An osmotic mechanism for exocytosis from dissociated chromaffin cells

Dissociated chromaffin cells from bovine adrenal medulla were stimulated to secrete epinephrine and dopamine beta-hydroxylase with a variety of secretagogues in a study designed to test the hypothesis that the chemiosmotic lysis reaction of isolated chromaffin granules might in some way be related to the mechanism of release during exocytosis. Increasing the osmotic strength of the incubation medium with either NaCl or sucrose led to suppression of secretion of epinephrine from the cells regardless of whether secretion was induced with veratridine or acetylcholine. Suppression of secretion was approximately exponential with respect to osmotic strength. Epinephrine secretion occurred only if the medium contained a permeant anion such as chloride, and secretion induced by veratridine was suppressed when Na isethionate replaced NaCl in the medium. In an extensive study with different monovalent anions veratridine supported epinephrine secretion according to the following activity series: Br-, I-, NO3- greater than methylsulfate, SCN- greater than Cl greater than acetate much greater than isethionate. A similar series, except for the potency of NO3-, was observed with A23187 as agonist. In general, the anion series for granule lysis was analogous. However, there was a poor quantitative correlation between the anion dependence of chemiosmotic granule lysis and the anion dependence of cell secretion. Anion transport inhibitors such as probenecid and pyridoxal phosphate also inhibited secretion while the stilbene disulfonates were inactive. The ineffectiveness of the stilbene disulfonates further distinguished chemiosmotic granule lysis from cell secretion. Secretion of catecholamines, induced by veratridine or nicotine, a cholinergic agonist, was suppressed when NaCl in the medium was replaced by isosmotic sucrose and unexpectedly low levels of dopamine beta-hydroxylase were observed in some cases. In sum, these properties of secreting chromaffin cells resembled some properties of isolated chromaffin granules incubated in ATP and Cl-, but were different in a number of instances. We, therefore, have interpreted our data to indicate that while some mechanistic relationships may indeed exist between the release event in exocytosis from chromaffin cells and the chemiosmotic lysis reaction characteristic of isolated chromaffin granules, an understanding of the energetics of exocytosis awaits the discovery of reasons for the quantitative differences between the two systems.