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 .
The cytoplasm of the megakaryocyte in the rat spleen possesses three zones, the perinuclear, intermediate and marginal. The perinuclear zone is characterized by the presence of Golgi apparatus, ribosomes, endoplasmic reticulum, and mitochondria. These organelles are found also in the more voluminous intermediate zone which in addition exhibits platelet granules and an extensive development of vesicles from smooth-surfaced endoplasmic reticulum to form demarcation membranes by coalescence. The marginal zone is almost devoid of the organelles and inclusions present elsewhere. Shedding of platelets appears to occur by extension of a paired demarcation membrane from the intermediate zone to the cell membrane and subsequent separation of its lamellae so that all of the essential organelles and inclusions of the intermediate zone may be included within the platelet. In addition, platelets contain vesicles which are probably pinocytotic in nature. Platelets are sometimes engulfed by the cytoplasm o f phagocytic cells.
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