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S S Rothman

Publications and source records attributed to S S Rothman.

At least 55 records · Page 3Linked to original sources
PubMed1980

Passage of proteins through membranes--old assumptions and new perspectives.

This article contains a summary of the ideas that led to the development of the equilibrium theory of protein secretion as an alternative to the current cisternal packaging-exocytosis paradigm. The argument has at its base the philosophical view that the simplest hypothesis should be favored until it is explicity excluded by the weight of evidence. For the transport of molecules across biological membranes this simplest hypothesis is diffusion, and the equilibrium theory simply includes proteins under this rubric. The article then goes on to consider a particular group of experiments that bear on one of three tests of a diffusion mechanism--evidence for the equilibrating nature of the transport process. The evidence that is presented is consistent with a diffusion-based process and does not support secretion by exocytosis. The article then speculates on the potential means by which individual proteins may pass through membranes and focuses on the observed interactions between a digestive enzyme and the membrane phospholipid, phosphatidylinositol. The potential significance of such interactions in the membrane transport of proteins is discussed.

Amylases↗
PubMed1979

Transpancreatic transport of digestive enzyme.

When porcine alpha-amylase or bovine chymotrypsinogen A was added to the medium bathing the rabbit pancreas in short-term organ culture, the secretion of these enzymes collected via the duct system increased greatly. To determine if it was indeed the amylase added to the bath that was recovered in secretion, endogenous enzyme stores were prelabeled during a 4 h incubation with [3H]-leucine and the specific radioactivity of amylase in secretion followed. The addition of unlabeled exogenous amylase to the bathing medium reduced the specific radioactivity of secreted amylase by about 90% suggesting that the response was due to the transpancreatic transport of the added enzyme. This inhibition was maintained over time, and was a result, not only of the increased secretion of unlabeled enzyme, but also of a 72% steady-state inhibition in the secretion of endogenous (labeled) amylase. This latter observation indicates that the exogenous enzyme crosses the acinar cell and mixes with endogenous cellular stores. A cellular route is also suggested by the observation that the addition of amylase to the bath increased the amylase concentration in ductal fluid relative to that in the bath by about 20 times; it did not reduce it as would be expected if paracellular shunts were involved. In addition, a cellular pathway is suggested by the observation that a 2 h prior incubation in bovine chymotrypsinogen resulted in a greatly augmented chymotrypsinogen response to a maximal cholinergic stimulus. In all, the data support the prediction of the equilibrium theory of digestive enzyme secretion that enzyme secretion should be responsive to mass action, and the prediction of the enteropancreatic circulation hypothesis that a capacity exists for a substantial transpancreatic flux of digestive enzyme.

Amylases↗
PubMed1979

Diffusion-like process can account for protein secretion by the pancreas.

When fluid secretion by the pancreas was mechanically blocked, amylase secretion into the duct ceased. When flow was reduced in a graded fashion by the application of a back pressure, amylase output was reduced proportionately and amylase concentration in secretion was maintained constant. Thus, the secretion of digestive enzyme from the cell into the duct appears to be dependent upon the concentration of enzyme in the duct system. This behavior is most simply explained by diffusion-like (concentration dependent, bidirectional) fluxes of digestive enzyme across the plasma membrane. A unidirectional process, such as exocytosis, whose rate should be unaffected by fluid flow, cannot readily explain these results.

Amylases↗
PubMed1978

Amylase transport across ileal epithelium in vitro.

Amylase transport was measured across the rabbit ileum in vitro employing a modified Ussing chamber. Amylase was moved preferentially in the mucosal to serosal direction. Its rate of transfer was 2--3 orders of magnitude greater than that for inulin. Mucosal to serosal transport of exogenous amylase was completely inhibited in the absence of oxygen. There was also a constant release of endogenous amylase from intestinal tissue into both mucosal and serosal compartments in the absence of an exogenous source. An estimate of the rate of amylase absorption indicates that it may be of sufficient magnitude to account for the enteropancreatic circulation of amylase secreted by the pancreas during augmented secretion.

Amylases↗
PubMed1977

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

The flux of alpha-amylase (1,4-alpha-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 bath was substantial at the initial rate (approximately three times the concurrent apical flux). With time, bath amylase approached a steady-state concentration, suggesting an equilbrating 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. Hourly 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 cholinergic 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.

Acetylcholine↗
PubMed1976

Equilibration of pancratic digestive enzymes across zymogen granule membranes.

