Polymer gel phase transition in condensation-decondensation of secretory products

Abstract

The molecular mechanisms of product storage and release in secretion remain unknown. Mucins stored in secretory granules form a highly condensed polymer network. An important condition for mucin condensation is the shielding of their polyionic charges, which depends on the presence of large amounts of Ca²⁺ inside the secretory granule. Experiments conducted on rabbit respiratory goblet cells, and in isolated giant mucin granules of the slug Ariolimax columbianus, indicate that upon exocytosis the mucin network undergoes dramatic swelling, expanding its volume as much as 250 to 1000 fold, respectively. Swelling of the granular matrix is governed by Donnan equilibrium, and follows a typical first-order kinetics. Depending upon the composition of the medium, the exocytosed mucin network can be recondensed. The observed volume changes are reversible and exhibit the characteristic discontinuity, as well as the temperature and pH dependence found in polymer gel phase transitions.

The evidence reviewed here is consistent with the idea that the condensed conformation of secretory products during storage in the cell, and their hydrated conformation upon release from the cell, reflect the corresponding condensed and decondensed phases of a polymer gel. Product release in exocytosis would result from a polymer gel phase transition that is probably triggered by a polycation Z^≥+2/Na⁺ ion exchange via the secretory pore.