Effects of calcium on intestinal mucin: implications for cystic fibrosis.

A major feature of the disease cystic fibrosis is the excessive concentration 
of mucus within ducts and glands of mucous-producing organs. Some mucous 
secretions also show an elevation in calcium concentration. Using purified rat 
intestinal goblet cell mucin as a model mucin, we have investigated 
the effect of millimolar additions (1-25 mM) of CaCl2 on the 
physical properties of the mucin. Isotonicity of incubation media was preserved 
in order to mimic in vivo conditions. CaCl2 (8-15mM) caused a 
15-33% decrease in viscosity, no change in electrophoretic mobility in acrylamide 
gels, and a 20-30% decrease in solubility of the mucin. Solubility 
changes were reversed by the addition of EDTA (20 mM) to 
incubations. Insolubility was also produced in incubations of mucin with a 
mixture of soluble intestinal contents (NaCl washings). These findings strongly suggest 
that the mucin became smaller and more dense as calcium was 
added, a process most probably achieved by loss of intramolecular water. 
