Structure and function of intracellular proteoglycans.

Proteoglycans are generally thought of in terms of extracellular function, but there is evolving evidence that they are also an important constituent of secretory granules. Although the mast cell appears to be uniquely endowed with granule proteoglycan heparin, the peripheral blood elements such as basophils, neutrophils, and platelets contain predominantly chondroitin sulfates which have not been fully characterized and are presumed to be of granular origin. Coupled activation-secretion of the rat mast cell and the human platelet, respectively, has demonstrated the release of a proteoglycan-protein complex. It is suggested that the intracellular proteoglycans serve as the granule matrix to allow the storage of structurally diverse chemical mediators, their transport during the cell secretory response, and, finally, their discharge from the granule to the microenvironment. The granule constituents of the rat mast cell differ after discharge in their bioavailability thereby suggesting that the proteoglycan may have a regulatory function in the extracellular environment.