Comparison of secretion of catecholamines from the rat adrenal medulla during continuous exposure to nicotine, muscarine or excess K.

The effects of continuous exposure of chromaffin cells to nicotine, muscarine and excess potassium on the secretion of catecholamines and uptake of radiolabeled calcium were investigated in the isolated perfused adrenal gland of the rat. Perfusion with 20 microM nicotine-Krebs solution increased catecholamine secretion from a basal value of 5 ng to 273 ng in the first 5 min. The secretion declined to about 20 ng after 30 min of continuous exposure to nicotine (slow desensitization). 200 microM muscarine increased the secretion to about 150 ng within the first 5-10 min, and remained at the level of 110 ng even after continuous perfusion with muscarine for up to 40 min (no desensitization). About 700 ng of catecholamines were secreted within the first 5 min of exposure to 55 mM K, the secretion declined to about 350 ng in the next 5 min, and remained almost at this level for 120 min during continuous perfusion with excess K (partial desensitization). When the secretion of catecholamines was depressed by continuous exposure to nicotine for 30 min, introduction of 55 mM K or 200 microM muscarine (along with nicotine) still led to a large increase in the secretion of catecholamines. Omission of calcium ions from the perfusion medium containing either nicotine or 55 mM K caused a rapid decline in the secretion of catecholamines, suggesting influx of external calcium, but muscarine-induced secretion was only modestly reduced by calcium removal, suggesting mobilization of internal calcium. Addition of EGTA to calcium-free medium completely abolished the secretion evoked by all agents.(ABSTRACT TRUNCATED AT 250 WORDS)