Restitution of granule stores in the rabbit parotid gland after isoprenaline-induced secretion. A stereological analysis of volume parameters.

A detailed stereological analysis has been made of the organelle content of rabbit acinar cells during the restoration of granule stores following extensive degranulation with isoprenaline (IPR). Rabbits were sacrificed 2, 4, 8, 12 and 16h after IPR administration and the volumes and proportions of intracellular organelles were compared with those of untreated glands. At 2h only 5% of cell volume was occupied by secretion granules, but there was already evidence of nascent granule formation. The volume per cell of secretion granules increased in sigmoid fashion and by 16h amounted to 350 micron3/cell, 36% of cell volume, which are values similar to those of the control replete glands. IPR treatment caused some initial swelling of the cells, and there were transient increases in the volumes of several compartments. However, the volume of mitochondrial and lysosomal compartments had returned to control levels by 4h and that of the nucleus by 8h. The greatest increase was in the volume of the rough endoplasmic reticulum which had increased by nearly 70% by 2h and remained enlarged throughout the period of restitution. However, neither the volume nor the proportion of the smooth membraned compartment varied throughout the period of analysis. The results are analysed in the light of the overall response of the cells to IPR and the interaction of the organelles during the synthetic phase of the secretory cycle. They are presented as a basis for ensuing studies of the granule populations and the membrane composition of the cells during the restoration of granule stores.