Cyclic AMPによる培養ウシ副腎髄質クロマフィン細胞からのカテコールアミン遊離増強機序に関する研究

Abstract

The mechanism by which cyclic AMP facilitates secretagogue stimulated catecholamine (CA) release from cultured bovine adrenal chromaffin cells was investigated. 8-Br cyclic AMP and forskolin enhanced CA release induced by acetylcholine (ACh). Ouabain, omission of extracellular K⁺ or veratrine enhanced ACh-evoked CA release and counteracted the effects of 8-Br cyclic AMP and forskolin. Cyclic AMP decreased Na⁺, K⁺-ATPase activity of microsomes from bovine adrenal medulla. 8-Br cyclic AMP, forskolin and catalytic subunit of cyclic AMP-dependent protein kinase (A-PK) reduced [γ-³²P]ATP hydrolysis in digitonin-permealilized chromaffin cells. This reduction of ATP hydrolysis was abolished in the presence of ouabain and thus suggested to be due to an inhibition of Na⁺, K⁺-ATPase. 8-Br cyclic AMP and forskolin alone slightly increased ²²Na uptake, intracellular Na⁺ concentration ([Na⁺]i), depolarization and raised intracellular free Ca²⁺ concentration ([Ca²⁺i), and encouraged these ACh-induced responses. 8-Br cyclic AMP and Forskolin enhanced CA release elicited by ACh in the presence of Ca²⁺ channel blockers. Potentiations by 8-Br cyclic AMP or forskolin of stimulation-evoked CA release, increase in ²²Na uptake, [Na⁺]i, depolarization and rise in [Ca²⁺]i were antagonized by tetrodotoxin, amiloride, or by the removal of Na⁺ from the medium, but not affected by K⁺ channel blockers. Ouabain, removal of K⁺ or veratrine mimicked the effects of cyclic AMP and these agents counteracted each other. 8-Br cyclic AMP enhanced CA release, ⁴⁵Ca efflux and [Ca²⁺]i rise induced by caffeine in the absence of extracellular Ca²⁺. These results may suggest that further enhancement of Ca²⁺ influx through the Na⁺/Ca²⁺ exchange mechanism and mobilization of intracellular Ca²⁺ from its stores due to reduction of the Na⁺ gradient as a result of inhibition of Na⁺, K⁺-ATPase by cyclic AMP, is involved in the facilitation of CA release elicited by cyclic AMP.