Salivary mucin modulation of buccal mucosal calcium channel activity

The effect of the low and high molecular weight salivary mucins on the activity of calcium channel isolated from buccal epithelial cell membranes was investigated. The 45Ca2+ uptake into the vesicle-reconstituted channels, while only moderately (15%) affected by the intact mucin forms, was significantly inhibited (60%) by the acidic mucin fractions. This effect was associated with the sialic acid and sulfate ester groups of the glycoproteins. The channel complex in the presence of epidermal growth factor (EGF) and ATP responded by an increase in protein tyrosine phosphorylation, and the vesicles containing the phosphorylated channels showed a 46% increase in 45Ca2+ uptake. The phosphorylation process was inhibited by the acidic mucins, 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, and the loss of the inhibitory capacity occurred following their removal. The results demonstrate the salivary mucins actively participate in the modulation of buccal mucosal calcium channel activity.