Electrolyte permeabilities of pancreatic zymogen granules: implications for pancreatic secretion

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Abstract
Zymogen granules from rat pancreas were prepared on a 40% Percoll gradient at free calcium levels less than 0.2 microM. We have previously shown [Am. J. Physiol. 246 (Gastrointest. Liver Physiol. 9)] that zymogen granules prepared by this method are stable in vitro for more than 1 h in "physiological buffers." The electrolyte permeabilities of the zymogen granule membrane were investigated to determine the basis for this stability. Ionic permeabilities were estimated from rates of osmotic lysis and measured as decrease in optical density (OD) of granule suspensions. OD correlated linearly with lysis, as indicated by release of amylase, except for the highest and lowest 10% of the OD of intact granules. Lysis of freshly isolated granules was slow in Na+ or K+ salt solutions (e.g., t1/2 approximately 3 h for Cl-) but was accelerated 5- to 50-fold when cation ionophores were present simultaneously. This behavior indicates that zymogen granules have low endogenous permeabilities to the cations Na+ and K+, but are highly permeable to a variety of anions. Both anion conductance and anion-exchange pathways were found. The relative selectivity of the anion conductance pathway was SCN- greater than Br- approximately NO-3 greater than SO2-(4) greater than acetate- approximately Cl- greater than isethionate-. The relative selectivity sequence for anion/-OH- exchange was acetate- greater than SCN- greater than Br- approximately NO-3 approximately Cl- much greater than isethionate- greater than SO2-(4). The anion transport blocker DIDS blocked the electrogenic pathway with a half-maximal effectiveness at approximately 2 microM. DIDS had little effect on the anion-exchange pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

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