Effects of Hypoxia on the Catecholamine Release, Ca2+ Uptake, and Cytosolic Free Ca2+ Concentration in Cultured Bovine Adrenal Chromaffin Cells

Abstract

The purpose of the present study is to clarify the effects of hypoxia on catecholamine release and its mechanism of action. For this purpose, using cultured bovine adrenal chromaffin cells, we examined the effects of hypoxia on high (55 mM) K⁺‐induced increases in catecholamine release, in cytosolic free Ca²⁺ concentration ([Ca²⁺]_i), and in ⁴⁵Ca²⁺ uptake. Experiments were carried out in media pre‐equilibrated with a gas mixture of either 21% O₂/79% N₂ (control) or 100% N₂ (hypoxia). High K⁺‐induced catecholamine release was inhibited by hypoxia to ∼40% of the control value, but on reoxygenation the release returned to control levels. Hypoxia had little effect on ATP concentrations in the cells. In the hypoxic medium, [Ca²⁺]_i (measured using fura‐2) gradually increased and reached a plateau of ∼1.0 μM at 30 min, whereas the level was constant in the control medium (∼200 nM). High K⁺‐induced increases in [Ca²⁺]_I were inhibited by hypoxia to ∼30% of the control value. In the cells permeabilized by digitonin, catecholamine release induced by Ca²⁺ was unaffected by hypoxia. Hypoxia had little effect on basal ⁴⁵Ca²⁺ uptake into the cells, but high K⁺‐induced ⁴⁵Ca²⁺ uptake was inhibited by hypoxia. These results suggest that hypoxia inhibits high K⁺‐induced catecholamine release and that this inhibition is mainly the result of the inhibition of high K⁺‐induced increases in [Ca²⁺]_i subsequent to the inhibition of Ca²⁺ influx through voltage‐dependent Ca²⁺ channels.