Ischemia-induced changes in catecholamine release and their mechanisms: a study using cultured bovine adrenal chromaffin cells

Abstract

Ischemia-induced changes in neurotransmitter release and their mechanisms were examined using cultured bovine adrenal chromaffin cells. When the cells were incubated in glucose-free media equilibrated with 0% O 2/100% N 2 (ischemia), ATP content decreased and reached the minimum level within 40 min. Control incubation was done in media equilibrated with 21% O 2 in N 2. After 10-min incubation under ischemic conditions, basal catecholamine (CA) release was elevated and the elevation persisted up to 90 min. High K +-evoked CA release was transiently enhanced at 10 min, but after that, it decreased to reach the minimum level at 60 min. At 10 min, cytosolic free Ca 2+ concentration ([Ca 2+] i) and 45Ca 2+ uptake of the resting cells (basal values) and high K +-evoked increases in these two parameters were unchanged, but CA release from permeabilized cells in response to Ca 2+ in media was augmented. After 60-min incubation under ischemic conditions, basal [Ca 2+] i was elevated: the elevation was observed even in the absence of extracellular Ca 2+. In contrast, high K +-evoked increases in [Ca 2+] i and in 45Ca 2+ uptake were suppressed, but basal 45Ca 2+ uptake into intact cells and CA release from permeabilized cells were unchanged. These results suggest that in an early phase (10 min) of ischemia, both basal and stimulation-evoked CA release are augmented because of increased sensitivity of exocytotic machinery to Ca 2+. In a late phase (60 min), basal CA release is augmented because of an increase in basal [Ca 2+] i, which is due to accumulation of Ca 2+ derived from intracellular Ca 2+ pools: stimulation-evoked CA release is suppressed because of inhibition of stimulation-evoked increase in [Ca 2+] i, which is due to functional disturbance of voltage-dependent Ca 2+ channels.