The Effect of Sodium and Calcium Ions on the Release of Catecholamines from the Adrenal Medulla: Sodium Deprivation Induces Release by Exocytosis in the Absence of Extracellular Calcium

1. Perfusing bovine adrenal glands with Na(+)-free Locke solution for 15-40 min did not modify the increase in the release of catecholamines from glands stimulated by acetylcholine. However, after 80-100 min of perfusion with Na(+)-free solution, the response to acetylcholine stimulation was decreased or abolished.2. Immediately after switching the perfusion medium to Na(+)-free solution, there was a sharp increase (6-10 times over control values) in catecholamine output.3. Graded substitution of Na(+) in the perfusion fluid enhanced the output of catecholamines. This increase in the output of amines was linearly related to the logarithm of the extracellular Na(+) concentration.4. The release of catecholamines in the absence of Na(+) was not reduced by the presence of atropine and hexamethonium nor by the omission of Ca(2+) in the presence of EDTA or EGTA.5. Excess of Mg(2+) in the perfusion fluid reduced (10 mM-Mg(2+)) or blocked (20 mM-Mg(2+)) the increase in the output of catecholamines induced by Na(+) deprivation in the presence or absence of extracellular Ca(2+).6. Na(+) deprivation induced release of catecholamines during perfusion of the glands with depolarizing concentrations (56 mM) of K(+).7. In the presence or the absence of extracellular Ca(2+), the increase in the output of catecholamines induced by Na(+) deprivation was accompanied by an increase in the output of dopamine beta-hydroxylase, but not of lactate dehydrogenase. In addition, during perfusion with Ca(2+) free solution, Na(+) deprivation induced a parallel increase in both catecholamine and adenosine triphosphate outputs.8. The ratios of catecholamines to dopamine beta-hydroxylase and catecholamines to adenosine triphosphate determined in the perfusates collected from glands during perfusion with Na(+)-free medium were similar to those measured in the soluble contents of isolated chromaffin granules. These results provided biochemical evidence in favour of exocytosis as the mechanism of secretion during Na(+) deprivation.