Zymogen granules isolated from the rat pancreas released digestive enzyms in response to lowering the enzyme concentration of the medium in which the granules were suspended, and reabsorbed enzyme when the enzyme concentration of the medium was increased. The evidence suggests that the distribution of digestive enzyme between zymogen granule and medium is the result of an equilibrium process. This equilibrium is apparently expressed across the granule membrane since the disruption of the membrane barrier eliminated the delay in establishing new equilibrium between the bound and free enzyme. The release of enzyme was continuous (equilibration did not occur) when the suspending medium was continuously filtered, thereby maintaining a constant state of disequilibrium between intragranular enzyme and enzyme in the medium. The data suggest the following: (1) that digestive enzyme passes through the intact granule membrane bidirectionally, and (2) that there is a free cytoplasmic pool of digestive enzyme in intact cells which is in equilibrium with the enzyme content of the zymogen granule.

Amylases↗
PubMed1976

Production of pancreatitis in rabbits by intestinal re-instillation of stimulated pancreatic secretion.

Pathological changes reminiscent of acute pancreatitis in man were observed in rabbits as a result of the intestinal instillation of pancreatic secretion produced by continuous cholinergic stimulation. Both the cause and nature of the pathological changes suggest a bloodborne disease produced by the absorption of digestive enzyme. It is proposed that acute pancreatitis can be the consequence of the absorption of active digestive enzyme in quantities and at rates sufficient to overwhelm natural defence mechanisms.

Albumins↗
PubMed1976

Secretion of new digestive enzyme by pancreas with minimal transit time.

A small amount of labeled digestive enzyme is secreted within 5 min of adding radioactive leucine to whole rabbit pancreas in vitro. This rapid secretion of labeled protein suggests that some new protein is able to bypass intracellular storage pools and is secreted immediately. The amount of protein secreted this way varies inversely with overall protein secretion in the unstimulated state and is decreased as the result of cholinergic stimulation of protein secretion as well. Under the conditions studied, a mean of 2.5% of secreted protein is apparently not stored prior to secretion.

Amylases↗
PubMed1976

Independent secretion of different digestive enzymes by the pancreas.

Pancreatic secretion was collected from the cannulated duct of anesthetized rabbits during the perfusion of the upper duodenum with a balanced salt solution with or without 5 mM glucose. The secretion of amylase, chymotrypsinogen, trypsinogen, and total protein was measured. While glucose did not increase overall digestive enzyme secretion, it did change the proportions of the enzymes in secretion. In addition, the following was observed: 1) non-0,0 intercepts when the output of one enzyme was plotted against that of another (an enzyme-pair plot), 2) changes in the variance of the slope and intercept of enzyme-pair plots, and 3) an increase in the variance around the slope of an enzyme-pair plot, concurrent with a decrease in the variance around the slope of a plot for another enzyme pair that contained a common member. These observations suggest that different digestive enzymes can be secreted independently of each other.

Amylases↗
PubMed1975

Enteropancreatic circulation of digestive enzymes.

Intact digestive enzymes can be absorbed by the intestine and resecreted by the pancreas. The pancreas, therefore, appears to be able to recycle proteins much as the liver recycles bile salts, although the magnitude of this process remains uncertain.

Amylases↗
PubMed1975

Kinetics of Na+ uptake and transcellular transit by the pancreas.

22Na uptake into strips of rabbit pancreas was measured for up to 10 min. The uptake curve was characterized by the presence of two plateaus separated by an inflexion point; a first "plateau" or an approximation of steady-state uptake was observed between 1 and 3 min; betwen 3 and 4 min the slope of the uptake curve increased again, finally decreasing to a new and higher steady-state uptake between 4 and 6 min. The data suggest that the first part of the uptake curve (from 0 to 3 min) represents uptake into most if not all cells, and the second part (from 3 to 10 min) represents the sum of "quasi" steady-state cellular uptake and of the equilibration of the ductal compartment in series with the cells. In this model a substantial delay (2.5-3.25 min) elapses between the filling of cellular and ductal compartments which is apparently of intracellular origin, implying restricted Na+ diffusion within the cytoplasm and an intracellular Na+ gradient. If this model is correct, then the mean transit time for Na+ across the whole organ should be approximately 3-4 min and be primarily the result of transcellular transit. The mean transit time for Na+ across the whole organ in vitro was measured and found to be 3.5 min on the average. The step that accounts for most of this time appears to be the transepithelial transit of Na+.

Animals↗