesicle-reconstituted channels, whil e only moderately (14%) affected by the intact mucus glycoprotein, was found significantly inhibited (59%) by the acidic glycoprotein fracti on. This effect was associated with the sialic acid and sulfate ester groups of the glycoprotein, as their removal caused a loss in the inhi bition. 2. The channel complex in the presence of epidermal growth fac tor (EGF) and ATP responded by an increase in protein tyrosine phospho rylation of 55 and 170 kDa proteins, and the vesicles containing the p hosphorylated channels showed a 50% increase in Ca-45(2+) uptake. The phosphorylation and the calcium uptake were susceptible to inhibition by a specific tyrosine kinase inhibitor, genistein. 3. The channel pro tein phosphorylation was inhibited by the acidic mucus glycoprotein, w hich also interfered with the binding of EGF to the channel protein. T he inhibitory effect was dependent upon the presence of sulfate ester and sialic acid groups, as evidenced by the loss of the glycoprotein i nhibitory capacity following their removal. 4. The results suggest tha t the acidic gastric mucus glycoproteins, by modulating the EGF-contro lled calcium channel phosphorylation, play a major role in gastric muc osal calcium homeostasis.