Secretory Protein Synthesis in the Stimulated Rat Parotid Gland

Abstract

Administration of the j3-adrenergic drug, isoproterenol (IPR), affects the release of 98% of stored amylase from rat parotid gland acinar cells . A period of 6 h elapses from the onset of secretion to the maximum [14C]phenylalanine (Phe) incorporation into total protein and amylase . 10 h after IPR administration the rate of [ 14C]Phe incorporation into total protein was no longer elevated above that of control . Incorporation into amylase, however, remained elevated above the control by 2 .3 times . This latent period may reflect :(a) reduced amounts of available ATP which occurs as a result of the process of secretion as well as (b) the time required for reorganization of cellular organelles and membranes after secretion. The latent period after IPR-induced secretion appears similar to the latent period which has recently been reported to occur after physiologic release of amylase from the parotid gland during the diurnal feeding cycle of the rat . These observations support the existence of a positive feedback system operant in the parotid acinar cell linking the release of secretory proteins with their synthesis . The period of greatest protein synthesis is, however, temporally dissociated from the secretory process .

Calcium-dependent inhibition of protein synthesis in rat parotid gland

“…Studies in vivo by Sreebny et al (1971 ) established that secretory protein synthesis is stimulated 6h after induction of salivation by mastication. Moreover, this stimulatory effect appeared to be mediated by the fl-adrenergic receptor, for isoprenaline, a f8-adrenergic stimulant, also produced a similar pattern of accelerated protein synthesis (Johnson & Sreebny, 1973;Lillie & Han, 1973). In the present study we were unable to observe any increase in protein-synthetic rates after stimulation with isoprenaline or dibutyryl cyclic AMP.…”

“…In the present study we were unable to observe any increase in protein-synthetic rates after stimulation with isoprenaline or dibutyryl cyclic AMP. The relatively short incubation period used in the present experiments may have precluded detection of stimulated synthesis, which becomes apparent only after 4h (Lillie & Han, 1973 In common with other a-adrenergic and cholinergic effects, inhibition of protein synthesis required extracellular Ca2+ and was probably mediated by a rise in intracellular free Ca2+. ,B-Adrenergic activation may also increase intracellular free Ca2+ by releasing Ca2+ from intracellular Ca2+ pools (Dormer & Ashcroft, 1974;Kanagasuntheram & Randle, 1976 …”

“…Activation of the fl-adrenergic receptor stimulates adenylate cyclase Wojcik et al, 1975) and induces a rapid and copious release of amylase (Bdolah et al, 1964;Butcher et al, 1975;Mangos et al, 1975). Moreover, the incorporation of radioactively labelled amino acids into trichloroacetic acid-insoluble proteins and amylase is also increased 4-6h after fl-adrenergic stimulation (Lillie & Han, 1973;Johnson & Sreebny, 1973). These findings have led to the suggestion that cyclic AMP serves as the second messenger for both biosynthesis and release of secretory protein.…”

Dynamic changes of the luminal plasmalemma in stimulated parotid acinar cells. A freeze-fracture study.

“…The ultrastructure of the rat parotid gland, as known from conventional electron microscopy studies (3,12,27), can be easily recognized in freeze-fracture replicas. In the unstimulated gland ( Figs 9 1-4), the acinar cells are characterized by a regularly ordered rough-surfaced endoplasmic reticulum, a large fenestrated Golgi complex, and the abundance of large secretion granules, which appear as rounded cavities (face P)~ or elevations (face E).…”

“…The fusion of the membrane of the secretion granules with the luminal portion of the ptasmalemma in the rat parotid acinar cells appears to satisfy this requirement because, in contrast to the situation observed in other exocrine glands, such as the pancreas of the guinea pig (13), the two membranes are clearly distinguishable in freeze-fracture. Furthermore, the system appears favorable for at least two additional reasons: (a) the secretory activity of the acinar cells can be easily synchronized by pharmacological stimulation (3,7,12,27); (b) the radius of the curvature of both participating membranes is large and, therefore, the intramembrane surfaces exposed by fracturing are also large, a situation which is of great help in this type of morphological investigation.…”