ISCHEMIA-INDUCED CHANGES IN CATECHOLAMINE RELEASE AND THEIR MECHANISMS - A STUDY USING CULTURED BOVINE ADRENAL CHROMAFFIN CELLS

Ischemia-induced changes in neurotransmitter release and their mechani sms 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 condition s, basal catecholamine (CA) release was elevated and the elevation per sisted up to 90 min. High K+-evoked CA release was transiently enhance d at 10 min, but after that, it decreased to reach the minimum level a t 60 min. At 10 min, cytosolic free Ca2+ concentration (Ca2+(i)) and Ca-45(2+) uptake of the resting cells (basal values) and high Kf-evok ed increases in these two parameters were unchanged, but CA release fr om permeabilized cells in response to Ca2+ in media was augmented. Aft er 60-min incubation under ischemic conditions, basal Ca2+(i) was el evated: the elevation was observed even in the absence of extracellula r Ca2+. In contrast, high Kc-evoked increases in Ca2+(i) and in Ca-4 5(2+) uptake were suppressed, but basal Ca-45(2+) uptake into intact c ells and CA release from permeabilized cells were unchanged. These res ults suggest that in an early phase (10 min) of ischemia, both basal a nd stimulation-evoked CA release are augmented because of increased se nsitivity of exocytotic machinery to Ca2+. In a late phase (60 min), b asal CA release is augmented because of an increase in basal Ca2+(i) , which is due to accumulation of Ca2+ derived from intracellular Ca2 pools: stimulation-evoked CA release is suppressed because of inhibit ion of stimulation-evoked increase in Ca2+(i), which is due to funct ional disturbance of voltage-dependent Ca2+ channels.