Maturation-related changes in mass and elemental contents of secretory granules as measured by electron-microprobe

Abstract

The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P less than 0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures.

Intracellular Transport and Secretion of Salivary Proteins

“…Experimental support for the operation of these processes during granule maturation is provided by observa-tions that the buoyant density or mass of parotid secretion granules increases as the granules mature (e.g., von Zastrow and Izutsu et al, 1991), and that granule membranes are permeated by various ions and also behave as osmometers (e.g., Arvan et al, 1984;Goddard et al, 1988). More significantly, a study by Wong et al (1991) clearly shows that maturation in situ is accompanied by removal of Na+, K+, and Cl-.…”

A review of electron probe X‐ray microanalysis studies of salivary gland cells

“…Of two particular secretory granule fractions obtained in this way, the lighter fraction labeled earlier in radio-leucine, pulse-chase studies, indicating that the lighter fraction matured into the heavier one. An analysis of the elemental contents of the two fractions found no significant differences, and that the S concentration was high in both fractions (Table 4 and Izutsu et al, 1991a). The S finding suggests that the two secretory granule fractions consist of granules which had matured beyond the condensing vacuole stage.…”

“…The findings from the above in vivo and in vitro EPXMA studies of secretory granule maturation indicate that the maturation process can be divided into an early (Wong et al, 1991) and a late stage (Iversen et al, 1985;Izutsu et al, 1991a;von Zastrow and Castle, 1987). The early stage involves changes that effect the transformation of the condensing vacuole into an early stage secretory granule, and results in an increased electron and mass density of the granule contents (Caro and Palade, 1964;Castle et al, 1972;Palade, 1975;Simson, 1969;Wong et al, 1991).…”

“…The mass density changes are correlated with significant increases in granule S content, and support the hypothesis that S-containing ions interacting with divalent cations (Ca and Mg) play an important role in the granule mass condensation process (Reggio and Dagorn, 1978;Reggio and Palade, 1978;Wong et al, 1991). In contrast to the early changes in secretory granule maturation, the later maturation changes have primarily been studied using preparations of isolated secretory granules (Iversen et al, 1985(Iversen et al, , 1987Izutsu et al, 1991a;von Zastrow and Castle, 1987), and involve transformation of a relatively mature, less dense secretory granule into a secretory granule with more mass density. Moreover, the late stage changes in granule density occur without significant changes in granule Ca or S content, but do involve addition of amylase and other proteins (Izutsu et al, 1991a).…”

“…In contrast to the early changes in secretory granule maturation, the later maturation changes have primarily been studied using preparations of isolated secretory granules (Iversen et al, 1985(Iversen et al, , 1987Izutsu et al, 1991a;von Zastrow and Castle, 1987), and involve transformation of a relatively mature, less dense secretory granule into a secretory granule with more mass density. Moreover, the late stage changes in granule density occur without significant changes in granule Ca or S content, but do involve addition of amylase and other proteins (Izutsu et al, 1991a). Studies from other laboratories indicate that removal of proteins also may occur during the later stages of secretory granule maturation (von Zastrow and Castle, 1987).…”