Transport of alpha-amylase across the basolateral membrane of the pancreatic acinar cell.

Abstract

The flux of a-amylase (1,4-a-D-glucan glucanohydrolase; EC 3.2.1.1) across the basolateral membrane of the acinar cell was measured in the cell-to-bath direction using the whole rabbit pancreas in organ culture. This in vitro preparation is polarized so that apical and basolateral secretions can be collected separately. The unstimulated amylase flux from cell to bathwas substantial at the initial rate (approximately three times the concurrent apical flux). With time, bath amylase au proached a steady-state concentration, suggesting an equsiibrating process. During the same time interval, ductal amylase secretion remained constant. At the steady state, the amylase concentration in the bath was at least an order of magnitude less than its ductal concentration. HQurly replacement of bathing medium reproduced the initial rate of amylase release into the bath for five consecutive hours. Pancreozymin (cholecystokinin), a peptide hormone, did not alter the steady-state bath amylase content, although it greatly augmented ductal amylase secretion. In contrast, a cholinergc agonist greatly increased both the flux from the cell to bath and the ductal secretion of amylase. Taken together, these results indicate a natural bidirectional permeability of the basolateral membrane to digestive enzyme and support evidence previously obtained suggesting that such a permeability might exist. The interior of the pancreatic acinar cell is highly polarized; the rough-surfaced endoplasmic reticulum is located predominantly at the basal or blood-facing side of the cell, while the digestive enzyme-containing secretion granules, the zymogen granules, are concentrated at the cell's apex or its duct-facing surface. A functional analog of this anatomical polarization for the secretion of digestive enzymes is assumed in the popular hypothesis that new protein is synthesized at the basal portion of the cell and moved through a series of membrane-bound compartments to the apical surface, where it is secreted into the duct system by the exocytosis of zymogen granule contents. This construct has been called "vectorial transport" because it proposes that the movement of the secretory product is constrained to one direction, i.e., from the basal part of the cell to the duct lumen. In this formulation, only the apical membrane of the acinar cell allows the passage of digestive enzyme, and only in the cell-to-duct direction (1).Studies that have examined the kinetics of the secretory process directly are in conflict with this "vectorial transport" concept in a number of areas. Not only is the apical membrane apparently permeable in both the cell-to-duct and duct-to-cell direction (2), but similar bidirectional fluxes have been described for digestive enzyme across the zymogen granule membrane (3). Indeed, in this paper we present evidence that the basolateral (blood-facing) membrane of the cell also is bidirectionally permeable to these large molecules. In addition, the evidence suggests that these fluxes are all related by a cy-The costs of ...

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Redistribution of amylase activity accompanying its secretion by the pancreas.

“…Regarding the former possibility, cholinergic stimulation and the peptide hormone cholecystokinin-pancreozymin can greatly augment the basolateral flux of amylase (7-10, 12, 13). This flux may be quite large and has been estimated to account for as much as 20% ofmaximal duct-directed secretion under certain circumstances (7,8,10,12,13 3-hr period and a proportional increase in degradative rate (10-fold), then there would also be a 10-fold increase in amylase activity in the gut. Thus, in order to explain the absence of such an increase in terms of degradation, degradation would have to have been enhanced to a substantially greater degree than secretion.…”

“…The first is that digestive enzymes can be reabsorbed intact although not necessarily in unaltered chemical form (1)(2)(3)(4)(5)(6) and that, at least after several hours of highly augmented secretion, enzyme appears to be conserved in this fashion (2). The second is that there is a substantial flux of at least some digestive enzymes from pancreas to blood-an endocrine secretion that can be greatly enhanced by natural secretagogues and that has been shown to be capable of rates as large as 10-20% of maximal duct-directed secretion (7)(8)(9)(10)(11)(12)(13).…”

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