Acetylcholine stores in rat diaphragm are increased by higher concentrations of dexamethasone

Abstract

In indirectly stimulated hemidiaphragms, dexamethasone (Dex, 2 μM) causes a significant increase in tissue acetylcholine (ACh) content. No increase in tissue ACh is found with 0.2, 0.6 or 6 μM Dex. Physostigmine (Physo, 15 μM) also causes an increase in tissue ACh, which is even greater in the presence of Dex (6–25 μM). There is no increase in ACh due to Dex, with 50 μM Dex plus Physo. The order of addition is important, as the Dex-induced increased in ACh is only found when Dex is added before Physo. No increase in twitch tension is found with any of these treatments. No Dex-induced increase in ACh is found with unstimulated hemidiaphragms. Similar increases in ACh are also found with other glucocorticoids (plus Physo), namely prednisolone (6–9 μM), corticosterone (2 μM) and hydrocortisone (2 μM). The mineralocorticoid aldosterone (2 μM), and other types of steroids cause no increase in tissue ACh. The increases in hemidiaphragm ACh are not found in a Na +-depleted medium, or in a medium containing 20 mM Mg 2+ extra. The increase in ACh due to Physo is Ca 2+-dependent, even though an increase in ACh due to Dex plus Physo is found in the absence of Ca 2+. No increase in ACh due to either Physo, or Dex plus Physo are found in the presence of the nicotinic antagonists (+)-tubocurarine (5 μM) or alpha-cobrotoxin (5 μg/ml), while the muscarinic ligands atropine or oxotremorine (10 μM) abolish the extra increase in ACh due to Dex. Perhaps the glucocorticoid-specific increase in ACh stores may be caused by a direct interaction of the corticosteroids with an ACh receptor (possibly presynaptic) which may regulate ACh synthesis and/or output.