Depletion of total acetylcholine by hemicholinium-3 in isolated rat diaphragm is less in the presence of dexamethasone

Abstract

Low concentrations of dexamethasone (Dex) stimulate the initial rate of radioactive choline (Ch) accumulation in the endplate-rich area (EPA) of indirectly stimulated hemidiaphragms, while higher concentrations (>0.6μM) inhibit. This biphasic concentration—effect curve is found even in the presence of 26 μM hemicholinium-3 (HC-3), an inhibitor of Ch accumulation. In incubations (3 min) where the total hemidiaphragm acetylcholine (ACh) content is not altered by 26 μM HC-3, the inhibition by HC-3 of both the Ch accumulation rate and the incorporation of radioactive Ch into ACh in the EPA of stimulated tissues is less in the presence of 0.2 μM Dex. In 120 min incubations with 15 μM HC-3 and without added Ch, the tissue ACh content is depleted in both stimulated and unstimulated hemidiaphragms. In both cases the depletion of ACh is significantly less in the presence of 0.2 μM Dex. In stimulated tissues a comparable depletion of ACh due to 15 μM HC-3 is also found with 1 and 10 μM Ch added to the medium. It is significantly less when 0.2 μM Dex and 1 μM Ch are added to the medium. In 120 min incubations with stimulated tissue, the amount of ‘bound’ ACh is increased by addition of 30 μM Ch to the medium, decreased in the presence of 0.2 μM Dex, and greatly decreased in the presence of 15 μM HC-3. In the presence of Dex plus HC-3, the decrease in the amount of ‘bound’ ACh due to either Dex or HC-3 alone, is abolished provided that 30 μM Ch is also present. The greater proportion of this ‘bound’ ACh is probably derived from the nerve endings, since it is reduced by approx. 60% 4 days after denervation. It is suggested that Dex is interacting with the effects of HC-3 in rat diaphragm via a neural Ch transport system, possibly at a place other than the hemicholinium binding site.