| Abstract: | Effects of adrenocorticotropin (ACTH) on cytoplasmic free calcium concentration, Ca2+]c, have been measured in adrenal glomerulosa cells using a calcium-sensitive photoprotein, aequorin. ACTH causes a rapid and transient increase in Ca2+]c. Dose response study demonstrates that 1 pM ACTH induces an elevation of Ca2+]c and that effect of ACTH appears to be saturated at 100 pM. ACTH action is greatly inhibited but not abolished by removal of extracellular calcium and is completely blocked in medium containing no added calcium and 1 mM EGTA. Under similar conditions, angiotensin II induces a remarkable rise in Ca2+]c. ACTH action is not affected by pretreatment with dantrolene, which considerably decreases angiotensin II action on Ca2+]c. One micromolar forskolin, which mimics 1 nM ACTH-mediated elevation of intracellular cAMP, does not increase Ca2+]c nor modulates changes in Ca2+] induced by a low dose of ACTH. One hundred micromolar forskolin or 1 mM 8-bromo-cAMP, however, increases Ca2+]c even in calcium-free medium containing 1 mM EGTA. When glomerulosa cells are co-loaded with aequorin and quin2, angiotensin II-induced change in aequorin signal is greatly reduced, and ACTH-induced change is abolished. Quin2 loading results in accumulation of calcium in the cell under both unstimulated and stimulated conditions. These results indicate that ACTH increases Ca2+]c by cAMP-independent mechanism, that ACTH action on Ca2+]c is exclusively dependent on extracellular calcium, and that quin2 is unable to detect the rapid change in Ca2+]c because of its calcium chelating activity. |
