Secretion of old versus new exportable protein in rat parotid slics. Control by neurotransmitters

The possibility that old and new secretory granules do not mix and that older exportable protein can be secreted preferentially was tested on parotid gland in vitro. Slices from fasted animals were pulse labeled for 3 min with L-[3H]leucine. Subcellular fractionstion showed that after 1 90-min chase period, the formation of new labeled secretory granules was mostly completed. The ratio of label in secretory granules to label in microsomes increased 250-fold during the period 5--90 min postpulse. After the 90-min chase, a submaximal rate of secretion was initiated by adding a low concentration of isoproterenol to the slices. Preferential secretion of old unlabeled exportable protein was evident from the finding that the percent of total amylase secreted was 3.5- fold greater than the percent of labeled protein secreted. Preferential secretion of old unlabeled exportable amylase was undiminished even when the chase period before addition of isoproterenol was extended to 240 min. Such long chase incubations were still meaningful due to the fact that the spontaneous rat of amylase release and radioactive protein release from the slices was negligibly low. A high isoproterenol concentration added to the slices after a 90-min chase produced the following results. An initial phase of preferential secretion of old unlabeled protein was soon replaced by secretion of a random mixture of new and old exportable protein. Electron micrographs indicated that high rates of secretion involved sequential fusion of secretory granules so that the lumen extended deep into the cell where the new labeled granules were presumably located. At low rates of secretion, the lumen showed no such deep extensions. Experiments were also conducted on slices from glands which had been largely depleted of old granules by prior injection of isoproterenol into the animals. Secretion of labeled protein from such slices stopped with the export of 80% of the labeled protein. This finding indicates that about 20% of the radioactive protein is cellular nonexportable protein and that the slices are capable of exporting the entire amount of secretory protein which was symthesized in vitrol. In addition to the beta-adrenergic receptor which mediates protein secretion, the parotid acinar cell also possesses an alpha-adrenergic and a cholinergic receptor both of which cause K+ release, vacuole formation, and water secretion. Activation of either of the latter two receptors in conjunction with the beta- adrenergic receptor increased randomization of the protein secreted. It is concluded that in the rat parotid acinar cell there is little spontaneous mixing between old granules near the luminal cell membrane and new granules coming up behind from the Golgi complex. The neurotransmitters which induce secretion produce the observed randomization.