Radioautographic Analysis of the Secretory Process in the Parotid Acinar Cell of the Rabbit

Intracellular transport of secretory proteins has been studied in the parotid to examine this process in an exocrine gland other than the pancreas and to explore a possible source of less degraded membranes than obtainable from the latter gland. Rabbit parotids were chosen on the basis of size (2-2.5 g per animal), ease of surgical removal, and amylase concentration. Sites of synthesis, rates of intracellular transport, and sites of packaging and storage of newly synthesized secretory proteins were determined radioautographically by using an in vitro system of dissected lobules capable of linear amino acid incorporation for 10 hr with satisfactory preservation of cellular fine structure. Adequate fixation of the tissue with minimal binding of unincorporated labeled amino acids was obtained by using 10% formaldehyde-0.175 M phosphate buffer (pH 7.2) as primary fixative. Pulse labeling with leucine-(3)H, followed by a chase incubation, showed that the label is initially located (chase: 1-6 min) over the rough endoplasmic reticulum (RER) and subsequently moves as a wave through the Golgi complex (chase: 16-36 min), condensing vacuoles (chase: 36-56 min), immature granules (chase: 56-116 min), and finally mature storage granules (chase: 116-356 min). Distinguishing features of the parotid transport apparatus are: low frequency of RER-Golgi transitional elements, close association of condensing vacuoles with the exit side of Golgi stacks, and recognizable immature secretory granules. Intracelular processing of secretory proteins is similar to that already found in the pancreas, except that the rate is slower and the storage is more prolonged.